Biorefinery for Water and Wastewater Treatment 3031208218, 9783031208218

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Maulin P. Shah   Editor

Biorefinery for Water and Wastewater Treatment

Biorefinery for Water and Wastewater Treatment

Maulin P. Shah Editor

Biorefinery for Water and Wastewater Treatment

Editor Maulin P. Shah Environmental Microbiology Laboratory Ankleshwar, Gujarat, India

ISBN 978-3-031-20821-8 ISBN 978-3-031-20822-5 (eBook) https://doi.org/10.1007/978-3-031-20822-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Contents

Wastewater as a Feasible Feedstock for Biorefineries . . . . . . . . . . . . . . . . . . Caroline Dalastra, Thamarys Scapini, Simone Kubeneck, Aline Frumi Camargo, Natalia Klanovicz, Sérgio Luiz Alves Júnior, Maulin P. Shah, and Helen Treichel An Overview of Nanomaterials—Synthesis, and Their Applications for Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Subhasis Ghosh, Sayan Mukherjee, Sk. Aakash Hossain, Poushali Chakraborty, Sanket Roy, and Papita Das

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Biogas as a Value Generation in Industrial Wastewater—A Review . . . . B. Saikat, S. Sivamani, and B. S. Naveen Prasad

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Nanotechnology-Based Solutions for Wastewater Treatment . . . . . . . . . . . Km. Sakshi and Navneeta Bharadvaja

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Municipal Wastewater as Potential Bio-refinery . . . . . . . . . . . . . . . . . . . . . . Shipra Jha and Nahid Siddiqui

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Bio-diesel Production as a Promising Approach of Industrial Wastewater Bio-refinery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 Avijit Chakraborty, Shreyan Bardhan, Sudip Das, Sagnik Roy, and Banani Ray Chowdhury Nutrient Recovery and Utilization from Wastewater for Soil-Less Agriculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Avijit Chakraborty, Medha Maitra, Banani Ray Chowdhury, and Chaitali Dutta Industrial Wastewater to Biohydrogen Production via Potential Bio-refinery Route . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 Pranjal P. Das, Deepti, and Mihir K. Purkait

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Development of a Novel Upflow Anaerobic Sludge Blanket (UASB) System for Treating Milk Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Khac-Uan Do, Dac-Chi Tran, and Gia-Khanh Nguyen Biofertilizer from Industrial Waste Water by Microalgal Treatment . . . . 197 N. Prabhu, M. Mounika, A. Sureja, M. Shareen Fathima, and N. Hiritha Membrane Bioreactor (MBR) Technologies for Treatment of Tannery Waste Water and Biogas Production . . . . . . . . . . . . . . . . . . . . . . 217 Mahadevan Vaishnavi, Kannappan Panchamoorthy Gopinath, and Praveen Kumar Ghodke Emerging Technologies for Separation and Recycle of Phosphorous from Sewage Sludge for Hydroponic Farming System . . . . . . . . . . . . . . . . . 249 Rashmi S. Shenoy, Prathibha Narayanan, and Savithri Bhat Recent Development and Innovations in Integrated Biogas-Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 Aishee Ghosh, Aishani Ray, Akash Goswami, Omar Aweis Ali, Puneet Kumar Singh, and Ritesh Pattnaik Integration of Membrane Technology in Microalgal Photobioreactor for Biodiesel Production Along with Industrial Wastewater Remediation: A Green Approach . . . . . . . . . . . . . . . . . . . . . . . . 299 Srijoni Banerjee, Shubhangi Gupta, Antara Dalal, Tanishka Hazra, Maulin P. Shah, and Sourja Ghosh Microbial Lipids as a Source of Value-Added Products: A Biorefinery Perspective . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Sunny Dhiman and Gunjan Mukherjee Bio-fertigation of Different Industrial Waste . . . . . . . . . . . . . . . . . . . . . . . . . 337 P. Malliga, N. Geetha, and G. Jenifer Biogas as a Value Generation from Dairy Industrial Waste Water . . . . . . 359 N. Prabhu, M. Shareen Fathima, N. Hiritha, M. Mounika, and A. Sureja Integration of Biogas Production from Wastewater as Value Generation in Biorefineries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377 T. R. Balbino, S. Sánchez-Muñoz, M. A. Yaverino-Gutiérrez, E. Mier-Alba, M. J. Castro-Alonso, J. C. dos Santos, S. S. da Silva, and N. Balagurusamy Bioprospecting of Microorganisms for Novel and Industrially Relevant Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 Sonia Sethi and Samvida Saxena Microbial Fuel Cell Usage in Treatment, Resource Recovery and Energy Production from Bio-refinery Wastewater . . . . . . . . . . . . . . . . 425 Rajesh Singuru, G. Praveen Kumar, and Adhidesh S. Kumawat

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Microbiotechnology-Based Solutions for Removal and Valorization of Waste in Pulp and Paper Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 445 Nada Verdel, Mija Sežun, Tomaž Rijavec, Maja Zugan, Dmitrii Deev, Iaroslav Rybkin, and Aleš Lapanje Promising Approach of Industrial Wastewater Bio-refinery Through Bio-diesel Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 481 A. Anuradha, Aakansha Singh, Somya Sadaf, and Muthu Kumar Sampath

Wastewater as a Feasible Feedstock for Biorefineries Caroline Dalastra, Thamarys Scapini, Simone Kubeneck, Aline Frumi Camargo, Natalia Klanovicz, Sérgio Luiz Alves Júnior, Maulin P. Shah, and Helen Treichel

Abstract Large volumes of wastewater are generated by industrial and urban activities and can be effectively used in other processes to reduce dependence on freshwater. To prevent risks to human and environmental health, the treatment of effluents before discharge is imperative. In conventional biofuel production, large volumes of fresh water are needed for the entire process. The development of technologies and strategies based on non-potable water resources is fundamental for optimizing biofuel biorefineries and value-added products in the market. Thus, wastewater application is an alternative to freshwater resources in a circular economy context. In addition to promoting wastewater management and treatment, energy recovery, value-added products, and reduced competition for freshwater, naturally occurring compounds in wastewater can function as nutrient sources for cell growth and maintenance of microorganisms. To carry out a comprehensive review on the alternative use of water for bioproduct purposes, in this chapter, we will address the most recent studies C. Dalastra · T. Scapini · S. Kubeneck · A. F. Camargo · N. Klanovicz · H. Treichel (B) Laboratory of Microbiology and Bioprocesses, Federal University of Fronteira Sul, Erechim, Brazil e-mail: [email protected] T. Scapini Department of Bioprocess Engineering and Biotechnology, Polytechnic Center, Federal University of Parana, Curitiba, PR, Brazil A. F. Camargo Graduate Program in Biotechnology and Bioscience, Federal University of Santa Catarina, Florianópolis, Brazil N. Klanovicz Research Group in Advanced Oxidation Processes (AdOx), Department of Chemical Engineering, Escola Politécnica, University of São Paulo, São Paulo, Brazil S. L. A. Júnior Laboratory of Biochemistry and Genetics, Federal University of Fronteira Sul, Chapecó, SC, Brazil M. P. Shah Industrial Wastewater Research Lab, Division of Applied & Environmental Microbiology, Enviro Technology Limited, Ankleshwar, Gujarat, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_1

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on the potential of wastewater as a viable raw material both for the prospecting of microorganisms and for the replacement of freshwater in biorefineries. Keywords Bioproducts · Biofuels · Energy recovery · Bioeconomy · Prospecting for microorganisms

1 Introduction In recent decades, the unrestrained use of fossil fuels for energy production has triggered a series of problems related to energy, environmental and human security (Yao et al. 2019). Carbon emissions have created significant climate change risks. Recently, in a report by the Intergovernmental Panel on Climate Change (IPCC 2021), scientists reaffirmed that it is imperative to reduce the emission of air pollutants to minimize the increase in global temperature. The consequences of pollutants emitted by burning fossil fuels to date will persist for centuries. They will continue to cause changes in the climate system to biodiversity, ecosystems, and human health (IPCC 2021). This scenario has encouraged government agencies worldwide to propose objectives and guidelines to mitigate the impacts caused using non-renewable resources and avoid or reduce air pollutants emissions. One of the most prominent instruments created by the United Nations is the 17 sustainable development goals, aiming to eradicate poverty, protect the environment and the climate, and guarantee the quality of people’s lives, to achieve global sustainability (UN 2021). Based on the linear economy, the current economic model is unsustainable and unstable, where raw materials are extracted and transformed into products discarded after use. The waste generation will not cease but increase and diversify due to agroindustrial activities and population growth, putting the planet’s biodiversity at risk and causing loss of aquatic systems and atmospheric quality since a large part of this waste is not adequately managed to cause significant damage to the environment. In contrast, as an example of (at least) a partial solution, lignocellulosic biomass arises as a valuable resource for the biorefinery industry, as it can be used in integrated processes to obtain several biologically based products (such as fuels, chemicals, and electricity) seeking to optimize the extraction of the compounds, minimize waste disposal and promoting the circular economy (Usmani et al. 2021). Choosing inputs is a relevant decision when designing sustainable processes, given the use of large volumes of water from cultivation to biomass processing. However, sustainable alternatives that use waste as a raw material are not enough; it is also necessary to consider other inputs, e.g., water resources. In this sense, it is essential to develop strategies to meet the demand for water in the biorefinery sector in an environmentally efficient manner. Wastewater is a resource-rich in nutrients and compounds, e.g., chloride, sodium, sulfate, and magnesium ions, which can act as a catalyst in biomass fractionation, a nutrient source, and a cultivation medium for microorganisms. The creation of

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industries, driven by the advancement of technology, affected all aspects of human life, with several adverse effects associated with water quality, such as the release of chemicals and pollutants in these resources. Different types of wastewaters are being produced daily and in large volumes in urban centers and different industrial sectors— such as food, textile, dairy, pharmaceutical, among others. However, some types of wastewaters may contain undesirable chemical compounds and pathogens that have adverse and unwanted effects on human and environmental health, thus requiring specific treatment before disposal (Nikolaou and Kourkoutas 2018; Louhasakul et al. 2021). Raising awareness of environmental safety issues and water pollution is one of the significant needs. The use of wastewater as a cultivation medium for microorganisms is interesting because of two scenarios: (a) the possibility of providing different, inexpensive, and readily accessible nutrients for the growth and maintenance of microorganisms’ cells; and (b) the production and recovery of compounds and bioenergy that can be commercialized. In these scenarios, wastewater from industrial processes and urban activities can be considered a valuable source of nutrients, an essential raw material, and a potential bioresource to the circular economy, reducing water consumption in bioprocesses and valuing economically through recovery purification and commercialization of compounds.

2 Wastewater as a Microorganism Source and Substrate Microorganisms are protagonists in biorefineries processes, but they played essential roles in nature long before this application. Fungi, bacteria, and microalgae are vital microorganisms in maintaining complex ecosystems and have the necessary adaptability to change with environmental fluctuations. This adaptability is already being exploited in favor of anthropic activities. Adaptive laboratory evolution (ALE) tools, based on Darwin’s natural selection theory, have been used to pressure microorganisms to modify their phenotype (biochemical activity, behavior, and morphology) and play roles in industrial processes under environmental conditions considered unfavorable (Mans et al. 2018; Kuroda and Ueda 2018; Shah 2020). In addition to cultivating microorganisms and their use in industrial processes, it is also possible to obtain them from unfavorable environments, such as wastewater. The purpose of prospecting microorganisms follows the inverse logic of ALE tools: microorganisms found in adverse environments are already adapted to such conditions and, therefore, have greater chances of maintaining their phenotype when removed from there. In this way, their chance of surviving in adverse conditions increases. Furthermore, their occurrence in these environments is an indication that the ecosystem has the necessary conditions for their cells and metabolism viability. When thinking about wastewater, some essential aspects for the growth and maintenance of microorganisms can be pointed out: (a) excess or lack of nutrients; (b) presence of toxic compounds; (c) unfavorable pH and temperature. Therefore,

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from prospecting to cultivating microorganisms, it is essential to know the chemical composition of wastewater and ensure that this composition remains constant or with low fluctuations. Table 1 presents some recent studies of prospected microorganisms in wastewater indicating the enormous potential they harbor, with a wide variety of genera often living in symbiosis. These waters are from industries and urban wastewater, preand post-treatment, having different chemical compositions and, therefore, other microorganisms. An exciting aspect of Table 1 is that some wastewaters contain heavy metals. After microorganism isolation, cells were tested in the presence of high concentrations of these metals and continued with excellent metabolic performance (Banala et al. 2021; Shah 2021; Cheng et al. 2021; Vélez et al. 2021, Arif et al. 2020; Baldisserotto et al. 2020). This demonstrates that phenotype is maintained even after isolation; these microorganisms are adapted to the conditions imposed by wastewater. Another important highlight of the works in Table 1 is the relevant role of intra and extracellular enzymes and the greater efficiency of living biomass when compared to dead biomass (Banala et al. 2021; Vélez et al. 2021). This finding is exciting as it indicates that the viability of vital functions is relevant in applying these microorganisms in biorefineries. Changes in environmental conditions, even when small, require an acclimatization phase and, therefore, can cause the retraction of metabolites and growth. Although in some cases shown in Table 1, the acclimation time and lag phase were long, e.g., more than 20 days for microalgal species (Arif et al. 2020), all studied microorganisms could return their metabolic activities or develop new activities based on imposed environmental pressure. This reinforces the concepts of Darwin’s theory and points out that microorganisms in wastewater have inherent metabolic plasticity, offering biorefineries a wide range of industrially relevant compounds, such as enzymes (Bernal et al. 2021). It is worth noting, still concerning Table 1, the large number of recent studies prospecting bacteria in wastewater, indicating the predominance of this microorganism in these environments. This fact can be explained by its resistance to the processes used in wastewater treatment plants (WWTPs) or its prevalence compared to other microorganisms, winning in the competition for nutrients or adaptability to adverse conditions. Some members of this genera are pathogenic, and therefore undesirable. While it is more appropriate for human health to eliminate these microorganisms, biotechnology can potentially produce compounds or enzymes of interest. However, when targeting processes with large volumes, a basic rule must be followed: prioritize the use of non-pathogenic microorganisms. Even if some pathogenic microorganisms have great biotechnological potential, before using them, the other prospecting possibilities for non-pathogenic microorganisms must be exhausted. Although fungi have not been highlighted in prospective studies in the last two years, it is known that WWTPs harbor communities of these microorganisms with great biotechnological potential, which has been evidenced over the years. At the end of the twentieth century, Yagüe et al. (2000) and González et al. (2000) pointed out

Wastewater characteristics

Presence of lead (Pb)

30 °C, pH 7.0

Alkaline pH

Wastewater source

River in an industrial zone with heavy metal contamination

Anaerobic reactor treating textile wastewater

Uranium mine wastewater

Bacteria: Arthrobacter, Bacillus, Micrococcus, and Kocuria

Bacteria: Ochrobactrum anthropi

Bacteria: Pseudomonas aeruginosa, Pseudomonas nitroreducens, and Pseudomonas alcaligenes

Isolated microorganism

Table 1 Prospection and isolation of microorganisms from wastewater

– Strain metabolism improvement under low carbonates/bicarbonate concentration – Capacity to tolerate concentrations up to 1500 mg/L of uranium – Capacity to tolerate several heavy metals (Cu, Cr, Cd, Co, and Mn), but under phenotype changes – Pollutant sequestration by cell membrane phosphate, amide, and carboxyl functional groups – Intracellular bioaccumulation of uranium – Live biomass removes more uranium than dead biomass

– Considerable influence of pH and temperature on strain growth – Strain has better adaptability in neutral or alkaline environments – The addition of electron donors and heavy metal ions increase the bacteria metabolism – Capacity to tolerate concentrations up to 400 mg/L of Reactive Black 5 and 100 mg/L of Cr (IV) – The pollutants removal mechanism was mainly dependent on biodegradation rather than biosorption – O–H, C–O, and C–H groups at the cell surface might be involved in the transport of enzymes from intracellular to extracellular

Catalase negative and oxidase-positive Pigment and fluorescence producers when exposed to UV Capacity to tolerate concentrations higher than 50,000 mg/L of Pb Live biomass removes more lead than dead biomass, independent of the time and strain – The tolerance can be attributed to the production of exopolysaccharides (EPS) and metal biosorption

– – – –

Microorganism characteristics

(continued)

Banala et al. (2021)

Cheng et al. (2021)

Vélez et al. (2021)

References

Wastewater as a Feasible Feedstock for Biorefineries 5

24–29 °C COD: 556 mg/L TN: 142 mg/L

Sequencing batch bioreactor and photo-bioreactor treating a mixture of municipal and molasses wastewater

Activated sludge from Aeration tank with an municipal WWTP occasional dosage of iron or aluminum salts COD: 27.4 mg/L TN: 4.59 mg/L TP: 0.39 mg/L

Bacteria: Alicyclobacillus hesperidum

pH 8.3 COD: 3887 mg/L TOC: 2023 mg/L TN: 266 mg/L

Organic acid fermentation wastewater from a sewage treatment plant

Bacteria: Candidatus Microthrix parvicella and Candidatus Microthrix subdominans

Bacteria: Klebsiella sp. and Escherichia coli

Isolated microorganism

Wastewater characteristics

Wastewater source

Table 1 (continued)

Zhang et al. (2021)

References

Filamentous microorganism Prevalence of lipid metabolism Triacylglycerol used as an energy storage compound Ca. M. parvicella prevails in winter (below 15 °C), while Ca. M. subdominants did not show a strong seasonal pattern – COD is essential for the microorganisms occurrence – Genomic potential to use nitrate or nitrite as a terminal electron acceptor under anoxic conditions – Floc properties, essential for wastewater treatment processes

– – – –

(continued)

Nierychlo et al. (2021)

– Fast growth Omar et al. – Manganese peroxidase-producing bacteria (2021) – Maximum cell density between 2 and 4 days, depending on medium composition – The strains requires glucose or peptone to reach the maximum growth rate – Capacity to tolerate concentrations up to 7000 mg/L of melanoidins (glucoseglutamic acid, glucose-aspartic acid, sucroseglutamic acid, and sucrose-aspartic acid) – The strains modify the pH of the medium during melanoidins decolorization, probably because of the sugars fermentation and the amino acid metabolism

Electrochemically active Good acid resistance (below pH 3.0) High-temperature resistance (up to 50 °C) High electricity generation ability Capability to use multiple organic substrates (e.g., glycerol, sucrose, trehalose, starch, and cellobiose) Omega-alicyclic fatty acids are the main membrane components and form a protective coating for cells

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Microorganism characteristics

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(continued)

– Round and unicellular shape Padri et al. – Ability to use various carbon sources (glucose, sucrose, fructose, (2021) mannitol, and galactose) in mixotrophic culture – In biogas wastewater, the logarithmic phase of growth occurred until day six, when the maximal biomass concentration was achieved (up to 2653 mg/L) – Microalgal growth was not negatively affected by detrimental effects or competition with other microorganisms – The strains utilize soluble organic carbon from the wastewater for biomass generation, achieving a COD removal efficiency of up to 73% – The microalgal harvesting processes by coagulation and flocculation utilizing FeCl3 were efficient with these strains

pH 7.6 COD: 205 mg/L TN: 70.61 mg/L TP: 37.26 mg/L

Cassava biogas effluent wastewater treatment system

Microalgae: Chlorella sorokiniana

Fungi: Sarocladium sp. and Penicillium sp. – Filamentous microorganism Bernal et al. – Antimicrobial activity against pathogenic bacteria (Gram-positive and (2021) Gram-negative) – Intra and extracellular protease activity – Presence of a proteolytic enzyme in intra and extracellular fractions – Sarocladium sp. presents collagenase activity – The microorganisms have pharmaceutical potential because of their enzymatic and metabolic profile

28 °C, pH 7.0

References

WWTP of a textile industry

Microorganism characteristics

Isolated microorganism

Wastewater characteristics

Wastewater source

Table 1 (continued)

Wastewater as a Feasible Feedstock for Biorefineries 7

pH 7.5 Microalgae: Chlorella-like Chlorophyta COD: 222 mg/L (based on morphological observations) TN: 61.8 mg/L TP: 24.8 mg/L Presence of heavy metals (Cr (VI), Pb, Hg, Ni, and Cu)

Thickening stage of an Urban WWTP

References

– Small-size (2–3 µm) and spherical cells, containing a large chloroplast with an evident pyrenoid – Vacuolations contain dark precipitates, ascribable to polyphosphate depositions, suggesting an intracellular P accumulation – Compared to a synthetic medium, the wastewater strongly promoted algal growth – The logarithmic phase of growth occurred until day seven – Strain already adapted to toxic elements and biological competitors (symbiotic metabolism with bacteria from the wastewater) – Ammonium and phosphates removal ability (up to 94%) at four cultivation days in wastewater – A possible activation of mixotrophic metabolism because of the wastewater characteristics – Microalgal biomass rich in chlorophyll, carotenoids, starch, and proteins, suggesting possible use as food/feed supplements and biofertilizers

Baldisserotto et al. (2020)

– The strains biocomponents ranged from 19.47–29.64%, Arif et al. 39.39–52.51%, and 15.08–22.75% for carbohydrate, protein, and lipid (2020) – Significant accumulation of lipid – Fractions of fatty acids (the predominant were palmitic acid, stearic acid, oleic acid, and linoleic acid) – The logarithmic phase of growth occurred until day twenty-eight – TN and TP removal ability (above 99%) after 28 cultivation days in wastewater – Higher carbon content and hydrogen content compared to rice straw and coal, making them potential candidates for biofuel production, emphasizing biodiesel – Unsaturation degree, saponification value, cetane number, iodine value, cold filter plugging point, and long-chain saturated factor profiles are acceptable according to internationally recognized standards of biodiesel

Microorganism characteristics

COD—Chemical Oxygen Demand; TOC—Total Organic Carbon; TN—Total Nitrogen; TP—Total Phosphorus; WWTP—wastewater treatment plant

Microalgae: Chlorella sorokiniana, and Parachlorella kessleri

Collected during the spring season, pH 7.1 COD: 196 mg/L TN: 27.6 mg/L TP: 3971 mg/L Presence of heavy metals (Cr, Fe, Ni, and Cu)

WWTP influent

Isolated microorganism

Wastewater characteristics

Wastewater source

Table 1 (continued)

8 C. Dalastra et al.

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the capacity of the fungus Corolopsis gallica and Trametes sp. to produce laccase using beer-factory wastewater and distillery wastewater, respectively. For both cases, laccase production increased considerably (up to 35 times) when the concentration of wastewater in the cultivation medium was increased. The recovery of this extracellular enzyme after cultivation may have relevant applications in the context of biorefineries, such as for pretreatment of lignocellulosic biomass (Widsten and Kandelbauer 2008). The importance of the fungus Trametes versicolor related to its ability to produce laccase was also highlighted by Tišma et al. (2021). The authors point out the role of the fungus and its enzymes in the lignin valorization for the bioenergy industry. The association of T. versicolor with lignocellulosic residues can generate an enzymatic pool containing dehydrogenases, ligninolytic enzymes, hydrolytic enzymes, among other enzymes of great importance for biorefineries. In addition to the production of extracellular enzymes, fungal biomass may have a favorable physicochemical composition for industrial applications, as pointed out by Nitayavardhana et al. (2013). In their study, the edible fungus Rhizopus oligosporus was cultivated in an air-lift reactor using vinasse from the ethanol production process as a culture medium. It was possible to obtain fungal biomass with approximately 50% crude protein and essential amino acids compatible with commercial protein for aquatic feed. Hashemi et al. (2021) also received exciting results in the biomass composition of the filamentous fungi Mucor indicus, Mucor hiemalis, Neurospora intermedia, and Aspergillus oryzae cultivated in baker’s yeast wastewater. In addition to obtaining biomass with up to 44% protein, the fungi used in the study could produce pigments and biogas. Although there is an enormous biotechnological potential, already explored in the literature, of extra and intracellular enzymes, cells, biomass, mycelia, and fungal hyphae, there is still a significant challenge to be overcome: to develop a largescale cultivation process and, thus, obtain fungal compounds in large volumes for biorefineries (Tišma et al. 2021). Indeed, this is a challenge that extends to other microorganisms and biotechnological processes in general: obtaining large amounts of metabolite or biomass products and keeping them viable on an industrial scale. In this sense, prospecting microorganisms in wastewater and replacing costly culture media with wastewater are essential steps to overcome such challenges. For this purpose, it is also necessary to understand the physicochemical characteristics of wastewater that maintain viable fungal communities and how these characteristics affect their metabolism. Assress et al. (2019) evaluated the distribution of fungi classes in three different urban WWTPs (affluent and effluent samples). They found that the physicochemical characteristics of the samples significantly influenced the diversity of fungal communities. Phosphate, chloride, dissolved organic carbon, nitrate, magnesium, and zinc concentrations influenced fungi from Pezizomycetes, Lecanoromycetes, Agaricostilbomycetes, Schizosaccharomycetes, and Dothideomycetes. In common with these clades of fungi, dissolved organic carbon, phosphate, chloride, and manganese levels also influenced the occurrence of members from Eurotiomycetes, Exobasidiomycetes, Orbiliomycetes, Glomeromycetes, Saccharomycetes, and Leotiomycetes. However, these last ones were also affected by fluoride, calcium, and nickel. Fungi from

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the classes Agaricomycetes, Pucchinomycetes, Atractiellomycetes, Sordariomycetes, and Archaeorhizomycetes were influenced by the levels of nitrate, sulfate, carbon monoxide, and iron. In the Assress and coworkers’ (2019) study, it was also possible to determine the factors that did not influence the fungi occurrence within the wastewater composition range: pH, dissolved oxygen, bromide, conductivity, and salinity. These results are very relevant as they demonstrate that each class within a fungal community has particularities and affinities with specific environmental conditions; therefore, they reinforce the importance of biochemical and metabolic studies of these microorganisms, aiming to determine a range of wastewater parameters to maintain viable cells and produce compounds of industrial interest. Establishing a range of physicochemical composition is a fascinating strategy for wastewater, given its inherent fluctuation, especially for urban WWTPs. Environmental factors also significantly affect microalgal biodiversity and growth rate. These photosynthetic microorganisms naturally occur in aquatic environments and are considered biofactories due to their metabolic flexibility and carbon dioxide fixation characteristics through light, thus becoming energy-conducting cells. Microalgae can absorb inorganic nutrients, as they need nitrogen (N) and phosphorus (P) to carry out protein synthesis; therefore, their use as wastewater bioremediation agents can remove N and P, as well as help to reduce pathogens (Chai et al. 2021). Some microalgae species can also carry out chemoheterotrophic or mixotrophic metabolism, which is of great value for treating wastewater with a high organic load. This versatility opens opportunities for the isolation of microalgae adapted to wastewater conditions, which is relevant for obtaining strains with potential application on several fronts, including energy production and nutrient removal from wastewater (Wollmann et al. 2019). Indigenous microalgae strain isolated from wastewater show easy adaptation when applied in systems similar to those prospected. In this sense, the isolated strains are very promising, as they are highly resilient and tolerant and can achieve relevant results in cellular growth and nutrient removal efficiency. The selection of microalgal strains is linked to the bioremediation efficiency during the biomass growth phase. It is possible to obtain several bioproducts and bioenergy (Goswami et al. 2021). The genus Chlorella deserves to be highlighted in this scenario: it is found naturally in these environments, overgrows using wastewater as a substrate, and can assimilate nutrients like nitrogen and phosphorus (Padri et al. 2021). Wastewater treatment with microalgae is based on biochemical mechanisms: fixation, assimilation, precipitation, and bio-adsorption accumulation. Microalgae assimilate carbon through their heterotrophic or mixotrophic metabolism for carbohydrate and lipid synthesis. Depending on the source, wastewater can contain heavy metals, and microalgae can capture them through adsorption and bioaccumulation in their cellular compartments for metabolic regulation (Mohsenpour et al. 2021). We can consider that wastewaters are an opportunity to obtain an environment rich in nutrients, mainly nitrogen and phosphorus. In general, systems based on the integration of microalgal biomass growth processes and wastewater treatment presents several advantages: (a) removal of N and P simultaneously with the

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obtention of microalgal biomass; (b) cultivation of microalgae through a low-cost route, with wastewater reuse without adding chemicals and drinking water; (c) the growth of microalgae will stimulate CO2 capture and release O2 to the environment, reducing the negative impact of CO2 emissions; and (d) value is added to effluent for the production of another product, within the scope of the circular economy (Baldisserotto et al. 2020). However, improvements are still needed in a large-scale integrated system, even with relevant advantages. The cost of using microalgal biomass to produce biofuels is still twice as expensive as those from fossil sources. Furthermore, an important issue is the high risk of contamination involved in cultivating microalgae in industrial and municipal wastewater. In this case, it would be prudent to use pretreatment strategies such as pH corrections, solids sedimentation, hydrolysis, ultraviolet light, filtration, among others. Therefore, it is crucial to choose a good strain or microalgal consortium working in synergy, that is, to be robust with a high growth rate even in the presence of contaminating compounds (Brasil et al. 2017). Most of the time, conventional microalgae cannot access the high load of compounds in wastewater. The extremophilic microalgae are adapted and tolerant to extreme environmental conditions, with a metabolism specialized in surviving in these conditions, thermophilic, psychrophilic, acidophilic, halophilic, and others. Galdieria sulphuraria stands out in the microalgae scenario with extremophilic potential and is a good candidate for wastewater treatment. It can grow in highly acidic environments (pH 1.8) and at high temperatures (56 °C). It still manages to acidify the environment, reducing costs with pH control and the risk of contaminating the system (Wollmann et al. 2019). Microalgae can play an essential role in a circular economy by providing products such as proteins, lipids, and dyes within the biomass produced by the wastewater bioremediation process. The integration of robust microalgal strains and biotechnology added to bioprocess engineering can economically and environmentally make large microalgae-driven nutrient recycling systems feasible. The potential of microalgae can be attributed to their flexible metabolism and the reduction of energy, water, and carbon footprints. This can be assessed through the life cycle analysis tool, indicating data on the general sustainability of wastewater processing through the use of microalgae (Mohsenpour et al. 2021; Goswami et al. 2021).

3 Use of Wastewater to Produce Biocompounds The recovery of biocompounds combined with wastewater management from industrial processes and urban activities has gained significant attention in the literature. Wastewater can be a promising alternative for the production and recovery of commercially available compounds, such as enzymes, organic acids, biosurfactants, biopolymers, among others (Fig. 1). However, proper wastewater composition and technical solutions to combining wastewater treatment with industrial-scale production.

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Fig. 1 Possibilities of biocompounds recovered from industrial and urban wastewater

The use of wastewater for biocompound recovery generally requires pretreatment processes to improve efficiency in bioproducts production. Processes such as centrifugation (to reduce solids content), filtration (to remove impurities), and pasteurization (to reduce the load of indigenous microorganisms) are the main pretreatments described for this purpose (Marami et al. 2022). Recently, the production of lipids was evaluated using wastewater from the palm oil processing industry, and the pasteurization process was essential to obtain efficient lipid yields, considering that in the absence of pretreatment, native bacteria in the effluent were favored to the yeast inoculum Yarrowia lipolytica (Louhasakul et al. 2021). The carbon availability in media composed of wastewater positively influences the production of lipids; the greater the concentration of carbon in the medium, the greater the lipid production. Therefore, wastewater mainly from industrial food and beverage processes can be an alternative in lipid production due to its load of nutrients and carbon (Santos Ribeiro et al. 2019). Wastewater from cassava processing is another medium rich in nutrients and carbohydrates and was evaluated by Santos Ribeiro et al. (2019) for lipid production by yeast Rhodotorula glutinis. Lipid production of 1.34 g/L was obtained, showing the capacity of the yeast to accumulate this type of compound. In addition to the production of lipids, which can be used to obtain bioenergy, the use of wastewater in the production process of other value-added biocompounds for various areas, such as food and pharmaceuticals, is also desirable to reduce the consumption of freshwater and to make the process more sustainable. For example, the biopolymers chitin and chitosan, which can be used as a food supplement, are usually obtained from crustaceans. However, this source and extraction process

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presents disadvantages to the environment (Namboodiri and Pakshirajan 2019), and alternatives and efficient production methods are desirable. Using wastewater and microorganisms to obtain these compounds allows greater control over the amount and time. Based on this, the fermentation media of the microorganisms Cunninghamella elegans and Penicillium citrinum with wastewaters from the food, dairy, and paper manufacturing industries can produce up to 50 mg/g of chitin and chitosan (Berger et al. 2014; Namboodiri and Pakshirajan 2019). Wastewater from the dairy industry and sewage sludge from urban activities are rich polysaccharides, proteins, and lipids sources, making them an excellent feedstock to biorefineries. Among the compounds that can be obtained from these sources, organic acids (e.g., lactic, succinic, and propionic acid) stand out, which are produced by microorganisms that can metabolize lactose, such as Kluyveromyces sp. and Lactobacillus plantarum (Venkata Mohan et al. 2007; Jing et al. 2021). In addition to organic acids, municipal wastewater can produce carotenoids such as astaxanthin, considered a natural antioxidant. This biocompound can be found in microalgae Haematococcus pluvialis and Chlorella zofingiensis, for example, due to their capacity to accumulate up to 5% of astaxanthin in their dry weight (Kang et al. 2006). These microalgae are predominant in freshwater, but they can be cultivated in wastewater, making it possible to obtain astaxanthin by an economically viable process. In addition, the residual biomass from astaxanthin extraction can be used as biofertilizer and as animal feed due to the portion of nutrients present in its composition (Nishshanka et al. 2021). The use of wastewater in the production of biocompounds is an alternative route to consuming freshwater and chemicals that could make these compounds less sustainable. In addition, the reuse of these waters has economic advantages by enabling the production of compounds with relatively low costs compared to the conventional production method.

4 Biomass Pretreatment Wastewater Recycling Biorefineries are considered advantageous because they make it possible to obtain different biocompounds and forms of bioenergy from the use of different types of biomasses. Lignocellulosic biomass has been widely studied for its exciting characteristics for converting biofuels, energy, and products of interest to various industrial sectors and being the most abundant raw material available globally (Usmani et al. 2021). However, there are still difficulties in converting lignocellulosic biomass into bioproducts with high yields due to the complexity of its structure (Vu et al. 2020). Lignocellulosic biomass has a complex matrix formed by cellulose, hemicellulose, and lignin that results in a recalcitrant structure that depends on fractionation processes, such as pretreatment, capable of breaking these structures, thus improving the accessibility of enzymes to the carbohydrates and facilitating the conversion of these components into simple sugars. However, during the pretreatment run, large volumes of wastewater are generated due to the need for varying amounts of solvents

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and other chemicals. Because of this, and in a context of a circular economy aimed at using all the resources available by the biorefinery in its production, the reuse of pretreatment wastewater to produce other compounds is a viable alternative, as it promotes the reduction of the use of freshwater and the consumption of chemical substances in the production process. The pretreatment of lignocellulosic biomass can be carried out chemically, physically, physiochemically, and biologically. The chemical process is one of the most applied as it is considered favorable in industrial processes, is generally carried out with acids and alkalis—for example, sulfuric acid (H2 SO4 ), hydrochloric acid (HCl), phosphoric acid (H3 PO4 ), nitric acid (HNO3 ) and sodium hydroxide (NaOH) (Scapini et al. 2021). Studies have been developed to recycle acids, alkalis, and solvents applied to pretreatment. The recycling of H3 PO4 from wheat straw pretreatment waste liquor was investigated. This process was considered efficient because it allowed reusing concentrated H3 PO4 up to 10 times to pretreat the biomass, with approximately 90% recovery and removal of hemicellulose and lignin of up to 95 and 50%, respectively (Yao et al. 2019). The reuse of liquors from biomass pretreatment, in addition to reducing the use of chemical substances, enables the recovery of mono, oligo, and polysaccharides such as xylose, xylooligosaccharides, and glucans. Zhu et al. (2015) carried out the liquor recycling from the pretreatment of rice straw with H2 SO4 . They evaluated the potential of the hydrolyzate from the recycling process in the recovery of xylose. As a result, the authors could reuse the hydrolyzate from the treatment with H2 SO4 five times without showing any difference in the biomass composition. The xylose recovery yield was 83.2%, which is attributed to the increase in the concentration of xylodextrin in the hydrolyzate after its recycling (Zhu et al. 2015). The application of the formilin process as a pretreatment of biomass, in which formic acid (CH2 O2 ) is used for delignification followed by treatment with NaOH to remove the formyl group from the biomass, ends up generating a liquor in the first stage of the CH2 O2 -rich pretreatment. Therefore, Zhao and Liu (2012) investigated the potential of reusing liquor containing CH2 O2 used in sugarcane bagasse delignification. This process made it possible to obtain high rates of polysaccharides such as glucans, reaching approximately 80% after six recycles and a delignification degree of 79.5% in five recycles, making this process an efficient alternative in reducing the use of chemical substances. NaOH is the most used when dealing with alkaline pretreatment because of its low cost. However, the use of alkaline compounds for this purpose generates large amounts of wastewater, also called black liquor, which has in its composition the permanence of most of the alkalis used at the beginning of the process. Based on this, black liquor recycling in the pretreatment process becomes an economically viable alternative (Chen et al. 2021). One of the ways of reusing black liquor is its reuse in conjunction with water from washing the biomass after being treated with alkali. Goshadrou (2019) investigated the application of this process in the pretreatment of the Congongrass weed for its subsequent use in obtaining biofuels. The author recycled 10 times the black liquor together with the residual washing water in the pretreatment and obtained, as

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a result, an increase in the accessibility of cellulose by the enzyme, a decrease in crystallinity, and removal of lignin and hemicellulose. There was a decrease in the enzymatic hydrolysis of biomass from 90.8%, with the alkali treatment, to 66.4% only after the ten recycling of the set of black liquor and wastewater. The recycling of liquor and wastewater in the pretreatment of lignocellulosic biomass is one of the most common processes. However, there are still other processes in which these substances can be reused. In wastewater with lower levels of toxic compounds, recycling the water in the enzymatic hydrolysis is possible. Monosaccharides generated from cellulose and hemicellulose degradation can function as a carbon source for microorganisms in the fermentation medium (Wang et al. 2017; Chen et al. 2021). Due to its richness in saccharides, lignin, and other compounds, the residual liquor can be applied to obtain other value-added products, such as xylose, which can be produced using the residual liquor from the acid pretreatment of rice straw with a purity of 90%, its production being possible due to the degradation of hemicellulose present in water (Zhu et al. 2015; Chen et al. 2021). The recycling of wastewater and liquors from the pretreatment of biomass makes the biorefinery process more economical and sustainable by reducing expenses with chemical substances and having the possibility of obtaining products with high purity such as xylose that can later be reduced into xylitol (Zhu et al. 2015).

5 Wastewater-Based Bioenergy Production Molding an energetic, sustainable, and secure future requires a set of transformations in the global energy sector. In 2020, when compared to 2019, the COVID-19 pandemic caused a drop of 8.5% and 8.7% in the worldwide consumption of fuels and biofuels, respectively, for transport (OECD-FAO 2021). As a result of policies and guidelines related to the reduction of air pollutants and dependence on fossil fuels, developed in most countries, projections indicate that the global demand for biofuels will recover in the coming years and that their production, until 2030, will continue to be supplied predominantly by primary agricultural commodities such as sugarcane, corn, and oils (OECD-FAO 2021). Such projections raise concerns about the potable water resources associated with raw materials for biorefinery. This is because the large-scale cultivation of energy crops would modify the demand for water in agriculture and bring competition for water for food and fuel, especially in arid and semi-arid regions. Different types of industrial and municipal wastewater have been studied to produce bioenergy simultaneously with the treatment of these resources using microorganisms such as fungi, yeasts, bacteria, and microalgae, thus promoting the concept of circular economy and further increasing the sustainability of the process. The cells of these microorganisms assimilate and accumulate different organic and inorganic compounds, such as carbohydrates, lipids, and proteins, which can be converted into biogas, bioethanol, biodiesel, and biohydrogen.

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Thus, wastewater management through bioprocesses to obtain bioenergy becomes an essential basis for a more renewable future concerning the global energy matrix, simultaneously providing environmental benefits both in the direction of wastewater treatment and sustainable generation bioenergy, in addition to reducing processing costs.

5.1 Biogas Biogas is produced by the anaerobic digestion of biomass, of plant or animal origin, from the breakdown of fats, proteins, and carbohydrates that make up organic matter. The fermentation process involves different microorganisms in the steps of hydrolysis, acidogenesis, acetogenesis, and methanogenesis, generating a mixture of gases whose most part is composed of methane (Louhasakul et al. 2021). Several factors affect the efficiency of anaerobic digestion, such as temperature, pH, volatile fatty acids, ammonium, and carbon/nitrogen (C/N) content. The pH depends mainly on ammonium and volatile fatty acids and is toxic to methanogen. Fatty acids can also inhibit the growth of methanogenic bacteria. Increasing the temperature improves the methane potential, but the C/N content must also be changed to avoid ammonia inhibition (Chou and Su 2019; Bhatia et al. 2021). In recent years, two main lines of biogas production have been discussed in the literature: (a) from microalgae cultivated in wastewater and (b) from the conversion of sewage sludge from municipal sewage treatment plants. Co-digestion of waste with municipal wastewater sludge can improve nutrient balance, increase the load of biodegradable organic compounds, and improve biogas recovery. Wastewater to produce microalgal biomass can minimize the demand for water and lower nutrient costs, and favor the production of biocompounds. Biomass with a higher content of C/N leads to a higher yield of methane, while biomass with a lower range causes an accumulation of ammonia and an increase in pH. Furthermore, the nitrogen and phosphorus concentration in wastewater can affect the biomass’s lipid, protein, and carbohydrate content (Bhatia et al. 2021). Arashiro et al. (2020) combined wastewater treatment with microalgae cultivation to produce natural pigments (phycobiliproteins) and generate biogas. After cultivation, efficient removals of COD, ammonium, and phosphorus were detected. The final biogas yield reached up to 199 mL CH4 /g-VS. These results highlight the beneficial relationship between wastewater management and biogas production. Municipal wastewater needs proper treatment to eliminate the potential for transmitting pathogens such as protozoa and bacteria. Electron beam treatment can change the characteristics of the sludge, increasing the availability of nutrients and, consequently, resulting in higher biogas yields. Chmielewski et al. (2021) obtained a biogas yield in 11–14 days in samples treated by electron beam comparable to the yield obtained in 21 days in untreated samples, interesting results for reducing process costs.

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In addition to these two main lines of research, other studies are being carried out with wastewater from different industries. Satisfactory methane yields (280 mL g-1 per COD consumed) were obtained when evaluating effluents from the palm oil industry as an influent in an anaerobic digester to produce biogas (Louhasakul et al. 2021). Co-digestion of dairy wastewater and raw glycerol from sludge cake transesterification was evaluated. The use of glycerol increased biogas production 225.6 mL/g-VSloaded and 1310.0 mL/g-VSremoved .

5.2 Bioethanol Bioethanol is usually obtained through anaerobic fermentation of carbohydrates using different microorganisms, fungi, bacteria, or yeasts, depending on the substrate content concerning the composition of pentoses and hexoses (Boboescu et al. 2018). This biofuel, refined from a range of biomasses, is one of the leading candidates for gasoline and has attracted significant attention in research (Yao et al. 2019). Most bioethanol is produced from first-generation commodities, that is, food sources, which raises debates about competition with food and arable land, increased deforestation, biodiversity loss, and large volumes of water for irrigation (Boboescu et al. 2018). Furthermore, considerable volumes of water are required to produce bioethanol. It is estimated that approximately 4 gallons of water are needed to make just 1 gallon of first-generation ethanol (Aden 2007). In the second-generation process, water consumption is even higher. In this sense, there is an urgent need to seek alternatives to replace primary commodities and replace drinking water in the production process. In general, the wastewater from the ethanol biorefinery is a rich source of nutrients and organic matter and has a low pH, the main limiting parameter for microalgae growth. However, the pretreatment application with ozone allows more significant biomass production, with a high content of neutral carbohydrates. Microalgal biomass with a high content of neutral carbohydrates can be used as a raw material to produce ethanol in sugarcane biorefineries (Heredia Falconí et al. 2021). Nannochloropsis gaditana was cultivated in different concentrations (0, 30, 60, and 100%) of municipal wastewater and evaluated for ethanol production from Saccharomyces cerevisiae. The carbohydrate content of microalgae cultivated in 30% of wastewater was higher than the other concentrations, where the maximum yield of ethanol, of 94.3 ± 5.5 mg/gbiomass , was observed (Onay 2018). Due to toxic compounds in some types of wastewaters, these can only be used to produce ethanol after undergoing a sterilization or dilution step. Dilution of wastewater from the oil mill and molasses from the sugar industry, in a 1:1 ratio, can increase ethanol production by up to 12.25 times (Nikolaou and Kourkoutas 2018). The feasibility of using whey permeate (WP), a by-product of the dairy industry, as a water substitute in the wheat fermentation process for ethanol production by S. cerevisiae, was evaluated. It was found that ethanol yield was not affected when WP was incorporated to replace process water. Most of the carbon source in wheat

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is glucose, being consumed simultaneously with galactose and fermentable sugars efficiently in the fermentation process. The abundant minerals and salt content present in WP did not affect yeast growth or influence ethanol production (Parashar et al. 2016). High salinity wastewater can inhibit several microorganism functions due to increased intracellular Na+ concentration. Recently, a process using effluent from shrimp production in fermentation was conducted by Wickerhamomyces sp. UFFSCE-3.1.2 and S. cerevisiae CAT-1, using papaya residues as substrate, were evaluated for potential for ethanol production. In fermentations with Wickerhamomyces sp. UFFS-CE-3.1.2 and with S. cerevisiae CAT-1, the ethanol yield was 0.42 g g-1 and 0.39 g g-1, respectively. In the order of 35 ppm, the high salinity of the wastewater did not affect the ethanol yield of the systems, indicating that the two yeasts are salt-tolerant (Bonatto et al. 2021).

5.3 Biodiesel Derived from renewable biological sources such as animal fats or vegetable oils, biodiesel has attracted worldwide attention as a direct replacement or blending component of diesel due to biodegradable, non-toxic, and environmentally benign nature. Because they do not need arable land and can use organic waste as a carbon source, microbial oils from yeasts, fungi, bacteria, and algae are receiving more attention than vegetable oils. The synergy between wastewater treatment and microalgae cultivation is interesting as it effectively reduces processing costs. Microalgae biomass is rich in lipids and can be used as an alternative source to produce biodiesel through transesterification reactions (Bhatia et al. 2021). The properties of biodiesel depend on the fatty acid composition. Thus, to improve the quality of this biofuel, it is necessary to reduce the content of polyunsaturated fatty acids, such as linolenic, and increase the content of monounsaturated fatty acids, such as oleic. Light intensity influences microalgae growth, lipid accumulation, and fatty acid content. When there is an increase in light intensity, there is a greater production of oleic acid and less linolenic acid (Arif et al. 2020). Scenedesmus oblique microalgae require high light intensity and daily lighting time for better growth and increased lipid accumulation. In 10 days of cultivation, S. opliquo presented an average growth rate of 0.20/d and biomass production of 380 mg/L (Fan et al. 2020). Bacteria can efficiently use low-cost raw materials as a carbon source. As carbon sources, lactose, sucrose, starch, glucose, and xylose have favored the accumulation of lipids in oleaginous bacteria. Behera et al. (2019) cultivated bacteria in wastewater from the dairy industry and obtained a lipid content of 72% and a productivity of 0.727 g/L d. The lipids accumulated by the bacteria during the exponential growth phase were neutral lipids, mainly composed of fatty acids of chain length C14:0C18:0. These results show the potential of using dairy wastewater as a raw material to produce biodiesel.

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5.4 Biohydrogen In recent years, the production of biohydrogen (BioH2 ) from a versatile range of renewable sources—such as biomass, organic waste, and wastewater—has received global attention, being considered a sustainable technology to produce biofuels, as there is no emission of air pollutants (Shanmugam et al. 2020). Combusting hydrogen produces a significant amount of energy, releasing only water as a byproduct. Furthermore, it has a high energy density (Venkata Mohan et al. 2007; Gudiukaite et al. 2021). However, despite the advantages, its diffusion in the market is hampered by technical and economic barriers. The main processes for the biological production of this biofuel are water biophotolysis, photofermentation, and dark fermentation (Paul et al. 2014). Among these processes, dark fermentation, where the biological conversion of organic compounds takes place in the absence of light, is more attractive as it does not require external energy, uses organic waste as a substrate, and has more industrial applicability, being considered an economical and high yield when compared to fossil fuel-based production (Venkata Mohan et al. 2007). In this process, microorganisms use complex hydrocarbons for the biosynthesis of pyruvate and subsequent production of BioH2 through the acidogenic pathway of glycolysis (Shanmugam et al. 2020; Gudiukaite et al. 2021). However, as the concentration of BioH2 increases, some by-products (e.g., ethanol and other alcohols) are observed due to metabolic changes, which can reduce BioH2 production (Shanmugam et al. 2020). Substrate characteristics are the determining factor in producing BioH2 , as they regulate the metabolic pathway involved, productivity, and the formation of byproducts. Substrates rich in organic compounds are highly potent to increase BioH2 production mainly via dark fermentation (Rajesh Banu et al. 2020; Gudiukaite et al. 2021). Municipal wastewater has a more significant amount of organic fraction that can be easily biodegradable and can be used directly as a substrate to produce BioH2 . Industrial wastewater rich in sugars, such as glucose, maltose, sucrose, arabinose, and carbohydrates, can be quickly metabolized by microorganisms (Rajesh Banu et al. 2020). In the study by Venkata Mohan et al. (2007), dairy wastewater was evaluated for the feasibility of producing BioH2 from wastewater treatment in a suspended growth sequencing batch reactor. The reactor was operated with variations in the organic loading rate (OLR), and after 12 h of operation, the H2 yield reached up to 0.118 mmol/min. When the OLR was lowered, a maximum BioH2 yield was observed after 16 h of operation (0.061 mmol/min). These results evidenced the influence of the substrate in the process since the BioH2 yield was higher in processes with a higher organic loading rate, that is, greater availability of substrate for microbial metabolism (Venkata Mohan et al. 2007). The influence of banana peel pretreatment on the photofermentative hydrogen production of Rhidobacter sphaeroides 158 DSM using brewery wastewater in a batch bioreactor was investigated. The results indicate that the pretreatment significantly increased the production of BioH2 , which can be attributed to the C/N ratio and

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the decrease in ammonium concentrations, which is responsible for the production of BioH2 . The maximum yield obtained in the study was 408.33 mL/H2 Lwastewater , 2.7 times greater than the tests without pretreatment. The authors report that a low content of C/N can inhibit the enzyme nitrogenase and increase the concentration of biomass, which results in reduced light penetration in the photobioreactor and, consequently, in a reduction in the production of BioH2 (Al-Mohammedawi et al. 2019). Yang et al. (2019) investigated the performance of co-fermentation of sewage sludge and fallen leaves. They observed that the maximum yield of BioH2 production was 37.8 mL/g of volatile solids added at a mixing ratio of 20:80 (sludge for leaves), 267% and 23.9% higher when compared to the mono-fermentation of sludge and fallen leaves, respectively. This result proved a synergistic effect of co-fermentation, which may be related to the acceptable C/N content in the 20:80 mixture ratio, increasing the microbial activity and providing the production of BioH2 . In addition, the mixture of leaves to the sludge may have diluted the content of inhibitors contained in the sewage sludge, which may also have enhanced the production of BioH2 (Yang et al. 2019). The use of microalgae Chlorella vulgaris, Scenedesmus obliquus, and Consortium C (a naturally occurring algal consortium) proved to be efficient for treating urban wastewater. After water remediation, the microalgal biomass remained in a photobioreactor for two weeks to induce sugar accumulation and was later converted into BioH2 by dark fermentation by Enterobacter aerogenes. The maximum production of BioH2 was 56.8; 40.8 and 46.8 mL/g of volatile solids for S. obliquus, C. vulgaris, and Consortium C, respectively (Batista et al. 2014). Furthermore, metal additives and nanoparticles play a considerable role in producing BioH2 . In dark fermentation, metal additives can increase electron transfer within the cell, enzyme production and provide vital nutrients for the growth of microorganisms (Rajesh Banu et al. 2020). Using ferrous ions and nitrate (100 mg/L) as enhancers improves BioH2 production from dairy effluent, with a hydrogen production rate of 5.729 and 5.208 mL/L.h in 24 h, respectively. Also, after fermentation and production of BioH2 , the effluent quality was improved (Paul et al. 2014). BioH2 production, combined with effluent management, is still in the development stage, where process control and optimization remain major challenges for the research sector.

6 Is Wastewater a Feasible Feedstock for Biorefineries? During biomass processing in biorefineries, valuable products can be generated and recovered. However, the implementation of full-scale biorefineries still needs to improve issues before making these processes successful. The technical operation, types of raw materials, efficient conversion techniques, inhibitor generation, and high economic cost are some of the problems. However, the use of other inputs

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must be considered, and one of the most significant challenges that have recently gained attention is the use of fresh water in the process. The conflict between the use of significant volumes of water in the most diverse industrial sectors with the scarcity of this resource has raised concerns today. In this sense, it is necessary to develop strategies to meet the demand for water in industrial sectors, especially in biorefineries, in an environmentally efficient manner. Wastewater can be considered a valuable source of nutrients, an essential raw material, and a potential bioresource in the circular economy, reducing water consumption in bioprocesses and making it economically more valuable through recovery purification commercialization of biocompounds. Wastewater-based biorefineries are a developing concept. These systems integrate wastewater treatment with obtaining value-added products, promoting the closing of the waste cycle, and further increasing the sustainability of the process (Fig. 2). This is a significant shift from the current linear economics model—extract, produce and the dispose. The potential of wastewater as a bioresource in biorefineries is widely investigated in the literature, and great success has been achieved in the recovery of bioenergy (e.g., biohydrogen, biogas, biodiesel, and bioethanol), bioproducts (e.g., acids and organic molecules), and in isolation and prospecting for microorganisms. Furthermore, replacing costly cultivation media with wastewater is essential in overcoming the challenges of setting up biorefineries. However, for the implementation of full-scale wastewater-based biorefineries, there are some challenges to be overcome: (a) integration of conventional wastewater

Fig. 2 Overview of wastewater potential in biorefineries. Blue arrows represent wastewater usage; black arrows represent agroindustrial waste applications. WWTP, wastewater treatment plant; UWW, urban wastewater (wastewater from urban activities); WP, whey permeate; LB; lignocellulosic biomass; SM, swine manure

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treatment processes with bioprocess technology; (b) proper composition of wastewater; (c) obtaining large quantities of products or metabolites/biomass and keeping them viable on an industrial scale; and (d) process costs.

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An Overview of Nanomaterials—Synthesis, and Their Applications for Wastewater Treatment Subhasis Ghosh, Sayan Mukherjee, Sk. Aakash Hossain, Poushali Chakraborty, Sanket Roy, and Papita Das

Abstract Nowadays scarcity of safe drinking water is a major point of concern, as more than 1.5 million people worldwide have not access to it. Millions of tons of industrial effluent are released into the water bodies every day, rendering the water unfit for both the aquatic life and human. As a result, there is a pressing need for the development of high-efficiency technology that can recycle industrial effluent to produce clean drinking water. Recent advancement has proved that nanotechnology can act as a bridge in proving efficient solution to this impeding issue of water crises. The characteristic nanoscale size of the material improved its adsorption, catalytic and reactivity properties. Many studies have reported that nanoparticles can effectively utilized to eliminate various pollutants from wastewater. This chapter casts light upon the nanomaterials like carbon/polymer/metal/zeolite and their underlying scientific principles for the wastewater treatment. A discussion on the processes of wastewater treatment both with traditional method and advanced method using nanomaterials has been presented. The risks associated with nanomaterials besides future perspective of these techniques is also investigated. Keywords Nanomaterials · Adsorption and photocatalysis · Membrane filtration · Waste water treatment · Sustainability · Future aspect

S. Ghosh · Sk. A. Hossain · S. Roy · P. Das (B) Department of Chemical Engineering, Jadavpur University, 188, Raja S. C. Mullick Road, Kolkata 700032, West Bengal, India e-mail: [email protected] S. Ghosh · S. Mukherjee Department of Life Science and Biotechnology, Jadavpur University, 188, Raja S. C. Mullick Road, Kolkata 700032, West Bengal, India S. Mukherjee · Sk. A. Hossain · P. Chakraborty · P. Das School of Advanced Studies for Industrial Pollution Control Engineering, Jadavpur University, Kolkata 700032, West Bengal, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_2

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1 Introduction For the survival of living beings and development of economy water is the most important resource. Nowadays it is hard to imagine that modern society can flourish without water. Earth, the only known planet in solar system with liquid water is named as the “Blue planet” because of its huge reserve of water. Furthermore, 68.9% of water is stored as ice, glaciers, and permanent snow, and 30.8% of fresh water is stored as groundwater, out of which only 0.3% is accessible. Nowadays one of the main problems humans are facing is the deterioration in water quality due to population growth, urbanization, extensive agricultural practices, and industrialization. Worldwide, almost 2.6 billion people are living in a lack of proper sanitation and millions of populations, especially children are facing life threats from various diseases caused by contaminated water. Diarrhea, a waterborne disease causes the death of 1.8 million children every year. Pollutant is a substance when introduced into the environment, cause adverse effect or spoil resources. When pollutants present on the earth’s surface enter underground and contaminate groundwater, the pollution is called groundwater pollution. Consumption of contaminated groundwater causes diseases like cholera and diarrhea. Besides different types of pathogens, nitrates also cause groundwater pollution causing diseases like blue baby syndrome. It is observed that the nitrate concentration of more than 10 ppm increases the chances of blue baby syndrome (Buitenkamp and Stintzing 2008). High level of fluoride contamination causes dental and skeletal problem. The term “nanotechnology” was coined in 1867 when James Clerk Maxwell published his observation about the technology and manipulating individual molecules In the twentieth century, with the development of the ultramicroscope it was possible to observe nanoparticles of a size of 10 nm for the first time. In 1959, Professor Feynman proposed a revolutionary theory about quantum mechanics which leads to the development of the first Scanning tunneling microscope (STM) by IBM in 1981. The nanoparticles are generally defined as materials and particles of at least one dimension in the range of 1–100 nm. Though the definition is incomplete. American National Nanotechnology Initiative defines nanoparticles with two main conditions. They are, a. Nanomaterials are characterized by their unique physical, biological and chemical characteristics different from their equivalent on the macro level. b. It must be possible to configure and control nanomaterials at the atomic level. For the synthesis of nanomaterials, there are two main approaches, a. Bottom-up approach: smaller molecules joined together to form more complex larger structures. b. Top-down approach: Nanostructures are made larger entities without configuration control at the atomic level.

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The procedure followed to synthesize the nanomaterials determine their structure, stability, and other characteristics. Thus, these materials have a completely different form of electrical, magnetic, optical, and reactivity properties. Most important use of nanomaterials is in wastewater treatment by removing and destroying pollutants.

2 Wastewater: Source and Composition Wastewater is described as any water that is contaminated due to human use. It generates from a variety of sources such as residential areas, commercial areas, industrial plants, and agricultural fields. Depending on the source the composition of wastewater varies. Various pathogens, agricultural pollutants, inorganic pollutants, sediments, organic pollutants, industrial pollutants, radioactive substances, different types of nutrients etc. can be found in wastewater. The source of wastewater is broadly classified into four types: a. b. c. d.

Wastewater from domestic/municipal. Wastewater from industries. Infiltration/inflow. Stormwater.

According to Jain et al. (2021) wastewater can be described as a matrix of 99.9% water and 0.1% suspended solids. The solid materials further can be divided into organic and inorganic substances. Organic substance includes proteins, carbohydrate, and fats. On the other hand, inorganic substances include sediment, salts, and metals. Apart from that wastewater contains several microorganisms.

3 Common Wastewater Treatment Methodology The increasing population and development of civilization demand immediate actions to mitigate water pollution and associated adverse effects. Wastewater treatment is not only about removing the pollutants from water but also recovering the micronutrients to eliminate environmental threats. The first step to treat the wastewater is preliminary treatment, that removes heavy and large debris. This step includes two stages: a. Screening: In this step, the large floating debris are removed. 60% of debris, 5% plastic, and 25% papers are removed using the screening process. b. Grit removal: In this step particles like gravels, sands, and particulate matters are removed by settling down in a grit channel. After that, the effluent undergoes primary treatment. In this step, the suspended solids get separated. During this process, the effluent is stored in the sedimentation

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tanks for a few hours so that either the suspended solid settles down by sedimentation process or forms a layer at the top. The settled solid is called the sludge. The primary treatment can remove about 40% of biological oxygen demand (BOD), 80% suspended solids and 55% fecal coliforms (Yoon et al. 2006). The secondary treatment includes the activated sludge processing, that derived from primary treatment by biological reactor, bio-filters and oxidation. For oxidation process single-cell organisms are used. They remove the organic contaminants by their metabolic activity. The secondary treatment is also capable to remove some micronutrients from water by nitrification and cell uptake. In tertiary treatment, effluents of secondary treatment are treated in this method by ultraviolet ray, ozone radiation, chlorine, chlorine dioxide, sodium hypochlorite etc. before releasing to the environment (Kalfa et al. 2020).

4 Nanomaterials and Nanotechnology in Wastewater Treatment Nanotechnology is a promising method to manipulate matter at the atomic and molecular level. By this manipulation, various new crafts can be developed with high optical, electronic, magnetic, mechanical, and conductive properties. Because of their large surface area, small size and functionalization nanomaterials offer various application opportunities including wastewater treatment. Recently more advancement occurs in the field of nanotechnology such as nano photocatalysis, nanomotor nanomembrane etc. Below these processes are discussed in details.

4.1 Nano Photocatalyst Photocatalysis is a phenomenon that includes the breakdown of compounds in the presence of light. This process can be carried out by using different lightUV/sunlight/visible. Photocatalysts are solid materials which absorb photon and induce a chemical reaction. As they alter the rate of photocatalytic reaction, they are widely used for wastewater treatment. Photocatalysis is a type of Advanced Oxidation Process (AOPs) as chemical oxidants of strong potential like hydroxyl radicals (;OH) are produced in this process with the help of H2 O2 , UV, Fenton’s reagent, ozone or a catalyst (Bethi et al. 2016; Shah 2020). Nano photocatalysts having minute size and greater surface area are used to remove dyes, heavy metals, toxic chemicals. Photocatalysts with nano range of size have increasing mechanical, electrical, magnetic, optical and chemical reactivity properties (Ong et al. 2018). Examples of some nanosized semiconductor photocatalysts are ZnS, TiO2 , ZnO, Fe2 O3 etc. Titanium oxide (TiO2 ) is one of the most used photocatalyst for its low cost, no toxic effects and easily availability. Other photocatalysts like ZnO and composites

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like CdS/TiO2 have been proved to be good adsorbents for removing toxic pollutants from water (Ali et al. 2019a, b). Inhibition of harmful microorganism in wastewater by using Pd incorporated ZnO nanocomposites were also reported (Yamakata and Vequizo 2019). Also, it has been found that Ag-doped TiO2 nano photocatalysts have the potential to degrade rDNA present in wastewater (Li et al. 2014). Cadmium oxide and zinc oxide nano photocatalytic composite has been prepared and shown the ability to remove organic dye acid orange 69% within a time span of 2.5 h (Berekaa 2016). As nano sized photocatalysts have properties like low cost, reduced particle size, reusability and complete degradation, they are now applied extensively. But the main limitations are toxicity caused by some of these nanomaterials and their lower recovery rate from reaction mixture (Mahmoudian-Boroujerd et al. 2019).

4.2 Nano and Micromotors Nano/micromotors are innovative motors in nanotechnology field which are a tool for converting energy from several resources into machine driven force. These motors can be operated by fuels or other sources like electric or magnetic field. Toxic pollutants removal from water can be achieved by these motors due to their high power, speed, self-mix ability etc. Nowadays nano and micromotors have been considered as a promising tool for purification of water. Diffusion boundary can be overcome by energetic mixing through these motors as they have self-propulsion competences. They can stimulate decontamination efficiency of water overlapping with nanomaterials that in turn increase the surface area and thus increase working activities (Gugushe et al. 2019). Some examples of nano/micromotors are polymer capsule motors for oil separation, Au/Pt nanomotors for DNA detection, Pd nanoparticle doped microspheres for pH dependence etc. H2 O2 fuel based nanomotor are Ag doped zeolite, Au NPs/TiO2 /Pt nanomotor, Biotin-functionalized Janus silica Micromotor etc. Though these motors are on the preliminary research stage, their advantages like decreeing the clear out time and entire cost and the relatively easy decontamination process made them a promising research topic for upcoming days.

4.3 Nanomembranes Nanomembranes are a type of membranes that is formed with different nanofibers which removes unwanted nanoparticles from aqueous phase. The composite nano membranes are formed by using different polymers and nanomaterials. Most commonly carbon-based material like graphene oxide is used with polymer matrix for treatments. Fouling resistance and hydrophilicity has been shown to increase by doping nanomaterials like TiO2 , zeolite etc. into ultra-filtration polymer membrane. Silver being an antimicrobial material are also doped in a polymer to form membrane

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that inhibits biofilm production (Mahmoodi and Arami 2009). However, limitations like clogging and fouling of membrane can be prevented by adding super hydrophilic nanoparticles like TiO2 , CNT etc. By introducing nano photocatalysts into the nanomembrane, it is made possible to degrade organic pollutants. Nowadays in the wastewater treatment field, anti-fouling membranes are top priority. Polyvinylidene fluoride (PVDF) membrane entrapped by TiO2 exhibited the ability to remove Escherichia coli from water (Waduge et al. 2015; Shah 2021). Due to selective permeability, polysulfone (PSf) and polyethersulfone (PES) membranes are used more in removal of toxic pollutants from water. For dichlorination process, metallic nanoparticles can be immobilized on various membranes. By membranes, toxic particles are separated from water resources and water safety level checkups are done. But the fouling problem of the membrane after few uses and less stability make membranes unable for using widely.

4.4 Nano Sorbents 4.4.1

Carbonaceous Nano Sorbents

The carbon-based materials have been studied in various disciplines of science and technology for decades. The nano derivatives of carbon are said to have various dimensions in its allotropes such as carbon nanotubes (CNTs), activated carbon (AC) and the buckyballs (C60 ), graphene and graphite. Carbon nanostructures have gained the attention of the scientific community because of their eco-friendliness, good thermal and chemical stabilities, high surface area, environmental friendliness and abundant availability (Sayed et al. 2020). Among these materials the ACs have been used most commonly due to its favorable porosity and surface area. But due to their high cost the scientific community have started to study other allotropes and functionalized forms of carbon as potential nano-adsorbents. Next, we come to the CNTs which have a cylindrical morphology comprising of hybridized carbons in a hexagonal arrangement. They are usually bulky molecules and which are formed due to the winding up of single and multiple sheets of graphene to form single walled (SWCNT) and multiple walled carbon nano tubes (MWCNT) respectively. The CNTs have exhibited superior performances over its counterparts due to its tunable surface chemistry permitting modifications, chemically inactive nature, good specific surface area, hollow structure, mechanical flexibility, which finally led to the efficient interaction with pollutants (Verma and Balomajumder 2020) making them one of the favorite candidates for waste water treatment. Yadav and Srivastava showed the adsorption and desorption abilities of CNTs towards Mn7+ ions using laboratory grade KMnO4 as the source. It was seen that the CNTs can adsorb Mn7+ ions efficiently which was revealed by UV–Visible Spectrophotometric analysis. The analysis showed the decrease of concentration from 150 to 3 ppm (Yadav and Srivastava 2017). In the recent times, the demand of pharmaceutical agents has increased due to the population growth and illness rate. The discharge of such compounds into

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the surrounding aquatic environment has also increased causing harm to the general flora and fauna and also to the mankind causing serious health issues when such water is consumed. Kariim et al. (2020) had produced MWCNTs from ACs derived from wood saw dust. The as-prepared MWCNTs were doped with nickel-ferrites (Ni–Fe) for adsorption of metronidazole and levofloxacin from contaminated waste water. The specific surface areas of pure ACs and Ni–Fe decorated AC-CNTs were 840.38 and 650.45 m2 g−1 . As a result, the adsorption ability of the AC developed CNTs was higher than that of decorated CNTs while removing the two target pharmaceuticals. Zhao and colleagues (2021) had successfully synthesized magnetic MWCNTs that was employed for removing tetracycline (TC). The prepared CNTs were up to 10–40 nm in size with a magnetic separation value of 10.8 emu g−1 with an elevated adsorption capacity of 494.91 mg g−1 at 308 K. The MWCNTs showed a about more than 80% efficiency was revealed from the systematic batch experiments that included adsorption kinetics, adsorption isotherm, effect of initial pH of TC solution. Yang and colleagues (2021) synthesized nitrogen doped CNTs which had surface oxidized magnetic nano-cobalt (NC) embedded within them (Co@CoO/NC). The as prepared adsorbent was employed to remove TC and rhodamine blue (RhB) where it was observed that Co@CoO/NC had an adsorption capacity of 679.56 mg g−1 and 385.60 mg g−1 for RhB and TC respectively. The reusability studies showed that after four cycles the magnetic Co@CoO/NC adsorbent had 75% and 84% affinity towards TC and RhB respectively. Recently in 2017, Bankole and co-workers (2017) have successfully synthesized a noble adsorbent which is purified CNT (P-CNT) functionalized with polymers to reduce the chemical oxygen demand (COD) of the waste water discharged from electro plating industry. The following polymers which were employed to functionalize the MWCNTs were poly hydroxyl butyrate (PHB), amino polyethylene glycol with poly hydroxyl butyrate (a@PEG-PHB), amino polyethylene glycol (a@PEG). Each of the functionalized CNT samples were given a stability period of 70 min. The order of removal performance was a@PEG-CNT (99.68%) > PHB-CNTs (97.89%) > P-CNT (96.34%) > a@PEG-PHB-CNT (95.42%). Graphene is a fascinating material which is two dimensional in nature and composed of sp2 hybridized carbon atoms arranged in a hexagonal assortment. The excellent specific surface area and porosity makes it a fascinating candidate for capturing gases like hydrogen, methane and carbon dioxide. Additionally, the excessive delocalized π-electrons, good chemical stability, increased active sites render it as a suitable adsorbent for waste water treatment. This material can be modified by changing the number of layers and stacking (Ali et al. 2019a, b). The graphene oxide (GO) is a single mono-molecular layer of graphite consisting of diverse oxygen functional groups such as carbonyl, hydroxyl, epoxide and carboxyl groups. The reason for which GO has garnered attention from the scientific community is because of its lucid design, eco-friendliness, easy to operate, good performance against the toxic pollutants. GO was transformed into an aerogel (AG) along with conjugation with aminated lignin (GO/AL-AG) thus forming noble adsorbents were prepared by Chen et al. (2020) which were employed for the adsorption of malachite green (MG) dye. The studies were conducted on the basis of aerogel dosage, pH, experimental

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time and reaction temperatures. The maximum adsorption capacity and removal percentage of the as-prepared GO/AL-AG was 113.5 mg g−1 and 91.72% respectively under optimum conditions. The GO/AL-AG gave a better performance than pure AG which was due to the good synergistic effect between the carboxyl group on the GO surface and the amine group in the aminated lignin. The stability studies showed that the adsorption–desorption efficiency of GO/AL-AG was maintained till 90% after five cycles of experimental runs. GO was also functionalized with thiosemicarbazide (TSC) (Bu et al. 2020) to form a noble adsorbent (GO-TSC) in order to remove methylene (MB) from simulated waste water. The GO and GO-TSC showed maximum adsorption capacities of 196.8 and 596.42 mg g−1 respectively. Zhen and co-workers have synthesized a hydrogel of GO with silver nanoparticles doped within them. The as-prepared adsorbent was integrated with a porphyrin complex for the adsorptive removal of dyes present in waste water. A series of adsorbents were prepared by forming complexes with different porphyrin precursors. The adsorption capacities were evaluated and it was found that tetra phenyl porphyrin modified hydrogel had the highest capacity (130.37 mg g−1 ) towards MB (Zheng et al. 2020).

4.4.2

Zero Valent Nano Metal Adsorbents

Silver Nanoparticle The silver nanoparticles (Ag NPs) have proved to be potentially virulent to the microorganisms and thus have strong anti-bacterial property against a versatile range of microorganisms such as viruses, bacteria and fungi. Henceforth, these nanoparticles have been used widely for the decontamination of water. Though the proper procedure of action of the Ag NPs have not been properly documented yet but in some literature works it has been reported that the Ag NPs attach to the bacterial cell wall and penetrate through it that causes certain structural changes of the cell membrane and thus increasing the membrane leakage (Sondi and Salopek-Sondi 2004). After penetrating into the cells, the Ag NPs can react with the DNA of the microorganisms that consists of sulfur and phosphorous elements and finally destroy the cells. Another theory states that when Ag NPs are in contact with the bacterial cells, they can generate free radicals which have the ability to damage the cell membrane causing the subsequent death of the cell (Danilczuk et al. 2006). A third mechanism specifically speaks about the dissolution of the Ag+ ions as released from the Ag NPs, which interact with the thiol groups of the bacterial enzymes and inactivate them which inhibit the normal functions of the bacterial cell (Dhanalekshmi and Meena 2016). The progress of nanotechnology has brought forward the opportunity to employ Ag NPs for water and waste water disinfection during the recent times. However, the direct application of this nano metal may cause some problems due to agglomeration in the aqueous environment that limits its long-term usage. The problem can be overcome if the Ag NPs are used in the filter media during water disinfection that would utilise its anti-bacterial property and cost effectiveness

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(Quang et al. 2013). Apart from this, the Ag NPs have been immobilised on the cellulose fibres of a blotting sheet by the in-situ reduction of silver nitrate. These Ag NP sheets have shown anti-bacterial properties towards E. coli and Enterococcus faecalis by inactivating them during filtration through the sheet. The possibility of silver being leached from the sheet and contaminate the drinking water was successfully controlled since the concentration of Ag in the drinking water was lower than the rejection quantity of silver contamination as standardised by the Environmental Protection Agency and World Health Organisation. Therefore, the disinfection of microorganism contaminated waste water via paper filtration deposited with Ag NP can be an effective measure. For membrane filtration application, the Ag NPs have been synthesized similarly via chemical reduction method and incorporated in to polyethersulfone (PES) microfiltration membranes. The bacterial activity around the adjacent areas of the membrane was significantly supressed. The PES-Ag NPs membranes have shown strong anti-microbial properties and had great potential to be used in waste water treatment (Ferreira et al. 2015). During the past two decades, the Ag NPs immobilised on ceramic membranes have been widely used for disinfection and biofouling reduction purposes for domestic water treatment. The removal of E. coli was improved by using clay and sawdust made ceramic filters with Ag NPs deposited on them. Additionally, the filters having relatively higher porosity achieved greater bacterial removal than those having lower porosity (Kallman et al. 2011). Cylindrical filters were prepared from clay rich soil mixing with grog, flour and water in different proportions and methods (painting and dipping). The colloidal Ag NPs have been combined with the as prepared filters. It was seen that the incorporation of Ag NPs enhanced the performance of the filter which can now remove E. coli to about 97.8 and 100% (Oyanedel-Craver and Smith 2008). The bonding of the Ag NPs onto the ceramic membranes has been evaluated by Derjaguin-LandauVerwey-Overbeek (DLVO) approximation method (Mikelonis et al. 2016). More research is required to unlock the full potential of the Ag NPs to applied for water and waste water treatment.

Iron Nanoparticle Neutral metal nanoparticles like iron (Fe), aluminium (Al), nickel (Ni) and zinc (Zn) have attracted the attention of the scientific community to be studied for water pollution control. Zero valent Al has extremely high reduction ability for which it is thermodynamically unstable in aquatic environment. This favours the generation of oxides/hydroxides that inhibit the transfer of electrons from its metallic facet to the contaminants. On the other hand, since Ni has a lower reduction ability than Fe (due to less negative standard reduction potential) so nano-zero valent Fe along with Zn can be used as reducing agents to remove a wide range of redox-labile pollutants. Although Fe has a relatively weaker reduction ability than Zn but it possesses certain advantages like oxidation in the presence of dissolved oxygen, outstanding adsorption properties, small size, favorable specific surface area and cost effectivity which are beneficial for water treatment application. So nano zero valent iron particles (nZVI)

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have been under extensive study. Under anaerobic condition Fe0 can get oxidised by H2 O or H+ to Fe2+ and H2 which are themselves potential reducing agents that can reduce the pollutants. During the reaction mechanism which is largely a redox reaction between the nZVI and contaminants, the Fe2+ will be oxidised to Fe3+ which forms Fe(OH)3 under an alkaline condition. Fe(OH)3 being a commonly used flocculant, causes the elimination of heavy metals like Cr(VI) (Wang et al. 2014). On the other hand, nZVI can transfer two electrons to O2 to produce H2 O2 (Eq. 1) which degrades and mineralizes the organic contaminants in the presence of dissolved oxygen. The H2 O2 which is produced is again reduced to H2 O by nZVI (Eq. 2). The combination of H2 O2 and Fe2+ can also generate hydroxyl radicals (HO. ) via the Fenton reaction which have profound oxidising ability towards a wide range of organic pollutants as well (Eq. 3) (Fu et al. 2014). nZVI + O2 + 2e → Fe+2 + H2 O2

(1)

nZVI + H2 O2 + 2e → Fe+2 + 2H2 O

(2)

Fe+2 + H2 O2 → Fe+3 + • OH + OH−

(3)

The cumulative effects of adsorption, precipitation, reduction and oxidation in aerobic condition has been proved to be a potential adsorbent for the removal of halogenated organic compounds, nitro aromatic compounds, radio elements and metalloids. It can be mentioned that the application of nZVI is not only limited to water and waste water treatment but it has been extended to soil abatement (Gueye et al. 2016) as well. The study has achieved a pilot scale and full-scale experimentation on real waste water polluted soil samples collected from the fields. However, there are some limitations of nZVI such as aggregation and difficulty to separate from the experimental solution. The solutions to these problems include surface modifications that would the performance of NZVI. Common modification techniques are doping, conjugating with supports, matrix encapsulation, surface coating and emulsification. The surface coating and conjugation with supports techniques can inhibit the particle agglomeration in nZVI and increase its dispersibility. The later technique along with matrix encapsulation provide efficient separation of the adsorbent from the experimental solution (Li et al. 2016). It should be mentioned that the emulsification technique of is employed to solve the delivery problem of nZVI in dense non-aqueous phase liquid medium.

Zinc Nanoparticle Zn has been considered as an efficient alternative (Bokare et al. 2013) to nZVI in the field of water and waste water treatment because of the disadvantages that nZVI brings with it as mentioned in the previous sub-section. Zn has a higher negative standard potential for which it can reduce compounds more than Fe and hence

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its degradation kinetics is greater than that of nZVI. The application of nano-zero valent zinc (nZVZ) focusses on dehalogenation reaction. In case of nZVZ the solution chemistry played a pivotal role in the degradation kinetics rather than particle size and surface morphology. While degrading CCl4 , a comparative study performed between the different types of nZVZ and nZVI where it was seen that nZVZ could break down CCl4 much faster and completely then nZVI under optimum conditions (Tratnyek et al. 2010). When nZVZ was compared with its other metal counterparts such as nano zero valent aluminium, nano zero valent nickel and nZVI for degrading octacholordibenzo-p-dioxin (OCDD) in water it was observed that nZVZ was the most efficient nanoparticle out of the four. It degraded OCDD into its lower chlorinated intermediates and finally became the first reported nano metal with zero valency nanoparticle that could dechlorinate OCDD at ambient conditions (Bokare et al. 2013). However, the application of nZVZ has only been upon the halogenated organic contaminants till now and its effectivity towards other contaminants are yet to be reported or are scarcely available in the literature. So, a pilot-scale and full-scale experimentation of nZVZ has to be conducted at the contaminated sites (Tratnyek et al. 2010) to gain a profound knowledge about its actual potential.

4.4.3

Metal Oxide Nanoparticles

The metal-based nano-adsorbents have been eye catching for the researchers in the recent times. The metallic oxides such as TiO2 , ZnO, Fe3 O4 , MnO2 , MgO and CdO have been applied in the research field on waste water treatment to remove pollutants like dyes, heavy metals etc. The metal oxides are considered to be relatively better adsorbents when compared with AC when the removal of radioactive elements and heavy metals are concerned. Moreover, the minute size and high surface area of the metal oxides provide a small diffusion length that can be suppressed without alteration of their surface area. The adsorption process is primarily governed by the formation of ion complex between the oxygen in the crystal lattice of the metal oxides and the dissolved metals (El-Sayed 2020). The metal oxides have also been extensively used as photocatalysts that bring about an efficient degradation of the organic pollutants present in waste water.

TiO2 Nanoparticle TiO2 nanoparticles (TiO2 NP) came to the attention of the scientific community in 1972 when Fujishima and Honda first showed an electrochemical photolysis of water by using TiO2 electrode. This experiment also brought forward photocatalysis as an upcoming and promising technology to the scientific community. During the recent times, this photocatalysis has been extensively used as a degradation technology in waste water treatment where the contaminants are completely oxidized into their nontoxic, low molecular weight intermediate products under the presence of light and

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the concerned catalyst. TiO2 is the most prominent photocatalyst among the semiconductors due to its excellent photocatalytic ability, cost effectivity, photostability, biological and chemical stability (Guesh et al. 2016). It has a band gap of 3.2 eV which requires ultraviolet irradiation for the electrons to jump to the conduction band. TiO2 can generate reactive oxygen species upon excitation and since TiO2 exhibits small selectivity so that it can degrade a variety of contaminants such as arsenic, phenols, dyes, cyanide, pesticide, heavy metals etc. The hydroxyl radicals that are generated from the TiO2 surface can cause damage to various cells by altering the structure and function of the cells. It has shown excellent anti-microbial activities against both gram positive and gram-negative bacteria as well as protozoa, algae, fungi and viruses (Foster et al. 2011). The difficulties and disadvantages of working with TiO2 should also be mentioned in this regard. Firstly, since it has wide band gap so it needs UV excitation since visible light excitation might not be that much successful. This makes the experiment more power or energy consuming for which studies have been going on to make the TiO2 get excited under visible light and improve its performance. For an instance, metal doping can be an effective measure to improve the absorbance of visible light by TiO2 and also to improve their performance under UV irradiation. Among the dopants used, Ag has gained significant attention as it can enable the excitation of the TiO2 NPs under visible light (Seery et al. 2007) thereby increasing the photocatalytic performance against bacteria and viruses (Kim et al. 2006). Another suitable measure can be the narrowing of the band gap of TiO2 which is possible by doping the NP with non-metallic elements such as N, S, F and C. This type of doping can also enhance the photocatalytic activity by enhancing the photon adsorption in the visible spectrum during dye degradation (Liu et al. 2006). Secondly, the production process and recovery of the as prepared TiO2 NPs from the experimental solution are both quite complicated. The recovery problem of TiO2 NPs has been solved due to the rigorous efforts of the scientific community. The NPs have been incorporated with a wide range of membranes and this technology has shown promising outcomes. Membranes materials such as poly (vinylidene fluoride), poly (amide-imide), polyether sulfone poly methyl methacrylate have been employed for the said purpose. During the conjugation of TiO2 with the organic materials, N,Nmethylenebisacrylamide reacts as the cross-linking agent and ammonium persulfate functions as the initiator pair. The acrylamide can undergo polymerisation in an aqueous medium under the presence of TiO2 to produce TiO2 /Poly [acrylamide-co(acrylic acid)] hydrogel. TiO2 NPs can be removed from the treated system as they have similar couplings with polymeric couplings with ease via a simple filtration process (Kangwansupamonkon et al. 2010). Much recently, TiO2 NPs have been doped with other magnetic NPs and have been synthesized using a spinning disk reactor for achieving an easy recovery using a magnetic trap (Stoller et al. 2009). This production process is suitable for large scale industrial applications (Stoller et al. 2009).

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ZnO Nanoparticle Apart from TiO2 NPs, the zinc oxide nanoparticles (ZnO NPs) have also emerged as an attractive photocatalyst due to its wide and direct band gap, excitation in the near ultra violet spectrum, excellent photocatalytic property and string oxidation ability. ZnO NPs are bio friendly which makes them very attractive as photocatalysts during waste water treatment (Schmidt-Mende and MacManus-Driscoll 2007). Besides, since the band gap energy of ZnO NPs is very close to TiO2 NPs it makes the photocatalytic ability of the former very similar with that of the later (Daneshwar et al. 2004). However, the ZnO NPs have an advantage of being relatively cheap than TiO2 NPs (Daneshwar et al. 2004) and secondly, ZnO NPs are capable to absorb a wider range of the solar spectra and capture more photons than many of its competitive semiconductor metal oxides (Behnajady et al. 2006). Just like TiO2 the ZnO NPs also comes with certain limitations. The first one being the wide band gap of ZnO NPs which limits it photo absorption within the UV region of the solar spectrum and the second one is the photo-corrosion that results in rapid charge recombination of the photogenerated species thus resulting in low photocatalytic efficiency (GomezSolis et al. 2015). To overcome these problems, doping of ZnO with metals seems to be a common strategy. Various anionic, cationic and rare earth metals have been employed as dopants (Lee et al. 2016). Apart from this, ZnO has also been coupled with other metallic oxide semiconductors such as CeO2 , CdO, SnO2 , TiO2 , GO and rGO which seemed to be a quite feasible approach to improve the photodegradation performance of ZnO NPs.

Iron Oxides Nanoparticle Easy availability and simplicity have put iron oxide nanoparticles in great demand for heavy metal removal from wastewater. Functionalization of iron oxide NPs increase adsorption efficiency. For this, functionalising agents such as Polymers like copolymer of acrylic acid and crotonic acid (Ge et al. 2012) and ligands such as ethylenediamine tetraacetic acid, mercaptobutyric acid L-glutathione, hepta (ethylene glycol), α-thio-ω-(propionic acid) and meso-2,3-dimercaptosuccinic acid (Warner et al. 2010), are used. A flexible ligand shell is beneficial since it has been reported to facilitate a wide range of functional groups into the shell that ensured the intactness of the properties of Fe3 O4 nanoparticles. In addition to this, the polymer shell can inhibit the particle agglomeration and improve its dispersibility (Ge et al. 2012). The polymer molecules can act as a binder for metal ions and can carry them from the treated water. Magnetic Fe3 O4 (MNPs) have been conjugated with polyacrylic acid (PAA) which were further treated with congo red (CR) azo dye (PAACR/MNPs). The PAA-CR/MNPs system had a good affinity towards heavy metal like Fe3+ , Cu2+ , Pb2+ . The experimental batch studies were conducted at various reaction conditions, pHs, temperature and times with a prime focus on Pb2+ . The optimum removal of Pb2+ was attained at a pH of 6.5 and at 45 min of reaction (Sadak et al. 2020). Fe3 O4 has also been coupled with GO as prepared by He et al. (2021a, b).

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The Fe3 O4 /GO-COOH based noble nano-adsorbent was synthesised through GO magnetization and carboxylation. The as-prepared adsorbents have removed 78.4 and 51% of calcium (Ca2+ ) and copper ions (Cu2+ ) that are usually present in oil refinery discharge. The stability and reusability studies showed that the adsorbent can still remove 72.3% of Ca2+ and 49.33% of Cu2+ and can shows a recovery percentage of 82.1% Ca2+ and 91.8% for Cu2+ after five cycles of adsorption and desorption. Lastly, we come to the brief discussion of hematite (α-Fe2 O3 ), which is considered to be stable and inexpensive in environmental application and also as sensors, catalysts (Liang et al. 2013). The nano hematite has proved to be an efficient adsorbent for removing heavy metals from contaminated tap water. The 3D flower like microstructure of α-Fe2 O3 that assembles into nano petal subunits can efficiently prevent further particle agglomeration and the improved surface area having multiple pores and spaces provide adequate active sites for contaminant interaction. The highest adsorption capacities of α-Fe2 O3 were relatively much higher than that of the previously reported nanomaterials (Liang et al. 2013).

Other Metal Oxide Nanoparticles Like TiO2 , ZnO and Fe3 O4 there are other metal oxides that have been reported to have good photocatalytic and anti-microbial properties to treat waste water. One of them is magnesium oxide nanoparticle (MgO NP) that have exhibited mentionable toxic effect on many bacterial species. The size is a dominant factor for anti-bacterial activity of MgO NPs (Nguyen et al. 2018). The two most accepted theories of antimicrobial activity for MgO NPs are the generation of ROS or reactive oxygen species during photocatalysis and the binding of the MgO NPs on the bacterial cell membrane causing subsequent damage to the cell (Nguyen et al. 2018). The last metal oxide that we shall discuss is the copper oxide nanoparticles (CuO NPs). Their anti-microbial properties are size-dependant as well with the toxicity getting increased at smaller sizes of the CuO NPs (Bezza et al. 2020). It was observed that the toxicity of CuO NPs is more severe on bacteria than on fungus which contradicts in the case of other metal and metal oxide NPs. The CuO NPs can diffuse into the porous microbial cell membrane with ease due to their small size. The endocytosis process also favours the entry of the NPs into the microbial cells through ion channels and transporter proteins. Once these CuO NPs are inside the cell they start interacting with the oxidative organelles, producing Cu2+ ions and also generates reactive oxygen species. The Cu2+ ions functions to deactivate the functional proteins and disturb the internal steady state of the metal ions, while the reactive oxygen species damage the microbial DNA (Bezza et al. 2020). The addition or doping of Ag NPs into the CuO NPs can improve the inactivation of the bacterial cell proteins.

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Polymer-Based Nano Adsorbents

For wastewater treatment, nano adsorbents are widely used as they have high sorption capability for different pollutants. Carbon-based materials are the most commonly used nano sorbents. Electrostatic interaction between nano adsorbents and dissolved metals present in wastewater are the main key for adsorption process. An important parameter like changing pH can alternate the rate of adsorption. Acidic surface nano adsorbents attract anions and negatively charged adsorbents attract cation from wastewater. Greater surface area, cost effectiveness stability and easy processability attracted the interest of researchers. Nanocellulose and lignin based nano adsorbents have showed their capability to degrade dyes, heavy metal removal and nitroarenes (Orozco et al. 2016). Starch grafted copolymers and addition of cellulose nano crystals made nano adsorbents highly effective for removing dyes like Crystal Violet and Methylene Blue (Abdi and Abedini 2020). Iron-aluminum layered reduced GO coated with sodium alginate are prepared and shown the capability to remove arsenic with a high adsorption ability. Carbon based CNT high surface area and hydrophobic properties are highly efficient for removing pollutants. Nano aerogels are now prepared for removing the Uranium from drinking water. Graphene oxide based nano adsorbents have presented themselves as a remarkable adsorbent for removing pollutants effectively from wastewater. Nanomaterials as adsorbents have decrease the time for decontamination process effectively. They are also used as a separating medium for organic and inorganic based pollutants.

5 Removal of Pollutants by Using Nanotechnology Scarcity of water in different regions of the world, due to the sudden boom in population and heavy industrialisation, has emerged in the past decades. Alongside that organic and inorganic contaminants have increased the severity significantly. Industry-released wastewater contains toxins including large organic compounds, dyes, heavy metals etc. which eventually led to changes in pH levels, saltiness, and turbidity in the surrounding water. Various technologies (commercial and noncommercial) are utilized on a day-to-day basis but nanotechnology has emerged as one of the most advanced and effective ways of wastewater treatment by removal of pollutants. It has been seen that nanoparticles and/or nanofibers can adequately resolve most of the water quality issues by removing dyes, heavy metals and harmful pesticides (Kokkinos et al. 2020).

5.1 Heavy Metal Removal Among various contaminants in wastewater, heavy metals possess the maximum threat to the surrounding population. Mercury (Hg), Cadmium (Cd), Arsenic (As),

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Chromium (Cr), Zinc (Zn), Nickel (Ni), Copper (Cu), and lead (Pb) are examples of such highly lethal heavy metals which are poisonous even in low concentrations. Along with industrial wastewater, municipal wastewater, mining waste, and urban runoffs are the major sources of heavy metals. Printed circuit board (PCB) manufacturing industries, paint and enamelling industries and electroplating industries release a significant amount of Zn, Cd, Ni, Pb, Ag, Ti, C, and Pd (Kouras et al. 1998; Kannan et al. 2020). Nanomaterials showed a greater potential in removing these heavy metals in comparison to activated carbon. In recent times, the concept of nanoparticle-mediated removal of heavy metals from wastewater has emerged and resulted in the development of many cheap adsorbents, widely used in the removal of these toxic elements from wastewater. For example, titanium dioxide nanocrystals have been used in the adsorption of different species of arsenic as a photocatalyst with high efficacy (Pearson 1990). Apart from arsenic adsorption, titanium dioxide nanocrystals engraved on a graphene sheet have shown potential to form Chromium III by decreasing Chromium VI in presence of light. Another study has depicted that nanoparticle of palladium helped in the complete removal of chromium in wastewater. Alongside, iron oxide nanoparticles were utilized in order to remove arsenic using high and particular surface areas. In another study, it was observed that magnetite nanoparticles could efficiently remove radioactive metal toxins, uranium dioxide from water. Along with that, it has been found that zero-valent iron or Fe-NPs are highly effective in the transformation of Arsenate, Arsenite, Pb (II), Cu (II), Ni (II), and Cr (VI). Cerium oxide nanoparticles were studied to be an efficient adsorbent of As and Cr. Chitosan nanoparticles were also identified as adsorbent of Pb (II) nanoparticles (Kolluru et al. 2021).

5.2 Removal of Pesticides and Dyes Low-cost adsorbents like fly ash are used to remove pesticides from water. Coal fly ash is used to remove various components like metolachlor, atrazine, and metribuzin. In a comparative study, it was observed among these three components atrazine was the most adsorbed component. The removal of a pesticide depends on the concentration of the pollutant in the mixture. In the case of herbicides, the highest removal occurs at the lowest concentration.

6 Nanocomposite Reuse Recovery of materials and reuse of nano-adsorbents from wastewater are significantly difficult and hazardous to the environment. To encounter that pre-treatment and process separation techniques must be utilized. Alongside that, ballistic electron discharge from the adsorbent (nanostructured) material may destroy the adsorbed

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compounds. To allow continued usage of chemicals due to the expense and general health concerns, nanomaterial holding and re-utilization using membrane filtration had been used. Different membranes and resin can control nanomaterials hence resulting in further removal of the residue. Extensive research and optimization are needed in order to easy to use, budget-friendly techniques for nanomaterials entrapment and immobilization without hampering their effectivity.

7 Risks Associated with Nanotechnology The effectiveness of the nanoparticles in treatment of wastewaters depends significantly on its size, shape and other physio-chemical properties. However, large-scale, commercial use of such nanoparticles are hindered by barriers like cost-effectiveness and more significantly its toxic effects on the ecosystem. If and when these risks and adverse effects are nullified, nanoparticle use would prove to be an efficient means of wastewater treatment.

7.1 Effect on Human Health Occurrence of toxicity in humans due to nanoparticles are dependent on the cellular uptake. The molecular structure and size of the nano particles are major contributing factors to this. The small size of the nano particles allows them to penetrate through the endothelial and epithelial barriers into different organs and tissues like brain, heart, CNS etc. Nanoparticles of sizes ranging from 1 to 100 nm can easily enter cells and its organelles owing to its similarity in size with DNA helix (2 nm), protein globules (2–10 nm) and cell membrane thickness (10 nm). Studies have shown invasion of nanoparticles of sizes less than 6 nm in the nucleus and that of 10–16 nm in the cytoplasm. Disturbance in intracellular transport, cell division and cell migration due to TiO2 nanoparticles by making conformational changes and inhibiting polymerization in tubulin have been reported. Differences in the shapes of nanoparticles have different effects on the cells. Endocytosis by spherical nanoparticles, blocking of calcium channels by SWCNTs, and nontumorigenic lung epithelial cell lysis by spherical and needle-like nanoparticles are a few examples of such.

7.2 Ecotoxicity Due to their high efficiency in Wastewater treatment, metal nanoparticle production is quite high. The process of nanoparticle production, usage, and disposal of such nanoparticles, carries the risk of leaching into the treated water. In natural surface waters, the concentration of nanoparticles are found to be in quite a limited scale.

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However, the increase in production and usage of metal nanoparticles changes the situation quite rapidly. This is known to cause growth inhibition in various aquatic organisms like duckweed etc. Damage to vital organs and DNA in small mammals like rats due to ingestion of TiO2 in nanoparticle contaminated waters have also been reported. Pulmonary inflammation, carcinogenicity, genotoxicity etc. have also been observed in a few engineered nanoparticles. Researchers have found nanomaterials to be potential water pollutants. Toxic effects in 2 fish species, Astyanax altiparanae and Danio rerio by MWCNTs were studied by Cimbaluk et al. which showed signs of CNTs-DNA crosslinking, oxidative stress generation and acute and sub-chronic neurotoxicity. Elevation in oxidative stress and genotoxicity were also observed in freshwater fish Labeo rohita due to effects of Ag NO-treated water. Dispersants like tetrahydrofuran, responsible for dispersing CNT and C60 fullerenes, are poorly soluble in water and have been reported to be toxic in nature. These raise concerns for its usage ethically at a large commercial scale.

8 Conclusion Wastewater management and water purification is a very important process in modern civilization. The limited amount of available water and increasing demand for resource creates a challenge of supply. Traditional methods of wastewater treatment technologies are being used since decade but is not fully compatible to satisfy the increasing demand. Here nanotechnology comes to play. Various newly developed processes like micromotors, nanomembrane etc. are well accepted methods for wastewater treatment. Though, some problems viz., cost effectiveness, toxicity etc. are associated with these technologies but nano-materials have a significant role to remove many types of pollutants like the heavy metal, dye and pesticides from wastewater.

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Biogas as a Value Generation in Industrial Wastewater—A Review B. Saikat, S. Sivamani, and B. S. Naveen Prasad

Abstract Biogas production by anaerobic digestion (AD) is the simplest method for extracting energy from waste materials, but it is complicated by a number of issues. Many technologies for the pre- and post-process of biogas generation have been developed in recent decades in order to achieve maximal biogas production. Pretreatment, microorganisms, process management and monitoring, co-digestion, phase separation, and biogas purification are all possible options. However, these technologies must be integrated in such a way that they improve bio-methane generation while being cost-effective and environmentally friendly. Due to the dynamic nature of AD process, the process control and real-time monitoring of the AD in the bioreactor is the most important thing to focus on innovations. This article answered a variety of concerns about the generation and use of biogas as an alternative energy source from three different types of industrial wastewater. Biogas is produced by anaerobic digestion of biomass, which occurs when lipids, proteins, and carbohydrates, which make up the majority of organic matter, are broken down. The page discusses the most prevalent biogas production systems, as well as its stages and characteristics. In addition, the article discusses some of the biogas’s potential applications. The article also discusses the important environmental and economic benefits of using biogas, including the reduction of methane emissions and the reduction of carbon monoxide and nitrogen oxides emissions into the atmosphere. Keywords Biogas · Methane · Natural gas · Anaerobic digestion · Biomethanation · Wastewater treatment

1 Introduction Anaerobic conversion of organic materials and contaminants is a well-established technology for waste and wastewater treatment that protects the environment. Biogas, a blend of methane and carbon dioxide, is the final result, which is a useful, renewable B. Saikat (B) · S. Sivamani · B. S. Naveen Prasad Chemical Engineering Section, Engineering Department, University of Technology and Applied Sciences (Salalah College of Technology), Salalah, Oman e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_3

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energy source. Biogas is combustible, smokeless, sanitary, colourless, odourless, and, if not desulfurized, has a horrible eggs door. It has a calorific value of 4740– 7500 kcal/Nm3 and has an energy content of 37.3 MJ/m3 , explosion limits of 6–12% biogas in air, ignition temperature of 650–750 °C, specific gravity of 0.847–1.004, and ignition temperature of 650–750 °C. Biogas is an environmentally beneficial, renewable, clean, inexpensive, high-quality, and adaptable fuel produced in digesters that are filled with feedstock. It is regarded as a green energy resource alternative. It can be used for a variety of energy services, including heat, combined heat and power, and automobile gasoline (Sarkar and Banerjee 2013). Anaerobic digestion is a low-energy, technologically easy method for converting organic material from a variety of wastewater types, solid wastes, and biomass into methane. In the current efforts towards sustainable development and renewable energy production, a far broader application of the technology is desirable. Several projects to manufacture biogas from trash were started in the Netherlands in the 1980s. Many initiatives were cancelled due to a lack of financial sustainability (Hartmann and Ahring 2005). The creation of methane from wastes is currently attracting increasing interest because it has the potential to reduce CO2 emissions by producing renewable energy while also limiting the emission of the greenhouse gas methane from wastes, particularly animal manure. The expanding market demand for green energy, as well as the significant optimization of anaerobic digestion technologies in recent decades, particularly the creation of modern high rate and co-digestion systems, has all contributed to this trend (Banerjee 2012). The organic waste is converted into electricity using biogas technology. The utilization of energy and manure can result in social and economic benefits, as well as a cleaner environment and a contribution to long-term development. Biogas technology produces nutrient-rich organic fertilizer, and the effluent slurry created by biogas technology is beneficial to algae development, fish production, and seed germination. The biogas technology is used for both small-scale and large-scale electric power generation. It’s a gas mixture whose composition is determined by substrates and AD process variables such as retention time, temperature, and pH. Biogas is one of the main products of the AD of organic substances (He et al. 2008). Anaerobic digestion (AD) is a biological process that destroys organic compounds in the absence of oxygen through the actions of microbial populations. In reality, as shown in Fig. 1, AD can be separated into four stages: hydrolysis, acidogenesis (which is also known as acid production), acetogenesis (which is also known as acetic acid production), and methanogenesis (which is also known as methane production). Purifying raw biogas and upgrading it to a high-quality fuel standard is critical in order to boost calorific value and eliminate undesirable components like H2 S and CO2 , which might harm utilization systems. Biogas cleaning and upgrading is a well-known technique. Biogas may easily be converted to biomethane or renewable natural gas (RNG), which is equivalent to natural gas derived from non-renewable fuel sources. It has a methane content of 90% or more. RNG could be used to replace natural gas as a fuel for cars that can run on it, as well as to supply gas to the natural gas grid. In the bioenergy business, upgrading biogas to biomethane is one of the

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Fig. 1 Steps of anaerobic digestion

technologies that has gotten a lot of interest. Biogas has the potential to play a vital part in the growing renewable energy sector, with global biogas usage predicted to increase in the next few years (Ganguly et al. 2006).

2 Renewable Energy Resources The term energy can be broadly defined as the amount of force or power that, when applied, can move an object from one position to another, or it can also refer to a system’s ability to perform work. The ability to convert one form of energy into another is the most important feature of energy. Energy technologies, in general, are man-made devices, equipment, and systems for capturing, converting, storing, and transporting energy from energy resources. As a means of promoting human development, productivity, and economic progress, energy is a significant requirement in our daily lives. The terms energy sources and energy resources relate to the output forms of energy from man-made energy technology, and the terms energy sources and energy resources refer to the naturally available forms of energy (Sivamani et al. 2018). Nuclear energy, renewable energy, and fossil fuels are the three types of energy resources available. Renewable energy is so named because the sources used to generate it constantly regenerate and replenish themselves over a very short period of time (i.e., months or years, not centuries). By replacing traditional energy sources with renewable energy techniques, it is possible to reduce greenhouse gas emissions and thus reduce global warming. Hydrothermal, geothermal, solar, wind, ocean (tide and wave), heat from the Earth’s interior, and biogenic (biomass) energies are examples of renewable resources that can be used to generate consistent power. These renewable energy sources are sometimes referred to as alternative energy sources. Traditional renewable energy technologies (such as wind turbines) as well as revolutionary new technologies, such as hydrogen internal combustion engines, are examples of alternative or renewable energy sources (Banerjee and Biswas 2004). Renewable energy has a direct association with sustainable development because of its impact on human development and economic growth. Renewable energy sources offer numerous advantages in terms of reducing environmental and health consequences, increasing energy access, ensuring energy security, mitigating climate

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change, and promoting social and economic growth. Renewable energy sources will be critical in the future of the world. Renewable energy technologies (RET), often known as clean technologies or green energy, are technologies designed to use renewable energies. Safe and sustainable energy resources are also required for long-term economic and industrial growth. In the solid waste management (SWM) sector, which has grown significantly over the last century, maximizing resource (material and energy) recovery and limiting environmental impacts such as contribution to global warming are major goals (Hema et al. 2010).

3 Biogas Source There are a range of wastes that can be fed into the digester to produce biogas called sources, or feedstock. The majority of biodegradable organic compounds can be converted to biogas through anaerobic digestion, and biodegradability is the criterion that determines how much of this is achievable. Biomass feedstock, such as municipal solid waste (MSW), industrial solid wastes and industrial wastewaters, food waste, livestock manure, sewage sludge, agricultural manures, catch crops, energy crops, and microalgae, are raw materials for creating biogas by anaerobic digestion. Animal dung and slurries from cow and pig production units are the most abundant supply. Animal dung, such as that from pigs, cattle, and chickens, is a major carbon source for biogas. The moisture content and volatile solids in animal dung make up 90% of the total solids. Because of its high buffering capacity, it works well as a substrate. For a long time, sewage sludge and agricultural manures have been the primary supplies in most countries, with butcher, dairy, and restaurant waste filling in the gaps. Recent years have seen an increase in the use of municipal solid waste (MSW), industrial solid wastes, and industrial wastewaters as feedstock for biogas plants (Mata-Alvarez et al. 2000). In Sweden, sludge from municipal wastewater treatment plants is now widely regarded as a major source of organic matter for biogas production. Source-sorted food waste and manure, slaughterhouse waste, and waste from the feed and food sectors are all common substrates for biogas generation in co-digestion systems. Typically, sewage sludge is utilized as a feedstock to give energy to sewage treatment plants. For many years, sewage sludge and agricultural manures were the primary inputs, accounting for more than 80% of the total. However, manufacturers have recently begun experimenting with biogas specific agricultural commodities such as maize and rapeseed. Both the crop and the fodder it produces (silage) are used. Animal manure is becoming more popular as a feedstock all around the world. There are presently over 750 biogas plants processing animal waste in the EU, with many of them operating on a considerable scale (Mohan et al. 2010). Organic waste created by families and municipalities can also be used to produce biogas. The creation of urban solid trash, also known as municipal waste, rises in tandem with population growth, increased economic activity, and the production of commodities. Agricultural residues (e.g., animal manure), landfill and food waste,

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as well as aquatic biomass and lignocellulosic raw materials, have the potential to be used to make biogas. The biogas technology uses wood, agricultural leftovers, and animal dung as energy sources. The use of wastewater from inorganic sources, such as chromium, as a source of energy has also been investigated as a more environmentally sustainable alternative to landfill disposal of these wastes. Due to its features, such as high organic load, industrial waste and wastewater have potential uses in biogas production. Algae is being looked at as a viable biomass feedstock for reducing our reliance on non-renewable energy sources for electric power, transportation, and heating. Livestock manure, i.e. cow dung, is an effective feedstock for biogas production, yielding a high cumulative biogas yield with consistent performance over time. As a result, cow manure is better for biogas production. Animal manures, municipal solid wastes, food wastes, industrial wastes, agricultural leftovers, poultry wastes, forestry wastes, microalgae, and some dedicated energy crops are all examples of biomass supplies for biogas production (Ganguly et al. 2006). Biogas is a versatile green energy source that may be used to replace fossil fuels in power and heat generation, as well as a gaseous transportation fuel. Biogas is a green energy supply that can be used in a variety of ways. The utilization of energy and manure can result in social and economic benefits, as well as a cleaner environment and a contribution to long-term development. By replacing renewable energy sources for traditional energy sources, renewable energy technologies offer a great chance to reduce greenhouse gas emissions and global warming (Gungor-Demirci and Demirer 2004). The most desirable management approach for organic waste is anaerobic digestion, which was examined in depth in this study. Biogas contains undesirable chemicals and other gases that are classified as biogas contaminants. Pretreatment to increase methane yields and/or post-treatment to eliminate H2 S are frequently required to improve the quality and amount of biogas. Pretreatment of organic waste is a critical stage in the biogas production process. Cleaning and CH4 enrichment are the two major treatment methods for biomethane (biogas upgrading) (Khan et al. 2017). AD is a waste treatment process that produces biogas by utilizing natural breakdown of diverse substrates. Over the last decade, there have been significant developments and technological advancements in AD processes. This research delved into the most current advancements in AD technology, including pretreatment, material selection for digestion, and digestion-related aspects. Challenges posed by process inhibition, AD reactor designs, and biogas upgrading to produce biomethane were also thoroughly examined. Finally, the process’s applications, current and future economics, and the AD process’s overall future were investigated. Co-digestion has been proposed as a priority for future AD technology research in order to reduce inhibition and improve process efficiency. To discover the best synergistic percentages and minimize inhibition, more substrate combinations must be investigated. Manure was given special attention because it is a commonly used and available substrate source all over the world. While AD is used all over the world, due to the intricacy of microbial community composition and substrate interactions, it is clear that a thorough knowledge of the process is lacking. More research is needed to determine

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their unique impacts on biomass structure, which will aid in the production of biogas (Nasir et al. 2014).

4 Process of Production of Biogas Biogas production provides a multipurpose carrier of renewable energy, as methane may be used to replace non-renewable fuels in both heat and electricity generation, as well as an automobile fuel. AD of wastes, energy crops, and leftovers is gaining popularity as a way to reduce greenhouse gas emissions and enhance long-term energy supply development. Anaerobic digestion is a well-established process for the treatment of industrial wastewater, municipal solid waste, animal manure, and sewage sludge. Mass reduction, door removal, pathogen reduction, decreased energy use, and, most importantly, energy recovery in the form of methane is just a few of the advantages of anaerobic digestion technology (Goel et al. 2010). The goal of the anaerobic digestion process is to produce methane-rich biogas in an oxygen-free environment by biological degradation of organic waste. Aerobic digestion is a low-cost, environmentally friendly waste treatment technology that minimizes greenhouse gas emissions. In the meantime, the wastes are stabilized and reduced. One of the most significant benefits of aerobic digestion is its flexibility to a wide variety of organic substrates. The biogas produced can be utilized to generate electricity and heat, or it can be improved and used as a car fuel in the transportation industry. Furthermore, the by-product of AD, known as digestate residue, can be used as a fertilizer on agricultural land. Dry and wet fermentation systems are two types of processes that can be used to generate biogas. Vertical stirred-tank digesters with various stirrer types are frequently used in wet digester systems, depending on the feedstock supply (Murto et al. 2004). As long as the biomass has hemicelluloses, cellulose, carbohydrates, proteins, and lipids as main ingredients, it can be used as a biogas substrate. Due to the delayed anaerobic decomposition, only powerful lignified organic materials, such as wood, are appropriate. Biogas composition and methane yield are affected by feedstock type, digestion mechanism, and retention duration. Carbohydrates 790– 800 (Nm3 /t TS) of biogas, 50% CH4 and 50% CO2 , carbohydrates exclusively in the form polymers from hexoses, not inulin and single hexoses, raw protein 700 biogas (Nm3 /t TS), (70–71)% CH4 and (29–30)% CO2 , and raw fat 1200–1250 biogas (Nm3 /t TS), (67–68)% CH4 and (32–33)% CO2 (Romano and Zhang 2008).

5 Biological Process Anaerobic digestion is the fermentation of organic wastes by bacteria in the absence of oxygen. Methane fermentation is a complicated process that involves four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis, which results in the

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breakdown of complex biodegradable organics (Sivamani et al. 2021). Large protein macromolecules, lipids, and carbohydrate polymers (such as cellulose and starch) are broken down into amino acids, long-chain fatty acids, and sugars in the first stage (hydrolysis process). Second stage (acidogenesis process): the first-stage products are fermented via acidogenesis to produce volatile fatty acids, valeric acid, propionic acid, lactic acid, and butyric acid. Bacteria consume these fermentation products and generate acetic acid, hydrogen, and carbon dioxide in the third stage (acetogenesis). The fourth step (methanogenic) occurs when organisms use hydrogen, acetate, and a small amount of carbon dioxide to produce methane. Three biochemical pathways are used by methanogens to achieve this (Sivamani et al. 2021). 4CH3 COOH → 4CO2 + 4CH4 (Acetotrophic reaction)

(1)

CO2 + 4H2 → CH4 + 2H2 O (Hydrogenotrophic reaction)

(2)

4CH3 OH + 6H2 → 3CH4 + 2H2 O (Methylotrophic reaction)

(3)

Biogas is a versatile renewable green energy source that may be utilized to replace non-renewable energy sources, generate heat and power, and serve as a gaseous automotive fuel. Biomethane can also be used to make chemical compounds instead of natural gas. Biogas production during AD has many advantages over other bioenergy producing systems. It is acknowledged as one of the most energy-efficient and ecologically friendly bioenergy generating technologies. Anaerobic digestion is a widely used technology that has several advantages over other biofuels production methods, including sustainable biogas production, the ability to use wastewater and sea water, lower operating costs, maximum biomass utilization, minimal sludge production, lower energy consumption, and the ability to recycle nutrients (Singh and Prerna 2009). Through pathogen inactivation, increased manure fertilizer quality, and significant door reduction, AD of animal manure gives certain socioeconomic, environmental, and agricultural benefits, including production of biogas, a green sustainable fuel with numerous uses. The reactor’s slurry or digestate is high in ammonium and other nutrients, making it ideal for use as an organic fertilizer. The European Renewable Energy Directive has set a target of substituting renewable energy sources for 27– 30% of overall energy consumption by 2030. Biogas from agricultural and forestry residues is estimated to contribute 14–26% of this renewable energy target. In Europe, biogas is being produced and used. In 2007, Germany was Europe’s greatest producer of biogas, primarily from energy crops, while the United Kingdom was the second largest producer, primarily from waste sources (Korbag et al. 2020). To transform biomass into green, sustainable products, three typical technologies (shown in Fig. 2) are used. Gasification, liquefaction, pyrolysis, and charcoal are prominent thermal ways for converting biomass into alternative fuels, while fermentation and anaerobic digestion are two biological approaches for converting biomass

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into bioenergy, as shown in Fig. 2. The focus of this study will be on anaerobic digestion to produce biogas. The anaerobic co-digestion system is a viable option for addressing the drawbacks of single substrate digestion systems, with substrate properties and chemical composition, operating parameters (pH, charge rate, temperature, etc.), bioavailability, biodegradability, and bioaccessibility all being important parameters to optimize (Solera et al. 2002). To boost biogas generation, some raw materials must be processed. Co-digestion has lately become a mainstream method in agricultural biogas production in several countries, while AD was formerly associated with a single substrate/single output process. The simultaneous digestion of more than one substrate with complimentary properties is known as anaerobic co-digestion, and it has gained popularity since the digestion of many materials can result in larger methane yields than when single materials are treated individually. Several of the reasons for the improvement are linked to substrate combinations that produce a beneficial interaction within the system, lowering negative effects of toxic or inhibitory substances, changing C/N ratio and reactor stability, supplementing nutrients, and balancing buffer capacity. Improved nutrient balance, microbial synergy, larger load of biodegradable organic matter, and higher biogas generation are all advantages of adopting co-digestion procedures (Baena-Moreno et al. 2019).

Fig. 2 Technology of biomass conversion

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6 Pretreatment Technologies Used in Biogas Production There are a variety of treatment procedures for organic waste management now available; some are more technologically advanced than others, and some are more established in countries where legislation and policy favour certain environmental goals. Landfilling, Aerobic Composting, Incineration, and Anaerobic Digestion are the four different processes now in use around the world (AD). The most desired management approach for organic waste is anaerobic digestion, which will be discussed in depth in this study (Khan et al. 2017). Organic waste is regarded exceedingly diverse, with large differences in moisture content and impurity levels. As a result, one of the most important processes prior to aerobic digestion is pretreatment. Pretreatment systems of various types have developed and are successfully implemented in many anaerobic digestion plants across the world. Glass, metals, stones, and sand are virtually always included in organic waste, necessitating the need of extra systems to deal with such heavy contaminants. Thus, mechanical (e.g., milling), chemical (e.g., acid or alkali treatment), or thermal (e.g., steam explosion) pretreatment procedures are commonly used. Novel pretreatment technologies based on ionic liquid or supercritical CO2 to solubilize and collect lignin are emerging, with the goal of improving biogas production while simultaneously generating additional cash through lignin collection. This phase is critical for better biogas generation from lignocellulosic feedstock, regardless of the pretreatment technology utilized. A mechanical pretreatment is installed in any AD application that deals with organic waste (Sarkar and Banerjee 2013). Mechanical pretreatment, which may include pulpers and shredders, is a critical unit in industrial biogas facilities. These devices are used to increase the surface area of tough solid substrates [such as municipal solid waste (MSW), cardboard, mixed industrial wastes, bulky trash, waste tires, waste wood, and waste papers, among others] by crushing and breaking them down, resulting in increased digestion and AD. To lower the size of organic waste and separate the plastic and packaging material from the biodegradable fraction of the trash, many pretreatment procedures are available. As a result, plants are generally very adaptable in their treatment of various types of organic waste, with no quality constraints. Due to their inflexible structure, sewage sludge and agricultural biomass, such as straw, are difficult to breakdown aerobically. As a result, while treating such forms of organic biomass, organic waste thermal treatment at high pressure and temperature values is more known. Food waste, on the other hand, can be efficiently converted to biogas in anaerobic digestion systems by mechanical processing. An efficient pretreatment of organic waste ensures the creation of high-quality fertilizers, allowing vital nutrients to be recycled back into the natural cycle while avoiding the need for additional, costly digestate processing following AD. If the biodegradable organic material is effectively crushed in the pretreatment and a significant surface area for microbial breakdown is accomplished, high biogas outputs in anaerobic digesters can be attained (Balcioglu et al. 2017).

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The content of organic waste collected from various sources (commercial, municipal, and industrial) varies greatly. The waste composition is the most important criterion for selecting an effective pretreatment technology. Furthermore, knowing which type of AD, i.e., dry or wet digestion systems, should be utilized to treat the organic waste is critical for selecting the most effective pretreatment system. Pretreatment technology is used in wet anaerobic systems to remove undesired pollutants prior to the anaerobic digestion process, and they operate at a lower solid concentration. After anaerobic digestion, the digestate can be used as high-quality fertilizer right away, and no further treatment (compost refinement, post-composting, etc.) is normally necessary. Because of the efficient organic waste preparation, a large amount of biogas is produced. Wet anaerobic digestion systems are the preferable technology for treating wet organic waste such as food leftovers, food trash, packaged food, and the organic part of MSW. Dry anaerobic systems operate at higher solid concentrations and use simpler pretreatment technologies before the anaerobic digestion process. Because impurity separation efficiency is insufficient to use the digestate as high-quality fertilizer right away, additional digestate treatment (i.e., compost refinement, post-composting) is usually required to determine whether the input chemicals are polluted (Prasad et al. 2019). Dry anaerobic technology is commonly used to determine whether organic waste has a large percentage of garden waste, and it can also be used to treat the organic component of MSW after a mechanical extraction procedure. Dry anaerobic digesters have higher solids loading and biomass retention, regulated feeding and geographical niches, and simpler pretreatment, but they have complex and expensive waste transport and processing, difficult material handling and mixing, and can only use structured material. Wet systems are designed to treat dilute organic slurry with a total solid concentration of no more than 10–15% (Sivamani et al. 2021). Substrates with a total solid content more than 15% will either be co-digested with co-substrates with a lower total solid content or diluted with recirculated or new process water. Wet AD technique has successfully treated a variety of low solid pollutants, including food industrial effluents and sewage sludge. In contrast, the substrates utilized in solid-state fermentation processes, also known as dry digestion, have a high solid content (25–40% TS), necessitating a fundamentally different technical approach to waste collection and treatment. Because heat and nutrient transport in dry digestion systems is less efficient than in wet processes because to the high viscosity, mixing is critical to avoid local overloading and acidity. Despite this, traditional mechanical mixers are ineffective for solid-state processes; instead, recirculation of waste or re-injection of produced biogas are frequently used to alleviate mixing issues in these reactors. Wet anaerobic digesters have some advantages, including diluting of inhibitors with fresh water, but they also have several disadvantages, such as scum development during crop digestion, high water and energy consumption, and sensitivity to shock loads (Baumann and Muller 1997).

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7 Biogas Production from PET Industrial Wastewater A polymer is a large molecule made up of small, repeatable chemical units. In certain cases, the chain repeat can be straight, while in others, it can be branched or interconnected to form three-dimensional networks. The repeat unit of a polymer is usually the same as or nearly the same as the monomer it is comprised of. The length of the chain is determined by the number of repeat units in the chain. The degree of polymerization is the term for this (Wallace et al. 2020). Farzi and Fotouhi divided polymerization into two types: condensation (step reaction) and addition (continuous polymerization) (chain reaction). Condensation happens between two polyfunctional molecules in the first case, resulting in the production of a larger polyfunctional molecule with the possible elimination of a minor component such as water. This reaction will proceed until one of the reagents is depleted (Farzi et al. 2019). Changing the temperature and altering the amount of reactants and products can alter the reaction’s equilibrium. The chain carrier in addition polymerization reactions is an ion or a reactive molecule with one unpaired electron called a free radical. This free radical is created by the disintegration of an initiator, which is a relatively unstable chemical. It can open the double bond of a single unit and bind to it, leaving one electron unpaired. In a few seconds or fewer, monomers are added to the growing chain, extending it. When two radicals unite or disproportionate, the chain comes to an end (Almeida et al. 2019). In general, polymerization is divided into three stages: initiation, propagation, and termination. In terms of reaction mechanisms, polymerization is mostly accomplished through two approaches. The step-growth and chain-growth mechanisms are two of them. Polymers are generated by an independent reaction between the functional groups of simple monomer units in step-growth polymerization. To generate a longer length molecule, each step in step-growth may consist of a mixture of two polymers of differing or equal length. The reaction takes a long time to complete, and the molecule mass increases at a glacial pace. Condensation polymerization is an example of step-growth polymerization, in which a water molecule is evolved as the chain lengthens (Furukawa et al. 2019). The production of the polymer happens when some tiny molecules are lost as byproducts of the reaction when molecules are linked together in condensation polymerization. Water or hydrogen chlorides are possible byproducts. Condensation polymers include polyamide and proteins (Gong et al. 2018). Some of the different types of condensation polymerization are given below: Nylons are polyamides, which are synthetic fibres. Between these polymers is an amide bond. A polyamide is formed through condensation polymerization of diamines with di-carboxylic acid, as well as amino acids and their lactams (Jenkins et al. 2019). Nylon 66: This polymer is made by condensation polymerizing hexamethylenediamine with adipic acid at high pressure and temperature. Nylon 6 is made by

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combining caprolactam and water at a high temperature. Tyre cords, textiles, and ropes are all made of it. Polyesters are generated when dicarboxylic acids and diols polycondensate. Using zinc acetate antimony trioxide as a catalyst, heat a combination of terephthalic acid and ethylene glycol at 460 k. Polyesters are most often known as Dacron or Terylene. They’re also employed in safety helmets as glass reinforcing elements. In certain conditions, condensation polymerization of melamine and formaldehyde produces formaldehyde polymer. They’re employed in the production of non-breakable tableware (Hahladakis et al. 2020). The monomer molecules are combined together to form a long chain in chaingrowth polymerization. The monomers that are added may be of the same kind or of a different type. Alkenes, alkadienes, and their derivatives are commonly used. Chain lengthening happens in this phase as a result of the production of free radicals or ionic species (Hiraga et al. 2019). In the presence of free radicals, many monomers, such as alkenes or dienes, and their derivatives, polymerize. A modest amount of benzoyl peroxide initiator is used in the polymerization of ethene to polythene by heating or exposing to light. The peroxide-generated phenyl free radical is added to the ethene double bond, resulting in the formation of a new, bigger free radical. PET is the most widely used polymer in the last two decades, with good chemical and physical qualities for a wide range of applications, including gas barrier, low diffusivity, superior mechanical and thermomechanical capabilities, extremely inert material, clarity, and fine process operation. PET waste, on the other hand, is already a source of concern for humans as well as the environment (Jaiswal et al. 2020). PET is a polymer-based thermoplastic polyester resin (ethylene terephthalate). There are two types of resins: low-viscosity and high-viscosity resins. PET with a low inherent viscosity has a viscosity of 0.75 or less, whereas PET with a high intrinsic viscosity has a viscosity of 0.9 or more. Low-viscosity resins, often known as staple PET, are used in a wide range of products, including garment fabric, bottles, and photographic film (when used in textile applications). High-viscosity polymers, often called as industrial or heavy denier PET, are used to make tire rope, seat belts, and other items. PET is commonly used in the manufacture of synthetic fibres (i.e. polyester fibres), which make up the vast bulk of the synthetic fibre market. Because PET is a pure and regulated material that passes FDA food contact guidelines, it is often used in food packaging, including as beverage bottles and frozen food trays that can be heated in a microwave or regular oven. PET bottles are used to package alcoholic beverages, salad dressings, mouthwash, syrups, peanut butter, and pickled foods. PET containers are used to package toiletries, cosmetics, and household and pharmaceutical supplies (e.g., toothpaste pumps). Moulding resins, X-ray and other photographic films, magnetic tape, electrical insulation, printing sheets, and food packaging film are some of the other uses for PET (Kawai et al. 2020). Between 1950 and 2015, over 6.3 billion tons of plastic waste were produced worldwide, with approximately 9% being recycled, 12% being burned, and 79% being stored in landfills or the natural environment (Sahota et al. 2018).

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Almost a million metric tons of PET waste is estimated to be deposited in the ocean or landfill per year. Plastic is currently recycled through landfill, incineration, energy recovery, and plastic recycling. Traditional landfill and incinerator technologies were worried because the plastic component may be discharged into the environment after processing. Landfill and incinerator operations release hazardous substances into the environment. According to the Environmental Protection Agency (2010), plastic recycling is divided into three categories: (i) direct use, (ii) physical reprocessing, such as grinding, melting, and reforming, and (iii) chemical processing, which involves isolating and reprocessing components for use in the chemical industry (Awaja and Pavel 2005). PET stands for polyethylene terephthalate, and it is a type of polyester that is commonly used in industry. While working for the Calico Printers Association, Whinfield and Dickson were the first to synthesize PET in the UK in 1942, at the commencement of World War II. Furthermore, Imperial Chemical Industries of the United Kingdom produced the fibre melt–spam terylene from the new polyester. In addition, DuPont launched the Dacron in the United States in 1953. PET has since eclipsed nylon as the world’s most frequently used synthetic fabric. The stretch moulding technique was created as a bulk chemical to make PET into long-lasting crystal-clear drinking bottles around the end of the 1970s. This use came in second as the most popular in fibre production (Chen et al. 2020). Only a little amount of polluted water is released into the environment by the plastics industry. The majority of the polluted effluent is cooling water, but small amounts of intermediate products, by-products, and end-products that are suspended particles or emulsions in solution make up a small fraction of the polluted effluent. Resin, organic acids, tetrahydric alcohol, pentaerythritol, formaldehyde, sodium formate, phenols, urea, and benzene may be present depending on the polymers being produced. The concentration of impurities varies from very low to quite high. BOD levels might be anywhere between 200 and 10,000 mg 02/1. The wastewater is acidic in general (Blank et al. 2020). In the plastics manufacturing, there are three forms of wastewater: (1) Biodegradable monomeric and polymeric compounds in wastewater, such as wastewater from the polyacrylics production process (Carniel et al. 2017). (2) In addition to non-refractory chemicals, wastewater contains nutrients as well as suspended refractory substances, which are frequently partially polymerized materials. This effluent is produced in the manufacturing of polystyrene and polyethylene (Carta et al. 2003). (3) Wastewater, which contains harmful chemicals such as cyanide as well as nonrefractory substances. This effluent is produced during the production of polyacrylonitrile, acrylonitrile-butadienestyrene copolymers, and other polymers (Bollinger et al. 2020). Natural habitats rely on the activity of anaerobic bacteria for the biological breakdown of organic substrates in the absence of molecular oxygen. In anaerobic conditions with no inorganic electron acceptors other than carbon dioxide, organic

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molecules are converted to gaseous methane and carbon dioxide as the ultimate step in their destruction. As previously noted, wastewater-treatment systems that use the anaerobic digestion process to remove organic contaminants from industrial wastewater streams, have significant advantages over aerobic methods (Hosaka et al. 2013).

8 Biogas Production from Municipal Sewage Sludge Environmental issues have been gradually expanded with concepts such as sustainability development, which means not only the environment, but also the economic and social aspect responsibilities. The treatment of sewage sludge is one of the most important challenges in wastewater management (Fytili and Zabaniotou 2008). Liquids and Solids are separated from the sewage sludge produced during wastewater treatment process. Liquids are discharged into the aquatic environment or reused after purification while solids are removed for further and final treatment removal. Components removed during wastewater treatment include granules and mud (Metcalf and Eddy 1991). Among these, sludge is the largest in volume. Therefore, treatment methods and techniques for their elimination are of great concern. Without a reliable sludge disposal method, the real concept of water protection will fail (Fytili and Zabaniotou 2008). Activated sludge is the solid or semi-solid material produced during industrial and municipal biological treatment residual water. It contains a variety of microorganisms in which organic and inorganic compounds are used in water as a source of energy, carbon and nutrients. Residual sludge containing 1–2% solids that is usually concentrated by gravity thickening or air fluctuation to about 10% solids. In many cases, concentrated sludge is introduced into aerobic or anaerobic digester to reduce the level of pathogens and odours (stability). In a sewage treatment plant, the activities associated with the treatment of sludge represent from 30 to 80% of the electricity consumed in factory (Water Environment Federation 2002). Biogas is produced by anaerobic digestion of organic matter such as manure, municipal solid waste, sewage sludge, biodegradable waste and agricultural sludge (Appels et al. 2008; Nasir et al. 2014). With the help of microorganisms, the AD process converts biomass into biogas (Bharathiraja et al. 2014). On the other hand, sewage sludge consists of wastewater treatment by-products (Usman et al. 2012). Organic matter from municipal solid waste can be digested to form biogas. The production of fuel from waste is a simple and economical process. The composition of local solid waste depends on the source of the waste; However, in all cases, the predominant components of urban solid waste are of organic origin, accounting for more than half of the MSW content. Social concerns for many civilizations revolve around the management of food and agricultural waste, food processing solid and liquid waste, and sewage sludge. Most biogas plants use manure or sewage sludge as an organic resource for soil control and also as fertilizer (Balat 2008). Approximately 16% of disposed municipal solid waste is incinerated; the rest is disposed of in

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landfills. Osorio et al. (2014) argue that municipal solid waste disposal in landfills is usually associated with Contamination of soil, surface and ground waters due to improper installation of the landfill. Municipal solid waste treatment has many advantages and limitations. Greenhouse gas emissions cannot be reduced due to the uncontrolled emission of methane from improperly disposed organic waste over a large area landfill (Demirbas 2006). The decomposition of landfills occurs in several stages, and each stage is characterized by the increase or decrease of certain bacterial populations, and the formation and use of certain metabolites. The first stage of decomposition usually takes less than a week and is characterized by aerobic bacteria that remove oxygen from the waste. The second stage is called the anaerobic acid stage, which includes many hydrolytic and enzymatic bacteria, which break down polymers such as cellulose, hemicellulose, proteins and lipids into soluble sugars, amino acids, long-chain carboxylic acids and glycerol. The main component of landfill gas is a by-product of organic decomposition, which is usually found in municipal waste and is produced by natural bacteria under anaerobic conditions (Demirbas 2008). A stoichiometric analysis of biogas production by anaerobic digestion of wastewater from cassava, wheat bran and sewage sludge is proposed. A wide variety of methods are available to study the stoichiometry of biochemical reactions. This work records the method of elemental balances for the resolution of stoichiometric parameters in the production of biogas from cassava effluent, wheat bran and sewage sludge. The method can be used for many substrates for biogas production and for reactions (Sivamani et al. 2020).

9 Biogas Production from Cassava Plant Industrial Wastewater Jiraprasertwong et al. (2019) developed a three-stage upflow anaerobic sludge blanket (UASB) system and tested for hydrogen (H2 ) and methane (CH4 ) production from cassava wastewater with an emphasis on CH4 production. The experiment was carried out at mesophilic temperature (37 °C) at different chemical oxygen demand (COD) loading rates from 5 to 18 kg/m3 d (based on total liquid holding volume) with a recycle ratio of the final effluent to both the first and second bioreactors at a constant 1: 1 flow rate ratio of feed: final effluent. The first bioreactor was maintained pH at 5.5 while those of the other two bioreactors were not controlled. At an optimum COD loading rate of 15 kg/m3 d, the system provided the highest COD removal level (92.5%) and the highest H2 and CH4 yields of 0.43 mL H2 /g COD applied, and 328 mL CH4 /g COD applied, respectively. The very high productivity of CH4 with the very low H2 productivity resulted from the recycled methanogen sludge from the third bioreactor to the first and second bioreactors. The process performance of the three-stage UASB system in terms of optimum COD loading rate and total energy yield was much higher than those of single and two-stage anaerobic processes.

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Watthier et al. (2019) aimed to evaluate the treatment of cassava starch wastewater in a horizontal anaerobic fixed bed reactor for the removal of organic matter and generation of biogas. For this purpose, two fixed bed reactors filled with different types of support materials were used: bamboo rings (R1) and flexible PVC rings (R2). The process of cassava starch extraction is associated with the generation of a large volume of liquid wastes from root washing and starch extraction (5–7 L/kg root). The reactors were constructed in polyvinyl chloride (PVC) with 90 cm length and 15 cm diameter. To evaluate the reactors were carried out 13 assays (A1 to A13) with increases in the organic loading rate (OLR, 1.7–15.0 g/L.d) and a decrease in the hydraulic retention time (HRT, 4.0–0.8 days). Biogas production followed a similar pattern in both reactors. The highest biogas productions of 1.4 L/L.d (R1) and 1.0 L/L.d (R2) were verified in the assay A6 in which were applied highest influent concentration (15.1 g/L), intermediates OLR (5.6 g/L.d) and HRT (2.7 d). The chemical oxygen demand (COD) removal increased with the OLR increase resulting in COD removal values of up to 99%. Total solids removal efficiencies of 86.2 and 85.5% were achieved in R1 and R2, respectively. In both reactors, the contact surface of bamboo (132 m2 /m3 ) and PVC (191 m2 /m3 ) supported biomass attachment even in the highest OLR (15 g/L.d) and low HRT (0.8 d). Both support material provided conditions to the reactors resist the variations in operating conditions and reestablish the equilibrium after possible hydraulic and organic shocks load, constituting a robust system for the treatment of cassava starch wastewater. In these conditions the COD and solids removal remained satisfactory. After 450 days of reactor running, no changes were observed in the composition and structure of the support material, indicating that bamboo and PVC are possible cheap and efficient alternatives for biomass immobilization. Peres et al. (2019) aimed to evaluate the process of anaerobic digestion using cassava wastewater, a pollutant and toxic residue of cassava processing, and sludge from a sewage treatment plant, to determine the best conditions for the production of biogas and to identify its chemical composition, specifically the levels of methane and carbon dioxide. To accomplish this, six anaerobic digestion media were produced and placed into 100-mL penicillin bottles. Due to the current need for alternative energy sources associated with the practice of actions to improve the environment, residual biomass has gained ground and importance in the energy sector, because of its high availability and cost–benefit, contributing to sustainable development. The experiments were carried out in triplicate, differentiated by the substrate/inoculum concentration (on a mass basis) and by the type of inoculum used (primary or secondary sludge). The best result was achieved using the following media: 80% cassava wastewater and 20% primary sludge, 4:1, which produced the highest methane content in the biogas over the shortest period of time, reaching 81.41% mol/mol of methane in 48 days of fermentation. In this way, biogas produced by cassava wastewater, both in large starch-producing factories and in smaller flour factories, can be used as a source of renewable energy, reducing production costs and providing an environmentally correct destination for this waste. Achi et al. (2020) investigated the impact of co-digestion of cassava wastewater (CW) with livestock manure (poultry litter (PL) and dairy manure (DM)), and porous

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adsorbents (biochar (B-Char) and zeolite (ZEO)) on energy production and treatment efficiency. Currently, there are challenges with proper disposal of cassava processing wastewater, and a need for sustainable energy in the cassava industry. Batch anaerobic digestion experiments were conducted, with 16 treatments of CW combined with manure and/or porous adsorbents using triplicate reactors for 48 days. The results showed that CW combined with ZEO (3 g/g total solids (TS)) produced the highest cumulative CH4 (653 mL CH4 /g VS), while CW:PL (1:1) produced the most CH4 on a mass basis (17.9 mL CH4 /g substrate). The largest reduction in lag phase was observed in the mixture containing CW (1:1), PL (1:1), and B-Char (3 g/g TS), yielding 400 mL CH4 /g volatile solids (VS) after 15 days of digestion, which was 84.8% of the total cumulative CH4 from the 48-day trial. Co-digesting CW with ZEO, B-Char, or PL provided the necessary buffer needed for digestion of CW, which improved the process stability and resulted in a significant reduction in chemical oxygen demand (COD). Co-digestion could provide a sustainable strategy for treating and valorizing CW. Scale-up calculations showed that a CW input of 1000–2000 L/d co-digested with PL (1:1) could produce 9403 m3 CH4 /yr using a 50 m3 digester, equivalent to 373,327 MJ/yr or 24.9 tons of firewood/year. This system would have a profit of $5642/yr and a $47,805 net present value. Andrade et al. (2020) proposed the stabilization of the cassava starch wastewater (CSW) pH and degrading cyanide (CN− ) to optimize biogas production. To control the acidity of the CSW, they used natural oyster shells as source of CaCO3 , and the photocatalytic degradation of CN− was achieved with Degussa P25 TiO2 . CSW poses a high polluting potential due to its high organic loading and CN− concentration, but this residue can be pretreated and reused. Natural oyster shells raised pH from 4.5 to 6.2 over 6 h of reaction, efficiently controlling the effluent acidity. After pH stabilization, the TiO2 photocatalyst tested in a degradation process under visible light was able to reduce CN− concentration by 73.02%. After these pretreatments (pH stabilization and CN− degradation), the CSW was inoculated with sewage sludge (SS) to produce biogas. The pretreatments were proved to be efficient at favoring biogas production as this was heightened by 27.6%. In addition, the pretreated CSW and digestate (anaerobic digestion) significantly reduced the toxicity of the effluent, assessed by investigating lettuce seeds (L. sativa) germination and root growth. Thus, pretreatments and reuse of residues may potentially provide socio-environmental and economic benefits. Wattanasilp et al. (2021) applied the optimization model of the biogas utilization pathway with the biogas utilization availability assessment to examine the effect of biogas system parameters on biogas utilization. The model analyzes the biogas utilization pathway availability and maximum profit to value added and productivity in biogas from industry wastewater in Thailand. The results showed that profit and availability of biogas utilization reduce biogas loss to flare, that it entails several conversion processes. The scenario for the biogas utilization pathway and storage with biogas production technology improves. Evaluated are operation time, waste and energy demand to the cassava starch usage during the production for 50–1000 tons per day. Five mature biogas production technologies were benchmarked evaluated based on the chemical oxygen demand removal efficiency and biogas yields. Subsequently,

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low-, medium-, and high-pressure storages and a battery storage were considered and discussed in this paper as suitable energy storage for each desired biogas plant operation. Five biogas utilization pathways, including converting biogas into thermal energy, generating electricity, and upgrading biogas to compressed biogas, were then compared. Those improved options in the scenario select suitable biogas technologies, storage, and application for value-added, reduce the environmental problems and renewable energy production from wastewater. Chavadej et al. (2019) investigated the separate production of hydrogen (H2 ) and methane (CH4 ) from cassava wastewater with added cassava residue using a twostage upflow anaerobic sludge blanket (UASB) system under thermophilic temperature (55 °C) in a continuous mode of operation and steady state condition. The two-stage UASB system was operated under an optimum chemical oxygen demand (COD) loading rate of 10.29 kg/m3 d (based on the total volume of both bioreactors) of the cassava wastewater with different concentrations of added cassava residue. The recycle ratio of the effluent from the second bioreactor to the feed flow rate was fixed at 1:1 (v/v). In addition, the solution pH in the first bioreactor was controlled at 5.5, while that in the second bioreactor was not controlled. Under the optimum cassava residue concentration of 1200 mg/L, the produced gas from the first bioreactor contained 42.3% H2 , 55% carbon dioxide (CO2 ) and 2.70% CH4 , while that from the second bioreactor contained 70.5% CH4 , 28% CO2 and 1.5% H2 . Apart from a high H2 and CH4 production performance (45.2 and 150% improvement, respectively, as compared to the system without added cassava residue) under the optimum cassava residue concentration (1200 mg/L) and the controlled COD loading rate (10.29 kg/m3 d) of the cassava wastewater, the degradation performance of cellulose and hemicellulose were 41% and 22%, respectively, for the first bioreactor and 23% and 11%, respectively, for the second bioreactor. The digestibility of the cassava residue at thermophilic operation was higher than that at mesophilic temperature.

10 Conclusion Biogas implementation decisions are based on careful planning of economic efficiency while taking into account potential dangers. The adoption of AD biogas will be determined by economic factors. The governments of European countries substantially subsidize AD, yet without these subsidies, AD may become less sustainable in the future. There are large, single-location dairy herds in the western United States, such as California, that can provide economies of scale and promote biogas production. With increased implementation and government subsidies, the United States might become a global leader in AD biogas production and contribute to the development of new technology that could be shared globally. AD biogas is a realistic choice in impoverished nations for generating power or heat for cooking, as well as for powering schools and other community structures. It also allows underdeveloped countries to have pumps, motors, and even freezers. Solar is another “green” option in developing countries, but it requires a greater initial capital investment, is

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not a constant source of energy, and requires more technical upkeep over time. In these developing countries, economics is not a valid factor; it is a social need and an opportunity for a better existence.

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Nanotechnology-Based Solutions for Wastewater Treatment Km. Sakshi and Navneeta Bharadvaja

Abstract The industrialization has exhibited a specific role in the growth and development of the nation. However, it also has a hazardous effect on the ecosystem. Industrial and domestic wastewater plays a very significant activity in water. The water cycle is a continuously regenerating system and makes drinking water one of the most valuable and scarce resources in the world. It is no secret that there are a number of contaminants in water, including organic, inorganic, bacteria, and industrial waste besides other harmful chemicals. Water pollution is a worldwide problem that poses a serious threat to health, the environment, and the economy. Due to the severe shortage of water resources, proper wastewater treatment is imperative. In this environment, it is critical to identify inexpensive, safe, and more efficient wastewater treatment technologies. Latest biological studies demonstrated that nanotechnology can be remarkably supported in offering prospering solutions towards the approximate hassle of the drinking water emergency. There are many advantages offered by nanotechnology, including the ability to detect and evaluate water pollutants as well as the ability to purify polluted water. A further interesting concept is bio-refinery, which uses wastewater as a source of raw materials. Which is converting waste nutrients into valuable products of value and providing a non-negotiable product, clean water. Keywords Nanotechnology · Pollutants · Wastewater · Industrial · Contamination · Ecosystem

1 Introduction The importance of water in human life can be seen in food, economic development, and health. Water is crucial to circulatory function, cellular homeostasis, and the movement of substances across cell membranes. The United Nations claims that drinking contaminated water leads to a higher death rate than any other cause of Km. Sakshi · N. Bharadvaja (B) Plant Biotechnology Laboratory, Department of Biotechnology, Delhi Technological University, Shahbad Daulatpur, Main Bawana Road, Delhi 110042, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_4

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death. The problem of excessive water pollution and lack of accessible water is hazardous issues for the universe in the present (US news 2017). It was estimated by the United Nations that by 2025, death rates from drinking polluted water would reach nearly 8.1 billion. Chemical and physical water purification methods are used. With the chemical methods, chlorination, coagulation, and flocculation are generally used. Filtration, reverse osmosis, irradiation with ultraviolet radiation, boiling, sedimentation, and coagulation take place in the physical method. Some procedures, such as sedimentation, flocculation, and adsorption on activated carbon, chlorination, and coagulation, are commonly employed, but generally, this kind of process is not robust enough to eliminate dissolved oxygen and toxic metal ions (Li et al. 2008). While there are a variety of different methods for eliminating pollutants present in water, such as ozone treatment, chlorine treatment, UV light irradiation, and ozone treatment, there are also some different methods that come at a cost. A variety of nanoparticles and Nanofibers are mainly used in the wastewater treatment process with nanotechnology being the most advanced, economic, and environmentally stable method to treat wastewater (Baruah et al. 2017). It is because nanoparticles penetrate deeper, have a high absorption capacity, and they have the ability to react, that nanoparticles are effective in therapy. Nanoparticles (NPs) are one structure that is dimensional significantly less than 100 nm in proportions. Different types of nanoparticles that can be used for wastewater treatment includes, including metal oxide nanoparticles like titanium dioxide (TiO2 ), zinc oxide (ZnO), and cerium oxide (CeO2 ) show high reactivity and photolytic properties against wastewater (Das et al. 2014). Due to the available surface area of these substances, they are ideal as an adsorbent for water purification as they have high affinity and maximum antibacterial activity. A functional Ag NP has maximum antibacterial activity and so it has a useful function. Hence, it is repaired so that materials can be filtered to adjust for water waste. Scientists have published research regarding fabricated nanostructured membranes that resemble ceramic zinc oxide and titanium and are also able to decrease pollutants photo analyzable and produce microorganisms by increasing the surface area (Theron et al. 2008). In this rapidly expanding field, researchers can study the unique properties of nanostructured materials that may be used to produce a wide range of products. Furthermore, it encourages researchers to investigate this unique and highly promising field. Due to their small size, nanomaterials have a large surface area to volume ratio, which means that they have more surface properties. Nanomaterials are also very small in size and also have dimensions between 1 and 100 nm that’s why they contain fewer atoms (Fig. 1). Nanomaterials absorb various water pollutants so efficiently due to their physicochemical properties (Sharma et al. 2014). One of the most straightforward methods for recovering useful products from wastewater is biorefinery. The bio-refinery concept includes the conversion of biomass into bio-based products and bioenergy. Biorefineries contribute to improving the industrial ecology of the wastewater industry in addition to promoting economic development and environmental sustainability. There are a variety of products extracted from wastewater such as poly-glutamic acid and lipids that make wastewater biorefineries an attractive option. Wastewater bioreactors are most suitable for products that are easily recovered and can play an important role in microbial ecology. The use of zeolites, biopolymers,

Nanotechnology-Based Solutions for Wastewater Treatment Fig. 1 Current nanotechnology-based solution for wastewater management

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metal nanoparticles, iron nanoparticles, nanoscale semiconductors, photocatalysts, and carbon-based nanomaterials in wastewater remediation is widespread. There have been many types of pathways developed based on nanotechnology. A—Zeolites. B—Nano adsorbents based on Polymer. C—Metal-based Nano-adsorbents based on Metal. D—Nano-adsorbents based on Carbon.

2 Carbon Based Nano-adsorbents Physicochemically favorable and inexpensive carbon-based nano adsorbents fall under the category of nano adsorbents. They have a lot of potential in wastewater treatment applications (Mauter and Elimelech 2008). The carbon allotropes are grouped into four distinct categories based on their coordination number and nuclear arrangement within the lattice. Carbon dots and fullerenes are two-dimensional carbon materials, whereas carbon nanotubes are one-dimensional carbon materials. All of Graphene’s carbon atoms exhibit sp2 hybridization, making it a two-dimensional allotrope with a planar structure. Carbon-based nanomaterials with different dimensions present surface chemistry that is conducive to the adsorption of various entities. Amorphous or porous carbon 3D materials, however, also play an important role in this application. Due to its enhanced exterior region, surface chemistry, and porous structure, activated charcoal provides a significant potential for adsorption of fine dust particles or other toxic chemical compounds, but also of other inorganic pollutants present in the atmosphere (Tomer et al. 2014). There are several papers that demonstrate the effectiveness of various allotropes of carbon, including graphene, CNTs, fullerenes, and exterior functionalized activated carbons, for the reduction of heavy metals and rock ions, such as iron, copper, nickel, and chromium, lead, and zinc as well as organic pollutants (Stafiej and Pyrzynska 2007). It has been proven that acid or alkali treatment boosts their adsorption potential to carbon nanotubes by enhancing their hollow and multilayered structure, resulting in a high adsorption efficiency. Several strong oxidizing chemicals, such as H2 SO4 , HNO3 , and KMnO4 , functionalize the carbon nanotube surface with various functional groups, such as –OH, –COOH, and –C5O. This facilitates the interaction of nanoparticles with heavy metal ions and allows for strong electrostatic interactions.

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As well as photocatalytic dye destruction, graphene and its composites with various metal oxides and metals have shown to be highly effective as metal ion adsorption agents. Because graphene surfaces are oxygen-rich, water cations that are detrimental are rapidly absorbed. Treatment of Permeable activated carbon sheets and fibers derived from plants or agricultural wastes can eliminate organic dyes, chlorinated hazardous materials, and metal ions such as mercury, lead, and cadmium present in wastewater. A commonly used icosahedral fullerene, C-60, is also advantageous as an adsorbent for hydrophobic compounds as they have superior cost, adsorption capacity, and recycling properties.

3 Metal-Based Nano-adsorbents Metal oxide-based nano adsorbents have been gaining attention in wastewater treatment during the past few decades (Hua et al. 2012; Shah 2020). In the past decade, various materials have been investigated for the treatment of wastewater, including metallic iron and its oxides, zinc oxide (ZnO), titanium dioxide (TiO2 ), magnesium oxide (MgO), and alumina. These metal oxides are extremely efficient, easy to recover after adsorption, and can be made smaller or different in shape because of their intrinsic affinity for metal ions. They also have altered magnetic properties. Iron oxides, metal oxides, and hydroxides work well to eliminate arsenic from water. In its purest form, metallic iron is able to remove polychlorinated biphenyls, pesticides, and other contaminants from wastewater. They have also been used to remove Co, Cr, Cu, Pb, and Ni-contaminated water due to the enhanced exterior region and the intrinsic ability of minor Fe2 O3 contaminants to bind with different valence ions (i.e., III and V). Burks et al. (2014) have investigated the binding of Cr from water to surface functionalized Fe2 O3 . Fe, Ni, Pb, Cu, Zn, and Cd are all tightly linked to Fe2 O3 nanotubes and Nanorods. In water, graphene oxide was used to bind dangerous mercury (II) ions using hematite nanoparticles. The removal of a variety of hazardous chemicals from water with nano-composite Zinc oxide, Magnesium Oxide (MgO), and Titanium dioxide (TiO2 ) is not limited to Fe-dependent compounds. The Nanoclusters have been extensively explored and well demonstrated. Manganese Arsenic, copper, lead iron, and cadmium are all effectively removed by hydrated amorphous. Alumina and zinc oxide are two other lower-charge nanoparticles that have been applied for adsorption, and thus elimination, of chromium ions, cadmium, and mercury in water.

4 Nano-adsorbents Based on Polymer The polymer structure of many organic and organic–inorganic hybrid polymers has been developed as well as utilized for the adsorptive removal of many contaminants from liquids. The porous metal-based structure, the large surface area, and the reactive compounds on the surface can effectively bind organic contaminants. Heavy

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metal ions, including Zn, Pb, and Cd, are bound to organic dyes in water (Yin and Deng 2015; Lofrano et al. 2016). Adsorbents have been developed to eliminate dyes and metal ions from water, but they have been limited by non-selectivity, limited adsorption capacity, and low recyclability. Intelligent and multipurpose components, included organic–inorganic. In order to solve these problems, hybrid polymers with enhanced recyclability, thermal stability, and adsorption capacity can be used (Lofrano et al. 2016). These polymeric materials are created through methods such as hydrogels, hierarchical structures, and interpenetrating systems. The sol–gel method, for instance, has been used to generate high surface area nano adsorbents that have heavy metals on their surfaces, such as mesoporous silica, and use them to remove different dangerous metal ions from water (Li et al. 2014; Shah 2021). Fe3 O4 magnetic NPs coated with acrylic acid-crotonic acid copolymers and 3-aminopropyltriethoxysilane were used by to remove heavy metal ions such as Pb21, Cd21, Zn21, and Cu21 from liquid solution at varied pH values. This allows for the fast substitution of these types of cations through other contaminants found in water. Various metal particles contain high absorption power under various acidic conditions and various pH.

5 Zeolites A zeolite with excellent porosity is among the most widely used natural adsorbents. The high porosity of zeolites renders zeolite the potential to adsorb various metal ions. The high porosity of zeolites makes them capable of adsorbing a wide range of metal ions. Due to their adsorption capability, both natural and manufactured zeolites have long been used to treat wastewater. A natural zeolite is a microporous alumina silicate framework that contains tiny cations like sodium, potassium, calcium, and magnesium. This was originally developed for the rapid exchange of these types of cations with pollutants found in water (D¸abrowski et al. 2004). Examples of natural zeolites include chabazite, analcime, heulandite, clinoptilolite, natrolite and stilbite, and phillipsite. D¸abrowski et al. (2004), reported that synthetic zeolites were commonly combined with metal ion surface assimilation as well as various organosilane moieties, including mercaptopropyl and aminopropyl, etc. Zeolites functionalized with cationic surfactants can be widely used to eliminate both natural impurities and harmful anions simultaneously. Silver and lead-loaded zeolites were employed in a few further trials to remove pathogenic bacteria and dangerous anions from water by forming insoluble complexes with arsenate, arsenate, chromate, and cyanide (Kwakye-Awuah et al. 2008; Chmielewská et al. 2003). Considering their unique designed passage systems regulatedpore condition and shape, and calculatedly changed surfaces, synthetic zeolites have now been demonstrated to have advantages over natural zeolites with regards to adsorption efficiency, selectivity, thermal equilibrium, and structural stiffness.

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6 Nanomaterials Role in Wastewater Treatment 6.1 TiO2 (Nanocrystalline Titanium Dioxide) Nanoparticles Titanium dioxide is the seventh most common metal on Earth with approximately 4.3 million tons produced every year. It is a major component of biomedical devices, metal alloys, and aerospace products (Pelaez et al. 2012). A key component of TiO2 ’s antibacterial potential is the presence of reactive oxygen species such as hydroxyl radicals, as well as TiO2 , which is produced through reductive pathways and from UV-An irradiation through oxidative processes. Researchers believe TiO2 may be widely used in the future due to its inert nature that disrupts damage to protein and DNA, disrupts electron transfer, and destroys the cell membrane. Studies have shown that paint releases an enormous amount of TiO2 (Monllor-Satoca et al. 2011). The photocatalyst activity of TiO2 is demonstrated in the splitting of water into hydrogen fuel that can be used to produce energy. It is also used in drinking water purification and air purification. Although titanium dioxide demonstrated long-term environmental friendliness and substantial efficiency, it did so through a process that is still under development: oxidation. Scientists are trying to boost the activity that is photocatalytic of TiO2 by use of mesoporous TiO2 surface treatments and consumption of various morphologies of titanium dioxide. Examples include Nanospheres, nanowires, and nanowires. It has been estimated that nitrogen-doped TiO2 nanoparticles catalysts possess high efficiency in degrading microbial contamination in water. With the use of nanostructured TiO2 films and membranes, biological impurities are destroyed by ultraviolet and irradiation, with the breakdown of biological impurities separating disinfecting microorganisms. Polypropylene in conjunction with TiO2 nanoparticles displays exceptional biocidal activities against Pseudomonas aeruginosa.

6.2 Nanomaterials Based on Carbon There are many different types of nanomaterials, which depend on a few factors, including diameter, size, length, and the total number of layers in the cage of fullerenes. Carbon nanomaterial is being manufactured using a production method that lacks full exactitude and regularity among growth conditions. Various aspects of carbon nanotechnology are being modified to improve the problems mentioned above, including temperature, purity, pressure, orientation, and catalyst (Cha et al. 2013). The use of activated carbon, fullerene, graphene, and carbon Nano sorbent in combination will reduce wastewater scents and organic wastes. These materials can be applied to the removal of particular impurities such as aromatic hydrocarbons, naphthalenes, trihalomethanes, and polycyclic hydrocarbons. Nanomaterials respond to aggregation state and solvent chemistry, as some researchers have focused on their physical properties. A variety of nanomaterial properties are influenced by

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the presence of impurities like metals, biomolecules, and vapor upon their surface, including physical, chemical, and aggregation properties (Madima et al. 2020). Carbon nanomaterials

Unique properties

Applications

References

Carbon nanotubes High porosity, hollow structure, a lightweight, specific area, and lightweight

Remove toxic pollutants, Corry (2008) heavy metals (zinc, cadmium, lead, and chromium), adsorption of recalcitrant contaminants

Graphene

Unique two magnitude structure as well as excellent physical and thermal property

Remove inorganic compounds Bacteria, algae, and solids

Yang et al. (2013), Su et al. (2009)

Graphite carbon nitrate

Extraordinary chemical stability, metal-free composition, low friction coefficient, high hardness

Remediation, low cytotoxicity electron transport ability, excellent stability, and large surface area

Liu et al. (2012)

Fullerenes

Photocatalyst, Eliminate and oxidation, antimicrobial availability of contamination incorporating endocrine disruptors and carcinogens

Tsydenova et al. (2015)

Zeolites

Molecular sieving, adsorption and ion exchange, conductivity

Cincotti et al. (2006)

Dendrimers

Physicochemical Utilized for removal of properties and works as organic and heavy biofunctional inner wall metals absorb organic compounds while outer absorbs heavy metals

Le et al. (1995)

Nanofiber membranes

Electrical conductivity and antifouling properties

Sorribas et al. (2013)

Eliminate heavy metals, cationic pollutants and also degrade aluminum

Oil–water separation

Potential unique properties and applications of nanoparticles in the treatment of wastewater

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6.3 Carbon Nanotubes (CNTs) In contrast to activated carbon, these tubes are cylindrical in shape and are divided based on their manufacturing into two categories: multiwall nanotubes and singlewalled nanotubes. These materials are characterized by a large surface area and a high adsorption capacity. In addition to the loose bundles of CNTs in the aqueous medium, it contains a hydrophobic surface. These surfaces are capable of attracting organic contaminants. Having a huge pore size bundle and a large number of additional absorption sites, the system is saturated in need for the elimination of organic contaminants. In order to eliminate organic contaminants, it may form a hydrogen bond and an electrostatic interaction between CNTs and the contaminant that may eliminate their hydrophobic effect. Currently, when comparing functionalized and non-functionalized CNTs for Cu2+ adsorption Mubarak et al. (2012) found they had a greater removal rate around 94.5% at pH 5 when compared to the non-functionalized CNTs. A number of types of raw CNTs and surface CNTs were also investigated for metal adsorption, and Rao et al. (2007) reported that different CNTs absorbed metals in the following order: Pb2+ > Ni2+ > Zn2+ > Cu2+ > Cd2+ in aqueous solutions. CNTs have been shown to be an effective adsorbent in removing Procion Red MX-5B from different types of aqueous solutions, at variable temperatures and pH levels. In addition to their antimicrobial effect, carbon nanotubes have been attributed cytotoxic, surface functional, and physical properties. Disturbances to the cell membrane and oxidative stress are the mechanisms by which CNTs eradicate bacteria. Even though multiwall CNTs are considerably more noxious than single-walled CNTs.

6.4 Silver Nanoparticles Ag and gold nanoparticles exhibit optical properties by which the amount of organic contamination can be detected. As the most significant nanoparticle in the modern era, they demonstrate optical properties by which trace quantities of organic contamination can be detected (Amin et al. 2014). Nanoparticle-based electrodes incorporating Au/Pt, Ag/Pt, and bimetallic nanoparticles make excellent contaminant monitoring, sensing, and photocatalysis instruments. Silver nanoparticles purify water through their biocidal properties. Surgical masks and textile fibers are disinfected by Ag nanoparticles. Ag/ZnO and Pt/ZnO nanocomposites, Ag/AgBr/graphene oxide nanocomposites (Esmaeili and Entezari 2016), and Au-CuS-TiO2 Nanobelts are the Nobel metals that exhibit photocatalytic activity and degrade several dyes, halogenated organic matters, and pesticides pollutants.

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7 Wastewater Treatment by Using Membrane Based Techniques The membrane layer is generally the porous and thin layered structure that permits water to pass but limits the transit of metals, viruses, salts, and bacteria through it. Membranes, in general, utilize two kinds of forces, electrical and pressure. Recently, membrane separation has become the most advanced technology for purification, since membrane pore and molecule size are factors that influence separation (Kumar et al. 2014). There are different kinds of membranes based on the mode of application, composition, and porosity, such as aquaporin-based membranes, own-arranging membranes, nano-composite membranes, and biological membranes. The following are some membranes that describe how they can be used in wastewater treatment.

7.1 Polymer-Matrix Nano-composite Membranes Polymeric membranes are used extensively in wastewater treatment because they possess an uncomplicated pore-creating mechanism. They contain nanomaterials in their polymer matrix, hence they are active in removing wastewater. This membrane can be used to separate solids from liquids or to separate gases from liquids. It can be used in proton exchange membrane fuel cells, lithium cell batteries, evaporation, and organic solvent nano-filtration (Soroko and Livingston 2009). Classification of Polymer-matrix nano-composite membranes (Yin and Deng 2015). Nanocomposite membranes

Conventional nanocomposite

Thin-film composite (TFC)

Thin-film nanocomposite(TF

Surface located nanocomposite

N);

Permeableness, healthy proteins situation, as well as fouling challenge

Higher initial permeability, maintaining high water permeability enhance hydrophilicity compounds (EDCs)and

Eliminate heavy materials, consistency, and organic micro contaminants these types of as pesticides

Hydrophilicity, pore shape, charge concentration, and roughness, separation, and antifouling

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7.2 Cellulose-Based Membranes Cellulose diacetate membranes respond better than CA membranes in terms of flux. Cellulose-based membranes are made primarily by inversion methods. Since the invention of the cellulose diacetate membrane research has expanded into the development of cellulose triacetate (CTA) membranes that demonstrate better chemical, biological, and chemical stability. The degree of acetylation plays an important part in the degree of filtration capacity of the cellulose diacetate membrane. The scattering of nanoparticles on the surface of a membrane would enhance the biological stability when adjusting the permeability, say some scientists. A phospholipid polymer coating on the membrane surface of CA membranes will convert them into fouling-resistant membranes that produce high water flux. Mineral filler is added to CA membranes to enhance compression resistance (Shaulsky et al. 2019).

7.3 Metal Oxide Membranes Ceramic membranes are those that contain pore sizes between 1 and 5 nm as well as inorganic membranes providing stronger mechanical characteristics and high chemical stability. Alumina is the most frequently used inorganic membrane, which consists of pore sizes between 2 and 5 nm. Furthermore, Titania and Zirconium are examples of ceramic membranes. In laboratories, zirconium alkoxides produced zirconium isopropoxide agglomerates which lead to zirconium sols. Zirconia membranes are subsequently synthesized using the gels formed when zirconia alkoxides are reacted with water. Adding glycerol to wastewater results in the phase transformation of ZrO2 , as it bonds to the surface of the nanoparticles (Santos and Galceran 2003).

7.4 GO-Based Membranes For reducing membrane fouling, the use of graphene oxide-based materials such as p-GO Pristine and f-GO Graphene Oxide is an effective method due to their unique properties for improving membrane performance (Alkhouzaam and Qiblawey 2021). Researchers are becoming more interested in graphene oxide (GO) nowadays due to its unique molecular sieving ability and fast molecular sieving speed (Fig. 2).

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High antifouling properties

Mechanical strength and stability

GO-based membrane

Antifouling properties

Antibacterial activity

Fig. 2 Schematic representation of various properties of GO-based membrane

8 Detection and Screening of Water Contaminants In the last few decades, rapid commercialization has negatively affected water quality. Nanotechnology-based and proactive sensor technology can be used to detect pollutants efficiently. The detection of a wide range of trace pollutants employs a number of nano-sensors, sensing techniques, and methods. However, nanomaterials have captured particular attention because of their high surface-volume ratio, sensitivity, and specificity. As a result of the method’s benefits, it requires a high understanding of the technique’s handling because it is a highly sensitive method for detecting trace levels of biological, organic, and inorganic contaminants (Xue et al. 2011a, b).

8.1 Biosensors When detection elements of the sensor interact with the analyte, biological responses arise. Biosensors are modeled as biological sensing compounds that are directly connected with the transducer. An electric signal is generated by the transducer after receiving the biological response, which is then amplified and quantified. The biological sensing elements can be classified into Bio catalytic (e.g. enzymes), Bio affinity (e.g. antibodies and nucleic acids), and Whole cells (e.g. various microorganisms). During bio-catalysis, several elements are immobilized in the same layer. Enzyme conversion occurs as the analyte is converted to the final product. A bio-affinity biosensor uses various bio-affinity agents to bind to the analyte macromolecule and create the corresponding complex that can be measured by a detector; they contain two types of affinity reactions; antibody-antigen interactions and receptor-ligand interactions. These nanocarriers have light-scattering properties because of their

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size, shape, and composition (El-Ansari and Faddah 2010). Nanocubes and nanorods have different kinds of light scattering properties, such as nanorods which can act as biosensors. Light scattering power in the visible range is excellent for nanoparticles whose diameter is greater than 30 nm. Besides nanoparticles, other small particles are also used in the detection process, such as DNA hybridization, electrostatic interaction, and avidin–biotin interaction. However, they have less scattering power than larger particles. In the urea detection area, nanoscale porous alumina is being used that has been fabricated using electrical anodization in acidic solution. Nano-sized materials have the advantage of improving sensitivity by increasing the surface area. In order to improve the detection of cadmium, some researchers have shown that gold nanoparticles functionalized with L-cysteine and 6-mercaptoethanol acid can enhance the sensitivity of biosensors (Xue et al. 2011a, b).

8.2 Biosensors Application in the Monitoring of Wastewater Pollutants 8.2.1

Biosensors for Organic Contaminants

In domestic organic pollution, hydrocarbons, chlorinated compounds, alkylbenzene sulfonates, and aromatic substances are dominant, while industrial organic pollution is dominated by polybrominated biphenyls (PBBs), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs), etc. (Santos and Galceran 2003). The results of gas chromatography-mass spectrometry (GC–MS) and Fourier Transform Infrared spectroscopy (FTIR) demonstrate a positive detection of hydrocarbons and alcohol, respectively (Lee et al. 2006). Several parameters must be considered for determining the level of organic pollutants, including BOD, total organic carbon, and chemical oxygen demand. Some researchers have demonstrated the biosensor built from Trichosporon cutaneum immobilized between organic contaminants and cellulosic compounds.

8.2.2

Biosensors for Microbial Pollution

There are three principal kinds of microbes that survive in wastewater: bacteria, viruses, and protozoa. These types of microbes cause contamination of the water, and also affect the food in the ocean). There are several types of microorganisms that cause life-threading diseases in the body, but E. coli bacteria, RNA bacteriophages, and coliforms are harmless bacteria present in wastewater. The conventional method is typically used for the detection but it has some disadvantages, including low sensitivity and low detection rate, as well as taking a long time to conclude the detection. Researchers have detected bacteria like Staphylococcus epidermidis, E.

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coli, Vibrio cholera, and Staphylococcus aureus through piezoelectric biosensors, which are cost-effective, simple, and efficient (Plomer et al. 1992).

8.2.3

Biosensors for Inorganic Contaminants

Cadmium, zinc, chromium, nickel, iron, and metals are the inorganic sources of water pollution. Lead and mercury are harmful substances that circulate from plating and finishing industries (Srivastava and Majumder 2008). There are a number of significant side effects on plants and animals. Researchers have demonstrated that an optical membrane sensor using membrane-based 4-hydroxy salophen can detect Cd (II) heavy metals in wastewater.

8.3 Fluorescent Chemosensors Consequently, it is a fast and reliable method for the selection of contaminants in wastewater. The method uses fluorescence to detect dissolved biological and organic contaminants in wastewater. It also detects humic-like fluorescence peaks in wastewater and prioritizes it over chromatographic techniques (Ahmad and Reynolds 1999). Detecting water pollution is very easy thanks to the light emission caused by energy gaps between fluorophores. Using IFE-based sensing assays, trace amounts of contaminants can be detected. It is possible to detect contaminants by using fluorescent nanoparticles Energy transfer, electron transfer, aggregation-induced quenching and emission, and electron transfer are the main mechanisms whereby analyte is affected by a fluorophores.

8.4 Gold Particle The shape of gold particles makes them suitable for monitoring wastewater pollutants based on the surface plasmon resonance, which is sensitive to shape. Cysteine together with gold nanoparticles showed extremely high sensitivity against E. coli (Raj et al. 2015). As a sensor of bacterial spores, Au nanoparticles are chelated with Tb31 and Eu31 which recognize the dipicolinic acid that is uniquely present in spores.

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9 Valuable Products Derived from Waste Water Activated Sludge: A Bio-refinery Concept By combining nanotechnology and bioprocessing engineering, it is possible to develop valuable bio-products through wastewater biorefinery technology. The concept is designed to increase the productivity of biomass and raw materials. By decreasing the environmental burden through reusing waste materials, raw substrates obtained from waste materials have reduced the environmental pressure. Wastewater treatment is a method that can recover costs, create profitable business models, and provide relief through financial benefits. Furthermore, wastewater treatment exhibits a sterile environment, making it ideal for micro-organism production.

9.1 Poly-glutamic Acid Production as a Bioproduct of Wastewater Treatment The use of poly-glutamic acid is a high cost and labor-intensive process, which results in a high cost and labor-intensive process. Their chemical structure provides a variety of industrial applications including cosmetics, medicines, food, thickeners, cry protection drug carriers, and biodegradable materials. Some researchers have studied the synthesis of y-PGA in B. subtilis and found that it is primarily produced from citric acid and ammonium sulfate by the TCA cycle (Buthelezi et al. 2009). It was recently studied that PGA is produced by bacteria while biofilm is formed by Bacillus species. Bacillus species have an important role in soil health and also in the field of natural pest control. A Bacillus strain was isolated from an oilfield that produces highly concentrated PGA. Wastewater contains a broad spectrum of metals, including magnesium and manganese, needed for the production of PGA. PGA protects the cells from osmotic pressure (Fig. 3).

9.2 Lipid Extraction from Sludge Two types of sludge have been found after the primary and secondary treatment processes. Primary sludge contains solids and floating grease. Secondary sludge exhibits activated sludge (microbial cells and suspended solids). A high concentration of biodiesel can lead to a high concentration of lipids in sewage sludge, which is the raw material used in biodiesel. Lipids are contained in wastewater fractions such as oil, fats, and grease.

Nanotechnology-Based Solutions for Wastewater Treatment Cheap treatment of wastes in dynamics and non – sterile environment

Have the ability to manage complex influent

Reduce overall cost and sustain natural capacity

Have the ability to produce several coproducts

Have the capacity to give additional benefits

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Wastewater biorefineries key characteristics

Bio – products which are cost effective

Fig. 3 Key characteristics of wastewater biorefineries (Todd et al. 2003)

10 Conclusion Organic, inorganic, and microbial contaminants pose serious health risks to humans as human demands for clean water increase. Water purification and treatment technologies are therefore becoming increasingly popular. The removal of pollutants from wastewater is often achieved through a variety of methods, but Nano-based technology has recently gained a great deal of attention and popularity. The use of nanotechnology for monitoring and sensing water pollution is broad and diverse since it utilizes a variety of modified biosensors for the removal of various pollutants. Nanotechnology is expensive, effective, and enduring in eliminating pollutants. Several nanomaterials that are beneficial for anti-pollutant flow at their shape in a nano-assortment, which forms that it is deserving to be used while maintaining an exterior area that is volume ratio, rapid dissolution, high reactivity, and the ability to adsorb, which means that it can be used for a wide range of applications.

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Municipal Wastewater as Potential Bio-refinery Shipra Jha and Nahid Siddiqui

Abstract Globally and locally, there is need of acceptable water quality and waste water treatments are precedence. The various conventional waste water treatment methods are used for the removal of particulate matter, organic matter and nutrient load before releasing into river. And these treatment methods include higher cost, higher energy consumption and impact on environment. With increasing research evidence for the impact of contaminated water on environment and human health, wastewater biorefinery is gaining interest. Certain technologies include biorefinery can convert wastewater into valuable product and reduce economic and environmental burden. Due to the potential, to fill the gap between wastewater treatment and biorefinery. This chapter will provide wealth of information on new research on technological interventions on the implementation, design and municipal waste water for biorefinery and promoting a green and cleaner environment. Keywords Municipal water · Environment · Technologies · Biorefinery · Green technology

1 Introduction In developing countries the waste disposal becomes big problem due to poor maintainance, budget limitations and lack of facilities to maintain practical standards. The waste pollutants become the reason behind air, water, and soil pollution, emission of green house gases and source of infection. Hence with the development of biorefineries, waste can be utilized and wide range of valuable products can be produced. With the increase in population, the demand for development of industrial sector and infrastructure also increases which in return dispose different waste effluent in environment. The wastewater management is difficult task due to the presence of excessive nutrients discharge in the environment leads to acidification and become S. Jha (B) · N. Siddiqui Amity Institute of Biotechnology, Amity University Uttar Pradesh (AUUP), Gautam Buddha Nagar, Sector-125, Noida, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_5

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risky for human health (Jerbi et al. 2020; Al-Zboon and Al-Ananzeh 2008; Arora et al. 2021; Bailey and Ollis 1986). Globally, municipal biorefinery are gaining attention due to the waste management and produce products includes heat, fuel, valuable products and energy. Many experimental data provided by scientist using wastewater containing heavy metal, biomass, chemicals, plastics, leather, oil, detergent used in biorefinery (Belinsky et al. 2005; Bhatia et al. 2017). The scientific studies show that there are many operational and technical challenges to achieve economical benefits of biorefinery. The wastewater biorefinery represents complex network and it’s important to identify the economical and sustainable development of biorefinery. The research study reported that agricultural waste, used cooking oil and poultry waste can also be used in biorefinery (Bhatia et al. 2020). Conventional wastewater treatment plant were designed for the treatment of suspended solids, particles and biological oxygen demand even along with trace elements to release treated waste water to land water. For the purpose of reuse of wastewater and complete breakdown of pathogenic microbes, tertiary treatment may be used along with the combination of conventional method. It is important to develop economically feasible methodology for complete removal of hazardous organic compounds and trace elements. Because presently municipal sludge constituents can be controlled by maintaining waste water quality before treatment (Fisher-Jeffes et al. 2014; Burton et al. 2009). Considering many scientific reports contributed by global research team, it is concluded that wastewater biorefinery has potential to develop various valuable products and provide solution to manage large municipal waste generate daily in urban areas. This chapter gives overview of the types of treatment, characteristic feature of Municipal wastewater biorefinery, potential of biorefinery and design of reactor (Narayanan and Narayan 2019; Carey et al. 2016; Coetzee 2012).

2 Potential of Municipal Waste Water Pollutant The research studies shows that the municipal waste water contains different pollutant includes pathogens, toxic contaminants, phosphorous, nitrogen, organic matter and dissolved minerals. Most of the cities started collecting waste water from commercial places, household etc. and treating municipal wastewater through centralized treatment plant. The waste pollutant differs from source to source. The composition of waste belongs similar to same sources. In urban areas varieties of toxic pollutant are discharged into water through human activities (Bruin et al. 2004; Kreuk et al. 2007a). In household wastewater pollutant contain number of microbes along with different toxic chemicals due to which chemical and physical methods are used to remove contaminant. Based on physical and chemical approach contaminant are categorised into dissolved impurities, settle able pollutant, colloidal pollutant and suspended pollutant. The main objective behind municipal wastewater treatment to clear off the waste and reuse the water (Kreuk et al. 2007b, 2010).

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With the vigorous research, scientist could develop municipal wastewater method to remove almost all the pollutant from municipal wastewater. The treatment plant consists of continuous series of system for the removal of complete pollutants (Details 2011).

3 Concept of Wastewater Biorefinery Biorefinery are effective way to utilize bioenergy in a durable manner to reduce the usage of chemicals, to secure, to minimize environmental changes. Biorefinery an essential built-in which can convert various substrates into different usable produce includes energy, compounds and substances after extraction or further treatments like chemical or biological (Donofrio et al. 2009). The operational condition to setup traditional Biorefinery plant considered to be expensive process in terms of social, environmental and economical concern. Due to the high cost of feedstock includes biopolymers, vegetables oil, glucose and diesels, pressurizing the agriculture sector for providing substrate and creating load on ecosystem by releasing heat, it become essential to explore wastewater biorefinery. The basic objective behind the construction of Waste water Biorefinery plant is to generate not only clean water but also the commercial valued products (Donofrio et al. 2009; Rabelo et al. 2011; Drosg et al. 2015; Sas et al. 2021). Depending upon the types of raw material and technology for product development. Biorefinery are categorized into different seven types which includes Lignocelluloses Biorefinery, Thermo chemical Biorefinery, whole crop Biorefinery, conventional Biorefinery, Two-platform Biorefinery, Green Biorefinery and Marine Biorefinery (European Union 1986, 2009) (Table 1). Municipal wastewater plants passes through primary, secondary and tertiary treatment or even sludge treatment either to further utilize for water reuse purpose or to dispose. After the treatment of municipal wastewater reuse for crop production, agriculture irrigation, and sludge used for landfill purpose, store as ground water and for construction sites (EUROSTAT 2014; Fux and Siegrist (2004); Shizas and Bagley 2004b). The wastewater treatment plant consist of preparatory treatment, primary, secondary treatment and if needed then advanced treatment to safeguard the quality of environment and public health known as tertiary treatment. A different phase of development used includes small particles or stone removal, waste screening, sedimentation and biological treatment (Fava 2012; Finger and Parrick 1980).

3.1 Initial or Preparatory Treatment The purpose of initial treatment includes removal of stone, inorganic solid or particles and screening of wastewater to enhance water quality to ovoid interference during further processes (Fisher-Jeffes et al. 2014; Fytili and Zabaniotou 2008).

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Table 1 Characteristic feature of Biorefineries Technology used

Sources of raw material Types of biorefineries

Stage of development

Cell disruption, extraction and product isolation

Seaweed, marine algae

Marine

Pilot plant, research and development stage

Biochemical and thermo chemical conversion

All types of raw material

Two platform

Pilot plant

Preliminary treatment, squeezing, partition, extract, isolate and digestion

Green crops and grasses

Green

Pilot plant, research and development stage

Initial treatment, enzymatic hydrolysis, fermentation and isolation

Material containing Lignocelluloses lignocellose like Wood, reed, straw and reed

Pilot plant, demo stage, research and development

Conversion through roasting, thermal decomposing, gasification, isolation, enzymatic synthesis

All types of raw material

Thermo chemical

Pilot plant, demo stage, research and development

Whole crop

Pilot plant and demo stage

Maize, rye, wheat and Wet or dry grinding, conversion through straw digestion, fermentation and harvesting or composting Biochemical conversion: enzymatic catalytic hydrolysis, fermentation, fractionation and product isolation

Plant oil, carbohydrate, Conventional terpenes, lignocelluloses rich material

Phase-III (advanced)

3.2 Primary Treatment In primary treatment screened sediments, suspended particles, Biochemical oxygen demands and settled particles are easily removed for economical purpose before secondary treatment. The research study shows that primary treatment may reduce pathogens, reduce nutrients concentration in wastewater, harmful organic compounds and trace elements (García-Martíneza et al. 2019).

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3.3 Secondary Municipal Wastewater Treatment The research study shows that chemical and physical method is not effective in case of secondary treatment. In secondary wastewater treatment, microorganisms used in activated sludge, pond or trickling filters known as biological method. In biological method oxidation of some portion of organic material takes place to release products and carbon dioxide, and rest part utilized by microbial growth and development (Ginni et al. 2021; Graedel and Allenby 2009; Harding 2009). The microorganisms form biological aggregate and separated from sediment tanks known as secondary sludge. Due to microbial aggregates, wastewater can linked to secondary sludge. The ammonia can be reduced by secondary treatment method (Guimarães et al. 2016).

3.4 Advanced or Tertiary Wastewater Treatment Tertiary treatment utilized for municipal wastewater when there is requirement for high quality water treatment after secondary wastewater treatment. For the removal of pathogenic microorganisms, trace elements, viruses and organic compounds advanced treatment method is used (Harding et al. 2007; Jung et al. 2009; Harrison et al. 2016; Heidrich et al. 2011). By adding coagulant Biochemical oxygen demand and suspended solid can be reduced. Trace elements and organic compounds can be removed by using activated carbon. Through chemical precipitation or microorganisms, phosphorous is easily removed and using nitrification nitrogen content can be removed from waste water (Henze et al. 2008) (Fig. 1). The characteristic feature of Municipal wastewater biorefinery includes: . To handle complicated and poorly managed wastewater effluent containing diversity in nature and concentration. . Municipal wastewater treatment require less amount of energy for its operation. . Easily adaptable system which can adjust internal and external environment. . Cheaper Cost of waste treatment in robust environment. . Comparison to conventional biorefinery, wastewater biorefinery deliver more effective ecological service. . Ability to give double benefit includes more important by-products along with preserving natural legacy. . Potential to allow easy construction of business model for Eco-Industrial systems.

4 Application of Wastewater Biorefinery Municipal wastewater nutrients used for producing biomass of Nannocholoropsis species. Municipal water nutrients open up new ways of enhancing economy for the

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Biofuel Material Water

Animal feed

Biorefinery

High Value chemical

Fertilizes

Combine d heat & power Material

Fig. 1 Modern biorefinery concept

production of valuable product from microalgae. A municipal water nutrient contains growth promoting nutrients and growth inhibiting pollutant (Huang et al. 2011).

4.1 Different Categories of Raw Material for Wastewater Biorefineries To design the reactor for biorefinery, it becomes very essential to classify the wastewater as feed. Based on the reactor, wastewater classify into three factors—quantity of constituents present, variable constituents and their number present in the wastewater and flow rate. Depending upon three factors, biorefinery scale up to produce desired products (Jackson et al. 2009; Iranpour et al. 2002). Large volume of waste water enters into streams per day and may get diluted with minor to major components. The heavily diluted waste water includes dyes, chemicals, detergent, acids, paints, cooking oil with changing concentrations enters from various sources becomes mega challenge for their treatment. Waste water Biorefineries can be designed based on the volume flow (Coetzee 2012).

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4.2 Waste Water Source for Production of Valuable Products The main objective behind biorefinery, to enhance water quality and number of products which yields after the complete treatment. Due to heavily dilute wastewater, desired product can be recovered by proving different growth condition through microbial activity (Jeong et al. 2010). The wastewater biorefinery products can be classified into three types: (a) to generate power or electricity, biogas produced under anaerobic tank (b) break down of large molecule into smaller one includes ethanol which can further utilized for complex industrial products and (c) third classified group includes super molecules with simple purification functional based products include soil conditioners, bioflocculants (Kleerebezem and Loosdrecht 2007; Kosaric et al. 1984). During the course of wastewater treatment, there are various valuable products includes alginate, pigments, organic acids, volatile substances, enzymes and polymers produced at the end of the treatment cycle. The behaviour of products always depends upon the bioactive agent and type of wastewater treatment method (Koskan et al. 1998; Lalloo et al. 2010).

4.3 Wastewater Biorefineries Source for Irrigation Globally, waste free irrigation water is main concern due to limited clean water. The scientific studies reported that with the advancement in drainage system, crop development process, waste water which is of low quality can be used for crop irrigation (Lettinga 1995; Li 2009; Liu and Tay 2004). Low quality water not only affects crop development but also interfere in soil properties (Libutti et al. 2018). Many research studies shows advantage and disadvantages of wastewater irrigation which affects the plant growth includes chlorophyll content, length of leaves, root length, seed size, soil quality in terms of nutrients and may increase salt concentration. Limited water resources are forcing to invite more studies to explore waste water treatment method. And the concept of Municipal waste water biorefinery used for treating contaminated water to improve water quality. The key aim of wastewater biorefinery to use distinct unit to release clean water and to produce variable products after complete treatment of wastewater (Baghel et al. 2018). The research studies indicated that multiple waste includes poultry waste, used edible oil, agro-industrial waste can also be used in biorefinery for water treatment and treated water can be utilize for fast-growing plant irrigation which may help soil to hold nutrients (Mitra and Mishra 2019; McCarty et al. 2011).

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4.4 Prevent Waterborne Pollution Water clarification is essential part of biorefinery treatment system which involves removal of floating matter and suspended particles. Through the process of adsorption, ion-exchange, reverse osmosis, precipitation and oxidation heavy metals are removed. Adsorption is considered as most efficient in removing heavy metal as compared to other methods due to high cost and operational limitations. Slit and sand can also be removed by using equipment and washed water may be reused for irrigating plants (McCarty et al. 2011; Mohan and Ramesh 2006; Wang et al. 2018).

5 Integration of Waste Water Treatment Increasing population and urbanization around the world has resulted in scarcity of water even in areas which were initially rich in water resources and water supply. This brought up the need to reclaim the wastewater and then reuse it for non drinkable purposes. Therefore the purpose of management of wastewater from different sources, gained importance which would decrease the burden of water pollution as well as use the treated waste water for purposes other than drinking and washing (Narala et al. 2016; Narayanan and Biswas 2015; Pandey et al. 2010). New technologies and methods are in place to help treat the waste water coming from different sources. Combination of the conventional and modern treatment processes are employed to reclaim the water to its original quality. The use and reclamation of waste water for irrigation, agriculture and landscaping is the most cost effective solution for protection of environment and also to mitigate the lack of water resources around the world. Integration of water and reclaimed water is done effectively. Planning with respect to the facilities like site of wastewater treatment plant, reliability of the treatment process, financial support and also future use of the reclaimed water, quality of water, regulatory mechanisms etc. is necessary for this to have a sustainable method in place (Rabelo et al. 2011). Treating wastewater is done by the combination of chemical, Physical and biological processes. Also, these treatment plants require high-cost infra structure, highly skilled workers, and lots of energy. Constant efforts are being made to find an alternative and cost effective approach for the wastewater treatment. Wastewater rich in nutrients is produced from different industrial sources such as municipality, textile, pharmaceutical, dairy, food and many others (Ramaswamy et al. 2013; Richardson 2011; Ren et al. 2019). Being rich in inorganic and organic substances, these cause eutrophication in the environment, which is harmful for the environment. Eutrophication mainly affects the irrigation, agriculture, fisheries and causes the growth of different microbes and pathogens in the environment (Li 2009; Liu and Tay 2004; Richmond and Cheng-Wu 2001) Above all, if left untreated, it will contaminate the ground water, soil and air as well, adding to the pollution woes of the environment and causing a potential damage to the ecosystem. Traditional methods use the

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physical and chemical methods of treatment but these are expensive and generate sludge/slurry, which is again needed to be, treated (Guimarães et al. 2016). Using the traditional method of treatment, pose a lot of challenge to the wastewater industry. Methods have to be adopted to valourize the wastewater, turn it into a useful resource and at the same time reduce the environmental and economic load. Researchers are exploring different methods for treatment and valourization of the components of the wastewater (Ramaswamy et al. 2013; Richardson 2011). Strategically optimization to enhance the effectiveness of the waste treatment, and better utilization of the bioresources with regard to the impact on environment and economy. Therefore, integration of environmental engineering and Bioprocess technology seemed to be the need of the hour which would help to produce useful and sustainable products from the waste and at the same time help improve the environment and help in the remediation of contaminants. Wastewater used as nutritive substrate followed by further treatment yields useful bioproducts which can be put to use (Saratale et al. 2020; Sheik et al. 2014). This can be termed as Wastewater biorefinery. When put into action, the wastewater biorefineries can extract the valuable components from the wastewater, valorize them and reinsert them for economic use and at the same time bring up the remediation of the pollutants. These products must be easily recoverable and should fulfil a role in ecology, for which sturdy treatment system should be maintained. The biorefineries should be able to produce fewer footprints, should be strong and resilient, should have the capability to generate more than one co-product and should need least energy for operation. In addition to these, the competitiveness of the biorefineries can be improved by considering systems based of renewable resources such as sunlight, gravity based flow systems for the economic, social and environmental issues. Waste water biorefinery can aim to enhance the wastewater industry by improving through industrial ecology. So that environmental and ecological sustainability can be achieved. Taking care to achieve the better final water stream is of top priority of the biorefineries (Shizas and Bagledy 2004a). One of the effective and sustainable approaches appears to be the integration of Microalgae in wastewater treatment (Ramaswamy et al. 2013; Richardson 2011). Microalgae grow in waste water and convert sunlight and carbon dioxide into biomass. The biomass produced contains different biomolecules like lipids, carbohydrates and other important organic compounds which are further utilized for the production of bio fuels. And these can have other applications (Bhatia et al. 2020; García-Martíneza et al. 2019) as these cells do not use the energy for the growth and development but store it within them. This stored energy with the biomass can be used for the production of biofuels, so, technologies based on such biorefineries have taken up a lot of interest of researchers. The Biological waste water treatment using microorganisms to degrade the pollutants is an integral step of the treatment system. Protozoa, Algae bacteria, fungi, nematodes are used for the breakdown of unstable organic wastes to convert them into stable inorganic forms, using aerobic or anaerobic methods (Shizas and Bagledy 2004b). Microalgae based technologies are the most viable methods of waste water treatment which allows almost 100% of the recovery of the nutrients from the waste water. The wastewater environment is

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non-sterile in nature, thus the microorganisms for the product formation should be selected accordingly such that the microbial ecology is maintained. Also beneficial culture conditions and products are selected which can have a selective contribution to the microbial community of interest (Show et al. 2017). Two-step degradation process is used for the production of biofuels. 1. Microalgae are grown in the wastewater aerobically 2. Biofuel is produced from the biomass anaerobically Biodiesel can be produced from lipid while as fermentation of other components of the biomass can be yield other biofuels (both liquid and gas) like as bioalcohol. Dark fermentation can be used for the production of biohydrogen and anaerobic co-digestion to yield biomethane (Stafford et al. 2013). Microalgae can be cultivated in different types of cultivation systems on a small scale as well as large scale (Stefanakis and Tsihrintzis 2012a, b) Choice of the system depends upon the type of the microalgae selected, availability of type of nutrients and the utilization of the biomass thus produced. Open and closed system of cultivation is most preferred but advanced method so cultivation is also available, but at times these might overlap. For large scale production, Open System cultivation uses open spaces such as ponds, tanks for cultivation. These are low cost, and more economical as compared to the closed systems, but have few disadvantages. Since these are open, evaporation of water, CO2 diffusing in the atmosphere causing pollution, poor utilization of light by the cells and requirement available land for the cultivation are a few disadvantages with this system (Verster et al. 2013). Open systems there are not preferred for pilot scale production. Closed systems, most appropriately calls Photobioreactors (PBR) are preferred as there is no direct exchange of gases and no contaminants in the surrounding environment can affect the system (Berg 2009). Another study based on the integration of the Willow as biorefinery was evaluated with primary effluent wastewater irrigation was done. Wastewater irrigation led to increase as lignin, phytochemical and glucose yield in the biomass. Also this could treat waste water in a more sustainable manner (Shizas and Bagledy 2004b) (Fig. 2).

6 Bioreactor Design Requirement of Wastewater Biorefinery includes Large volume reactor . Semi continuous or Continuous flow . Large commodity . Decouple hydrofluric and sludge retention time Complex variable . Targeted non-sterile or microbial community . Create environmental niche and target to product benefits

Municipal Wastewater as Potential Bio-refinery Fig. 2 Flowchart shows potential wastewater biorefinery

Raw material

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Operational Treatment

WASTE BIOMASS

Chemical

• Organic waste

• Acid hydrolysis • Alkaline hydrolysis • Enzymatic Fermentation

• Animal manure • Food waste

• Corn stove • Crop residue • Rice straw • Tree • Muncipal sludge

• Biological • Fungal • Bacterial

Biologiacl Treatment

ANAEROBIC TRANFORMATION

• Mixed digester • Anaerobic lagnoon • Anerobic filter

• Physical • Ultrasonic • Steam explosion

Chemical

• Dyes • Solvents • Adhesives • Pigments • Detergent

Fuel

• Renewal Diesel • Ethanol

Powder

Environment . Allow to flow water into environment. Downstream processing . Product can formed in different phase . Recovery of product . Design reactor for load balancing and elimination Reactor designing to increase residence time . Recovering before cell settling . Recycle after settling . Rectors in parallel

• Power • Electricity

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7 Designs Bioreactor refers to any device or system which may be manufactured or engineered which a biological system with active environment, in which growth of microorganisms can take place. Recovery of the useable resources from the wastewater can be done using a Biorefinery reactor, the design of which is optimized as per the requirement. Downstream processes (DSP) are more developed, easily adaptable in a given reactor in which the separation mode is depended upon a few properties such a size, charge, solubility, separation properties and volatility (Sterr and Ott 2004; Stuart and El-Halwagi 2012). Primary objective of an environment efficient and a cost effective DSP is to obtain easy recovery from the bulk material and to reduce the amounts of different unwanted components. Design of the reactor towards product recovery is done which will reduce the loading on the DSP operation units and at the same time efficiency can be increased. The flow rates of gas (i.e., air, oxygen, nitrogen, carbon dioxide), pH, temperature, and agitation speed/circulation rate and dissolved oxygen levels, need to be monitored and controlled (Takkellapati et al. 2018). A few challenges are encountered with respect to the current design of the reactors. Firstly, Optimization of the system as a single unit will pose a challenge as it will not ensure desired results, therefore, a systems approach is needed where other productions are also considered along with the reactor design to have the maximum utility of the reactor functionally as well as deliver high productivity. Aerobic reactors working at low substrate concentration are the primary source of employment of energy, with increasing cost of energy supply this design needs to be reconsidered (Sung et al. 2007; Taniguchi et al. 2005). Various types of Biological waste water treatment Biorefinery reactors are in application as below.

7.1 Stirred Tank Reactor for Aerobic Treatment of Waste Water This process which has undergone large number of diversifications and modifications is one of the oldest methods of biotechnology for wastewater treatment. An activated sludge process employs a tank which is an agitated vessel in which the inoculum of the microbial sludge is introduced. Air at high pressure is introduced from the bottom to provide sufficient amount of dissolved oxygen to the medium in the tank. Due to large volume of the tank and less solubility of atmospheric oxygen in the medium, large volumes of gas has to be introduced in the tank, requiring huge compressors so that aerobic conditions inside the tank are maintained. The constraint with this type of a system is the high cost of the compressors although the system can be easily designed and installed (Tsihrintzis et al. 2007). Oxidation of the dissolved organic matter, denitrification and nitrification can be achieved in this process. Conventionally, the system requires two stirred tanks in

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series-in the first one has aerobic conditions in which carbon removal and nitrification occurs and in the second tank denitrification is done anoxically. Using a number of small sized tanks in series or dividing the tank in various compartments are the different types of modifications in this technique, where in the cascade remains the same as in a single tank, but improves performance, as increased BOD destruction occurs, also reducing the deadzones and bypass streams (Tsihrintzis and Gikas 2010).

Source C. M. Narayanan and Vikas Narayan, Biological wastewater treatment and bioreactor design: Sustainable Environmental Research (2019)9, Article number: 33 (Kreuk et al. 2010)

1. Stirred tank reactor for Anaerobic treatment of waste water: This process involves the treatment of wastewater using culture of microbes which are acidophilic, methanogenic or acetogenic. The products thus produced are converted into useful products such as Biogas. The sludge digested anaerobically can be used as a fertilizer directly or can be used in the production of phosphate rich fertilizer. This process this slow and the microbes especially the methanogenic ones can be sensitive to pH and temperature changes (Berg 2009). Although not that economically cost effect, this process can be used with thermophilic microbes as well, the cost of the installation of the heating pipes may prove a constraint (Belinsky et al. 2005) Mesophilic bacteria can also be used. These can be horizontal, vertical or tube like are the close systems, considered easiest to scale up. Algae and the growth media are continuously circulated through the tubes using a mechanical pump. A whole range of algae such as Chlorella, Porphyridium, etc. can be successfully grown on a pilot scale using tubular photobioreactors. Problems like unfavourable CO2 and pH gradients and high levels of

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Dissolved oxygen are a few problems encountered with this system (Verster et al. 2013).

7.2 Flatplate Photobioreactors These are ideal for large scale indoor and outdoor conditions. Less accumulation do dissolved oxygen, lots of solar light on the plate and easy to use modular designs make these a better choice. Temperature controls, algal film formation on the plates, hydrodynamic stress are some of the drawbacks with this method (Sung et al. 2007).

7.3 Plastic Bag Photobioreactor Plastic bags with a diameter of 0.5 m with aerators attached to it are use as photobioreactors, vertically hung inside plastic or metal cages and kept exposed to sunlight. Air is pumped from the bottom and the microbes are continuously mixed with the air. The drawback with this is the poor mixing of the microbes and air leading to the destruction of the cultures (Verstraete and Vlaeminck 2011).

7.4 Packed Bed Biofilm Reactors Support particles such as activated carbon particles, silica granules, polymer beds etc. Microbial cells surround each particle forming a biofilm. The aggregation of particle and biofilm complex form a distinct phase in the Bioreactors. Microbial cells in the biofilm grow and multiply till the thickness of, δ = 0.3–0.5 mm is reached after which they slough off from the particle surface and get replaced by fresh cells on the particle. High rate of bioconversion is achieved as the biofilm thickness is low and the concentrated cell mass on the biofilm is high. Since the unconverted substrate and the product accumulation in the biofilm does not occur, the substrate and product inhibition for the growth of microbe is low in such bioreactors (Vymazal 2002).

7.5 Moving Bed Bioreactors As the name indicated in this system, the bed of the particles is not fixed in the column rather it is a fluidized type of a reactors in which the particle-biofilms aggregates move against the current of water. The aggregate is fed to the stirred tank bioreactor and remain suspended in the liquid substrate (wastewater) in the tank.

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Under aerobic conditions, Air sparged in at high pressure also keeps these aggregates suspended and moving in the liquid, thus the name moving bed. The microbes grow attached to the support particles so there is no need of the sludge to be recycled as the microbes do not leave the bioreactor. Batchwise method of operation of this system is preferred although it can be operated in the continuous mode as well. The biomass concentration is substantially high in the biofilms, BOD removal is high but a little resistance in the transfer of substrate in the biofilms is encountered (Yen et al. 2019). The denitrification and aerobic tank bioreactors can be operated for the activated sludge process using the moving bed system.

7.6 Fluidized Bed Biofilm Reactors These bioreactors can be operated at high velocity and flow rates. Used for large scale production, the industrial effluent enters from the bottom of the column at high velocity due to which all aggregates remain suspended due to the upstream flow of the fluid. The bioreactor performance is enhanced as all the biofilms are surrounded by the fluid on all sides making an intimate contact with the aggregates. The total volume of the reactor increases due to the expansion of the bed. Advantage with this reactor is that the pressure drop across the bed is close to constant (Show et al. 2017; Yen et al. 2019).

Source Neha Baghel*, Satyam Sopori, Bagwan Mohamadazrodin, Ashpak Rafik Saudagar, Saudagar Ajharoodinournal of Advances and Scholarly Researches in Allied Education Vol. XV, Issue No. 2 (Special Issue) April-2018, ISSN 2230–754 (Stafford et al. 2013)

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8 Conclusions Many wastewater treatment programme and waste segregation at community level result in the removal of contaminant from sewage water. Treated waste may increase the load of contaminant in soil which comes from sewage sludge, agricultural pollutant. The final product of municipal wastewater treatment is sewage solid or mud containing contaminant removed from wastewater effluent. In sewage mud containing nitrogen and phosphorous as nutrients similar to other organic compost which can be used as soil conditioner and improves soil properties. Currently, financial benefits for sewage mud are argumental issue. With the increasing rise in biofinery technology, municipal wastewater treatment plant releases improved water quality for non-portable use. It has been found through many research studies that land irrigated continuously with treated polluted water which motivates municipal wastewater biorefinery to use for crop irrigation. Municipal wastewater biorefinery can be new key source to solve irrigation water deficit problem worldwide. In future with proper reactor designing, improved operational conditions, varieties in raw material may significantly enhance the irrigation water problem.

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Bio-diesel Production as a Promising Approach of Industrial Wastewater Bio-refinery Avijit Chakraborty, Shreyan Bardhan, Sudip Das, Sagnik Roy, and Banani Ray Chowdhury

Abstract Global industrialization is the infamous leading cause for generating many wastes such as petroleum fuel combustion and unchecked wastewater discharge. Unrestrained emissions of greenhouse gas and their impacts on global warming have become significant social, economic and environmental menaces. Wastewater is not solely the cause of inconvenience nowadays and may be utilized for value-added products and microbial fermentation. Nutrients such as nitrates and phosphates, as well as other organic chemicals, are supplied to wastewater to promote the growth of microbes. Biofuels are a promising alternative and an electromotive force for the modern world. Different feedstocks have been assessed for biofuels production tilldate. However, microalgae have been found to be the most productive due to their high potency, increased lipid profile, non-competitive nature with human nutriment, and ability to grow on non-arable land using brackish or wastewater. Microalgae are single-celled organisms living in water and are extensively used for wastewater remediation. Several methods like open, closed, and integrated cultivation has been reported for simultaneously microalgal treatment of wastewater and resource recovery. The biomass crude product extracted from wastewater remediation can be productive in generating carbon-based biofuels, feed, bio-fertilizer, bioplastic, and exopolysaccharides. Wastewater management, along with microalgal bio-refinery resolves the wastewater treatment complications, produces profits, and promoting a feasible bio-economy. The significant rise in the global biodiesel market during the past decade has caused a stoichiometric escalation in the crude glycerol coproduction. The biorefinery development by chemical production and power generation with transformation processes of biomass into biofuels is a way to achieve cost-effective production chains, obtain residues and co-products, and reduce industrial waste disposal. By the next decade, India’s fuel exhaustion in the industry of transportation is expected to have doubled. To subsidize the fuel supply, renewable energies are an engaging source for biodiesel production in upcoming years. A. Chakraborty · S. Bardhan · S. Das · S. Roy · B. R. Chowdhury (B) Department of Biotechnology, Bengal Institute of Technology (BIT), Kolkata 700150, India e-mail: [email protected] B. R. Chowdhury Baranagar Baghajatin Social Welfare Organization, Kolkata 700036, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_6

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Keywords Bio-diesel · Microalgae · Biofuels · Wastewater remediation · Biorefineries · Microbes

1 Introduction The two greatest issues that humanity faces in this century are energy access and change of climate. The quick growth of the inhabitants and rising income have resulted in an increase in energy consumption. Process of globalization is largely reliant on the use of energy, which plays an important role in socioeconomic growth through raising living standards. Every country’s economic development is dependent on the availability of energy. Agriculture, industrial, transportation, commerce, and home sectors, among others, all demand energy (Degfie et al. 2019; Bilgin et al. 2015). For a number of years, fossil fuel-based fuels including natural gas, coal and petroleum have been the primary sources of energy all over the world. Because of the extensive use of fossil fuels, the high energy consumption in the industrialized world, as well as in the residential sector, has resulted in environmental contamination. Fossil fuel consumption has a number of public health and environmental hazards, as well as having widespread and likely permanent effects on global warming (Bilgin et al. 2015). As a consequence, environmental concerns have grown, prompting researchers to look into alternative energy sources. Hydropower, wind power, solar power, biofuels and biomass are all examples of renewable energy. Due to various economic and environmental considerations, all of these resources must contribute, and biodiesel may be one of the solutions (Li et al. 2014). In the world where petrol fuels are getting limited and costly, the generation of biofuels and chemicals from renewable sources is required to meet energy demand. Among the most significant issues with biofuels is the high cost of manufacturing, which can be decreased by converting biofuels production leftovers into valuable coproducts (Almeida et al. 2012; Zhang 2011). Biodiesel is a renewable fuel that minimizes net greenhouse gas emissions and is now required in several nations (O’Connor 2011). It’s made by trans-esterifying fats and vegetable oils in the presence of a catalyst with a primary alcohol (typically methanol) to produce a fatty acid methyl ester (FAME), which is then utilized as a biofuel. Soybean, Sunflower, palm and rape oils are the most common sources of biodiesel around the world, however there are regional differences (Almeida et al. 2012). Direct usage and raw oil mixing, pyrolysis, micro-emulsions, and transesterification employing edible oils, non-edible oils, and reusable wastes such as feedstock are the four basic procedures utilized in biodiesel manufacturing (Vyas et al. 2010; Daud et al. 2015; Siddiquee and Rohani 2011). Transesterification is a prevalent process for making biodiesel nowadays (Siddiquee and Rohani 2011; Abbaszaadeh et al. 2012). Through use of edible oils in producing biodiesel may result in a food shortage and unneeded forest destruction for crop purposes (Talebian-Kiakalaieh et al. 2013). As a result, several researches have emphasized on non-edible oils such as algae, microalgae, and jatropha oils (Karatay and Dönmez 2011). WCO from

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households and restaurants can be utilized to make biodiesel, reducing water pollution and preventing drainage system obstructions (Yaakob et al. 2013). The utilization of WCO in biodiesel manufacturing can help save expenses by 60–90% (Sari-Erkan 2019; Daud et al. 2015). Microalgae are natural bio factories that are increasingly garnering attention because to their long sustainability and versatility in being used as feed and food for biochemical and bioenergy generation, and climate change mitigation (Mutanda et al. 2020; Ratnapuram et al. 2018; Vu et al. 2018). Many major aquaculture creatures, such as mollusks, shrimps, and fish, use microalgal biomass as a natural food source (Selvarajan et al. 2015). Biorefinery, in which a quantity of products, such as lipids, chlorophylls, carbohydrates, carotenoids, nucleic acids, nutraceuticals and proteins, are generated from the same harvested microalgal biomass within a few days, has sparked interest in the microalgal biotechnology industry in recent decades (Vu et al. 2018). Numerous microalgae produce these high-value chemicals as storing constituents and functional compounds, with uses in the bioenergy and pharmaceutical industries (Figueroa-Torres et al. 2017). Because of its medicinal implications, such as possible cancer treatment agents, the production of omega-3 fatty acids, carotenoids, and phycobiliproteins (PBPs) from microalgal biomass is currently attracting a lot of attention. Microalgal biotechnology industrialization is hampered by high costs and low biomass efficiency associated with microalgal bioprocessing, hence high-value products are recommended for microalgal biorefinery (Banu et al. 2020; Bhatia et al. 2021). Because the manufacturing of microalgal biodiesel is now uneconomical, it is necessary to look into the possible uses of the unique microalgal intracellular biomolecules and chemicals (Wang et al. 2018). Furthermore, integrating lipid synthesis with other uses, such as extraction of high-value metabolites and CO2 sequestration, might result in a direct large decrease of microalgal lipid production costs (Mutanda et al. 2020; Lee et al. 2018). Due to the use of renewable feedstock in its production, biodiesel is considered as a sustainable and clean fuel that is recyclable and ecologically responsible. These qualities also allow this liquid fuel to reduce exhaust emissions from diesel vehicles, such as particulate matter (PM) (Kolesárová N, Hutnan M, Bodík I, Spalková V 2011), unburned hydrocarbons (HC), and carbon monoxide (CO), with the exception of nitrogen oxides (NOx). Because of the oxygen concentration in biodiesel, nitrogen oxide emissions normally increase. Additional benefits of using biodiesel include the utilization of agricultural surplus and the reduction of reliance on crude oil. The features of biodiesel with a self-ignition exceeding 93.3 °C, making combustion process safer and easier to use, manage, and preserve. The greater energy level, also referred to as thermal efficiency, of biodiesel renders it equivalent to petroleum diesel. The good influence on the environment may be the primary reason why biodiesel is becoming more popular as a transportation fuel (Fig. 1). Nevertheless, the high cost of pure biodiesel in comparison to petroleum gasoline has stifled the growth of this green fuel. Biodiesel’s commercialization is hampered by high cost of production due to high feedstock costs. The use of edible vegetable oil is another barrier to biodiesel growth. It presents the issue of global food competition, which might result in food scarcity, degradation, and fuel supply management challenges, such as ensuring that

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Fig. 1 Usage of biodiesel

oil market is sufficiently managed for food intake and commercial commodities. (Daud et al. 2015). Thus, our present work focuses on various methods of biodiesel production and utilization of the wastewater for biodiesel production.

2 Classification Based on Different Sources and Properties of Biodiesel On the basis of the form of feedstock utilized, Biofuels are categorized into 3 classes generation biofuels square measure ready straightly from food crops like corn, sugarcane, wheat and helianthus etc. Biofuels may be categorized into two categories according to their chemical nature: bioethanol and biodiesel. Bioethanol, which are being used in production of gasoline (10–15%), can be made from digesting plastic biomass to reduced sugars or by directly extracting sugars and then converting them to alcohol. Bioethanol accounts for over a quarter of all automotive gasoline in Brazil, but it’s the most frequently utilized renewable transportation biofuel in the United States, with 13.3 billion gallons produced in 2012 (Westpheling 2014). In Brazil, sugar cane is the primary crop, although other crop species such as cassava, sugar beet, wheat, rice, corn, barley, potatoes, and sorghum are used for bioethanol production in various parts of the world (Lee and Lavoie 2013). Biodiesel is made by esterifying oil taken from various seed crops such as sunflower, soybean, oil-palm, oilseed and canola (Avinash et al. 2014). It may be utilized straight in diesel engines or combined with regular diesel. It is reported that the concerned nations have granted areas with coverage restrictions for producing crops for biodiesel production (Singh and Ahalavat 2014). Algae and a variety of non-crop seed oil plants, such as Pongamia pinnata, Jatropha curcas, and a variety of halophytes, are also recommended for this purpose (Murugesan et al. 2009). Currently, the United States consumes 50 million gallons of biodiesel per year, is the country with the highest percentage of bio-diesel

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Fig. 2 Classification of various methods for biofuel extraction

in its fuel. Bio methanol, syngas, biochar, and bio-hydrogen are among the alternative biofuels that are gaining popularity (Khan et al. 2014) (Fig. 2).

2.1 Different Sources of Biodiesel Alternative diesel fuels, like vegetable oil and fats, are derived from natural, renewable sources. Jatropha, palm, sunflower, rapeseed, soybean, canola, and cotton seed are the most often utilized oils for Biodiesel production. Non-edible crude vegetable oils and Waste vegetable oils are preferred as prospective cheaper sources of biodiesel because edible vegetable oils are more expensive than diesel fuel. In India, using such edible oil to make biodiesel is likewise not practical due to a large gap in supply and availability of these oils. Only plants that generate non-edible oil in significant quantities and can be cultivated on a big scale on non-cropped marginal and waste lands can be considered for Biodiesel in India. Due to natural property variations, animal fats

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have not been investigated to the same level as vegetable oils, despite their frequent mention. Because animal fats include more saturated fatty acids, they solidify at room temperature. Biodiesel is usually made by utilizing crops which are suitable in the local climate. In the U.S. soybean oil is the most widely used biodiesel feedstock, while palm oil and canola (rapeseed) oil are the most prevalent Biodiesel feedstocks in tropical and European countries, respectively. Certain high-priced applications, such as pharmaceutical raw resources, should not be consistent with an effective feedstock for making biodiesel. The need for medicinal raw materials, on the other hand, is lower than that for energy resources. The biodiesel resource should, as much as feasible, its criteria: reduced production expenses and a huge output. Biodiesel can also be produced by growing microalgae in wastewater (Singh and Singh 2010).

2.2 Properties of Biodiesel Biodiesel is a type of fatty acid alkyl ester generated by alcohol-catalyzed lipid transesterification. When methanol is utilized as the alcohol, it is referred to as FAME. It contains no sulphur and has a strong lubricating capability (Aransiola et al. 2014; Kamel et al. 2017). Its physical properties can change depending on the feedstock used. Its flash point is higher than that of petroleum diesel (Aransiola et al. 2014; Mahmudul et al. 2017). Its calorific value is 37.27 mJ L−1 (Aransiola et al. 2014). The differences in biodiesel energy density are given the different feedstocks used instead of the process of manufacturing (Aransiola et al. 2014; Mahmudul et al. 2017). Because it is highly adapted to the engine, biodiesel can be employed as a substitute in a variety of engines without alteration (Mahmudul et al. 2017; Soccol et al. 2017). To indicate the quantity of biodiesel in any gasoline blend, the most of countries employ a mechanism known as the “B” factor. With no or modest modifications, around 20% or less can be used in diesel machinery (Mardhiah et al. 2017). To prevent any technical issues, biodiesel can also be utilized in its pure form (B100) with particular modifications of engine (Mardhiah et al. 2017). It can be used to replace diesel fuel for a variety of purposes, for example internal combustion engines and boilers, because of its reduced toxicity and excellent biodegradability. The calorific value (MJ/kg), cetane number, density (kg/m3 ), viscosity (mm2 /s), cloud as well as pour points (°C), acid value (mg KOH/g-oil), flash point (°C), copper corrosion, ash content (percent), water content, carbon residue, and sulphur content, sediment, glycerin (percent m/m), distillation range, phosphorus (mg/kg), and oxides (Mahmudul et al. 2017). Biodiesel’s physicochemical qualities are heavily influenced by the type and fatty acid makeup of the feedstock (Atadashi et al. 2010; Lin et al. 2011; Jena et al. 2010; Atabani et al. 2012). Biodiesel with a flash point of more than 93.3 °C is simpler to manage, utilize, and storage, making it safer to use. It also has a heating value or comparable energy content, to petroleum diesel. However, users may encounter issues such as fuel pumping issues, poor low temperature flow, cold storage and increased copper strip corrosion (Yaakob et al. 2013; Mahmudul et al.

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2017). The greatest impediment to the growth and efficient deployment of biodiesel is the higher expense of feedstock (Mahmudul et al. 2017; Srivastava et al. 2018).

3 Microalgae as a Precursor of Bio-diesel Production Global climate shift and rising greenhouse gas outputs, and the exhaustion of common fuel sources, have become major concerns in recent years. The burning of oil, natural gas and coal releases over 6 billion tonnes of CO2 into the atmosphere every year. The CO2 contained in microalgae as lipids can be removed and utilized as a biofuel. Equation (1) describes the process catalyzed using the carbonic anhydrase enzyme: + CO2 + H2 O →CA → HCO− 3 + H

(1)

Green plants can capture CO2 physiologically via photosynthesis. Nevertheless, due to the sluggish rates of growth of traditional terrestrial plants, the CO2 collected by agricultural plants has been projected to be just 3–6% of fossil fuel outputs (Skjånes et al. 2007). Microalgae, on either hand, may present a potential due to their quantity and quicker development rate. However, biofuel obtained through edible oil, waste cooking oil, food crops as well as other vegetable oils is insufficient to supply the growing need for fuels of transportation (Ghosh et al. 2016). Furthermore, biofuels derived from food crops assist land clearance when generated on cropland or freshly harvested land. As a result, biofuel derived from CO2 extracted by microalgae may be considered one of the viable choices for reducing the use of food-based lignocellulose.

3.1 Current Microalgae Biofuel Production Scenario In relation to blue-green algae, green microalgae generate more biofuel. Chlorella sp., Chlorococcum sp., and Neochloris oleoabundans have been identified as promising biodiesel feedstocks. Nevertheless, Haematococcus pluvialis, a red microalga, appears to be a viable biofuel generation possibility (Lei et al. 2012). The access of feedstock and the technological choices that can be adopted considerably influence the generation of multiple kinds of biofuel. At the moment, the generation of biofuel via microalgae is limited to laboratory and small companies.

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3.2 Impacts of Different Culture Parameters on Microalgae CO2 Sequestration and Lipid Accumulation Maximum CO2 fixation rate (252.883 0.361 mg L−1 day−1 ) and highest biomass (4.975 0.003 g L−1 ) were attained at 13.8 1.5% CO2 and 25 C, which was superior to several other comparable investigations. There is a link connecting the biomass concentration and the rate of CO2 fixation in microalgal species. As the concentration of the biomass rises, more cells fix CO2 , increasing the fixation rate of CO2 of the culture. Scenedesmus obliquus has a high biomass concentration and a high rate of CO2 fixing. SA1 produced the highest biomass of 0.883 0.001 g L−1 at increased temperatures of 40 C with 13.8 1.5% CO2 . As a result, SA1 showed to be a promising option for CO2 extraction from flue gas and also biofuel generation (Basu et al. 2013). It has been found that microalgal culture grows faster in an airlift bioreactor. Aeration improves microalgal culture interaction, that aids in sustaining uniform settings. Cells and nutrients enjoy greater interaction in aerated cultures. It also keeps silt from accumulating.

3.3 Economic Overview 1. Microalgae proliferate quickly and produce a lot of algae per acre. 2. Utilizing microalgae to generate feedstock for biofuels will not interfere with food supply. 3. Water utilized to cultivate microalgae may contain waste and saline water which can be employed in traditional farming or for home purposes. 4. Via the mechanism of biological carbon capture, microalgae have enormous technical promise for GreenHouse Gas abatement. 5. No requirement to remove the CO2 once it has been retained by the microalgae. CO2 is broken down producing lipids and carbohydrates. 6. A microalgal biorefinery could provide a variety of value-added products, notably oils, protein, and carbohydrates, in addition to biofuel. Transformation processes may also be utilized to make biofuels like green diesel, biodiesel, aviation fuel, methane, and green gasoline.

3.4 Microalgae Cultivation in Wastewater for Biodiesel Production A significant amount of water contamination is produced globally as a result of the global increase of human civilization and people’s enhanced standard of living. Wastewater is a term used to describe the end product of home, industrial, municipal,

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and agricultural resources (Ellis 2011). The content of wastewater is a representation of the contribution in the current societal lifestyles and innovations. Organic matter such as carbohydrates, proteins, volatile acids and lipids are found in wastewater, as well as inorganic matter such as calcium, sodium, potassium, chlorine, magnesium, sulphur, bicarbonate, phosphate, heavy metals and ammonium salts (Abdel-Raouf et al. 2012). Eutrophication or algal blooms are caused by an exceeding of such load of nutrients in nearby streams, which is commonly caused by anthropogenic waste accumulation (Bhatt et al. 2014). Annually, biodegradable industrial wastes, residential and domestic-like wastes, and other wastes of greater than 300 million tonnes are produced in European countries, with the majority of them going unused (European Nations 2012). Annually, humans produce around domestic wastewater of 3 billion tonnes. Annual migration into cities in India is predicted to surpass 600 million by 2030, putting pressure on urban return flow (wastewater), which accounts for 70–80% of the supply of water (Amerasinghe et al. 2013). Agricultural, Domestic, industrial and municipal activities are the primary outlets of wastewater creation in most of underdeveloped countries, and wastewater is discharged into the environment without treating it adequately. Numerous microalgae species are able to develop well in wastewater environments as they can use plentiful organic carbon as well as inorganic P and N (Pittman et al. 2011). Algae absorb these nutrients, as well as CO2 , and use them to generate biomass via photosynthesis. Microalgae are the most common microorganisms employed in oxidation ponds and oxidation ditches to clean domestic wastewater. Algae has also been used to treat wastewater at a low cost and in an eco-friendly way. The concept of using wastewater as a platform for production of algal biofuel is not new, as indicated in a report by the Aquatic Species Program (ASP) from 1978 to 1996 in the USA. The fundamental challenge in developing a wastewater-based algal biofuel production system is identifying optimal microalgae strains that can thrive in a wastewater environment while removing considerable amounts of nutrients and producing high biomass and productivity of lipids (Bhatt et al. 2014). Scientists from all over the world have worked hard to investigate the possibility of employing microalgae in the production of biofuel from wastewater with nutrient removal properties, particularly nitrogen and phosphorus from effluents. In contrast to industrial wastewater, researchers focused more on culture of microalgae for P and N elimination from home sewage. The cause for this is that wastewater of several industries, such as chemical industry wastewater, tannery wastewater, contains more metal ions in addition to diverse organic P and N substances (Zhen-Feng et al. 2011), and heavy metal pollution makes it more hazardous, which inhibits algae growth. Whenever algae is cultivated in residential wastewater produced by 1000 Indian cities, the overall biofuel potential is 0.16 Mt/annum (Craggs et al. 2011), assuming a 20% lipid percentage (Bhatt et al. 2014).

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3.5 Production of Biomass Utilizing Microalgae Grown in Wastewater The commercialization viability of algae biofuels is heavily influenced by species identification, growing efficiency, lipid productivity, and large-scale harvesting. The algal biomass formed and gathered by these wastewater treatment systems could be converted to biofuels via a different pathway, including anaerobic digestion to biogas, lipid transesterification to biodiesel, carbohydrate fermentation to bioethanol, and high temperature alteration to biocrude oil. When contrasted to industrial algal production by HRAPs, which consume freshwater and fertilisers, high rate algal ponds (HRAPs) can reach financial viability of algal biofuel synthesis from wastewater treatment with little ecological consequence (Craggs et al. 2011; Park et al. 2011). The main difficulty in microalgal investigation for current high-rate ponds is to build an effective and cost-effective carbonation mechanism that can meet the high CO2 demand while also improving biomass efficiency (Putt et al. 2011). Viswanath and Bux (Viswanath and Bux 2012) extracted Chlorella sp. from a wastewater pond and tested it for lipid production rates in a bioreactor under photoautotrophic and heterotrophic conditions. The optimum level of biomass was retrieved from Chlorella sp. cultivated under heterotrophic growth conditions with 8.90 gL−1 especially in comparison to photoautotrophic growing conditions with about 3.6-fold less biomass, generally results in the deposition of high lipid composition in cells especially in comparison to autotrophic development by 4.4-fold increasing production of lipid (Fig. 3).

4 Mechanism of Biodiesel Production Presently, over 90% of energy is produced by fossil fuels, whereas renewable energy sources produce only 10%. According to projections, conventional oil sources will be depleted by 2050 due to rising energy demands. Because it possesses no aromatics or sulphur, biodiesel is an effective renewable fuel source, and it’s burning significantly lessens emissions of unburned carbon monoxide, hydrocarbons and particulates. Efforts have been undertaken to generate biofuel using agricultural products, however the ‘food or fuel’ argument forces all to look for another option. Microalgae is an alternate source since it grows quickly and eliminates the food vs. fuel debate (Mondal et al. 2017). The availability of feedstock and the technical choices that may be adopted considerably influence the generation of several biofuel forms. Microalgaebased biofuel generation is currently limited to laboratories and limited companies (Mondal et al. 2017).

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Fig. 3 Microalgal lipid production is depicted schematically. The enzymes are highlighted in red. Inside the chloroplast, free fatty acids get produced (Srirangsan et al. 2009)

4.1 Algal Biology Microalgae are microscopic algae which may be observed in both freshwater and marine habitats. They are unicellular organisms that may survive alone or in groups. They are responsible for producing more than 50% of the atmospheric oxygen. The richness of microalgae is enormous. The development of the photosynthetic organelles, the plastids, is perhaps the clearest foundation for understanding the connections between the various algal types. Algae have one thing in common: they all do oxygenic photosynthesis. The principal photosynthetic pigment in all algae is chlorophyll a, however the related photosynthetic pigments and plastid structure differ amongst algal taxa (Borowitzka 2018). Algae have a wide range of nutrition strategies. The majority of algae are photoautotrophs, which means they solely employ water, CO2 and light to generate organic substances via oxygenic photosynthesis. For photosynthesis, algae may utilise both bicarbonate (HCO3 ) and dissolved CO2 as inorganic carbon resources (Beardall and Raven 2016). Many algae, however, require additional specialised chemical compounds, particularly vitamins, for growth, indicating that they are auxotrophs. Other algae may also thrive in the dark by utilising simple organic substances like glucose or acetate as an energy and carbon source. This is referred to as heterotrophy or chemoorganotrophy. The biology of microalgae is as diverse as the creatures that make up this polyphyletic group. We know a great deal about a few species, but we don’t know much

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about the rest of the microalgal species. As a result, microalgae are both exciting and rewarding subjects for basic and practical study. Microalgae are employed in health nutritional supplements, as sources of important biochemicals, biofuel, and as yet unknown sources of novel chemical substances like new medications, as well as for wastewater treatment and as fertilisers and soil enhancers in agriculture. In other words, although it is rarely recognised, microalgae are a part of people’s lives (Beardall and Raven 2016).

4.2 Growth of Microalgae In Vitro We carried out an experiment wherein pure microalgae is separated using the pour-plate technique using microalgae agar medium, followed by the creation of a microalgae subculture in 50 mL of microalgae culture broth (inoculation from agar plate to broth and incubation for 2 days at room temperature). In order to produce microalgae, a 5% (v/v) subculture is inoculated into the production medium, which is then incubated at room temperature for 4 days in a 250 ml production medium. After 4 days, the product microalgae is flocculated and filtered to get a concentrated microalgal suspension. It is then trans esterified at 60 °C using diethyl ether as the solvent and hydrochloric acid as the catalyst, followed by a 24-h settling period. The upper surface of crude biodiesel is retrieved after decantation, with glycerin at the bottom. After that, the product is washed in water and dried to produce pure biodiesel.

4.3 Algae Culture Systems Algae are organisms that grow in water and produce biomass using the light and carbon dioxide (CO2 ). There are two kinds of algae: macroalgae and microalgae. The three primary components of algal biomass are carbohydrates, proteins, and lipids/natural oils (Perez-Garcia et al. 2011). For algae to thrive, a few simple conditions must be met: light, a carbon source, water, nutrients, and a temperature that can be controlled. Algae are often farmed in open ponds known as high-rate ponds (HRP) or confined systems known as photobioreactors. Each system has its own set of advantages and disadvantages (Mohammadi and Azizollahi-Aliabadi 2013).

4.3.1

Open Ponds

Open ponds are the most basic and oldest technology for bulk microalgae cultivation. The small pond in this approach is generally roughly 1 foot deep; algae are generated under circumstances comparable to the natural environment. A pond with a “raceway” or “track” form is popular, with a paddlewheel generating circulation and

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mixing of the algal cells and nutrients. Examples include large open ponds, circulating ponds with a rotating arm, and raceway ponds. This cultural framework is not without flaws. Because these are open-air systems, evaporation causes a significant amount of water loss. In open ponds, microalgae cannot utilise carbon dioxide as efficiently, and biomass output is restricted. The presence of undesired algal species, as well as organisms that feed on algae, further limits biomass output. Furthermore, maintaining adequate growth conditions in open ponds is difficult, and extracting biomass from such a low cell yield is expensive (Mohammadi and Azizollahi-Aliabadi 2013).

4.3.2

Photobioreactors (PBR)

An algal bioreactor or algae photobioreactor is a bioreactor that is used to cultivate algae with the goal of producing biomass to fix carbon dioxide. The major benefit of Photobioreactors (PBR) is their ability to generate a huge amount of biomass (Salvi et al. 2021). On average, the biomass production of photobioreactors may be more than 12 times that of a typical raceway pond. Photobioreactors have significant drawbacks as well. Fluctuations in temperature and light, which are typical in all photoautotrophic systems, can lead to poor microalgae proliferation. The scale-up is extremely detrimental in these systems and necessitates a large fee to do so. Because of their intricacy, as well as differences in design and combination, singles have a high initial capital cost. When manufacturing a high-value product, such as a medicine, this expense can be justified; but, a low-value commodity, such as diesel, cannot repay the original cost of construction in any acceptable period (Rawat et al. 2011) (Table 1).

Fluid Dynamics Some of the most significant aspects for building a photobioreactor are CO2 input, hydrodynamics, photon intensity and distribution, and mass transfer. Computational fluid dynamics (CFD) is a technology that uses a computer and numerical equations to handle fluid flow difficulties in various PBRs. CFD models for PBRs are built by taking physicochemical qualities into account while evaluating various aspects in order to decrease the number of tiresome trials. The choice of turbulence model (flow model), grid resolution, and bubble size is crucial for improving prediction accuracy. CFD software has been designed with 229 models that are based on the phase of the fluids. Because there are no moving elements in tubular PBRs, simulations are simpler than in baffled reactors. The main flow models for running simulations are two-phase and multiphase, with the latter being the most appropriate for realistic modelling (Ramasamy et al. 2020). With a gas hold up of up to 30%, the two-phase model is used. Choosing a technique for calculating flow is critical and is dependent on the gas hold up of the distributed fluid. Pfleger et al. studied the several flow regimes in a bubble column PBR and discovered that a turbulent regime matches well with experimental performance, whereas a laminar domain exhibits a chaotic

10 L tubular column

Airlift photobioreactor

Airlift tubular photobioreactor

Photobioreactor

Chlorella zofingiensis

Cholorella sorokiniana

S. obliquus

C. vulgaris

Saline wastewater

Domestic wastewater

Bioindustrial wastewater

Mixed biogas slurry and municipal wastewater

Synthetic municipal wastewater

Membrane photobioreactor

Chlorella vulgaris

Photoautotrophic

Mixotrophic

Mixotrophic

Mixotrophic

Phototrophic

Waste substrate Cultivation conditions

Reactor type

Species

Table 1 Different algal photobioreactor studies on a few algal species

Light intensity 2000 lx Hydraulic retention time (HRT) 2 days

Operation conditions

Biomass concentration stage 3 1.0380 mg/L Lipid productivity stage 3 54.25 mg/L day, 40% lipid content

Maximum areal productivity 21.76.26 6 0.3 g SS/m2 day 20.80 6 0.22 wt%

0.023 g dw/L day

Batch cultivation 20 days

HRT 5 days

Temperature 25 °C 6 1 °C 70 day incubation Sunlight

0.28 g/L day Light intensity biomass, 150 μmol/m2 s 96.3 mg/L day lipid Photoperiod 12:12 12 day incubation Temperature 25 °C 6 1 °C

2 mg/L biomass

Biomass productivity

(continued)

Wang et al. (2018)

Selvarajan et al. (2015)

Talebian-Kiakalaieh et al. (2013)

Karatay and Dönmez (2011)

Karatay and Dönmez (2011)

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Municipal wastewater

Tubular photobioreactor

Neochlorisoleoabundans

Mixotrophic

Waste substrate Cultivation conditions

Reactor type

Species

Table 1 (continued)

Dry biomass concentration of 0.47 ± 0.03 g L−1

Biomass productivity

Operation conditions Valev et al. (2019)

References

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flow pattern. K – ε is the most used model in CFD software, where K denotes turbulent kinetic energy (m2 /s2 ) and denotes the rate of kinetic energy dissipation (m2 /s3 ). Turbulence flow characteristics are determined using mixture properties and mixture velocities. K – ε is classified into three sorts based on the state of the phases namely, dispersed K – ε, two-phase K – ε, and mixture K – ε.

4.3.3

Heterotrophic Culture Systems

Some microalgal species may also be grown in a heterotrophic environment. In this case, the algae get their carbon from an organic carbon source in the medium rather than carbon dioxide, and because they don’t undertake photosynthesis, they don’t need a light source. Heterotrophic algae frequently produce more lipids and less protein than photosynthetic algae. Heterotrophic cultivation is most effective in monocultures of a single algae species and involves meticulous cleaning of the medium and equipment. Many factors, including culture age, medium nutrients, and environmental conditions such as temperature, pH, and salinity, impact heterotrophic lipid production by microalgae. The accumulation of lipids in the freshwater C. sorokiniana may be due to an excess of carbon in the culture media rather than a deficiency of nitrogen. As a consequence, accumulation in autotrophic or heterotrophic cultures might be attributed to the consumption of sugars at a rate larger than the rate of cell production, stimulating the conversion of excess sugar into lipids. This process is often carried out in two steps: exponential cell proliferation, followed by slower development owing to dietary constraints, culminating in fat storage. It might be linked not only to increased lipid-synthesizing enzymes in nitrogen-deficient cells, but also to the pausing of other enzymes involved in cell growth and proliferation, as well as the activity of enzymes specifically involved in lipid accumulation (Bitog et al. 2011). E. gracilis was employed as a model in another suggested process for lipid accumulation in heterotrophic circumstances. The mobilisation of lipids from chloroplast membranes under nitrogen deprivation is connected to the relocation of chloroplastic nitrogen by 1,5-bisphosphate carboxylase/oxygenase (E.C. 4.1.1.39, Rubisco) (Perez-Garcia et al. 2011). The fact that chloroplast development is nitrogen-dependent adds weight to this theory. In any of the aforementioned settings, energy storage molecules, lipids, or carbohydrates accumulate. Glycerol is one of the prospective carbon sources for heterotrophic biodiesel production. Currently, glycerol is a cheap and plentiful carbon byproduct of biodiesel fuel manufacturing. It is required to discover methods for converting this low-value glycerol into higher-value goods. Because of the highly reduced nature of the carbon atoms in glycerol, more fuel and reduced chemicals may be created than from ordinary sugars such as glucose. When fed glucose, fructose, or glycerol, Schizochytrium limacinum produced palmitic acid (16:0) as ~45–60% of their dry weight, which might be used to create biodiesel.

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Algae Turf Scrubbers (ATS)

Algae Turf Scrubber (ATS) systems, for example, are an alternate method of cultivating algae. Instead of suspending the microalgae in a culture medium, they rely on keeping the algae in place and feeding it nutrients. This method offers various advantages, including the ability to collect the algae directly from its substrate rather than sifting it out of the media. ATS systems are widely used to treat river water. Turf scrubbers are easy to scale and have been utilised in massive systems (Perez-Garcia et al. 2011). The Algal Turf Scrubber (ATS) is a Smithsonian Institution-developed engineering device that consists of a substrate for attaching and growing algae in the presence of a constant or periodic wastewater flow. Algae attachment occurs naturally, as a result of a complex biological succession of species that originated in the wastewater itself. ATS’s initial purpose was to model, in micro and mesocosm systems, the intricate relationship between ecosystem primary productivity and water quality factor management, allowing for the ex-situ preservation of living aquatic ecosystem simulations for extended periods of time. In recent decades, new applications have evolved, including the removal of nutrients and contaminants in aquatic production environments (Perez-Garcia et al. 2011), sewage, and agricultural runoff. These investigations retained the basic approach of association with algae found naturally in effluents, but experiments on the development of a monospecific culture planted on substrates were majorly ignored (Adey and Loveland 2011).

4.4 Microalgae Harvesting Centrifugation, flocculation, filtering, and flotation are some of the most common microalgae harvesting and recovery processes used today.

4.4.1

Flocculation

Flocculation is the process of making algal flocs, which are commonly used as a pretreatment to destabilise algae cells in water and increase cell density via natural, chemical, or physical methods. Chemicals known as flocculants are frequently used to enhance flocculation, and common flocculants include inorganic flocculants like alum and organic flocculants like chitosan. Because of ionization of functional groups on cell walls and adsorption of ions from the growth media, the surface charge of microalgal cells is generally negative, which can be neutralized by using positively charged electrodes and cationic polymers, which are also widely used to flocculate the microalgal biomass. Because of the high cost of flocculants, this technique of harvesting is incredibly expensive; hence, flocculants must be economical, easily manufactured, and nontoxic (Adey and Loveland 2011).

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Centrifugation

Centrifugation is a popular method for removing particles based on particle size and density. The size of the chosen algae species influences separation efficiency. Various centrifugal methods, such as tube centrifuges, multi chamber centrifuges, imperforate basket centrifuges, decanters, solid retaining disc centrifuges, nozzle type centrifuges, solid ejecting type disc centrifuges, and hydrocyclones, have been used depending on the application. Despite being an energy-intensive approach, it is fast and preferable for microalgal cell retrieval, but cell viability was discovered to be highly dependent on microalgal species and centrifugation technique (Ray et al. 2015). Despite its effectiveness, centrifugation is deemed impracticable in large-scale algal production systems due to significant investment and operational expenses.

4.4.3

Filtration

Filtration captures microalgal biomass by passing the liquid medium through filters on which the algae congregate, resulting in thick algae paste. The four types of filtering systems are microfiltration (pore size of 0.1–10 m), ultrafiltration (pore size of 0.02–2 m), and reverse osmosis (pore size of 0.001 m) (Salvi et al. 2021). Dead end filtration, microfiltration, ultrafiltration, pressure filtration, vacuum filtration, and tangential flow filtration (TFF) are all filtration processes (Uduman et al. 2010). Nonetheless, it has substantial running costs and is time consuming.

4.4.4

Flotation

After being entangled in minute air bubbles, microalgae cells float towards the water’s surface (Harun et al. 2010). The size of the created bubble is generally related to the flotation effectiveness: nanobubbles (1 m), microbubbles (1–999 m), and fine bubbles (1–2 mm) (Valev et al. 2019). Dissolved air flotation is a common technique in which microalgal cells are flocculated first, followed by the passage of air bubbles through the liquid, allowing the flocs to float to the surface for simple collection. Microalgae surface charge and hydrophobic interaction both play important roles in microalgae adhesion to bubbles.

4.5 Transesterification Reaction Transesterification is a three-step reversible reaction in which triglycerides are converted to diglycerides, diglycerides are converted to monoglycerides, and monoglycerides are converted to esters (biodiesel) and glycerol (by-product). Some of the alcohols that can be used in the transesterification process are methanol, ethanol,

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Fig. 4 Transesterification reaction

propanol, butanol, and amyl alcohol. Both homogeneous and heterogeneous catalysts can catalyse transesterification processes (Kim et al. 2013). Transesterification can convert microalgal lipids that have aggregated as triglycerides to fatty acid alkyl esters. The ester exchange reaction of microalgal lipids is another name for this process. The lipid biomasses are composed of 90–98% triglycerides by mass, with trace amounts of monoglycerides, diglycerides, and free fatty acids (1–5%). Phospholipids, phosphatides, tocopherols, carotenes, sulphur compounds, and water molecules are also present in trace amounts. TAGs are kept in the cell’s cytoplasm by particular oil structures. They act as an energy store and are preferable to phospholipids and glycolipids for biodiesel generation due to their higher fatty acid concentration and lack of phosphate (Adey and Loveland 2011). Transesterification, also known as alcoholysis, is a multi-step process that occurs in the involvement of a catalyst wherein triglycerides are processed in methanol. Transesterification is the chemical process of neutralising free fatty acids in a triglyceride molecule by removing the glycerin and creating an alcohol ester. In the first stage, triglycerides are converted to diglycerides. The diglycerides are subsequently converted into monoglycerides. Eventually, the monoglycerides are converted to esters (biofuel) and glycerol, which is mostly a byproduct. Transesterification is seen in Fig. 4, where the radicals R1, R2, and R3 represent long-chain hydrocarbons classed as fatty acids. Although methanol and ethanol have been used successfully for supercritical fluid harvesting, they are hygroscopic, corrosive, and have a low energy concentration. Farobie et al. (2016) advocated replacing methanol and ethanol with 1-propanol. More research is required, and the best alcohol may be established by evaluating the findings of each trial. In the influence of methanol, transesterification processes may be acid or base catalysed to yield matching fatty acid methyl esters. Lately, it has been proposed that an enzyme-catalyzed transesterification process can boost biofuel output. The lipase enzyme is an example. Pseudomonas fluorescence, Candida rugosa, Rhizopus oryzae, and Candida antractica are sources of lipase enzyme (Teo et al. 2014).

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4.6 Ignition Delay and Cetane Number (CN) The ignition delay in a diesel engine is defined as the period among the beginning of combustion and the beginning of injection. The physical delay produced by mixing of air fuel, atomization and vaporisation, as well as the chemical delay induced by pre-combustion reactions, make up this delay time. At the same time, physical and chemical delays occur. Shortening the ignition delay is the current technique for reducing NOx emissions in modern engines. One such key characteristic that is responsible for the delay period is the fuel’s cetane number (CN). When burnt in a typical engine under intended functioning circumstances, the cetane number of a fuel is characterized as the percentage by volume of standard cetane in a mixture of α-methyl naphthalene and normal cetane that has the same ignition characteristics (ignition delay) as the test fuel. A gasoline with a higher cetane number has a shorter delay time and allows for smoother engine operation. Biodiesel has a greater CN than petro-diesel due to its higher oxygen content (Teo et al. 2014).

5 Biodiesel Wastewater and Its Treatment 5.1 Biodiesel Wastewater Biodiesel manufacturing generates relatively little waste. Based on the feedstocks used and methods, this can result in effluent, resins, minerals, particles extracted from spent oil, and glycerin. Major manufacturers can sometimes afford to spend on the machinery required to recycle and reuse several of these items, thus trash is not an issue for them. For small manufacturers, disposal of waste is a serious issue. Pollutants including soap, residual methanol, residual catalyst and glycerin are typically “washed” out of biodiesel during production. For each gallon of biodiesel produced, one gallon of effluent can be produced.

5.2 Biodiesel Wastewater Treatment The chosen feedstock is treated chemically, defined as transesterification in order to produce biodiesel. This technique produces a lot of waste, with a ratio of 20–120 L per 100 L of biodiesel. The most pressing environmental problem is the development of an efficient technology for treating and reusing washing water. Conventional refinery techniques are unable to fully recover the chemicals contained in wastewater. As a result, an alternate and effective treatment approach for removing carbonaceous chemicals, are perhaps the most environmentally harmful substances, is required (Veljkovi´c et al. 2014).

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Crude biodiesel’s wet washing can be done with: (a) mineral acid and water, (b) deionized water, and (c) organic solvents and water to remove soluble contaminants such as methanol, residual catalyst, glycerol and soap. Distillation is occasionally used to remove surplus alcohol before the wet washing stage. The benefits and drawbacks of various crude biodiesel refining processes have been thoroughly reviewed elsewhere (Veljkovi´c et al. 2014). After separating crude biodiesel from the glycerol stage, water washing is commonly used to purify it. To avoid the creation of an emulsion, this method involves adding hot water to crude biodiesel while gently agitating it. The use of hot distilled water for washing eliminates the precipitate of saturated biodiesel and prevents the creation of emulsions. Bioprocessed biodiesel is supplied with fresh water after the aqueous phase has settled and separated. The washing process is frequently repeated 2–5 times until the wasted washing water (biodiesel effluent) is colourless, indicating that all contaminants have been eliminated. The water washing process creates a substantial volume of biodiesel effluent, based on the washing method utilised (Veljkovi´c et al. 2014). Biodiesel wastewaters should not be released into the public sewerage system, since the leftover oil might cause issues including microbial activity reduction and system blockage. Biodiesel plant wastewater should be properly handled before disposal or reuse to protect the environment. Because biodiesel wastewaters are very stable emulsions comprising oil, grease, and soap, a grease trap tank is unsuccessful in treating them. The alkaline pH of this effluent necessitated an effective treatment to lower it to a level that would benefit the later processing steps. In most cases, an upstream physicochemical preprocessing is also required to ensure that the biological treatment goes smoothly and efficiently (Teo et al. 2014). Furthermore, the factory management may find the recycling of all processed wastewaters in the synthesis of biodiesel to be an appealing option. A physicochemical procedure should be accompanied by floatation or sedimentation, a biological treatment, and a reverse osmosis system in a typical treatment method for recycling wastewater. For the wastewater produced by biodiesel production via alkali catalysed transesterification, physicochemical treatments, coupled chemical and electrochemical treatments, electrochemical treatments, biological treatments, advanced oxidation technologies, and integrated treatment incorporating previous treatment procedures have all been established (Veljkovi´c et al. 2014).

5.2.1

Physico-chemical Treatments

Adsorption, acidification (pH adjustment), and flocculation/coagulation procedures, or their combination, are used to treat biodiesel wastewater, Following that, physical treatment such as floatation, sedimentation or filtration may be used. This approach assumes the employment of appropriate chemicals to change the pH, produce coagulation, and promote flocculation. The injected chemicals have the primary purpose of destabilising the oil-in-water emulsion and creating flocs that will coalesce and settle

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swiftly. The method is often utilised as a preliminary process prior to the therapeutic treatments (Veljkovi´c et al. 2014).

5.2.2

Electrochemical Treatments

This technique is appealing for processing industrial wastewaters for its benefits, which include equipment that is simple, ease of operation, quick treatment time, lack of or decreased chemical use, rapid sedimentation, and lower sludge production. Electrocoagulation and hydrothermal electrolysis are two types of electrochemical therapy (Veljkovi´c et al. 2014).

5.2.3

Advanced Oxidation Techniques

Sophisticated oxidation mechanisms are presently thought to be extremely effective physico-chemical processes that generate highly reactive oxidising species, primarily hydroxyl radicals (HOd), capable of degrading organic molecules. Among these are ozonation and ozone-related processes (O3 /H2 O2 , UV/O3 ), heterogeneous (TiO2 /UV) and homogeneous (photo-Fenton process) photocatalysis, and others. Electro-oxidation methods are advanced treatment processes because they produce hydroxyl radicals from water electrolysis. Many advanced oxidation techniques can be used to degrade various types of organic contaminants (Veljkovi´c et al. 2014).

5.2.4

Coupled Chemical and Electrochemical Treatment

The electrocoagulation treatment is frequently paired with a chemical treatment to boost its effectiveness. This chemical and electrochemical processing mixture can be done in a one-step or two-step method. The electrochemical treatment is achieved by the addition of a coagulant and/or an oxidant in the first example. In the previous case, the combined treatment technique began with acidifying the biofuel wastewater using a mineral acid in order to retrieve crude biodiesel, followed by electrochemical treatment. In most circumstances, the two-step technique is insufficient to reduce contaminants like grease and oil, COD, and BOD to levels below the allowable limits, necessitating post-treatment of the released effluent (Veljkovi´c et al. 2014).

5.2.5

Biological Treatment

Biodiesel wastewaters appear to be a particularly interesting raw material for microbial breakdown, given their high amount of biodegradable organic components. Nonetheless, biological treatment of wastewater from biodiesel factories is difficult

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owing to the effluent’s composition, which is not conducive to microbial development. With the exception of the carbon source (residual oil, methanol, and glycerol), the wastewater has a high pH and is low in nutrients essential for microbial growth, therefore only a modest amount of key nutrients should be provided. Methanol is a fantastic microorganism substrate. It stimulates fast microbial development due to high concentrations in biodiesel wastewaters (Valev et al. 2019). This might be advantageous in the bioremediation of biodiesel wastewaters including slow-degradable residual oil. A high methanol concentration, on the other hand, is poisonous to methanogens, and a relatively high oil content in the biological treatment can stifle microbial growth. The anaerobic digestion process is said to be inhibited by free fatty acids. Furthermore, high salt in biodiesel wastewater might harm some bacteria, such as methanogens. As a result, biological treatment of biodiesel wastewaters, comprising biodegradation of residual oil, should be carried out under optimal operating conditions. Nevertheless, biological treatment of biodiesel wastewaters, including for biogas production, has yet to be thoroughly researched (Veljkovi´c et al. 2014).

5.2.6

Coupled Physico-chemical, Electrochemical, Advanced Oxidation and Biological Processes

Considering the COD content of the raw wastewater, both anaerobic and aerobic procedures can be used for biodiesel wastewater treatment. The anaerobic method was also used to treat biodiesel wastewater with COD levels less than 100,000 mg/L, whereas the aerobic method was utilised to treat biodiesel wastewater with COD levels under 100,000 mg/L. To eliminate free fatty acids, residual oil, and esters from raw biodiesel effluent, it is acidified first. Subsequent to the biological process, the effluent wastewater was treated using floatation/electrocoagulation, flocculation/coagulation or sedimentation/coagulation. The aerobic and photo-Fenton processes were also combined (Veljkovi´c et al. 2014).

6 Conclusion In our contemporary age, where population and pollution have both reached critical levels, emerging technologies are increasingly becoming an essential part in safeguarding the earth’s ecological sustainability. Waste creation has grown as a result of growing urbanisation and a lack of knowledge among people, and there is no effective mechanism for its disposal. A wastewater refinery intends to produce a variety of commercial products from wastewater utilising innovative integrated processing systems at a single location or across a network of facilities. According to several studies, biofuel generation from wastewater has attracted a lot of attention due to the scarcity of fossil fuels. Furthermore, biodiesel manufacturing has piqued

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the interest of scientists due to the broad variety of feedstock available and their potential qualities. Microalgal biodiesel is theoretically achievable, as illustrated here. It might be the only renewable biodiesel capable of completely replacing petroleum-derived liquid fuels. The economics of manufacturing microalgal biodiesel must improve significantly to compete with petrodiesel, although the amount of improvement required looks to be possible. Currently, algal-biofuel manufacturing is prohibitively costly for commercialization. Because of the operating costs of oil recovery and biodiesel processing, as well as the uncertainty of algal-biomass output, future cost-cutting measures for algal-oil production should focus on the oil-rich algae itself. This must be addressed by breakthroughs in algal biology, as well as technology developments or culture-system engineering. Major developments in photobioreactor design, microalgal biomass harvesting, drying, and other downstream processing systems, for example, are critical steps that may contribute to improved cost effectiveness and, as a result, effective adoption of the biofuel from microalgae vision.

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Nutrient Recovery and Utilization from Wastewater for Soil-Less Agriculture Avijit Chakraborty, Medha Maitra, Banani Ray Chowdhury, and Chaitali Dutta

Abstract Recovery of nutrients from wastewater and recycling nutrients as soil fertilisers is a potential circular economy challenge. Bio-omics wastewater treatment is among the most potential remediation strategies for municipal, household, and industrial effluent. Biological Nutrient Removal is a method of extracting phosphorus and nitrogen-enriched chemicals from wastewater prior discharging them into bodies of water such as ponds, rivers, and oceans. Human-derived urine, nitrogen and phosphate-rich urine are excellent sources for fertilizer use in soil-less agriculture. A conservative solution is to extract nutrients from wastewater using microalgal cultures. Wastewaters are enriched in P and N and can be regarded as a microalgal culture medium. The elimination of nitrate from wastewater is aided by bio-electrochemical denitrification. Because of its low sludge output and costeffectiveness, electrocoagulation is the best technology for extracting phosphate and nitrate. Nutrients derived from wastewater processing by conventional and microbial degradation are assimilated by plants in wastewater hydroponic operations via biomass harvest action from the anaerobic bioreactor of the decentralised wastewater management. The high levels of phosphorus and phosphates eliminated at the time of purification of sewage are connected to the use of iron sulphate following biological processing in the reactor, which is related to secondary phosphorus contamination of sewage from dead sections of algae or plants. Because plants have the potential to absorb nutrients, harmful metals, and new pollutants, hydroponic systems can be utilised to clean inadequately treated wastewater before it is released into the ecosystem. The ability of wastewater to boost crop production in hydroponic systems is attributed to its multi-nutrient constituent, which enhanced plant chlorophyll level

A. Chakraborty · M. Maitra · B. R. Chowdhury (B) Department of Biotechnology, Bengal Institute of Technology (BIT), Kolkata 700150, India e-mail: [email protected] C. Dutta Kazi Nazrul University, Asansol, West Bengal, India e-mail: [email protected] B. R. Chowdhury Baranagar Baghajatin Social Welfare Organization, Kolkata 700036, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_7

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and rate of photosynthesis and could thus be connected with higher heavy metal concentration. Keywords Nutrient-recovery · Electrocoagulation · Microalgae · Wastewater-treatment · Hydroponic system · Electrochemical-denitrification

1 Introduction Soilless agriculture is considered as a reasonable opportunity for agronomy, especially in regions distinguished by restricted water availability and acute soil degradation (Sambo et al. 2019). Hydroponic wastewater systems are the combination of wastewater treatment into the production process of hydroponic plants (Norstrom et al. 2003). Plants will absorb nutrients produced by the wastewater treatment process via microbial and physical breakdown in a hydroponic wastewater system. By providing value-added crops, such synergistic utilisation offers various benefits over previous phytoremediation/bioremediation systems which including artificial wetlands (Magwaza et al. 2020a; Roosta and Hamidpour 2011a). Since microalgae are photosynthetic autotrophic microorganisms, they utilize solar power to transform inorganic nutrients to organic matter, allowing them to produce biomass. Microalgaes have been suggested for various utilizations, including treating wastewater and flue gases (Gabriel et al. 2018; Spolaore et al. 2006; Acién Fernández et al. 2012). To regulate eutrophication, traditional wastewater treatment plants usually eliminate nutrients from wastewater as phosphaterich sludge and transform nitrogen-containing substances into nitrogen gas via nitrification–denitrification (Yamashita and Yamamoto-Ikemoto 2014). Microalgaes can recover the nutrient content from the waste water (Olguín 2012). Due to its high growth rates, growth in warm areas such as deserts and tropics to cold areas such as poles and mountains, biomass is good sources of protein and lipids among other beneficial substances and no requirement of fertile land and available water, microalgaes become valuable (Gabriel et al. 2018; Chisti 2012). The production of microalgae in these wastewater streams, such as centrate, sewage, and manure, aids in the recovery of the nitrogen and phosphorus encapsulated in these wastewater streams as recoverable biomass. As a result, microalgae have been proposed as a technology of recovering nutrients. The nutrient recovery in this scenario is restricted by the biomass production of the microalgae; the larger the biomass yield, the higher the nutrient recovery (Acién et al. 2016). Since these compounds lead to eutrophication in natural water sources, separating phosphorus and nitrogen from wastewater is becoming a universal issue. Furthermore, nitrate poses a risk to people, particularly as a potential matter of neonatal methemoglobinemia (Campbell 1952). The activated sludge method is frequently employed in wastewater treatment, and raw sewage from wastewater treatment plants frequently contains residual phosphorus and nitrogen in the form of ammonium and/or nitrate. A post-treatment method is required to recover

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phosphate and nitrogen from wastewater. However, since those effluents contain very few organic substances, a source of energy must be removed by adding the nitrogen (Yamashita and Yamamoto-Ikemoto 2014). The method of separating nitrogen and phosphorus from wastewater before it is released into groundwater or surface water is recognised as biological nutrient removal (BNR). Extrinsic inorganic fertilizers for example potassium, nitrogen (ammonium nitrate and urea) and phosphorus have been used to boost global food production (Rittmann et al. 2011; Ciceri et al. 2015; Ganesapillai et al. 2016). Nitrogen fertilizers are derived from ammonia produced by the Haber–Bosh process, even though phosphorus and potassium fertilizers are obtained from the mining industry (sedimentary and phosphate rocks, respectively) (Ciceri et al. 2015; Paepe et al. 2018). Ammonium nitrate yields are likely to diminish in the coming years, making urea a feasible nitrogen fertilisation substitute (Ganesapillai et al. 2016). High-grade phosphate rock mines are expected to run dry in the near future (Jurgilevich et al. 2016; Rittmann et al. 2011; Freguia et al. 2019a; Tao et al. 2019; Cunha et al. 2020; Liu et al. 2020). Phosphate is an essential raw substance (Jurgilevich et al. 2016), and there is currently no recoverable outcome that can be used to replace phosphate rock (Cunha et al. 2020). Potassium, being a nonrenewable resource with an inconsistent worldwide spread, could hinder agricultural development in areas where it is lacking (Liu et al. 2020). Thus our present review work focuses on the recovery of nutrients from various sources of wastewater and the utilization of the recovered nutrients in the hydroponic system for sustainable soil-less agriculture.

2 Recovery of Nutrients from Wastewater Using Microalgae, Bacteria and Other Sources 2.1 Recovery of Nutrients from Wastewater Using Microalgae 2.1.1

The Principle of Microalgal Consortia

With so many techniques to treat wastewater, microalgae have gotten a lot of recognition now a days as an alternative strategy for treating wastewater (Tredici et al. 1992; Kaya and Picard 1996; Craggs et al. 1997; Kong et al. 2010; Su et al. 2011). Microalgae are photosynthetic microorganisms with a unicellular or multicellular morphology that allows them to grow over time and survive in harsh settings (Mata et al. 2010). They propose a new method in elimination of carbon, phosphate, and nitrogen from wastewater even as generating biomass that can be used to achieve exorbitant chemicals including biogas and/or algal metabolites via anaerobically digestion process (Muñoz and Guieysse 2006). Furthermore, microalgae can reduce eutrophication in marine habitats and attenuate the detrimental effects of wastewater contaminants (Abdel-Raouf et al. 2012; Delgadillo et al. 2016).

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Fig. 1 Major biological events in microalgae/bacteria consortia for recovery of nutrients from wastewaters

In high rate algal ponds (HRAPs), combined bacteria-microalgae colonies are frequently cultivated to control industrial, agricultural and municipal wastewaters. In general, HRAP technique comprises a narrow racetrack reactor where bacteria and algae proliferate. The wastewater is injected into the HRAP and mixed with the algal and bacterial cultures developing within the bioreactor. Mixing enhances algal growth while limiting the accumulation of biomass (Oswald 1991; Delgadillo et al. 2016). Because microalgae retrieve nutrients from wastewater, they have been suggested as a solution to traditional wastewater treatment techniques based dependent on activated sludge. Nevertheless, when recognising use of microalgae in the treatment of wastewater, it is important to remember that which persists is a consortium of bacteria and microalgae (Muñoz et al. 2006). Because sterilized environments are not possible in such frameworks, the consortium that eventually prevails in the reactors will be that which usually happens like a characteristic of the constituents of wastewater, reactor design, environmental factors and operational parameters. Irrespective of the consortium’s biological content, a standard process is presumed (Fig. 1). The efficiency of microalgal wastewater treatment systems is primarily determined by light accessibility in the reactor, as well as the availability of solar emission and the depth of the culture. Because the biomass present within the culture reduces the amount of light affecting the reactor exterior as culture depth increases, the mean illuminance where the cells are introduced in the culture develops a feature of culture depth (Molina Grima et al. 1996). This parameter’s significance in the effectiveness of microalgal wastewater treatment. To maximize recovery of nutrients, the culture depth must be diminished to less than 0.2 m. The greater the proportion of nutrients ultimately framed as beneficial microalgal biomass, the shorter the culture depth.

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Function of Microalgae in the Retrieval of Nutrients

The mixing of microalgal generation and wastewater treatment was originally introduced as a cost-cutting measure, now it is regarded as a substitution of the use of traditional systems of wastewater treatment. The primary cause seems to be that microalgae permits for nutrient recovery from wastewater while reducing greenhouse gas emissions and conserving energy (Olguín 2012; Craggs et al. 2013; Fernández 2018). The ability of microalgae to grow and yield biomass is determined by photosynthetic efficiency and the availability of solar radiation. Daily solar radiation varies from 150 to 350 W/m2 based on area and day of the year. The photosynthetic efficiency is determined by how near the conditions of culture (pH, temperature and so on) are to the ideal level needed by the strain utilized. The ability of microalgal processes to retrieve nutrients can be predicted using solar radiation accessibility and photosynthetic effectiveness, as well as the usage of minerals by microalgae per unit mass generated. Conventional P and N-based fertilizers can be predominately replaced by microalgal nutrient recovery systems. Thereby, soluble instances of phosphorus and nitrogen are synthesized in vast amounts all over the world, as they are the pillars that support agricultural food production. In terms of nitrogen, practical industrial techniques utilize nitrogen from the atmosphere and huge quantities of energy (10– 15 kWh/kgN) to convert that to nitrate and ammonia through the Haber process, that contributes significantly to global warming. Despite the numerous advantages of microalgal processes in the retrieval of nutrients from wastewater, this technique is yet in its early stages, there are only a handful examples in use around the world. The main motive is due to the current technique’s minimizing potential. The wastewater treatment technology that relies on microalgae still needs to be improved. To make this technique more appropriate for wastewater deposition on an industrial level, these variables must be greatly optimized, with the time of hydraulic retention reduced to one day and the needed exterior area reduced to one m2 for each person-equivalent. Because of the importance of the field of wastewater treatment, huge corporations are focusing their attention on this struggle (Fernández 2018).

2.2 Recovery of Nutrients from Wastewater Using Bacteria 2.2.1

Recovery of Nutrients from Wastewater Using Electroactive Bacteria

Wastewater is generally acknowledged as a source of activated phosphorus and nitrogen, as well as the retrieval of both P and N as fertilizers has received a great deal of attention now a days. Electroactive bacteria are gaining popularity in this sector due to their ability to generate an electric field in microbial electrochemical processes to

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concentrate phosphate and ammonium for retrieval. Relevantly, this one-of-a-kind bacteria can transform nitrite and nitrate straightly to ammonium, increasing the amount of active nitrogen species available for recovery. In unbiased wastewaters, ferric ions formed by electroactive bacteria can be precipitated with phosphate and recovered as vivianite. Electroactive bacteria were used in all of these procedures as a nitrate and iron reduction as well as a bioelectric field generator. The methodology and techniques are outlined, as well as the obstacles to enhance their achievement (Li et al. 2020).

2.2.2

Recovery of Nutrients from Wastewater Using Photosynthetic Bacteria

Photosynthetic bacteria (PSB) wastewater treatment has emerged as an innovative technique capable of both nutrient recycling and wastewater treatment (Chen et al. 2019; Lu et al. 2019). Even though PSB has been used to treat certain types of wastewaters from the 1960s, primarily nontoxic wastewaters from various industries, it has received increased focus in the last decade due to its ability as a waste recycling technology (Meng et al. 2018). PSB, particularly purple nonsulfur bacteria (PNSB), have substantial capability for sustainable wastewater treatment due to their sensitivity of toxicity, large hydrogen yield, temperature volatility, and more flexible metabolic processes than microalgae which can integrate P, C and N in a single phase (González et al. 2017). The observations essentially demonstrated that, while microalgal systems demonstrated better nutrient recovery efficiency levels, PSB demonstrated greater robustness versus dynamics in operating conditions. The efficiency of nutrient recovery and useful biomass yield of systems that are based on photosynthesis are usually determined by characteristics of wastewater, environmental factors, phototrophic organisms, and process technology (Robles Martínez et al. 2019). As a whole, Microalgae and PPB cultivated on agricultural wastewaters have the potential to be utilized as efficient organic fertilizers. The recycling and retrieval of such ingredients on cropping and industries pasture has the capacity to expand on-farm economic viability, efficiency, and revenue growth (Zarezadeh 2019).

2.3 Retrieval of Nutrients from Food Waste by Anaerobic Digestion Process Significant amounts of wasted food caused by unutilized consumable food products or rejected yield from producers can have a harmful effect on the surroundings if not properly managed (Buhlmann et al. 2019). Approximately 97% of the total wasted food is dumped in landfills, where it quickly breaks down, causing odor, effluent pollution of deep and shallow waters, and methane production (Melikoglu et al.

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2013). Food waste management intends to enhance organic waste reuse by applying waste to soil (Tampio et al. 2016). Anaerobic digestion of numerous bio waste or food for sustainable energy, as well as the generation of solid digestates or/and nutrient-rich liquid, are well-structured methods for reducing landfill loads while recovering nutrients from industrial wastes (Zarezadeh et al. 2019). Because of its plant-available nutritional content, such as several required macro- and micronutrients, digestate produced from food waste has some benefits over synthetic N applications. Furthermore, digestate includes other organic elements, such as phytohormones and/or other materials that are difficult to identify, which can have a desirable impact on plant growth and development (Möller et al. 2008). The utilization of digestate results in higher content of organic N (present as NH4 + ) and C compounds, that can serve a key function in greatly increasing crop production and soil C balance (Nielsen et al. 2011; Alburquerque et al. 2012; Ren et al. 2020).

2.4 Metals and Nutrients Recovery from Wastewater Using Bioelectrochemical Systems Mechanisms that not only treat wastewater and moreover recover resources like metallic materials and nutrients are appealing for the implementation of appropriate techniques. Ammonium, metal, phosphorus, and water are all recovered using bioelectrochemical systems, which are regarded as promising innovation platforms. Metal ion separation and retrieval from wastewaters, process streams and metallurgical wastes has piqued the interest of bioelectrochemical systems. Bioremediation of organic compounds at the anode is coupled with metal ion decrease at the cathode in these processes. Metal mobilization and immobilization from synthesized mixtures have been shown in bioelectrochemical processes (Yarlagadda et al. 2019).

3 Hydroponic System: Type, Media Substrate and Uses 3.1 Hydroponics System The cultivation of plants in a liquid nutritional solution, with or even without the utilization of artificial media, is known as “HYDROPONICS.” Hydroponics has been regarded as an efficient technique of growing crops (lettuce, tomatoes, peppers and cucumbers) and also ornamental plants like herbs, roses, foliage plants and freesia. Because of the methyl bromide ban in soil culture, the popularity for hydroponically grown crops has skyrocketed in recent years (Dunn 2013) (Fig. 2).

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Fig. 2 Hydroponic system

3.2 Types of Hydroponics System Hydroponic systems are classified into seven categories: • • • • • • •

Deep Water Culture (DWC) Wick Drip (recovery or non-recovery) Ebb and Flow (Flood and Drain) Aeroponics Nutrient Film Technique (NFT) Fogponics (Modu et al. 2020). Characteristics of each type of hydroponics are discussed in Table 1.

3.3 Uses of Hydroponics Rice, Tomato, wheat, chrysanthemum, tobacco, potato, spinach, gerbera and lettuce have all been studied using hydroponic systems. The researches were conducted to determine the optimal nutrient concentration in solution in order to prevent stress caused by nutrient deficiency or overload. Temperature, conductivity, and pH were also investigated as physical variables. Furthermore, the utilization of wastewaters which is treated in hydroponic systems to yield plants which are commercially sustainable have earlier been reported. In recent times, treated wastewater has been proposed as a viable water resource for producing barley fodder in a hydroponic system. The proliferation responses and mineral nutrient content of barley provided with two distinct treated wastewaters were evaluated using a hydroponic culture investigation. The effect of treated wastewater on the deposition of heavy metals was also investigated (Cifuentes-Torres et al. 2020).

The wick type hydroponics, as its name indicates, feeds the nutrient solution to the plants via a Honeydew (2016) wick. Vermiculite, Pro-Mix, Coconut Fiber and Perlite are the most commonly used wicks. Since these plants’ roots are not immersed in the solution of nutrients, this technique is simple to sustain as this is a non-active mechanism with no parts that move, just not an air pump. The wick, on the other hand, can just supply the plants a quantity of water at once. As a result, larger plants may go hungry

Wick

(continued)

Drip (non-recovery or recovery) The system in this type of hydroponics utilizes drip to nourish the plants with nutrients from Joshi (2018) several reservoirs. Extra minerals are returned to the reservoir, or they are permitted to flow or vaporize. Previous one is referred to as recovery drain hydroponics, whereas the latter is referred to as non-recovery drain hydroponics. This approach has the advantage of being adaptable to any plant variety because the flow rate of nutrients can be modified. Nevertheless, due to the change of pH in the recovery method, it is tough to sustain

Hydroponics Water culture hydroponics is another name for DWC. This model’s growing media is Butcher et al. (2017) composed of Styrofoam, which travels straight in the solution of nutrients. The most challenging aspect of this mechanism is oxygen delivery. Because the plants’ roots are fully immersed in nutrients, the airstone system and an air pump supplies them oxygen. DWC is generally utilized in plants that require a lot of water to develop, like lettuce

DWC (Deep Water Culture)

References

Characteristics and substrate media

Types

Table 1 Different types of hydroponics, their characteristics and their substrate media

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Patten (2004)

Nutrient Film Technique (NFT) A nutrient solution is pumped into a slanted growth tray by this type of hydroponic system. The nutrients travel to the plate’s opposite end and are discharged into the tank. This scheme does not need a growing media or a timer. Moreover, because the system’s growth medium is air, the air pump is unnecessary. Apparently, when the nutrient flow stops, the plants wilt since these roots dry out comparatively quickly

Fogponics is a more effective version of aeroponics. The growth medium is also air in this case. Wootton-Beard (2019) Nevertheless, in aeroponics, a fog transmitter (referred to as a fogger) is utilized to create smaller droplets (generally range from 530 m) than those in fog emitters. The resulting fog transports nutrients and water to the roots of the plants. Fogponics is advantageous over aeroponics due to the following reasons: smaller (fog) particles stimulate greater nutrient absorption (Freguia et al. 2019a), fog can achieve additional components of the plant’s root than spray droplets, and the lack of a flow of nutrient solution permits efficient crop management

Calvin (1975)

This technique reduces the barrier that existed among the reservoir and the growth tray. The roots are left exposed onto the solution in the reservoir. At frequent intervals, a mist of the solution is spritzed onto the roots. The foremost benefit of this process is aeration. In addition, when contrasted to other systems, a smaller portion of nutrients is consumed. The system, on the other hand, requires a relatively short time span for the nutrient cycle. This equates to increased energy ingestion. Furthermore, if the timer or pump fails, the plants’ roots will quickly dry out

Aeroponics

Fogponics

A submersible pump is equipped in the nutrient to flood it, pumping the minerals up and into the Jones (2016) growth plate in this sort of hydroponics. A method is designed to help in the ebb and flow of the solution back into the tank. A timer controls the pump, turning it on to fill the growing tray and then turning it off to allow the solution to gradually trickle back into the reservoir. This flood-ebb cycle enables the root to obtain both a solution and oxygen (during the flood phase) (during the ebb cycle). Plant roots, on the other hand, are sensitive to disease. Because the plants’ lives are entirely dependent on the flood-ebb cycle, they are also dependent on the pump, the timer, as well as the drain system, and the system’s efficiency is severely reduced

Ebb and flow (drain and flood)

References

Characteristics and substrate media

Types

Table 1 (continued)

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• This technique can be utilized in the areas where in-ground crop production or planting trees are not feasible (e.g., cold climate regions or dry desert areas). • In this system pH, growing environment and nutrient content can be controlled more precisely. • Hydroponics can be used for growing plants faster due to more oxygen supply in the root area as nutrients are supplied directly to the roots. • Elimination or reduction of soil related insects, fungi and bacteria. • Hydroponics can be used for abolition or control of soil-borne bacteria, insects and fungi. • In this system fallowing or rotating of the crops is not required (Dunn 2013). • These systems produce the healthiest crops with the high yields and are reliably good; here crop production is easy and neat, needing less endeavor. As a whole, soil-less culture or hydroponics systems offers improved nutrient control, higher density farming, and higher yield per acre, as well as better quality traits. It is also useful in areas of the world where there has been a limited availability of arable or fertile land for production of crops (Sardare 2013).

4 Use of Wastewater Nutrients in Hydroponic System The utilization for wastewater in agricultural production is gaining popularity because it offers a better source of water and nutrients for crop varieties. Domestic wastewater is being used in farming in a number of developed and emerging nations. Aside from water reuse, there has been a rise in the seek for agricultural production processes that make the best usage of water (Carvalho et al. 2018). Studies based on the use of hydroponics for the treatment of wastewater generated some promising results. The emphasis was on the analysis of cost–benefit of a hydroponic wastewater treatment system versus traditional wastewater treatment, as assessed in terms of used resources, required work, consumption value or generated goods. Despite the fact that these studies found substantial distinctions among the two schemes, hydroponic wastewater treatment was found to need a higher initial expenditure, energy costs and labor (Schrammel 2014). The cost–benefit ratio, on the other hand, differs by geography, climate, and social awareness of the impact of wastewater on the ecosystem. For example, a scheme placed in a hot climate will expense additional costs of cooling the suitable environment of growth (Magwaza et al. 2020b). By discharging numerous metabolites, the plant root system modulates the living environment for the microorganisms observed in municipal wastewater, thereby improving the treatment of wastewater biologically through denitrification and nitrification techniques. Aside from removal of nutrients, the inclusion of hydroponics systems and municipal wastewater can aid in biomass yield for important crops, enhancing food security. This method is incredibly well-suited for agricultural recycling of municipal wastewater since it reduces the issues related to health posed by wastewater contact to workers, harvested crops, and consumers (Qadir et al.

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2010). Pathogen risks involved in wastewater reuse have been observed to be lowered when edible crops are harvested in hydroponic systems (Magwaza et al. 2020b). The benefit of this scheme in terms of pathogen contamination is influenced by the assertion that it enables distinct kinds of methodologies such as nutrient film technique (NFT), drip irrigation technique (DIT) and water culture, all of which are potent in diminishing risks when contrasted to various functionalities where other irrigation schemes such as sprinkler irrigation are commonly used (Oyama et al. 2008). Hydroponically produced tomatoes utilizing NFT showed no evidence of contamination of pathogens when compared to tomatoes produced in inoculated nutrient solution. Moreover, these observations highlight the significance of wastewater pre-treatment, hydroponic technique, crop selection and selection of irrigation systems, if treatment of wastewater is to be integrated with hydroponics (Magwaza et al. 2020b). The implementation of treatment of wastewater into the hydroponic plant development is referred to as a wastewater hydroponic system (Norström et al. 2004). Nutrients derived from wastewater treatment via microbial and physical decomposition are soaked up by plants in wastewater hydroponics. By producing value-added crops, such synergies provide numerous benefits over other bioremediation/phytoremediation strategies such as constructed wetlands (Roosta and Hamidpour 2011b). It needs less space, is less expensive, and can be executed onsite as a hydroponic technology based on wastewater (Norström et al. 2004). The hydroponic constituent functions as a tertiary or secondary treatment process for wastewater treatment, reducing the need for additional treatment of wastewaters to levels acceptable for disposal norms. As a result, integrating the two systems may provide viable strategies for crop production using renewable resources such as nutrients and water retrieved from domestic wastewater. This will save input expenses (irrigation and fertilizer) and energy, which are often used in traditional wastewater treatment plants and commercial hydroponic crop production systems, respectively (Azad et al. 2013). As a context of domestic wastewater dumping, the utilization of wastewater pollutants as nutrient materials for hydroponic agricultural yield has been extensively used (Yang et al. 2015; Oyama et al. 2005; Haddad and Mizyed 2011). This technology has been acknowledged as one of the most environmentally friendly wastewater management methodologies. Numerous researches have proven the potential of various forms of wastewater as an origin of fertiliser and irrigation water for hydroponic vegetation farming (Khan et al. 2011; Monnet et al. 2002). In such studies, a variety of different crops produced in hydroponic systems, ornamental plants (e.g. rose bushes and carnations) and including leafy vegetables (silver beets, lettuce and spinach), fruit crops (pepper, tomatoes and eggplant) were found to be appropriate for wastewater hydroponic systems. Wastewater’s ability to maintain growth of plants in hydroponics is because of its multi-nutrient composition, which enhances tomato plants’ chlorophyll content and rate of photosynthesis. It could be due to high quantities of micronutrients like zinc, copper, magnesium, and iron. The fertigation of tomato plants in a hydroponic system with ABR pollutants is insufficient to sustain plant growth of plant, according to studies. Plants fed a commercial fertilizer mix have poor development and output efficiency due to low concentrations of key nutrients such as Ca, P, N, Zn, and K in

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Fig. 3 Hydroponic wastewater treatment plant (Schrammel 2014)

wastewater. Plants cultivated in a hydroponic system with a fertilizer blend incorporated to ABR effluent, on the other hand, exhibited improved plant growth, yield efficiency, and shoot nutrient composition (Partially treated domestic wastewater as a nutrient source for tomatoes (Lycopersicum solanum) grown in a hydroponic system: effect on nutrient absorption and yield 2020). Microalgae can depollute wastewaters by removing mineral pollutants such as nitrate and phosphate (Delrue et al. 2016). In this perspective, microalgal growth on hydroponic farm wastewaters (HFWW) can be advantageous both in agricultural and the microalgal sectors in a win–win situation. On the one hand, microalgae can assist HFWW in reaching the concentration limits that allow it to be discharged into the environment in accordance with the regulations. Microalgae, moreover, can reveal the majority of their required nutrients in HFWW, which normally accounts for a significant amount of the expense of microalgal growth on artificial medium (Delrue et al. 2021) (Fig. 3). Risks of using wastewater in hydroponics system One of the key considerations is the effect of recirculating hydroponic feed water on pathogenicity. In hydroponics, frequent water replacements are anticipated to be costly and time-consuming. As an outcome, owing to uptake and evaporation, hydroponic operators often assess real time nutrient levels or through regular monitoring, and addition of fertilizers and water of necessary amounts. As a result, frequent microbiological testing of feed water as well as the preparation of solutions of nutrients using treated water are required to restrict diseases from spreading quickly through plants. Moreover, aside from the apparent cases of plant disease outbreaks, there are no established rules for how often fertilizer solution should be drained to trash and refill, rather than replenished as required. To see if such labor-intensive approaches have a positive impact on food safety in hydroponic systems, more research is necessary (Riggio et al. 2019).

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5 Nitrogen, Phosphorus Removal in Hydroponic Wastewater Treatment Plant Since phosphorus and nitrogen induce eutrophication in aquatic systems, eradicating unwanted substances from effluent seems to have become a widespread problem. Nevertheless, nitrate is indeed a hazard to public health concern, especially as a causative agent of infant methaemoglobinemia. In treating wastewater, influent is widely utilized, whereas effluent from sewage treatment plants frequently contains residual phosphorus and nitrogen. Residual phosphorus and nitrogen present in the form of ammonium and/or nitrate. An after-post-treatment scheme is needed to eliminate nitrogen and phosphorus from contaminants. However, since these effluents contain so few organic molecules, a source of energy (carbon source) must be introduced to eliminate nitrogen (Yamashita and Yamamoto-Ikemoto 2014). Phosphorus and nitrogen are two elements of worry in treating municipal wastewater, and nitrogen can be found in two forms including organic and inorganic forms (Ammonia–nitrogen, nitrate nitrogen and Total Kjeldahl Nitrogen (TKN)). Utilizing hydroponic systems with different plant species, nitrogen and phosphorus removal efficiency ranged from 47 to 91% (Vymazal et al. 2008). The ability of this system to remove nutrients is reliant on the relationships of bacteria, organic matter, plant roots, sediment, groundwater, and light. These elements help to remove contaminants from municipal wastewater, both in direct and indirect ways (Magwaza et al. 2020b). Methanol is frequently used in traditional post-treatment nitrogen removal techniques to remove nitrate (Timmermans and Haute 1983). Even though methanol has a high denitrification rate, there are issues about the dangers associated with outflow from a treatment system incorporating extreme the percentage of organic carbon existing, as well as the ignitability of the substance (Bill et al. 2009). In general, wastewater management administrators strive to establish operational costs for post-treatment as low as possible. Post-treatment methodologies that permit the utilization of waste products such as lumber waste, household trash, and agribusiness contaminants are thus cherished. Formerly, in an anoxic bioreactor employing timber as a natural substrate, nitrate has been efficiently removed from synthetic wastewater. Surprisingly, it was assumed that denitrification efficiency would improve sulfur denitrification by sulfate reduction via wood decomposition (Yamashita et al. 2011). Before devouring a wide variety of organic substances, sulfate-reducing bacteria use wood pellets or animal wastes as electron donors and carbon suppliers (Liamleam and Annachhatre 2007). Desulfovibrio sp. CMX, sulfate-reducing bacteria is often applied to reduce nitrogen oxide (NO) from iron/ethylene diamine tetra acetic acid (FeEDTA) solutions (Chen et al. 2013). Sulfide production arises in aquatic ecosystems as wood decays (Yücel et al. 2013). Sulfate-reducing bacteria are considered to have an essential role in the removal of nitrogen from wood. With either side, iron polarization produces ferrous ions that combine with phosphate to make vivianite as well as other ferrous phosphates, depleting phosphorus (Emerson and Widmer 1978). It was illustrated that steel wool was effective in the removal of nitrogen from denitrifiers that were capable of photosynthesis (Till et al. 1998). However, the weathering

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of iron is hastened in extreme nitrate environments. As a result, the inclusion of some other electron donor is beneficial in bolstering resistance to corrosion. As a result, We anticipated that a bioreactor containing iron and timber would be able to derive nitrogen and phosphorus in a homogeneous way. Evidently, our previous research demonstrated that both denitrification and phosphate recovery happened in a bioreactor packed with synthetic wastewater, loaded with iron and wood, and permitted to react for an extended period of time (Yamashita and Yamamoto-Ikemoto 2008, 2014). Physiological settlements, denitrification, and plant assimilation are the primary mechanisms in hydroponic wastewater treatment to eliminate nitrogen. Nevertheless, unless the plants are cultivated on a regular basis, plant nitrogen utilization doesn’t really depict this entire recycling method (Healy et al. 2007). Microorganisms degrade inorganic nitrogen primarily through denitrification, where nitrate is transformed into nitrogen gas, which is then emitted from sewage, resulting in nitrate disposal (Gebeyehu et al. 2018). Furthermore, microorganisms represent a significant approach in nutrient elimination by the modification and conversion of nutrients into their own feedstock. Various researches have already been carried out in evaluating plant adsorption capacities in hydroponic systems. Several factors are identified as influencing the extraction process of nitrogen in the hydroponic system (Saeed and Sun 2012). This comprises plant type, substrate parameters (oxygen content, pH, temperature and electrical conductivity), chelating agent additions and root zone (Prasad and Oliveira Freitas 2003; Merkl et al. 2005). This is because both plants and microorganisms are sensitive to all the parameters that impact the eradication of contaminants in soil or aqueous system. A link among temperature and nitrogen removal effectiveness was discovered while treating swine wastewater in hydroponic built wetlands. The inclusion of pig effluent in hydroponic constructed wetlands has really been attributed to temperature (Kadlec and Reddy 2001). For ammonification, nitrification, and denitrification, the optimal pH levels for nutrient removal are 7–9, 6.5–8.5 and 8–9. Both nitrification and denitrification can be harmed by a pH below the necessary range. This indicates that agronomic approaches should be developed that allow for temperature and pH modification, improving both wastewater purification and plant growth in hydroponic wastewater systems (Magwaza et al. 2020b). Photosynthetic microorganisms, such as microalgae have also been thoroughly researched for their ability to remove Nitrogen and/or Phosphorus from commercial, residential, and traditional farming effluents. Nevertheless, few researches have been carried out about the usages of microalgae for treating hydroponic effluent. Furthermore, there is no indication of the combination of chemical and microbial mineral precipitation for nutrient recovery. It’s possible that highly concentrated substances will form as precipitation., where under saturated elements, along with micronutrients and chemical substances might be utilized for microbial growth. In addition to the ability of photosynthetic microorganisms to remove nutrients, the biomass cultivated after microbial production has the ability to provide precious resources (added income streams) such as biodiesel, feed additives, and cosmetics (Lee et al. 2018).

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With a grownup discharging up to 500 L of urine per year, human urine is a valuable source for nutrient recovery among waste streams. Ammonia recovery is a less energy-intensive replacement to the Haber–Bosch process. Moreover, the process of regeneration implies an expense and reduction in energy consumption in fertilizer manufacturing as well as nitrogen removal from wastewater. P recovery provides a valuable alternative to phosphate mining. Besides that, one of the most appealing aspects of nutrient removal is it not only shuts the nutrient cycle but also lessens pollutants in the water habitats. In addition, human pee can be treated as a soil amendment for crops, and promising results have been reported when processed for nitrogen and phosphorus derivation as a fertilizer. Apart from faeces, urine is virtually aseptic, and thus poses a low ability to spread microbial ailments. Human urine contains a plethora of key nutrients, with a dried solid content of 13% C, 14–18% N, 3.7% P, and 3.7% K (Harder et al. 2019); however, it is high in salinity. Significant technologies are often used to retrieve urine, including electrochemical concentration, membrane distillation, struvite precipitation, stabilization plus distillation and ammonia stripping (Freguia et al. 2019b). Thus far, slight consideration has been devoted to the use of nitrogen- and phosphate-rich human excrement throughout hydroponics systems, to cultivate crops (El-Nakhel et al. 2021). Tall fescue uses a hydroponic system to lessen nutrients from sand-filter treated wastewater in a little span of time. Hydroponic system also presents a potential strategy for long-term wastewater purification. Plant-based processes may gain higher retrieval of N and P through biomass cultivation. While plants mature at distinct rates, multiple harvests of biomass are aimed at keeping the crops at the most optimum state for nutrient extraction. In part, nitrate and phosphate removal was aided by plant uptake via digestion and absorption. Microbes, together with plant root system and exposed burlap, are also suspected to have had a role in nutrient assimilation and nutrient absorption (Xu et al. 2014).

6 Conclusion The numerous researches on the feasibility of hydroponic systems for treating wastewater is currently on the rise, owing to their ability to decentralize wastewater treatment as well as enable agricultural manufacture of products that, when combined, can have a significant benefit leading to a positive contribution to environmental safety and food security. Moreover, it has been discovered that a variety of issues relating to the specifications for treatment methods in hydroponics wastewater systems. System type, types of plants, habitats, and processing conditions are only a few examples, which are obstructing the progress. The selection of each design appears to be influenced by operational aspects such as pH, EC, nutrient preservation duration, and irrigation facilities As a result, determining the best operational parameters is critical to ensuring maximum pollution elimination and efficient usage of the

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hydroponic wastewater scheme. The effectiveness of these systems in removing nutrients (phosphorus and nitrogen), contaminants, and pathogens is primarily determined by microbial decomposition, plant absorption, and filter medium adsorption. In this context, it was discovered that using hydroponics could help with nitrogen removal through microbial activities. The utilization of agricultural products in sewage water or plant cultivation systems, on the other hand, is linked to the risk of diseases such as microorganisms infecting humans. This theory highlights the need of selecting the appropriate substrates and plant species, as well as operational issues that can limit the risk of the scheme becoming infected by viruses.

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Industrial Wastewater to Biohydrogen Production via Potential Bio-refinery Route Pranjal P. Das, Deepti, and Mihir K. Purkait

Abstract Extensive and inappropriate use of water from industrial and other activities produce substantial quantity of wastewater worldwide. Wastewaters from different industries consist of significant quantity of nutrients viz. phosphorous, nitrogen and carbon. As such, the recovery of such nutrients through adequate sustainable technique has become a necessity. Amongst various available techniques, bio refinery routes utilizing dark fermentation and microalgae-based technologies have gained considerable recognition over the last few decades, along with its strategies for sustainable and cost effective treatment which allows degradation of more than 75% nutrient loads from wastewater. Comprehensive studies on the mechanism of bio refinery approach, its associated technologies and various microbial catalyst involved in bioenergy production from wastewater are extensively discussed and summarized in this chapter. The significant presence of value-added biomolecules in dark fermentation and harvested microalgae biomass along with its subsequent application in biohydrogen production has also been demonstrated. More apparently, the two stage coupling process and its possibilities towards potential bio refinery systems have been reviewed comprehensively. Comparative energy and economic aspects of biohydrogen production from industrial wastewater based on techno-economic analysis and life cycle assessment are also taken into consideration. Taken together, this chapter effectively summarizes the modern developments and enhancement strategies for improving the potential of low-cost bio refinery system for wastewater treatments and resource recovery, which can present new insights on assisting the bio refinery approach towards promising environmental applications. Keywords Biohydrogen · Dark fermentation · Industrial wastewater · Microalgae · Resource recovery

P. P. Das · M. K. Purkait (B) Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam, India e-mail: [email protected] Deepti Centre for the Environment, Indian Institute of Technology Guwahati, Guwahati, Assam, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_8

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1 Introduction Due to the exhaustion of fossil fuels, the demand for global energy has increased. Fossil fuels vary from biomass and as such that they are not contemplated as a renewable source, hence their use is restricted in some cases. The rise in global energy consumption and the diminishing supply of fossil fuels have pushed for the development of alternate sources of renewable energy. Thus, hydrogen is considered as the alternative energy since it emits no greenhouse gases (Banu et al. 2018). Various ways for producing biohydrogen from industrial effluents have been used; amongst them, the dark fermentation method seems like a viable option for pilot scale application from both practical and cost-effective standpoints. Despite the fact that biohydrogen generation by dark fermentation is a lucrative and low-energy method, its commercial relevance can only be realised by increasing the productivity and yield (Sridevi et al. 2014; Chandrasekhar et al. 2015). The most important criterion in biohydrogen generation is the selection of acceptable substrates. The use of a suitable substrate could boost the process productivity. Because of their high organic content, low nutritional requirement and positive net energy output, several biomass and few renewable sources are contemplated as promising substrates for the production of biohydrogen. Industrial wastewaters, in particular, are easily accessible sources which are classified as low-cost and widely available biodegradable substrates (Mishra and Das 2014). The food processing industry, sugar industry, rice mill, starch processing, paper mill, beverage, citric acid, cheese whey, chemical and pharmaceutical industries have all employed industrial effluents as substrates for biohydrogen generation. Bioconversion of complicated wastewaters need a large variety of microbial biocatalysts. The existence of methanogenic bacteria in fermentation procedures reduces the efficacy of microbial biocatalyst. In such scenario, choosing the right microbial biocatalyst along with its pretreatment is very critical for suppressing the methanogenic populations and promoting the development of hydrogen-forming microorganisms. To date, many well-known pretreatments viz. chemical, biological, physical and mechanical methods have been documented to improve the biohydrogen generation. Many studies have demonstrated the use of industrial wastewater as a biohydrogen substrate, along with the processing challenges and improvement tactics, as well as hybridization for the reclamation of products from dark fermentation effluents. The biological remediation procedure can achieve the goal of wastewater treatment while minimising natural resource use and maintaining the environment (Rajesh Banu et al. 2020). Using various microorganisms, several biological wastewater treatment techniques have been devised. Microalgae-based therapies are particularly advantageous because of their high biomass productivity rate, bioaccumulation efficiency and nutrient consumption. Microalgae are defined as eukaryotic, single-celled aquatic microbes that may be employed to remediate a variety of wastewaters through bioaccumulation, biodegradation, and bioadsorption methods. Microalgae absorb wastewater nutrients to create new biomass which may be utilised as a feedstock to produce

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valuable bioproducts viz. bioenergy (Wen et al. 2016). This might be a viable technique for achieving long-term socioeconomic growth through the use of the circular bioeconomy idea. However, there is still a lot to learn about building microalgaeassisted water treatment methods that are both effective and affordable. As a result, the focus of this study is on careful examination and assessment of various forms of industrial effluents as a potential substrate for bioenergy generation. The chapter also discusses the possibilities of different microalgae-assisted treatment techniques for effective recovery of wastewater resources. Finally, the limitations and enhancement strategies of microalgae are outlined for the generation of bioenergy from different effluents with a discussion on ecobiotechnological approach.

2 Microalgal Biorefinery Approach The use of wastewater for microalgae culture has various advantages, including (i) lower nutrient costs and (ii) removal of contaminants from effluents. Different microalgae species, including Tetraselmis sp., Chlorella sp., Chlorella sorokiniana, Scenedesmus sp., Chlorella pyrenoidosa and Nannochloropsis sp. are employed for bioremediation, and their biomass is used to make various bioenergy products. The bioremediation efficiency rate of microalgae strains is determined throughout the growth stage. The chosen microalgae species can lower the amount of nutrients in the effluent and employ the nutrients that are accessible for biomass formation. The biomass is collected and the biomolecules are recovered for bioenergy generation. According to several research, microalgae uses a variety of wastewater types, including industrial, municipal, domestic and agricultural effluents. The use of wastewater in conjunction with flue gas (atmospheric CO2 ) boosts the microalgae biomass production rate. Mehar et al. (2019) showed that microalgae may be farmed by utilising the food industrial effluents and ambient CO2 , with the biomass being employed to create bioenergy and beneficial compounds. Daneshvar et al. (2019) showed that the dairy wastewater may be exploited for biomass production by Scenedesmus quadricauda (S. quadricauda) and T. suecica (marine water) microalgae. The study also showed how to recycle dairy effluent three times for microalgae development (two mixotrophic and one heterotrophic). It was reported that, the biomass productivities of T. suecica and S. quadricauda were 570 mg/L and 420 mg/L, respectively, during the first cultivation (mixotrophic) cycle. Moreover, the biomass generation for T. suecica and S. quadricauda during the second cycle (mixotrophic) were found to be 640 and 0.40 mg/L, respectively. Furthermore, various microalgae and bacteria consortia (symbiotic association) may be used to clean the wastewater efficiently. Makut et al. (2019) examined the microalgaebacteria combination and cultivated Chlorella sp. DBWC7 and C. sorokianiana DBWC2 with two bacterias viz. Acinetobacter calcoaceticus strain ORWB3 and Klebsiella pneumoniae strain ORWB1, using synthetic wastewater and dairy effluent. In comparison to monoculture, the results demonstrated that microalgae-bacteria combination significantly aid in the decrease of nitrate and chemical oxygen demand.

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Fig. 1 Utilization of different effluent for microalgae cultivation and bioproducts application (Reproduced with permission from Goswami et al. (2021) © Elsevier)

The study also reported that, the microalgae-bacteria combination is a popular strategy for wastewater treatment and microalgae biomass production. Furthermore, valuable metals viz. uranium, silver and gold can be found in different types of wastewater. The removal of these expensive compounds possess long-term economic advantages for the society. Metal bio-adsorption by microalgae is a long-term solution for recovering extremely valuable minerals. Heavy metals get bio-adsorbed by microalgae throughout the growing phase via an intracellular mechanism. Tuzun et al. (2005) employed Chlamydomonas reinhardtii to recover lead (II), mercury (II) and cadmium (II) at pH 4–7 while optimising other parameters viz. temperature at 23 °C, light: dark cycle ratio at 16:8 and light intensity at 4000 lx. The results revealed that cadmium (II) and mercury (II) had higher absorption capacity at pH 6.0, whereas lead (II) showed higher absorption capacity at pH 5.0. According to this study, microalgae may be able to recover different types of metals from wastewater. Also, production of microalgae biomass throughout the effluent treatment can be utilised for a variety of purposes as shown in Fig. 1.

3 Mechanism of Wastewater Treatment via Biochemical Approach Microalgae is one of the most crucial biochemical route for the treatment of wastewater. Majority of the wastewater comprises a high nutritional load, as well as organic and inorganic carbon, phosphate, micronutrients, nitrite, and heavy metals. The direct absorption of surrounding contaminants or nutrient loads by microalgae wastewater treatment involves many biochemical pathways such as precipitation, bio-adsorption

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accumulation fixation, and assimilation. Symbiosis between microalgae and bacteria also aids in polymeric compound assimilation. Microalgae absorb both organic and inorganic carbon from different effluents in either mixotrophic or heterotrophic manner for metabolic control and biomolecule production, like lipids and carbohydrates. Additionally, microalgae absorb inorganic carbon such as bicarbonate ions as well as carbon dioxide via carbon concentrating processes and photosynthesis (Maheshwari et al. 2020). Equations (1)–(4) depicts the biological reaction of carbon nutrient assimilation and biomolecule synthesis. Where, Eq. (1) depicts the usage of bicarbonate ions with the help of carbonic anhydrase, Eq. (2) represents fixation of CO2 by carbohydrates formation and photosynthesis process, Eq. (3) shows the utilization of glucose to form pyruvate and acetyl-CoA, and Eq. (4) shows the formation of fatty acid (Goswami et al. 2021). + HCO− 3 + H ↔ CO2 + H2 O

(1)

light energy

H2 O + CO2 −−−−−−−→ Rubisco → Glyceraldehyde 3 − phosphate → Carbohydrates

(2) Glycolysis

Glucose −−−−−→ pyruvate → Acetyl CoA

(3)

Acetyl CoA → Malonyl CoA → FFAs → TAGs

(4)

Depending upon the source of wastewater, phosphate exist as either orthophosphate (inorganic) or polyphosphate (organic). Phosphate binding units in the cell wall of microalgae ingest several types of inorganic phosphate. Furthermore, the existence of some ions such as magnesium and carbonate in the effluent increases the precipitation of either magnesium or calcium phosphates at higher pH (around 8.5). In microalgae cells, polyphosphate is converted into orthophosphate, which is needed for the control of metabolic pathways that lead to the generation of ATP, phospholipids, nucleic acid and other compounds as indicated in Eq. (5). Furthermore, P uptake is affected by cell requirements and ambient circumstances. Surplus of phosphate is deposited in volutin granules, which are then employed to regulate metabolic pathways (Wang et al. 2017). phosphorylation

− Polyphosphate → H2 PO− −−−−−−−−−→ ATP + Nucleic acid + phospholipids 4 /HPO4 −

(5) ATP, nucleic acid and phospholipids is formed from polyphosphate, mono hydrogen, and dihydrogen assimilation from wastewater as shown in Eq. (5). Due to continuous reduction of nitrate to nitrite and then to ammonium ion, the inorganic nitrogen is transferred into the plasma membrane of microalgal cell. Later, the ammonium ions involved in the formation of amino acids transformed into proteins (Ghosh and Kiran 2017a). The net biochemical reaction where assimilation of nitrogen from the effluent into microalgal plasma membrane, including the formation of amino acid is given in Eqs. (6) and (7) respectively.

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L − glutamine (6)

L − glutamine → amino acids → Protein

(7)

Nitrate reductase

Nitrite reductase

− NO− −−−−−−−→ NH+ 3 −−−−−−−−→ NO2 − 4

→

Depending upon the source of effluent, different types of heavy metals viz. zinc, lead, cadmium, copper, and nickel may exist in varying amounts. Microalgae primarily absorb heavy metals via bio-adsorption process and collect them in various cellular chambers to control metabolism. In addition, microalgae cells possess different sets of binding groups such as, OH− , COO− , NO3 − , PO4 3− and RO− that effects the bio-adsorption process or aid in heavy metal binding. Microalgae absorbed the heavy metals and transferred them to cell vacuoles, where it binds to metallothioneins protein and prevents growth inhibition. Nevertheless, heavy metals particularly cadmium and lead at higher concentrations restrict algae growth by altering cellular protein, substituting critical components, and destroying algal cells (Zeraatkar et al. 2016). Microalgae bacteria consortia are another wastewater mechanism. Through respiration, microalgae might potentially produce oxygen, which is then ingested by aerobic bacteria in order to oxidise and convert the polymeric nutrient loads in the effluent into simple contaminants. At the same time, bacteria generate carbon dioxide, which is used by microalgae to build biomass. A schematic diagram of biochemical mechanism for wastewater treatment has been shown in Fig. 2.

4 Industrial Effluent as a Potential Renewable Substrate for Biohydrogen Generation via Biorefinery Approach The metabolic process involved in biohydrogen production, as well as the creation of by-products, are all influenced by the substrate features. Organic compound-rich substrates are extremely effective for increasing the biohydrogen generation via dark fermentation. Low-cost and easy availability of feedstock are seen as critical parameters for better biohydrogen production. In this regard, industrial wastewater has been described as a promising organic-rich, renewable, and low-cost substrate for the production of biohydrogen (Sivagurunathan et al. 2017). Because industrial effluent includes highly degradable organic materials, it results in the formation of net energy. Microbes digest industrial wastewaters differently depending on their organic content and properties. For example, effluents from the sugar industry, which are high in sugar and carbs, are easily digested by the bacteria. The ideal substrate for the production of biohydrogen was thought to be sugar-rich industrial effluents. Glucose, maltose, sucrose, and arabinose are the sugar molecules that play a major part in the formation of biohydrogen (Arimi et al. 2015). Food processing wastewater is regarded as an ideal candidate for biohydrogen generation since it quickly biodegrades and include highly hydrolysable compounds such as sugars and carbohydrates, apart from having a low inhibitor content. Although reasonable amounts of biohydrogen have

Fig. 2 Wastewater treatment mechanism via biochemical route (Reproduced with permission from Goswami et al. (2021) © Elsevier)

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been obtained without any pre-processing treatments from wastewaters generated by various food processing industries, however most of the food processing industry wastewaters require dilution to decrease the organic load, that may be inhibitory to biohydrogen generation (Ntaikou et al. 2010). Cappelletti et al. (2011), for example, used high organic rich cassava effluent as a feedstock for the production of hydrogen. Due to the increased organic load, the study reported lower biohydrogen generation and substrate conversion efficiency. Nevertheless, when the organic load of the substrate was lowered, they were able to reach a greater hydrogen formation rate of 2.41 mol H2 /mol glucose. According to Karadag et al. (2014), the substrate transformation prospect to biohydrogen was reported to be poor, which might be due to the partial biohydrogen consumption throughout protein breakdown. The study found a biohydrogen output of 15–145 mL H2 /g COD, which is quite low. Ramprakash and Muthukumar (2018) examined the impact of rice mill effluent’s biohydrogen generation potential. To liquefy the macromolecules, they pre-treated the rice mill effluent with sulphuric acid at a concentration of around 1.6%. The study reported a greater biohydrogen generation rate of 1.61 mol H2 /mol sugar. Glycerol-rich wastewater from olive mill, palm oil mill and biodiesel plants, produced as a byproduct of oil refineries are all considered as promising substrates. It can also be utilised as a substrate for the formation of biohydrogen owing to its high fat content (Mahapatra et al. 2013). According to Eq. (1), Akutsu et al. (2009) assumed that the theoretical maximum hydrogen generation yield from glycerol was 3 mol/mole of glycerol: + C3 H8 O3 + 2H2 O → CH3 COO− + HCO− 3 + 2H + 3H2

(8)

Industrial wastewaters have been considered to be appropriate substrates for successful biohydrogen formation from an economic standpoint. Based on their organic content, various types of industrial effluent biodegrade in distinctly separate ways.

5 Microbial Biocatalysts 5.1 Pure Strain Biocatalysts Pure strains are of specific interest in biohydrogen formation because of their unique characteristics, viz. selective utilisation of substrate, ease of metabolic pathway modification in growth conditions and increased output. Various pure strain biocatalysts have recently been investigated in order to manufacture biohydrogen using industrial wastewater as a substrate. Methylotrophs, rumen bacteria and clostridia are some of the stringent anaerobes, while others are facultative anaerobes, viz. Escherichia and Enterobacter coli. Clostridium and Enterobacterium have been identified as the important bacterial species engaged in dark fermentative hydrogen generation

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(Ghimire et al. 2015; Li et al. 2012). Cappelletti et al. (2011) tested the biohydrogen generation capacity of Clostridium acetobutylicum, a pure strain isolated from effluent of cassava processing, in a bioreactor at a mesophilic temperature and pH of 7. The organic load of the reactor was observed to be 5 g COD/L, and the resultant biohydrogen production was 2.40 mol H2 /mol glucose. In another study carried out by Mishra and Das (2014) used a pure biocatalyst, Enterobacter cloacae IIT-BT 08, to treat distillery effluent as a substrate in a batch reactor at pH 5.5, and mesophilic temperature, produced a biohydrogen yield of 175.3 mL/g COD. Ramprakash and Muthukumar (2015) studied the possibility of employing rice mill effluent as a substrate in a batch reactor with pure strain Enterobacter aerogens RM08 at temperature 33 ± 2 °C and pH 6–7. The yield of biohydrogen was found to be 2.0 mol of H2 /mol of reducing sugar.

5.2 Mixed Culture Biocatalysts Mixed cultures are mostly suitable for use on a large scale. This might be due to the lack of a demand for medium sterilisation, which reduces the cost of the process. The most promising source of biohydrogen from industrial wastewaters has been demonstrated to be mixed cultures generated from anaerobically digested sludge, soil, and slaughterhouse sludge. Using mixed cultures in large-scale industrial processes might improve biohydrogen production. When compared to pure cultures, mixed cultures have a healthier activity, such as the direct wastewater utilisation, utilisation of a larger range of organics and resistance to process factors such as, pH, organic load and temperature (Niu et al. 2010). O-Thong et al. (2011) studied the biohydrogen generation of an enhanced thermophilic mixed culture in a continuous stirred tank reactor (CSTR) utilising cassava starch processing effluent as a substrate. The working conditions of a reactor were stated to be temperature, pH, organic load and HRT as 60 °C, 5.2, 9.2 g starch/L, 5 days respectively. Table 1 represent the potential of biohydrogen formation by pure strain and mixed culture microbial biocatalyst.

6 Microalgae-Based Technologies 6.1 Low-Cost Microalgae Cultivation Strategies The mass culture of microalgae using various cultivation tactics is the first step in the downstream biorefinery process. The fabrication of a simple, low-cost photobioreactor might provide widespread microalgae farming while also simultaneously treating the targeted wastewater. This integrated method transforms the contaminants into nutritional medium, thereby increasing the algal biomass and ensuring a long-term viability of low-cost mass microalgae culture (SundarRajan et al. 2019).

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Table 1 Biohydrogen generation prospect of pure strain and mixed culture biocatalysts Type of wastewater

Bioreactors involved

Operating conditions

Microbial biocatalysts

Hydrogen yield

References

Pure strain biocatalysts Rice mill wastewater

Batch reactor pH: 6.5–7; Enterobacter 1.97 mol temperature: aerogens RM08 H2 /mol sugar 33 °C; organic load: 10.2 g starch/L

Ramprakash and Muthukumar (2015)

Distillery effluent

Batch reactor pH: 5.5; Enterobacter temperature: cloacae 37 °C; organic IIT-BT08 load: 59.0 ± 2.0 g COD/L

165.3 mL/g COD

Mishra and Das (2014)

Rice mill wastewater

Batch reactor pH: 6.5–7; Enterobacter temperature: aerogenes 33 °C; organic load: 10.2 g starch/L

1.74 mol H2 /mol sugar

Ramprakash and Muthukumar (2014)

Cassava processing wastewater

Batch reactor pH: 7; Clostridium temperature: acetobutylicum 36 °C; organic load: 5 g COD/L

2.41 mol H2 /mol glucose

Cappelletti et al. (2011)

Mixed culture biocatalysts Beverage wastewater

Continuous stirred tank reactor (CSTR)

pH: 6.3; temperature: 37 °C; HRT: 1.5 h; organic load: 20 g COD/L

Enriched mixed 1.50 mol/mol culture substrate

Cassava processing wastewater

Continuous stirred tank reactor (CSTR)

pH: 5.2; temperature: 60 °C; HRT: 5 days; organic load: 9.2 g starch/L

Enriched thermophilic mixed cultures

2.49 mL H2 /g O-Thong et al. starch (2011)

Tofu processing wastewater

Continuous stirred tank reactor (CSTR)

pH: 5.5; temperature: 60 °C; HRT: 8 h; organic load: 15 g starch/L

Anaerobically digested sludge

1.2 mol H2 /mol hexose

Sivagurunathan et al. (2015)

Kim et al. (2011)

(continued)

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Table 1 (continued) Type of wastewater

Bioreactors involved

Operating conditions

Microbial biocatalysts

Hydrogen yield

Palm oil mill effluent

Anaerobic sequencing batch reactor (ASBR)

pH: 5.5; temperature: 60 °C; HRT: 48 h; organic load: 60 g COD/L

Enriched sludge 0.27 L H2 /g COD

References Prasertsan et al. (2009)

Microalgae production in the mixotrophic mode is an effective way to remove both the organic carbon and nutrients from the wastewaters. As previously mentioned, wastewater includes a variety of macro and micronutrients, along with high organic and inorganic carbons. These nutrition sources considerably assist in the growth and formation of more biomass and biomolecules. To reduce the cultivation costs, this technique may be paired with a hybrid carbon dioxide-wastewater system. Microalgae possess the capacity to fix CO2 from the atmosphere, as well as flue gas, through photosynthesis. It increases their biomass by using CO2 as a carbon source (Ghosh and Kiran 2017b; Lowrey et al. 2015). It can fix CO2 faster (10– 20 times) compared to the terrestrial plants, while consuming 172 tonnes of CO2 to generate 100 tonnes of microalgae biomass. According to Prathima Devi and Venkata Mohan (2012), supplementing CO2 with residential wastewater increased the lipid accumulation. Similarly, Nayak et al. (2016) found that using wastewater and CO2 to cultivate Scenedesmus sp. results in increased lipid accumulation. The lipid can then be utilised to make biofuel, whereas Tu et al. (2019) reported that fuel gas from power plants, may be employed. The results revealed that microalgae can fix CO2 (210 mg/L/d) in C. pyrenoidosa, leading to an increased biomass concentration (84.92%) and lipid productivity (74.44%). Another option is to recycle the glycerol that is produced as a byproduct of the transesterification method. Glycerol may be employed as a carbon source to impact the microalgae development viz. mixotrophic and heterotrophic microbes (Ehimen et al. 2009). Abomohra et al. (2018) investigated the effects of glycerol and lipid-free algal hydrolysate on biomass production and lipid accumulation in S. obliquus. The study found that, glycerol increased the production of biomass and accumulation of lipid (59.66 mg/L). According to Chen et al. (2020), using glycerol for mixotrophic culture increases both the lipid and biomass output up to 75% in Thraustochytrium sp. BM2. Furthermore, Zhang et al. (2020) showed that glycerol increased the lipid production by boosting the fatty acid synthesis and astaxanthin agglomeration during the growth of Heamatococcus pluvialis in low light conditions with glycerol. Furthermore, carbohydrates are fermented to generate bioethanol (the major product) and carbon dioxide as a by-product. The carbon dioxide produced as a byproduct can be employed as a carbon source for the development of microalgae. Rashid et al. (2019) found that growing Chlorella sp. and Ettlia sp. together optimises the productivity of biomass. The rate of biomass productivity in monoculture was 250 mg/L, which increased to

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700 mg/L in co-culture, thereby indicating that mixotrophic agriculture is more effective compared to autotrophic farming. Furthermore, for biohydrogen production, Fakhimi and Tavakoli (2019) co-cultivated Chlamydomonas sp. with Escherichia coli, Pseudomonas stutzeri and Pseudomonas putida. It was observed that bacteria consume oxygen and generate an anaerobic environment, which triggers the biohydrogen generation. Nevertheless, the growth of Chlamydomonas sp. was reduced by E. coli while simultaneously improving the biohydrogen productivity. A simplified representation of low-cost biorefinery process of microalgae has been shown in Fig. 3.

6.2 Downstream Processes Harvesting of biomass might be dependent on different species of microalgae, bioactive chemicals, and other factors. As a result, sedimentation technologies can lower the cost of biomass harvesting, whereas incorporation of organic content externally, helps in the improvement of sedimentation method. Moringa oleifera seed powder was used by Ogbonna and Edeh (2018), which improved the sedimentation process by 70% within 30 min. Additionally, Moringa oleifera seed powder combined with sedimented algal biomass can be used for anaerobic co-digestion to produce biogas. Besides, bio-flocculation is an advanced cost-effective approach that harvests microalgae biomass using filamentous fungus and bacteria. According to Zhou et al. (2017), filamentous fungus spores are implanted in microalgae culture vessels or bioreactors and incubated for 48–72 h. Pellets generated after incubation can be smoothly harvested via filtration. Likewise, Nguyen et al. (2019) reported that the production of microalgae-bacteria based bio-flocs significantly assist in the removal of wastewater nutrients (88 ± 2.2%) and provides high flocculating efficiency (92 ± 6%). Bio-flocculent is a cost-effective harvesting technique which can lower the operating costs while simultaneously allowing the biomass to be utilised for biofuel production. Furthermore, biofilm development is a low-cost harvesting strategy in which numerous nutritional stressors aid the formation of biofilm in microalgae (Schnurr and Allen 2015). Furthermore, for bio-metabolites extraction, numerous sophisticated eco-friendly alternative technologies have been produced, including supercritical fluid extraction, enzyme based extraction, ultrasound based extraction and microwave based extraction. Di Sanzo et al. (2018) extracted astaxanthin via supercritical CO2 extraction methods (carotenoids). Osmotic shock aided the liquid biphasic systems, and can be considered as an excellent approach for protein extraction. This is a unique extraction method for microalgae-derived compounds. The biological extraction process has a lot of advantages over other techniques. Cellulase, endo-β-1,4-mannanase, pectin-lyase, lysozymes, endo-β-1,4-xylanase, polygalacturonase, β-D-glucosidase and endo-1,3 (4)-β-glucanase are some of the biological enzymes utilised to retrieve the biomolecules from microalgal biomass. The optimised enzyme mixture aids in the recovery of various biomolecules. Simple purification procedures are required for the manufacturing of biodiesel and bioenergy

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Fig. 3 Systematic illustration of low-cost microalgae cultivation strategies (Reproduced with permission from Goswami et al. (2021) © Elsevier)

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products, which may be performed utilising a solvent system (Zuorro et al. 2015). However, advanced purification processes are necessary for biomolecule purification of various bioproducts. The purification of PUFAs and lipids necessitates rupture of the cell wall in order to produce a high oil yield. Traditionally, lipids have been extracted using various solvent ratios (methanol, chloroform, isopropanol or hexane). Winterization and fractional distillation are also employed to separate a segment of PUFAs from the total lipids. Furthermore, these extracted PUFAs include a hazy appearance, foul odours, contaminants and a terrible taste that make them unfit for human ingestion. To improve the quality and shelf life of PUFAs, deodorization, bleaching, advanced filtering, polishing and the incorporation of antioxidant compounds are necessary. Furthermore, the use of such type of solvents poses major environmental and health risks. The utilization of green solvent as well as solvent recycling approach to replace the traditional extraction techniques (organic solvent) helps in enhancing the biorefinery of microalgal-assisted purification and extraction of PUFA (Wynn et al. 2010).

7 Limitations and Improvement Routes 7.1 Microbial Ecosystem Issues 7.1.1

Methanogens

During the utilization of mixed culture for dark fermentation, the hydrogenotrophic methanogens are considered to be the highest hydrogen-consuming methanogens. The use of hydrogen as the principal substrate to reduce CO2 for CH4 formation is a very crucial process during hydrogen reduction. Hydrogenotrophic methanogens, such as Methanobacteriales and Methanomicrobiales mediate this process. As a result, the hydrogen produced in the dark fermentation process is used via the hydrogenotrophic methanogenic route, resulting in a lower hydrogen production yield (Hwang et al. 2004). The pretreatment procedures such as freezing/thawing, aeration and sonication are insufficient to prevent CH4 generation. Methanogenic bacteria are mostly destroyed by heat shock pretreatment of the inoculum. Furthermore, methane-producing microbes are often low pH-sensitive. According to Yang et al. (2007), the presence of methanogens in constantly fed hydrogen generating reactors was found to be greater at longer hydraulic retention time due to their reduced growth and development. Nonetheless, it has been found that in a large lab-scale fermenter, methanogen activity cannot be completely suppressed.

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Propionic Acid Producers

For the accumulation of propionic acid, microorganisms that produces it require hydrogen as an electron donor. Furthermore, microorganisms such as Clostridium propionicum and Clostridium homopropionicum produces propionic acid by the decomposition of lactic acid via the propionate route and consuming NADH. As a result, biohydrogen production undergoes adverse effects. Propionic acid-producing bacterias may also utilize hydrogen directly from NADH, which suppresses dark fermentation by reducing the biohydrogen generation and increasing the propionate production. Fermentation of propionic acid is a method for obtaining hydrogen. With negligible gas production and a long retention period, this route mostly produces valeric acid, acetic acid and propionic acid (Guo et al. 2010). Pattra et al. (2011), for example, found that propionic acid generation is inextricably linked to HRT, and the increased retention time results in an increased propionic acid accumulation. Sivagurunathan and Lin (2016), for example, reported on propionic acid accumulation in a continuously stirred tank reactor (CSTR) used for the processing of real industrial effluent from a beverage facility. The study claimed that propionic acid was accumulated as a result of the activities of propionic acid bacteria such as Bifidobacterium catenulatum and Selenomonas lacticifex.

7.1.3

Sulfate-Reducing Microbes

Microbes that reduces sulphate to hydrogen sulphide typically utilize hydrogen as an electron source. In the hydrogen consumption route, sulphate reduction is the main process. The agglomeration of additional SO2− 4 in the substrate, viz. effluent from pulp and paper companies, distilleries, marine food processing businesses and swine effluent, facilitates the growth of a highly competitive sulphate-reducing bacterial population. So far, there has been relatively little research on the influence of sulphide inhibition on hydrogen formation via dark fermentation (Guo et al. 2010). Dhar et al. (2012) evaluated the direct effects of sulphide on hydrogen-generating bacteria and found that the concentrations of sulphide (more than 100 mg/L) in soluble form may completely hinder the dark fermentative hydrogen generation process. Sulphide concentrations of around 25 mg/L, on the other hand, have been found to be crucial for increasing the biohydrogen generation. Furthermore, Hwang et al. (2011) examined the formation of biohydrogen from effluent containing high SO2− 4 concentration and found that SO2− 4 content greater than 2 g/L can severely inhibit the hydrogen formation, possibly due to the competition for substrates amongst the sulphate reducers and the hydrogen formers.

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7.2 Enhancement Strategies 7.2.1

Culture Enrichment

Culture enrichment is an improvement procedure for selection and enrichment of specific microbes. Mixed microflora enrichment strategy boosts the biohydrogen output while inhibiting the growth of hydrogen-consuming bacteria like methanogens and homoacetogens. Many studies have described various enhancement procedures for obtaining an augmented and versatile bacterial species with distinct functional characteristics from a mixed population of bacteria. For appropriate acclimatisation of mixed microflora to particularly less biodegradable substrates in both continuous and batch reactors, community shifts and adaption techniques are used (Wang and Wan 2009). Wang et al. (2010) reported on a novel enrichment strategy known as “dilution to extinction”. The authors used this method to determine the particular microbial community, demonstrating an intriguing unique function by serially diluting the initial mixed microbial population of bovine rumen fluid. Furthermore, three strains viz. Succinivibrio sp., Butyrivibrio sp. and Ruminococcus sp. were determined at a dilution of 107, however at a dilution of 110, the bacterial strain Ruminococcus sp. disappeared, resulting in inadequate cellulose biodegradation and biohydrogen yield. Ruminococcus sp. appears to have a crucial symbiotic function with the two other prevalent strains. Another method of culture enrichment is to place the culture in a nutrient-rich media. In this context, Sen and Suttar (2012) used heattreated cultures that were enriched in peptone yeast extract (PY) agar media to treat sago-processing effluent, thereby yielding a highest hydrogen output of 130.5 mL/g COD. Likewise, Sivagurunathan et al. (2014) used PY media for culture enrichment in another investigation. By applying a PY medium supplemented culture for treating beverage effluent, they were able to obtain a hydrogen generation rate of 1.8 L/L-d.

7.2.2

Co-culture Fermentation

Owing to the collaborative link between the numerous metabolic pathways of all the strains involved, fermentative biohydrogen generation by co-culture appears to be more helpful than single culture fermentation. This might have enhanced the product formation and the capacity to use low-cost substrates like industrial effluents. To begin with, these symbiotic positive interactions take place between diverse hydrogen-producing bacterias belonging to the Clostridium genus. Vatsala et al. (2008) for example, conducted a detailed investigation on co-culture fermentation. During fermentation of molasses water (batch process) obtained from a glutamate producing factory, the study reported on the cooperative interaction of glutamatebased strain C. sporosphaeroides with carbohydrate-degrading strain C. pasteurianum, along with an improvement in the biohydrogen output by 10–200% compared to pure strain biocatalyst. These strains have a variety of metabolic pathways and substrate biodeterioration capabilities, thus, reducing the substrate competition and

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forming a successful symbiotic mixed-culture interaction. Injection of prospective acidogenic bacteria with local anaerobically acclimatised bacteria by Goud et al. (2014) demonstrated an elevated biohydrogen generation rate of around 2.0 L, which is about 170 times more than the local inoculum of 0.13 L with an initial organic content of 60 g/L.

7.2.3

Ecobiotechnological Approach

Ecobiotechnology can be used as an alternative technique to boost the biohydrogen generation. The ecobiotechnological technique depends on the natural selection and competition concept, which involves applying selective stress to a chosen metabolism in a diverse microbial community, depending upon the substrate and the bioreactor operating parameters. The environment (ecology) gets bioengineered, and not the bacteria. Valdez-Vazquez and Poggi-Varaldo (2009) used a biokinetic control to select fermentative bacterial species. Apart from the relevance of examining the structure and quantity of fermentative bacteria, a better understanding of the working potential of hydrogen-consuming microorganisms would significantly aid in improving the operational settings of the reactor to inhibit their proliferation and as such, intensify the biohydrogen generation. Few Clostridium sp. homoacetogenic bacteria, viz. C. aceticum, exhibit the same growth characteristics as hydrogenproducing bacteria. As a result, eradicating such hydrogen-consuming microorganisms from Clostridium derived ecosystems poses a big threat. Homoacetogens, on the other hand, have been found to consume hydrogen only when hydrogen accumulation is near saturation. Thus, homoacetogen activity can be suppressed by maintaining a constant supply of hydrogen or lowering the partial pressure of hydrogen (Chaganti et al. 2012).

8 Conclusion When compared to other processes, hydrogen generation via biological approach has the advantage of being able to employ organic-rich industrial effluents while reducing the greenhouse gas emissions. Several studies have investigated the traditional factors that influence the hydrogen generation. The microbial biocatalysts responsible for biohydrogen generation, as well as its enrichment, have been discussed in order to enhance the process output. The low substrate conversion (typically less than 30%) and the limited production of 4 H2 /glucose are the two key challenges that drive up the cost of the procedure. As a result, process integration is the most effective way to overcome such constraints. Microalgae-based techniques have a high nutrient recovery efficiency and can reduce the environmental contaminants significantly. Bioenergy generation and nanoparticle synthesis can both benefit from the recovered biomass. The microalgae biorefinery concept can significantly lower the effluent treatment cost, while also providing long-term solutions for achieving a circular economy

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and greener production. Cost-effective microalgae biorefineries need careful bioflocculants selection for harvesting and enzyme-assisted biomass extraction, in order to maximize the resource recovery and biofuel conversion along with other valueadded products. For the fabrication of industrial microalgae biorefinery systems, however, pilot-scale research is needed. In order to maximise the utilisation of renewable resources, generation of microalgae biomass and efficiency of its end products, special focus should be given to the hybridization of microalgae-derived biorefinery approach with different effluent treatment techniques.

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Development of a Novel Upflow Anaerobic Sludge Blanket (UASB) System for Treating Milk Wastewater Khac-Uan Do, Dac-Chi Tran, and Gia-Khanh Nguyen

Abstract Milk processing wastewater flows come from many sources with different characteristics, mainly detergent, condensate, washing water and product parts discharged into the common sewer system. It can be seen from the production process that cleaning water accounts for the largest proportion, in terms of quantity and pollutant concentration. The upflow anaerobic sludge blanket (UASB) reactor can be used as part of a high-load wastewater treatment system. It produces biogas that can be converted into energy. Also, it generates less sludge compared to aerobic systems. UASB could be a suitable technology for highly polluted wastewater. UASB technology has been applied in food and milk wastewater treatment. UASB could be operated at different conditions. Sludge recirculation from sedimentation column to the anaerobic reactor could be a good solution to enhance the performance of the UASB system. Keywords Milk wastewater · Biogas · UASB · Treatment · Energy

1 Introduction Milk contains high nutrient contents such as proteins (3.5%), casein (2.8%), fat (3.7%), mineral (0.7%) and other vitamins (Kaur 2021). During the production process, a large of water was used to wash the pumps, tanks and cans. Besides, water was used to rinse the floor at the end of each operation cycle (Demirel et al. 2005; Struk-Sokołowska et al. 2018). It can be seen from the production process that cleaning water was accounted as the largest proportion (Slavov 2017). Therefore, wastewater was generated from those activities. In particular, it was come from equipment washing, condensate water, and factory washing. It was reported that milk K.-U. Do (B) · D.-C. Tran School of Environmental Science and Technology, Hanoi University of Science and Technology, Ha Noi, Vietnam e-mail: [email protected] G.-K. Nguyen Environmental Engineering K58, School of Environmental Science and Technology, Hanoi University of Science and Technology, Ha Noi, Vietnam © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_9

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industry generates large amount of wastewater (Alayu and Yirgu 2018). Basically, it could produce about 0.2–10 L of wastewater per one liter of milk. Milk wastewater flowrate was varies significantly depending on the milk production processes. The compositions of milk wastewater were also depended on the type of milk products (Passeggi et al. 2009). Whey is the most polluting wastewater that contains high organic matter compositions (i.e. lactose, protein, phosphorous, nitrate, nitrogen). It was polluted than 60–80 times higher compared to domestic wastewater (Zandona et al. 2021). The characteristics of milk wastewater could contain high concentration of organic matter and nutrients (Rivas et al. 2010; Tikariha and Sahu 2014). More importantly, it can contain some leaking milk. Therefore, it contain fats, proteins, sugar which contribute high pollutants to the waste stream (Ekka et al. 2021). In addition, during cleaning process, chemicals were used to clean equipment, so that, it can also increase the pollution of washing water. Generally, milk wastewater could be diluted or mixed with domestic wastewater of workers. The main elements contributing to COD and BOD in milk wastewater are lactose, butter, protein and lactic acid (Sinha et al. 2019). It was reported that 1 mg of lactose was equivalent to COD concentration of about 1.13 mg, whereas, 1 mg of protein was corresponding to COD value of about 1.36 mg. Milk wastewater contains mainly the organic compounds (Menchik et al. 2019). It also has high nutrient, such as nitrogen in the organic form, while nitrogen in ammonium and nitrate forms are much lower than in the organic form. Besides that, the phosphorus in mild wastewater is normally lower than the nitrogen. It should be noted that, TN and TP in milk wastewater were strongly depended on the milk products (Gil-Pulido et al. 2018). Nitrogen was existed mainly in proteins, particularly in amino groups. Other forms of nitrogen compounds were also found in the wastewater, isuch as urea, uric acid, NH4 + , NO2 − and NO3 − (Licata et al. 2021). Phosphorus compounds are mainly in the inorganic forms, including phosphate (PO4 3− ) and diphosphate (P2 O7 4− ). It also existed in the organic form. It was reported that TN in milk wastewater could be about 4.2–6% of BOD concentration, whereas TP was only about 0.6–0.7% of BOD. It is obviously that TN and TP in milk wastewater could contribute a risk of eutrophication in the receiving water (Gil et al. 2019). Basically, milk wastewater is neutral or slightly alkaline. However, in the low dissolved oxygen condition, it tends to become acidic quickly. This is due to the sugar was fermentated into lactic acid, which reduces pH value. In some factories, wastewater was then collected and discharged into the common sewer system. Many milk factories discharge wastewater into domestic sewer system. Some of them could discharge milk wastewater by irrigation in the field which could result in pollution of surface and ground waters. Milk wastewater could have high COD concentrations which could contribute high pollution loads discharged to the environment (Tikariha and Sahu 2014). The average COD of milk wastewater is about 3800 mg/L. Finally, it could contribute the pollution in the receiving water bodies. Therefore, treating milk wastewater is important to protect the environment. In fact, several treatment technologies such as chemical precipitation (Banu et al. 2008), biological anoxic–oxic (Do et al. 2009, 2013), anaerobic-anoxic–oxic (Uan et al. 2013), membrane bioreactor (Chen and Uan 2013; Erkan et al. 2018; Saddoud

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et al. 2007), or combination of biological processes with membrane technologies (Banu et al. 2009, 2011; Rajesh Banu et al. 2009; Ribera-Pi et al. 2020) could also be applied to treat milk wastewater. UASB could be used for domestic wastewater at low temperature of only 10 °C (Xu et al. 2018). Combination of anaerobic digestion with membrane bioreactor was used to treat milk wastewater (Lutze and Engelhart 2020; Szabo-Corbacho et al. 2021). Multi-section horizontal flow reactor was used to evaluate the dairy wastewater. In this reactor, dairy wastewater was digested anaerobically (D˛ebowski et al. 2020). So far, anaerobic process, such as UASB, can be used to treat high strength wastewater effectively (Dawood et al. 2011; Gavala et al. 1999; Yahi et al. 2014; Shah 2020). This process will produce biogas which could be converted into energy for the plant (Anukam et al. 2019; Birwal et al. 2017; Kavacik and Topaloglu 2010). More importantly, this process will generate less sludge compared to the aerobic systems (Ahmad et al. 2019; Ozturk et al. 2019). Therefore, UASB systems could save operating costs compared to the aerobic systems (Sinha et al. 2019). UASB has been used widely in treating wastewater from food and milk industry (Buntner et al. 2013; Goli et al. 2019). UASB technology has also been applied for food wastewater treatment (Arunadevi and Saravanaraja 2020; Talaiekhozani 2019). During treatment process, to the organic matters will be converted into biogas (Oz and Uzun 2015). The biogas generated from UASB can have a relatively high CH4 content (60–70%). Therefore, it can be used in plants as an alternative energy for incinerators, generators. Thus, it helps to save the operational cost in the wastewater treatment plants. Example, in the United Kingdon, a UASB system of 2000 m3 for whey’s effluent treatment could produce biogas which was used enough for the total required energy for the plant (Ahn et al. 2001). Improving the efficiency of milk wastewater treatment with anaerobic technology using UASB is interesting and necessary. It should be noted that, the anaerobic process could take long time to achieve high efficiency (Bella and Rao 2021; Sarkar et al. 2006). Besides, this process could not remove nutrients (i.e. Nitrogen, Phosphorus) which were important elements in wastewater generated from the food and milk industry (Carvalho et al. 2013; Numviyimana et al. 2020, 2021). Therefore, it is necessary to combine UASB with aerobic treatment to remove nutrient effectively. The aim of this work is to provide a development of a novel UASB system to treat synthetic milk wastewater. The potential of the anaerobic process by using this novel UASB was examined. In particular, the effective of UASB for treating milk wastewater was determined at different operational conditions. The possible development of the UASB system for the treatment of milk wastewater was also investigated.

2 Development of Novel Lab-Scale UASB System A novel lab-scale UASB system was developed and presented in Fig. 1. The UASB system include several components as follows. A feeding tank (1) is made of steel. The working volume is about 80 L (The dimension (length × width × height) was 600

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Fig. 1 A novel lab-scale UASB system. where: 1. Feeding tank; 2. Mixing pump; 3. Heater; 4. Influent pump; 5. Recirculation pump; 6. Blending container; 7. UASB reactor; 8. Sludge layer; 9. Water layer; 10. Sampling valve; 11. Sedimentation column; 12. Gas valve; 13. Gas recirculation valve; 14. 15. 16. Exhaust valve

× 310 × 465 mm). A cover is used to prevent the smell from wastewater contained in the tank. A cylindrical blending container (6) with a volume of 3.2 L, made from uPVC plastic was used to distribute the influent wastewater in to the bottom of the UASB reactor. In this system, a cylindrical UASB (7) made of steel has a total volume of 68L in which the working capacity is 60 L. The UASB reactor is divided into 2 parts with different diameters (30 and 28 cm diameter). A sampling point (10) is placed at middle of UASB reactor to check the sludge (SS) concentration. A tube was placed at the top of UASB reactor to collect the generated biogas. A sedimentation column (11) has a volume of 35 L (with 1.38 m in height, and the diameter of 0.15 m). The UASB was operated at a relatively constant temperature by covering with an insulating foam layer. Besides, two mixing pumps were put in the feeding tank to maintain the wastewater characteristics uniformly. Two heating devices were put in the UASB reactor and in the feeding tank to maintain a stable warm temperature to facilitate microbial activity. In addition, two Masterflex peristaltic pumps were used to pump wastewater from the feeding tank to the UASB reactor, and to recirculate wastewater from the sedimentation column to the blending container. Milk wastewater was prepared from the expired milk powder. In particular, it was prepared by disolving the expired whole milk powder with tap water. Initially, 1 g of milk powder was dissolved and diluted with 500 mL of boiled water to get the

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No.

Parameters

Unit

Value

1

pH

–

7

2

BOD5

mg/L

2650

3

COD

mg/L

3200

4

TN

mg/L

50

5

TP

mg/L

4.8

6

Ammonium

mg/L

4.8

7

SS

mg/L

1300

desired concentration. Similarly, 2 g of milk was dissolved with 1 L of tap water to determine the organic concentration. In this study, COD concentration of 3200 mg/L was selected as the influent characteristics (Table 1). Prepared milk wastewater was put into the feeding tank. It was then pumped with a flow rate of 20 L/d into UASB through the blending container. The wastewater flow was downward into the UASB. After that it was come from the bottom to the top of UASB to mix with the sludge layer. In this reactor, the anaerobic digestion was taken place with the mixture of wastewater and sludge. The wastewater was overflowed through the sedimentation column. In this process, an amount of organic was converted to biogas, which was collected and stored by Testla bag (with a volume of 10 L). At the sedimentation column, sludge was settled down while the treated wastewater was come out through a tube. In this system, a part of wastewater was recirculated from the bottom of sedimentation column to the blending container. The recirculated wastewater will be mixed with new wastewater before coming into UASB reactor. The recirculation flow could increase the mixing ability inside the UASB reactor (McAteer et al. 2020). It could enhance the biogas production and wastewater treatment efficiency. In this study, the recirculating flow was adjusted to evaluate the effect of the wastewater circulation and effluent quality. In addition, a part of biogas in the storage bag was recirculated to the bottom of UASB reactor. This could also help to mix the wastewater and sludge inside the reactor.

3 Functions of Operational Conditions of UASB System A feeding tank is used to store 20 L of the milk wastewater. The synthetic milk wastewater is prepared daily. At the weekend days, the volume of the prepared milk wastewater is increased to about 40 L. An amount of NaHCO3 is added into the wastewater container to maintain relatively constant pH in the range of 7.0–7.3. The feeding tank plays a role in providing wastewater stably for microorganisms in the UASB system. The influent milk wastewater can be sampled at the top of blending container. The influent milk wastewater was mixed with the wastewater recirculation in the blending container before entering into the UASB reactor. Milk wastewater

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is gradually raised up from bottom to the top of the UASB reactor. The anaerobic process of milk wastewater was taken place. The retention time of wastewater in the reactor was about 3 days. It is controlled by a fixed flow rate of the influent pump. In the UASB, main pollutants were digested and composited by the anaerobic microorganisms to the biogas (Bella and Rao 2021). During operation, the suspended solid or sludge concentration in the UASB was tested by sampling through a valve which was placed in the middle of the UASB reactor. The treated milk wastewater was then overflowed through the sedimentation column. The sludge was settled down in the sedimentation column. A part of sludge was recirculated by pumping from the bottom of sedimentation column into blending container. The recirculation flowrate could be adjusted at the ratio from 1 to 3 times comparing with the influent flowrate. The sedimentation column plays an important role to ensure the sludge and suspended solids to settle at the bottom effectively (Dawood et al. 2011; Shah 2021). It helps to enhance the effluent quality. The anaerobic sludge in the UASB was cultivated within 4 weeks. 20 L of milk wastewater was prepared by dissolved 40 mg of milk per liter to make the COD concentration of 3200 mg/L. Milk wastewater in the feeding tank was pumped into the UASB. The hydraulic retention time of wastewater in the UASB was maintained at 3 days. Sludge was not withdrawn during the cultivation period. This period helps to increase the anaerobic sludge and helps the operation of the UASB reactor stably. Once the system has become stable, the sludge in the sedimentation column was recycled to the UASB. The recirculation flowrates were controlled at several different conditions, i.e. 20, 30, 40, and 60 L/day. Each recirculation flowrate was lasted for 1 weeks. Under each recirculation flowrate, the performance of the UASB was evaluated. The effect of the sludge recirculation to biogas yield was also determined. Similar to wastewater circulation, the biogas recirculation is carried out by pumping the produced biogas from the top back to the bottom of the UASB reactor. This period was conducted for 1 week. It could help to evaluate the effect of biogas recirculation on UASB performance. Biogas produced from anaerobic digestion was collected by Testla bag at the top of UASB. The volume of biogas was used to calculate biogas yield coefficient (Chou and Su 2019). Percentage of methane in the biogas was determined as well. In order to evaluate the system performance, several parameters were analyzed following the standard methods (APHA 2017). In particular, COD was analyzed following a standard (TCVN 6491:1999), using several chemicals, such as H2 SO4 98%, Ag2 SO4 (5.5 g/500 mL of H2 SO4 98%), K2 Cr2 O7 (0.25 N), HgSO4 salt (NH4 )2 Fe(SO4 )2 .6H2 O (FAS) 0.025 N, Ferroin indicator solution. BOD was measured by using Oxytop method, with phosphate buffer solution (KH2 PO4 8.5 g/L, K2 HPO4 21.75 g/L, Na2 HPO4 .7H2 O 33.4 g/L, NH4 Cl 1.7 g/L), MgSO4 .7H2 O solution of 22.5 g/L; CaCl2 solution of 27.5 g/L; FeCl3 .6H2 O solution of 0.25 g/L, HCl and NaOH solution to pH adjustment. VFAs was measured by standard method (TCVN 4589:1988) with Phenolphthalein 0.1% and NaOH 0.1 N. SS was determined by a standard method (TCVN 6625: 2000). Percentage of CO2 in the biogas was determined by a standard method (TCVN 353-89), using Barryte solution of

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(Ba(OH)2 6.4 g/L, 0.32 g/L BaCl2 ), Oxalic acid of 0.56 g/L, and Phenolphthalein solution 0.1%.

4 Performance of UASB Under Different Sludge Circulation Flowrates COD removal efficiency in the UASB was presented in Fig. 2. It could be seen that during operation, the COD removal of UASB was reached at high level. At the first stage, UASB was started in 22 days. During this period, the biogas yield was stable. In this stage, the influent COD concentration in the feeding tank was frequently analyzed to see the variation of the organic compounds which could be utilized by the microorganisms (Birwal et al. 2017). The COD removal after 3 days was approximately 97%. High COD removal efficiency at this period could be due to the UASB reactor was operated steadily. The effect of the sludge recirculation flow at the ratio of 1:1 (recycling flowrate and influent flowrate) was started from the 23rd date to the 35th date. This period was used to evaluate the COD removal. It was found that the COD removal was almost unchanged when comparing with the initial period. The effluent COD concentration was found to be about 30–40 mg/L. After that, the sludge recirculation flowrate was increased to various ratios, i.e. of 1.5:1, 2:1; 2.5:1; and 3:1. Those stages were carried out from the 36th the 59th date. Those periods were conducted shorter than the first period due to the operation of the UASB reactor was already stable.

Fig. 2 COD removal in the UASB treating milk wastewater

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It should be noted that when milk wastewater was prepared in the laboratory, the influent COD concentration in the feeding tank was found to be unstable. This was due to the effect of mixing and the nature of the milk wastewater (Kaur 2021). In the tank, milk was easily to form as a solid layer on the surface which could accumulate some amount of COD, resulted in reduction of the influent COD concentration. When mixing was used, the solid layer was dissolved. This could help to partly increase the influent COD concentration (Ahmad et al. 2019; Carvalho et al. 2013). Therefore, during operation, mixing should be frequently checked to ensure the influent COD concentration stably. Based on the observations, it could be seen that the performance UASB was almost unchanged under the effect of the ratio of sludge recirculation and influent flowrate. It indicated that COD removal efficiency in the UASB was not depended on the sludge recirculation. The COD removal efficiency was found to be similar in each period, i.e. in the range of 97–98%. In fact, during the period from the 40th to 50th date, the COD removal efficiency was found to increase to over 99%. This could be caused by the strong mixing in the reactor, resulted in enhancing the microorganisms contacting with milk wastewater (Sarkar et al. 2006). Actually, the anaerobic digestion is not used for nitrogen removal (Passeggi et al. 2009). This is due to the nutrient demand for the anaerobic microrganisms is not low. In addition, the nitrification process could not happen in the absence of oxygen condition (Struk-Sokołowska et al. 2018). In the anaerobic condition, nitrogen was mainly existed in the organic nitrogen compounds. They could be converted to ammonium by the hydrolysis process. Based on the real observation, it could be seen that the color of milk wastewater is white initially. After that, it couldbe changed to yellowish. After recirculation process the yelllow color was found to decrease significantly. At the first stage, the effluent ammonium concentration was found to be 52 mg/L. It was interesting that, the ammonium concentration was gradually decreased when the sludge recirculation ratio was increased. At the recirculation ratio of 1:1, the ammonium concentration was strongly decreased down to about 26 mg/L (reduced about 50%). However, from the day 37th on ward to the end of study period, the ammonium concentration was decreased slowly. The ammonium concentration in the effluent was reduced to about 22 mg/L when the sludge recirculation ratio was increased from 2:1 to 3:1. The decease in the effluent ammonium concentration indicated that microorganisms have consumed more nitrrogen in the anaerobic digestion. It meant that when sludge was recirculated at low ratio, the ammonium concentration in the effluent was strongly decreased. However, when the recirculation ratio was increased, the ammonium concentration was not decreased sigificantly. Therefore, the recirculation ratio of 1:1 could be considered as the suitable ratio for minimizing ammonium concentration in the effluent. Suspended solids (SS) concentration in the effluent was also varied with the different recirculation ratios. The initial SS in the influent was fluctuated and SS concentration in the effluent was slighly decrased. The SS removal efficiency was increased from 65 to 78% at the sludge recircualtion ratio of 1.5:1. When the sludge recirculatiomn ratio was increased to 3:1, the SS removal efficiency was not increased only about 2%. It should be noted that the anaerobic slugde has mainly contributed as SS with dark color in the effluent (D˛ebowski et al. 2020). In addition, the sludge could

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contain some untreated organic compounds which could increase the COD concentration in the effluent up to 200–300 mg/L. During operation, the SS concentration in the sedimentation column was monitored. It was varied depending the height of the sedimentation column. It was found that that the SS concentration was gradually decreased with the increase in the height of the sedimentaion column. The SS concentrations were 14,202 mg/L, 1296 mg/L and 273 mg/L at the height of 10 cm, 45 cm and 90 cm, respectively. The SS concentration at the bottom was much higher than that at other positions of the sedimentation column. This shown that the SS removal was very effective in the sedimentation column. These results also indicated that the a lot of slugde from UASB reactor was overflowed to the sedimentation column. It meant that the increase in the recirculation ratio could be a cause making a loss of biomass in the system. Therefore, if the sedimentation column was not installed in the system, the performance of UASB could be effected strongly due to a loss of biomass. Thus, it is necessary to use a sedimentation tank after the UASB reactor when sludge recirculation was applied to ensure the highest treatment performance (Dawood et al. 2011). Volatile fatty acids (VFAs) could be seen as one of the important parameters to evaluate the acidification process in the anaerobic system (Begum et al. 2018). The acidification in the reactor was more strongly when the VFAs concentration was high. However, high concentration of VFAs could inhibite the growth of the anaerobic microorganisms due to decreasing pH. As a result, high VFAs concentration could have an adversely effect on the UASB performance. During operation, VFAs concentration in the UASB at the normal operational condition was found to be about 53 mg/L. However, VFAs concentration was slightly increased to 71 mg/L whenthe sludge recirculation ratio was increased to 1.5:1. The lowest VFAs concentration was about 45 mg/L when the sludge recirculation ratio was increased to 3:1. Figure 3 presents the volume and the biogas yield coefficient at different sludge recirculation ratio. As seen from Fig. 3, during operation, the higher biogas yield coefficient was obtained at higher recirculation ratio. At the beginning, The biogas yield was found to be about 0.143 L/g COD. The obtained biogas yield was much lower when comparing with the theoretical biogas yield (0.35–0.5 L/g COD). The total volume of produced biogas was about 10 L. Low biogas yield could be affected by High C:N:P ratio in this study (600:10:1) could cause a decrease in biogas production. It was reported that low biogas yield could be affected by high C:N:P ratio (Choi et al. 2020). In addition, at the beginning period, a layer of floating scum was formed on the top of UASB reactor. During operation, a scum layer could be formed in the UASB or in the sedimentation tank. Therefore, a suitable design for scum removal should be considered (D˛ebowski et al. 2020). The scum layer has inhibited the biogas production of the UASB system (Xu et al. 2018). It could be seen from Fig. 3 that the volume of biogas was significantly incrased from 9 to 18 L/day when the sludge recirculation was applied. The biogas yield was also increased from 0.143 L/g COD to 0.31 L/g COD at the sludge recirculation ratio of 1:1. The biogas yield was increased significantly to 0.5 L/g COD which is similar to

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Fig. 3 Variation of biogas volume and biogas yield coefficient at different sludge recirculation ratio (Notes on the horizontal axes: (1) is for ratio of 0:1; (2) is for ratio of 1:1; (3) is for ratio of 1.5:1; (4) is for ratio of 2:1; (5) is for ratio of 2.5:1; (6) is for ratio of 3:1)

the theoritical biogas yield when the sludge recirculation ratio was increased to 2.5:1 and 3:1. The obtained results showed that the sludge recirculation has a strong effect on the biogas production of the UASB system. This could be due to the ammonium concentration was decreased when the sludge recirculation ratio was increased. It was reported that ammonium ion was one of the inhibitor of methanogenic bacteria (Kavacik and Topaloglu 2010). At low ammonium concentration, these bacteria was not inhibited, resulted in increasing the biogas production (Adghim et al. 2019). Furthermore, wastewater and slugde in the system could be well mixed at high sludge recirculation ratio. This could help biogas to be separated from the wastewater better (Chou and Su 2019). Biogas compositions were detected. The results showed that the content of CO2 in the biogas was increased significantly from 12.4 to 22.1%. High CO2 could reduce the content of CH4 in the biogas. The conversion CO2 to CH4 was found to be decreased. The sludge recirculation ratio could enhance the CH4 content in the biogas. The sludge recirculation ratios of 2:1 and 2.5:1 could be used as the suitable condition to improved the biogas proction. The biogas production was increased at high sludge recirculation ratio. Similarly, the CO2 content in the biogas was gradually increased when sludge recirculation ratio was increased.

5 Performance of UASB Reactor with Biogas Recirculation The performancce of the UASB reactor was evaluated when a part of biogas was recirculated. The biogas flow produced at the sludge recirculation ratio of 3:1 was about 38 L/day. The COD removal efficiency was almost unchanged when biogas

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Table 2 A comparison of COD, SS, ammonium VFAs, biogas, CO2 in two operational conditions Operational conditions

COD (mg/L)

SS (mg/L)

Ammonium (mg/L)

VFAs (mg/L)

Biogas (mg/L)

CO2 (%)

With sludge recirculation

99

81

22

45

38

22

Biogas recirculation

99

66

20

33

40

13

recirculation was used. However, SS removal efficiency was reduced significantly, from 81 to 66%. This could be due to mixing in the UASB reactor was increased when biogas was recirculated. SS concentration in the effluent was increased at high mixing condition. VFAs concentration in the effluent was decreased significantly when biogas was recirculated. VFAs concentration was decreased of 30%, from 45 to 33 mg/L. This could be due to two reasons (i) the acidification was weakened; or (ii) the conversion of organic acids to CH4 was strengthened (Shi et al. 2022). The ammonium concentration in the effluent was slightly drecreased, from 22 to 20 mg/L. This decrease was coressponding to increasing the sludge recirculation ratio of 0.5. The obtained results showed that the recirculation of biogas could enhance the nitrogen consumption by the anaerobic microorganisms. Thus, biogas recirculation could be considered as an alternattive rmethod for reducing ammonium concentration in the effluent (Chou and Su 2019; Kavacik and Topaloglu 2010). Table 2 shows that CO2 content in the biogas was decreased significantly, from 22 to 13% when the biogas recirculation was used. It meant that the biogas recirculation could enhance the conversion from CO2 to CH4 (Adghim et al. 2019). Besides, an amount of VFAs could be converted to CH4 when biogas was recirculated.

6 Conclusions and Remarks Based on the obtained results, it could be concluded that UASB reactor could be used for milk wastewater treatment. In the normal operational condition, COD and TS removal efficiencies were about 97% and 65%, respectively. During operation, ammonium removal was not effectively. The biogas production in the stable condition was only about 9 L/day. The biogas yield was 0.143 L/g COD. The CO2 content in the biogas was quite low, about 12.4%. These results pointed out that wastewater recirculation did not affect on COD removal was not effected when sludge recirculation was used at various ratios of 1:1, 1.5:1, 2:1, 2.5:1 and 3:1. SS removal efficiency was increased significantly, from 66 to 81% when sludge recirculation ratio was increased. Ammonium removal was not improved when sludge recirculation ratio was increased from 1.5, 2:1, 2.5:1, 3:1. Increasing in the sludge recirculation could enhance the biogas production. The biogas yield was gradually increased from 0.31 L/g COD to 0.534 L/g COD when

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the sludge recirculation ratio was increased from 1:1 to 3:1. It is noteworthy that the CO2 content was also increased with increasing in the biogas volume. VFAs concentration was not changed significantly in the sludge recirculationcondition. Biogas recirculation shows that the COD removal efficency was stable. However, SS removal efficiency was reduced from 81 to 66%. Ammonium and VFAs concentrations in the effluent were reduced slighly. However, CO2 concent was decreased significantly. However, the volume of biogas was almost unchanged. In conclusion, the obtained results in this study could provide valuable information for application of UASB technology for treating milk wastewater effectively. Acknowledgements The authors would like to thank the facility supports from School of Environmental Science and Technology, Hanoi University of Science and Technology.

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Biofertilizer from Industrial Waste Water by Microalgal Treatment N. Prabhu, M. Mounika, A. Sureja, M. Shareen Fathima, and N. Hiritha

Abstract Due to rapid industrialization and the depletion of non-renewable fossil fuels, alternative feasible renewable alternatives are being sought to supply rising energy demand while reducing carbon dioxide emissions. Microalgae cultivation has to meet these criteria in today’s world energy strategy, which is centred on costeffective and environmentally friendly alternatives. Microalgae has been discovered as a promising and long-term solution for wastewater treatment and the generation of valuable products. Microalgae, which have a short life cycle, a rapid growth rate, and a high CO2 usage efficiency, are one of the most feasible renewable resource technologies for producing biomass from wastewater nutrients. Technology and cost are now the key issues limiting industrial-scale use, which necessitates an optimum downstream process to reduce manufacturing costs. These issues have become feasible and economically viable thanks to the utilisation of microalgae for wastewater treatment and biofuel generation at the same time. The efficacy of microalgae for the removal of ammonia, phosphorus, and heavy metals, as well as the creation of biofuel and biofertilizer, is examined. It also aims to concentrate on current breakthroughs in wastewater microalgae growth, as well as the response of microalgae to various stimuli and their implications on the quality and quantity of high-value products. Keywords Waste water · Treatment methods · Nutrients · Bio fertilizer · Environment · Effluents · Microorganisms

1 Introduction Wastewater is produced when fresh water is used in a number of applications, and it typically entails leaching, flushing, or washing away waste items and nutrients that have been introduced to the water during those uses. Used water from any combination of household, industrial, commercial, or agricultural activity, surface runoff/storm water, and any sewer inflow or sewer infiltration, according to a more N. Prabhu (B) · M. Mounika · A. Sureja · M. Shareen Fathima · N. Hiritha Department of Biotechnology, Vivekanandha College of Engineering for Women, Elayampalayam, Tiruchengode 637205, Tamil Nadu, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_10

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thorough definition of wastewater. Wastewater is sometimes used interchangeably with sewage (also known as sewerage, household wastewater, or municipal wastewater), which is wastewater generated by a group of people. It is usually disposed of in a sewage system (Tilley et al. 2014). To avoid eutrophication, liquid wastewater streams containing nitrogen must be treated before being released into the environment. There are already a number of conventional treatment systems that can remove nitrogen from wastewater using a mix of procedures. A portion of nitrogen will be released into the atmosphere, depending on the processes involved. Several species of microalgae, on the other hand, have a voracious appetite for nitrogen and can absorb waste-bound nitrogen in a single step, primarily as intrinsic proteins. The minerals inside the microalgae cells remain available for plants once they are isolated from the water, and they may be utilised as fertiliser. Microalgae may grow in wastewater and extract nutrients; this wastewater-grown biomass can be utilised as a biofertilizer for crops. Furthermore, by removing microalgal biomass from the wastewater at the conclusion of the process, the waste water can be treated fully, or at least partially, reducing the time and expense of traditional treatment. Qatar’s climate and lack of arable land make it perfect for growing microalgae (Abdel-Raouf et al. 2012).

2 Waste Water Water that has been contaminated by home, industrial, or commercial use is referred to as wastewater. As a result, the composition of all wastewaters is always changing and extremely variable. The makeup of wastewater is 99.9% water, with the remaining 0.1% being removed. Organic materials, bacteria, and inorganic chemicals make up the 0.1%. Wastewater effluents are discharged into lakes, ponds, streams, rivers, estuaries, and seas, among other places. Storm runoff comprises dangerous elements that wash off roadways, parking lots, and rooftops. Human excrement, protein, fat, vegetable, and sugar material from meal preparation, as well as soaps, make up the organic composition of wastewater. Some of this organic matter dissolves in water, while others remain as distinct particles. Suspended solids are the parts of organic material that do not dissolve but stay suspended in water. The organic material in wastewater is removed as much as possible. Sodium, copper, lead, and zinc, among other inorganic minerals, metals, and compounds, are abundant in sewage and wastewater. They can come from a variety of places, including industrial and commercial facilities, rainwater, and inflow and infiltration through damaged pipes. The majority of inorganic compounds are stable and cannot be easily broken down by bacteria in wastewater. Eutrophication is caused by excess nutrients such as phosphorus and nitrogen, which can be hazardous to aquatic creatures. This also encourages excessive plant growth and lowers oxygen availability, changing ecosystems and perhaps putting certain species at risk (Tuser 2020).

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2.1 Waste Water Classification 2.1.1

Industrial Wastewater

These waters are created by various industrial operations and include any unit operation’s undesirable liquid result. The main worry with these wastes is the potential for direct or indirect interactions with the environment. Some may deplete the environment’s oxygen supply, while others may be poisonous. Industrial Water Pollutants from industry that run off into streams, rivers, or lakes can harm wildlife, plants, and humans. The amount and type of pollutants that industries can discharge into bodies of water are strictly regulated in the United States. These regulations aren’t always followed, and chemical and oil accidents are a major source of industrial water contamination. Depending on the sort of company that produces it, industrial wastewater has a wide range of quality and volume. It could be extremely biodegradable or not, and it could contain or not contain components that are resistant to treatment. Organic synthetic chemicals or heavy metals, for example, whose content in developing country wastewater may differ significantly (in quantity and quality) from that in developed countries. The main source of worry with industrial wastewater is the growing amount (in terms of both quantity and variety) of synthetic substances included in and discharged to the environment (Cisneros 2011). Wastewater is produced by almost every industry. The recent trend has been to reduce such production or recycle treated wastewater in the manufacturing process. Some industries have been successful in reducing or eliminating pollutants by restructuring their manufacturing processes. Battery manufacturing, chemical manufacturing, electric power plants, food processing, iron and steel industry, metal working, mines and quarries, nuclear power plants, oil and gas extraction, petroleum refining and petrochemicals, pharmaceutical manufacturing, pulp and paper industry, smelters, textile mills, industrial oil contamination, water treatment, and wood preservation are all sources of industrial wastewater (Pollution prevention case studies 2021).

2.1.2

Domestic Wastewater

Domestic wastewater has a grey colour, a musty odour, and a solids concentration of roughly 0.1% on a physical level. Faeces, food particles, toilet paper, grease, oil, soap, salts, metals, detergents, sand, and grit make up the solid stuff. The solids can be dissolved as well as suspended (approximately 30%) (about 70%). Chemical and biological processes can precipitate dissolved solids. When suspended materials are discharged into the receiving environment, they might result in the formation of sludge deposits and anaerobic conditions. In terms of chemistry, wastewater is made up of organic (70%) and inorganic (30%) components, as well as different gases. Carbohydrates (25%), proteins (65%), and fats (10%) make up the majority of organic molecules, which reflects people’s diets. Heavy metals, nitrogen, phosphorus, pH, sulphur, chlorides, alkalinity,

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hazardous chemicals, and other inorganic components may be present. However, because wastewater has a higher percentage of dissolved solids than suspended solids, approximately 85–90% of the entire inorganic component is dissolved, whereas approximately 55–60% of the total organic component is dissolved. Hydrogen sulphide, methane, ammonia, oxygen, carbon dioxide, and nitrogen are all gases that are regularly dissolved in wastewater. The degradation of organic materials in the wastewater produces the first three gases. Biologically, wastewater comprises a wide range of microbes, but those classed as protista, plants, and animals are of particular significance. Bacteria, fungus, protozoa, and algae are all classified as Protista. Ferns, mosses, seed plants, and liverworts are examples of plants. The animal category includes both invertebrates and vertebrates. Protista, particularly bacteria, algae, and protozoa, are the most important category in wastewater treatment. In addition, wastewater contains a large number of pathogenic organisms, most of which come from individuals who have been affected with a disease or who are carriers of a disease. Typical faecal coliform concentrations in raw wastewater range from several hundred thousand to tens of millions per 100 ml of sample (Henze 2001).

2.1.3

Storm Wastewater

Stormwater, usually called storm water, is water that comes from extreme precipitation (storm), such as heavy rain and hail and snow meltwater. Stormwater can penetrate into the soil and form groundwater, be held in ponds and puddles on depressed land surfaces, evaporate back into the atmosphere, or contribute to surface runoff. The majority of runoff is discharged untreated as surface water into neighbouring streams, rivers, or other big water bodies (wetlands, lakes, and oceans). Soil absorbs a lot of stormwater in natural settings like woods. Plants also help to reduce stormwater runoff by enhancing infiltration, intercepting rain as it falls, and absorbing water through their roots. Unmanaged stormwater in developed environments, such as cities, can cause two significant issues: one relating to the volume and timing of runoff (flooding), and the other related to potential toxins carried by the water (water pollution). In addition to the chemicals carried by stormwater runoff, urban runoff is now being recognised as a pollution source in and of itself. Stormwater has become a valuable resource as the human population and demand for water has increased, especially in arid and drought-prone areas. Stormwater collecting and purification techniques have the potential to make some metropolitan surroundings self-sufficient in terms of water (Schueler 2014).

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2.2 Characteristics of Waste Water Wastewater is classified based on its physical, chemical, and biological characteristics. Physical, chemical, and/or biological treatment will be used depending on the level of contaminants and local requirements. To get the best water quality, the three procedures are usually combined. The qualities of wastewater differ greatly depending on the industry. As a result, the treatment approaches to be employed to meet the compliance discharge criteria will be determined by the unique features. Because of the enormous number of contaminants, each substance’s properties are rarely considered. Pollutant or characteristic classes are made up of materials that have comparable polluting impacts (Classification of wastewater 1h2o3 GmbhLangens and strasse 10 6005 Lucerne Switzerland).

2.3 Waste Water Treatment Wastewater treatment is the process of removing impurities from wastewater and converting it into effluent that may be recycled back into the water cycle. Once returned to the water cycle, the effluent has a low environmental impact or can be utilised for a variety of uses called water reclamation. A wastewater treatment facility is where the treatment takes place. Various types of wastewaters are treated by wastewater treatment plants of the proper type. The wastewater treatment process is divided into three stages: primary, secondary, and tertiary water treatment. More sophisticated treatment, known as quaternary water treatment, is necessary in some applications. This step deals with contamination levels of a few parts per million to billions of parts per billion, and it frequently includes oxidation or fine filtering. Each of these stages targets a different contaminant, and as the water progresses through the

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stages, it becomes cleaner (Encyclopedia Britannica wastewater treatment process, history, importance, system and technologies 2020).

2.3.1

Primary Treatment

Water is briefly stored in a settling tank during primary treatment, when heavier particles sink to the bottom and lighter solids float to the surface. These components are kept back until they have settled, while the remaining liquid is released or transferred to the more stringent secondary phase of wastewater treatment. Mechanical scrapers at the tank’s base continuously push collected sludge to a hopper, where it is pumped to sludge treatment facilities. Material that will float or settle out by gravity is removed during primary treatment. Screening, comminution, grit removal, and sedimentation are examples of physical processes. Long, closely spaced, narrow metal bars make up screens. They block floating waste like wood, rags, and other bulky materials from clogging pipes and pumps. The screens are cleaned mechanically in contemporary facilities, and the waste is quickly disposed of by burial on the plant grounds. To grind and shred debris that goes through the screens, a comminutor can be utilised. Later, sedimentation or flotation techniques are used to remove the shredded debris (Nathanson 2010).

2.3.2

Secondary Treatment

Secondary wastewater treatment is meant to significantly degrade the biological component of the waste through aerobic biological processes, and it acts at a deeper level than primary treatment. Secondary wastewater treatment reduces common biodegradable pollutants to tolerable levels, allowing for safer discharge into the surrounding ecosystem. It can be done by biofiltration, aeration, Oxidation ponds. The soluble organic matter that escapes basic treatment is removed in secondary treatment. It also eliminates a higher percentage of suspended solids. Biological procedures are typically used to remove organic pollutants, with bacteria consuming them as food and converting them to carbon dioxide, water, and energy for their own growth and reproduction. The sewage treatment facility provides an appropriate environment for this natural biological process, albeit one made of steel and concrete. The removal of soluble organic matter at the treatment plant aids in the preservation of a receiving stream, river, or lake’s dissolved oxygen balance (Brown and Roberts 2007).

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Tertiary Treatment

The goal of tertiary wastewater treatment is to improve the water’s quality to satisfy household and industrial standards, as well as to fulfil particular criteria for water discharge safety. In the case of municipally treated water, tertiary treatment also includes the elimination of pathogens, ensuring that the water is safe to consume. The final stage of the multi-stage wastewater treatment process is tertiary water treatment. Inorganic chemicals, bacteria, viruses, and parasites are all removed during the third stage of treatment. The treated water is safe to reuse, recycle, or discharge into the environment after these dangerous substances have been removed. In most cases, tertiary wastewater treatment entails final filtration of the treated effluent. It may be necessary to use alum to eliminate phosphorus particles from the water when necessary. Alum also causes any particulates that were not removed by primary and secondary wastewater treatment to clump together, allowing filters to extract them. The filters are backwashed as needed to remove the build-up of floc, allowing them to continue to function properly (AOS Treatment Solutions 2018).

3 Biofertilizer A biofertilizer is a product that contains live microorganisms that colonise the rhizosphere or the inside of the plant when applied to seeds, plant surfaces, or soil, and encourage growth by increasing the supply or availability of primary nutrients to the host plant (Vessey 2003). Biofertilizers supply nutrients to plants through natural processes such as nitrogen fixation, phosphorus solubilization, and the creation of growth-promoting chemicals. Biofertilizers contain microorganisms that restore the soil’s natural nutrient cycle and increase soil organic matter. Healthy plants may be developed with the application of biofertilizers while also improving the soil’s sustainability and health. Biofertilizers will likely minimise the need of synthetic fertilisers and pesticides, but they will not be able to completely replace them. Biofertilizers are organic agro-inputs that are “eco-friendly.” Rhizobium, Azotobacter, Azospirilium, and blue green algae (BGA) have long been used as biofertilizers. Leguminous crops benefit from Rhizobium inoculant. Wheat, maize, mustard, cotton, potato, and other vegetable crops can all benefit from Azotobacter. Sorghum, millets, maize, sugarcane, and wheat are among the crops for which Azospirillum inoculations are advised. Nostoc or Anabaena or Tolypothrix or Aulosira, blue green algae belonging to the cyanobacteria genus, fix atmospheric nitrogen and are utilised as inoculants for paddy crops produced in both upland and lowland settings (Listing 17 bio-fertilizer microbes and their effects on the soil and plant health functions 2016).

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3.1 Types of Biofertilizer 3.1.1

N2 Fixing Biofertilizer

Microorganisms including Rhizobium, Actinobacteria, Azotobacter, and Azospirillum are found in nitrogen-fixing biofertilizers. They aid in the conversion of nitrogen to organic molecules. One method of turning elemental nitrogen into a form that plants can use is biological nitrogen fixation. It is the process of converting nitrogen (N2 ) to ammonia (NH3 ). As a result of growing public awareness of water pollution and nitrate emissions, alternative sustainable sources such as nitrogen-fixing biofertilizers are becoming more important (Biofertilizers 2019). Biofertilizers that include nitrogen assist to balance nitrogen levels in the soil. Because plants require a particular quantity of nitrogen in the soil to flourish, nitrogen is a limiting element for plant development. Because various biofertilizers work best in different soils, the type of nitrogen biofertilizer to employ is determined by the farmed crop. Rhizobia is utilised in legume crops, Azotobacter or Azospirillum is used in non-legume crops, Acetobacter is used in sugarcane and blue-green algae, and Azolla is used in lowland rice fields (Stancheva et al. 2013).

3.1.2

Phosphate Solubilizing Biofertilizer

Phosphorus, like nitrogen, is a limiting element for plant growth. Phosphorus biofertilizers assist the soil in reaching its optimum phosphorus level and correcting phosphorus levels. Phosphorus biofertilizers, unlike nitrogen biofertilizers, are not dependent on the crops grown on the soil. Phosphatika is utilised in all Rhizobium, Azotobacter, Azospirillum, and Acetobacter-infected crop (Motghare and Gauraha 2012). PSB is a phosphate biofertilizer that has been presented to the agricultural sector. Phosphorus (P) is one of the most important macronutrients for plants, and phosphate fertilisers are used to provide it to the soil. However, a considerable amount of soluble inorganic phosphate used as a chemical fertiliser in the soil is quickly immobilised and inaccessible to plants (Malboobi et al. 2009). Currently, the primary goal of soil phosphorus management is to maximise crop yield while minimising P loss from soils. PSB have piqued agriculturists’ interest as soil inoculums to boost plant growth and productivity. When PSB is used with rock phosphate, it can save up to 50% of the phosphatic fertiliser needed by the crop. Pseudomonas, Bacillus, Micrococcus, Aspergillus, Fusarium, etc. are the major phosphate solubilizing bacteria (PSB) in soil (Baas et al. 2016).

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3.2 Biofertilizer Production Wastewaters are rich in nitrogen and phosphorus, and therefore have the potential to be used as low-cost microalgae medium. These organisms have the capacity to thrive in a wide range of settings and digest a wide range of nutrients. This type of biomass production is advantageous since the absorption of nitrogen and phosphorus from wastewaters can aid in the recycling of algal biomass as a biofertilizer while also reducing the usage of chemical fertilisers and sewage disposal (Pittman et al. 2011). Microalgae may grow in wastewater and extract nutrients; this wastewatergrown biomass can be utilised as a biofertilizer for crops. India is one of the world’s greatest producers and users of fertilisers, but the country’s gradual growth in fertiliser usage has caused severe environmental issues, as well as being insufficient in light of present industrial capacity. The availability of macronutrients and micronutrients is a critical component in increasing crop yields (Bashan 1998). The key important macronutrients in crop nutrition are nitrogen, phosphorus, and potassium. Because microalgal biomass contains more nitrogen and cyanobacterial members can fix atmospheric nitrogen, such microalgae/cyanobacteria can be used as biofertilizers in a variety of agricultural systems. Biofertilizers are micro/macroorganisms that can colonise the soil, rhizosphere, or plant interior to boost the plant’s growth and nutrition (Prasanna et al. 2012). They also play a vital role in preventing soil erosion by controlling water flow into soils and promoting soil fertility, as well as in the

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reclamation of waste-lands, salty soils, and other arid environments. Microalgae boost the availability of soil nutrients to plants and function as plant growth promoters by releasing growth hormones. Certain cyano bacteria also help the plant develop by boosting its endogenous hormones (Hussain and Husnain 2011). The protein content of microalgal biomass is significant, and their multiplication need a lot of nitrogen as a fertiliser. However, the use of wastewater-grown biomass is debatable due to the accumulation of hazardous heavy metals, even if trace amounts might function as micronutrients, if present above acceptable limits. The presence of dangerous bacteria is a major downside of wastewater, however microalgal development is known to raise the pH of the media and restrict bacterial growth. This demonstrates that sewagegrown microalgae can provide a competitive advantage for the direct application of sewage sludge or wastewater in agricultural activities. Microbial biofertilizers are advantageous in cereal crops because they minimise the usage of chemical fertilisers while also improving the general health and nutritional state of the soil (Singh et al. 2011).

4 Microalgae Production and Harvesting 4.1 Production 4.1.1

Suspended Growth

In wastewater, the most typical microalgae production method is a suspended growth system, which can be open or closed (photobioreactor or PBR). Biocoil, horizontal tubular, and vertical PBRs are the most commonly investigated PBRs for wastewater treatment (Kong et al. 2009). Microalgae growing in the PBR has the potential to reduce or eliminate undesired pollution, evaporative water loss, and CO2 loss. Inside a PBR with a low optical depth, high volumetric biomass productivity can be achieved; as the optical depth of the PBR increases, the biomass productivity decreases (Huang et al. 2017). In general, the cost of PBR materials for wastewater treatment may be excessively expensive. Furthermore, the energy required to mix the culture inside a PBR may be several times greater than the calorific energy of the microalgal biomass generated. Despite various obstacles (evaporation water loss, pollution, etc.), open growing of microalgae in wastewater could be highly promising (White and Ryan 2015). Earthen lagoons, concrete tanks, and raceway ponds were used to investigate microalgal bioremediation of wastewater. The open type cultivation systems, such as the high rate algal pond (HRAP) and the corrugated raceway pond (CRP), were specifically intended for treating wastewater. The depth of the open cultivation system could vary from 0.15 to 0.45 m because microalgae are good at absorbing light, the top layer of the culture would absorb the majority of it, leaving the cells below in the dark (Murthy 2011). Microalgal culture mixing in a large-scale open system is

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frequently insufficient, hence microalgal bioremediation in a deeper pond may be ineffective (Eltanahy et al. 2018).

4.1.2

Attached Growth

To address the difficulties of harvesting microalgal biomass from suspended cultures, microalgae cells could be immobilised or attached to a support medium where the attached cells come into contact with wastewater, where the nutrients are absorbed and used by the microalgae to form biomass. The immobilised cells could be put on a revolving paddle or on a static surface (Roostaei et al. 2018). The support media may be removed from the wastewater and the biomass scraped out once the appropriate degree of bioremediation had been achieved or when the biomass growth on the surface had reached a certain thickness. In addition, the leftover cells on the biofilm could serve as inoculum for the following batch. In a microbial fuel cell, a biofilm of Chlorella sp. was grown as a biocathode to treat TWW and generate power (Logroño et al. 2017). Another type of connected microalgal growing system is the algal turf scrubber (ATS), which allows benthic microalgae to be grown on a solid support while wastewater is circulated through it. Similarly, an algal biofilm might be formed on the surface of a floating conveyer belt made of a dimpled metal sheet. However, one of the greatest impediments to commercialising this approach is the cost of producing the support medium (Hoffmann 2002).

4.2 Harvesting One of the most important processes in microalgal bioremediation of wastewater is the removal of microalgal biomass from treated wastewater. If the treated wastewater is to be used for other purposes, efficient preparatory harvesting of microalgae is also required. Although there are a variety of strategies for separating biomass from the rest of the culture, the choice of harvesting technique is largely determined by the intended use of the biomass and the energy required per unit of biomass production. To create a biomass, paste with a solid content of 20% or greater, a two-phase harvesting procedure is usually used. Harvesting processes such as sedimentation, flocculation, filtering, and others are employed in the early step to create a biomass slurry (usually 1–4%), which might then be refined further (usually above 20%) using a centrifugation (Al-Jabri et al. 2020).

4.2.1

Sedimentation

Some microalgae cells were unable to stay afloat in the growth media due to their size or the medium’s pH value; in either case, the cells sank to the bottom. As a result, the sedimentation technique might be used to separate microalgae biomass in

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large-scale operations as a low-cost, variable harvesting method. Several microalgal strains, such as Scenedesmus and Chlorella sp., have large enough cells to remain suspended in the growing medium without being mixed. The negative charge on the surface of microalgal cells inhibits cells from adhering to one another. As the density of the microalgal culture grows, the cells utilise the soluble carbonate, raising the pH of the growth fluid and potentially neutralising the microalgal surface charge. At high pH, certain inorganic substances precipitate; this process could potentially cause microalgal cells to coprecipitate. Because the slow pH increase and natural precipitation can take time, a base solution (i.e. sodium hydroxide) is sometimes used to speed up the microalgal sedimentation process (Olguın 2003).

4.2.2

Auto-flocculation

Several cyanobacterial strains join together to create flocs, which are held together by the EPS they make. Several additional cyanobacteria, such as Phormidium sp., Leptolyngbya sp., and Pseudoanabaena sp., are filamentous in character and tangle together to create floc (Iasimone et al. 2020). Several factors (e.g., light intensity, temperature, nutritional deprivation, etc.) may influence EPS productivity, resulting in the destabilisation of the microalgal cell surface and auto flocculation (GonzálezFernández and Ballesteros 2012).

4.2.3

Bio-flocculation

The combined biomass harvesting efficiency of a non-settling microalga and a selfsettling microalga could be improved by co-cultivation. A few fungal strains (for example, Aspergillus sp.) can create gelatinous pellets in which negatively charged microalgal strains can bind and precipitate together. The fungus might exploit the available organics in wastewater as a substrate to grow and make the pellets. Similarly, bio-flocs generated by microalgae and bacteria might be extracted from treated wastewater by gravity sedimentation (Hende et al. 2014).

4.2.4

Coagulation–Flocculation

In the harvesting of microalgae from wastewater, multivalent metal salts (Fe, Al) and cationic polymers (both synthetic and natural) were found to be the most successful. For numerous microalgal strains, the effectiveness of ferric chloride and alum has been proven. Natural polymers (e.g., tan floc, chitosan, tannin, starch, gamma glutamic acid, guar gum, and tamarind kernel polysaccharide) could also boost microalgal harvesting efficiency if used at a lower dosage (Gutiérrez et al. 2016). Recent research has shown that ferric chloride and alum collected biomass can be used to recover iron and aluminium. Furthermore, the extracted metals, in combination with a little amount of fresh coagulant, might be used to harvest biomass from

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the next batch of microalgal culture. Alternatively, the bacterial strain was grown separately utilising lignocellulosic materials to produce specialised bio-flocculants (e.g., xylanase and cellulase) that were shown to be very effective in harvesting microalgal strains (e.g., Chlorella minitussima) (Liu et al. 2017).

4.2.5

Filtration

Membrane filtration of microalgae culture might theoretically achieve 100% cell recovery, and unlike other methods of harvesting, membrane filtration could be used on a wide range of microalgae strains, resulting in a biomass slurry with no degradation in biomass quality. A vibrating screen setup could be utilised to separate the biomass from a filamentous strain (e.g., Arthospira sp.) (Vonshak and Richmond 1988). Another promising biomass separation technique is tangential-flow-filtration (TFF), in which the culture is pumped through the TFF module, where a fraction of the clear water passes through the membrane while all the cells remain in the reduced volume of the culture, resulting in an increase in biomass density in the water. The TFF is used to pass the concentrated culture through until the necessary biomass density is achieved. Biomass density, cell shape, and culture salinity are some of the essential characteristics that influence membrane filtration energy consumption. Membrane fouling would occur during TFF operation; the frequency and extent of membrane fouling would be determined by the strain type, growth media composition, and TFF operation mode. Backwashing is typically performed during the TFF operation for cleaning the membrane and recovering the biomass for a short period of time. The TFF filtrate would be of good quality and may be reused for a variety of purposes (Das et al. 2019).

4.2.6

Electrocoagulation

The coagulants are created in situ from the anode when the electrodes (aluminium, steel, etc.) are linked to a DC power source in the electrocoagulation process. In addition to low coagulant requirements, the electrocoagulation technique has a low energy consumption (e.g., 0.3 kWh/m3 ). The coagulants subsequently divide the microalgal cells into big flocs, which are separated by sedimentation or floatation. Microbubbles of oxygen and hydrogen gas are created at the anode and cathode, respectively, during the electrocoagulation process. These microbubbles can cling to the coagulated algae cells and float to the surface. When comparing aluminium electrodes to steel electrodes, the efficiency of algal separation was substantially greater for aluminium electrodes (Curteanu et al. 2011).

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5 Factors Influencing the Growth of Microalgae 5.1 Light One of the most important limiting elements in microalgae production is light intensity. Microalgae photosynthesis is directly affected by light duration and intensity, which also has an impact on the biochemical composition of microalgae and biomass yield. Growth rate and biomass productivity are predicted as a function of light in models of outdoor or indoor algal culture systems (Huesemann et al. 2013). Light intensities fluctuate within the culture and decrease as culture depth increases; this should be considered when modelling a bioreactor or open pond system. The amount of light that algae require for optimum development and biomass buildup varies by species. Microalgae cannot grow efficiently at both extremely low and very high light levels. Net growth is zero at the compensation point, where photosynthetic CO2 uptake perfectly matches respiratory CO2 release. Higher light intensities will boost photosynthetic rate to a maximum point, after which it will level off until photorespiration and photoinhibition balance the photosynthetic rate. As a result, the appropriate light intensity in each situation must be found experimentally in order to maximise CO2 assimilation while minimising photorespiration and photoinhibition. Algal photosynthesis necessitates a specified duration of light/dark intervals (Ye et al. 2012).

5.2 Temperature Temperature is another key component in the growth of microalgae, as it has a direct impact on biochemical processes in the algal cell factories, including photosynthesis. Each species has its own ideal temperature for growth. Rises in temperature to the optimum range exponentially boost algal growth, but increases or decreases in temperature beyond the optimum point slow or stop algal growth and activity (Bechet et al. 2017). For most algae species, the ideal temperature range is 20– 30 °C. Microalgae cultures grown at non-optimal temperatures lose a lot of biomass, especially in outdoor culture systems. Temperature is a crucial element in largescale production, particularly in open-pond culture, and it requires close monitoring because algae endure significant temperature changes over time (Bechet et al. 2010). Low temperatures inhibit photosynthesis by inhibiting carbon assimilation, whereas high temperatures inhibit photosynthesis by inactivating photosynthetic proteins and disrupting the cell’s energy balance. Cell size and respiration both shrink when the temperature rises. A drop-in growth rate is caused by a decrease in photosynthesis. The main effect of temperature on photosynthesis is a decrease in the activity of the dual-function enzyme ribulose-1,5bisphosphate (Rubisco). Depending on the proportional levels of O2 and CO2 in the chloroplasts, it can serve as an oxygenase or a carboxylase. Rubisco enzyme CO2

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fixation activity increases with increasing temperature up to a point, then decreases. As a result of its effect on the affinity of ribulose for CO2 , temperature is a limiting factor for algal growth rate and biomass output (Salvucci and Crafts-Brandner 2004).

5.3 Nutrients The nutritional requirements of different microalgae species may differ, but the essential requirements are the same for all. Nitrogen, phosphorus, and carbon make up the backbone of microalgae and are classed as macronutrients necessary for algal growth. Silicon is also required as a macronutrient by some marine microalgae species. Microalgae use water to absorb oxygen and hydrogen. Varied species of microalgae may have different amounts of macronutrients like nitrogen and phosphorus (Juneja et al. 2013). The micronutrients Mo, K, Co, Fe, Mg, Mn, B, and Zn are only needed in minimal levels, yet they have a big impact on microalgae growth because they affect a lot of enzymatic activity in algal cells. Inorganic nitrogen and phosphorus are usually absorbed in the form of nitrates and phosphates. Other inorganic nitrogen sources, such as urea, are also a good supply and a cost-effective option. Carbon can be given to the algal culture in the form of organic compounds like glycerol or acetates, or in the form of CO2 . However, in order to grow microalgae on a big scale, environmental CO2 must be used as a carbon source, which is not only inexpensive but also has the added benefit of CO2 mitigation. The key inorganic nutrients required for microalgal development are P, N, and C.

5.4 Mixing In microalgae production, mixing and aerating ensure homogeneous dispersion of nutrients, air, and CO2 . They also allow for the penetration and even dispersion of light throughout the culture, as well as the prevention of biomass settling and aggregation. If all other parameters are met but no mixing occurs, biomass productivity will be drastically reduced. As a result, microalgae cultures must be constantly mixed to keep all cells suspended and exposed to light. In a photo-bioreactor, a correct mixing system not only allows for nutrient dissolution and light penetration into the culture, but it also allows for efficient gaseous exchange.

5.5 pH and Salinity Another key factor impacting microalgae growth is the pH of the culture media. The pH needs of different microalgae species vary. Most thrive in a pH range of 6–8.76.

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The pH of different sources of growth medium varies. The salt of the culture media will rise as the pH rises, which is particularly detrimental to algae cells (Khan et al. 2018).

6 Conclusion Wastewater provides required nutrients in an aqueous media for microalgae production while also removing contaminants such as heavy and toxic metals, TSS, TDS, FOG, BOD, and COD. Another simulated technique using granular activated microalgae pellets proven to be a viable alternative for effective wastewater treatment. During improved cultivation in wastewater, the microalgae’s natural lipid, carbohydrate, and protein contents are preserved. These natural ingredients can be used to generate energy. Microalgae’s tremendous productivity, combined with a typical biofertilizer production technology, would eliminate the financial and environmental difficulties associated with chemical ones. As a result, the design of suitable highrate algal ponds or photobioreactors for large-scale cultivation and harvesting of microalgae biomass during wastewater treatment in the context of biofertilizer generation is critical. The key problem now is to show these types of procedures on a big scale, under various conditions, and with various effluent types, as part of many industrial scale projects that are currently underway. The biomass produced is a useful resource for a variety of purposes. Biodiesel and bioethanol, as well as biogas/biomethane, can be made from microalgae biomass, with the latter being the most recommended. Furthermore, microalgae biomass can be used to provide intermediates for chemical synthesis. Microalgae biomass, on the other hand, is best used for animal feed and agriculture. Microalgae includes useful components (proteins, fatty acids, bio stimulants, and so on) that allow food production operations to increase yield while also improving sustainability and economic balance. There’s no doubt that additional microalgae-based processes will be implemented in the next years, combining nutrient recovery with biomass production, increasing the relevance of microalgae biotechnology.

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Membrane Bioreactor (MBR) Technologies for Treatment of Tannery Waste Water and Biogas Production Mahadevan Vaishnavi, Kannappan Panchamoorthy Gopinath, and Praveen Kumar Ghodke

Abstract Leather is one of the traditional and prominent industries in India, contributing substantially to the economy in view of export earnings and intensive employment. However, tannery processes are known to consume large quantities of water and thus subsequently producing huge volumes of highly complex, potentially toxic and enormously polluted effluent. Currently, the Activated Sludge Process used for the treatment of tannery waste water is an energy intensive operation accounting to almost 60% of the maintenance and operational costs of the effluent treatment plant. Alternative treatment technologies that employ Upflow Anaerobic Sludge Blanket (UASB) reactors, though partially advantageous by virtue of biogas production, present with suitability issues in cases of effluents with hyper salinity and high suspended solids concentration. Membrane bioreactors (MBRs) are a promising technological option, with potential to overcome the shortcomings of UASBs along with several benefits viz. achieving high retention of solid particles inside the reactor, thus eliminating the need of secondary clarifier and also enhancing the biodegradation potential of organic pollutants. This chapter summarizes the fundamentals of membrane separation processes, principles of MBR systems and its configurations, with special emphasis on submerged anaerobic systems for biogas production. To facilitate a more holistic view, a comparison of MBRs with other effluent treatment processes and the challenges and limitations involved with submergered anaerobic membrane systems are also outlined. Keywords Tannery waste water · Biogas · Submerged anaerobic membrane bioreactor

M. Vaishnavi · K. P. Gopinath (B) Department of Chemical Engineering, SSN College of Engineering, Rajiv Gandhi Salai (OMR), Kalavakkam, Thiruporur 603110, Tamil Nadu, India e-mail: [email protected] P. K. Ghodke Department of Chemical Engineering, National Institute of Technology, Calicut, Kozhikode 673601, Kerala, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_11

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Abbreviations UASB MBR USD CAGR UK UAE BOD COD CETP STP ASP AD TDS ZLD HRT SRT MF UF NF RO nm µm TSS SO4 NO3 Na TMP DO MLSS AeMBR SAD MLVSS dMBR eMBR AnMBR Deg C % g/L kg COD/L kWh/m3 L/m2 h CSTR

Upflow Anaerobic Sludge Blanket Membrane bioreactor United States Dollars Compound Annual Growth Rate United Kingdom United Arab Emirates Biological Oxygen Demand Chemical Oxygen Demand Common Effluent Treatment Plant Sewage Treatment Plant Activated Sludge Process Anaerobic Digestion Total Dissolved Solids Zero Liquid Discharge Hydraulic Retention Time Solid Retention Time Microfiltration Ultrafiltration Nanofiltration Reverse osmosis Nanometer Micrometer Total Suspended Solids Sulfate Nitrate Sodium Transmembrane Pressure Dissolved Oxygen Mixed Liquor Suspended Solids Aerobic Membrane Bioreactors Specific Aeration Demand Mixed Liquor Volatile Suspended Solids Diffusive Membrane Bioreactor Extractive Membrane Bioreactor Anaerobic Membrane Bioreactor Degree Celsius Percentage Gram per litre Kilogram COD per litre Kilo watt hour per cube metre Litre per square metre hour Completely stirred tank reactor

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AFBR EGSB EPS iSub eSub Ext PVDF PE C-PE PET P-Ceramic A-Ceramic PES PTFE m/s TN TP

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Anaerobic fluidized bed bioreactor Expanded granular sludge bed Extracellular Polymeric Substances Internally Submerged Externally submerged External crossflow Polyvinylidene fluoride Polyethylene Chlorinated Polyethylene Polyethylene terephthalate Pyrophyllite-based Ceramic Alumina-based Ceramic Polyethersulfone Poly-tetrafluoroethylene Metre per second Total nitrogen Total phosphorus

1 Introduction Leather is one of the labour intensive employment procuring, traditional industry in India playing a prominent role in the world’s economy, and is valued at USD 394.12 billion in 2020 and is expected to grow at a compound annual growth rate (CAGR) of 5.9% from 2021 to 2028 (Leather Goods Market Size 2021). China is the largest producer of leather and leather goods holding up to 25% share of the global production, while India ranks at the 4th place with a global production share of 6.4%, producing 1560 million square feet of leather annually (Global Leather Industry Factsheet 2020). The Indian leather industry occupies a place of prominence in the Indian economy in view of its substantial export earnings and growth, besides the fact that it is an employment-focused industry of almost 4.42 million people (URL 1). In fiscal year 2021, the value of leather and leather products exported from India amounted to over 244 billion Indian rupees (Export value of leather and leather products from India 2021). The major markets for Indian Leather & Leather Products are USA with a share of 17.22%, Germany 11.98%, U.K 10.43%, Italy 6.33%, France 5.94%, Spain 5.01%, Netherlands 3.52%, U.A.E 3.35%, China 2.61%, Hong Kong 2.15%, Belgium 2.21% and Poland 2.11% and exports of leather, footwear and leather products touched $3.67 billion during 2020–21 (URL 2). More than 2,400 numbers of tanneries and leather products manufacturing units are existing in India (Hides and Skin 2016), using up to 17,200 tonnes of raw hides and skins per day to produce finished leather, and releasing around 60 million litres of wastewater per day and around 510 tonnes per day of solid waste per day (Sreeram and Ramasami 2003).

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2 Background Tanning is the chemical process that converts animal hides and skin into leather and related products. The transformation of hides into leather is usually done by means of tanning agents. It is classified into two forms depending upon the functional needs of the various operations and are known as Beam house operations and Tan Yard operations. Beam house operations and Tan Yard operations include chemical processes like Trimming and desalting, Soaking, Liming, Pickling, Vegetable and Chrome tanning (Alighardashi et al. 2017; Giacobbo et al. 2015).

2.1 Tannery Effluent Tanning industry has been viewed to be a major source of water pollution and tannery effluent is among one of the hazardous pollutants of industry and are typically characterized as pollution intensive industrial complexes, generating widely varying, high-strength wastewaters (Kanagasabi et al. 2013; Durai and Rajasimman 2011). They generate wastewater in the range of 30–35 L/kg skin/hide processed with variable pH and high concentrations of suspended solids, BOD, COD, tannins including chromium (Daryapurkar et al. 2001; Shah 2020). Tannery waste are uniquely identified as an activity generating pollution of mixed character in the sense that both organic and inorganic constituents occur at concentrations higher than other wastes. Major problems are due to wastewater containing heavy metals, toxic chemicals, chloride, lime with high dissolved and suspended salts and other pollutants (Midha and Dey 2008). Tannery wastewater treatment is complex due to the variety of chemicals added at different stages of processing of hides and skins. Organic and other ingredients are responsible for high BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) values and represent an immense pollution load, causing technical problems, sophisticated technologies and high costs in concern with effluent treatment. Tanneries are thus obligated to treat effluent to a level that cause less impact on the environment. Manufacturing of finished leather, finished leather goods and leather boards produces numerous by-products, solid wastes and generates highly turbid, colored, foul smelling wastewater. The major components of the effluent include sulphide, chromium, volatile organic compounds, large quantities of solid waste, suspended solids like animal hair and trimmings (Bhaduri et al. 2020a; Midha and Dey 2008). Major problems arise as tannery wastewater contains heavy metals, toxic chemicals, chloride, lime with highly dissolved and suspended salts and other pollutants and thus their uncontrolled release into natural water bodies increases health risks for human beings and environmental pollution leather industry is being urged to search cleaner, economically and environmentally sustainable wastewater treatment (Lofrano et al. 2013).

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2.2 Treatment Methods Currently, the tannery wastewaters are treated by Common Effluent Treatment Plants (CETPs) in India since 1986. Technology used for the CETPs are mostly based on the design of conventional Sewage Treatment Plants (STPs) for domestic wastewater treatment. The preliminary treatment options included grit chambers, equalisation and pre-aeration or chlorination. The physio-chemical treatment operations consisted of units such as sedimentation, dissolved air flotation, flocculation cum clarifloculation, granular media filtration, land treatment, neutralisation and precipitation. Secondary biological treatment processes were stabilization ponds/lagoons, activated sludge process and trickling filters. The activated sludge process (ASP) is efficient in BOD, COD and nutrients removal and the process itself has flexibility and numerous modifications can be tailored to meet specific requirements (e.g. for nitrogen removal). Other advantages are good quality effluent, loss of head is small, high quality of treatment and free from fly and odor nuisance. ASP is the best documented and most widely used form of secondary wastewater treatment. Prior to ASP, many conventional processes were carried out to treat wastewater from tannery industry such as oxidation process, chemical process and biological process. The presence of sulphide, chromium, chloride and fluctuation in temperature has adverse effects on nitrification process and has adverse impact on a full-scale industrial activated sludge plant treating leather tanning wastewaters (Guo et al. 2016; Shah 2021). Also, the activated sludge process (ASP) uses mechanical and diffused aerators, to treat tannery effluents, is an energy intensive operation, amounting up to 60% of the overall operating and maintenance cost of the entire treatment process (Umaiyakunjaram and Shanmugam 2016).

2.3 Drawbacks of Conventional Wastewater Treatment In addition to the above, the drawbacks in the existing wastewater treatment comprising of primary and secondary biological treatment system are (Choudhary and Pandey 2021; Hussain and Head 2012). i. Space requirement is high. ii. A sudden increase in the volume of effluent or a sudden change in the character of effluent, causes adverse effects on the working of the process and consequently efficiency of the process is affected. iii. Addition of chemical is necessary in the primary treatment which increases the TDS and also increases the generation of primary sludge. iv. Large scale sludge storage and disposal is required. v. Skilled continuous supervision is required to run the ASP. vi. An additional reverse osmosis plant is required for the removal of TDS and achieve zero liquid discharge system (ZLDs).

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2.4 Need for Anaerobic Treatment Alternatively, many researchers have extensively investigated the usage of Upflow Anaerobic Sludge Blanket (UASB) reactors, to treat tannery effluents through anaerobic digestion (AD), and thereby generating biogas, power and at-least partial revenue from the organic pollutants (Dogruel et al. 2006; Rajamani et al. 1997). Compared to aerobic treatment, the anaerobic digestion showed numerous advantages (Mainardis et al. 2020): i. ii. iii. iv.

It does not need aeration. It requires lower amounts of essential nutrients. Most of the substrate is converted into a clean source of energy, i.e. methane. It protects the environment by reducing the amount of solid residues and in the contrary, yields solids of good quality for fertilization.

However, in the case of wastewater with high suspended solids concentration, the UASB becomes less suitable, as the hyper-salinity significantly hinders the biogranulation process (Lefebvre and Mosaddouda 2006). Salinity conditions and its subsequent stress has toxic effect on the biomass, affecting the cell activity leading to cell plasmolysis and is regarded as one of the major limiting factors for anaerobic systems (Ismail et al. 2010). Thus, in the sludge blanket of UASB reactor, poor biogranulation, high washout of active biomass and deader biomass results in inefficient biogas yield (Tare et al. 2003; Visser et al. 1993; Alphenaar et al. 1993; Lettinga et al. 1984). In addition, the conventional anaerobic reactor does not exercise a selective solid recycle which causes the solid retention time (SRT) to be identical to the hydraulic retention time (HRT). This limits the volumetric loading rate of the digester and requires longer SRT (20–30 d) (Saddoud et al. 2007).

3 Membrane Separation Processes Filtration involves the separation (removal) of particulate and colloidal matter from a liquid. In membrane filtration the range of particle size is extended to include dissolved constituents (typically 0.0001–1.0 µm). The membrane serves as a selective barrier that will allow the passage of certain constituents and will retain other constituents found in the liquid. Membrane processes, rely on hydraulic pressure to achieve separation, includes microfiltration (MF), ultrafiltration (UF), Nanofiltration (NF) and Reverse osmosis (RO). Table 1 describes a brief overview of various membrane processes (Abdullah et al. 2018; Ghernaout 2019; Yusuf et al. 2020). Whereas, dialysis process relies on concentration gradient to develop an osmotic pressure to achieve separation by means of chemical equilibrium. Electro dialysis processes uses an electrical potential to drive ions through an ion permeable membrane. Membrane processes are classified based on (Abdullah et al. 2018)

Micro pores (8

Stuckey (2012), Berkessa et al. (2018)

Energy consumption (kWh/m3 )

~2

0.03–5.7

Martin et al. (2011)

Water flux (L/m2 h)

20–30

5–12

Wang et al. (2018)

Sludge retention time (day)

5–20 Low

>100 High to moderate

Liao et al. (2006), Skouteris et al. (2012)

Operational temperature (°C)

20–30

20–50

Martinez-Sosa et al. (2011), Hai et al. (2018)

Effluent quality

Excellent

High

Lin et al. (2013)

Alkalinity requirement

High

Low

Lin et al. (2013)

Temperature sensitivity

Low

Low to moderate

Lin et al. (2013)

Startup time

Mn, respectively (Radziemski et al. 2019).

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4.6 Paper About 240–250 chemicals have been identified in effluents, produced at different stages of paper making in the pulp and paper industry (Hossain and Ismail 2015). The manufacturing of paper and pulp industry has a harmful effect on the environment. The process uses intensive energy, water and various chemicals in which some are hazardous (Latha et al. 2018). The effect of pulp and paper mill effluents on the physicochemical and biological structure and the intrinsic ecological capabilities of the receiving watercourses was studied (Karrasch et al. 2006). Disposable Paper cups are composed of 90% high strength paper with 5% thin coating of polyethylene and are a threat to the environment. This polyethylene prevents the paper cup from undergoing degradation in the soil (Arumugam et al. 2018). Most of the paper and pulp mills discharge their effluents directly into the water bodies which lead to serious environmental problems. These effluents contain Bleach and Black liquor as a by-product in waste from production of paper (Uma and Tripathi 2020). Although numerous studies have looked at ways by various researchers to remove COD, BOD, color etc. of pulp and paper effluents, the problem still persists (Kashif et al. 2019). The major pollutants in a pulp and paper industry are the various gases like sulfur, nitrogen oxides, chlorinated organic compounds, nutrients and metals emitted to the air which are discharged to the wastewater (Sharma et al. 2021). Pulp and paper production, consumption and wasting have many negative environmental and social impacts and are responsible for denuding of forests (Kumar et al. 2014). A large amount of water and various chemicals required for paper production, which is produced at different steps of papermaking in paper mills (Sangeetha et al. 2018). Although the physical and chemical methods are on the track of treatment, they are not on par with biological treatment because of cost ineffectiveness and residual effects. The biological treatment using microorganisms including bacteria, fungi and actinomycetes is known to be effective in reducing the organic load and toxic effects of paper mill effluents (Karrasch et al. 2006). Microcystis spp., can decolourize diluted bleach kraft mill effluents and the study observed that pure and mixed algal cultures removed up to 70% of colour within 2 months of incubation. White rot fungi such as Phanerochaete chrysosporium, Trametes versicolor etc., are used to decolorize the paper and pulp mill effluents. Due to the production of hemicellulase, lignin peroxidase, manganese peroxidase and laccase in Gliocladium virens, a saprophytic soil fungus decolourises paper and pulp mill effluents by 42% (Punmia and Ashok 1998). Number of bacterial species have been assessed for their decolorization abilities and a few of them have also been used commercially. The dominant original microbes like Bacillus subtilis and Micrococcus luteus were found competent in reducing BOD (87.2%), COD (94.7%) and lignin content (up to 97%) after 9 days. It has been observed that numerous bacteria can decompose monomeric lignin. Substructure models, only a few strains are able to attack lignin derivatives obtained from different Pulping processes. Pseudomonas aeruginosa are capable of reducing kraft

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mill effluent color by 26–54% or more under aerobic conditions were tested Bacillus cereus and two strains of Pseudomonas aeruginosa for the decolorization of leach kraft effluent (Tyagi et al. 2014). Increase the plant height, biomass, grain yield and chlorophyll content in wheat plants on irrigation with diluted effluent of the paper industry. Paper industry effluent can be utilized for irrigation purposes after proper dilution; the increased soil column height provides better growth and yield of wheat plants (Anoop et al. 2003). The different concentrations (10, 20, 30, … 100%) of paper mill effluent on growth and production of rice, mustard and peas for three years has been recorded. The paper mill effluent has deleterious effects on the growth of crops at higher concentrations. The lower concentration (viz. 10–40% in rice, 10–50% in mustard and 10–60% in pea) of effluent, beneficial impact on general welfare of the crops was noticed (Medhi et al. 2011).

4.7 Petroleum Petroleum effluents comprises grease and Petroleum compounds which consists of three main groups of Hydrocarbon; Naphthenic, Paraffin and Aromatics. In addition to this, Naphthenic acid are one class of compounds present in wastewaters from Petroleum Industries which is known to cause toxic effects and their removal from oil field wastewater is a challenge for remediation of Petrochemical effluents (Aljuboury et al. 2017). Petroleum is composed of hundreds or thousands of aliphatic, branched and aromatic hydrocarbons (Alexis 2018) and other organic compounds including some organometallic constituents (Ali 2020). Most of them are toxic to humans, animals and vegetation in the long term. They are classified as priority environmental pollutants by the US Environmental Protection Agency due to the adverse impact of these chemicals on human health and environment (Yuniati 2018). Microbial degradation is the major and ultimate natural mechanism by which one can clean the petroleum hydrocarbon pollutants from the environment. The recognition of biodegraded petroleum-derived aromatic hydrocarbons in marine sediments was reported by Nilanjana and Preethy (2011). The extensive biodegradation of alkyl aromatics in marine sediments which occurred prior to detectable biodegradation of n-alkane profile of the crude oil and also the microorganisms, namely, Arthrobacter, Burkholderia, Mycobacterium, Pseudomonas, Sphingomonas, and Rhodococcus were found to be involved for alkyl aromatic degradation. Microbial degradation of petroleum hydrocarbons in a polluted tropical stream in Lagos, Nigeria was reported by Ali (2020). Totally nine bacterial strains, namely, Pseudomonas fluorescens, P. aeruginosa, Bacillus subtilis, Alcaligenes sp., Acinetobacter lwoffii, Flavobacterium sp., Micrococcusroseus and Corynebacterium sp. were isolated from the polluted stream. Moreover, soil microorganisms naturally live and survive in microenvironments (Ali 2020).

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4.8 Pharmaceutical Pharmaceutical industry production involves raw materials, a variety of medicines, antibiotics and cosmetic products, which is generated from effluents containing constituents harmful to aquatic life and human life. These effluents are toxic and hazardous which also has intensive color and smelly odor (Rajender et al. 2017). Many Industrial Pharmaceuticals effluent (PE) has implications for public health and these PE contains an aromatic compound that contaminates the groundwater and gets accumulated in plants/animals tissues which leads to serious toxicological problems (Enick and Moore 2007). The presence of pharmaceutical residues in the environment was reported for the first time in the 1970s (Oliver et al. 2005). Pharmaceutical waste is introduced into the environment in different ways; in this one of the important ways is through human excretion. The other way is when a medication expires people will dispose of it in the toilet or in garbage (Thomas et al. 2002). Special care must be taken regarding their accumulation in the environment for four reasons (Enick and Moore 2007). Biological wastewater treatment is a widely appeared process due to its ease of handling and economically inexpensive. In the PE discharged soil, two bacterial strains were isolated and identified as Klebsiella pneumonia and Pseudomonas aeruginosa which carry out biological degradation. In these two, Pseudomonas aeruginosa can be suggested for controlling the pollution caused by PE (Prince 2016). Wastewater treatment plants (WWTPs) are not designed to remove pharmaceutical compounds from domestic wastewaters. The treatability of pharmaceutical compounds in WWTPs varies depending on the compounds biodegradability. Fungi and algae-treatments are helpful in eradicating the harmful chemicals in PE (Andreia et al. 2019). In recent years, improper disposal of PE, which contains chemical compounds that lead to great environmental damage. An alternative disposal for PE has been studied that can be reused for agricultural purposes. Research has been conducted on PE as fertilizer for plants in Brazil, evaluating the potential use of PE for fertilizing Schinus molle, a native Brazilian species locally known as aroeira-salsa. Here the PE was pretreated by chemical precipitation, and the solid phase was tested as fertilizer in the plant. So, this fertilizer increases the nutrient absorption of plants and the plant growth (Carina et al. 2019). Here 5 rates of PE were used (0, 25,000, 50,000, 75,000 and 100,000 L/ha were used). Results show that PE had an effect on some soil chemical properties and also helped in the growth of Maize (Osaigbovo et al. 2006).

4.9 Textile The textile wastewater is rated as the most polluting among all in the industrial sectors (Syeda et al. 2009). The wet process uses a considerable volume of potable

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water and releases contaminated wastewater. This wet process includes sizing, desizing, sourcing, dyeing, mercerizing, printing, bleaching and finishing techniques. The dyes and chemicals used in textile industries are toxic when it is released in the environment by untreated effluents (Yaseen and Scholz 2019). Textile industrial wastes and effluents are being discharged at random without treatments directly to soil, canals, and rivers. They pollute our soils and natural water systems as well as ground water endangering human health, aquatic lives, and crop production in the environment (Asma et al. 2013). Removal of colored compounds from textile industry effluents by physic-chemical and biological methods is currently available. Microorganisms such as bacteria, fungi, actinobacteria, Yeast, algae, and plants, the biological decolourization of dye effluent is much consideration due to low cost (Teshale and Amare 2021). Textile sludge can increase the nutrient contents of pot soil and growth of red amaranth, which is revealed by pot experiments. Therefore, it can be used as soil improver if Pb, Cr, Zn and Fe content can be controlled in the textile sludge (Badrun et al. 2016). The dilution of wastewater has significantly increased growth and germination of black gram, green gram, rice, groundnut, sunflower and maize (Elarajan and Bupathi 2006). Moreover the treated waste water from textile could safely be used to irrigate crop plants and solve the problem of effluent disposal in an eco-friendly manner (Gufran et al. 2011). Biologically treated wastewater exerted no deleterious effects on the studied parameters, as compared with untreated wastewater and clean irrigation water, but significantly increased the plant biomass and yield (Tahira et al. 2014). Textile sludge can improve the nutrient contents of soil and growth of red amaranth. Therefore, it can be used as soil improver Pb, Cr, Zn and Fe content can be controlled in the textile sludge (Badrun et al. 2016). The pot-trail was conducted to evaluate the suitability of untreated textile wastewater at different dilution levels for improving growth, physiology and yield of Triticum aestivum L. (Najam et al. 2017).

4.10 Tannery Tannery industry is one of the major important and developing areas in the industrial sector but it has a lot of disadvantages in the management of the waste which is coming out of the tannery industry. It affects the ecosystem and causes a lot of trouble to agriculture due to the chemicals present in the tannery waste. The waste produced by tannery industry is one of the main reason of the soil contamination not only in the pollution of the tannery industry it also increase the presence of the toxic substance in the soil, waterways and sediments it also affect the food chain and disturb the biological process (Subramanian et al. 2014). Most tannery industries use chromium salts in the tanning process to provide good leather. When chromium is released in tannery wastes (effluent or sludge), it can cause carcinogenic, teratogenic and mutagenic effects in humans, plants and animals in the environment (Geremew and Tekalign 2017). Tannery effluent is also

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known as water waste because it contains fine leather particles, residues from various chemical discharges and reagents from different waste liquors, huge pieces of leather cuts, trims, and coarse shavings, fleshing residues, solid hair debris, and paper bag remnants (Robin et al. 2012). The tannery effluent mixed with Ganga river water and different concentrations (5, 15, 25, 50, 75 and 100%) were applied on seed germination of Vigna radiata. The 100% of seed germination was observed in 15 and 25% of dilutions but the 100% of tannery effluent given 0% of germination (Singh et al. 2014). Cristina et al. (2008) reported that the application of treated tannery wastewater on Trifolium pratense and Phragmites australis at different concentrations (2, 5, 10, 25, 50 and 100%). Low concentrations of treated effluent showed high % of germination and high concentrations of treated effluent showed low % of germination.

4.11 Poultry Waste disposal is one of the most serious problems facing the poultry processing industry and the characteristics of poultry processing wastes depend upon the manner in which blood, feathers and are handled, the degree of efficiency of screening and liquid solids separation at the pre-treatment level (Sarita and Neeraj 2010). By treating Chlorella vulgaris and Chlorella protothecoides for the production of feed, fertilizer and/or biofuels in the poultry industry has been revealed by Catarina et al. (2016). Both undigested poultry manure influent TKN (46% NH4+ -N) and digested poultry manure effluent TKN (88% NH4+ -N) were compared with urea at equal rates of TKN application (146 kg ha−1 ) for corn (Zea mays L.) yield (Caldwell et al. 1986). Fertilization of maize gave significantly (P = 0.05) higher seed yields. Fortified poultry manure gave an average yield of 3.97 t ha−1 while fortified Pacesetter fertilizer had an average of 3.78 t ha−1 . Inorganic fertilizer gave a yield of 3.70 t ha−1 while a significantly lower yield of 2.48 t ha−1 was given by the unfertilized plants. Maize growth and yield from the enriched organic manures were comparable with inorganic fertilizer, indicating the potential of the use of fortified organic manures as alternatives to inorganic fertilizers. Poultry manure required lesser N-fortification to give comparable seed yields as cow dung (Ayoola and Makinde 2009). The composts were of high quality, characterized by high levels of nutrients, a relatively low C/N ratio of 15–17 and a fertilizing value similar to that of conventional cattle manure, however without phytotoxicity. Application of compost (poultry manure, olive mill wastes and mineral-rich wastewater) showed an increase in potato production of 31.5–35.5 t/ha, compared to 30.5 t/ha using cattle manure (Salma et al. 2006).

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5 Deliverable and Value Added Products 5.1 Biofertilizer The use of treated industrial liquid and solid waste materials for biofertilizers production is an environmentally friendly approach in the integrated management as they are cheap and renewable sources of nutrients for sustainable plant growth and yield. Biofertilizers comprise living microorganisms that, upon application, provide almost all the nutrients necessary for the growth of the plants, minimizing the environmental impact of land use. Formulations based on nitrogen fixers/photosynthetic/mobilization of nutrients microorganisms, such as bacteria, fungi, cyanobacteria and microalgae, are of particular interest due to the valuable biomass production. Further, many studies could be concentrated on the development and commercialization of carrier based biofertilizers from different industrial waste for managing sustainable agriculture and land development.

5.2 Biogas The abundant biomass wastes generated by different industries are used for biogas production by biomethanation process. Wastes consisting of rumen and paunch contents, dung, agriculture residue, fat and blood are processed in biomethanation plants. Power plants have been designed to produce biogas by digestion of animal waste.

5.3 Manure The leftover feed, dung from the lairage, ruminal and intestinal contents, blood, meat trimmings, floor sweepings, hair, feathers, hide trimmings can be stabilized by composting with agro waste. It produces very good quality bio-manure which may be utilized as fertilizers/manure for the agriculture land and gardens.

5.4 Irrigation During drought periods in some places, agricultural production often occurs under water deficiency or causes the depletion of the existing water resources. Hence, reuse of treated wastewater for crop irrigation could contribute to balance water shortage, support the agriculture sector and protect groundwater resources.

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Fig. 1 Treatment of different industrial waste and application on plants to improve growth and yield

6 Conclusion Biofertigation is a viable alternative process for waste treatment and recovery of nutrients of industrial wastes that support the plant growth. Utilization of these waste by-products as fertilizer contributes a lot in organic farming and could reduce our dependence on synthetic fertilizers. The application of the above treated industrial wastes had a positive impact on the growth and yield. Degradation of different industrial waste using microorganisms has a high ecological significance that depends on the indigenous process to convert into manure/biofertilizers. The industrial waste water may be treated and applied on various plants as a manure as well as foliar spray is depicted in Fig. 1. It may be concluded that microbial degradation can be considered as a key component in the cleanup strategy for industrial wastes. The conversion of the industrial wastes into completely mineralized fertilizer by biological methods could be applied for improving the crop yield. Acknowledgements The corresponding author acknowledges Rashtriya Uchchatar Shiksha Abhiyan (RUSA 2.0), MHRD, New Delhi for financial assistance during the period of study.

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Biogas as a Value Generation from Dairy Industrial Waste Water N. Prabhu, M. Shareen Fathima, N. Hiritha, M. Mounika, and A. Sureja

Abstract Dairy industry wastewater has a high chemical oxygen demand, a high biological oxygen demand, nutrients, and organic and inorganic components. If these wastewaters are not properly treated before being released, they will significantly contaminate recipient water bodies. There are various physical, chemical, and biological techniques for treating dairy waste water. Dairy waste, on the other hand, responds well to biological treatment. When microorganisms come into touch with the strongly aerated effluent, they oxidise the organic stuff to carbon dioxide and water. Microorganisms convert organic materials to biogas and cell biomass in anaerobic processes. Anaerobic digestion is a complicated chemical and biological process that is influenced by a variety of variables. Biogas is a renewable energy source that can be used as a long-term replacement for fossil fuels. The major objectives of this paper is to examine Bio-gas Generation and variables impacting Bio-gas Generation from dairy industry wastewater, such as pH, temperature, alkalinity, and so on, in order to maximise biogas release by biological breakdown. Biogas is the cheapest renewable energy source created in an engineered fashion from dairy sector effluent, according to the results. Keywords Industrial waste water · Types · Dairy waste water · Anaerobic digestion · Biogas production

1 Introduction Industrialisation plays a significant part in the growth of a country, but it also contributes to major pollution issues across the world. With rising demand for milk and milk products, the number and scale of dairy farms has exploded in many nations across the world. The dairy sector is a key source of food processing, and it uses a lot of water. Pasteurized and sterilised milk, yoghurt, ayran, cheese, cream, better, ice cream, and milk powder are only a few of the goods available in the dairy sector. N. Prabhu (B) · M. Shareen Fathima · N. Hiritha · M. Mounika · A. Sureja Department of Biotechnology, Vivekanandha College of Engineering for Women, Elayampalyam, Tiruchengode 637205, Tamil Nadu, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_17

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Both the manufacture of goods and the packing of those goods generate waste water (Srinivasan 2009). Biogas may be made through anaerobic digestion of nearly any sort of organic material, including agricultural waste, industrial waste, and residential waste. The dairy sector, like most other agricultural industries, produces huge amounts of refractory effluent with high biological oxygen demand (BOD) and chemical requirement of oxygen concentrations (COD), indicating a high organic matter content. The release of industrial milk effluent has a major environmental effect, since in addition to the high organic matter concentrations, the effluents contain high oil and graft levels and the presence of suspended particles and odours generated by casein degradation (Monali et al. 2011). The adsorption of anaerobic digestion procedures in wastewater treatment facilities in the dairy sector stands out in this situation as the biological approach more suited to treating and pre-processing produced waste effluents (Ramesh 2007). India is a major producer of milk and dairy products, with annual milk output exceeding 85 million tonnes in 2002 and increasing at a pace of 2.8% per year. Dairy effluent has significant levels of organic materials, namely Lactose, fat, and protein. Because of the fortified nutrients in cheese whey, a favourable habitat for Lactobacillus species is produced, which is beneficial in converting organic materials into methane via anaerobic process. The anaerobic treatment procedure is an effective method for converting dairy effluent to biogas (Deshannavar and Basavaraj 2012).

1.1 Industrial Waste Water We are now preoccupied with the liquid portion of industrial waste, which is usually referred to as industrial waste water. Waste water, which is produced as a by-product of process unit operations, contains components that can be detrimental to humans, animals, plants, aquatic life, and microbiological life/different life forms on the planet (Shrirame 2017).

1.2 Sources of Industrial Wastewater See Fig. 1.

1.3 Characteristics of Industrial Waste Water The properties of industrial waste water differ from one industry to the next, as well as from one process to the next, even within the same industry. They have either too high proportion of suspended solids, dissolved inorganic and organic solids, BOD

Biogas as a Value Generation from Dairy Industrial Waste Water Fig. 1 Sources of industrial wastewater

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Dairy industry

Agricultural waste

Iron and steel industry

Mine and quarries

Food industry

Complex organic Chemical industry

Nuclear industryTextile industry

alkalinity or acidity/and their different constituents will not be in the same proportion as the exist in a normal domestic sector (Arthur et al. 2011).

2 Waste Water Treatment Levels 2.1 Preliminary Treatment Waste water components such as rags, sticks, seaworthy grit, and lubricant are removed to avoid reliability issues with treatment systems, processes, and auxiliary systems (Fig. 2).

2.2 Primary Treatment A percentage of the colloidal matter and organic debris in the wastewater is removed. Improved wastewater elimination of suspended particles and organic materials. Primary treatment consists of following processes are sedimentation, coagulation and flocculation.

2.3 Secondary Treatment Decomposable organic materials and colloidal solids are removed. In most cases, disinfection is included in the concept of typical secondary therapy. Secondary treatment consists of following processes are activated sludge process, oxidation ponds and lagoons and trickling filter.

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Preliminary treatment Primary treatment Secondary treatment Tertiary treatment Fig. 2 Waste water treatment levels

2.4 Tertiary Treatment After secondary treatment, remaining suspended particles are removed using solid matrix separation or micro screens. Disinfection is usually included in tertiary therapy. This section frequently includes nutrient removal. Tertiary treatment consists of following processes are chlorination/ozonation/UV, filtration, reverse osmosis, evaporation, post aeration.

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3 Dairy Industrial Waste Water Treatment Pasteurization or homogenization of milk, butter, cheese, or yoghurt creates waste water with significant BOD and COD loads, which can only be decreased and being sent to municipal treatment facilities. Buttermilk, whey, and derivatives are common by-products. During the process, a large volume of water is consumed, resulting in effluents comprising dissolved carbohydrates and proteins, lipids, and perhaps pharmaceutical remnants. (@2018 Ecologix Environmental Systems, LLC | 11800 Wills Rd Alpharetta GA 30009).

3.1 Operations in a Dairy Industry Basic process of raw milk: • Receiving • Pasteurizing Various manufactures: • • • • • •

Bottling Condensing Dry milk manufacture Cheese manufacture Butter making Casein making

3.2 Dairy Wastes Are Made up of (Rao and Datta 2012) 1. Milk solids composed of BOD of 1 kg milk fat—0.89 kg, BOD of 1 kg milk protein—1.03 kg, and BOD of 1 kg of milk sugar—0.69 kg. 2. Dilutions of whole milk and by-products composed of Bod of whole milk90,000–1,05,000 mg/l, BOD of skim milk-65,000–75,000 mg/l, BOD of buttermilk-55,000–65,000 mg/l and BOD of whey-25,000–35,000 mg/l. 3. Waste water from: Equipment cleaning, Floor washing, Water softening, Boiler house and, Refrigeration plant. 4. Chemicals and detergents. 5. Broken glass pieces, torn bags and aluminium foil.

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3.3 Composition of the Waste Water of Typical Dairy Industries (Rao and Datta 2012)

Items

Influents

pH

7.2

Alkalinity

600 mg/l

Dissolved solids

1060 mg/l

Suspended solids

760 mg/l

BOD

1250 mg/l

COD

84 mg/l

Nitrogen

84 mg/l

Phosphorous

11.7 mg/l

Oil and grease

290 mg/l

Chloride

105 mg/l

3.4 The Sourced of the Dairy Industry Wastewater (Tawfika et al. 2008; Yonar et al.)

Dairy processes

Sources of waste

Preparation stages Milk recovering/storage

• • • • •

Pasteurisation/ultra-heat treatment • • • • •

Poor damage of tankers Spills and leaks from hoses and pipes Spills from storage tanks Foaming Cleaning operations Liquid losses/leaks Recovery of downgraded product Cleaning operations Foaming Deposits on surfaces of pasteurisation and heating equipment

Homogenisation

• Liquid losses • Cleaning operation

Separation (centrifuge, reverse osmosis)

• Foaming • Cleaning operation • Pipe leaks

Product processing stages (continued)

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(continued) Dairy processes

Sources of waste

Market milk

• • • • • • • • •

Foaming Product washing Cleaning operation Over filling Poor damage Sludge removal from clarifiers/separators Leaks Damaged milk packages Cleaning of filling machinery

Cheese making

• • • • •

Overfilling vats Incomplete separation of whey from curd Using salt in cheese making Spills and leaks Cleaning operations

Butter making

• Cleaning operations • Product washing

Powder manufacture

• • • • • •

Spills of powder handling Start-up and shut down losses Plant malfunction Stack losses Cleaning of evaporation and driers Bagging losses

3.5 Effects of Dairy Effluents The fast breakdown of dairy waste water lowers the dissolved oxygen level of receiving water and encourages the growth of sewage fungus by lowering the dissolved oxygen level of receiving water and lactose, which is a significant ingredient of waste (Deshannavar and Basavaraj 2012). When water is heavily polluted by dairy waste water, it provides a breeding ground for disease-carrying flies and mosquitos. The breakdown of casein precipitation produces a foul-smelling black sludge that is hazardous to aquatic life. They contribute to eutrophication, turbidity, and a strong unpleasant odour. The impact on the environment varies depending on the biodegradability and solubility of waste. Dairy waste water processing contains organic components that are highly decomposable (Naik 2012).

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4 Biogas Production 4.1 Anaerobic Digestion Anaerobic digestion is a method of decomposing organic materials in the absence of oxygen. In the lack of oxygen, anaerobic bacteria consume organic molecules to generate methane and CO2 . During the biogas generation in anaerobic digestion plants, a tiny quantity of hydrogen sulphide and ammonia, and therefore a trace of other gases, is generated (Bhuyar 2013). Anaerobic digestion is a natural method that entails the transformation of organic material to different end products, such as methene and carbon dioxide, in a stepby-step manner (Sorathia Harilal et al. 2012). Pathogens will be killed during the production of bio gas, resulting in a cleaner atmosphere. During anaerobic digestion, and thus producing fertiliser enriched in NPK (nitrogen, phosphorus and potassium). Bio gas is used in cooking and illumination. Bio gas, like natural gas, may be compressed and utilised to power automobiles, among other things (Arthur et al. 2011).

4.2 Theory Acetic acid-forming bacteria (acetogens) and methane-forming bacteria are among the microorganisms that impact anaerobic digestion (methanogens). These organisms stimulate a variety of chemical reactions as they convert biomass to biogas (Chelliapan and Sallis 2011) (Fig. 3).

Liquid effluents produced from cleaning, washing and sanitizing Operation

Pretreatment Options Physical

Chemical

Biological

Thermal Hydrolysis

Dairy Industries

Waste water treatment in dairy industries

BIOGAS

INFLUENCING PARAMETERS pH, Temperature, Hydraulic Retention Time (HRT), Organic loading rate (OLR), CN ratio

Methanogenesis

Co – digestion Substrates Various product residues lost during milk processing steps

Animal manure

Agrowastes

Fig. 3 Waste water treatment in dairy industries

Municipal waste

Sewage sludge

Acidogenesis

Acetogenesis

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The process includes four stages they are Hydrolysis, Acidogenesis, Acetogenesis, and methanogenesis (Dioha et al. 2013).

4.2.1

Hydrolysis

Large polymer nanocomposites make up the majority of biomass. Fewer component pieces must be created by trying to break down into chains for the bacteria in anaerobic digesters to access the material’s energy potential. Other bacteria may easily access these component elements, such as glucose. The act of unravelling these chains and dispersing the smaller molecules into solution is known as hydrolysis. As a result, the initial step in anaerobic digestion is to hydrolyse these high-molecular-weight polymeric components. Complex organic compounds hydrolyse to produce simple sugars, amino acids, and fatty acids (Kumar 2008).

4.2.2

Acidogenesis

The components of the system are broken down by acidogenic bacteria, such as fermentative bacteria, in the biological process of acidogenesis (Demirel and Yenigun 2004). VFAs, as well as ammonia, carbon dioxide, hydrogen sulphide, and other by-products, are produced here (Bhadouria and Sai 2011).

4.2.3

Acetogenesis

Acetogenesis is the third step of anaerobic digestion. Simple molecules produced during the acidogenesis phase are digested further by acetogens, which generate acetic acid, carbon dioxide, and hydrogen (Pittule 2011).

4.2.4

Methanogenesis

Methanogenesis is the final stage of the anaerobic digestion process. Methanogens utilise the by-products from the previous stages to produce methane, carbon dioxide, and water. The bulk of the biogas released from the system was made up of these components. Methanogenesis occurs between pH 6.5 to pH 8, and is sensitive to both high and low pH (Monali 2011). The following equations describe various products, by-products, and intermediate products that are generated during the digesting process of an anaerobic methane generation. The acids produced are processed by methanogenic bacteria to generate methane, which is described in the following equations (Gotmare et al. 2011). CH3 COOH → CH4 + Aceticacid

methane

CO2 carbondioxide

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2CH3 CH2 OH + Ethanol

CO2 Carbondioxide

CO2 + 4H2 Carbondioxide Hydrogen

→ CH4 + 2CH3 COOH Methane

Aceticacid

→ CH4 + 2H2 O Methane

Water

4.3 Methodology (Onyimba and Nwaukwu) See Fig. 4.

4.4 Biogas Production Procedure from Anaerobic Digester The anaerobic digestion process is divided into three stages. The first stage is to use hydrolysis to coagulate granular feedstock material. The soluble solids that resulting from hydrolysis are then digested in the second stage. Acid-producing anaerobic bacteria do this process at the molecular level (primarily acetic, propionic, and butyric acid). The microorganisms engaged in this step are organisms that can both utilise oxygen and produce energy anaerobically. These organisms can survive in a wide range of pH conditions (Lebrator and Pérez Rodríguezr 1990). Gasification is the final and most time-consuming process. Certain bacteria employ the organic acid generated in the second phase as a substrate, resulting in the production of methane and carbon dioxide emissions. Methanogenesis is the name given to the process that results in the creation of methane (Maqueda and Morillo 1990). Biogas is composed mostly of methane and carbon dioxide, with trace quantities of hydrogen sulphide and other gases. After the biogas is created, it is sent through a series of scrubbers and upgrading equipment to remove any remaining greenhouse gases. PSA (pressure swing adsorption) units or membrane filters are common gas upgrading technologies that are being used to improve gas to pipeline quality requirements and remove greenhouse gases (Srinivasan et al. 2009) (Fig. 5). Anaerobic digesters are impermeable tanks that run at a temperature of 95°–105° F or 135°–145° F. Covered lagoon, plug flow, and full mix are three common types of anaerobic digesters. Fixed film, induced blanket, and two-phase digesters are less frequent digester types. Which method is acceptable depends on the properties of the manure, how it is handled, and whether or not bedding is used? Manure may be co-digested based on digester technology (Ramesh et al. 2012). Mixing is sporadic in standard digesters, and holding durations can range from 30 to 60 days, resulting in sludge formation and reduced gas output. Due to constant mixing as well as a more effective sludge feeding and removal mechanism, modern digesters have better turnover and digestion rates. Modern digesters may retain food for as little as 15 days or fewer (Deshannavar et al. 2012).

Biogas as a Value Generation from Dairy Industrial Waste Water Fig. 4 Waste water treatment steps in dairy industries

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Collection of samples (dairy waste water, cow dung)

Testing the initial characteristics of samples

Mixing of samples

Anaerobic digestion process

Feeding the feedstock in the digester

Production of biogas

Final parameter testing

Results and conclusion

4.4.1

Throughput Products and Byproducts

Many value-added products may be produced by anaerobic digestion. Biogas may be used to create power or fuel a boiler, and it can be improved to meet renewable natural gas requirements by removing contaminants. Biogas that has been enhanced can indeed be put into the natural gas transmission or utilised as a car fuel on farms. A solid–liquid evaporator can separate digestate into a polymer solution (filtrate) and

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Fig. 5 Biogas production procedure from anaerobic digester

a heterogeneous structure (fibre) for use as a nutrition source on the ground (Qazi et al. 2011). Thesupernatant can be utilised as fertiliser or processed even more to fulfil effluent quality criteria. A commercial dairy mix digester may generate 30 pounds of nitrogen, 10 pounds of phosphate, and 30 pounds of potash every 1,000 gallons of liquid digestate (filtrate). The nutritional analysis will vary based on the digester’s management, the feedstocks employed, and other variables. The solid fraction’s fibre can be used as a soil supplement, livestock bedding, or compost feedstock, or it can be utilised to create medium-density fibre board. These are just a few of the value-added alternatives that may be used with a digester (Baig and Syed 2011).

4.5 Factors Affecting Biogas Production 4.5.1

Carbon to Nitrogen Ratio

Carbon (C) and nitrogen (N) are two important nutrients for species development and nutrient elimination (N). The carbon to nitrogen ratio must be kept between 20 and 30 for efficient anaerobic digestion (Perera). Ammonia builds up in the digester as a result of a decreased C:N ratio, which limits microorganism activity. Lower gas output is caused by a higher C:N ratio. Different types of material are mixed together to maintain the appropriate C:N ratio of the influent feed. Numerous different materials are mixed together in the influent feed. The microbial community engaged in aerobic digestion need enough resources to grow in a predictable manner. If the C/N ratio is too high, the method is governed by N availability, and the resulting acidification

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slows methanogenesis activity; if it is too low, ammonia may be found in abundances large enough to be toxic to the bacterial population (Dioha et al. 2013).

4.5.2

Temperature

Fermentation temperature has a significant impact on biogas generation. The optimum conditions for anaerobic fermentation and methane-forming bacteria are temperatures of 29–41 °C or 49–60 °C, with a pressure of 1.1–1.2 bar absolute. This is because two distinct types of bacteria reproduce best in these two temperature ranges, yet the high-temperature bacteria are far more susceptible to environmental effects. The rate of gas production increases as the temperature rises, but the percentage of methane produced decreases. Temperatures between 32 and 35 °C have been shown to be the most efficient for producing methane in a steady and continuous manner (Perera).

4.5.3

pH Value

Because methane formers are acidic, a pH of more than 7.0 is maintained during the methane production stage. According to McCarty, the optimal pH range for anaerobic treatment is about 7.0–7.2, although it can also work effectively at pH levels ranging from 6.6 to 7.6. In terms of chemical makeup, organic materials differ from one another. That mixture may not always be ideal for optimal bacterial growth and methane generation (Asha 2014). Lactose, for example, the significant aspect of whey solids, encourages the development of acid-producing bacteria under anaerobic environments. Lactose is broken down by these bacteria into short-chain fatty acids like acetic, propionic, butyric, and other acids, causing a fast drop in medium ph. Reduced pH has a detrimental impact on methane-producing bacteria, resulting in low biogas production (Demirel and Yenigun 2004).

4.5.4

Hydraulic Retention Time, in (Days), HRT

The average amount of time a soluble chemical remains in a built bioreactor is measured by hydraulic retention time, also known as hydraulic residence time. The capacity of the sedimentation tanks split by the prominent fluid velocity equals hydraulic retention time (Rajeshwari et al. 2000). [ ] volume of aeration tank m3 [ ] HRT [d] = Influent flow rate m3 /d where, HRT is hydraulic retention time (d) and usually expressed in hours (or sometimes days), the V is the volume of aeration tank or reactor volume (m3 ), and Q is

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the influent flow rate (m3 /d). The hydraulic retention time is the amount of time that the substrate and the components that are being removed keep in interaction with the biomass within the reactor (Awad et al. 2014).

4.5.5

Organic Loading Rate

On anaerobic treatment systems, the volumetric Organic Loading Rate (OLR) is proportional to the retention time via loading (Adebayo et al. 2015). The organic loading rate is significant in anaerobic wastewater treatment. Because the chemical oxygen demand (COD) is commonly employed to determine the amount of organic matter in wastewater, the OLR for biological systems is expressed as COD per reactor volume per unit time (i.e., kg COD/m3 day (Meisam et al. 2011)). The OLR may be changed by altering the permeate proportion and the flow rate. As a result of altering the HRT and the fluid velocity, OLR can be represented in the following ways under these conditions. OLR =

Q ∗ COD V

where, OLR is organic loading rate (kg COD/m3 d), Q is flow rate (m3 /d) COD is chemical oxygen demand (kg COD/m3 /d), COD is chemical oxygen demand (kg COD/m3 ), and V is reactor volume (m3 ) (Ramesh et al. 2012).

4.5.6

Alkalinity

Alkalinity refers to the digestive medium’s capacity to absorb protons or neutralise overly acidic or basic conditions. The power of water to neutralise acid is known as alkalinity (Sivakumar and Asha 2012). The typical proportion of carbon dioxide in the gas phase of anaerobic digestion is 25–45%. A pH greater than 6.5 necessitates bicarbonate alkalinity of at least 500–900 mg/L CaCO3 . When adequate carbonate buffering is not present in the wastewater, alkaline materials are added to keep the pH in the acceptable range for anaerobic digestion (Karthiyayini et al. 2017).

4.6 Merits of Biogas Biogas is a sustainable and clean form of energy, Biogas is a sustainable and environmentally friendly energy source that helps to reduce greenhouse gas emissions, Biogas does not pollute the environment, Biogas production helps to clean up the

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environment by lowering pollution, Organic fertiliser is produced via biogas production, it’s a low-cost, low-tech solution that promotes a circular economy and for underdeveloped regions, a healthy cooking option (Bhadouria and Sai 2011; Despande et al. 2012).

4.7 Disadvantages of Biogas Currently, the biogas generation system is inefficient, impurities remain in biogas after it has been refined and compressed, weather has an impact on biogas production (Jactone et al. 2009). Although bacteria require a temperature of about 37 °C to digest garbage, digesters in cold areas require heat energy to ensure a consistent biogas supply and unsuitable for densely populated urban regions (Kolhe and Pawar 2011; Gavala et al. 1999).

5 Future Aspects A new approach using an open-source low-cost system for monitoring and controlling biogas production from the dairy industry may be expected in the future: a system to monitor/control was developed, the use of low-cost electronic components and Arduino was efficient for the process monitoring, the digestion of dairy waste inoculated with sewage sludge was performed, COD removal and biogas production took place in the AnSTBR and digestate analysis showed higher toxicity.

6 Conclusion Anaerobic treatment is a tried-and-true method for producing biogas (methane), which can be used to generate renewable heat and power as well as compact output. Temperature, pH, organic loading rate, sludge retention time, hydraulic retention time, up flow velocity, and size distribution all have a significant impact on anaerobic treatment efficiency. Therefore, anaerobic treatment needs especially kind of setting because anaerobic processes successfulness depends on bacteria living and growth inside the reactor and the investigated results show that biogas is the cheapest nonconventional energy source produced through an engineered way from dairy industry wastewater.

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References Adebayo AO, Jekayinfa SO, Linke B (2015) Effects of organic loading rate on biogas yield in a continuously stirred tank reactor experiment at mesophilic temperature. Br J Appl Sci Technol 11(4):1–9 Arthur R, Baido MK, Antwi E (2011) Biogas as a potential renewable energy source. A Ghanian case study. Renew Energy 36:1510–1516 Asha B (2014) Influence of pH condition on the performance of fixed film fixed bed reactor in dairy wastewater. Direct Res J Eng Inf Technol (DRJEIT) 2(3):33–36 Awad A, Chen X, Liu J, Xie X, Zhang J, Zhang K, Wang H, Liu N (2014) Characteristics, process parameters, and inner components of anaerobic bioreactors. Hindawi Publishing Corporation. BioMed Res Int Article ID 841573 Baig S, Syed Q (2011) Anaerobic fixed film biotreatment of dairy wastewater. Middle-East J Sci Res 8(3):590–593 ISSN 1990-9233 Bhadouria BS, Sai VS (2011) Utilization and treatment of dairy effluent through biogas generationa case study. Int J Environ Sci 1 Bhuyar KD (2013) Treatment of domestic wastewater in an up flow anaerobic packed bed reactor (UAPBR). Int J Adv Eng Res Stud IJAERS II(III):122–124. Research Paper. E-ISSN 2249-8974 Chelliapan S, Sallis PJ (2011) Performance of an up-flow anaerobic packed bed reactor system treating pharmaceutical wastewater. IPCBEE 1 (© IACSIT Press, Singapore) Demirel B, Yenigun O (2004) Anaerobic acidogenesis of dairy wastewater, the effects of variations in hydraulic retention time with no pH control Deshannavar UB, Basavaraj (2012) High rate digestion of dairy industry effluent by upflow anaerobic fixed-bed reactor. J Chem Pharm Res 4(6):2895–2899 Deshannavar UB, Basvaraj RK, Naik NM (2012) High rate digestion of dairy industry effluent by upflow anaerobic fixed-bed reactor. J Chem Pharm Res 4(6):2895–2899 Despande DP, Patil PJ, Anekar SV (2012) Biomethanation of dairy waste. Res J Chem Sci 2(4):35– 39. ISSN 2231-606X Dioha IJ, Ikeme CH, Nafi’u T, Soba NI, Yusuf MBS (2013) Effect of carbon to nitrogen ratio on biogas production. Int Res J Nat Sci 1(3):1–10 Gavala HN, Kopsinis H, Skiadas IV, Stamatelaton K, Liberatos G (1999) Treatment of dairy wastewater using upflow anaerobic sludge blanket reactor. J Agric Eng Res 73(1):59–63 Gotmare M, Dhoble RM, Pittule AP (2011) Biomethanation of dairy wastewater through UASB at mesophilic temperature range. Eng Sci 8:1–9 Jactone AO, Wen Z, Ignosh J, Bendfeldt E, Collins ER (2009) Biomethane technology, vol 3. Virginia Cooperative Extension Publication, pp 442–881 Karthiyayini S, Sivabharathy A, Sreehari C, Sreepoorani M, Vinjth V (2017) Production of biogas from dairy waste water. Int J Eng Res Mod Educ Spec Iss 131–134 Kolhe AS, Pawar VP (2011) Physico-chemical analysis of effluents from dairy industry. Recent Res Sci Technol 3(5):29–32 Kumar CB (2008) Dry continuous anaerobic digestion of municipal solid waste in thermophilic conditions. A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering in Environmental Engineering and Management Asian Institute of Technology, School of Environment, Resources and Development, Thailand Lebrator J, Pérez Rodríguezr JL (1990) Cheese factory wastewater treatment by anaerobic semicontinuous digestion. Resour Conserv Recycl 3:193–199 (Elsevier Science Publishers B.V./Pergamon Press plc-Printed in The Netherlands, 193, Instituto de Ciencia de Materiales, C.S.I.C., Aptdo. 1052, 4,1080 Sevilla (Spain)) Maqueda C, Morillo E (1990) Cheese factory wastewater treatment by anaerobic semicontinuous digestion. Resour Conserv Recycl 3:193–199 (Elsevier Science Publishers B.V./Pergamon Press plc-Printed in The Netherlands, 193, Instituto de Ciencia de Materiales, C.S.I.Cz, Aptdo. 1052, 4,1080 Sevilla (Spain))

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Meisam T, Sulaiman A, Nikbakht AM, Yusof N, Najafpour G (2011) Influential parameters on biomethane generation in anaerobic wastewater treatment plants. Altern Fuel 227–263 Monali G (2011) Biomethanation of dairy waste water through UASB at mesophilic temperature range. IJAEST 8(1):001–009 Monali G, Dhoble RM, Pittule AP (2011) Biomethanation of dairy waste water through UASB at mesophilic temperature range. IJAEST 8(1):001–009 Naik M (2012) High rate digestion of dairy industry effluent by upflow anaerobic fixed-bed reactor. J Chem Pharm Res 4(6):2895–2899 Onyimba IA, Nwaukwu IA. Generation of biogas from cow dung Perera KUC. Investigation of operating conditions for optimum biogas production in plug flow type reactor. Master of science thesis, KTH School of Industrial Engineering and Management, Energy Technology EGI2009-2011 Division of xxx, SE-100 44 Stockholm Pittule AP (2011) Biomethanation of dairy waste water through UASB at mesophilic temperature range. IJAEST 8(1):001–009 Qazi JI, Nadeem M, Baig SS (2011) Anaerobic fixed film biotreatment of dairy wastewater. MiddleEast J Sci Res 8(3):590–593 ISSN 1990-9233 Rajeshwari KV, Balakrishnan M, Kansal A, Lata K, Kishore VVN (2000) State-of-the-art of anaerobic digestion technology for industrial wastewater treatment Renew Sustain Energy Rev 4:135±156. www.elsevier.com/locate/rse Ramesh T (2007) Performance evaluation of fixed bed fixed film anaerobic bioreactor for treating dairy effluent. J Ind Pollut Control 23(1):11–14 Ramesh T, Nehru kumar V, Srinivasan G (2012) Kinetic evaluation of fixed film fixed bed anaerobic reactor by using dairy wastewater. Int J Pharm Biol Arch 3(4):835–837 Rao MN, Datta AK (2012) Waste water treatment, 3rd edn, pp 254–258 Shrirame N (2017) Environmental engineer Sivakumar MS, Asha B (2012) Effect of organic loading rate on dairy wastewater using anaerobic bio-film reactor. J Ind Pollut Control 28(1):21–24 (© EM International Printed in India) Sorathia Harilal S, Rathod PP, Sorathiya AS (2012) Bio-gasgeneration and factors affecting the biogas generation—a review study. Int J Adv Eng Technol IJAET III(III):72–78. E-ISSN 0976-3945 Srinivasan G (2009) A study on dairy wastewater using fixed-film fixed bed anaerobic diphasic digester. Am-Eurasian J Sci Res 4(2):89–92 Srinivasan G, Subramaniam R, Nehru kumar V (2009) A study on dairy wastewater using fixed-film fixed bed anaerobic diphasic digester. Am-Eurasian J Sci Res 4(2):89–92 Tawfika A, Sobheyb M, Badwya M (2008) Trearment of a combined dairy and domestic wastewater in an upflow anaerobic sludge blanket reactor followed by activated sludge (as system). Desalination 227(1–3):167–177. https://doi.org/10.116/j.desel.2007.06.023 Yonar, T., Sivrio˘glu, Ö., & Özengin, N. (2018). Physico-chemical treatment of dairy industry wastewaters: A review. Technological approaches for novel applications in dairy processing, 179.

Integration of Biogas Production from Wastewater as Value Generation in Biorefineries T. R. Balbino, S. Sánchez-Muñoz, M. A. Yaverino-Gutiérrez, E. Mier-Alba, M. J. Castro-Alonso, J. C. dos Santos, S. S. da Silva, and N. Balagurusamy

Abstract In recent decades, the high emission of greenhouse gases and fossil fuel uses has worsened environmental problems and strongly impacted society. The increased demand for renewable fuels is a result of this socio-environmental impact. According to this scenario, the use of municipal and industrial organic waste to generate renewable energy and fuels has become an attractive alternative. Biogas production can replace natural gas and fossil fuels and allows obtaining renewable energies and organic waste treatment simultaneously through a sustainable process. The anaerobic digestion (AD) pathway is used to produce biogas through a process with less emission of greenhouse gases. In this pathway, several microorganisms are responsible for the degradation of organic matter in low-oxygen conditions. Wastewater can be a source of carbohydrates, lipids, proteins, and organic matter to microorganisms to produce biogas by AD. The integration of biogas production from wastewater in biorefineries can be an alternative for industries to recover bioenergy and reduce biomass generation after treating these wastes. This strategy can contribute obtaining a complete and self-sufficient bioprocess. This chapter focuses mainly on the effects of the main operational parameters of the process in the production of biogas, the interaction between microorganisms presents in the wastewater to benefit the AD process, challenges found in the biogas production from wastewater, and alternatives to overcome these barriers and achieve higher yields of this biofuel. Keywords Anaerobic digestion · Renewable energy · Bioprocess T. R. Balbino · S. Sánchez-Muñoz · E. Mier-Alba · M. J. Castro-Alonso · S. S. da Silva Bioprocesses and Sustainable Products Laboratory, Department of Biotechnology, Engineering School of Lorena, University of São Paulo (EEL-USP), 12.602.810, Lorena, SP, Brazil M. A. Yaverino-Gutiérrez · N. Balagurusamy (B) Bioremediation Laboratory, Biological Science Faculty, Autonomus University of Coahuila (UAdeC), Torreón Campus, 27276 Torreón, Coahuila, México e-mail: [email protected] J. C. dos Santos Biopolymers, Biorreactores and Process Simulation Laboratory, Department of Biotechnology, Engineering School of Lorena, University of São Paulo (EEL-USP), 12.602.810, Lorena, SP, Brazil © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_18

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1 Introduction The world’s population has become dependent on fossil fuels to obtain energy. Energy consumption from fossil fuels contributes to a decrease in the availability of energy resources and a rise in CO2 emission levels, resulting in environmental problems, such as an increase in the greenhouse effect, which are reflected in climate change (Cherubini 2010; Abdollahbeigi 2020). Thereby, environmental pollution and a growing acceleration in the reduction of energy resources are driving a worldwide concern to find alternative procedures to obtain a cleaner and renewable energies (Parsaee et al. 2019). Around one-fourth of primary energy is predicted to be supplied by biofuels in 2050 worldwide, which the renewable energy market is being driven by biogas production (Ahmed et al. 2021). Biogas is a mixture of gases (mainly methane (CH4 ) and carbon dioxide (CO2 )) produced by anaerobic digestion (AD) of waste generated from urban, agricultural, and industry activities. It can be used as biofuel or to produce heat and cleaner energy, and residues resulting from the biogas production process also can be used as biofertilizers (Ingole and Dhawale 2021; Fu et al. 2021). In addition to these applications, the reuse of wastes to produce biogas helps to reduce environmental pollution. For example, urban and industrial wastewater is generated in high amounts worldwide and those wastes have high chemical oxygen demand (COD) and total solids concentrations; thus, it is needed a treatment before being discharged to the environment or may cause harmful impacts because it can provide growth of various pathogenic microorganisms (Parsaee et al. 2019; Shah 2020). However, the treatment of wastewater is still made by expensive methods. Thus, the use of wastewater as feedstock to produce biogas is an interesting alternative for reducing the volume of this biomass, reusing the remaining residues, and in the meantime, generating energy and biofuels. Furthermore, these products generated by the production of biogas from wastewater can still be returned to the biorefinery, resulting in an integrated and beneficial system for all parts involved (Luostarinen et al. 2009). This chapter discusses how wastewater can be used for biogas production and the integration of biogas production from wastewater into biorefineries, highlighting the advantages and challenges encountered in achieving this integration and some strategies that can be applied to improve the process.

2 Overview of Biogas Production Biogas is mainly composed of methane (CH4 ) in a range of 50–70% and carbon dioxide (CO2 ) in a concentration of 30–50% obtained by AD process. In addition to these gases, biogas may also contain small amounts of nitrogen (N2 ) (0– 3%), water vapor (H2 O) (5–10%), oxygen (O2 ) (0–1%), hydrogen sulfide (H2 S) (0–10,000 ppm), ammonia (NH3 ), hydrocarbons (0–200 mg/m3 ) and siloxanes (0– 41 mg/m3 ) (Angelidaki et al. 2018). The types and concentrations of the compounds

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that make up the biogas are variable and depend mainly on the nature of the substrate; for example, biogas produced from effluents from cosmetic industries may have a higher concentration of siloxanes (Muñoz et al. 2015). The AD process carried out by symbiotically living microbial consortium results in the production of biogas. AD is a biodegradation well-established technology and economically profitable for sludge recovery, but there are still some limitations when compared to fossil fuel-based energy technologies, such as high operating costs and maintenance/upgrade expenses. However, it is a key energy source in the emerging renewable energy market that enhances the transition to the use of clean energy and the consequent reduction in the use of fossil fuels (Aryal et al. 2018; Shah 2021; Cao and Pawłowski 2012). The purpose of AD is to convert waste, such as wastewater, into biogas involving four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Parsaee et al. 2019). In the hydrolysis step, compounds with higher complexity are converted into less complex compounds (monomers), in other words, large organic polymers, such as carbohydrates, lipids, and proteins, are converted into sugars, fatty acids, amino acids (monomeric compounds), this conversion occurs through the activity of hydrolytic bacteria that produce extracellular enzymes, as cellulase, amylase, protease, and lipase (Van et al. 2020). The second step is the acidogenesis, performed through the action of acidogenic microorganisms, which degrade the monomers generated in the hydrolysis step into volatile fatty acids, such as acetic, butyric, propionic, and valeric acids. Then, acetogenesis is performed by two coexisting acetogenic groups: syntrophic acetogenic bacteria group and homoacetogenics group. The dominant group consists of syntrophic acetogenic bacteria, which produce acetate, CO2 , and H2 from volatile fatty acids, and the group of homoacetogenics are responsible for converting CO2 and H2 into acetate (Bajpai 2017; Anukam et al. 2019). Finally, the methanogenesis step is realized by methanogenic microorganisms. The acetoclastic methanogenic converts acetate into CH4 and CO2 , and the hydrogenotrophic methanogenic produces CH4 from H2 and CO2 . In general, 70% of the methane generated by AD is from acetate, and 30% is from CO2 and H2 , and proportions between the acetoclastic and hydrogenotrophic communities are the same. However, the proportion of microorganisms depends mainly on the operating conditions of the reactor, presence of inhibitors, and the feedstock composition (Li et al. 2019). The performance of AD is affected by various factors related to the microbial consortium cultivation conditions, such as pH, temperature, C/N rate, organic loading, and total suspended solids (TSS) (Mao et al. 2015). Even with different configurations of alternative reactors evaluated to perform AD, the use of up-flow anaerobic sludge blanket reactors is still preferentially selected over other technologies due to the ease of controlling all parameters and being successful in the process. However, the implementation of other alternative reactors in the AD process can reduce operating costs and even improve the yield of biogas production (ArreolaVargas et al. 2016). Thus, studies of the configuration of reactor parameters, the composition of the microbial communities, and the organic matter to be used as a feedstock in the AD can clarify how are the bests conditions to perform the process.

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Regarding the influence of pH on AD performance, the ideal pH of the reactor should be kept between 6.5 and 7.5 to balance the coexistence of microbial populations and promote the growth of methanogenic microorganisms (Elalami et al. 2019). The fermentative bacteria, responsible for the acidogenesis step, show metabolic activity in the pH range between 4.0 and 8.5, the optimum pH being between 5.0 and 6.0. While, the microorganisms involved in methanogenesis show metabolic activity in the pH range between 5.5 and 8.5, with the optimum pH being around 6.5 (Lohani and Havukainen 2018). Lowering the pH below 6.5 can initiate an inhibition of the growth of methanogenic microorganisms and consequently reduce the yield of AD. Throughout the AD process, the products generated by the methanogenic bacteria act by neutralizing the reduction in pH caused by the acid production carried out by the acidogenic bacteria (Alavi-Borazjani et al. 2020). The accumulation of acid in the reactor, which can occur if the amount of organic material increases drastically, favors the growth of acidogenic bacteria, which produce more amounts of acid, further acidifying the medium and inhibiting the growth of methanogenic microorganisms (Yang et al. 2015). On the other hand, alkalinization of the medium by raising the pH above 8.0 results in inhibitory conditions for most anaerobic microorganisms. The increase in pH during the AD process can occur due to prolific methanogenesis, increasing the concentration of ammonia that would prevent acidogenesis (Lohani and Havukainen 2018). Temperature is another key factor for the success of AD that influences the physical and chemical properties of the substrate, affecting the thermodynamic and kinetic reaction of biological processes (Nie et al. 2021). The AD process can take place over a wide temperature range: psychrophilic (300 km3 /year of municipal wastewater that are globally generated would be theoretically enough to produce biogas to supply energy for millions of households (Mateo-Sagasta et al. 2015). This conversion from waste to energy is possible owing to wastewater holding in its rich nutrient composition, around five-fold the quantity of energy that is needed for its treatment (Tarallo et al. 2015; Qadir et al. 2020). Moreover, the biogas production process has the potential to be circular and ecological when the biogas energy is used as thermal energy to heat the effluents and generate electrical energy (Rasi et al. 2010; Tarallo et al. 2015; Maktabifard et al. 2018; Qadir et al. 2020). In order to be exploited, it is necessary to consider the amount of nutrients present in wastewater, also its composition and physical and chemical characteristics. Those characteristics and nutrient composition would vary depending on the source and activity it came from (Dey and Islam 2015; Libardi et al. 2017; Fazal et al. 2019; Ao et al. 2021). Urban effluents generally could contain a combination of wastewater from different sources, being the most present: • Domestic effluent; • Industrial wastewater; • Water from commercial or public establishments and institutions. Commercial and domestic wastewater usually would be a mixture of blackwater and greywater, which present mainly organic load. Blackwater is mainly composed of bathroom wastewater that contains organic matter and suspended solids (mostly urine, faeces, and toilet paper) rich in nitrogen, phosphorus, and other nutrients. Greywater is more abundant in wastewater and is composed of cooking, laundry, and cleaning activities. Thus, this effluent normally contains organic matter from wastes of food and cleaning products (detergent, soap, beauty products, shampoo, etc.), giving around 20 and 30% of pollutants from the total in domestic wastewater (Mateo-Sagasta et al. 2015; Boutin and Eme 2016). On the other hand, industrial wastewater composition may vary depending on the activity of each industry, presenting more inorganic compounds (chemical, steelworks, etc.) or organic compounds (food, agroindustrial, etc.) (Mateo-Sagasta et al. 2015; Boutin and Eme 2016; Choi et al. 2017). As expected, is common to get mixtures from different effluents in diverse proportions (Bouallagui et al. 2009; Carvalho et al. 2013; Boutin and Eme 2016; Arantes et al. 2017; Ardley et al. 2019; Vian et al. 2020). For example, domestic wastewater has a mixture of black and greywater. Big industries could mix effluents with diverse compositions from different processes. Municipal effluent would be the mixture of domestic, industrial, and commercial wastewater, being an important source of water,

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nutrients, and energy (Boutin and Eme 2016; Qadir et al. 2020). Thus, it is important to understand that each kind of wastewater has to be characterized to get better performance for biogas production. Moreover, even using a unique kind or source of wastewater, its composition may vary due to variation of process conditions, human activities, or natural raw material variation (Boutin and Eme 2016; Choi et al. 2017; Qadir et al. 2020). As a consequence, characterization becomes an essential step of the process. In Table 2, can be observed examples of the variation in composition and characteristics in different types of wastewaters. It was collected information on common types of effluents, such as effluents from the food industry and domestic wastewater from around the world. The rank of nutrients and relevant characteristics as pH or TSS is large and would vary in the same kind of effluent, industry, or home, even more, when comparing different sources. During AD, mineralization of organic matter, pH increased (Bouallagui et al. 2009), so it becomes necessary to begin the process with an adequate pH. In effluents like textile wastewater, it can be noticed that pH variation can achieve 10 units of difference, as well in the food industry, the pH of the effluent can vary until 7–8 units. On the other hand, the number of TSS from a brewery, textile and fruit and vegetable wastewater, presents large differences within due to raw materials and processes. These variations implicate adaptations of upstream processes, as pH adjustment or applications of unit operations before the biogas production process. It is noticeable that some wastewater from the food industry like the fruit and vegetable industry, brewery, dairy, etc., is deficient in nitrogen, potassium, and phosphorus. In some cases, chemical precipitation can improve the utilization of these nutrients when present in the organic fraction, another option is to appeal to codigestion with rich substrates to improve biogas production (Bouallagui et al. 2009; Carvalho et al. 2013; Puchlik and Ignatowicz 2017; Fazal et al. 2019; Ardley et al. 2019; Vian et al. 2020). Moreover, this kind of effluents (food industry), as well domestic wastewater, present high concentrations of biochemical oxygen demand (BOD) and COD due to a composition rich in organic matter that subserves microbial growth and biogas production (Libardi et al. 2017; Fazal et al. 2019; Qadir et al. 2020; Vian et al. 2020). However, it is possible that some organic compounds are actually refractory organic compounds as organic dyes or compounds with aromatic ring structures, chlorinated organic compounds, lignin and its derivatives, fulvic-like products, present low bioavailability due to low solubility or affinity with the microorganisms. Hence, these organic compounds (mostly present in industrial effluents) cannot be treated by biological interventions due to its biodegradation resistance (Huang et al. 2010; Choi et al. 2017). In this context, it is noticeable to consider the whole composition of the effluent and the microorganisms present in the wastewater during the process to achieve great yields and also, treat the effluents appropriately. Wastewater composition influences methanogenesis process Microbial communities present in the effluents also vary according to the nutrients and characteristics of the effluent, and the stage of the process that is analysed.

BOD (mg/L)

0–6

500–6100

1200–3600

Effluent type

Dairy production

Fruit/Vegetal processing

Brewery

2000–32,500

806–163,500

0–5052

COD (mg/L)

200–3000

165–3269

0–150

TSS (mg/L)

5–21.6

2–106

0–0.56

N (mg/L)

5–35

7.6–178.4

0–0.014

P(PO4 3− ) (mg/L)

–

–

0–0.8

K (mg/L)

3–12

3.2–10.2

4–12

pH

Methanosaeta concilii, Methanosarcina mazei, Methanospirillum hungatei, Methanobrevibacter

Methanosarcina sp, Anaerolineaceae sp, Cloacamonas sp, Anaerobaculum sp, Methanosaeta sp, Methanosarcina sp, Methanothermobacter sp Defluviitoga, Thermoanaerobacterium, Sphaerochaeta, Bacillus

Methanosarcina barkeri, Methanosarcina harudinacea, Methanosarcina sp., Methanosarcinaceae spp., Methanoculleus palmolei, Methanosarcina thermophila Methanosaeta spp., Methanococcus spp., Methanobrevibacter sp.

Microorganisms present

Table 2 Composition of different types of wastewaters and the main microorganisms present involved in biogas production

(continued)

Tabatabaei et al. (2010)

Bouallagui et al. (2009), Puchlik and Ignatowicz (2017), Ardley et al. (2019), Ao et al. (2021), Vian et al. (2020)

Tabatabaei et al. (2010), Carvalho et al. (2013), Fazal et al. (2019)

References

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25–1583

20–756

10–786

Greywater

Textile

24.9– 3950

20–361

126–400

TSS (mg/L)

3–75

43.7– 70

N (mg/L)

0–101

3.7–7.8

P(PO4 3− ) (mg/L)

5–23

13.86–16.5

K (mg/L)

3.9–14

6.1–9.6

7.2–7.25

pH

BOD—biochemical oxygen demand; COD—Chemical Oxygen Demand; TSS—total suspended solids

41–2430

105–880

38–350

Domestic wastewater

COD (mg/L)

BOD (mg/L)

Effluent type

Table 2 (continued)

Parcubacteria, Proteobacteria, Saccharibacteria, Candidatus Methanoperedens, Candidatus Methylomirabilis, Methanomethylovorans, Candidatus_Moranbacteria, Chlorobium, Methanosaeta

Chlorella sp., Merismopedia sp., Closteriopsis sp., Scenedesmus sp.

Dey and Islam (2015), Bai et al. (2020)

Boutin and Eme (2016), Kumar et al. (2017)

Huang et al. (2010), Libardi et al. (2017), García et al. (2017), Qadir et al. (2020), Gao et al. (2020)

Arantes et al. (2017)

Deferribacteres, Nitrospira, Chloroflexi, Clostridium Acinetobacter, Luteolibacter, Thauera, Aquamicrobium, Cyanobacteria, Pseudomonas, and Aeromonas Methanolinea, Methanospirillaceae, Methanoculleus, Methanolinea, Methanospirillum, Candidatus Methanoregula, Methanomicrobiales, Methanobacterium, Methanoregulaceae, Methanolinea

References

Microorganisms present

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Methanogens reported in most of the research cited in Table 2 are Methanosarcina and Methanosaeta and bacteria such as Clostridium are also present. Degradable organic matter as complex or large molecules like proteins, sugars and lipids present in domestic wastewater can achieve values up to 30 or 50% of COD and total organic carbon, respectively, or even higher. Rich organic matter effluents have multiple possibilities to fit in biogas production, when proteins, lipids and carbohydrates are present, they are usually coupled at the hydrolysis process in biogas production (Cai et al. 2016; Rabii et al. 2019). Hydrolytic bacteria such as Bacteroides, Ruminococcus, Selenomonas, and Acetivibrio, degrade these polymers into monomers (Balagurusamy and Ramasamy 1999). Hydrolysis stage would be one of the most important links between wastewater (raw material) and biogas (product). As shown in Fig. 1, components and organic matter can direct the metabolic process. After hydrolysis of these complex biomolecules, biogas production would go forward to fermentation, oxidation and finally, methanogenesis. Thus, according to the effluent, the metabolic process would present variations (Cai et al. 2016; Rabii et al. 2019). Great illustrations are industrial effluents with degradable organic matter as domestic or commercial wastewater and food or agro-industrial industries (Arantes et al. 2017; Qadir et al. 2020; Gao et al. 2020; Vian et al. 2020). Dairy effluents, for example, when present more lipids and proteins in their composition, would follow a hydrolysis process, followed by fermentation and beta-oxidation, before methanogenesis. On the other hand, effluents rich in carbohydrates, would follow acidogenic fermentation (Rabelo et al. 2018; Fazal et al. 2019). Beyond the hydrolysis stage, wastewater composition manages microbial and metabolic interactions. It was reported by Tabatabaei and colleagues (2010), that a suitable acetate concentration in brewery wastewater favors the presence of Methanospirillum hungatei and Methanosaeta concillii. Acetate presence improves the production of methane through Acetotrophic methanogenesis, while soluble organic solids are being hydrolysed. Other effluents as pulp and paper mill wastewater, produce methane and hydrogen in the presence of cellulolytic and fermenting bacteria as Klebsiella, Rautella and Clostridium, that probably act in synergism for cellulose conversion without commercial cellulase application, through acetic-butyric fermentation. During the process, hydrogen consumption promoted methane production through hydrogenotrophic methanogenesis (Rabelo et al. 2018). When volatile fatty acids increase their concentration, methanogens are inhibited causing an imbalance between methanogenesis and acetogenesis. Under these conditions, Methanosarcina, Cloacamonas, Anaerobaculum, Methanosaeta, Anaerolineaceae and Syntrophaceticus were the dominant acidogens, methanogens and syntrophus in mesophilic and thermophilic stages of vegetable wastewater digestion (Ao et al. 2021). Wastewater is an excellent source of nutrients and chemical energy. It is important to highlight that each effluent has to be treated considering all characteristics as possible in order to improve biogas yields. The continuous research in this area can approach better and more efficient processes in the way of sustainability. Even

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Fig. 1 Methanogenesis from wastewater effluents

though it is necessary to improve its utilization as substrate for biogas production and to develop alternative or complementary treatments to solve the presence of no-degradable compounds present in urban effluents.

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5 Biogas Integration from Wastewater to Biorefinery: Use and Production The wastewater treatment is a remarkable process to obtain treated water that can be used on anthropogenic activities, and also biogas can be produced from the sludge formed in this process. This process includes several steps for converting raw wastewater to treated wastewater, such as pretreatments (physical or chemical methods), sedimentation, coagulation for removing big solids, generating a pretreated effluent, primary treatment as flocculation or precipitation, generating treated effluent. Furthermore, a biological treatment can be performed for biodegradation or filtration, resulting in a post-treatment effluent, and finally, by physical or chemical methods, a final treatment happens for a water discharge (Fig. 2) (Crini and Lichtfouse 2019). Biogas production from wastewater can be approached as a process of obtention of energy, biofuels or heat, and more valuable products, but first, the purification of the biogas is crucial to making it more valuable and possible a better yield. Biogas can consist of methane (50–70%), CO2 (30–50%), and a minor concentration of hydrogen sulfide (H2 S), water vapor, ammonia (NH3 ), and siloxane (Mattioli et al. 2017; Angelidaki et al. 2019). However, the presence of CO2 and other components makes it less efficient, limits the capability for the conversion of the methane into other products, and reduces the potential for producing energy. The presence of CO2 decreases the energy content of methane, for example, the Lower Calorific Value (LCV) of methane is 36 MJ/m3 -CH4 (under normalized conditions of temperature and pressure), but LCV of biogas with 65% of methane has a value of 20–25 MJ/m3 biogas. In addition, chemicals present in biogas, such as NH3 and H2 S, are very corrosive and toxic, so they represent damage to the combined heat and power unit and metal parts of the system (Angelidaki et al. 2018). Therefore, the production of methane with fewer contaminants, especially CO2 , will increase the biogas purity and improve it for further use. There are various treatments to remove all these polluters from biogas to increase the value and methane purity. One of the treatments is called “biogas cleaning” as a first step to remove toxic contaminants, such as NH3 , H2 S, siloxanes, and organic compounds. This process is characterized for removal of H2 S in a biological way which occurs with the oxidation of H2 S by aerobic sulfate oxidating bacteria. Another treatment is called “biogas upgrading” which the clear objective is to convert biogas into fuel with more quality. The primary difference between these two treatments is that the first includes the remove of toxic components and also the removal or conversion of CO2 into methane (Adnan et al. 2019). The “biogas upgrading” constitutes four stages: biogas upgrade, cleaning, desulfurization, and drying. In the first step, the purpose is to remove or transform CO2 to methane to increase the raw of biogas, meanwhile is also performed the cleaning to remove all the impurities like water, ammonia, hydrogen sulfide, and siloxanes present in the raw biogas. Besides the cleaning step, in the desulfurization step, the main focus is to remove all the hydrogen sulfate, and finally, the drying is where

Fig. 2 Biogas production from wastewater integrated into a biorefinery. There are schematized on the image the steps on the Wastewater Treatment Plant, biogas upgrading by chemical scrubbing or membrane separation, biogas cleaning by biological strategy, and respective products generates during the process reused on the biorefinery

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raw biogas must keep within no moisture from water (Kapoor et al. 2019). There are various general technologies for the biogas upgrading focus is in the first stage. One of the most common is the physical/chemical technology, in which the main goal is the separation/transformation of CO2 from CH4 . Moreover, upgrade technologies also include processes, as chemical and physical absorption, adsorption, cryogenic separation, pressure swing adsorption (PSA), and membrane separation (Toledo-Cervantes et al. 2017). In Fig. 2 is schematized the different steps on the Wastewater Treatment Plant to obtain biogas, biogas upgrading using methodologies as membrane separation (one of the newest and more eco-friendly technologies) and chemical scrubbing (the most effective and used technology) to the obtention of purified biogas, and biogas cleaning by biological process, in addition to the integration on the biorefinery of various products, generates during the process, such as the implementation of biomethane (upgraded biogas) to energy production, heat generation and other approaches. The mechanism for the physical absorption is to utilize solvents for separation of CO2 and CH4 by the difference of solubility between the gases, so a solvent that has a high solubility for CO2 is selected, at the same time that solvent does not have absorbing CH4 , and then the gases would be separated. On the other hand, on chemical absorption, the mechanism is based on the chemical reaction between absorbents and molecules of CO2 . Thus, chemical absorption could be more effective than physical when the concentration of CO2 is lower. Furthermore, the adsorption specifically on PSA is another option that can be applied to the process and is based on the selectivity of the gas molecules that can be adsorbed to solid surfaces depending on the molecular size, considering that molecules of CH4 are bigger than CO2 , N2 , and O2 , it can be separated by this technology. Another option is the cryogenic separation, based on the condensing temperatures through condensation and distillation, in which CO2 can be separated from CH4 because CO2 has a boiling point of −78 °C and methane is −160 °C, so it is possible to separate CO2 from biogas by a cooling gas mixture at elevated pressures. Finally, membrane separation is one of the newest system form upgrades consisting of the use of a membrane that has a selective permeability allowing the separation of the component of biogas, so depending on the permeability rates, it is possible to make a separation of the different molecules of biogas (Angelidaki et al. 2019; Bauer et al. 2013; Zhao et al. 2021).

5.1 Barriers and Challenges in Production and Utilization of Biogas The production of biogas has a great impact in the world due to its environmental and economic relevance, and the use of organic matter as a substrate for AD is the key to success in obtaining biogas in an economically viable way. These substrates can be obtained from various sources, mainly as wastes of anthropogenic activities and

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industrial or agricultural production processes (Lora Grando et al. 2017). The pollutants present in wastewater have a deep impact on different aspects of the world’s life and are a subject of concern to society, public authorities, and the whole industrial world. The disposal of wastewater into the environment without prior treatment can cause bad odors, illnesses, bioaccumulation of toxic compounds, among other problems (Crini and Lichtfouse 2019). Moreover, the necessity to avoid water pollution makes the treatment of wastewater an essential option. In this way, the integration of treatment of industrial wastewater and biogas production in a biorefinery can be a strategy for avoiding problems due to discarding of wastewater in the environment and turning the process more environmentally and economically rentable. However, some challenges and barriers still are encountered during the process and need to be resolved. Sewage Sludge (SS) is biomass resultant of the process of treatment of wastewater. This biomass is composed of a mixture of organic and inorganic carbon compounds, such as nitrogen and phosphorus components, heavy metals, and pathogens (Samolada and Zabaniotou 2014). SS is produced in the EU, USA, Brazil, Canada, China, and Iran on about 8910–370 thousand metric toneless sludge on the dry weight (Singh et al. 2020). The dry mass of SS contains a lot of organic compounds that can be used to produce a considerable amount of biomass energy. Moreover, the SS treatment reduces the amount of this material that will be discarded in the environment and the safety and health issues associated with SS. Nevertheless, the treatment of the SS represents a high cost-operation, about 40–60% of the budget for wastewater treatment, according to the U.S. Environmental Protection Agency (EPA 2004). Another issue is in the process of AD, in which the synergic between the microorganism can be disrupted for the decreasing of pH caused by the accumulation of volatile fatty acids (VFA) on the stage fermentation, and it will affect the yield of the AD. Thereby, strategies should be implemented to strengthen the power and interaction between the microorganisms on AD, and the addition of conductive carbon-based materials (CCBMs) can be an effective method to resolve this issue (Peng et al. 2018). The biogas yield also can be improved using biochar that can higher the adhesion on the surface of the methanogenic microbial consortium and promotes better growing, beyond helps to adjustment of the pH to maintain it in an optimal range to the production of biogas or biochar also can be applied directly to SS. The biochar is obtained from the pyrolysis of biomasses, such as manure, agriculture residues, and other solid waste, and is a by-product rich in carbon. Pyrolysis is an interesting thermochemical method for its capacity to destroy carbon-based materials to smaller materials by a process in a reactor, in which it is heating in an anoxic atmosphere at 400–800 °C to obtain biochar, bio-oil, and biogas. Biochar can have various properties depending on pyrolysis, such as surface area, adsorption capacity, redox activate of moieties (e.g., quinones or phenolics), and activate metals (e.g., Fe and Cu). Therefore, once obtained biochar it can be added during to the AD and will act adjusting the pH due to the balance between biochar redox activate moieties and metals and the interspecies electron transfer (IET) of fermenting bacteria producing hydrogen; a diffusive electron that can affect pH (Wang et al. 2020).

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In addition to the IET, the temperature and the C/N rate are key factors in AD that influence process efficiency and stability. Regarding the temperature, both mesophilic (20–45 °C) and thermophilic conditions (45–60 °C) are used during the AD, depending on some circumstances. For instance, many regions with Wastewater Treatment Plant have a low range of temperature, and the mesophilic condition is widely used instead of thermophilic digestion due to the lower request of energy and better stability for the process. Nevertheless, thermophilic digestion has some advantages over mesophilic, such as higher metabolic rate, higher rate-deactivation for pathogens, and higher yield of biogas. However, it also has some disadvantages, such as the complex system of control during a high rate of AD, which can be resolved by biochar addition and other elements aggregates, enhancing the production of methane in the biogas, as mentioned above. In this case, depending on the type of the biochar, it can be more effective in mesophilic or thermophilic conditions (Wang et al. 2020, 2018; McKeown et al. 2012). Concerning the C/N ratio, about 20–30 is the ideal range of the C/N rate for occurring the AD. However, the SS is a feedstock with a low C/N ratio (less than 10), and it is not appropriate for the growth of microorganisms of AD. Thus, the codigestion process can be realized for the supplementation of SS with other sources of carbon. Moreover, the activated sludge generated during the process has some resistance to biodegradation and can be a problem in the efficiency of AD. Under these conditions, pretreatments, such as alkali treatment, chemical treatment, and biological treatment, have been employed to improve the hydrolysis and release of organic matter from SS (Liu et al. 2018; Kumar et al. 2021; Mishra et al. 2021). In summary, biogas production from wastewater, especially using SS as a feedstock, have some challenges and barriers to be faced to improve the production, as mentioned above, related to the elimination of biogas pollutants and other parameters to be adjusted during the process, such as pH, temperature, C/N ratio, and supplementation with other feedstocks by a co-digestion process. Thereby, studies are still being carried out to develop technologies that make AD from organic waste (specifical wastewater) a key strength of the bioenergy sectors shortly, improve the use and production of biogas, and enable its production on a commercial scale, not just on a pilot scale (Mishra et al. 2021). Moreover, the determination of how is an appropriate use of feedstock, an adequate pre-treatment, how to control parameters on the reactor, and how to do a purification step, not just it will make possible to obtain purified biogas and increase the yield of the process, but will also help in mitigation of greenhouse gases emissions. Nevertheless, the production of biogas not only has these barriers in the aspect of the parameters and conditions for efficiency of AD to overcome, but there are also other barriers and challenges depending on the condition of the country, challenges are present in different ways focus on developing countries, these challenges are associated with technical services, policy, funding, and awareness (Patinvoh and Taherzadeh 2019). Some countries are rich in biomass but unable to use these sources effectively due to technical and infrastructural challenges, such as the lack of information about the process of AD, conditions to operate, problems with feedstock composition, temperature to operate the digestor, and, as a consequence, they have lower effectiveness

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in the biogas implementation (Patinvoh and Taherzadeh 2019). Moreover, the financial part also represents a major dare for the biogas implementation. For example, building a small industrial bio-digester with a daily input capacity of 2000 kg require a huge investment, about $500,000–$1,000,00, plus other additional costs, such as storage tanks, operational cost, purification, biogas compressor, and maintenance, that have a high cost due to few technical experts available (Morgan et al. 2018). In addition, politics also impact biogas implementation and is another challenge. The environmental policy is inadequate due to the lack of a policy for renewable energy; thus, the government has a poor commitment and does not continuity to programs as biogas production or other ways to produce energy from renewable sources. As a consequence, the companies are not focused on implementation strategies to use renewable sources, and the collaboration between the public and private sector to improve the biogas implementation as a strategy for energy production and mitigation of greenhouse gasses still is small (Mahachi et al. 2015; Nevzorova and Kutcherov 2019). Beyond these issues, the socio-culture impact is another limitans to the use of biogas. The low public participation and lack of customer interest in biogas implementation due to lack of information about technologies for biogas uptake, added to the lack conscious about the importance to develop a green and sustainable process with fewer costs, worsens the problem to implementation of biogas. Even in developing countries exist problems with the socio-cultural factor. For example, in some rural regions of China is limited awareness about environmental protection, management of resources, and health improvement for rural households (Chen and Liu 2017). These communities do not feel responsible to contribute to the use and maintenance of new eco-friendly strategies that can improve economically biogas production. Therefore, the uptake and production of biogas can have a significant impact on the energy industry, but exist different barriers and challenges in various sectors, such as technical (production) and infrastructure, economic, institutional (politics), and socio-culture that make a challenge the implementation of biogas (Nevzorova and Kutcherov 2019). All these barriers influence the implementation of biogas into society as a new way to produce energy from a renewable source, but new strategies to obtain “clean” energy coupled with the circular economy are very attractive and a necessity these days.

5.2 Strategies to Improvement of Biogas Production Anaerobic co-digestion, the simultaneous digestion of two or more substrates, has proved to be a potential strategy to relieve the bottlenecks of biogas production of conventional AD from different aspects. Nutrient balance and dilution of toxic compounds are the most important limitations of conventional AD (Mata-Alvarez et al. 2014; Rabii et al. 2021). The nutrient balance, especially C/N ratio, of co-substrates is one of the most critical parameters in microbial activities of anaerobic co-digestion systems (Tyagi

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et al. 2018; Xu et al. 2018). In general, similarly to AD, a C/N ratio of 20–30 is considered optimal for microbial activities in anaerobic co-digestion (Zhen et al. 2015; Xu et al. 2018). In mono-digestion, a single substrate is either carbon-rich or nitrogen-rich, which can difficult the maintenance of C/N ratio balance and affects microbial processes (Neshat et al. 2017). Thus, anaerobic co-digestion of multifeedstocks provides better macro and micronutrient balance, dilution of inhibitory compounds, moisture regulation, and better-buffering capacity (Khalid et al. 2011; Tyagi et al. 2018). In anaerobic co-digestion systems, co-digestion of wastewater sludge with a low C/N ratio of 6–10 with co-substrates with a higher C/N ratio can equilibrate the nutrients and reduce the production of inhibitory compounds that leads to instability of the microbial processes (Chow et al. 2020). For example, Yen and Brune (2007) evaluated the effect of co-digestion of algal sludge and high carbon content of wastepaper at different fraction in methane production. They observed that adding 50% (based on volatile solid) of wastepaper in algal sludge feedstock increased the methane production rate to 1170 ± 75 ml/l day, as compared to 573 ± 28 ml/l day of algal sludge digestion alone. The authors found that an optimum C/N ratio for co-digestion of algal sludge and wastepaper was in the range of 20–25/1. Furthermore, they also discussed that co-digestion resulted higher methane gas yields, due to synergistic effect. The synergistic effect is related with the balanced nutrients, dilution of inhibitory compounds and increased buffering capacity (Yen and Brune 2007). In other studies, was demonstrated that the pH is another critical parameter in improvement of biogas production in anaerobic co-digestion systems. For example, Dai et al. (2016) analyzed the effects of pH and C/N ratio in simultaneous enhancement of methane production and methane content in biogas from waste activated sludge and perennial ryegrass anaerobic co-digestion. They showed a maximum methane production of 310 mL/gVS add at optimum conditions (C/N 17/1 and initial pH 12). Meanwhile, the methane content in biogas was about 74%, which was much higher than that of sole waste activated sludge (64%) or sole perennial ryegrass (54%) AD. In other study, Simioni et al. (2020) evaluated the effect of nutrient balance for anaerobic co-digestion of tannery wastes in terms of energy efficiency, wastewater treatment, and cost-saving. The authors showed that the nutrient balance in the microbial processes of anaerobic co-digestion can reduce 71% of electric or 35% of thermal energy consumption, which can decrease considerably the costs of the processes in anaerobic co-digestion systems. Furthermore, the authors found that concentrations of volatile fatty acids were far below the inhibitory concentration (>1500 ppm). Other studies have been showed that municipal solid wastes (Nikulina et al. 2021), agricultural wastes (Muscolo et al. 2018), crude glycerol (Sittijunda et al. 2021), and fat, oil, and grease (Hao et al. 2020) are potential co-substrates that can be used in anaerobic co-digestion of wastewater sludges. Although the nutrient balance and the concentration of inhibitory compounds can be equilibrated by anaerobic co-digestion as discussed above, the presence of complex organics, microbial flocs and extracellular polymeric substances in wastewater sludges can inhibit the either AD and anaerobic co-digestion process because of

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its low biodegradability, which generates a long retention time in the process (Rabii et al. 2021). Furthermore, anaerobic co-digestion process requires much longer time for conversion of the organic substrates compared with AD. Specifically, hydrolysis is the rate-limiting step since hydrolytic enzymes are absorbed on the surface of the organic substrates and convert it into smaller compounds for further degradation into volatile fatty acids along with other byproducts (Naran et al. 2016). In order to overcome this limitation, other technologic strategies are required, such as pretreatment techniques (Rabii et al. 2021; Pasalari et al. 2021; Tsigkou et al. 2021). Pretreatment methods facilitate metabolic activities of the microbial communities in anaerobic co-digestion systems by making the organic substrates more accessible and utilizable (Carlsson et al. 2012). The main aim of pretreatment methods is increased the porosity and expose more surface area of organic substrates to enzymes and increase the hydrolysis step efficiency (Baral and Shah 2017). This can accelerate the subsequent steps during biological treatments (AD/anaerobic co-digestion) and reduce the solid retention time required during microbial processes (Anjum et al. 2016). Pretreatment methods include mechanical, chemical, physicochemical, and biological techniques and they have been applied individual or combined for improvement of both AD and Anaerobic co-digestion systems using wastewater sludges (Kumar and Sharma 2017). Mechanical pretreatments Mechanical pretreatment methods are applied to reduce crystallinity, particle size, and degree of polymerization of biomass materials. In this type of pretreatment, mechanical tools and techniques, such as millers, ultrasonic, and microwave radiations are generally employed (Mankar et al. 2021). Alagöz et al. (2018) evaluated the effect of ultrasonication and microwave sludge disintegration/pre-treatment techniques on the anaerobic co-digestion efficiency of wastewater sludges with olive and grape pomaces. They showed that the ultrasonication and microwave wastewater sludge pre-treatments speeded up the rate limiting “hydrolysis” step and enhanced the anaerobic biodegradability of organics substrates leading to an increase in biogas and methane yields. Comparing pretreatments using, the authors found that ultrasonication was more effective wastewater sludge pre-treatment method than microwave irradiation based on applied specific energies. Although the specific energy applied in microwave was almost 9 times higher than that of ultrasonication; microwave pretreatment led to only 10–15% increase in the biogas/methane yield. More recently, Wang et al. (2021) analyzed the effect of mechanical cutting pretreatment in wastewater sludge fermentation and AD. They observed that a wastewater sludge disintegration degree of 6.7% was achieved when the sludge was mechanically cut for 8 min. After the pretreatment, the soluble COD (SCOD) production in the sludge fermentation system was as high as 3052 mg/L, which was 2.28 times higher than that of the control group and biogas production increased by 3 times compared with unpretreated sludge. In the recent years, hydrodynamic cavitation has been employed for the enhancement of biogas production from co-digestion. For example, Ezz et al. (2021) investigated the influence of hydrodynamic cavitation pretreatment in anaerobic co-digestion of rice straw and thickened waste activated sludge. They obtained

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highest biomethane production with both pretreated with HC pretreatment (300 L/kgvs) compared with 190 L/Kg-vs for co-digestion without pretreatment. Furthermore, the authors observed that the amount of energy production from the co-digestion of TWAS and RS with pretreatment was evaluated to be 9156 MJ/t, 62.4% increase in comparison with the co-digestion without pretreatment. Despite the mechanical pretreatment of wastewater sludge produce an effect in increase the biogas production, other studies showed that this type of pretreatment needs other secondary or tertiary chemical and biological agents are necessary to facilitate the disintegration of complex substrates for obtain higher biogas and methane yields in AD and anaerobic ˇ co-digestion systems (Cater et al. 2014; Elalami et al. 2019; Mankar et al. 2021). Chemical pretreatments In chemical pretreatments, reagents such as acids, alkalis, and oxidants are used to hydrolyze recalcitrant compounds of complex organic substrates (Neumman et al. 2015; Wang et al. 2019). For example, Devlin et al. (2011) studied the effect of chemical pretreatment using HCl on subsequent batch digestion. The biogas yielded behind 13 days compared to untreated biosolids by 21 days, followed by semi-continuous digestion experiments. The authors demonstrates that methane yield increased by 14.3% compared to control (untreated biosolids). In other study, Shao et al. (2012) evaluated the influences of alkaline pretreatment on AD and sludge dewaterability after AD. The authors showed that when they compared with the control (pH 6.8), total suspended solids and volatile suspended solids reduction following pretreatment at pH 9–11 increased by 10.7–13.1% and 6.5–12.8%, respectively, while biogas production improved by 7.2–15.4%. More recently, Achouri et al. (2021) evaluated the influence of pretreatment with H2 O2 to improve the performance of the AD process for a partial treatment of tannery wastewater. They showed that the H2 O2 pretreatment of tannery wastewater promotes an improvement of AD process, up to increase the methane yield by 94% compared to unpretreated tannery wastewater. Biological pretreatments In biological pretreatments methods, microorganisms including bacteria and fungus are used for microbial degradation of biomass. These microorganisms have the ligninolytic enzyme system, mainly comprising laccases and peroxidases with a high reduction potential for oxidation of the lignin structure (Kumar et al. 2020). Damtie et al. (2021) evaluated the interaction between various biological pretreatment conditions and the co-digestion of microalgae and primary sludge. The results of that study revealed an increase of 36% of methane yield by co-digestion of a substrate pretreated by thermophilic aerobic conditions (55 °C and a hydraulic retention time of 2 days). More recently, Avila et al. (2021) studied enzymatic pretreatments of microalgal biomass and further anaerobic co-digestion system with waste activated sludge. The authors observed that the pretreatments increased the methane production of the raw algal biomass 3.6- to 5.3-fold. The methane yield was 9–27% higher at the lower enzyme dose. Hence, microalgae pretreated with commercial enzymatic cocktails at a 1% dose were co-digested with waste activated sludge. According with results

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of that study, co-digestion may achieve the goals of a waste recycled bio-circular economy. Combined pretreatments Combined pretreatments are based on combination of different pretreatment methods (Karimi et al. 2013). Biomass pretreatment via combined pretreatments demonstrate more favorable for crystallinity reduction of complex substrates efficiency in comparison with the individual utilization of physical, chemical, and biological pretreatment techniques (Ballesteros et al. 2004; Han et al. 2015; Kim et al. 2017). For example, Solé-Bundo et al. (2018) studied the influence of the anaerobic co-digestion of microalgal biomass grown in wastewater and wheat straw. The authors evaluated the co-digestion of both substrates after applying a thermo-alkaline pretreatment (10% CaO at 75 °C for 24 h. The results of co-digestion process of 50% microalgae—50% wheat straw in mesophilic lab-scale reactors showed that the methane yield was increased by 77% with the co-digestion as compared to microalgae mono-digestion. In other study, Lee et al. (2019) investigated the optimum conditions of combined pretreatment (thermal-alkali method) and the performance of ammonia stripping for enhancing solubilization efficiency and methane yield in the anaerobic co-digestion of food waste and sewage sludge. The results of the optimum pretreatment conditions (140 °C, 60 meq/L and 60 min) demonstrated higher methane yield (84%) compared with the untreated substrates. However, after thermal-alkali pretreatment, the NH4 + concentration of the thermal-alkali pre-treatment liquid showed a concentration that could inhibit anaerobic co-digestion. The authors discussed that the ammonia stripping can be applied to remove NH4 + . More recently, Coura et al. (2021) investigated the effect of double pretreatment solution, including ultrasonic treatment and struvite precipitation of anaerobic co-digestion of cattle slurry liquid fraction and sewage sludge. They observed that the application of ultrasound (with an energy input of 218 kJ L−1 ) and struvite precipitation (NH4+ : Mg2+ of 1:3) increased the methane content in the biogas by 82% and reduced hydraulic retention time by 28%, when compared to the anaerobic co-digestion assays without pretreatment. The most recent and relevant investigations show that pretreatment is an efficient technology for improving anaerobic co-digestion by increasing biodegradability and hydrolysis rate of recalcitrant compounds in co-substrates and improving biogas yield. However, as Rabii et al. (2021) correctly mention, greater efforts are still required from different aspects of implementation of pretreatment technologies in AD and/or anaerobic co-digestion systems such as cost–benefit analysis.

6 Conclusion The production of biogas from waste is an alternative with a high socio-economic and environmental impact that favors the production of renewable energy, production of other compounds that can be reused for the benefit of the community, such as biofertilizers, and reduction of pollution of the environment. However, the costs

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involved with assembling the platform to carry out the AD, equipment maintenance, and updating the technical team are still obstacles that need to be overcome to improve the process implementation in the energy market. Thus, the importance of integrating the biogas production process from wastewater in biorefineries is highlighted to recover bioenergy and reduce biomass generation after treating these wastes, reuse all the wastes and products generated throughout the process, contribute to reducing costs and obtaining a complete and self-sufficient bioprocess.

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Bioprospecting of Microorganisms for Novel and Industrially Relevant Enzymes Sonia Sethi and Samvida Saxena

Abstract Enzymes obtained from microbes have become a basic part of the industrial process and have a major impact on our lives. The exploration for microbial enzymes has seen various improvements over the past few years, such as directed evolution, omics research, recombinant DNA technology and metagenomics. A recent report suggests that the establishment of a collection of relevant enzymes from the environmental genome to the industry is a routine procedure. Metaproteomics could be used for bioprospecting microbial communities to query for the most active enzymes to improve the selection process of industrially relevant enzymes. This process involves the direct isolation of DNA from various gaps, the construction of metagenomic libraries, and the extraction of information through sequence and function-based approaches. This approach has resulted in the acquisition of many new biocatalysts and metabolites. Therefore, mining the microbial genome can broaden our horizons for finding industrially important enzymes. This chapter seeks to provide comprehensive information for metagenomics and comprehensive information on the potential for new microbial enzymes. Keywords Bioprospecting · Novel antimicrobial metabolites · Bacterial antagonists · Enzyme bio degradation · Biocatalyst

1 Introduction Climate change caused by human activity, such as the burning of fossil fuels, which releases greenhouse gases into the atmosphere, is a serious issue. Sea levels will rise, soils will dry out, weather extremes will increase, and temperatures will rise (Danielsen et al. 2009). As knowledge and efficiency improves, switching from fossil fuels to a more sustainable resource while maintaining access to existing resources should reduce man-made emissions (Department of Energy and Climate Change 2012). Bioprospecting is the process of finding, collecting, and extracting genetic S. Sethi (B) · S. Saxena Dr. B. Lal Institute of Biotechnology, Malviya Industrial Area, Malviya Nagar, Jaipur, Rajasthan, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_19

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material from samples of biodiversity for use in a variety of industrial purposes. It is used to describe the actions of microbes in decomposing plant matter to generate goods that are sustainable, such as medications, chemicals, fuels, cosmetics, textiles, heat, and power (Beacon 2011). Biorefining, which is similar to petroleum refining, is the long-term integration of biomass conversion processes and equipment to create fuels, heat, and value-added chemicals from biomass (Langeveld et al. 2010). A biorefinery is a concept adopted from the petroleum oil refinery, which converts biomass into a variety of goods. Sustainability, cascade, non-conflict with food, and a neutral carbon footprint are the four principles that guide biorefineries. The last three can be regarded ramifications of the core notion of sustainability. Biomass is tempting as a carbon–neutral and sustainable alternative to present fossil fuels, particularly petrol, which can be substituted by bioethanol and biobutanol (Nakayama et al. 2011; Shah 2020). Through a network and cascade of processes, the cascade principle mandates the creation of as many bioproducts (biofuels and added-value products) as feasible from biomass (Escamilla-Alvarado et al. 2012). This idea may be seen from two perspectives: direct and inverse cascading. On the idea that biochemicals (e.g. enzymes, biomass protein, organic acids, and antibiotics) have higher added value than biofuels, direct cascading focuses on generating first bioproducts and then biofuels (Angenent et al. 2004). A biorefinery may modify the sequence of production in response to energy shortages or market demands, emphasising biofuels above added-value bioproducts; this is known as an inverse-cascading scheme. Furthermore, following the creation of bio-products, any waste streams that still include organic material may be utilised to produce more biofuel, increasing energy output (Poggi-Varaldo et al. 2013). Many industrial biobased products, such as starch, lignin, oil, protein, cellulose, and terpene, emerge from physical or chemical treatment and processing of biomass. Other compounds, including as ethanol, lactic acid, acetone, butanol, and amino acids, can be made from feedstocks using biotechnological processes (Cheng and Wang 2013). Dedicated energy crops (such as Miscanthus), trees, aquatic plants, wood residues, waste materials, and agricultural food and feed crop residues may all be used as feedstocks in biorefineries (Chin and H’ng 2013). Waste biomass from landscape gardening is one example of waste material that may be used to extract valuable organic chemicals (Kamm and Kamm 2004). The enzymatic machinery of microbes are at the heart of the lignocellulose biofuel biorefinery, appearing at practically every stage of the process, from pretreatment through biomass hydrolysis and fermentation. The majority of biorefinery research is still in the idea and lab-scale stages, with a few pilot-scale and demonstration facilities across the world that usually consist of two steps: pretreatment and fermentation. Biorefineries use a variety of technical approaches to transform biological raw materials into finished products and industrial intermediates, including physical, chemical, and biological mechanisms. Most feedstocks, however, require pretreatment because saccharification requires the lignin/hemicellulose/cellulose matrix to be loosened (Chin and H’ng 2013; Kamm and Kamm 2004; Shah 2021). There are a variety of techniques to achieve this, including thermal, thermo-mechanical, or

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thermo-chemical treatment, all of which demand energy and so boost prices (Kamm and Kamm 2004). Biorefinery’s ultimate goal is to accomplish effective fractionation of lignocellulosic biomass, which may then be routed into different streams to separate value-added chemicals, considerably improving the economics of biofuel production. Separate Digestion and Fermentation, Sequential Saccharification and Fermentation, Synchronous Saccharification and Co-fermentation, and Combined Bioprocessing are the four types of biocatalytic process configurations. Separate Digestion and Fermentation avoids issues in fermentation or hydrolysis from interfering with other processes, allowing microorganisms and enzymes to perform at their best (Cheng and Wang 2013). Sequential Saccharification and Fermentation on the other hand, allows both enzymatic hydrolysis and fermentation to take place at the same time, cutting down on time. To achieve the aim of a sustainable economy based on bio-resources, the development and implementation of biorefinery processes is critical. The biorefinery concept is to manufacture biofuels, cellulose, hemicellulose, lignin, and byproducts from inedible lignocellulosic material. The replacement of petroleum-derived chemicals with those obtained from biomass will be critical to the chemical industry’s long-term viability. Unlike petroresources, which are restricted in type and composition, bioresources are made up of a diverse range of chemicals including cellulose, hemicellulose, oils, lignin, starch, and proteins. (Octave and Thomas 2009). Each ingredient in the biomass (plant) may be functionalized to yield non-food and food fractions, intermediate agro-industrial products, and synthons. As a result, a comprehensive set of particular technologies must be created in order to transform each fraction into value-added goods as effectively as possible. These fractions can either be employed directly as needed biochemicals or chemically transformed. Converting these wastes to high-value co-products will offset biofuel costs, improve lignocellulose biorefinery economics, reduce waste discharge, and lessen reliance on petroleum-based goods. By producing a wide range of chemicals, transportation fuels, and electricity, the biorefinery will provide new economic prospects for agricultural and chemical businesses (Fitzpatrick et al. 2010). The complete cycle of biomass production, i.e., breeding, growing, and harvest, as well as its (pre)treatment and conversion to products, should be addressed when developing technologically viable bio-refinery routes. The low energy content, seasonality, and limited geographic availability of biomass feedstocks have been identified as roadblocks to meeting large-scale energy and fuel demands. Furthermore, it will be demonstrated that small-scale (pre)processing of biomass may be preferable to large-scale processing (Sanders et al. 2007). A biorefinery, in theory, would combine technology from many sectors such as polymer chemistry, bioengineering, and agriculture. The US Department of Energy (DOE) has discovered high-volume commodity compounds that may be made from biomass and used as starting materials for a variety of chemical products. Although ethanol is likely the most well-known bio-based chemical produced worldwide, a wide number of bioproducts are created and employed in a wide range of industrial applications. Naturally occurring carbohydrate polymers, lipids and oils from

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plants, terpene-based materials, chemical products of carbohydrate-containing materials, and fermentation products of carbohydrate-containing sources are only a few examples. Fuel, timber, mechanical pulps, and textiles are all made from cellulosic plant resources. Wood-free paper, cellophane, photographic film, membranes, explosives, textile fibres, water-soluble gums, and organic-solvent-soluble polymers used in lacquers and varnishes are all made from purified cellulose. Photographic film, acetate rayon, different thermoplastic goods, and lacquers are all made from cellulose acetate, which is a biodegradable cellulose derivative (Patel et al. 2005). According to the existing circumstances, demand for food, energy, mobility, chemicals, and materials will skyrocket in the near future. Energy supply and mobility consume 80–90% of fossil resources. Knowing that there is a link between international markets for energy and fuels, raw materials for chemicals and materials, feed and food, and biomass, one must address raw material supply issues for food production, as well as materials, fuels, and energy production. Innovative research aiming at the creation and deployment of biorefineries-multi-step, multi-product facilities built for specific bio-sourced feedstocks will evolve into the successful use of biomass feedstocks, notably lignocellulosic materials in large-scale applications. Two main techniques may be found in the development of a biobased chemical sector. Value added chemicals in biomass are discovered and extracted in distinct processing and (bio) conversion processes in the first approach, the value chain approach. The residual biomass is subsequently converted into a universal substrate for the production of chemical goods. It is considered that extracting valuable chemicals and polymers from biomass, rather than manufacturing these compounds from universal building blocks, is more technologically and economically advantageous. It may be stated that biomass refining, separation technology, and bioconversion technology are the primary technological difficulties that must be overcome in order for this strategy to be economically viable. Furthermore, extensive integration of the food, feed, and chemical sectors, as well as significant infrastructural investment, are necessary. The integrated process chain strategy, on the other hand, is modelled after the petrochemical sector. In this strategy, a “universal” substrate is first converted into universal building blocks, which are then used to create chemical products. Building chemicals in highly integrated manufacturing facilities is regarded to be economically and technologically advantageous in this method. The high-efficiency translation of biomass into generally known building blocks for the petrochemical sector is the key technological hurdle for this strategy (Fitzpatrick et al. 2010). According to the existing circumstances, demand for food, energy, mobility, chemicals, and materials will skyrocket in the near future. Energy supply and mobility consume 80–90% of fossil resources. Knowing that there is a link between international markets for energy and fuels, raw materials for chemicals and materials, feed and food, and biomass, one must address raw material supply issues for food production as well as materials, fuels, and energy production (Octave and Thomas 2009). Decentralized biomass feedstocks will be more convenient for localised biorefinery approaches than large-scale, centralised facilities driven by cost-intensive technologies. Large centralised facilities as well as smaller dispersed plants may provide the

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most cost-effective option for a biorefinery, depending on geographical and technological boundary conditions (Lyko et al. 2009). In terms of energy and greenhouse gas emissions, bio-based polymers save 20–50 GJ/t polymer and 1.0–4.0 t CO2 eq/t polymer, respectively. Bio-based polymers are appealing in terms of particular energy and carbon reductions, but they can’t compensate for the increased environmental burden caused by the expansion of petrochemical polymers. Bio-based polymers will not be able to compensate for the economy’s overall environmental consequences during the next two decades. While environmental consequences in particular terms are considerable, effects in absolute terms relative to entire industry or society are minimal, according to the possible environmental and socio-economic implications analysed. There is a limited possibility for employment generation. It is important to note that these minimal contributions are due to the relatively small manufacturing quantities of bio-based polymers (Shen et al. 2010).

2 Biocatalytic Valorization of Lignocellulose 2.1 Cellulose Saccharification Systems The field of enzyme systems for lignocellulose bioconversion has sparked a lot of scientific interest and effort during the last few decades. Cellulases catalyze the hydrolysis of cellulose to soluble fermentable sugars, which is an important step in the enzymatic process. Fungi, bacteria, and plants all produce cellulases. Endoglucanases or 1,4-b-D-glucan-4-glucanohydrolases (EC 3.2.1.4), exoglucanases (EC3.2.1.74) and 1,4-b-D-glucan cellobiohydrolases (cellobiohydrolases) (EC 3.2.1.91), and bglucosidases or b-glucoside glucohydrolases (EC 3.2.1.91) are thought to be useful (Lynd et al. 2002). Cellulolytic systems can be linked together to form multienzymatic complexes (cellulosomes) or can exist independently as individual enzymes. The fungus Trichoderma reesei, the most researched cellulolytic microbe in the last 60 years, is the most often reported source of cellulases. T. reesei generates an extracellular, stable, and effective cellulase enzyme system among the different bacteria capable of manufacturing cellulase enzymes. However, the enzyme system from T. reesei has a poor glucosidase activity, which causes partial hydrolysis of cellobiose in the reaction mixture and, as a result, significant inhibition of the enzymes. Submerged fermentation technology (SmF) is used in the majority of research on microbial cellulase production, and the highly studied T. reesei is likewise produced in liquid medium. In submerged fermentation, proper monitoring and treatment of cultures are still critical. The fundamental technical constraint in fermentative cellulase production is the long fermentation period combined with low output. Solid state fermentation (SSF) for cellulase production is gaining popularity as a cost-effective

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approach for not only enzyme synthesis but also bioconversion of lignocellulosic biomass using cellulolytic microorganisms. Despite reports of SSF for cellulase manufacturing, commercial scale production continues to rely on the proven technique of SmF. The employment of enzymes, such as cellulases and hemicellulases, to hydrolyze polysaccharides to monomeric sugars is an environmentally benign method. Since the early 1970s, researchers have been studying the breakdown of cellulose by cellulolytic enzymes with the goal of establishing an ethanol manufacturing process. Even though soluble substrates for measuring endoglucanase and b-glucosidase activities have been developed, there are very few substrates for estimating exoglucanase activity. The hydrolysis of soluble substrates cannot be used to predict the hydrolysis of insoluble substrates. A pretreatment procedure separates the lignin component from the cellulose and hemicellulose to make the lignocellulosic biomass susceptible to enzymatic hydrolysis. Lignin obstructs hydrolysis by preventing cellulases from accessing the cellulose and binding irreversibly to hydrolytic enzymes. As a result, removing the lignin can greatly improve the rate of hydrolysis (Lynd et al. 2002). Enzymatic hydrolysis methods have demonstrated distinct advantages over acid-based hydrolysis methods; the very mild process conditions result in potentially higher yields, and the utility cost is low (no corrosion issues). As a result, this is the method of choice for future wood-to-ethanol processes. There are two types of variables that influence enzymatic hydrolysis: substrate-related factors and enzyme-related factors. The link between cellulose structural characteristics and enzymatic hydrolysis rates has been extensively researched, and various reviews have been published (Zhang and Lynd 2004). The complete hydrolytic machinery, high specific activity, high rate of turn over with native cellulose/biomass as substrate, thermostability, decreased susceptibility to enzyme inhibition by cellobiose and glucose, selective adsorption on cellulose, synergism among the different enzymes, and ability to withstand shear forces are the most desired attributes of cellulases for lignocelluloses bioconversion (Maki et al. 2009). These requirements are satisfied by protein engineering techniques, overexpression techniques, and the development of suitable enzyme combinations and hydrolysis conditions. The hydrolytic efficiency of a multi-enzyme complex for lignocellulose saccharification is determined by both individual enzyme characteristics and the ratio of enzymes in the multi-enzyme cocktail (Gusakov et al. 2007). The ideal cellulase complex should be highly active on the biomass feedstock, able to completely hydrolyze it, operate well at a mildly acidic pH, withstand process stress, and be cost-effective. Regulating the hydrolysis process, improving cellulase activity, optimising the reaction conditions, enzyme and substrate cocktail composition, enzyme recycling and recovery procedures have all been studied to increase the yield and rate of enzymatic hydrolysis. The substrate concentration has the greatest impact on the yield and beginning rate of enzymatic cellulose hydrolysis. Realistic approaches must be based on industrial substrates that are both physically and chemically relevant. Employing a statistical technique of factorial design, the enzymatic hydrolysis of lignocellulosic biomass has recently been improved using enzymes from various

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sources and mixed in an optimum proportion (Zhou et al. 2009). The total protein required to achieve glucan to glucose and xylan to xylose hydrolysis objectives (99% and 88% conversion, respectively) was reduced by twice, confirming this approach to enzyme optimization and lignocellulose hydrolysis process cost reduction. Enzymes that can withstand both acid and heat are also being studied in the hopes of improving the processing of lignocellulosic biomass. Extremophilic microbes/extremophiles generate these enzymes in their native state (Carvalho 2011). Many of the cellular traits and biosynthetic characteristics sought after in an ideal biofuel-producing microorganism, such as the ability to degrade lignocellulosic materials and resistance to substrate and product inhibition, can be found in isolated native organisms that can biosynthesize specific biofuels with high yield, can be found in isolated native organisms. Natural species’ inherent potential should not be underestimated (Ferrer et al. 2005). Existing culture collections and a bioprospecting survey of extreme environments are two large sources of microorganisms that bioprospecting efforts can tap into. Bioprospecting for better key enzymes can be random or guided by evolutionary or ecological principles. It can take the form of isolating microorganisms that grow better on biomass substrates, mining databases of sequenced genomes, using polymerase chain reaction (PCR) to clone variations of known enzyme genes, or using metagenomics to discover novel genes (BCC Research 2014). On a range of soluble and insoluble substrates, microbial degradation of lignocellulose includes synergistic catalytic activity. Unique metabolic pathways, cellular sensitivities to hazardous compounds, and an unusual variety of enzymes are involved in natural breakdown of lignocellulosic biomass by a consortium of microbes from termite guts, leaf litter, or forest floor (Ferrer et al. 2005). Many bacterial and fungal plant diseases are known to produce a variety of more stronger plant cell wall disintegrating enzymes. As a result, enzyme prospecting research continues to look for ways to improve Trichoderma enzyme formulations by supplementing with enzymes from other species (Kaul and Asano 2012). Developing enzyme mixtures to supplement the enormous amounts of commercial enzymes now employed is one strategy to improve conversion. Supplementing cellulases with supplementary enzymes has been demonstrated to improve hydrolysis in recent investigations (Mathé et al. 2002). Actinomycetes have been shown to generate thermostable cellulases in several species. Cellobiohydrolase is one of the cellulases produced by Thermomonospora fusca (Oshiro et al. 2002). Cellulase and beta glucosidase are both produced by Streptomyces sp. Streptomyces transformant T3-1-produced thermostable cellulases have been employed in the food sector and for cellulose conversion. Cellulase-producing, culturable bacteria have been identified from a wide range of settings throughout the years, including composting heaps, decaying plant material, ruminant faeces, soil, and severe conditions. Strains of Paenibacillus campinasensis, Bacillus subtilis, and Brevibacillus, among others, have been identified as novel cellulase producers (Voet et al. 2013). These are said to offer beneficial qualities for lignocellulosic bioconversion. Rational bioprospecting might lead to the discovery of organisms that could be used in biorefineries.

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2.2 Microbial Sources of Lignocellulolytic Enzymes Organisms that feed on lignocellulose can be found in compost, ruminant and termite digestive systems, and decomposing wood. Saccharification does not require any pretreatment because, despite the fact that it takes a long period, enzymatic processes have developed spontaneously to allow them to destroy material (EPOBIO 2006). Plant biomass hydrolysis is more complicated than cellulose hydrolysis. The plant cell wall is a heterogeneous matrix made up of polysaccharides of various compositions and connections, as well as aromatic lignin components (King et al. 2009). The lignin concentration and structure change significantly between C3 and C4 grasses, as well as between angiosperms and monocots (Shrestha et al. 2011). Even in basic bacteria, which typically form vast communities of hundreds of species, microorganism sequencing has revealed that more than 50 genes target polysaccharide breakdown. Such creatures have enzyme systems that are highly specialised and diversified (EPOBIO 2006). It has been demonstrated that fungi that break down grasses, such as Trichoderma reesei, and those that degrade wood, such as Phanaerochaete chrysosporium, are not the same (Ahmad et al. 2011). They don’t have the same degrading approach, thus they might not be the best choice for various plants. As a result, cellulosic enzymes from one system may not be able to depolymerise another (Shrestha et al. 2011). Clostridium thermocellum is the most commonly cited microbe for cellulose degradation, despite the fact that it is anaerobic, requiring the addition of nitrogen (N2 ) or carbon dioxide (CO2 ) instead of air to the fermentation (Tomes et al. 2011). Cellulomonas uda has hemicellulolytic enzymes and may be an aerobic equivalent. Plant pathogens appear to represent a new source of hydrolytic enzymes that has yet to be discovered. Many enzymes are produced by the fungus Gibberella zeae, which causes Fusarium head blight, and Ustilago maydis, which are not produced by the industrially employed Trichoderma reesei, including glycosyl hydrolase families (Couturier et al. 2012). To circumvent plant defences, plant diseases must be able to swiftly infiltrate the cell wall. Endocellulases, exocellulases, Beta-glucosidases, hemicellulases, and ferulic acid esterases are among the enzymes produced by G. zeae for biomass hydrolysis. When compared to T. reesei, the phytopathogens Magnaporthe grisea and G. zeae have more cellulases, hemicellulases, pectinases, carbohydrate binding modules, carbohydrate esterases, and polysaccharide lyases. During fermentation, Clostridium thermocellum hydrolyzes cellulose and generates ethanol, avoiding the need for a separate saccharification phase. It also produces hydrogen, acetic acid, formic acid, and butanol, which are all industrially useful fermentation products (Nakayama et al. 2011). As a result, C. thermocellum is seen as a potential candidate for metabolic engineering, in which energy and carbon flow may be steered toward the desired fermentation products. Because of their adaption to more harsh settings, proteins extracted from thermophilic bacteria are known to be innately thermostable. Bacteria useful in biorefining have been isolated from hot springs in Japan, such as Thermus thermophilus.

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A number of microorganisms that produce hemicellulolytic genes, such as Klebsiella pneumonia, are recognised human pathogens and hence would not be suitable for biorefining. Clostridium thermosulphurigenes (now Thermoanaerobacterium thermosulfurigenes) has been shown to have xylanase enzymes attached to the cell membrane non-covalently. This species’ xylA protein has also been found to bind two cobalt ions per subunit, with a total of four (homotetramer) (UniProt Consortium 2014). At higher temperatures, metal ions are necessary for the stabilisation and activation of the enzyme. B. subtilis, K. pneumonia, Lactobacillus brevis, and Streptomyces sp. are among the bacteria that can degrade xylose because they have the xylA gene (xylose isomerase). The xylA protein is particularly important in platform chemical manufacturing since it converts xylose to xylulose directly, bypassing D-xylose reductase (XYL1) and xylitol dehydrogenase (XYL2). For early depolymerisation of the xylan backbone, endo-xylanase, exo-xylanase, and beta-xylosidase enzymes are required.

3 Different Strategies for Prospecting Novel Enzymes Nonetheless, the fact that a huge number of protein sequences derived from the genomes of these cultured organisms have yet to be given a function suggests that scanning already sequenced genomes might be a lucrative option. New genes and gene products for biotechnological applications can be discovered by screening genomic libraries of culturable species. Genomic and metagenomic-based methodologies have been effectively established as strong tools for finding enzymes with fibrolytic activity from distinct terrestrial microbial ecosystems to overcome the lack of culture method.

3.1 Function-Based Screening of Microorganisms Composting heaps, decaying plant material from forestry, agricultural waste, ruminant faeces, soil and organic matter, and severe settings have all been used to extract culturable cellulase-producing bacteria over the years. Growing on microcrystalline cellulose as the only source of carbon, followed by extraction and analysis of the 16S rRNA sequence to detect the cellulase generating species, can be used for functionbased screening for cellulase production. An effective plate-screening process is required to obtain the pure isolate. Furthermore, most function-based cellulase activity testing is done on plates containing carboxymethylcellulose (CMC). Kasana et al. (2008) modified the plate-screening process by adding hexadecyltrimethyl ammonium bromide or Congo red for the gram’s iodine. However, due to low enzyme activity, the plate-screening approach utilising diazo dye is not sensitive enough for screening. As a result, several screening methods using chromogenic/fluorogenic groups were devised in order to obtain quantitative procedures

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with improved sensitivity (Maki et al. 2009). The tendency for products to fail to diffuse widely in the medium is a fundamental constraint in the inclusion of fluorescent substrates into agar plates, and as a result, these compounds are not as commonly employed. Different test techniques have been used to screen recombinant proteins in E. coli in order to locate the new protein (Duan et al. 2009). Expression screening is a useful tool for identifying new cellulose-producing bacteria in complicated microbiomes such as the rumen, termite digestome, pulp and paper mill effluent sediments, and so on. A number of cellulases with unique properties have been reported or are still being identified using various screening approaches. Using metagenomics techniques, researchers have been searching for new genes in unculturables. The newly isolated enzymes must be able to tolerate enzymatic activity in biotransformation processes while also being resistant to harsh environmental conditions such as a wide pH and temperature range. Enzymes must also reduce the cost of converting complicated polysaccharides to ethanol by reducing enzyme use in the biotransformation process while maintaining their enzymatic character. In functional screening, a metagenomics library must be constructed and functionally screened with a specified character in order to uncover a novel enzyme from a complex microbiome.

3.2 In Silico Sequenced Based Screening Karr et al. (2012) revealed the world’s first comprehensive phenotypic prediction from a genotype of Mycoplasma genitalium, the world’s smallest free-living bacterium. As a result, in silico sequence-based screening for candidate genes is based on known conserved sequences and hence fails to find novel genes. Unlike functional-based screening, however, sequence-based screening can uncover target genes regardless of the gene’s sequence completeness. The introduction of new sequencing technologies, such as next-generation sequencing, has altered gene cloning’s limitations. Tyson et al. (2004) reported the first successful metagenome experiment, which looked at the microbial communities in acid mine drainage.

3.3 Role of Metagenomics in Biorefinery Over the last two decades, considerable progress has been made in using metagenomics methods to expand our understanding of the microbiome of diverse ecosystems, opening a magnificent avenue for examining this information for efficient lignocellulosic biomass conversion. Because “saving our environment” is a pressing requirement, just converting copious biomass will not sufficient unless and until we build a comprehensive system for capturing value-added products in order to establish a biorefinery. This industry accounted for around 8% of total industrial growth, and

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biofuels are anticipated to account for 54% of the worldwide bioeconomy. Metagenomic techniques have extended our understanding of microbial metabolic pathways for lignocellulosic biomass. Metagenome studies that use modern sequencing methods now not only yield more total base pair reads, but also have more consistent coverage of species within the community (http://www.genomesonline.org/). Many online bioinformatics resources have been made accessible to analyse metagenomic data (Huson et al. 2007), but new bioinformatic methods for data mining based on sequence homology, protein structures, catalytic sites, and particular activities are still needed. Protein structural models may be created using protein prediction and classification methods to investigate protein folding mechanisms. The formation of putative active sites and their function can be anticipated based on their structural folding (Cantarel et al. 2009). Several papers on searching metagenomes for cellulolytic genes and active enzymes that will be important in the development of the biofuel industry are available (Gilbert et al. 2012). Schluter et al. (2008) sequenced a metagenome library of an agricultural biogas plant’s microbial population. They sequenced 141 million base pairs and discovered a group of bacteria that play a prominent role in methanogenesis, as well as genes encoding cellulolytic activities in Clostridia spp. (Warnecke et al. 2007) collected 71 million base pairs of sequence data by sequencing a metagenome library of hindgut microbiome data from the biggest family of wood-feeding termites (Termitidae). They discovered a large number of probable cellulases and hemicellulases with over 700 domains homologous to glycoside-hydrolase catalytic enzymes, corresponding to 45 distinct carbohydrate active enzymes (CAZy) families, using the global alignment approach. There are already around 2500 glycoside hydrolases discovered and divided into 115 families. The enzyme family includes members from bacteria, fungi, and plants, each with its own set of features and substrate preferences in hydrolysis processes. Cellulases and hemicellulases are lignocellulolytic enzymes that belong to a category of enzymes known as glycoside hydrolases (Dashtban et al. 2009). Though metagenomics has traditionally focused on harsh conditions, the focus has now switched to the investigation of particular habitats in order to find microbial consortiums. Various environments have recently been explored using metagenomic approaches (Luetz et al. 2008) including wheat straw, rice straw, corn stover, agave fibres, sugarcane bagasse, wood feeding insects, wet tropical forest, peat swamp forest soils, switch grass-adapted compost community, yak rumen, air-metagenome, and biogas reactors. The investigation revealed a diverse microbial flora and their genes encoding enzymes (cellulases, xylanases, ligninases, and esterases/lipases) for the depolymerization of lignocellulosic with a greater working range at different temperatures, pH levels, and oxygen levels. The creation of metagenomic libraries for the production of desired sequences necessitates the use of a wide variety of hosts. Industrial workhorses such as

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E. coli and Saccharomyces cerevisiae have been extensively used for heterologous expression to date. The lack of posttranslational modification limits the efficacy of expressing desired metagenomic sequences in E. coli for lignocellulosic biomass degradation, creating a significant gap in the field of functional metagenomics for lignocellulosic biomass conversion. As a result, several research papers theorised that the discovery of novel hosts from the metagenome or the exploration of available species for host provides fresh energy to metagenomic research. Various prokaryotic systems have been used as suitable hosts, including Thermus thermophilus, Burkholderia graminis, Ralstonia metallidurans, Sulfolobus solfataricus, Caulobacter vibrioides, Pseudomonas putida, Bacillus subtilis, Streptomyces, and Proteobacteria spp. have been used as suitable hosts for compatible enzyme assays (such as esterase, phosphatases, lignin peroxidases) and heterologous gene expression for metagenomic studies (Bosch and McFall-Ngai 2011). Apart from bacterial systems, fungal cells have recently been favoured due to the ease of posttranscriptional and posttranslational modification, as well as efficient signal peptide recognition/secretion for lignocellulosic enzymes such as xylanases, cellulases, xylose isomerase, and endoglucanases for lignocellulosic degradation (Ferrer et al. 2008). Because the content of lignocellulose biomass varies, different types of enzymes are required for successful bioprocessing. As a result, bioprospecting for new enzymes from the environment is an interesting field of metagenomic study (Valenzuela et al. 2006). Until now, the most prevalent enzyme, cellulose, has been thoroughly studied in a variety of settings to see if it can work in a variety of physiochemical circumstances. Cellulase has been extracted from a variety of habitats, including rice straw compost, sugarcane bagasse, and biogas digesters, which can work at high temperatures, while cellulase obtained from mangrove soil and soda lake can survive high salt concentrations. Furthermore, cellulase generated from compost soil showed optimal activity at temperatures ranging from 10 to 40 °C. Endoglucanases genes such as GH5, GH9, GH12, GH44, and GH45, which are responsible for cellulase depolymerization, have been found in a variety of settings including soil, spill water, insect stomach, and faeces, as well as in Clostridium, Bacillus, Vibrio, and Cellulomonas. Furthermore, thermo-alkaliphilic enzymes (cellulases and hemi-cellulases) have been extracted from plants using a multisubstrate technique (Poretsky et al. 2005). The advancement of computational biology has hastened the development of a genetic repertoire of various enzymes engaged in lignocellulosic biomass breakdown. CAZymes (carbohydrate-active EnZymes), a web-based system for the structural and functional annotation of complicated enzymes, has made lignocellulosic research much easier to navigate (Poretsky et al. 2005). The CAZy database’s six groups, which include glycosyl hydrolases, carbohydrate esterases, glycosyltransferases, polysaccharide lyases, carbohydrate-binding modules, and AAs, are a useful tool for utilising lignocellulosic biomass information (Li et al. 2009). As a result, existing methodologies, together with technological advancements such as synthetic biology, will undoubtedly assist in a thorough knowledge of genes for the mining of biocatalysts that may play a part in biorefinery development.

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Bioprospecting has the best chance of uncovering unique ligninolytic bacteria and enzymes from various ecosystems in order to develop high yields of lignin-based goods. Proteobacteria and actinobacteria have large proportions of lignin-degrading genes, according to a bioinformatics search for ligninolytic enzymes and aromatic degradation pathways in bacteria (Poretsky et al. 2005). Gram-positive Actinobacteria such as Streptomyces viridosporus, Streptomyces coelicolor, Amycolatoposis sp. 75iv2, R. jostii RHA1, Nocardia autorphica, Microbacterium phyllospharae, Micrococcus sp., and Frimicutes such as Bacillus sp. and Paenibacillus sp. All of these species have been demonstrated to mineralize milled wood, and colorimetric tests have revealed variations in lignin concentration. Gram-negative Protobacteria such as Sphingobium SYK-6, Ochrobactum sp., Cupriavidus basiliiensis, Comamonas sp. B-9, P. putida mt-2, P. putida KT2440, Acinetoacter sp., Enterobacter lignolyticus, Citrobacter freundii, and Bacteroidetes Shingobacterium Although preliminary research has shown a wide range of lignin and aromatic degradation routes, there are likely to be many more pathways and enzymes still to be discovered. With further screening studies, relevant genotypes and phenotypes for improving biological valorization of lignin in the pursuit of value-added products can be found.

3.4 Synergistic Metabolic Activities of Microbial Consortium Given the limited capacity of certain bacteria to breakdown lignin, the synergistic metabolic activities of microbial communities have a significant potential to depolymerize and degrade compounds for its value (Herbst et al. 2016). Because of its cheap cost and economic security, the microbial community has the potential to solve the issues in lignin valorization. The biological valorization of lignin or aromatic compounds is a multi-step process involving a number of ligninolytic enzymes. White rot fungi are thought to release phenol oxidases in reaction to environmental factors in order to degrade native lignin from plant biomass in soil, whereas bacteria are thought to play a key role in the mineralization of low-molecular-weight lignin derivatives. In comparison to fungus, bacteria have an edge in terms of environmental tolerance. The lignocellulolytic enzyme system establishes a structurally stable consortium capable of effectively decomposing plant biomass substrates. After 3 days of culture, a microbial consortia WCS-6 has been shown to degrade filter paper, cotton, and rice straw by 99.0%, 76.9%, and 81.3%, respectively, by sequential subcultivation on the medium containing rice straw. After 15 days of static culture at 30 °C, the microbial consortium was successfully screened to directly consume lignin, converting 60.9% lignin at 30 °C (Barber 2007). The microbial community DM-1 was isolated by serial fermentation for over 5 generations to yield microbial consortia for efficient lignin breakdown. Within 16 days of fermentation, the microbial community DM-1 showed high degradation selectivity.

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According to the microbial consortium DM-1’s diversity study, the most common genera are Achromobacter, Cellulosimicrobium, Mesorhizobium, Stenotrophomones, and Pandoraea. For lignin breakdown, a microbial consortium consisting of two inter-kingdom fusants (PE-9 and Xz6-1) and two indigenous bacteria (Bacillus sp. (B) and P. putida (Pp)) has been created (Speda et al. 2016). On the breakdown of lignin, P. putida (Pp) and PE-9, as well as Bacillus sp. (B) and PE-9, exhibited a substantial interaction. Multiple bacteria may produce laccase and had great environmental adaptability and lignin breakdown capacity in alkaline circumstances, according to the findings (Speda et al. 2016). To decolorize black liquor, a bacterial consortium composed of Citrobacter sp., Serratia marcescens, and Klebsiella pneumoniae was utilised. This consortium’s ligninolytic activities have been linked to growth, resulting in an 85% reduction in colour. The lignin depolymerization caused by bacteria’s ligninolytic activity is revealed by the reduced colour intensity. Bacterial treatment consumes the aromatic chemicals in the control, and new metabolic products are created in the decolorized Kraft black liquid, lowering toxicity by roughly 70%. The favourable effects of microbial consortia on lignin breakdown are highlighted in all of the research above, indicating a potential for overcoming the obstacles.

3.5 Synthetic Biology Creating the Pathways to High Value End Products Systems biology provides great tools for revealing lignin’s basic metabolic cycle. Synthetic biology may be used to design cells, resulting in building block molecules that can be utilised in biofuels and materials. Previous research has shown that engineered lignin-degrading microbes can manufacture and enhance a variety of valueadded products from lignin or aromatics. By studying alternative degradation mechanisms, Johnson and Beckham designed P. putida to convert aromatic compounds from lignin (Bae et al. 2013). When an exogenous meta-cleavage route in P. putida mt-2 was used to substitute the endogenous catechol ortho catabolism system in P. putida KT2440, the pyruvate output from lignin aromatics increased by around 10%. The endogenous protocatechuate ortho route was replaced with a meta-cleavage pathway, which resulted in a five-fold increase in pyruvate production. The synthesis of muconic acid from a lignin-rich stream has been proven by designing a P. putida KT2440. The muconate was collected and converted to adipic acid by catalysis. The research found that utilising synthetic biology techniques, aromatic compound degradation pathways may be efficiently designed to boost target product yield. The functional modules boost cell growth by a factor of two and increase lignin aromatics intake. The up-regulation of oxidation of fatty acids was also found when lignin and vanillic acid were used as carbon sources. The third functional module responsible for PHA synthesis was then constructed based on these findings by rechanneling oxidation products. Using aromatic compounds as a carbon source, a

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record PHA yield of 0.73 g/g dry cell was produced. The addition of these functional modules dramatically increased the amount of PHA produced from lignin.

4 Future Perspectives The aim to employ microorganisms to convert plant biomass into biofuels and chemicals has sparked a need for microbial “biorefinery” techniques that can power future economies and contribute to the creation of sustainable enterprises by exploiting renewable and carbon–neutral resources. Plant biomass lignin has the potential to be a renewable source of alternative fuels and hence a supply of value-added chemicals. According to recent research, biological lignin processing is a viable option for increasing lignin conversion efficiency and improving biorefinery sustainability by lowering lignin breakdown chemical consumption and process costs in biorefineries. Despite considerable advances, research over the last decade has indicated that biological lignin valorization issues persist due to a lack of knowledge about the lignin breakdown route and low product yield in microorganisms.

5 Conclusion The scarcity of energy with the fossil fuels continues to explore the alternative fuel resources. Biofuels have been considered as an alternative energy source throughout the world. Microbial enzymes are known to be used as biocatalysts in bio-refinery industries however, only a few enzymes are currently employed for commercial applications. The inefficiency and low activities of currently available enzymes for production of biofuels has limited their industrial application. In this scenario, the metagenomic data provides a new unexplored treasure of genomic wealth that can enhance the enzyme inventory by the discovery of novel useful enzymes. Number of functional screening approaches in metagenomics has been implemented to emphasize uncultivated microbes and their potential application in biofuel development, concerning their specific functions in their environments. Metatranscriptomics and metaproteomics are the newest development in metagenomics that gives further promises for functional screening of uncultivated microbes. With the current technological advances in the next generation sequencing technologies, remarkable price drop in sequencing large data within short time has greatly makes metagenomics as a great tool to access the inaccessible organisms.

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Microbial Fuel Cell Usage in Treatment, Resource Recovery and Energy Production from Bio-refinery Wastewater Rajesh Singuru, G. Praveen Kumar, and Adhidesh S. Kumawat

Abstract The horizons of industrialization and economic growth have expanded, resulting in enormous global energy consumption. Fossil fuels induce environmental challenges such as CO2 emission. Nuclear, solar, wind, and biomass energies are alternatives with limitations. In the light of utilizing biomass energy sources, microbial fuel cells (MFC) is a relatively novel approach for generating energy. Electricity is generated from biomass using microbes in MFC. The microbes oxidize organic matter and generate electrons for the current generation. The MFC consists of bacteria as exoelectrogens generating electrons by electrogenesis while employed in a reactor. MFC could be constructed in different arrangements facilitating separated environments for both anode and cathode. The separation is necessary due to the presence of oxygen as it inhibits the generation of electrons by microbial processes. Thus, the system should be designed in such a way as to keep the microbe environment free from oxygen. This separation is usually achieved by placing a membrane between anodic and cathodic chambers. A voltmeter or potentiostat connected to the circuit measures the current and potential drop. The prime parameters that affect bacterial activity in MFC are bacterial culture, temperature and organic matter. The electrode material, pH, electrolyte, proton exchange system, operating conditions in cathodic and anodic chambers, substrate composition, oxidant, and catalyst in the MFC system also impact performance. Understanding the parameters enable optimization of process and ensure system sustainability. Wastewater treatment is one of the most researched areas in engineering. MFC explores the treatment of wastewater along with electricity generation. Various researchers have been developing an MFC system that is more sustainable to generate electricity using wastewater by employing various materials substrates and system architectures. With the advancing development, MFC Technology could be employed to generate enough electricity to power even a city, partly or entirely. Thus the development of MFC technology could enable us to achieve energy sustainability. R. Singuru · A. S. Kumawat (B) Department of Chemical Engineering, NIT Rourkela, Rourkela, Odisha, India e-mail: [email protected]; [email protected] G. Praveen Kumar Department of Chemical Engineering, NIT Calicut, Kozhikode, Kerala, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_20

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Keywords Microbial fuel cell · Bioelectric energy sources · Industrial wastewater · Wastewater treatment · Electrogenesis

1 Introduction World energy requirements have been increasing continuously since the industrial revolution in the last century. It has been judiciously estimated in the World Energy Council 2019 meeting that the energy requirement would double from the present level in the next 40 years (Kober et al. 2020). Due to the continuous increase in demand for energy, the fossil fuel reserves would deplete drastically, lest a shift in the energy consumption towards renewable and non-conventional sources is made. In addition to the increasing demand for energy, addressing global climate change is an equally important topic that requires shifting towards non-conventional and renewable energy sources (Owusu and Asumadu-Sarkodie 2016). One of the most significant environmental challenges is simultaneously addressing energy requirements and atmospheric carbon dioxide concentration levels. It is also because carbon-positive methods suffice primary energy requirements. It is thus quite relevant to mention here that there is an urgent need for devising methods to produce energy in carbon– neutral or rather carbon-negative methods. Various carbon-negative methods such as solar, wind, nuclear, hydrogen and bioresources have already been investigated by researchers. Solar energy is the ultimate source of energy, providing Giga amounts of energy, depending upon the methods of energy utilization. It is worth mentioning that the current usage of solar energy stays far below that of maximum utilization due to limitations of capturing and utilizing solar energy through panels. The generation of hydrogen through solar energy increases the efficiency of utilization. Additionally, efforts are being put into renewable energy technologies to generate and efficiently utilize fuels such as wind and solar to generate hydrogen. Furthermore, several techniques such as fermentation and other biological methods utilize bacteria and algae to produce hydrogen (or smaller molecules such as methane). The hydrogen produced by alternative methods utilizing microbes has several efficiencies and handling limitations due to the inherent complexity of the process and technique (Logan et al. 2008). Due to the advancement in technology and development related to fossil fuels, the efficacy of generating electricity from fossil fuels has continuously improved. Developing the resources and techniques for generating utilities such as electricity with comparable efficiencies from renewable sources would require higher research inputs. One of the industrial operations that need the urgent attention of researchers is wastewater treatment. Though colossal research is going on to improve wastewater technologies, the high treatment cost is still a challenge with the current level of wastewater treatment technologies. Furthermore, with the advent of novel technologies such as microbial fuel cells (MFC), it has been possible to recover high values products such as heavy metals and chemicals such as hydrogen and methane.

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Due to these advantages, MFCs have become an attractive solution to produce bioelectricity from biowaste resources such as wastewater from biorefineries (Jadhav et al. 2020). Additionally, the increased attention of researchers and policymakers towards bio-based processes and products has increased the bio-based materials into the atmosphere. It has caused a requirement to increase awareness of bio-based processes and products. Bio-based products have the potential to increase global economic growth significantly. It includes liquid fuels, polymers, building materials, adhesives, and lubricants. Additionally, rural economies gain from biomass agriculture. Domestic production and processing contribute to the conservation of natural resources. Apart from CO2 emissions, items made from biomaterials are often less polluting than those made from petroleum. Despite these benefits, bio-based products remain unpopular. Bio-based products cannot compete on price with petrochemical or conventional products, which is one of the several impediments to a bio-based economy (Lee et al. 2021). Feedstock costs are expensive, and the intensive processing required for feedstock contributes to the high cost of bio-based commodities. Biochemical conversion, more commonly referred to as bioconversion, converts biomass into several products. The pretreatment process is done to separate the biomass into desired categories. Hydrolysis is carried on the separated biomass that breaks down the segregated biomass. The cellulose and hemicellulose components of the biomass are processed into carbohydrate monomers. Further, fermentation is employed to obtain desired products from the carbohydrates (Bhatia et al. 2021). Stillage is the remainder of the separation process and constitutes the bulk of wastewater in bioconversion facilities. One liter of product can be utilized for the production of about 20 L of stillage. The organic matter concentration of effluent from lignocellulosic biorefineries varies according to feedstock and technique. Around 85% of wastewater is composed of flash condensate from pretreatment processes such as steam explosions, blowdown of boiler and cooling water, and water for cleaning (Morales et al. 2021). The stillage effluent from conventional starch to distilleries for ethanol and alcohol is identical to lignocellulosic stillage. Numerous research has been conducted on treating various contaminants found in wastewater. A typical biorefinery wastewater’s average characterization contains components such as solids (volatile, suspended, dissolved), phosphorus, ammonia, nitrates, nitrites, nitrogen and sulfates. Henceforth, it is justified to input the research efforts in a technology that can generate electrical energy from these waste components. MFC is the technology that can treat a considerable number of bio-resources at a relatively low cost and make a sustainable addition to the electricity production to meet the increasing energy demands, as shown in Fig. 1. Moreover, a higher understanding of MFC is required to develop and increase its technology readiness level. In this chapter, an attempt is made to introduce MFC technology, materials, applications and advancements.

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Fig. 1 Representation of MFC technology process

2 MFC and Its Working Principle An MFC, a fuel cell, generates electricity by converting the available energy in biodegradable substrates, low molecular weight organic acids, and amino acids with a catalytic process of electrochemically active bacteria over the electrode. At first, Potter (1911) described the electrical generation from the microbial breakdown of organic substances. Until the next century, just a few reports were prepared on the MFCs. Later, Schroder (2011) gave significant steps in the technological development of MFCs, envisioning a great future scope in this area. Tamboli and Eswari (2019) presented different types of MFCs: Dual-chambered, Single-chambered, Up-flow, Stacked, Flat plated and Paper type MFC. A significant consideration is directed towards development of MFCs cells during these days due to their operating conditions at ambient temperatures and a variety of biodegradable substrates as fuel, which gives an advantage over conventional methods in operating parameters (Do et al. 2018). A typical dual-chambered MFC arrangement contains two chambers in which an anode and a cathode are placed, as shown in Fig. 2. The figure represents the electrodes to be separated by a proton exchange member (PEM), through which the protons produced at the anode reach the cathode for maintaining ionic balance. The anode and cathode are connected by an electrical circuit with connecting wires such as titanium or copper wires to make it a closed system. The organic substances are oxidized in the anode chamber by the microorganisms producing electrons, protons and carbon dioxide. Due to the potential microbial metabolic activity of microorganism’s redox reactions, the electrons are generated and transferred from the anode to the cathode through the electric circuit. Redox reactions occur at both the electrodes as described, oxidation reactions occur at the anode chamber and reduction takes place at the cathode chamber due to electrons in the MFC (Mohan et al. 2014; Shah 2020). MFCs are encouraging substitutions for generating electric current from different materials such as natural organic matter waste, and can be beneficially employed with various applications in industrial, agricultural municipal solid waste and wastewater treatment. MFCs have the potential to handle a broad variety of fuels and oxidants in principle, but they are currently focused on common fuels such as natural gas (and derivatives) and hydrogen, which are synthesized using air as an oxidant. An MFC

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Fig. 2 Working principle of MFC

processes with a continuous supply of fuel to the anode and an oxidant to the cathode (usually oxygen from the air). A current travels through an electrolyte formed by electrochemical processes at the electrodes, while a corresponding current drives a load that generates work at the electrodes. A typical representation of MFC has been shown in the Fig. 1. It represents the anodic matter conversion in the anodic chamber as well as water oxidation in the cathodic chamber (Bard and Faulkner 2001). The anode is a vital component of an MFC because it offers the essential substrate for growth of bacterium. Additionally, these bacteria create electrons and protons that are sent to the anode. However, creating anode materials that function well in MFCs continues to be a difficulty. Recently, there has been a spike in demand for high-performance MFC anode combinations, materials, and design. To achieve high-performance standards, anode materials must possess a number of critical properties including biocompatibility, high conductivity, chemical stability, thermal and mechanical stability, and a wide surface area. While a huge variety of anode materials have been employed in MFCs, the limitations of these materials render this technology unsuitable for practical applications. The configuration of the anode is crucial because it provides the required surface area on which the bacteria generate and produce electricity. However, no treatment has been approved for extensive applications till date. While both electrodes (anode and cathode) are critical in MFCs, the anode is responsible for bacterial growth, clearance rate, electron generation, and cathode conversion according to ElMekawy et al. (2017). The authors have made an attempt to evaluate and analyze the performance of graphene derivatives as anode

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and cathode electrodes. The scientists concluded that an anode manufactured of graphene derivatives outperformed a cathode in their experiment in terms of energy output (Santoro et al. 2017; Shah 2021). On the other hand, carbon-based materials are the most significant and rapidly developing anode materials. The most effective carbon-based anode material is graphene derivatives. It is now feasible to synthesize graphene oxide from household and industrial wastes. Similarly, several metals and conducting polymers prevail as improved composites. Additionally, these composite materials are viewed as a preferable choice for enhancing electrode performance, particularly in the manufacturing of anode electrodes. Despite this improvement, it is not recommended to use unmodified graphene derivative materials because of their bactericidal characteristics. On the other hand, unmodified metal derivatives demonstrate corrosion concerns in MFCs as well. As a result of this, graphene derivatives including metal/metal oxide nanoparticles or conductive polymer-based composite materials have been utilized as anodes in MFCs to increase efficiency. Composite materials’ greater surface area and conductivity create a perfect environment for bacteria to develop long-lasting connections with the anode material. Thus, when graphene derivatives are used to increase MFC performance, it is crucial to modify the anode (rather than the cathode).

3 MFC Construction and Its Components A MFC consist of electrodes and electrolytes similar to an electrochemical cell. Likewise, the development of a MFC includes research on the components and efficiency of the process similar to an electrochemical cell. The electrode materials should be compatible to process in terms of variety of factors such as biological activity, chemical stability, mechanical strength and electrical conductivity. Further discussion of the electrodes is covered in the succeeding sections.

3.1 Anode Materials Numerous materials may be utilized to generate an optimal anode for MFCs requiring a higher surface area to maximize extracellular electron transfer via a biofilm. On the other hand, anodic components are critical because they enable anaerobic microorganisms to oxidize organic waste more rapidly. Bacteria are widely accepted to have a significant effect on the power density of MFCs (both in terms of concentration and species). As a result, the anode material must be suitable. MFCs require high-performance anode materials. Furthermore, some research has concentrated on improving anode materials using various modification techniques. As a result, additional resources for preparing anode material are required. The most often employed

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sources include carbon-based materials, metals or metal oxides, conducting polymers, or composite materials, all of which are recognized as crucial anode preparation materials (Li et al. 2017).

3.1.1

Carbon-Based Materials

Recently, Carbon-based materials have gained popularity as electrode fabrication materials due to their chemical suitability, mechanical durability, low cost, better electrical conductivity, biocompatibility, and efficient kinetics of electron transfer. Carbon has been employed and tested in various forms including rod, fiber, cloth, felt, mesh, and paper. Further, processed forms of carbon have been utilized such as activated carbon powder, activated carbon cloth, glassy carbon, brushes, and reticulated vitreous carbon. Additionally, graphite has been applied for MFC applications in the form of block, felt, 3D graphite, graphene oxide, and granular graphite (Li et al. 2017). Graphene, a recently discovered substance, has garnered considerable interest for its potential application as an electrode material in MFCs. Wang et al. (2011) observed that carbon mesh is less expensive and has a greater current density than other forms of carbon. However, the performance of the carbon mesh could be enhanced by treating with ammonia (or other types of) gas. As a result, no raw material has a higher power density today. Borsje et al. (2016) studied the capacitive bio-anode properties of single carbon granules. The results were obtained by measuring the charge storage capacity and current output. Charges can be stored in the activated carbon granules through an electric double layer, which increases the function of the bioanode. Granular and activated graphite carbon grains were employed to determine the untapped potential of these materials. When compared to Ag/AgCl anodes, single activated carbon-based granules generated a current density of 0.6 mA/cm2 at 300 mV applied potential. Capacitive granules are capable of generating around 1.3–2 times the additional charge in comparison to graphite granules with smaller surface areas, depending on the charge or discharge procedure. Similarly, Kalathil et al. (2011) explored activated granule carbon, which generated double the energy of conventional carbon materials. According to the researchers, granule-activated carbon might be a feasible alternative source of anode material to be employed in MFCs. Carbon cloth/sheets are flexible materials that provide a sufficient surface area for bacterial growth. However, due to the material’s high cost, it is unsuitable for large-scale applications. The researchers have observed that activated carbon cloth had a higher surface area and a better adsorption capacity for sulphide removal and current production in electrochemical oxidation of sulphide materials at the anode. Wang et al. (2016) demonstrated the preparation of a doped carbon fabric with a high current efficiency of 2777.7 mW/m2 . The carbon fabric was doped with nitrogen gas to result in a significant increase in power efficiency. Another component that is frequently utilized as an electrode material in MFCs is graphite. Graphite is a crystalline form of carbon that has been hybridized with a sp2 atom. Graphite is a viable anode material for MFC applications due to its

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excellent conductivity and durability. Graphite sheets, clothes, plates, granules and brushes are all suitable materials for constructing a simple anode. Ter-Heijne et al. (2008) discovered that graphite with rough surface structure had a greater current density than smooth surfaced graphite for employment as an electrode in MFCs. This material, however, is inappropriate for commercial energy generation because to its high cost and relatively lower surface area. Lowy et al. (2006) evaluated the performance of a graphite brush and advocated for its usage as the anode in MFCs to maximize energy output while minimizing harmful emissions. Later that year, Yazdi et al. (2016) employed two graphite brushes with different diameters (2.5 and 5.0 cm2 ) and enormous surface areas of 18,200 and 7170 m2 /m3 , respectively. These extremely small brushes with a diameter of 2.5 cm2 exhibited a maximum current density of 2400 mW/m2 and a columbic efficiency of 60%. This proved that the large surface area of the anode materials contributed greatly to the high removal efficiency and current output of the anode by boosting bacterial growth on its surface. Similarly, Zhang et al. (2016) was able to achieve a power density of 1430 mW/m2 utilizing a graphite brush with a diameter of 5 cm2 as the anode electrode. This means that brushes with a smaller diameter can generate more energy than brushes with a bigger diameter. Similarly, graphite was calculated to have a power density of 1771 mW/m2 when used as an electrode in the Cassava mill’s waste water treatment. Additionally, carbon-based materials are employed in a packing form to enhance the accessible surface area for bacteria. Due to the increased specific surface area of graphite carbon, it is also employed in packaging. Doping graphite with metals/metal oxides is one way for improving graphitic materials’ performance as anodes. It was found that calcium sulphide doped graphite may be used to create an anode material that increased bacterial interaction with the anode while reducing the electrical potential. While investigating graphite-doped iron compounds, researchers such as Yasri et al. (2016) found that they outperformed previously created anode materials. Many people are interested in graphene, a recently discovered carbon allotrope found in a two-dimensional hexagonal lattice. Graphene is seen as a viable anode material because to its high electrical conductivity and remarkable thermal and mechanical durability qualities. Graphene has stronger nonlinearity and diamagnetism than graphite. The anode use of graphene and its derivatives in MFCs is currently being studied. Graphene may be made utilising a wide range of methods. Commercially available graphene is expensive, however waste materials may be used to produce graphene at a reduced price. Graphene is an excellent anode material for MFCs because of its high energy output. Graphene-based electrodes are more efficient anodes than traditional carbon-based electrodes because of their outstanding performance qualities. When used in MFC operations, graphene is non-toxic to bacteria. It is thus possible to reduce the poisonous properties of copper and other materials, such as metals and conductive polymers, by combining them together. Carbon allotropes with altered properties might transform the energy and wastewater treatment industries.

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Materials Composed of Natural Biomass for the Anode

The materials employed as an electrode exhibit a wide variety of chemical, physical, and biological properties. The electrode materials must be physiologically compactable with bacterial species in order to influence microbe adhesion, electron transfer, reaction rate, and electrode resistance on the electrode surface. However, in recent years, the selection and manufacture of electrode materials have emerged as exciting and expanding research subjects. In electrode manufacture, the utilization of waste materials in MFC production has been limited. Although converting waste resources to usable materials for electrode manufacture takes a lengthy time, this process surpasses commercial materials on a number of characteristics. Cheng et al. (2018) examined for improving the energy production and wastewater treatment efficiency of MFCs by the use of waste material in the fabrication of a reduced graphene oxide (rGO) composite anode. The effective synthesis of green rGO was accomplished utilizing a byproduct of dried eucalyptus leaves. Consequently, by stacking the materials and then coating them as layer-by-layer assemblage with gold nanoparticles, it was possible to make nanocomposites of rGO and gold nanoparticles. With this modified electrode, current densities of 69.4 A/m3 and power densities of 33.7 W/m3 were achieved. The work by Cheng et al. (2018) describes that nanocomposite electrode has a rougher surface, which makes it easier for bacteria to colonise. As a consequence, a large number of electroactive sites are formed in the composite, allowing electron transfer from the biomaterial to the anodic site. Singh et al. (2018) has shown that carbon nanoparticles made from candle soot were used to construct an MFC electrode that was quite efficient. The carbon nanoparticles were able to operate as electrodes directly because the candle soot was created on the surface of a stainless-steel disc. When tested for physicochemical, electrical, and chemical characteristics, the electrode materials exhibited exceptional electrochemical and mechanical stability. Carbon nanoparticle electrodes fabricated from candle soot are recyclable, economical, scalable, and reliable. Similarly, Bose et al. (2019) employed biomass to prepare activated carbon cathode conversion in MFCs for bioenergy generation. This was a one-of-a-kind system of electricity generation and water purification that had no adverse effect on the environment. Typically, platinum is the most popular material to be employed at the cathode as a catalyst for oxygen reduction. Additionally, the researchers examined the stability, use, and cost of activated carbon derived from sugarcane refuse. This refuse was carbonized at three distinct temperatures (300, 400, and 500 °C) over a 60 min period. The current density was determined to be 0.40 mA/m2 and the power density to be 110.58 mW/m2 . Additionally, electrodes manufactured from a variety of biomass resources may be utilized to treat environmental contaminants while delivering energy. To our knowledge, very few studies have been published on biomass-based anodes in MFCs. Recycling waste biomass materials is an economically feasible approach of boosting the efficiency of MFCs without incurring large expenses. Graphene and its derivatives are the most widely used and promising electrode materials, and they can be easily synthesized via a variety of methods, including the scotch tape method, epitaxial growth, CO reduction, chemical vapour deposition, electrochemical synthesis, confined self-assembly,

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and exfoliation. Hummer’s methodology is critical as well as most popular because of its enormous benefits over alternative ways. For instance, it is an ecologically friendly technology; it produces no hazardous gases during the preparation process; the result has a well-organized structure; and a bigger quantity of product is delivered. Hung et al. (2019) shown how to boost the power density of MFCs by utilizing a waste coffee anode. The authors minimized environmental waste by turning waste materials into valuable carbonized materials and used them as anodes in MFCs. The power density reached was 3800 mW/m2 , much more than that of typical materials. Our ecosystem contains a variety of waste materials, each of which poses substantial problems. As a result, repurposing biomass waste products is a cost-effective strategy. On the other hand, Hummer’s technique involves carbonizing a range of waste products (biomass, home, and commercial) at a temperature of 1050 °C under the argon gas inert atmosphere to create fine carbonized powder materials. Graphene oxide is formed by oxidising the resultant graphitic powder with KMnO4 /H2 O2 . Polymeric binders like as nafion, polyethyleneimine, and polylactic acid may be used to further functionalize graphene oxide to create an anode electrode. Graphene oxide may be utilised either as an anode or as a cathode, however the anode is the preferred use for this material. Material that has been manufactured in this manner may be more cost-effective while also improving the performance of the material. Utilizing low-cost synthetic composite materials in conjunction with metal oxides such as TiO2 /GO, ZnO/GO, and CuO/GO is an efficient way to address a range of contemporary challenges. The systematic synthesis paths of the anode electrodes for MFCs are shown in Fig. 2.

3.1.3

Metal and Metal Oxide-Based Materials

Corrosion substantially limits the use of metal and metal oxides as anode electrodes, notably for MFC anodes. Compared to carbon-based materials, metals are more conductive because of their preference for efficient electron flow. However, owing to the procedure’s necessity for non-corrosiveness, not all metals are suitable for electrode fabrication. Bacterial adhesion is inhibited by some metals as well. Non-corrosive stainless steel, for example, has a lower power density than graphene and graphite, which are both carbon-based materials. Metals’ flat surfaces make them unsuited for bacterial adherence in the broadest sense. It is not possible to get higher power densities with non-corrosive materials like stainless steel. In the anode chamber, the power density of stainless steel was determined to be 23 mW/m2 . An anode made of stainless steel outperformed a graphite electrode. Nevertheless, certain metals, such as platinum, gold, and silver, perform substantially better when used as anode materials. Due to their expensive cost and low bacterial adhesion, noble metal-based anode electrodes are restricted in their use in MFCs. Catalysts such as titanium and platinum are routinely used to improve the performance of anode electrodes. However, the high cost and limitations of pure metal-based anodes in MFCs

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make large-scale commercialization difficult. Metal oxide nanoparticles and nonprecious metals exhibit catalytic activity equal to that of precious metals, lowering resistance and boosting bacterial adhesion to surfaces, respectively. Nanometallic particles may also help to reduce the toxicity of bacteria when used in pharmaceutical formulations. These challenges can be overcome by coating metal/metal oxide nanoparticles (ZnO, Ag, TiO2 , and others) with other materials such as carbon-based or conductive polymers.

3.1.4

Conductive Polymer-Based Composite Material

Several materials including conductive polymers such as polyaniline, polypyrrole, and polythiophene, among others, can be employed as anode materials attributed to the high values of their electrical conductivity. When coupled with other carbonbased materials, conductive polymers perform brilliantly. For instance, after being treated with polyaniline, carbon fibre generated more power than untreated materials. As anodes, graphite felt and polyaniline composites exhibited a power density of 2.9 W/m3 , which was much more than the value for a standard anode. Additionally, this combination resulted in a significantly increased surface area for bacterial growth. When covered with carbon paper, polypyrrole, another conductive polymer, achieved a 452 mW/m2 power density. Polypyrrole is capable of penetrating bacterial cell membranes and transferring electrons via the metabolic pathway, according to the literature. Electrodes may be considerably improved by using composites of conductive polymers and other materials, such as carbon-based polymers, metals, and their derivatives, as a consequence. Dumitru et al. (2018), for example, studied polypyrrole and polyaniline in the presence of carbon nanotubes (CNTs) in the form of a nanocomposite anode. The power density of the carbon nanotube/polyaniline (202.3 mW/m2 ) and carbon nanotube/polypyrrole (167.8 mW/m2 ) nanocomposites was greater than that of the carbon nanotubes in their natural state (145.2 mW/m2 ). Due to their synergistic effect, carbon nanotubes and conducting polymer nanocomposites exhibit acceptable performance, particularly in electrochemical applications. Various metals, such as Ag, Zn, TiO2 , and Cu, as well as a ZnO composite comprising conductive polymers (especially polyaniline and polycarbazole), may provide a good chance to enhance the performance of MFCs. Regrettably, inadequate effort has been devoted to the development of polymeric composite electrodes for MFCs.

3.2 Cathode Materials The electrode material used for preparation of cathode for a MFC is equally important as that of the anodic electrode. Carbon based materials are most popular for

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cathodes. Majorly, the cathodic materials are required to catalyze the oxygen reduction reaction occurring in the cathodic compartment. Though, many materials are suitable as cathodic materials but the search for better materials is still going on. An aqueous air cathode with or without catalyst is the most frequent cathode type. The catalyst is the main difference between these two systems. Platinum and titanium are the two most often used catalysts for performance improvement. Additionally, air cathodes are constantly exposed to oxygen at the moment they are inserted into a device. Because of its improved electrode design and lack of aeration, this set-up has drawn a lot of attention. The power output of an air cathode with MFCs may be greatly increased. Electrodes in aqueous air cathodes are made from conductive materials such as platinum meshes, carbon felt, carbon fibre, and carbon cloth. With the help of this layer, the electrodes are connected to the low-oxygen zone, which is already present in the water. Carbon fabric is thought to be the best conductor to utilise as an air cathode. Binder chemicals are used to form air cathodes in order to connect the electrodes’ catalysts (platinum, titanium, or copper), Poly(tetrafluoroethylene) and perfluorosulfonic acid are the most attractive binders (nafion). Zhang et al. (2009) studied the cathode electrode performance of activated carbon and carbon cloth in the presence of poly(tetrafluoroethylene). When platinum is utilized as a catalyst, activated carbon has a greater power density than carbon cloth (1220 mW/m2 vs. 1060 mW/m2 , respectively). As a result, activated carbon may be a feasible choice for cathode material preparation. Zhao et al. (2019) created a cathode by combining a platinum catalyst with carbon fabric. According to statistics, this catalyst has a power efficiency of 1.2 W/m3 . Cu when employed, is a more selective catalyst than Pt at lower temperatures, owing to Cu’s superior selectivity retention. Otherwise, Pt is a superior and more wellestablished catalyst at room temperature. Wang et al. (2018) discovered carbon paper with a power efficiency of 457.8 15.2 mW/m2 in the presence of a Pt catalyst. As a consequence, the anode and cathode materials can act as catalysts for oxygen reduction. Due to their low overpotential, platinum and gold are considered appropriate catalysts; nevertheless, their expensive cost precludes their utilization. Because of their cheap cost, high degree of stability, and lack of toxicity to microbiological life inside cells, primary transition metals are an ideal solution to this issue. There are a number of composite materials that may be used, including tungsten carbide and molybdenum. While nanocomposites (such as palladium nanoparticles and nickel nanomaterials) are less costly, their potential to improve MFC efficiency is significant (for example, palladium nanoparticles and Ni nanomaterials). Nanoparticles have a larger surface area, improved mechanical and thermal stability, and increased electrochemical activity when compared to conventional materials. Oxygen reduction has been sped up recently by the addition of materials to electrodes. Research into new materials that might improve electrodes, especially anodes, is critical, according to the literature. It is possible to transform the MFC industry by combining highquality anode materials like graphene and its derivatives with metal oxide in the form of a composite material. The most effective composites are GO/ZnO, GO/TiO2 , and GO/Ag, all of which have a considerable effect on energy generation. Both

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graphene oxide and metal oxide may be synthesized affordably from waste materials, contributing to the anode’s cost effectiveness.

4 Effects of Anodes in MFC Anodes in an MFC have the critical function of generating electrons. Additionally, it performs various additional functions such as decreasing toxicity through oxidizing materials during operation of MFC. The anodic surfaces hosts the microbes to grow and sustain while providing us with output in the terms of electric potential and treatment of wastewater.

4.1 Effect of Anode on Pollutants Removal MFCs are expected to be highly effective in bioremediating wastewater. There are various conventional wastewater treatment methods, but all are prohibitively expensive, difficult to operate, and environmentally dangerous. Animal waste, vegetable and food processing waste, slaughterhouse effluent, maize stover waste, dairy effluent, surgical cotton industry effluent, cassava mill effluent, and petrochemical industrial wastewater may be bioremediated using MFC technology. Using exoelectrogens in the anode chamber, organic pollutants are removed from the water. Instead of travelling outside the cell, photons are delivered to the cathode or via membrane sources. The electrodes’ efficacy is critical to this method’s success. Using electrodes, bacteria are able to breathe and thrive because they transfer electrons and protons from anode to cathode. Zhang et al. (2012) lowered Cr (VI) and V in double chamber microbial fuel cells by employing vanadium-based wastewater as an electron acceptor (V). Two of the most harmful and plentiful metals found in vanadium wastewater are Cr (VI) and V (V). After ten days of operation, with a power density of 970.2 20, the average reduction efficiency of Cr (VI) was 75.41% and V (V) reduction efficiency was 67.93%. Qiu et al. (2017) demonstrated a 60% clearance rate for Dysgonomonas and Klebsiella when a biocathode was used in MFC (biocatalyst). The power density reached 529 12 mW/m2 after seven days of operation. Carbon fibre felt was used for both the anode and cathode electrodes (40 40 10 mm3 ) (100). Jiang et al. (2013) evaluated the energy generation capability of MFCs used to treat oil sand tailings. After 35 days with a 1200 resistance loaded, the maximum voltage measured was 0.726 V. It was roughly 392 mW/m2 after 70 days. MFCs were successfully utilized for removal of heavy metals from oil sands wastewater with an efficiency of 66.9 % (Cu), 4.9 % (Cr), and 32.5% (Pb). The removal efficiency was low due to the anode and cathode being carbon cloth. Although the carbon felt outperformed the carbon cloth in terms of removal efficacy, the anode’s quality and accessible surface area to bacteria were critical considerations. Additionally, bacteria require a surface area with an active

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surface area to degrade pollutants. As a result, Habibul et al. (2016) explored the electrokinetic bioremediation of heavy metals from polluted soils using graphite anodes. After 108 and 143 days of operation, respectively, removal efficiencies of 44.1 and 31% were achieved. The permeability of Cd is 7.5 mW/cm2 , while that of Pb is 3.6 mW/cm2 . Due to the electrodes’ lower efficiency for Pb and Cd, bacteria generated electricity at a slow rate. However, research on the decomposition of toxic metals such as lead, mercury, and cadmium is limited. Bacteria require healthy anodes to breakdown harmful metals in water, and the authors used MFC anodes to decolorize ecologically toxic organic dyes. Fang et al. (2015) investigated the possibility of MFCs employing an activated carbon anode and a stainless steel mesh cathode to process and eliminate azo dye from CAS. The power density was 0.852 W/m3 and the rate of decolorization was 95.6%. The activated carbon acted as an anode, speeding up the process of decolorization. Kawale et al. (2017) used an unpolished graphitic rod to decolorize anaerobic sludge. Ecolorization was 73.4% in double chamber MFCs. The electrode had an influence on the degree of decolorization and the amount of energy produced. Nonetheless, several studies have been conducted on the removal of organic pollutants utilising MFCs equipped with a variety of anodes. Kabutey et al. achieved a 28.2% removal efficiency using carbon fibre brushes for the process for both the anodic and cathodic chambers in a macrophyte cathode sediment microbial fuel cell. This is because the microorganisms formed, Euryarchaeota and Proteobacteria, are acidic, necessitating the use of a non-corrosive electrode. According to Marks et al. (2018), MFCs efficiently removed 22% of nitrate from anaerobic sludge. Anode and cathode electrodes in this experiment were made of graphite plates. Various anode materials are utilized in a variety of configurations due to a number of factors affecting MFC performance, according to the authors. Electrons are sent from the bacterium to the cathode through the external circuit via anodes to help bacteria in respiration. On the other hand, the use of a high-quality anode is projected to boost performance and reduce environmental impact. Long-term stability issues may be solved using this chemical. Composite composites may benefit from the addition of high-surface-area materials like graphene and metal/metal oxides to improve the anode’s ability to resist metal corrosion. For wastewater treatment to be successful, the anode must meet functional as well as desired characteristics.

4.2 Anode as Energy Production Factor MFCs have ushered in a new age of pollution control. MFCs utilize microbes as exoelectrogens generate energy from organic waste. The energy output of both the anode or cathode electrode has been enhanced from less than 1–4 W/m2 as a result of design advances, single chamber MFCs, 3D electrode materials, and enhanced and highly valued anodes. Initially, several materials and operating factors were adjusted, making it impossible to determine which operational characteristics improved the current generation over previous generations. Due to the evolution of the system, further attention is necessary to increase power generation. The electrode is critical

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in the energy generation process. The quantity of energy generated grows in direct proportion to the electrode’s quality and conductivity. Although the power output was low (457.8 15.2 mW/m2 ), Wang et al. (2017) demonstrated high efficiency of carbon felt employed as an anode in conjunction with platinum as the catalyst. Similarly, Zhang et al. (2012) exhibited the integration of adsorption to recover chromium from anaerobic digestion sludge, achieving a current power of 343 mV. The potential value mentioned was achieved with the use of an MFC system equipped with a carbon felt anode. Liu et al. (2019) demonstrated a 1092 mW/m2 energy production using synthetic liquids, carbon brushes, and fabric as anode and cathode electrodes. Santoro et al. (2017) sought to improve the material by incorporating graphite brushes with platinum catalyst SMFCs as anodes. The maximum power density of 1280 mW/m2 was achieved utilizing natural wastewater as an inoculum source. The electrodes’ efficacy has an influence on the amount of energy generated; graphitebased materials have a greater surface area and conductivity than carbon felt. As a result, graphite-based goods beat carbon felt by a factor of three. Nguyen et al. (2019) improved the anode’s material quality. Carbon nanotube composite activated carbon paper and biocatalysts were employed to generate electricity. Energy generation was 3.9 W/cm2 in a two-chamber MFC. Zhang et al. (2011) recently proved that carbonaceous allotrope graphene oxide may be used as an anode in MFCs. In electron transport, graphene oxide outperforms other carbon-based materials. Graphene is the most promising MFC electrode material. On the other hand, modern research makes anodes from natural resources since they are less costly and function better than commercial versions. Yang et al. (2019) has proven that banana peels and subaqueous wetland sediments could be utilized as anaerobic sludge may generate direct electricity (inoculum). It was measured and reported that 78.2 mA/m2 for banana peel and 91.3 mA/m2 were the values of current densities for subaqueous wetland sediments using carbon felt electrodes in both MFC chambers. Thus, natural materials employed as anodes represent a realistic option for overcoming current challenges and generating high-quality anodes such as graphene oxide and its metal oxide composites. Composites of graphene oxide and metal oxides may increase anode performance. MFCs frequently utilise anodes composed of ZnO/GO, TiO2 /GO, and CuO/GO.

5 Opportunities and Challenges MFCs are relatively novel in terms of advent of technology. Following this, there are various components and techniques having further scope of research and development. The final aim of all the development in the MFC is towards increasing the efficiency of the technology to make it commercially viable. There are various challenges associated with the MFC technology, a few of which are discussed as below:

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1. The economic stability and viability of MFC as a system is dependent on the anode materials utilized. As a result, cost reduction of anode materials is critical for the long-term viability of MFC installations. To overcome this limitation, better anodic materials such as carbonizing waste materials to create anode rods, brushes, and plates needs to be developed. In waste materials, carbon-based compounds abound. Another approach is to develop metal–metal composites and increase their efficiency through the use of polymers. Material selection is essential, as the majority of MFC researchers tend to be unfamiliar with highly conductive or composite materials. 2. The binder material is crucial in the production of anodes because it enables the material to be moulded into the correct form. The binder increases cohesion and stability of the material by acting as a binding agent. It is an urgent requirement to develop better, more active and cost-effective anode electrodes. Additionally, anode binders are the materials are requires development. 3. Size and form of anodes are critical factors in the manufacturing process. Anode– cathode electron transport in MFCs is facilitated by the separation of electrodes and the presence of a high surface area. 4. The MFC’s power generation and wastewater bioremediation were both improved as a result of the anode electrode’s modification. However, the necessary methods and criteria are still a puzzle. Researchers need to come up with a faster way to respond. 5. The anode’s long-term industrial stability is another issue. Aside from energy production, there are presently no established rules or debates about the stability of electrodes over time. Stability is a major impediment to MFC adoption in the industrial sector. This means that researchers should concentrate on developing an electrode manufacturing process that takes into consideration the stability of the anodic material in their designs. The anode electrodes can be strengthened by coating them with a stable binder such as nafion or polysulfones.

6 Conclusion Anodes in MFCs have been discussed in this article. Investigations have been conducted into the anode materials for MFCs: carbon, metal/metal oxide, polymers with conductivity in them, and composites. Bacterial adhesion and biofilm growth are significant factors in anode electrode development. To increase the density of biofilms, the surface area of anode materials has been increased. Numerous materials are proposed for use as anodes, as mentioned in this overview. However, a large gap still exists in the development of anode materials. MFC anodes may be composed of a variety of materials, including 3D graphene and metallic composites. MFC anodes must be highly stable in wastewater over time. If these characteristics remain constant, they add to the industrial value of an anode. Anode materials must have wide pores to minimize clogging during wastewater bioremediation. MFCs are unfeasible due to uncertain material costs and surface modification techniques. Businesses should be

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exposed to low-cost materials and procedures for making MFC electrodes from metal or polymeric composites. Upscaling resourceful anodes should be demonstrated in the future. It is vital to develop an anode/membrane assembly that operates successfully as an electrode assembly based on a membrane. As a result, future research should focus on the fabrication of anode electrodes from waste material and the optimization of existing difficulties.

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Microbiotechnology-Based Solutions for Removal and Valorization of Waste in Pulp and Paper Industry Nada Verdel, Mija Sežun, Tomaž Rijavec, Maja Zugan, Dmitrii Deev, Iaroslav Rybkin, and Aleš Lapanje

Abstract The pulp and paper (P&P) industry is an important industrial sector and the third largest producer of industrial wastewaters in the world. Although the industry has attempted to reduce water consumption by completely enclosing their processes, the water (whitewater) cycles and release of pollutants into the environment, this current and past water treatment solutions have failed it to reach their goals. Bioaugmentation of systems for wastewater treatment is an evolving microbiologically-based strategy with a high potential for industrial use. Use of specific microorganisms can help remove even the most resistant organic additives and transform large amounts of the readily available waste compounds, but has a minimal impact on the environment and the reduction of treatment costs. The classical state-of-the-art microbiological treatment approaches and their drawbacks are discussed and the advanced treatment solutions based on cell aggregation and immobilization to engineer artificial microbial communities capable of degrading or transforming a wide repertoire of wastewater components are presented. We describe how the natural properties of microbiological agents can be exploited and present several possibilities showing how microbes can degrade persistent pollutants or transform natural polymers like cellulose, hemicellulose and lignin into novel added-value compounds. Keywords Bioaugmentation · Environmental microbiology · Pulp and paper industry · Biorafination of lignin · Biofilm · Phenol formaldehyde resin

N. Verdel (B) Fenolit d.d., Breg Pri Borovnici 22 a, 1353 Borovnica, Slovenia e-mail: [email protected] M. Sežun Pulp and Paper Institute, Bogiši´ceva Ulica 8, 1000 Ljubljana, Slovenia T. Rijavec · M. Zugan · D. Deev · I. Rybkin · A. Lapanje Group for Colloid Biology, Department of Environmental Sciences, Jožef Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_21

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1 Introduction The global demand for paper is increasing at an annual rate of 5–6%, and in 2021 reached an annual global production of paper and cardboard of 400 million metric tons (Statista 2021). The world’s largest paper company by revenue generated more than $20 billion in revenue in 2020 (Statista 2021). Paper is a renewable resource with one of the best recycling rates of any waste material, but it generates over 20% of all municipal solid waste globally and about half of all municipal solid waste recycled in the United States (Statista 2021). The pulp and paper (P&P) industry accounts for more than 40% of wastewater production globally and is the world’s third largest producer of industrial wastewater (Gopal et al. 2019). The contaminants in the wastewater of the P&P industry are closely related, quantitatively and qualitatively, to the source of cellulose that is used for paper production and the papermaking additives that are used in the paper production process. With respect to the source of cellulose, delignified wood biomass or recycled fibers from paper waste are used in the integrated paper industry, while purchased virgin cellulose, produced as part of a separate delignification production line, is the raw material in the non-integrated wood-free paper industry. The trends in the P&P industry are directed towards reduced water consumption and have seen closing of water cycles, utilizing cheaper sources of cellulose and solving the problems caused by different types of waste. To reduce the effects of the increasing price of virgin cellulose and to ensure the inflow of cellulose fibers, increasing the use of recycled paper and cardboard is being complemented by the delignification of lignocellulosic biomass included in waste from the lumber industry and by green clippings, the waste generated in agronomy and by harvesting of certain plants like invasive plant species. To address the issues associated with the different types of waste it is necessary to implement novel biorefinery strategies, which have a high potential to introduce new end products other than paper or its derivatives, into paper production. We already have some pipelines that transform the waste generated in the P&P industry into by-products, such as different uses of sludges, but there is also waste and wastewater that we do not know how to transform, reuse or detoxify. Some compounds, such as lignin waste and cellulose rich sludge that are not environmentally problematic, are ideal for biorefinery treatment. These can produce novel compounds with added value in the food industry, medicine and biotechnology, while others that are toxic are not removed by conventional wastewater treatment or other treatments at our disposal. These materials include toxic recalcitrant compounds in process water and need novel approaches for their complete degradation. In order to understand the various problems associated with waste treatment in the P&P industry, such as transformation of lignin originating from the delignification process, degradation of recalcitrant compounds in wastewater or biotransformation of sludges, we first need to understand the origin and chemical characteristics of each particular waste component (see Sect. 2). To see how these problems are currently being addressed, we must examine the advantages and disadvantages of classical treatment approaches. The microbiological processes are mainly uncontrolled and

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have been developed stochastically over a longer period of time. The strains that are directly involved in the degradation of recalcitrant pollutants are not necessarily in the majority. According to the Pareto principle, only 20% of the strains are performing the majority of the degradation (Cooper et al. 2019), and it is reasonable to assume that we simply need to artificially increase their quantity and persistence in the system in order to accelerate the process (see Sect. 3). With the understanding that the other 80% of the waste community is also important for the formation of specific niches and the occurrence of complex cross-feeding interactions, we need to develop advanced bio-based treatment approaches in which artificial microbial consortia are engineered by cell aggregation and immobilization onto carrier-matrix systems, keeping in mind the complex functioning of the community (see Sect. 4). Several advanced treatment methods are already at different stages of development. These include cavitation-based pretreatment of sludge aiming to improve nutrient availability to microbes (Sezun et al. 2019), biotreatment of whitewater (Verdel et al. 2021) and lignin biotransformation into novel active compounds (Venkatesagowda and Dekker 2020) (see Sect. 5).

2 Generation of Waste in the P&P Industry It is estimated that the P&P industry produces about ten billion tons of highly pigmented and hazardous wastewater effluent each year, with the majority of the dissolved content being high molecular weight lignin (Molina-Sanchez et al. 2018). P&P production wastes are otherwise diverse and include different polymers, inks and dyes, toxic metals, deinking chemicals and recalcitrant compounds. The conventional integrated P&P mill controls three major processes: generation of cellulose pulp, production of paper and wastewater treatment, the first two being the main sources of water pollution (Fig. 1).

2.1 Industrial Lignin and the Delignification Process The P&P industry is a manufacturer of pulp, paper, and other cellulose-based products obtained from lignocellulosic biomass. Lignocellulosic compounds are important components of plant biomass (15–40% of dried biomass). Lignin interconnects the cellulose fibers in the wood and is a prominent biopolymer found in cell walls and tissues of most terrestrial plants. About 20% of lignocellulose by weight is composed of lignin (Kumar and Sharma 2017), depending on the type of lignocellulosic biomass and the region where it was obtained. Lignin structure is heterogeneous and varies from plant to plant. It is composed of three main subunits derived from three cinnamyl alcohol precursors, which makes lignin water insoluble, chemically inert and, compared to other naturally occurring polymers, very resistant to degradation (Lu and Ralph 2010). The guaiacyl-type (G) lignin is found in hard woods; the

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Fig. 1 Generation of waste in the P&P industry. Three types of waste (brown), including black liquor, paper production wastewater and deinking wastewater, are generated by the different P&P processes depending on the raw input material, such as plant biomass (yellow) or recycled fibers (green) and papermaking additives. Black liquor produced by the delignification process contains high levels of lignin, while deinking wastewater contains inks, adhesives and remains of cellulose. The whitewater used in the papermaking process is subject to a buildup of recalcitrant and readily biodegradable additives due to the closing of the water cycles, which influences the characteristics of process wastewater. The conventional wastewater treatment plants, producing different types of sludges and a cleaner water effluent are not exploiting the full potential of microbes and are leaving unresolved several problems of waste degradation or transformation. The advanced treatment approaches (blue) try to address these issues resulting in improved water treatment and new biovalorization opportunities

guaiacyl-syringyl-type (GS) is found in soft woods; while in grasses p-hydroxyphenyl subunits also occur in addition to GS. These subunits can be the basis for a variety of new compounds generated after biotransformation. In the P&P industry, lignin is a by-product of the delignification process, also known as pulping. Conventional pulping can be divided into two major classes: chemical- and solvent-based, in which the two chemical processes, Kraft (alkaline) and sulfite (acidic) pulping, are industrially the most widely used (Shah 2020). Additionally, a physical method, steam explosion, has been described as a method for the separation of biopolymers and was shown to be efficient in the pretreatment of biomass prior to solvent treatment, distillation or biological treatment (Pielhop et al. 2016). The physical and chemical properties of industrially available lignin are strongly influenced by the industrial delignification process that is used for its extraction from the lignocellulosic biomass. The alkaline Kraft lignin, a byproduct, has a higher percentage of phenolic hydroxyl groups, lower content of sulfur in the form of thiol groups and a lower molecular weight in comparison to the acidic lignosulfonates, which are sulfonic acids and therefore water-soluble. The organosolv, i.e., solvent-based, pulping techniques are, on the other hand, still either in the early stages of development or are being employed in small-scale commercial production. Organosolv lignin is sulfur-free and has a homogeneous structure due to its low polydispersity, a higher availability of free phenolic hydroxyl groups, and low molar mass (Vásquez-Garay et al. 2021; Shah 2021). A total of 80% of cellulosic pulp worldwide is produced by the Kraft delignification process. During the Kraft pulping wood chips are digested in “white liquor”,

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an aqueous sodium sulfide (Na2 S) and sodium hydroxide (NaOH) solution, at high temperature and pressure, producing cellulose pulp and suspended lignocellulosic waste. After several other mechanical processing steps the pulp is removed from the cooking fluid in a washing step to produce “black liquor”. A total of 50% of the fiber raw material remains dissolved in the black liquor and after the precipitation of Kraft lignin from the liquid, black liquor is composed of inorganic chemicals used for cooking, lignin residues, hemicellulose, cellulose breakdown products, resin acids, starch and organic acids. Black liquor finally consists of around 30–35% inorganic and 65–70% organic substances. Black liquor is 10–15% of total P&P industry wastewater volume, but in terms of the chemical oxygen demand (COD) it accounts for about 90% of all compounds to be degraded. Many of these compounds can be converted into novel compounds and can also be used to replace various fuels (Al-Kaabi et al. 2018).

2.2 Deinking Process To reduce the overuse of wood, recycling of paper waste has also been implemented in the P&P industry. Cellulose fibers are extracted during the de-inking process, in which ink and adhesives are removed from the waste paper. Using a mixture of water and solvents, oils and particles of ink are broken down, and this is followed by a flotation technique that removes larger particles and adhesives by injecting air, surfactants and cleaning chemicals into the recycled pulp suspension. The released inks, adhesives and other impurities float to the surface of the liquid and are removed as waste. The remaining wastewater contains the leftover printing inks and adhesives, convertible chlorinated organic compounds and large amounts of chemicals that were used in the deinking process. It is treated in wastewater treatment plants (WTP) to further remove all particulate matter in the form of deinking sludge, which ultimately will contain small amounts of toxic metals such as zinc, lead, chromium, mercury and nickel, process chemicals, cellulose fibers and fiber fines and a large portion of inorganic fillers, such as kaolin clays, talc and calcium carbonate (Kloekke 2018).

2.3 Papermaking Process The recycled or delignified cellulosic fibers are used as the main raw material in the papermaking process along with fresh process water, aka whitewater and all the papermaking additives. A ton of paper requires roughly 60 m3 of water, and produces at least 50 m3 of whitewater effluent (Molina-Sanchez et al. 2018), which is composed of azo dyes used for paper coloring, sizing agents (e.g., artificial or natural resin acids and alkyl ketene dimers—AKD), stilbene-based whiteners, fillers and antimicrobials, as well as leftovers of cellulose and starch, mainly cationic starch (Kamali and Khodaparast 2015).

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These organic waste compounds can be divided into three large groups, based on how microorganisms are able to degrade them: (a) recalcitrant compounds (RC) that mineralize slowly and persist in the environment causing toxic effects in humans and animals, (b) natural polymeric substances (NPS) that are insoluble in water and must be extracellularly degraded to oligomers and monomers prior to their intracellular mineralisation, and (c) readily biodegradable compounds (RBC), such as oligo- and monosaccharides, which are soluble, need no special uptake systems and whose degradation is chemically undemanding. In order to optimize environmental safety and water reuse, priority should be given to the wastewater treatment processes, particularly in closed water cycles. It is most important that the degradation of the RCs is facilitated in order to prevent their accumulation in whitewater, decreasing the quality of the final product or causing malfunctions of the papermaking devices, or to prevent their later release into the environment. The NPSs such as starch and cellulose induce the cross feeding process within the biofilms formed within paper production lines and interfering with paper quality. The RBCs are the most simple to remove, but are also crucial for the development of complex communities as they are also involved in the cross feeding processes. Depending on the different requirements facing a treated water, different levels of purity of the treated water are achieved. They either have to be in compliance with legal standards for safe disposal or released back into the environment or for reuse in the papermaking process.

3 Approaches to Classical Microbiological Valorization 3.1 Wastewater Treatment Plants In the wastewater treatment plants, reduction of chemical oxygen demand (COD) index is usually treated as a measure of the effectiveness of the wastewater treatment process. In most cases, prior to the main wastewater treatment processes, some kind of mechanical or chemical pre-treatment is performed, either by sedimentation, filtration, adsorption or flotation. In the pre-treatment processes suspended particles are mostly removed, while dissolved particles remain in the water cycle. In order to remove the dissolved particles, various treatment strategies can be employed and have different rates of efficiency. For the reduction of color and other pollutants in paper manufacturing effluent, coagulation, chemical oxidation, ozonation, and biological treatments are applied. Adsorption, ozonation, and membrane filtration procedures effectively decrease chlorinated phenolic chemicals and adsorbable organic halides (AOX), while combined

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anaerobic and aerobic biological treatment processes effectively eliminate soluble biodegradable organic contaminants (Singh and Tripathi 2020). Biological treatment is a practical solution that uses microorganisms such as fungi, bacteria, algae or enzymes alone and it can be combined with other treatment technologies such as, for example, membrane filtration. For P&P industry’s wastewater treatment the biological method has the following advantages when compared to other treatment methods: – According to the characteristics of the paper mill wastewater, such as the large BOD7 /COD coefficient, it is the most suitable technology. – Low operating costs are essential, since only low-value bio and/or chemical augmentation is needed and, in the case of aerobic treatment, expensive construction and operation costs, particularly the energy costs of aeration systems can be incurred. – The process is environmentally friendly, since the final products are expected to be completely degraded to CO2 and H2 O. In many cases however this cannot be achieved, and consequently effluents must be monitored. A total of 60–75% of all biological wastewater treatment plants in the P&P sector employ activated sludge systems and activated sludge is the most common secondary treatment technology, even in new plants. The advantages of aerated activated sludge systems include high removal efficiency, good control of the process, requirement for less surface area, and microorganisms that are acclimated to receiving wastewater (Hynninen 2008). However, all the aforementioned conventional treatment processes have some drawbacks, such as a high rate of sludge creation requiring the addition of nutrients, high energy consumption, the release of secondary pollutants and low biodegradation of recalcitrant compounds, or membrane clogging (Priyadarshinee et al. 2016). Currently, other biobased technologies are emerging and may be implemented in the future. These include microbial fuel cells (MFC), biological-electrical systems (BES) and biomimetic flat cells (BFC) that can produce electricity from easily bioaccessible organic compounds or utilize electric energy in order to facilitate degradation of recalcitrant wastes. These methods are well established in laboratory conditions but currently however, have not been considered for implementation on a large scale in the P&P industry.

3.2 Treatment, Disposal and Valorization of Sludge Paper mill sludge (PMS) is a solid waste which is generated as a side product in wastewater treatment plants, either as primary sludge after mechanical and chemical treatments in “settlers” to remove solid particulate matter, or as secondary sludge after various biological treatments. Solid waste of the P&P production, which includes de-inking, and primary and secondary sludge, accounts for over 10% of total paper production (Likon and Trebse 2012). The chemical composition of PMS depends on

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the P&P production process, but basically the three types differ as follows: (i) primary sludge contains mostly inorganic additives, (ii) de-inking sludge additionally contains de-inking chemicals, ink and adhesives residues, while (iii) secondary sludge mostly contains undegraded RCs, partially biotransformed organic compounds and microbial cell biomass (Frias et al. 2015). Primary sludge has a water content of ~40–50%, 30% inorganic matter, like kaolin, calcium carbonate, toxic metals including zinc, lead, chromium, mercury and nickel, and 20–30% organic matter, mostly cellulose (Joshi et al. 2017). Compared to the primary sludge, secondary sludge has more inherent moisture, due to the higher amount of bound water in the organic matter. PMSs are a major economic and environmental problem in the paper industry, as their disposal in landfills is problematic due to the presence of toxic compounds and high organic matter content (Shi et al. 2017). Therefore, alternative approaches for its assessment have been studied. PMSs can also have added value, if they are used as a partial energy source. The rising cost of fossil fuels, as well as the associated environmental challenges has recently inspired much interest in examination of alternative fuel sources. Paper mill sludge, particularly secondary sludge, is an appropriate substrate for biogas production, at least for semi-commercial uses (Pervaiz and Sain 2015). PMS is recognized as a viable resource for conversion by various processes of resource recovery into lower-value products, such as sludge-based adsorbents, bioflocculants, construction materials, composts and as activated carbon. In the study by Cavdar et al. (2017) the use of secondary sludge in the production of wood cement boards was investigated. They found that raw material use and energy efficiency were improved in a sustainable manner. Chun et al. (2004) found that incorporation of PMS from de-inked pulping into biodegradable thermoplastic composites can be used in civil engineering. Accordingly, it has much potential for finding further value-added uses and recycling enormous amounts of PMS. Another similar solution for PMS reuse is utilization of paper sludge as an organic filler in wood adhesives and a reinforcement filler (Geng et al. 2007). However, the existing solutions are only temporary and in the long term do not solve degradation or removal of PMS-related toxic compounds. Research by Zambare and Christopher (2020) demonstrates an integrated strategy as part of a larger attempt to improve the development of lignocellulose bioprocessing technology to convert primary PMS into value-added goods such as fermentable sugars, biofuels, lipids and additive compounds with many applications in various industries. The key focus of their research was cost reduction of proposed technologies as a means of improving their economic feasibility for industrial applications. A recent study (Du et al. 2020) was directed at providing a sustainable approach to efficiently convert PMS to cellulose nanofibrils and cellulose nanopaper by microfluidization and hydrolysis with formic acid. Cellulose nanofibrils, a type of renewable and biodegradable nanomaterial, have gained the interest of both academia and business (Keplinger et al. 2019). Due to their unique nanostructure, they have a high aspect ratio and high specific surface area and thus excellent physicochemical and mechanical properties (Zhang et al. 2000). Great potential has also been reported for

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their application in the fields of reinforcing nanofillers (Bian et al. 2018), biomedicine (Luo et al. 2019), and optoelectronic devices (Hoeng et al. 2016).

3.3 Classical Bioaugmentation Based Approaches Bioaugmentation is currently seen as an appealing approach to lowering the pollutant load of contaminated water. This is a type of environmental remediation that utilizes selected microorganisms to catalyze the degradation or transformation of certain toxic compounds into simpler compounds. As a result, bioaugmentation was proposed as a good option for the total degradation of a variety of industrial effluents, such as wastewater from P&P mills (Yang et al. 2008). Bioaugmentation was found to have minimal impact on the environment, reduce treatment costs and help to remove even the most recalcitrant organic additives (Herrero and Stuckey 2015; Santisi et al. 2015). State-of-the-art studies on the bioaugmentation of the P&P industry’s effluents have mainly focused on the biodegradation of some recalcitrant compounds (see Table 1). We can categorize these approaches as (i) specifically directed toward particular compounds such as lignin (Yadav and Chandra 2015), resin acids (Yu and Mohn 2002) or (ii) general, in which the aim was simply to lower COD (Hailei et al. 2006). Additionally, the study of Tyagi and coworkers (2014) clearly demonstrates that large P&P mill effluents may be treated with native bacterial and fungal isolates. After 9 days of incubation with optimum shaking and stationary settings, the native microorganisms were found to utilize lignin as a carbon source and to lower the COD and BOD7 values. For suspended bioaugmented biomass, where different cells are not physically attached to one another, the nutrient sources or other solid supports (Machineni 2019), or multispecies cooperation of bacteria results in the formation of flocs, filaments or granules, which vary in cell composition, buoyancy and their ability to degrade various pollutants (Li and Pagilla 2017). Suspended cells were reported to have high throughput and efficiency for removal of RBCs and to be cost-effective in large-scale setups (Machineni 2019; Rout et al. 2021), but less information on the removal of RCs is available. However, suspended cells are (i) prone to washing out from the system, decreasing the augmented bacterial biomass and affecting the quality of the effluent, (ii) not shielded from grazing and (iii) not ensuring or maintaining the needed anaerobic conditions in several technological setups (Goli et al. 2019; Machineni 2019; Rout et al. 2021). Furthermore, high buoyancy of flocs makes removal of precipitated recalcitrant pollutants inaccessible to microbes (Li and Pagilla 2017). The classical bioaugmentation therefore needs several improvements to address these drawbacks. The development of advanced treatment solutions will provide the necessary answers.

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Table 1 Studies of bioaugmentation of P&P mill effluents for bioremediation of recalcitrant organic compounds Pollutant

Set-up and chemical/microbial analysis

Bioaugmented microorganism

Dehydro abietic acid (DhA)

Bioaugmentation of aerated Bacteria Zoogloea lagoon samples in flasks: gas resiniphila DhA-35 chromatography (GC) for DhA determination, total organic carbon content (TOC), competitive polymerase chain reaction (PCR) and the reverse transcriptase PCR assays for strain DhA-35 determination, total bacterial abundance by acridine orange direct count and identification of bacterial communities using ribosomal intergenic spacer length polymorphism (RIS-LP)

References Yu and Mohn (2002)

500-mL sequencing batch reactors (SBRs): GC for DhA determination, TOC, ribosomal intergenic spacer assays (RISA) for identification of microbial community structure and most probable-number (MPN) PCR assay to detect the number of bacteria

Bacteria Pseudomonas Yu and Mohn abietaniphila BKME-9 (2001) and Zoogloea resiniphila DhA-35

COD

SBR by aerobic granular systems: COD, BOD5 , integrality coefficient of granules, granulation rate determination and morphological studies of the sludge using bio-microscope

Bacteria Azotobacter sp. and white rot fungi Coriolus versicolor and Phanerochate chrysosporium

Hailei et al. (2006)

Lignin

SBR: UV–VIS spectroscopy for lignin determination, COD, BOD5 and microbial community analysis by denaturing gradient gel electrophoresis

Bacteria Gordonia strain JW8

Chen et al. (2012)

Aerated bench-top bioreactor with immobilized fungi on nylon mesh: color by UV–VIS spectroscopy (465 nm), lignin by modified nitrosation method, COD, BOD5 , chlorides and total phenols by standards, pH and conductivity

Fungi Merulius aureus syn., Phlebia sp., unidentified genus and Fusarium sambucinum Fuckel MTCC 3788

(Malaviya and Rathore 2007)

(continued)

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Table 1 (continued) Pollutant

Set-up and chemical/microbial analysis

Bioaugmented microorganism

References

Rayon grade pulp mill Bacteria Bacillus subtilis Yadav and wastewater in Erlenmeyer and Klebsiella Chandra (2015) flasks: COD, BOD5 , color pneumoniae determination using spectrophotometer at 465 nm, GC combined with mass spectrometry (GC–MS) and high-pressure liquid chromatography (HPLC) for lignin determination, 16S rRNA gene sequencing for bacterial identification, ligninolytic enzyme activity assays and toxicity assessment by seed germination tests

4 Advanced Treatment and Valorization Approaches 4.1 Pre-treatment of Bulk Waste Using Cavitation Cavitation, the formation of an empty within an object, is a mechanical pre-treatment method for the treatment of wastewater and solid waste like the PMS that is becoming increasingly popular (Petkovšek et al. 2015; Sezun et al. 2019). Due to the sound waves, microjets, radiated pressure pulses, highly intense shockwaves and high local temperature, which can reach ~1000 K, which are used during cavitation treatment, mechanical and chemical changes of the substrate occur. At the moment of final bubble compression, the vapor inside the formed bubble and the liquid around it enter into a supercritical state, which can be predetermined with models that examine microflow features in inter-bubble space and calculate local velocity, pressure fields, and dynamic effects (Pavlenko and Koshlak 2021). In this type of process, large molecules are converted into smaller ones making them more degradable and susceptible to microorganism degradation. Sezun et al. (2019) studied the hydrodynamic cavitation treatment of secondary paper mill sludge in order to transform and decrease the amount of the initial material. Laboratory and pilot scale tests using chemical and microbiological characterization, as well as studies on energy consumption and economic feasibility demonstrated that hydrodynamic cavitation has a variety of beneficial impacts on secondary sludge disintegration in P&P mills, resulting in better nutrient release. These primary results indicate that cavitation is a potential technology for the pre-treatment of sludge produced in P&P industry and could be applied in combination with further biological treatment approaches.

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4.2 General Characteristics of Microbes In many industries it is preferred that the microbes should be absent from the production process due to their undesired interference. In paper production systems, as in natural environments, microbes are present by default in suspensions, or water interfaces or are attached to material surfaces, mainly aggregated as flocs and biofilms (Davey and O’toole 2000). These can be described as multicellular communities, embedded in extracellular polymeric substances (EPS) containing proteins, carbohydrates, and DNA. Their formation is the cause of biofouling which results in microbiologically influenced corrosion (MIC) of materials present in the production process. Biofouling and MIC cause several problems for the paper industry, and require costly countermeasures (Klahre and Flemming 2000). Primary bacterial colonization of the metal surfaces of pipelines and mechanical equipment creates favorable conditions for the development of anaerobic strains that can use iron or sulfur as electron donors, producing iron oxides, sulfuric acid or hydrogen sulfide, all of which are highly corrosive. The second important nutrient source that stimulates H2 S production are short fatty acids, which are produced from the fermentation of readily available organic substrates present in industrial waters from the paper production line. Biofilms in the inner parts of mechanisms not only lead to increased wear and tear of equipment, and downtime during slime-related cleaning, but also significantly affect the quality of paper products. For these reasons the flocs have been regarded and biofilms solely as an unwanted nuisance. In order to decrease the load of microbes on different surfaces by preventing microbe attachment or killing the microbes, different strategies have been developed, including the application of biocides. Despite all the problems microbes cause on the production line, they hold numerous solutions for wastewater treatment through the metabolic ability of individual microbial strains or communities to degrade specific compounds. They develop naturally in paper production microenvironments and their metabolic potential covers the wide range of pollutants present in the paper production wastewaters (see Sect. 4.3.2). Moreover, the methodology for the construction of artificial multicellular structures with a predetermined composition of strains has been developed as part of an eco-evo approach to the control of biofouling (Rijavec et al. 2019; Deev et al. 2021). By selecting the appropriate candidates to be included in these artificial structures, it is possible to transform or completely degrade several unwanted compounds present in the paper production wastewaters. The natural microbial communities found in different microenvironments of the paper production process are adapted to metabolism of specific chemical compounds present in the production system (see Sect. 4.3.1) and as these compounds represent sources of C and N, they influence the formation of specific ecological niches, which become occupied by specific microbes able to perform the necessary metabolic transformation. Fungi have been more commonly sought in past remediation studies due to their known ability to degrade lignin, but in a broader sense, bacteria have a wider metabolic potential than fungi, as they are not as dependent on simple sugars, can grow faster and can survive in a wider range of environmental conditions. Additionally, the ability to degrade lignin has also been

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described for bacteria. In the light of the above, bacteria already are and will be the preferred candidates for paper waste treatments, for example in the classical treatment (see Sect. 3) as well as in the advanced approaches that implement engineering of microbial communities.

4.3 Starting a WTP Using Synthetic Bacterial Consortia 4.3.1

Bacterial Consortia from Paper Production Systems—Learning from Nature

There are at least two possible advanced strategies that might be used separately or combined together for engineering microbial communities and can be used in environmental services: (i) introducing a synthetic consortium as the initial inoculum that is going to direct microbial community succession and (ii) structuring either the environmental conditions via augmentation nutrients or the physical properties of the microenvironment by depositing cells onto the surfaces of carriers. To prepare a robust and efficient synthetic inoculum we can select from a range of specialist microbes that are capable of degrading only a specific substrate or generalist phenotypes and are able to degrade most or several different compounds. Bacterial populations growing in complex media do not become dominated by a single generalist phenotype which can utilize all the substrates present with maximum efficiency, nor do they consist of a set of specialist phenotypes, restricted to utilizing only a single substrate. Usually, they are composed of a mixture of generalist phenotypes with overlapping metabolic properties and a range of specialist phenotypes focused on the degradation of less accessible and recalcitrant substrates. How the community is finally arranged and how niche separation takes place depends on the types and amounts of specific pollutants, temperature fluctuations, water flow that influences washing off and the flow of other nutrients. We assume that the most robust community having also the highest responsiveness to physicochemical fluctuations will be the one with the highest diversity of specialists and the presence of generalists which by different interactions support the whole community. The macro-environments present in the P&P process, such as whitewater, surfaces of piping or sludge, incorporate different microenvironments with specific physical properties and sets of nutrients. As is explained in Sect. 4.3.2, all the chemicals present in these microenvironments represent specific carbon and nitrogen sources for different types of bacteria and we can expect to find in specific microenvironments specific bacteria that are able to degrade or transform these compounds. Several studies have already been carried out to elucidate the phylogenetic structures of bacterial consortia at different places in the paper production process and to assess if the specificity of the environment reflects the community structure. Whitewater bacterial communities, for example, have proven to be spatially homogeneous along the production process and varied only slightly over time, with Proteobacteria, Bacteroidetes, and Firmicutes being the predominant groups (Chiellini et al.

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2014). Similarly, next-generation sequencing has shown that the biofilms and slimes in a production process are composed of only specific bacterial genera (Zumsteg et al. 2017). The high specificity of bacterial communities from the paper production process was demonstrated through the comparison with other industries (Boon et al. 2002). Paper mill pollution affected the composition of microbes of the river sediment (Guo et al. 2016) and more importantly, it was demonstrated that this specificity translates to a potential to degrade different carbon sources (Tripathi et al. 2014), or that native communities could be used in membrane bioreactors to treat whitewater (De Sousa et al. 2011). Most recently, Verdel et al. (2021) have shown that several of these bacteria can be isolated and cultured in the laboratory to test their ability to degrade the additives of the wood-free P&P process (see Sect. 5.1), but prior studies have already confirmed that active bacterial cultures can be collected from the P&P microenvironments (Maukonen et al. 2006; Sachan et al. 2019). Although there are very similar processes in the industrial paper production, it is expected that different sorts of species are predominant in different industrial setups due to the small changes in the local temperatures, volume designs, water fluxes and predominant paper types and therefore we only need to determine what sort of synthetic communities would respond most efficiently to different ecological interventions, such as initiating the water treatment process, executing bioaugmentation during an established treatment process or management of the development of natural biofilms. The presence of different anaerobic bacteria in the whitewater, pulp and slime (Maukonen et al. 2006) reveals the establishment of anaerobic niches, and this has important metabolic implications. The presence of aerobic and anaerobic niches means that the compounds that need to be removed from wastewater will be transformed by different metabolic pathways and only in the presence of oxygen will they be converted to CO2 and H2 O. Conversely, anaerobic transformation can have several outcomes: it can be completely ineffective as the primary transformation step of certain compounds, it can lead to formation of unwanted end products or it can be the basis, and the crucial step in the formation of novel compounds with added value, such as methane, butyrate and lactate. Several transformation processes are possible at the same time due to the diverse and complex substrates and the phylogenetic diversities that have been detected in paper production microenvironments. A naturally formed wide metabolic potential is expected in these systems.

4.3.2

The High Metabolic Diversity of Bacteria

The RCs and POCs in whitewater and deinking process wastewaters as well as the different forms of lignin produced by the delignification process (see Sect. 2) are a chemically diverse set of compounds. Microbes are able to degrade them, but not via the common degradation pathways used for the universal organic compounds like sugars and amino acids. The RCs have a very low degradation kinetic constant due to (i) the inefficient enzymatic degradation, caused by enzymes’ low affinity or nonspecific activity (Fischer and Majewski 2014), (ii) the high energy barrier caused by strong bonds or the absence of downstream metabolic pathways linking

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the initial breakdown with energy generation in core metabolic pathways (Helbling et al. 2012), (iii) the inadequate C:N ratio (Polman et al. 2021) and (iv) the asynchronized intracellular and intercellular pathways leading to the buildup of intermediate products, for example diterpenoids and azo dyes (Martin and Mohn 2000; Sarkar et al. 2017). To date, metabolic pathways for the degradation of pitch-forming sizing agents (Kumar et al. 2016), such as binders (Miranda et al. 2008), resin acids (Yu and Mohn 2001) and highly visible azo-dyes (Hubbe et al. 2016) and toxic, fluorescent stilbene-dyes (Salas et al. 2019) have been reported. Due to their high visibility, biodegradation of azo dyes has been extensively studied (and reviewed in Varjani et al. 2020), while other studies also report on microorganisms able to utilize synthetic binders, like polyvinyl alcohol (PVA) (Ahmed et al. 2018) and latex (Ali Shah et al. 2013). Recently, Verdel et al. (2021) isolated adapted bacteria that efficiently degrade alkyl ketene dimers and also a previously “non-biodegradable” stilbene derivative, a fluorescent dye. The high chemical complexity of RCs is reflected in the diversity of the degradation enzymes and metabolic pathways. Initial metabolic reactions are performed by different unspecific oxidoreductases, resulting in the formation of smaller products or the oxidation of side groups, which in turn increases solubility and thus the availability to other microbes, e.g. abietic acid in rosins (Martin and Mohn 2000). For azo dyes in particular, the degradation products are accumulated, some being toxic or capable of inhibiting other metabolic reactions, for example, L-quinate inhibits the synthesis of aromatic amino acids (Caspi et al. 2020). For instance, the oxidative product from acid orange is 1,4-benzoquinone which inhibits the D-glucose dehydrogenase involved in harvesting glucose (Le et al. 2016; Caspi et al. 2020). The main problem for POCs is that they are large insoluble molecules and as substrates the cell cannot simply uptake them into the cell and an initial extracellularly degradation is necessary, so that the degradation products can be taken up and transformed further. Ubiquitous microorganisms have been isolated to hydrolyze starch (de Souza and Magalhães 2010) and cellulose (Wierzbicka-Wo´s et al. 2019). Cellulose and starch, cellulases and amylases, respectively, participate in this transformation to produce glucose, while for hemicelluloses and lignin a wider range of enzymes is needed, producing a diverse set of monomers (see Sect. 5.2.2). For the RBCs, enzyme kinetics can also play an important role, demonstrated by the difference between the α(1–4) and α(1–6)-glycosidic bonds in starch versus the β(1– 4)-d-glucoside bond in cellulose, which causes es a three-fold difference in the rate of degradation (Polman et al. 2021). Neither excretion of enzymes or the uptake of degradation products are highly efficient, due to diffusion effects. To increase the efficiency of degradation, microbes will attach to the POCs and produce a highly efficient uptake system for the monomers that are produced, such as glucose. The POC degrading organisms that are able to attach to the POCs have the capability of biofilm formation, one of the most frequent problems encountered in the paper production line. As the POCs are degraded much faster, the RCs are accumulated in the wastewater especially in closed whitewater systems where the wastewater treatment plant is located within this closed circle (Jidong et al. 2011; Kamali and Khodaparast 2015).

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To summarize, the chemical diversity of the compounds present in the wastewater calls for several bacterial strains to form a community to provide the metabolic diversity needed to transform or degrade these compounds. It is crucial that the bacterial community is structured accordingly, so that the metabolic pathways are active and that substrates are passed from one bacterium to the other.

4.3.3

Structuring a Bacterial Consortium

To degrade a complex POC molecule it is preferable that several bacterial species join to construct the degradation pathway, because: (i) the chance to evolutionarily develop the whole multi-step degradation pathway within a single microbial strain is lower than when distributing the different steps between different strains, (ii) niche specialization of a strain will increase its survival and (iii) the C:N ratio is high in these compounds, meaning that the N can be raised in the system by a collaboration of strains, POC degraders and nitrogen fixators. In the field of ecology, it is well established that single strains do not support the different environmental services as well as the intertwining of the redundant and specific pathways in heterocultures. Still in place is the debate about whether it is better to select a generalist degrading all the pollutants present or a collection of microbes specialized for the degradation of particular pollutants or even a single step in a degradation pathway. It is obvious that selecting a diverse set of microbes is important when: (i) there are multiple pollutants, calling for the presence of different metabolic pathways, e.g. for azo dyes and AKD, (ii) the pollutant is composed of different chemical units or the same units that are linked with different types of bonds, e.g. stilbenes, or (iii) the pollutant is structurally very variable from batch to batch, e.g. lignin. Even though it is important to select phylogenetic groups with the most diverse metabolisms when structuring a consortium, the wastewater treatment plants naturally do not harbor sufficiently high microbial diversity to achieve the maximal degradation of a single pollutant (Johnson et al. 2015). Different pollutants enable diversification of metabolic niches, but for a single pollutant it is not straightforward to predict that a single generalist or several specialists will perform the degradation process better, because in actual remediation systems the availability of nutrients and adaptation of strains to fluctuating physicochemical conditions must be taken into account. Using azo dyes as an example, to carry out complete degradation of a compound a consortium of at least three organisms will be needed: Stenotrophomonas acidaminiphila APG1, Pseudomonas stutzeri APG2 and Cellulomonas sp. APG4 (Nanjani et al. 2021). They carry out oxidative and reductive reactions in aerobic and anaerobic conditions by producing different laccases, peroxidases, azo-reductases and other enzymes. In this consortium glycerol is the initial growth substrate and source of reductive power for some consortium members. After the initial activity of azo-reductase several new intermediates occur and must be further deaminated or sulfonated and these are then used as the source of reductive power by a consortium member that is unable to initially utilize glycerol. The intermediate products of this tripartite cross feeding process, namely 1-naphthylamine, 1-naphthol and aniline-2-sulfonic acid are more toxic than

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the initial azo dye, but they are degraded by each of the consortium members separately via anaerobic degradation pathways or via aerobic degradation in the strain APG1 specifically by vanillyl-alcohol oxidase or nonspecifically by certain oxidoreductases. As the three strains are able to utilize different substrates, have different efficiencies for utilizing the same substrates and produce or harvest electrons, this promotes niche differentiation rather than competition. Additionally, chemotactic motility and not only metabolic pathways are important in this cooperative process, indicating that degradation and consequently the interaction between species can be started by a strain possessing these traits (Nanjani et al. 2021). The exemplary cross-feeding process that was described will occur when the intermediate products are shared among the partners and there are no toxic substances produced to cause competition between the partnering cells. Factors that limit cooperation are: (i) transport systems ensuring the transition of compounds between the intracellular and the extracellular space, (ii) the equilibrium between the rates of production and uptake of substances, (iii) the distances between the cells involved in the cross-feeding process, and (iv) spatial distributions of cells and substrates influencing spatial and temporal gradients of compounds. For example, extracellular enzymes such as cellulases or lignocellulases are involved in the production of large quantities of oligosaccharides supplying nutrients for large communities and promoting loose associations between partners (Munoz et al. 2020). Conversely, when the products are more specific and produced in lower quantities, tight associations between cells are promoted, especially in the absence of efficient uptake systems (Ude et al. 2021). For the majority of compounds, efficient cross feeding due to the diffusion effects can probably take place at a very close distance, smaller than a few micrometers (van Tatenhove-Pel et al. 2020). Taking this into account, the selection of the appropriate partners, and appropriate spatial and temporal structuring of a consortium has begun to guide the development of advanced cell immobilization techniques to support the execution of novel bioaugmentation approaches.

4.3.4

Novel Bioaugmentation Approaches

The classical bioaugmentation approach is in need of improvement, since bacterial biomass should not merely be added to the system as a suspension (see Sect. 3.3). Bacteria should be introduced as attached biomass, as this increases the interactions between cells and consequently the gradient of physicochemical factors which diversity niches and consequently increases the efficiency of wastewater treatment (Wolff et al. 2021). As we have learned in Sects. 4.3.2 and 4.3.3, advanced cell immobilization techniques that are currently being developed can guide the construction of artificial consortia of selected microbial strains. We have to bring the cells together physically by cell aggregation and/or attachment to carriers before introducing them into the system for wastewater treatment. How we prepare this attachment depends on the throughput requirements, costs and the type of inflow and effluent, but only after building a heterogeneous consortium can we remove recalcitrant pollutants (Machineni 2019). Setups with attached biomass show a high removal efficiency and the

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ability to remove a complex mixture of pollutants and allow the reuse of the biomass (Kanaujiya et al. 2019). Initial attachment of cells does result in the formation of sludge with higher thickness and settling ability (Loupasaki and Diamadopoulos 2013), which can result in overgrowth of the membranes and clogging of the filters (Goli et al. 2019; Hayder et al. 2017; Kanaujiya et al. 2019), but these problems can be overcome with several technical solutions, such as backflushing or regular filter exchange. To really improve the efficiency of wastewater treatment a combination of both aerobic and anaerobic processes is required. This is currently achieved in bioreactors but not in treatment plants (Loupasaki and Diamadopoulos 2013). Granulated aggregated biomass can solve this problem, since very specific biological niches can be established due to the formation of nutrient and oxygen gradients within the granules. Such a system is populated by bacteria performing different functions, resulting in a system capable of degrading complex pollutants (Li and Pagilla 2017). To prevent cells washing off from the system, the cell biomass is attached to different matrices or carriers (Kanaujiya et al. 2019). Matrices can be composed of various biocompatible and biodegradable components as natural polysaccharides, proteins, polypeptides, lipids or synthetic polymers (Martín et al. 2015; Vemmer and Patel 2013). The application of matrices increases mechanical stability and protects bacteria from colonization by other microbes (Covarrubias et al. 2012). Moreover, confined cells are protected from external abiotic influences, surviving very low pH (Ding and Shah 2009) and tolerating changes in temperature or mechanical stress (Abbaszadeh et al. 2014). Inside matrices they can stay alive, can grow and can retain their metabolic activity (Mirtiˇc et al. 2018). Mechanical properties of such systems can be improved by combining the matrix with a natural highly porous degradable or non-degradable carrier, so that the cells are embedded in the matrix that is covering the surface of the carrier (Bayat et al. 2020). This allows the bacteria to attach to the carrier surface, to form biofilms, to multiply and to populate the substrate surrounding the carriers. Using a combination of carriers and matrix reduces shear stress and grazing of bacteria by protozoa (Horemans et al. 2016). Furthermore, to improve the attachment of biomass to both the matrix and the carrier surface, a cell surface can be modified with polyelectrolytes, which leads to improved attachment even for bacteria that can form biofilm (Deev et al. 2021; Rybkin et al. 2019) (Fig. 2).

5 Microbiological Valorization: Case-by-Case 5.1 Case 1: Microbiological Treatment of Process Water The current classical wastewater treatment technologies in paper mills are efficient in their decrease of COD, but for elimination of RCs we believe that advanced biological approaches are more appropriate. Recently, an advanced bioaugmentation-based strategy was developed in which bacterial degradation of all possible contaminants in the process water (whitewater) of wood-free paper mills was examined with the

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c

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Fig. 2 Advanced cell aggregation and immobilization approaches support the development of advanced bioaugmentation approaches. a Appropriate partner strains are selected to form an artificial consortium and are b aggregated into flocs or biofilm to be immobilized onto c different types of porous carriers. The carriers serve a protective function and represent a hot-spot where the artificial consortia multiply to colonize the surrounding substrate. Also being a delivery system the consortia are loaded into d column filters or e water treatment basins

aim of closing the whitewater cycle (Verdel et al. 2021). Since in the wood-free paper industry, cellulose fibers are added as purchased pulp, contaminants in the whitewater are mostly papermaking additives, such as starch, cellulose, resin acids, alkyl ketene dimers, polyvinyl alcohol, latex, as well as azo and fluorescent dyes. By isolating 318 bacteria from three interrelated sites of a wood-free paper mill, an actual microbial system in the whitewater was simulated (see Fig. 3). By agar plating and spectrophotometric methods, the individual isolates were tested for their ability to degrade separate papermaking additives. By using multivariate statistical methods, such as principal component analysis (PCA), a correlation between carbon source use, genera, and inoculum source of isolates was established. In most cases, the sources of the inoculate determined the genera of the isolated bacteria. Finally, by means of co-culturing tests in synthetic and industrial whitewater, Verdel and coworkers (2021) selected a consortium of four strains. Among the four selected bacteria Xanthomonadales bacterium sp. CST37-CF and Sphingomonas sp. BLA14CF are specialists that complement each other in different niches, while Cellulosimicrobium sp. AKD4-BF and Aeromonas sp. RES19-BTP are general degraders of all the papermaking additives that were tested. The degradation results of the four selected isolates were in accordance with a review of the published work on their closest relatives. The efficient degradation of carbohydrates, resin acids, PVA and AKD by Xanthomonadales bacterium sp. strain CST37-CF is in accordance with the reported (Guo et al. 2018) high activity for PVA utilization by its closest relative, [Pseudomonas] boreopolis. The utilization of azo dyes by Sphingomonas paucimobilis (Ayed et al. 2011) confirmed the results for Sphingomonas sp. strain BLA14-CF. Lignocellulolytic enzymes detected from Cellulosimicrobium cellulans (Dou et al. 2019) and use of diverse metabolites like azo dyes and hydrophobic plastic materials

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Fig. 3 The steps for constructing an effective bacterial consortium for the bioaugmentation of wood-free whitewater (Verdel et al. 2021): 1. Isolation of 300–400 bacteria with stable catabolism at low nutrient concentrations (Horemans et al. 2017); 2. Testing the activity of the isolates with respect to the degradation of the targeted organic additives and finding the best combinations using, 3. Statistical tools, like principal component analysis (PCA) and 4. Co-culturing tests, and 5. testing the best combination in the desired unsterile environment for proof of concept

by Aeromonas hydrophila (Thomas et al. 2020) are in accordance with degradation results for Cellulosimicrobium sp. strain AKD4-BF and Aeromonas sp. strain RES19-BTP. For a proof of concept, the four bacteria selected for bioaugmentation were immobilized on carriers and placed in a 33-L column filled with whitewater. On day 21 of the proof-of-concept pilot trial, an 88% decrease in COD was measured with a retention time of approximately 15 h. These results proved that the selected bacteria can effectively purify the whitewater of a wood-free paper mill (Verdel et al. 2021). The bioaugmented strains indeed remained on the carriers for twenty days under non-sterile conditions and even spread onto the surfaces of fresh carriers. However, the following questions regarding such treatment in the case of a real whitewater closure arise: What could happen with the consortium on the carriers after a longer time and in a different production type when different papermaking additives would end-up in the whitewater? Would it be necessary to add some more specialist strains for the complete degradation of the azo dyes that are visually so disturbing for the paper end-product? How important is the initial inoculum? Is a one-time inoculation enough for the bioaugmentation or is it necessary to continuously add carriers with the bioaugmented organisms? How large an inoculum do we actually need to achieve long-term efficiency in order to completely close the whitewater circle of the paper mill without a decline in paper productivity? Even though many questions remain unanswered, the strategy of building an artificial consortium has potential for treatment of P&P industry’s whitewater as well as for valorization of its by-products such as lignin and papermill sludge.

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5.2 Case 2: Valorization of Lignin Waste Using Bacteria Lignin is difficult to isolate in its natural condition, therefore a key source of lignin appears as the precipitate from the P&P industry’s black liquor (see Sect. 2.1). Lignin has proven to be a natural resource with high potential for aromatic bioproducts, and it is one of main focuses of research into biorafination in the P&P industry. Lignin is a promising source of high value organic molecules, but its utilization is still limited by its complex structure. Lignin valorization is believed to be the key for successful development of lignocellulosic biorefinery (Ragauskas et al. 2014).

5.2.1

Lignin Valorization for Sustainable Phenolic Resins

One of the most promising end uses of lignin valorization is to replace phenol in phenol formaldehyde (PF) resins. PF resins are thermosetting polymers resulting from the condensation of petroleum-based phenol with formaldehyde. With their versatile properties and good performance in a wide variety of applications, PF resins dominate the resin industry even more than 100 years after their first synthesis. The properties of PF resins that have made it the material of choice in automotive, computing, aerospace and the building industry are their excellent mechanical properties, flame retardancy, flexibility, low cost, high thermal stability, and water and chemical resistance (Gardziella et al. 2000). PF resins are also widely used in the manufacture of engineered wood products, such as plywood, particle-board, and laminated lumber (Hong et al. 2018). There are three motivations for replacement of phenol in PF resins. First, the unsustainable utilization of petroleum for phenol production causes environmental issues, such as increased emissions of greenhouse gases. Second, the cost and availability of phenol depend heavily on petroleum prices while the Kraft delignification process where lignin ends up as a byproduct, will reach 1.7 million tons by 2025 and the estimated average price for technical lignins in 2025 will range from 600 to 800 e per ton, i.e. half that of phenol. The third motivation is related to health and safety, since according to the European Chemical Agency (ECHA) phenol and formaldehyde are mutagenic, carcinogenic and reprotoxic (Lourençon et al. 2020). In the P&P industry, sulfur-containing lignins in the form of Kraft lignin and lignosulfonates are mainly produced (see Sect. 2.1). The mechanical properties of PF resins where Kraft lignin and lignosulfonate are added without prior activation are inferior to those of conventional PF resins because of the lower ability of lignin in comparison with phenol to form bonds with formaldehyde. Lignin is restricted by its high molecular weight, irregular structure and steric hindrances (Sarika et al. 2020). To improve the reactivity of lignin, various chemical modifications, such as methylation, demethylation, phenolation, sulfonation, hydrolytic depolymerization, thermochemical treatments, homogeneous and heterogeneous catalysis and reductive depolymerization are performed prior to its incorporation in the lignin-based PF resins (Farag and Chaouki 2015). However, all these chemical conversion processes

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require a large amount of energy or expensive catalysts to break the lignin’s recalcitrant structure, and this hinders its use in industrial applications. On the other hand, the conversion of lignin by microorganisms is an alternative option because this method has low costs and generates environmentally friendly products (Hermosilla et al. 2018). The microbial degradation of lignin has been well studied in white-rot and brownrot fungi, but there is still no commercial biocatalytic process for lignin depolymerisation (Lee et al. 2019). In comparison to fungi, bacteria are more adaptable to the environment and have faster growth rates. Recent reports have suggested that bacteria play an increasing role in breaking down lignin (Bugg et al. 2011b). Bacteria use versatile metabolic pathways for the breakdown of lignin into highvalue chemicals (Jimenez et al. 2014) (see Sect. 5.2.2), but studies on lignin phenol formaldehyde resins produced from bacterially modified industrial lignin, either Kraft lignin or lignosulphonates, are still rather scarce. Recently, Venkatesagowda and Dekker (2020) studied wood-rot fungi for their ability to demethylate Kraft lignin by removing the O-methyl/methoxy groups which liberated methanol. They found that demethylating enzymes in fungi offer potential for their application in the production of demethylated lignins enriched in vicinal hydroxyl groups for possible synthesis of lignin-based formaldehyde polymers. In most cases, biovalorization of enzymatically hydrolysed lignin originating from biorefinery residues from ethanol production was studied as raw material, which due to the difference in delignification process disallows direct comparison. Jin et al. (2010) managed to incorporate 5–20 wt% of enzymatically hydrolysed lignin into PF resins and met the standard criteria for commercial adhesives for the resulting resin adhesive. The exploitation of lignin PF resins based on full utilization of biorefinery residues has been reported (Zhang et al. 2013). Four kinds of biorefinery residues (ethanol, butanol, xylitol and lactic acid biorefinery residues) were compared according to their utilization potential as lignin PF resins by copolymerization. It was found that the bioethanol biorefinery residue exhibited the highest reactivity due to its high content of hydroxyl groups and 50% phenol could be replaced by bioethanol residue with no prior purification, without diminishing the properties of adhesives and plywoods.

5.2.2

Biovalorization of Lignin for High Added Value Products

Lignin can often be found in wastewater treatment plants, where different physical, chemical and biological approaches for its removal are used. So far biological methods have been shown to be most successful, not only based on sustainability, but also regarding costs and labor intensity (Haq et al. 2020). Biological methods consist of either enzymatic or microbial degradation of lignin and while much work has been published on fungal lignin degradation; bacteria were shown to be more often used in situ in wastewater treatment. Bacteria have higher physiological resistance, are more robust and produce a wider range of enzymes able to oxidize lignin to valuable compounds (Nicolella et al. 2000).

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Lignin degrading microorganisms can be found in a variety of different ecosystems, from forest soils to aquatic environments and anaerobic gut microbiome. There are far more fungi species than bacteria that are known to oxidize lignin. Also, fungi are often living as saprotrophs and parasites in relation to plants and wood and in this regard, they are able to degrade lignin fully and have been studied much more than bacteria (Xu et al. 2019). There are nevertheless several soil bacteria, often aromaticdegrading bacteria, that are able to break down lignin. In this process, extracellular peroxidase and laccase enzymes seem to play a major role (Bugg et al. 2011a). Several species of bacteria with versatile metabolic pathways for depolymerizing lignin have been isolated (Brown and Chang 2014). Streptomyces viridosporus T7A, using an extra-cellular lignin peroxidase enzyme, degrades lignin only in the presence of hydrogen peroxide (Ramachandra et al. 1988). Comparable lignin-degrading activity was found also for Sphingobium sp. SYK-6 (formerly known as Sphingomonas paucimobilis SYK-6) (Masai et al. 2007). While for Pseudomonas putida mt-2 and Rhodococcus jostii RHA1 no need was demonstrated for stimulation of lignin degradation by addition of hydrogen peroxide (Ahmad et al. 2010). Ahmad and coworkers suggest that lignin degradation mechanism in these two bacterial strains is performed either by excretion of oxygen-utilizing laccase enzymes, or by extracellular enzymes for hydrogen peroxide generation. In addition, Pseudomonas putida has also been studied using lignin-derived aromatic compounds for the synthesis of polyhydroxyl-alkanoates (PHAs) (Linger et al. 2014). Bacteria only recently received attention with respect to lignin degradation because of specific metabolic pathways and their ability to produce low molecular weight compounds from lignin. The majority of the enzymes produced by both fungi and bacteria are extracellular and their mechanism of transformation is oxidative. Lignin peroxidase, mangan peroxidase, laccase and laccase-like multicopper oxidase are some of the most common enzymes found in lignin degrading microorganisms (Silva et al. 2021). Not all enzymes are equally represented in fungi and bacteria domains. Lignin peroxidases are mainly found among fungi, while DyP-type peroxidases are more common among bacteria (Sugano and Yoshida 2021). While fungi produce more lignin degrading enzymes, bacterial enzymes are more applicable to industrial and wastewater use due to their higher halo- and thermotolerance (Table 2). The specific and detailed metabolic pathways of oxidation are still largely unknown because lignin is highly heterogeneous in structure. Generally, it is believed that in nature, fungi perform the first steps of lignin decomposition, as some fungi can grow on lignin as their sole carbon source. While cases of bacteria growing solely on lignin have been reported (Riyadi et al. 2020), most of the strains degrade depolymerized lignin to low molecular weight products or even to CO2 . Biological treatment of wastewater plants in terms of lignin removal is not limited to experimental in vitro research and many applications have already been established. Single culture fungal ligninolytic systems of degradation have been shown to be highly effective, achieving up to 100% of degradation of lignin (Costa et al. 2017). While the efficacy in terms of lignin removal is very high, the use of fungal strains often encounters problems when the conditions are not optimal. Paper plant

PEROXIDASES Dye-decolorizing peroxidases LACCASES Lignin-modifying bacterial laccases Glutathione-dependent β-etherases Superoxide dismutases

Bacillus, Pseudomonas, Saccharomonospora, Thermobifida

Streptomyces, Amycolatopsis, Pantoea, Bacillus, Thermus

Sphingobium, Thiobacillus

Sphingobacterium

Bacteria

Catalase-peroxidases Bacterial dioxygenases

Amycolatopsis

Streptomyces, Sphingomonas

PEROXIDASES Lignin peroxidases Manganese peroxidases Versatile peroxidases Dye-decolorizing peroxidases FUNGAL OXIDASES Alcohol oxidases Glyoxal oxidases Carbohydrate oxidases LACCASES

White-rot fungi

Fungi

Enzymes

Example organisms

Group

Cinnamic acid Salicylate Phenol p-coumarate Ferulate Benzoate Vanillin Catechol Syringic acid

CO2

End products

Table 2 Overview of lignin degrading enzymes produced by different microbes. The two microbial groups, bacteria and fungi produce different enzymes to degrade lignin into novel added value compounds. Summarized after de Gonzalo et al. (2016), Chauhan (2020) and Haq et al. (2020)

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wastewaters often do not reach optimal pH or temperature and contain toxic chemicals that can limit fungal growth. Fungi are, therefore, not considered to be ideal for lignin removal from wastewaters and bacteria are more often used. Aside from their good pH, temperature and salt tolerance, bacteria multiply faster than fungi and, since the amount of extracellular enzymes produced can be correlated with the amount of bacteria, similar efficiency of lignin degradation can be achieved (Haq et al. 2020; Mathews et al. 2015). Currently, aerobic and anaerobic bioreactors, activated sludge and biological filters are used most. Despite that, bacterial lignin removal faces its own challenges, the main one being the complexity of the substrate. Bacteria can be isolated from different sources where lignin is present. Many strains were isolated from paper industry wastewaters (Zainith et al. 2019), from rotting wood material (Yang et al. 2017) or directly from forest soils (Umashankar et al. 2018). Other sources of bacteria include composts, animal dung and gut of termites and other wood eating insects. The latter were shown to contain many anaerobic bacteria with lignin degrading abilities. Because bacteria are able to degrade lignin only to a limited extent, immobilization techniques were adapted to improve bacterial stability, activity and overall degradation efficiency. Ojha and Tiwari (2016) showed that three bacterial strains, Bacillus subtilis, Bacillus endophyticus, Bacillus sp. with 18 ± 0.5% separate degradation rates can degrade lignin up to 40% when combined together in a consortium. Bacteria, as well as their enzymes, can be immobilized, encapsulated, entrapped or absorbed using various hydrogel matrices and organic or inorganic carriers. Guo et al. (2021) demonstrated lignin degradation by immobilizing Aspergillus fumigatus and Bacillus cereus on mycelial pellet, while Paliwal et al. (2015) showed how to immobilize two Bacillus and Pseudomonas strains on corncob cubes in order to perform lignin degradation. Furthermore, bioreactors can be used for lignin and chemical oxygen demand reduction and decolorization, as seen by Chuphal et al. (2005) where bacteria and fungi were immobilized on sand and gravel at the bottom of a bioreactor. In order to fully utilize lignin as a substrate, advances have been made in building biological systems, from which, in addition to lignin removal, valuable products could be extracted. As discussed previously, lignin is the largest natural resource of aromatic compounds and its degradation products (Table 2) can be used in a wide range of applications. The most notable products derived from bioprocessing of lignin are flavors and fragrances (e.g. vanillin), fuels, nylon, various organic acids used subsequently as precursors in mass production of industrial chemicals as well as pharmaceutically and medically relevant precursors and compounds (Kohlstedt et al. 2018; Wang et al. 2019). While the production of these compounds from lignin by bacterial transformation has been well documented, most of the research has been conducted in batch or bioreactor controlled settings and few reports have been made in which wastewaters are used in this manner. Upscaling can be achieved with high yields and published in which purity was stressed, as Kohlstedt et al. (2018) demonstrated by production of kilograms of cis,cis-muconic acid using a Pseudomonas putida soil isolate with 98% purity. Cis,cis-muconic acid was subsequently used to

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produce nylon. Many other reports described successful breakdown of lignin from wastewater using bioreactors with appropriate lignin degrading microorganisms, often exploiting the use of immobilization techniques (Kuang et al. 2018). Advanced methods include the use of electrical current in bioreactors through graphite electrodes, where the best lignin removal, electron transfer and smallest molecules in effluent were achieved. Functional lignin degrading bacteria were enriched using the appropriate time intervals, resistance and current strength (Zhang et al. 2019). Lignin transformation into novel compounds is an ideal case, in which engineered microbial consortia can be implemented. Lignin chemical composition offers the highest number of metabolic transformation steps allowing the use of a diverse set of microorganisms to be involved in the cross-feeding processes. As lignin-derived compounds are currently being produced from fossil fuels (L’udmila et al. 2015), advanced biorefineries with targeted lignin transformation will further gain their attractiveness.

6 Conclusions 1. Since the pulp and paper (P&P) industry is a large producer of chemically diverse waste containing materials, like lignin, cellulose, hemicelluloses and starch, which can all be biotransformed into novel low molecular weight bioactive compounds or polymers, the P&P industry has a large potential for biorefinery development. The chemical diversity of compounds present in waste and the complexity of metabolic pathways calls for the engineering of whole microbial communities instead of the conventional use of a single strain in biotechnological transformation processes. 2. The conventional treatment approaches fail to exploit the microbial potential of new chemical generation to the fullest. Toxic chemicals, recalcitrant compounds and other organic pollutants are typically reduced in quantity but not completely removed from wastewater as it is released back into the environment. These chemicals are mostly concentrated in sludges, the by-products of conventional treatment processes, which can be used as adsorbents, construction materials, solid fuels for burning and as biogas. What remains are large volumes of active secondary sludge, which is not appropriate for landfills and needs to be further biotransformed. The conventional solutions are set to fulfill the regulatory obligations postponing addressing the initial problem, which is the release of toxic chemicals into the environment. 3. Advanced microbe-based treatment approaches, which are in development, can increase the sustainability of the industry, and reduce treatment costs. The microbes that are, from the perspective of a clean production line, unwanted in the P&P industry, prosper in the paper production related microenvironments, where they execute metabolic pathways that mineralize or biotransform recalcitrant and polymeric organic compounds. From these environments, we can collect the generalist and specialist strains and study their individual metabolic

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traits in order to cover the chemical diversity of different pollutants in wastewater and use them to engineer artificial microbial systems for waste biotransformation and degradation as well as to produce new industrial chemicals. 4. The engineering of microbial consortia includes: (i) the selection of appropriate strains or larger communities that are metabolically coupled through crossfeeding, (ii) the development of strategies for cell immobilization, and (iii) the selection of different supports for delivery as well as for augmentation with additional nutrients. In the engineered immobilized consortia, cell interaction is promoted and optimized, the cells are protected from grazing and hydrodynamic washing-out, which fulfills all the niche-related criteria for the efficient microbial activity in a biotechnological process. 5. Artificially engineered systems can be directly used for the treatment of whitewater to remove recalcitrant and toxic compounds or in combination with physical treatments like cavitation, which increases the level of pollutants in sludge. The latter is suitable for valorization of lignin, which can be modified by microbes for the production of resins or biotransformed into novel compounds with added value for the food and pharmaceutical industries. Acknowledgements NV acknowledges financial support from the Ministry of Education, Science and Sport of the Slovenia (research program No. P2-0150) and the European Regional Development Fund (ERDF) within the Operational Program for the Implementation of European Cohesion Policy in the period 2014–2020. TR, MZ, DD, IR and AL acknowledge the Slovenian Research Agency, projects (J4-7640, J1-6746, J3-1762, J1-9194, J7-9400, J4-4556, J4-4561 and P1-0143), project CROSSING (grant PIE-0007), Urban Innovative Actions project Applause (UIA02-228), European Commission, projects GREENER, Grant No.: 826312 and SurfBio, Grant No.: 952379. Contributions First authors NV, MS and TR conceived the structure of the manuscript and equally contributed to the writing of the manuscript. All authors contributed to the writing of the contents of the manuscript. NV, MS, TR and AL critically reviewed the contents of the manuscript. Ethics Declarations The author NV is currently employed by Fenolit, d.d., industrial company. The author MS is employed by a research & development center doing contract research and publicly funded research projects. The authors TR, MZ, DD, IR and AL are employed by a public research institute. All authors declare no conflicts of interest.

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Promising Approach of Industrial Wastewater Bio-refinery Through Bio-diesel Production A. Anuradha, Aakansha Singh, Somya Sadaf, and Muthu Kumar Sampath

Abstract With population expansion, the shortage of conventional fuels and contamination of water resources has become a global issue. As a result, it necessitates the development of sustainable alternative fuels as well as the treatment of water through numerous methods. Limited energy supplies and significant growth in energy consumption have led to the exploration of biodiesel in recent decades. Microalgae have the potential to be a sustainable feedstock for generating biodiesel since they can grow in a variety of environmental situations. The most expensive and technically difficult stage in the production of microalgal biofuel is their mass cultivation. Rapid industrialization all over the globe is producing several types of wastewaters and the treatment of industrial wastewater is critical for protecting downstream populations from health hazards. Wastewater management and treatment are expensive procedures. As a result, it is critical to employ appropriate technology to make it feasible and cost-effective. Wastewater treatment coupling with microalgal technology has the potential to be a worldwide solution and may serve as a low-cost feedstock for bioenergy production. Therefore, using this technology can help in treating the wastewater and producing biodiesel efficiently. Potential advancements in microalgal culture, along with wastewater treatment in open and closed systems, have resulted in an increased algal biomass output. However, significant researches are still required for the improvement and optimization of a combined system that can create biomass and treat wastewater at the same time. The systematic overview of the technologies necessary for the effective incorporation for treatment of waste water with microalgal culture for production of biodiesel will be discussed in this chapter. This chapter will also examine several microalgal growth systems, as well as their key qualities and limitations. Keywords Wastewater · Microalgae · Biodiesel production · Bioenergy

A. Anuradha · A. Singh · S. Sadaf · M. K. Sampath (B) Department of Bioengineering and Biotechnology, Birla Institute of Technology Mesra, Ranchi 835215, Jharkhand, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 M. P. Shah (ed.), Biorefinery for Water and Wastewater Treatment, https://doi.org/10.1007/978-3-031-20822-5_22

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1 Introduction The increased usage of fossil fuels has resulted in energy depletion and significant greenhouse gas emissions into the environment. To mitigate the effects of global climate change and evade a new energy crisis scientists are concentrating their efforts on developing a new renewable energy source such as biofuels. However, since the 1970s, there has been widespread public concern about the long-term viability, price volatility, and negative environmental impact of crude oil. Thus, in recent decades, biooil-based biodiesel has arisen as alternatives to crude oils and crude oil-based Petro diesel fuels respectively. As a result, the development of algal biodiesel fuels as the third-generation fuels has piqued the public interest. However, it is vital to cut the total cost of biodiesel manufacturing by using wastewater instead of fresh water to lower feedstock costs. Microalgal biomass for biofuel generation holds a lot of promise in terms of replacing fossil fuels. Microalgae have excellent features for use as a biodiesel feedstock including greater lipid content, growth rate and cost-effective cultivation in wastewater. This chapter critically examined recent advancements, prospects, and challenges in the wastewater treatment and microalgae bio-industry. Biofuel manufacturing is not a novel phenomenon but due to increased demand and the rising cost of fossil fuels it is currently being thoroughly researched. Numerous commercialization challenges, such as a lack of energy and expensive methods for growing algae and harvesting with significant amounts of nutrients required, such as phosphorous (P) and nitrogen (N) (Mahapatra et al. 2009). Membrane integrated systems are also developing as viable options for producing and recovering biofuels such as biodiesel, biohydrogen, bioethanol, biogas and other value-added products in addition to microalgae culturing and harvesting (Maaz et al. 2019; Aslam et al. 2018). Treatment of waste water coupled with microalgae cultivation has piqued interest as a cost-effective and ecologically friendly method of producing microalgae-based biofuels. Algae, for example, consume nutrients which can be “sourced” from the wastewater, resulting in bioremediation and also reduces the cost of treatment (Clarens et al. 2010). Carbon capture and sequestration would be more efficient if this was combined with CO2 emission facilities the biomass created by this method can be used to make biofuel such biodiesels and other byproducts illustrated in Fig. 1. Microalgae cultivation in wastewater is a well-established technique during cultivation microalgae produce oxygen via photosynthesis. The oxygen produced in waste water can be used to break down the organic and inorganic chemicals. Microalgae can also recover the resources from wastewater, which could be useful in bio-refineries (Green et al. 1995). Cultivating microalgae in wastewater is a promising approach but it has a number of drawbacks including limited biomass yield, complex nutrient removal procedures, and contaminations in the biomass. Future research should focus on addressing these challenges in order to create a sustainable and cost-effective biorefinery. This chapter offers an overview of current movements and advances in microalgae-based wastewater biorefinery. The feasibility of combining treatment of wastewater with microalgae farming and biofuel creation is examined.

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Fig. 1 Production of biodiesel with microalgal cultivation

2 Wastewater Characteristics The physical chemical and biological changes involved are more important for wastewater management. Understanding this will help in development of proper technologies for the treatment. The asset and characteristics of the wastewater are the first and leading data collected for the design of any wastewater treatment system. The strength of wastewater is based on pollution load, which is the usual method of how it is expressed which can be examined from the concentrations of estimated chemical, physical and biological contents of the wastewater (Muttamara 1996). As a result, it’s evident that wastewater features or quality are stated in terms of its physical, chemical, and biological properties using the metrics listed in Table 1. All the Characteristics of wastewater mentioned above hinge on the water quality used by the population, community, type of industries and treatment adopted by industries and their wastewater. There is a well-defined correlation among all the above parameters. For example, dissolved gases concentration and organisms present in wastewater and its relationship with temperature.

3 Water Sources Water that has been contaminated by human activity as a result of industrial, commercial, residential, or agricultural applications is referred to as wastewater. It could be a complicated mixture of common inorganic and organic compounds that were

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Table 1 Physical, chemical, and biological aspects that are important to consider Physical characteristics

Chemical characteristics

Biological characteristics

Colour

Organic constituents (fats and oils, carbohydrates, pesticides, proteins, phenols etc.)

Animals

Odour

Inorganic constituents (heavy metals, nitrogen, alkalinity, chlorides pH, sulphur, phosphorus, and toxic compounds)

Plants

Solids (suspended, total and volatile, dissolved and fixed or mineral solids)

Dissolved gases (hydrogen sulphide, oxygen)

Protista

Temperature

Viruses

discharged into the environment and creating pollution. Various sort of trash is generated depending on social cognition and the types of industries that exist. Type of sewer system technology used also have an important effect on the final configuration of wastewater. Different wastes are frequently discharged into separate sewer systems in some developed countries but in majority of countries it is discharged into a joint sewer system. Wastewater is classified based on its source such as the dairy industry waste, municipal or household waste and pharmaceutical waste illustrated in Fig. 2. The textile sector is one of the world’s fastest expanding and largest industries with yearly sales of over 1 trillion dollars and up to 7% of worldwide export (Lellis et al. 2019). The wide variety of synthetic dyes used to colour garments produces massive amounts of coloured effluent. Textile industries use more water than other

Fig. 2 Different sources of waste water

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sectors leading to highly contaminated effluent discharge. The amount of water used and the types of wastewaters produced are also affected by the stages involved in fabric processing. Alkalis, acids, dyes, metals, soap, and surfactants make up the majority of textile industrial effluent (Holkar et al. 2016). However, municipal wastewater is generated by domestic actions. Biological oxygen demand (BOD), total dissolved solids (TDS), Chemical oxygen demand (COD), pH, total suspended solids (TSS), phosphate, metals, total nitrogen, potassium and total microbial load are all used to identify the characteristics of wastewater. Untreated wastewater can damage water and land that creates human health problems and eutrophication of aquatic bodies. Wastewater management is a major concern in metropolitan areas. As a result, wastewater treatment is necessary to ensure that it is safe before it is reused for various purpose (Hwang et al. 2014). Since the previous few years, the global pharmaceutical business has been quickly expanding, and contributing to great economic growth while also creating serious environmental damage. Biological or chemical methods are commonly used in pharmaceutical technology. Microbial fermentation, organic material extraction, and the addition of antibiotics, vitamins, and amino acids are all procedures used in the biopharmaceutical industry. Chemical-based pharmaceutical manufacture employs a variety of chemical processes which produce wastewater with such a high salt concentration and low nutrient value which is not biodegradable. Wastewater from various pharmaceutical enterprises is not similar and the composition of each type of wastewater is affected by the procedure used (Guo et al. 2017). Because of its toxicity, genotoxicity, and mutagenic character, pharmaceutical wastewater has negative consequences on human health and the environment. Pharmaceutical businesses are clearly addressing these issues by adopting a variety of tactics to treat wastewater. As the global demand for milk and milk products rises, so does the amount of wastewater generate by dairy processes. According to the International Dairy Federation’s World Dairy Report 2016, global milk output was predicted to be around 818 million tonnes (Koca 2018). Nutrients, COD, chlorides, TSS, lactose, lipids, BOD, sulphates, inorganic, and organic components are all present in wastewater released by dairy processing plants. Depending on the milk product type the effluent composition also varies (Davarnejad et al. 2017). Dairy effluent also has a low alkalinity, which necessitates the use of extra chemicals during fermentation to keep the pH stable. Direct discharge of untreated dairy effluents into the water bodies is not suggested. Hazardous volatile compounds, resulting in the extinction of aquatic life and environmental degradation since it can cause issues such rapid depletion of dissolved oxygen and the creation of hazardous chemicals which can lead to the extinction of aquatic life and environmental damage.

4 Wastewater Treatment In order to have a clean environment, wastewater treatment must be prioritized. To develop and manage efficient treatment procedures it is critical to first characterize the

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wastewater. Elimination of solid material, organic debris, and nutrients from wastewater is typically accomplished by a number of stages that include physical, chemical, and biological treatments. Such consecutive steps are sometimes referred to as preliminary, primary, secondary, and tertiary treatment. As a last treatment step disinfection was frequently performed on severely contaminated wastewater. Wastewater treatment seeks to lower BOD and COD levels as well as nutrient concentrations (mostly P and N) in order to reduce pollutants and improve water quality (Costa and Morais 2011). The treatment process begins with the collection of raw wastewaters which is then transported to a treatment plant and treated to a series of operations including preliminary, secondary, tertiary, and disinfection. Mineral elements namely dissolved nitrogen and phosphorus are becoming increasingly important in wastewater treatment frameworks. The primary treatment procedure is described as the physical settling of suspended particles. The main goal of primary wastewater treatment is to eliminate suspended materials and make the treated water a little clearer than before. The water is then delivered into sedimentation tanks which use gravity to eject the heavier settleable materials (this procedure evacuates between 50 and 75% of the total suspended solids). It is accomplished through the use of devices that slow down and disperse the effluent stream. By simply settling the suspended materials in a finely designed sedimentation tank BOD can be decreased by roughly 40%. The leftover solids in wastewater will be subjected to further treatment in the next phase (Costa and Morais 2011; Gupta and Bux 2019). However, secondary treatment primarily reduces BOD produced by a mixed population of heterotrophic bacteria utilized for their growth reducing organic substances present in wastewater. Microbes are given the appropriate ideal conditions to proliferate in order to achieve good treatment operations. Aeration tanks and sedimentation tanks are two techniques that can be used for secondary treatment. Then there are two processes in biological treatment that use bacteria to eliminate nitrogenous waste: nitrification and denitrification. With the help of autotrophic bacteria ammonia is converted to nitrite and then to nitrate in the nitrification phase. Nitrosomonas and Nitrobacter are two of the most well-known autotrophic bacteria species. Heterotrophic bacteria such as Flavobacterium and Pseudomonas reduce nitrate to gaseous nitrogen (N2 ) in the denitrification process. Settling tanks separate clean effluent water from biomass that has grown during treatment processes which is then pumped to drying tanks in the secondary treatment plant (Gupta and Bux 2019). Tertiary treatment also known as advanced treatment is described as method that comes after the primary and secondary stages have been completed. It aims to eliminate all ions while also using primary and secondary treatments to remove silt and oxidize organic compounds respectively. This pure water can be discharged into waterbodies. However, the presence of inorganic nitrogen and phosphorus in freshwater bodies such as lakes can promote eutrophication that can lead to harmful microalgal flora. Tertiary wastewater treatment is used when secondary treatment fails to eliminate a specific pollutant that needs to be separated. Tertiary treatment generally contains processes in addition to traditional biological treatment. The core

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resistant materials are nitrogen and phosphorus compounds that lead to an increase in planktonic algae in water. Tertiary treatment is based on technologically complex techniques that include several steps. Tertiary treatment is typically accomplished through the use of biological or chemical methods (Gupta and Bux 2019; Sawayama et al. 2000). Bioremediation is the biological means of tertiary treatment and it is defined as the process of using microorganisms to remove pollutants. The mechanism of bioremediation is based on the release of CO2 and N2 as a result of the oxidation of carbonaceous organic substances as well as reductions in nutrient content like phosphate and nitrogen. Even after three stages of treatment many microorganisms remains and thus, to prevent infection of pathogens these effluents are primarily treated for pathogen killing. The process of eliminating these pathogens is referred to as disinfection. There are numerous methods for killing pathogens both chemical and physical. After tertiary treatment disinfection step will be administered to the effluent. Chlorination is a traditional chemical approach for pathogen destruction since chlorine is an excellent disinfectant. Chlorine was originally less expensive however, its associated side effects such as fish poisoning induced by the development of chlorinated hydrocarbons have become a severe concern (Abdel-Raouf et al. 2012). This reduces the need for chlorine and the use of ozone (ozonation) or ultraviolet radiation for pathogen killing is now widely promoted. As a result, the use of ozone and ultraviolet radiation for pathogen killing is successful while being non-toxic to the environment (Gupta and Bux 2019).

5 Wastewater Treatment and Algal Biofuels Around the world wastewater treatment is becoming increasingly important. The use of algae for wastewater treatment was first investigated in California in the 1950s. Algae was used as a micro-aerator to provide oxygen to microorganisms while also digesting organic waste in wastewater. Bacteria produce CO2 and other nutrients that microalgae require for photosynthesis (such as P and N) (Sawayama et al. 2000). The symbiotic system effortlessly absorbs nutrients from the system. Algal ponds were initially intended to sieve secondary effluent before dumping it to prevent eutrophication (Park et al. 2011). Algae may be able to extract nutrients like N and K from sewage more effectively than current treatment methods. Water treatment, algal cultivation, and biofuel generation all have a lot of potential with the sewage process. Aquatic Species Programs established that the concept of algae growth in wastewater for biodiesel generation is cost-effective when compared to petroleum-based diesel in a classified study supported by the US Department of Energy in 1978 (Lau et al. 1995). This paper clearly demonstrates that algal-biofuel generation is economically feasible when wastewater treatment and cultivation are integrated. Photoautotrophic microalgae had a high biomass growth rate but heterotrophic microalgae used organic effluent to boost their growth rate and yield more biomass and lipids. Wastewater contains both inorganic and organic carbon sources allowing microalgae

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to convert carbon in both heterotrophic and mixotrophic ways. These microalgae production strategies have several advantages over photoautotrophic mode including faster growth and productivity (Min et al. 2011), lower light (Javed et al. 2019), and lower contamination rate (Zhou et al. 2012).

6 Microalgae Cultivation Microalgae are prokaryotic photosynthetic microorganisms that can fix atmospheric CO2 by using organic carbon from wastewater and receive light from the sun in their natural environment (Javed et al. 2019). To nurture microalgae in an artificial environment the resource input should be similar to that of the natural environment. The most significant impediment to commercialized algae production is a lack of sunlight. To reduce this factor an artificial source of light such as fluorescent light is used in the algae cultivation. However, artificial light sources derived from petroleum energy undermine the primary goal of developing a cost-effective method while also increasing the carbon emissions (Borowitzka 1997). There are various culture systems available to conduct microalgal production on a large scale while also treating wastewater such as photobioreactors, oxidation ponds, polybags, open raceway ponds and vertical reactors. Open ponds have been discovered to be a cost-effective method for nutrient removal from wastewater via algae cultivation. Despite the fact that photobioreactors have a high level of productivity, remediation cannot be motivated in photobioreactors due to the cost associated with it. The cost-effectiveness of microalgal-based biomass production as a subsequent form of energy is an essential factor.

6.1 Photoautotrophic Open Cultivation System Phototrophic cultivation is the most cost-effective method of microalgae cultivation. Phototrophic cultivation can be done at the lab scale in both open ponds and closed bioreactors (Borowitzka 1997). Although open pond systems are more advantageous because they are less expensive than photobioreactors they only cultivate a limited number of microalgae species. HRAPs, also known as open raceway ponds were first developed in the late 1950s (Chisti 2007). HRAPs were simplistic, about 30– 60 cm deep, and shaped like a raceway, with a large paddle wheel to generate a flow of water and gentle mixing. HRAPs are shallow and provide maximum light penetration for increasing algal growth. It has a short hydraulic retention time (HRT) ranging from 4 to 10 days depending on the season and needed area. Continuous mixing is bound to maintain cells from settling and to allow sufficient light penetration. HRAPs are the most cost-effective system for wastewater treatment and for efficient solar capture. Open culture methods have been shown to be a cost-effective and longterm cultivation method. These need less energy and are simple to maintain and

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clean. Hence, have the potential to deliver a high net energy production. Open pond systems on the other hand have several drawbacks, including water loss, inadequate light use, and a huge necessary space (Borowitzka 1997; Chisti 2007). Furthermore, the technology limitations include limited types of algae production, impure culture development, poor mixing, and low biomass yield.

6.2 Photoautotrophic Closed Cultivation System Photobioreactors are generally closed containers used to grow phototrophic microalgae in which energy is supplied by an artificial light source. They provide uniform dispersion and efficient light use leading in high gas mass transfer (O2 and CO2 ). Closed systems typically comprise of four phases: liquid, gas, solid and light. Microalgae are in the solid phase, growth media are in the liquid phase, CO2 and O2 are in the gaseous phase and light is in the visible phase, Tubular, flat plate and column photobioreactors are examples of closed systems. A closed system with a high transparent surface and a low number of lit parts would be optimal (Grobbelaar 2007). For cultivating a specific species in a controlled environment closed systems are preferred. Closed systems were created using flat-plate reactors. They have a large surface area and microalgae cell densities of more than 80 g/L (Grobbelaar 2007). When compared to tubular bioreactors, they are made of a transparent material and have a high solar energy capture rate. It’s difficult to scale up a tubular reactor. The only way to make a large tubular reactor is to tie smaller units together which creates operational and maintenance issues. Column bioreactors were presented as a solution to these issues. These offer a high volumetric mass transfer, adjustable growing conditions, a small footprint, and are simple to use (Molina et al. 2001).

6.3 Photoautotrophic Closed Cultivation System Microalgae growth in photobioreactors and open ponds is combined in hybrid farming. Photobioreactors are employed in the first step where regulated conditions are supplied to reduce the risk of contamination and promote cell division. Cells are exposed to nutrients in the second stage which aid in increasing lipid output (Slegers et al. 2011). The second step should ideally be an open pond, where environmental factors support microalgae production.

6.4 Heterotrophic Cultivation System Heterotrophic culture can successfully create algal biomass. In this technique algae grows on carbon substrates. The growth of algae is independent of light energy that

490 Table 2 Heterotrophic microalgae species and their products (Javed et al. 2019)

A. Anuradha et al. Species

Culture

Product

Chlorella protothecoides

Batch

Biodiesel

Crypthecodinim cohnii

Batch

Docosahexaenoic acid

Chlorella protothecoides

Batch

Biodiesel

Chlorella protothecoides

Batch

Biodiesel

Chlorella protothecoides

Batch

Biodiesel

Chlorella protothecoides

Batch

Biodiesel

Chlorella protothecoides

Batch

Biodiesel

Crypthecodinim cohnii

Batch

Docosahexaenoic acid

makes the process simpler and easier to scale up. Because of the increased cell density algae have faster growth rate which lowers the harvesting costs (Eriksen 2008). The cost of installation is also low. Many other investigations have found that heterotrophic plants produce more than photoautotrophic plants (Table 2).

7 Advantages of Microalgae as a Biofuel Source Microalgae are photosynthetic prokaryotic or eukaryotic microorganisms found throughout nature that can form a chain or colony ranging in size from a few micrometers to a few hundred micrometers. Algae are a broad grouping that lacks a classification (Samson and Leduy 1985). They are primitive plants (thallophytes), meaning generally they don’t have roots, leaves or stems and use chlorophyll as their principal photosynthetic pigment (Hu et al. 1998). They can be divided into two types: macroalgae, which are multicellular and microalgae which are microscopic creatures ranging from 0.2 to 100 m (Samson and Leduy 1985). Microalgae have a number of intriguing characteristics that make them attractive for biofuel generation. Atmospheric carbon dioxide is the main carbon source for microalgae development. Numerous microalgae species can grow in nonportable water (seawater, wastewater, and brackish water). Production of biofuels may be coupled with one of these systems in the future. This combination does not compete for arable area that can be utilized for agriculture, and it also does not deplete freshwater supplies. Biofuel production from algae can be combined with CO2 mitigation from flue gas, wastewater treatment, and the manufacture of high-value compounds (Richmond et al. 2003). Many microalgae species produce considerable amounts of lipids, which can be turned to biodiesel via the transesterification process. Microalgal biodiesel possesses density, flash point, viscosity, cold flow, and calorific value similar to petroleum-based diesel. Microalgae may be collected in batches almost all year, ensuring a consistent and reliable supply of oil (Samson and Leduy 1985; Hu et al. 1998). Unlike terrestrial plants which require herbicides or pesticides microalgae do not require the use of chemicals which are harmful to the environment and increase the cost of manufacturing.

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8 Wastewater Microalgal Farming for Biofuel Production Wastewater is produced from the discarded materials of home, agricultural, municipal and industrial sources (Samson and Leduy 1985). The composition of wastewater is a reflection of the producing society’s lifestyles and technologies (Cheng et al. 2020). Organic matter such as lipids, carbohydrates, proteins and volatile acids are found in waste water, as well as inorganic matter such as calcium, potassium, magnesium, sodium, chlorine, Sulphur, phosphate, ammonium salts, bicarbonate and heavy metals (Richmond et al. 2003). Eutrophication or algal blooms are caused by an excess of these nutrient loads in nearby water bodies, which is commonly caused by anthropogenic waste production (Lang et al. 2001). Researchers from all over the world have worked hard to investigate the feasibility of employing microalgae for biofuel production from wastewater with nutrient removal properties particularly removal of phosphorous and nitrogen from effluents. In comparison to industrial wastewater researchers focused more on microalgal culture for N and P removal from home sewage (Richmond et al. 2003; Lang et al. 2001). The commercialization potential of algae biofuels is heavily influenced by species selection, growth optimization, lipid content, and large-scale harvesting. The algal biomass produced and harvested by these wastewater treatment systems might be converted to biofuels in a number of ways including anaerobic digestion to biogas, lipid transesterification to biodiesel, carbohydrate fermentation to bioethanol, and high temperature modification to biocrude oil.

9 Biodiesel The term “biodiesel” denotes to any diesel-equivalent biofuel produced from biological feedstock that further converted into fuel by utilizing unique methods. Biodiesel has gained a lot of attention from people all over the world. Biodiesel is made by trans esterifying oxoalkyl esters of fatty acids with or without a catalyst. The method can be used to make biodiesel from oil derived from a variety of renewable sources including oil crops and microalgae. The scarcity of fossil fuels has compelled people to look into renewable energy sources that could give an alternative to fossil fuels. Biodiesel feedstocks are divided into three generations. Rapeseed (Çelikten et al. 2010), sunflower, palm oil and soyabean (Rattanaphra and Srinophakun 2010) are considered first-generation biodiesel feedstocks because they were the first crops explored for biodiesel manufacturing. Because food oils make up roughly 95% of this biodiesel feedstock generation, relying on first-generation biodiesel feedstocks has caused a slew of issues primarily due to global food markets and food safety. For example, soy and palm are food crops whose oils are crucial to human nutrition. Redirecting these food crops to produce biodiesel on a wide scale may result in

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food market imbalance. The demand for arable land to create biodiesel is a significant disadvantage of biodiesel production from edible oil. Second-generation feedstock, which uses non-edible biomasses for biodiesel synthesis is concentrated to overcome the problems associated with edible oil (Brennan and Owende 2010). As second-generation biodiesel feedstocks, energy crop wastes such as wheat straw stalks, corn stover, rice straw, and rice husks were utilized. Furthermore, secondgeneration feedstocks include restaurant grease and animal fat. Despite the fact that second-generation feedstocks have little impact on human food and can be grown on non-arable soil they are insufficient to provide much of the total transportation energy. As previously said, biodiesel produced from these terrestrial crops lacks the potential to be a long-term replacement leading to the development of biodiesel from microalgae. Biodiesel is planned to replace petroleum-based transportation fuels, and output is likely to increase. Because of the high cost of the oils, biodiesel remains a key barrier for large-scale commercial usage (Brennan and Owende 2010). Third-generation feedstocks, primarily generated from algae are discussed as a key alternative source of oil for biodiesel synthesis and they have various advantages over lipid for biodiesel production. Fatty acid alkyl esters (FAAEs) are produced by trans esterifying oils with alcohols in the presence of catalysts to produce biodiesels. In terms of wastewater treatment, nutrient recovery, biomass growth, and lipid storage, microalgae have a lot of promise. Microalgae farmed on a small scale under regulated conditions may produce over 760 tons ha−1 year−1 biodiesel compared to oil seeds yielding just 0.4 tons ha−1 year−1 for soybean and 0.7 tons ha−1 year−1 for canola oil (Mehrabadi et al. 2016). Apart from light intensity, season and wastewater nutrient concentration, microalgal species have different nutrient recovery, lipid accumulation capacities and growth. Researchers evaluated the year-round biodiesel production potential of wastewater treatment plants and found variance in biomass (2.0–11.1 g VSS m−2 d−1 ) and lipids (0.5–2.6 gm2 d−1 ), respectively (Liu et al. 2020). Algal growth and lipid buildup are both influenced by light intensity. Scenedesmus obliquus and Spirulina platensis were studied in wastewater and it was discovered that S. obliquus needed high light intensity and daily illumination duration for optimal growth and lipid synthesis (Su et al. 2011). Wastewater from the food and beverage industry is thought to be better suited for microalgal culture due to its low concentration of heavy metals and other harmful substances. SPW is high in saccharides, proteins, organic acids, lipids, calcium, iron, phosphate, and other nutrients, and it may be used as a good medium for microalgal cultivation without harming the microalgae. Hongyang et al. cultured Chlorella pyrenoidosa in SPW without adding any fertilizers and were able to remove TN (89.1%), COD (77.8%), and TP (70.3%) in 120 h with biomass and lipid productivity of 0.64 g L−1 h−1 and 0.40 g L−1 h−1 , respectively (Choudhary et al. 2020). Livestock waste (dairy waste) is high in phosphorus and nitrogen, and algal culture in this medium resulted in an increased carbohydrate content of (24.8– 27.7%) biomass while decreasing lipid content to 45–34% (Su et al. 2011). Some contaminants give wastewater color while also inhibiting algal growth by reducing light intensity. Cheng et al. pretreated swine wastewater with TiO2 photocatalysts and evaluated the cultivation of two algae Synechocystis sp. and Tribonema sp. found

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that pretreated wastewater had a greater nutrient removal efficiency than nontreated wastewater. They found that these species lipid buildup increased by 40–42.4% and 23.9–26.3%, respectively (Cheng et al. 2020). Integrating wastewater treatment with microalgae cultivation for bioenergy production lowers wastewater treatment costs while improving energy output.

10 Potentials, Challenges, and Future Prospects Microalgae are able to acquire these nutrients and grow in wastewater without the need for extra nutrients because wastewater is high in organic content. During the pretreatment process microalgae collect CO2 and produce oxygen which aids aerobic microbe growth and decreases the requirement of energy for mechanical mixing, resulting in higher organic load removal (Khazraee Zamanpour et al. 2017). Sludge management is important because treatment of waste water requires the addition of numerous chemicals which results in formation of sludge which is released into the environment. Whereas, wastewater treatment by using microalgae does not require the addition of chemicals and the resulting sludge is primarily made up of algal biomass. Microalgae could aid in greenhouse gas assimilation in addition to resource recovery and treatment of waste water from waste. Microalgae can fix CO2 10–15 times better than land plants, requiring 183 tons of CO2 to produce 100 tons of algal biomass. However, there are numerous obstacles to overcome in order to develop a more effective algal technology. Microalgal growth is affected and inhibited by wastewater contamination (protozoa, bacteria and fungi). Pretreatment is a necessary step that can be accomplished on a small scale with filtration and autoclaving but is not practicable at a commercial size. Various types of wastewaters have varied structures that may affect the microalgae proliferation. As a result, it is critical to recognize and choose appropriate microalgae species that are more durable resistant to a variety of environmental variables and able to handle high nutrient loads as well as accomplish the desired outcome. Generally, microalgae grow on the surface of the water and a dense culture constrains light from accessing the water that further restricts the growth of microalgae. New bioreactors must be designed immediately in order to improve reactor efficiency. Turbidity and suspended particles in wastewater also limit light penetration limiting microalgae growth and productivity. Algal growth is influenced by temperature and light availability. As a result, utilizing this technology in high-altitude areas with colder winters and shorter daylight hours is ineffective. The recovery of algal biomass from wastewater is also a challenge. Despite the introduction of numerous technologies, the harvesting procedure remains costly. To overcome all of these difficulties additional procedures are required, increasing expenses; thus, it is vital to develop technology that is more efficient and cost-effective. Despite these obstacles, the algal-based wastewater treatment system still has a lot of advantages. Microalgae are nutrient-dense (lipids, proteins and carbohydrates) and quickly digested making them a good aquaculture feed. With all of these rewards of mixing

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algal biomass with treatment of waste water and combining it with other innovative technology can increase the income and improves the overall economics of the wastewater treatment process.

11 Conclusion This review discusses the importance of using a combined method to promoting the long-term viability of microalgae bio-refinery. Microalgae are believed to have great properties for cleaning wastewater, generating power, and recovering valueadded bio-products simultaneously. These have the potential to replace conventional fuel sources. Hence, advancements to the entire microalgae bioprocessing chain are required to ensure their long-term viability. In this case, microalgae can thrive by absorbing the nutrients in wastewater, effectively eliminating the nutrient cost of biomass production. As a result, microalgae could be a great way to achieve the dual aims of waste management and energy production and the next step is to find the right conditions to make it more relevant and economically viable. Instead of depending on traditional microalgae cultivation the microalgae bio-refinery should be oriented toward resource recovery and value-added products.

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