Microplastics Pollution in Aquatic Media: Occurrence, Detection, and Removal (Environmental Footprints and Eco-design of Products and Processes) 9811684391, 9789811684395

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Environmental Footprints and Eco-design of Products and Processes

Mika Sillanpää Ali Khadir Subramanian Senthilkannan Muthu   Editors

Microplastics Pollution in Aquatic Media Occurrence, Detection, and Removal

Environmental Footprints and Eco-design of Products and Processes Series Editor Subramanian Senthilkannan Muthu, Head of Sustainability - SgT Group and API, Hong Kong, Kowloon, Hong Kong

Indexed by Scopus This series aims to broadly cover all the aspects related to environmental assessment of products, development of environmental and ecological indicators and eco-design of various products and processes. Below are the areas fall under the aims and scope of this series, but not limited to: Environmental Life Cycle Assessment; Social Life Cycle Assessment; Organizational and Product Carbon Footprints; Ecological, Energy and Water Footprints; Life cycle costing; Environmental and sustainable indicators; Environmental impact assessment methods and tools; Eco-design (sustainable design) aspects and tools; Biodegradation studies; Recycling; Solid waste management; Environmental and social audits; Green Purchasing and tools; Product environmental footprints; Environmental management standards and regulations; Eco-labels; Green Claims and green washing; Assessment of sustainability aspects.

More information about this series at https://link.springer.com/bookseries/13340

Mika Sillanpää · Ali Khadir · Subramanian Senthilkannan Muthu Editors

Microplastics Pollution in Aquatic Media Occurrence, Detection, and Removal

Editors Mika Sillanpää Aarhus University Aarhus, Denmark

Ali Khadir Islamic Azad University of Shahre Rey Branch Tehran, Iran

Subramanian Senthilkannan Muthu SgT Group and API Hong Kong, Kowloon, Hong Kong

ISSN 2345-7651 ISSN 2345-766X (electronic) Environmental Footprints and Eco-design of Products and Processes ISBN 978-981-16-8439-5 ISBN 978-981-16-8440-1 (eBook) https://doi.org/10.1007/978-981-16-8440-1 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 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 Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Contents

Microplastics Occurrence in Different Regions Around the World . . . . . . Ajith Nithin, Arumugam Sundaramanickam, Amra Bratovcic, Parthasarathy Surya, and Manupoori Sathish

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Microplastics Pollution in Rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Amra Bratovcic, Ajith Nithin, and Arumugam Sundaramanickam

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Microplastics in Freshwater Environments and Drinking Water . . . . . . . Décio Semensatto, Geórgia Labuto, Fabiano Nascimento Pupim, and Marilia da Rocha Peloso

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The Role of Rivers in Microplastics Spread and Pollution . . . . . . . . . . . . . Yulianto Suteja and Anna Ida Sunaryo Purwiyanto

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Microplastics Pollution in Coastal Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Arthita Ray, Venkatalakshmi Jakka, Shubhalakshmi Sengupta, and Aniruddha Mukhopadhyay

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A Critical Review on Separation, Identification, Quantification and Removal of Microplastics in Environmental Samples: Developments and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 Dhanaraj Sangeetha, Ainala Shivani, Jogannagari Anusha, J. Ranjitha, and Vani Narayanan Pretreatment Methods for Further Analysis of Microplastics in Wastewater and Sludge Samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 C. Bretas Alvim, M. A. Bes-Piá, and J. A. Mendoza-Roca Microplastics Sampling and Recovery: Materials, Identification, Characterization Methods and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 P. Snega Priya, M. Kamaraj, J. Aravind, and P. Muthukumaran

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Contents

Ecotoxicity Assessment of Microplastics on Aquatic Life . . . . . . . . . . . . . . 177 Beatriz Pérez-Aragón, Juan Carlos Alvarez-Zeferino, Arely Areanely Cruz-Salas, Carolina Martínez-Salvador, and Alethia Vázquez-Morillas Occurrence, Fate and Removal of Microplastics in Wastewater Treatment Plants (WWTPs) and Drinking Water Treatment Plants (DWTPs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Dhruba Jyoti Sarkar, Soma Das Sarkar, and Basanta Kumar Das Microplastic Pollution in Water and Their Removal in Various Wastewater Treatment Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Ali Khadir, Mehrdad Negarestani, Asiyeh Kheradmand, and Mika Sillanpää An Overview of Physical, Chemical and Biological Methods for Removal of Microplastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Christian Ebere Enyoh, Oluniyi O. Fadare, Marcel Paredes, Qingyue Wang, Andrew Wirnkor Verla, Leila Shafea, and Tanzin Chowdhury Microplastics and Anaerobic Digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Flora N. Ezugworie, Godwin O. Aliyu, and Chukwudi O. Onwosi Role of Microplastics as Attachment Media for the Growth of Microorganisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Megha Ukil, Srinjoy Roy, Atun Roy Choudhury, and P. Sankar Ganesh Bioremediation Techniques for Microplastics Removal . . . . . . . . . . . . . . . . 327 Samaneh Hadian-Ghazvini, Fahimeh Hooriabad Saboor, and Leila Safaee Ardekani

About the Editors

Prof. Mika Sillanpää is engaged in synergistic collaboration with over 50 research partners from the world’s leading laboratories in six continents, and he is currently working at the Department of Biological and Chemical Engineering, Aarhus University, Nørrebrogade 44, 8000 Aarhus C, Denmark. He has received numerous awards for research and innovation. For example, he is the first Laureate of Scientific Committee on the Problems of the Environment (SCOPE)’s Young Investigator Award, which was delivered at the UNESCO Conference in Shanghai 2010 for his “significant contributions, outstanding achievements and research leadership in Environmental Technological Innovations”. Dr. Ali Khadir is an environmental engineer and a member of the Young Researcher and Elite Club, Islamic Azad University of Shahre Rey Branch, Tehran, Iran. He has published/prepared several articles and book chapters in reputed international publishers, including Elsevier, Springer, Taylor and Francis, and Wiley. His articles have been published in journals with IF of greater than 4, including Journal of Environmental Chemical Engineering and International Journal of Biological Macromolecules. He also has been the reviewer of journals and international conferences. His research interests center on emerging pollutants, dyes, and pharmaceuticals in aquatic media, advanced water and wastewater remediation techniques and technology. At present, he is editing other books in the field of nanocomposites, advanced materials, and the remediation of dye-containing wastewaters. Dr. Subramanian Senthilkannan Muthu currently works for SgT Group as Head of Sustainability and is based out of Hong Kong. He earned his Ph.D. from The Hong Kong Polytechnic University and is a renowned expert in the areas of Environmental Sustainability in Textiles and Clothing Supply Chain, Product Life Cycle Assessment (LCA) and Product Carbon Footprint Assessment (PCF) in various industrial sectors. He has five years of industrial experience in textile manufacturing, research and

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About the Editors

development and textile testing and over a decade’s of experience in Life Cycle Assessment (LCA), carbon and ecological footprints assessment of various consumer products. He has published more than 100 research publications, written numerous book chapters and authored/edited over 100 books in the areas of Carbon Footprint, Recycling, Environmental Assessment and Environmental Sustainability.

Microplastics Occurrence in Different Regions Around the World Ajith Nithin, Arumugam Sundaramanickam, Amra Bratovcic, Parthasarathy Surya, and Manupoori Sathish

Abstract Microplastics (MPs) have been detected in diverse aquatic environments, including oceans, seas, lakes, rivers, beaches, coastlines, and even in mangroves. A recent study has brought to light that almost 22% of the countries have carried out studies on MPs. From these it is evident that MPs have been observed in almost 44 countries globally. However it cannot be taken for granted that the remaining countries are pristine to MPs. More studies on MPs need to be carried out across all countries by employing uniform protocols right from collection to analyses and units of expression. The microplastics may be accumulated in all environments across different countries of the world. However, the quantum of MPs present globally remains under estimated since there is lacuna of information in many parts of the world. All countries in unison should minimize plastic discharges and find alternatives to reduce MP accumulation. Keywords Marine environments · Implications · Aquatic organisms · Table salts

1 Introduction The discovery of plastic in the early 1900s has made it the most revolutionary material invented by man which has currently become an unavoidable commodity in human lifestyle [98]. The mechanical properties of plastics such as low density, resistance to corrosion, conductivity, and economic benefits during manufacture have paved way toward plastic usage in a wide range of applications ranging from packaging to technology [40] which are equally utilized in domestic and industrial arenas [101]. The long-lasting and non-degradable nature of plastics have led to plastic pollution which is a major environmental concern [116]. Over the past few decades plastic A. Nithin · A. Sundaramanickam (B) · P. Surya · M. Sathish Centre of Advance Study in Marine Biology, Annamalai University, Parangipettai, Tamil Nadu, India A. Bratovcic Faculty of Technology, Department of Physical Chemistry and Electrochemistry, University of Tuzla, Urfeta Vejzagica 8, 75000 Tuzla, Bosnia and Herzegovina © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Sillanpää et al. (eds.), Microplastics Pollution in Aquatic Media, Environmental Footprints and Eco-design of Products and Processes, https://doi.org/10.1007/978-981-16-8440-1_1

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pollution in the coastal and marine environment has increased alarmingly due to waste mismanagement. If this mismanagement of plastic trash continues, by 2025 plastic rubble in the maritime environments may increase by several folds [52]. Several pathways transfer plastic debris from the land based environments to marine environments [61]. Rivers and lakes are essential carriers of plastic trash into the maritime environments since copious magnitudes of debris are dumped into these water bodies [24, 50]. Plastics accounted for 348 million tons in 2017 which is several folds superior to its initial production in the 1950s [83]. An annual estimate of about 12.7 million tons of plastic trash makes its way into the maritime environments [52]. A study pointed out that about 6300 million metric ton of plastic waste was generated in 2015 out of which only 9% was recycled while 79% was dumped into terrestrial and marine environments [44]. The longer persistence of plastic shreds in nature [107] paves the way for several natural factors to act upon them and carry out its degradation [7, 24]. The persistence of larger plastics invites biotic and/or abiotic activity [57] which causes their breakdown into minute plastics [7, 107]. These are called as microplastics (lesser than 5 mm in size) which exists as primary and secondary MPs [60, 69]. Primary MPs are manufactured in the particular size range and are commonly used in personal use commodities such as toothpastes and cosmetics [18]. The secondary MPs are the end product of disintegration of larger plastic materials by several factors such as ultraviolet radiation, mechanical abrasion, and microbial activity [57, 60, 107]. Fibers are unidirectional microplastic shapes which are soft, tough, and nondeformable [123] which arises from washing clothes [19], fragmentation of vehicle tires [48]. Fragments are bi- or tri-directional microplastics which tend to lose buoyancy after six weeks of floatation hence settle in the sediments [37] which are irregularly shaped with uniform thickness and hardness [123]. Films are bi-directional microplastics [123] which originate from disintegration of plastic covers and bags [75], packaging materials and plastic containers [86]. These are also brought in by discharges from agricultural fields [43, 119]. Pellets are commonly primary microplastics which are cylindrical or disc shaped [77]. They may originate from oceans and terrestrial sources [46] arising out of accidental spillage from ships as well as sewage input from inland processors [23]. These are the most common microplastic shapes reported globally Fig. 1. The color of microplastics is an important criteria to be studied since most aquatic organisms are visual predators [73, 125]. The observation of color may provide information on the origin of microplastics [73] hence different colors indicate different origin [92]. Colored microplastics occur as resultant of washing of clothing and packaging materials while the wear and tear of fishing lines and nets yields colorless microplastics [32]. Some colors such as blue and red lose color over time thus becoming colorless [32]. The most commonly referred upper size range for microplastics is 25 mm), meso- (25–5 mm), micro- (5 mm– 0.1 μm), and nano-plastics ( 75 μm **Grab bucket (B-10104, Ravenep)

St. Lawrence River

North America

**Canola oil extraction *100 μm nylon mesh

Brisbane River

Australia **Ponar stainless-steel grab sampler

832 (±150 SE) plastics /kg (dw)

PVC and PP [24] fragments, PE and nylon fibers, PE microbeads

**0.18 to PE, PA, PP and 129.20 mg kg−1 or PET **10 to 520 items kg−1

[38]

(continued)

Microplastics Pollution in Rivers

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Table 1 (continued) River

Country

Sampling methodology

Average concentration of MPs

Netravathi River

India

*Sieve (5 mm *288 pieces/m3 and 0.3 mm) **96 pieces/kg **Stainless-steel spoon

Type of References polymer/Chemical composition PE and PET, fibers, films and fragments

[3]

* Water phase ** Sediment phase

eastern and northern Germany before it mixed into the North Sea. The Elbe has a total catchment area of 148,268 km2 with 96,932 km2 of German state territory. They have found that MPs concentrations were 600,000 times higher in the sediments (mean: 3.35 × 106 pm−3 ) when compared with water (mean: 5.57 p m−3 ). These findings evidenced that sediments can serve as a retention area for contaminants and toxic components [49, 83], which may interact with MPs and increases their potential risk of bioaccumulation [69]. Sekudewicz et al. studied MP pollution in surface water and sediments in the Vistula River in the urban section of Warsaw (capital of Poland). The Vistula River is the second-largest river in the Baltic catchment in Poland. The collected water samples were treated through different sizes of sieves (5, 2, 0.63, 0.30, 0.10, and 0.05 mm), and particles higher than 5 mm were not considered as MPs. To remove organic matter from water samples the formaldehyde and 30% hydrogen peroxide were used, while sediment samples were treated with 1.2 g mL−1 of NaCl solution. After that, both samples, water and sediment, were filtered through the glass fiber filters (Munktell MG/A, diameter: 47 mm, pore size: 1.6 μm). In the water samples the concentration of MPs was ranged from 1.6 items L−1 to 2.55 items L−1 , whereas, in the sediments, it ranged from 190 items kg−1 to 580 items kg−1 . Fibers were the main type of MPs in both examined samples water and sediment composed from PS, PP, and other materials. The MPs concentration in the surface water of the Wei River which is located in China ranged from 3.67 to 10.7 Nos/L, and in the sediments, it was ranged from 360 to 1320 Nos/kg [26]. In another study, also done in China, Hu et al. [42] investigated 15 surface water samples and 15 sediment samples collected from Dongting Lake and its four affiliated rivers (Xiangjiang River, Zishui River, Yuanjiang River, and Lishui River). The concentration of MPs found in plankton net samples was in range of 0.62–4.31 items/m2 and 21–52 items/100 g dry weight (dw) in sediments. The highest concentration of microplastics (4.31 items/m2 ) was detected in the area of west Dongting Lake with many factories and plants. In both water and sediment, fibers were dominant MPs fraction with size 0.9–0.333 mm in surface water and 50% (917.28 ± 1011.90 μm), followed by the fibers ranging from 13.25% to 37.80% (1193.52 ± 1462.66 μm), films (606.62 ± 368.40 μm), and pellets less than 10% (279.50 ± 208.97 μm). It was found that the concentration of MPs in the main body of the Qing River was higher in November (0.26 ± 0.20 particles L−1 ) than in July (0.17 ± 0.11 particles L−1 ), while in the effluent outfalls were similar in both months. These results indicate that the concentration of MPs in the river might be diluted by surface runoff during the high-flow period (July) compared with the normal-flow period (November). The concentration of MPs found in the Qing River is lower comparing to the other urban rivers such as Yongjiang River (2345 ± 1858 particlesm−3 ) [87] and in the Suzhou River (7.4 particles L−1 ) in Shanghai City [57] but higher than the concentration in the North Shore Channel in Chicago (1.94 ± 0.81 to 17.93 ± 11.05 particles m−3 ) [61] and in the River Seine in Greater Paris (4–108 particles m−3 ) [28]. Pan and co-workers [62] studied MPs pollution in the Zhangjiang River of southeastern China. They used a bulk sampling method for the collection of water samples. The concentration of MPs in water samples were ranged from 50 to 725 items m−3

Microplastics Pollution in Rivers

33

with an average of 246 items m−3 . PP and PE were the major polymers (~75%) in size of fragments (0.5–1 mm). Zhou and co-workers [89] investigated MPs distribution in urban water of seven cities in the Tuojiang River basin in southwest China and found that the concentrations varied from 911.57 ± 199.73 to 3395.27 ± 707.22 items m−3 . The highest MPs concentration was found in Ziyang urban water. The main polymer type of MPs found was PP in shape of fibers (34.88–65.85%) of small sizes (0.5–1 mm). They reveal high impacts of the secondary industries on the MPs pollution, and the results showed that Fushun sites had the highest risk in regard to MPs. Huang and co-workers [41] studied MPs pollution in the downstream area of West River which is the longest river and the main stream of the Pearl River system, long 2,214 km and with a catchment area of 353,120 km2 in China. West River is surrounded by industries and is densely populated. Water and sediment samples were collected in summer 2019. Surface water samples were filtered using a stainless steel screen of 75 μm, while sediment samples were collected by the grab bucket (B-10104, Ravenep). They found the concentration of MPs in range of 2560–10,240 items/kg in sediment and 2.99 to 9.87 items/L in surface water, respectively. In both water and sediment, fibers were dominant MPs with size 20 µm [53]. As described in the study, the method is effective for MPs >20 µm while less effective for MPs 1 cm) which can be taken out utilizing a straightforward and, thusly, more financially savvy filtration measure [27]. This method was recently applied for the removal of PE and PP from aqueous solution by Sturm et al. [52] and they reported a mean removal rate of 95%. The removal rates were generally influenced by the precursor used and for optimal removal, the gel formation must be critically monitored. According to Sturm et al. [52] the formation of gel should occur at the time when it is possible for large agglomerates to be formed by the MP particles and fixed in the course of gel formation. Therefore, if the formation of gel occurs too fast, the agglomeration won’t be possible and if it is too slow then the MP particles fixing cannot be possible.

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Fig. 1 Example of formed agglomerate by MPs under optical coherence tomography (Adapted from [52]). In the image, the MPs are PP (left) and PE (right)

2.3 Biological Methods 2.3.1

Reverse Osmosis (RO)

Disinfection is one of the key processes for the separation of emerging pollutants and within which reverse osmosis (RO) stands out, membrane technologies are a good option for the retention of micropollutants. RO is a process that removes contaminants from a liquid solution, usually water, by applying a certain pressure through one or more semi-permeable membranes that separate a contaminated solution from the clean or purified solution [9]. Compared to other physicochemical and biological technologies, membrane separation has great advantages such as efficiency, little chemical addition and ease of operation [9, 29]. This technique has been gradually accepted and spreading its uses in domestic water conditioning installations. RO membranes have been shown to have better performance not only in disinfection or desalination processes, but also in retention of micropollutants such as MPs [18]. This technique, applied to water, allows 95% of the salts to be separated and in wastewater, it allows eliminating color, dissolved solids, organic load, microorganisms and concentrating acids, bases and micropollutants such as MPs. The application of RO method in the removal of MPs from three major WWTPs in Sydney, Australia was demonstrated by Ziajahromi et al. [64]. After the RO process, the authors reported that the sampling device captured between 92 and 99% of PS MP while significant concentration of MP fibers remain in the water which was attributed to some membrane defects or simply small openings between pipework, indicating the necessity to ad hoc design the processes for MP removal [46, 64]. To improve the efficiency of MPs removal by RO process, a pretreatment process including screening and sedimentation, biological treatment, flocculation, disinfection/de-chlorination processes and ultrafiltration may be required. Another way is to use filters that can filter down to 0.001 micron, but these filters are more expensive and require maintenance [55].

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Dynamic Membrane (DM) Technology

Improved/advanced treatment technologies have been shown to effectively remove possible harmful compounds that conventional treatments could not remove [5]. Some of the advanced technologies are becoming the preferred alternative to the conventional process in wastewater treatment [5, 61, 63]. In the last two decades, the use of dynamic membrane (DM) technology in WWTPs for water and wastewater treatment has considerably grown because of the potential benefits it offers compared to the conventional microfiltration (MF) membrane, ultrafiltration (UF) membrane in membrane bioreactor (MBR) system [61]. The membrane formed in DM is often referred to as secondary membrane because it is formed as a secondary layer over the underlying support/primary membrane such as filter cloth, membrane, or mesh [22]. The secondary membrane is derived from the filtered solution that contains suspended solid particles like microbial cells and flocs and has the capability of trapping organic and colloidal particles that causes membrane fouling thus preventing fouling of the primary membrane [4, 22, 23]. In the removal of MPs from WWTPs, the efficiency of the DM technology and effect of influent concentration were studied by Li et al. [33]. Based on the reduction in the turbidity of the wastewater sample which was filtered through the DM technology under gravity compared to conventional method, the author suggested that the DM is more efficient. The DM formation process was strongly affected by the influent particle concentration. Higher influent particle concentrations resulted in more MPs being filtered through the supporting mesh, laying the foundation for the rapid formation of the DM layer and a faster effluent turbidity reduction. As a result, increasing flux and influent particle concentration can be used for the control of the DM formation process. Overall, Li et al. [33] demonstrated the capacity of DM in wastewater treatment to remove low-density and non-degradable MPs such as plastics that are not easily removed by the conventional process. Dynamic membranes have the following advantages over the conventional membranes [4, 22, 23, 63]. • Require flexible and comparatively cheap materials • It is easy to clean using water and/or air backwash, or via brushing without using chemical reagents • It has good antifouling properties • Relative reduction in the size of equipment required • Reduced energy requirement • Low capital cost (require low or no chemical usage) • The system is environmentally friendly and has easy accessibility. • Allow reuse of wastewater with less energy requirement. • Membrane can be formed and re-formed. Dynamic membranes have the following disadvantages over the conventional membranes [22, 23].

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• Repeated DM formation and removal could lead to reduction or loss of membrane permeability • Subject to the support material used, cleaning of the secondary membrane might lead to temporary loss of effluent quality. • Cleaning/removal of the foulants may be irreversible when deposited in the pores of the support material. • Membrane fouling affects the membrane performance of support material, thus hindering permeates flux. • Higher pressure is usually required to ensure permeate flux resulting in high energy consumption and more downtime. 2.3.3

Cross-Flow Membrane (CM) Technology

Cross-flow filtration is a vital technique used to separate undissolved solids from supernate slurries in many planned and operating wastewater treatment [5]. In a cross-flow membrane technology, filter cake formation is either limited or almost totally suppressed by a flow of the suspension parallel or tangential to the filtration surface as opposed to the conventional dead-end filtration system in which the suspension is made to flow perpendicularly to the membrane surface making the retained particles to accumulate on the membrane surface (Fig. 2) [1, 5]. Applying crossflow membrane in water and wastewater treatment involves pumping the suspension tangentially at high velocity over the membrane surface [5]. Due to the high pressure involved in the system, clear liquid permeates the filter and is recovered as permeate while the solids form a fouling layer or cake at the filtration barrier [1, 5]. In this system, the retentate (the fouling layer or cake) can be recirculated back to the feed flow [16]. Cross-flow membrane filtration is mostly used for the filtration of liquids that contain high solids content [5].

Fig. 2 Schematics of cross-flow membrane technology

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C. E. Enyoh et al.

The advantages of CM technology are as follows; (1) The use of cross-flow membrane in microfiltration and ultrafiltration has the following advantages over the conventional dead-end filtration [1, 5, 12]. (2) The robustness of the system enables it to run continuously for a long period before cleaning, making it more reliable. (3) The growth of cake flux is limited due to the flow of suspension parallel to the filtration. (4) The membranes have an extended lifetime because filter aids are not required and (5) It is efficient and energy-saving because particles that are deposited on the primary filter are removed/cleaned by the cross-flow velocity action. While the disadvantages include, (1) Cross-flow filtration has the following disadvantages over conventional dead-end filtration [1, 12]. (2) The system must be designed specifically for the task for the shear forces to which the fluid is exposed not to get damaged. (3) The system may require a big footprint for the pump and a big diameter for the piping.

2.3.4

Membrane Bioreactors (MBR) Technology

Membrane bioreactors technology is a modern and advanced wastewater treatment technology combining an improved solid/liquid separation process with conventional activated sludge (CAS) treatment process [4, 28]. It consists of a biological reactor with suspended biomass and solids removal by means of low-pressure membrane filtration (ultra- and microfiltration, UF or MF membranes, with pore sizes in the range 0.05–0.4 µm). The MBR process joins aeration, clarification and filtration into a single unit (Fig. 3), based on its working mechanism, therefore, the MBR solution results in a simplified process, with low space requirements and low visual impact. MBR can effectively produce a high-quality clarified effluent, due to the fact that the membrane pore size may be below 0.1 µm. Typical applications of the MBR technology are related to recycling of wastewater from downstream activities. However, recent studies have used MBR technology in separation of MPs and other microparticles from wastewaters.

Fig. 3 MBR process (Adapted from https://www.lenntech.es/processes/mbr-introduction.htm)

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Studies have been conducted to assess the efficiency of MBR in the removal of MPs from water. Talvitie et al. [53] showed that MBR method was able to remove 99% of MPs after secondary treatment of samples from WWTPs in Finland. The author compared the MBR method with other methods such as disc filter, rapid sand filtration and dissolved air flotation and reported that the highest reduction of MP in the final effluent was shown by MBR, demonstrating that the method is the most efficient. Similarly, Lares et al. [30] also found that the MBR process had a slightly better removal efficiency of MPs (99.4%) compared to the overall conventional activated sludge (CAS)-based process (98.3%) in their study on the removal of MPs in samples from WWTPs. More recently, Lv et al. (2019) investigated MPs removal at a fullscale WWTP, Eastern China, with membrane bioreactor (MBR) and reported that influent MPs were removed by 99.5% in MBR system on the basis of plastic mass while 82.1% on the basis MP number. The removed MPs accumulated in sludge phase. They concluded that MBR system has much higher MP removal efficiency than oxidation ditch system (which was also tested), attributed to membrane filtration involved in the MBR process. Recent development in the MBR technique involves the use of bacteria to degrade plastic. Poerio et al. [46] in their review explained that the isolation of a novel bacterium (Ideonella sakaiensis) was able to efficiently convert PET in the less dangerous monomers (terephthalic acid and ethylene glycol). In another study, exposure of MPs to Antarctic Krill (Euphausia superba) was able to reduce MPs from 31.5 µm to less than 1 µm [15]. The degradation of MPs by the organism is possible from the enzymes produced and are now been integrated with the MBR process which was demonstrated for PET degradation by Barth et al. [6]. Advantages • Separation of solids by membrane filtration eliminates the need for secondary sedimentation, and small pore size prevents the discharge of most pathogens. • The MBRs can be operated with a longer solids retention time, allowing for more complete oxidation of organics and the maintenance of a population of slowgrowing bacteria capable of nutrient, EDC and PPCPs removals and reduced biosolids generation. • They offer a competitive alternative when nutrient removal is required • It has high removal efficiency for pollutants, space-saving and less sludge production. • It ensures that microorganisms are completely trapped into the bioreactor resulting in better control over the biological reactions and modifying the conditions of the microorganisms in the aerated tank. • It enables long sludge retention time (SRT) and high mixed liquor suspended solid (MLSS) concentration. Disadvantages • MBRs is capital intensive • Routine/periodic membrane replacement • High energy costs and need to control membrane fouling.

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3 Conclusion The studies discussed in this chapter have shown that in order to limit microplastic pollution of the environment, ad hoc microplastic treatment processes must be designed and developed. Currently, the water industry and the WWTPs do not have expertise or technology to effectively distinguish MP from waste. From several papers, it arose that exceptional tertiary treatment is expected to eliminate the plastic for wastewater. The membrane processes, MBR specifically, have all the earmarks of being the most encouraging with 99.9% of MP elimination, additionally offering the likelihood to diminish the quantity of stages involved in the WWTPs processes. Moreover, a more nitty–gritty and uniform compound actual portrayal of the plastic is obligatory to choose fitting systems that ensure a more effective expulsion of the plastic from the effluents. From writing arose that the information are not effectively practically identical to one another because of the absence of normalized portrayal conventions. This portrayal ought to likewise incorporate nanoplastics that could have a more genuine organic effect. Conflict of Interests There are no competing interests to declare.

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Microplastics and Anaerobic Digestion Flora N. Ezugworie, Godwin O. Aliyu, and Chukwudi O. Onwosi

Abstract The majority of microplastics originating mainly from the utilization of different products: toothpaste, foodstuff, skincare products, vehicle tyres, synthetic fabrics, etc., end up in sewage wastewater. Microplastics in wastewaters (e.g., polyethylene, polystyrene, polyethylene terephthalate and polypropylene) appear in different forms and shapes (e.g., fibres, pellets, films, fragments, etc.). These recalcitrant microplastics in wastewater pose some challenges during sludge stabilization— anaerobic digestion. Due to their adsorptive potential, microplastics also harbour toxic substances (e.g., antibiotic residues, heavy metals, some resilient organic pollutants) as well as antibiotic-resistant-gene-bearing pathogenic organisms. This scenario complicates the anaerobic stabilization of pollutants found in wastewater and often reduces biogas yield. Therefore, poor management of wastewater could lead to the introduction of microplastics and their co-pollutants into water bodies, thereby causing serious public health issues. The present chapter will summarize the impacts of microplastics on anaerobic stabilization and the techniques currently developed to mitigate these concerns. Also, efforts aimed at reducing the continuous influx of these pollutants into the wastewater through global regulations and policies, as well as future research prospects will be presented herein. Keywords Anaerobic digestion (AD) · Microplastics · Wastewater · Biogas yield · Antibiotics residue · Heavy metals

F. N. Ezugworie · G. O. Aliyu · C. O. Onwosi (B) Department of Microbiology, Faculty of Biological Sciences, University of Nigeria, Nsukka, Enugu State, Nigeria e-mail: [email protected] Bioconversion and Renewable Energy Research Unit, University of Nigeria, Nsukka, Enugu State, Nigeria

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Sillanpää et al. (eds.), Microplastics Pollution in Aquatic Media, Environmental Footprints and Eco-design of Products and Processes, https://doi.org/10.1007/978-981-16-8440-1_13

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Abbreviations AD ARGs ARMs DLS EPS FPA FTIR HGT MGEs MRGs NTA PCBs PE PET PET PHA PP PS PVC Pyr-GC-MS RBC ROS SEM WWTPs

Anaerobic digestion Antibiotic-resistant genes Antibiotic-resistant microorganisms Dynamic light scattering Extracellular polymeric substances Focal plan array-based system Fourier transform infrared spectroscopy Horizontal gene transfer Mobile genetic elements Metal-resistant genes Nanoparticles tracking analysis Polychlorinated biphenyls Polyethylene Polyethylene terephthalate Polyethylene terephthalate Polyhydroxyalkanoate Polypropylene Polystyrene Polyvinyl chloride Pyrolysis-gas chromatography-mass spectrometry Resistant-bacterial communities Reactive oxygen species Scanning electron microscopy Wastewater treatment plants

1 Introduction The utilization of plastics and its accessories have resulted in microplastics (0.05–5 mm) which have caused environmental pollution at varying scales [1]. The introduction of microplastics into the environment has drawn huge concern in the past decade due to their impacts on the ecosystem and human health [2]. The majority of microplastics and other miniature anthropogenic litters (microbeads, fibres, particles, etc.) emerging from the domestic wastewater, landfills, stormwater and industries accumulate in sludge generated during wastewater treatment [2–7]. The treatment of municipal wastewater (screening, grit chamber, primary and secondary sedimentation tank, advanced treatments units) results in the generation of sludge harbouring large amounts of microplastics [8–10]. The wastewater treatment options, the amount of industrial wastewater, servicing area and sludge dewatering are the wastewater treatment parameters that determine the concentration of microplastics in sludge [11]. Zhang et al. [10] further noted that the source of wastewater, economic status

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and people’s living style determines the composition, size distribution and shapes of microplastics found in wastewater. The existence of microplastics in different forms, shapes and sizes, makes their removal from sludge a challenging task [12]. While some investigators believe that microplastics are randomly distributed in wastewater [13–16], others claim the distribution follows a defined pattern. For instance, Sun et al. [17] reported that microplastics of particle size 0.05 mm are more frequently detected in wastewater than those with sizes greater than 0.1 mm. Microplastics most detected in wastewater treatment plants (WWTPs) derived sludge include polystyrene (PS), polyvinyl chloride (PVC), polypropylene (PP), polyethylene (PE) and polyethylene terephthalate (PET) [17–19]. Other forms found in the WWTPs are the principal components of textile products (polyamide (PA) and polyester (PES)), acrylic fibres, polyolefin [10, 11]. Some cosmetic ingredients and personal care products are considered emerging pollutants since their utilization continuously introduces a large number of microplastics—plastic microbeads—into the environment [20, 21]. The estimated number of microplastics extracted from the sludge ranged from 4,196 to 15,385 particles kg−1 DM according to the reports of Mahon et al. [2]. It has already been observed that the amounts of microplastics present in sludge are 10 times greater than that found in the aquatic environment [18]. According to Li et al. [11], the average concentration of microplastics detected in sludge was 22.7 ± 12.1 × 103 particles kg−1 DS. It is estimated the amount of microplastics entering the natural environment is 1.56 × 1014 particles per year [11]. However, the amount of microplastics found in sludge differs from the location. For instance, the microplastics in the sludge derived from WWTPS in China are greater than that from sludge in the United States and Europe and several orders higher than that from the freshwater sediment [11]. Although a cocktail of toxic chemicals voluntarily used as additives during their production, surface morphology of microplastics makes them potential vectors of hazardous and persistent organic pollutants, inevitably introduced into WWTPsderived sludge [4, 9, 11, 20, 22–25]. Microplastics found in the environment (terrestrial, aquatic) pose unusual toxicological risks due to their potential to adsorb toxic chemicals present in wastewater and to release toxic chemicals into the sludge. The toxicity of microplastics depends on their sizes, bioaccessibility, absorption capacity and desorption kinetics [3, 26, 27]. The sorption behaviour of organic pollutants to microplastics is influenced by the solution chemistry (e.g., ionic strength, pH and dissolved organic matter) [28]. The majority of persistent organic pollutants (e.g., polychlorinated biphenyls, herbicides, polycyclic aromatic hydrocarbons, synthetic musks and pesticides), heavy metals, antibiotics, often attached to microplastics, end up in the waste activated sludge [29–31]. Microplastics also present platforms for biofilm formation by pathogenic microorganisms thereby aiding the proliferation of antibiotic-resistant genes in the environment [2]. The complex physico-chemical attributes of microplastics make them multifaceted stressors, and the understanding of their environmental dynamics has been somewhat intriguing [22]. The main additives in plastics are diethylhexyl phthalate (DEHP), Irgafos 168 phosphate and acetyl tri-n-butyl citrate (ATBC) [18]. Other chemicals commonly used in plastic manufacture include brominated flame

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retardants, phthalates and bisphenol A (BPA). Different forms of heavy metals are primarily used as fillers, colourants, stabilizers and flame retardants during plastic production. Well-known flame retardants comprise the oxides aluminium (Al) and antimony (Sb) as well as zinc borate. Others such as cadmium (Cd), cobalt (Co), lead (Pb), zinc (Zn) and chromium (Cr) are used as inorganic pigment-based colourants in plastics [22]. The incorporation of additives to plastics during production process results in their persistence in sludge [18]. Biocides (e.g., arsenic (As), Sb and tin (Sn)) are deliberately used as additives in plastics to make them resistant to microbial attack [22]. Apart from some flame retardants (reactive organic additives) polymerized with plastic moieties during the production process, usually the majority of the additives are not chemically bound to the plastic polymer [32]. Overall, these additives are leach from the microplastics and inevitably introduced into the sludge during wastewater treatment [18, 33]. Before sludge generated during the wastewater treatment is disposed, the nutrient (organic matter) content is further stabilized via anaerobic digestion (AD). AD is described as a sequence of interconnected processes (e.g., hydrolysis, acidogenesis, acetogenesis and methanogenesis) where microorganisms breakdown organic matter under anaerobic conditions (i.e., absence of oxygen) to produce biogas and digestate [34, 35]. Therefore, besides reducing the environmental contaminants during wastewater treatment, another merit of AD is that it provides a sustainable platform for the production of methane-rich biogas [36–38]. However, the performance of anaerobic digesters towards sludge stabilization is often challenged by the presence of microplastics in sludge as well as the additives released from the microplastics [39]. In light of the fact that sludge derived from WWTPs is often contains large amount of microplastics, it is important to understand their impacts on anaerobic sludge digestion. Therefore, this chapter considers the sources and detection of microplastics in WWTPs. The present chapter also delves into the impacts of microplastics in the overall performance of the anaerobic digester-sludge stabilization and energy recovery. Finally, the focus of future investigations on AD of sludge containing microplastics is highlighted.

2 Origin of Microplastics Microplastics originate from two main sources depending on the source and the size of the fragments. • Primary microplastics • Secondary microplastics The primary microplastics are plastic particles produced intentionally and used as additives in our daily care products such as toothpaste, cosmetics, face scrubs, bubble bath, shampoo, soap and hand-cleaners, where they replace other natural materials formally used like pumice, apricot husk and so on [21, 40]. They are also

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known as microbeads mostly made of polyethylene and are also used for dead skin removal, deep cleansing and as decorations on personal care products [41]. The lowdensity microplastics made of polyethylene, polystyrene, alkyd resin, polyamide, acrylic fibres and polyolefin are found float in on the water surface while those made of polyester, polyvinyl chloride and polyethylene terephthalate (high-density microplastics) accumulate in the sediment [11, 21, 42]. The comminution process is also used to generate some primary nano/microplastics (powder, granulates, fillers and industrial pellets) by crushing solid plastic particles to fragments intentionally [8]. The secondary microplastics are fragments from large plastics disintegrated either by ultraviolet radiation, wind, water, sunlight, chemical or mechanical force [43]. Fragments of microplastics can also be introduced into the environment through certain mechanical activities such as shipping, fishing, shoreline activities, manufacturing industries, untreated sewage, treated effluents from industries, tear and wear of tyres, laundry wastewater and sludge from sewage or WWTPs [2, 3, 11, 44, 45, 46]. Certain factors affecting the fragmentation of plastics include; the type of polymer material used in the production of the plastics, the type of product additives (bisphenol-A and Nonylphenol) used to increase the life-span of the plastics and the environmental condition surrounding the plastics [32, 47]. The formation of fracture and cracks on the surface of a solid plastics can be induced by photo-oxidation, weathering process, hydrolysis and other shear forces from water turbulences, tear and wear or collision thus, promoting the weakening and disintegration of the solid plastics into nano/microplastics [15, 48, 49]. Exposure to salt marsh can also weaken the solid plastics hence facilitating fragmentation and degradation into nano/microplastics [8]. The size, composition and lifespan of microplastics determine the level of risk it can cause to the environment [50, 51]. The overall process involved in the formation of microplastics is shown in Fig. 1. Microplastics originate from numerous plastics products (e.g., tyre, plastic containers, plastic water cans, etc.) as a direct result of biodegradation, fractionation, photodegradation and weathering of different plastic products [8, 52]. However, microbeads, (a category of microplastics usually less than 2–3 mm in size) are manufactured polyethylene plastics included as exfoliants and cleansers in personal care and health products [53–55]. Owing to their tiny nature, microbeads end up in wastewater and usually pass through filtration during wastewater treatment processes [55]. Therefore, microplastics found in the environment are either by-products of the disintegration of larger plastic products or a result of the introduction of microbeads (constituents of personal care products), via effluents from wastewater treatment systems [55, 56].

3 Regulation Policies on Microplastics Globally, legislative actions are being taken by different countries to tackle increasing global concerns over the occurrence of microplastics in coastal water bodies [53,

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Fig. 1 Processes involved in the formation of microplastics

56]. For instance, there is widespread clamour to ban the utilization of microbeads in personal care products due to its potential impacts on human health as well as the environment [56]. Environmental and human health risks linked to microplastic exposure are of growing concern, globally. Microplastics could affect human health through numerous exposure routes such as drinking water, inhaling dust particles and intake of food contaminated by microplastics [57]. The microplastics intake via the inhaled dust in children and adults was estimated at 3223 and 1063 particles per year, respectively [58]. Also, sludge from sewage/WWTPs was previously disposed in landfills but this sludge is a source of manure that can be used for agricultural purposes. Given this fact, the EU legislature introduced the Landfill Directive (1999/31/EEC30) and the renewable Energy Directives (2009/28/EC31) [2]. These directives were to divert all sewage/wastewater sludge initially deposited in landfills or incinerators to be used for energy production and agriculture [2]. In Ireland, more than 80% of sewage sludge is used for agricultural purposes [2]. The use of wastewater sludge as manure makes land another reservoir for microplastics as more than 90% of the microplastics in wastewater are not removed after treatment [2]. Due to the absorptive capacity of microplastics, it interacts with soil nutrients, organic matters and other contaminants in the soil like heavy metals, antibiotic residues and organic pollutants, thereby hindering the progress of crops and increasing the risk of microplastics pollution in the environment [2, 59]. Although sunlight and weathering help in the breaking down of some microplastics in the soil, microplastics are still found in the soil many years after the application of sewage/wastewater sludge in the land because they are recalcitrant and very difficult to degrade [2, 60]. According to the reports of den [61], successive sewage sludge application on agricultural soils results in microplastics accumulation. They noted that unlike, the soil without sewage addition with microplastics load of 1100 ± 570 microplastics kg−1 (heavy density plastics (HDP))

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and 930 ± 740 microplastics kg−1 (light density plastic (LDP)), sewage addition to soil resulted in microplastics load of 3060 ± 1680 microplastics kg−1 (HDP) and 2130 ± 950 microplastics kg−1 (LDP). Compared to the soil with sewage sludge application, the soil with a history of sewage sludge application showed a 256% increase in the content of microplastics. According to de Souza Machado et al. [62], microplastics impacted the functional association between water-stable aggregates and soil microbial activities, water holding capacity and soil bulk density. Also, the introduction of microplastics altered the soil microbial activities as it interfered with microbial community dynamics and remarkably decreased the rate of substrate-induced respiration [63]. Sewage/wastewater is not the only source of microplastics, the atmosphere also harbours a large amount of microplastics dispersed by wind to different parts of the globe and then settle finally on land [2].

4 Detection Methods for Microplastics A series of imaging and spectroscopic analysis has been deployed to reveal the significant insights into the abundance, composition and distribution of microplastics present in the WWTPs [64]. Thus, the protocols deployed for the detection and quantification of microplastics comprise pyrolysis-gas chromatography-mass spectrometry, Fourier transform infrared spectroscopy, Scanning Electron Microscopy, Raman and micro-Raman spectroscopy, dynamic light scattering, Focal plane array-based system micro-FTIR, nanoparticles tracking analysis [8, 65].

4.1 Scanning Electron Microscopy (SEM) SEM uses electron to detect nano/microplastics in sewage/wastewater and can measure nano/microplastics as small as 10 nm. The actual shapes of the nano/microplastics are revealed but quantification is limited as only a few samples can be analysed at a time [8]. That led to the employment of a more suitable technique that can analyse the whole sample at once.

4.2 Dynamic Light Scattering (DLS) This is a technique that uses scattered light to detect very small particles (nano/microplastics 10 nm) by generation hydraulic diameter of nano/microplastics assuming the shape of a spherical model. DLS is a more accurate method than visual counting and SEM because of the ability of DLS to analyse the whole sewage/wastewater sample at the same time but the conversion from intensity to

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volume to number weighted distribution gives room for errors to occur [65]. DLS is cost-effective and it is mostly used for the detection of nano/microplastics in sewage/wastewater [8].

4.3 Nanoparticles Tracking Analysis (NTA) Nanoparticles tracking analysis (NTA) is another light scattering technique used to quantify nano/microplastics in sewage/wastewater. NTA measures particle size as small as 10 nm, analyses the whole sample at a time and provides the concentrations as well as the sizes of the nano/microplastics detected [66]. NTA is more efficient and précised than DLS because based on a particle to particle analysis, NTA generates a number weighted distribution. It is expensive and not commonly used [8]. The capacity of NTA could not access the chemical structure of the nano/microplastics detected.

4.4 Fourier Transform Infrared Spectroscopy (FTIR) Fourier transform infrared spectroscopy (FTIR) is a technique that can detect the chemical structures of microplastics in sewage/wastewater samples. It is not suitable for the detection of nanoplastics as it cannot detect any particle lower than 10 mm. FTIR is efficient in analysing the chemical properties and the additives used in the production of microplastics [8]. FTIR can detect polymers such as polyethylene, polyester, polyethylene terephthalate (PET), etc. [2]. The results generated by FTIR are not accurate because impurities and other contaminants present in the sewage/wastewater are easily absorbed by the FTIR and this influences the outcome [8].

4.5 Raman and Micro-Raman Spectroscopy The process and mechanism of Raman spectroscopy is similar to FTIR and is not suitable for detecting nanoplastics and abodes impurities from sewage/wastewater sample. To access smaller particles and generate accurate results, more efficient techniques were used.

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4.6 Pyrolysis-Gas Chromatography-Mass Spectrometry (Pyr-GC-MS) Pyrolysis-gas chromatography-mass spectrometry (Pyr-GC-MS) is commonly used that analyses the thermal degradation products of microplastics to determine the chemical composition. It is not suitable for the quantification of nano/microplastics because Pyr-GC-MS can only analyse a limited mass (10 mg) at a time. The limited mass analysis is for proper penetration and detection of the thermal degradation product of nano/microplastics by the GC-MS sensor [11].

4.7 Focal Plan Array-Based System (FPA) Micro-FTIR Unlike Pyro-GC-MS, FPA micro-FTIR analyses the chemical composition of microplastics by analysing (mapping) the surface of the filter used to recover microplastics. FPA micro-FTIR cannot be used to detect nanoplastics as plastic particles below 10 nm cannot be detected. This technique is similar to DLS as the actual shape of microplastics is not revealed due to the scattered light used during the analysis [8].

5 Presence of Microplastics in Sludge Impacts AD Since microplastics are not usually degraded during the treatment steps used in wastewater management, the sludge becomes the sink for the microplastics as concentration as high as 13,460 particles/kg sludge has been reported [67]. In short, majority of microplastics (>90%) that pass through wastewater treatment processes accumulate in the wastewater sludge [11, 12]. Wei et al. [18] added that the amount of microplastics present in sludge is 10 folds greater than that found in the aquatic environment. Before the disposal, the microplastics/nutrient-rich sludge is subjected to AD [64]. Due to the participation of diverse microbial communities, AD of sludge results in multiple benefits—stabilization, volume reduction, pathogen and odour reduction and bioenergy recovery. AD has been described as a sustainable approach for mitigating energy crisis as well as environmental pollution [68]. Through bioenergy (methane or biohydrogen) production, AD ensures that the capital operation cost of WWTPs is reduced [12]. AD comprises four important steps—hydrolysis, acidogenesis, acetogenesis and methanogenesis. These stages provide conditions that permit a succession of microorganisms to participate in the bioconversion of sludge to biogas and ultimately reducing the sludge volume. Although under debate, AD has also been shown to reduce the abundance of microplastics in the sludge [2]. However, Fu et al. [39] and Zhang et al. [10] noted that the presence of microplastics as well as other co-contaminants (e.g., antibiotic residues, heavy metals, persistent organic

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pollutants, etc.), effects the overall digester performance. The outcome of various investigations shows that the size, concentration of the microplastics as well as the presence of toxic chemicals—additive agents bioleached from the microplastics or harmful chemicals adsorbed to the microplastics—determines the overall effect on anaerobic digester performance (i.e., volatile solids (VS) destruction and methane production) [10].

5.1 Effects of Microplastics on Methane Formation During AD A series of process is involved in methane formation during AD of sludge. These stages of AD—solubilization, hydrolysis, acidogenesis, acetogenesis and methanogenesis—are affected by the presence of microplastics or their bioleached additives. The impacts that microplastics would have on AD depend on the concentration and distribution of microplastics or their additives in the sludge. For instance, diethylhexyl phthalate, a plasticizer commonly found in the WWTPs, disrupted the solubilization step during sludge digestion. However, they did not have any known impact on the composition or total yield of short-chain fatty acids as well as microbial population within the anaerobic digester [23]. On the other hand, the retention of polyethersulfone hinders AD of wastewater-derived sludge [69]. According to Wei et al. [70], about 10 particles of PVC microplastics per gram total solid (10 particles MP/gTS) enhanced methane production (5.9 ± 0.1%) while the trend reversed when the digester contained ≥20 MP particles/gTS. They concluded that the amount of a toxic additive agent for plastic production, bisphenolA, bioleached from the microplastics either promoted or inhibited the methane production. In another study, Wei et al. [18] demonstrated that short term exposure of wastewater sludge to lower levels of PE microplastics (≤60 particles MP/g-TS) did not hinder digester performance but there was a significant reduction (27.5 ± 0.1%) in the methane yield when the concentration rose to 200 MP particles/gTS. Li et al. [12] hinted that polyester microplastics, at doses of 1 × 103 –2 × 105 particles/kg sludge reduced methane production (88.53 ± 0.5–95.08 ± 0.5%). They added that retention of microplastics in the sludge retarded the anaerobic stabilization process as the resulting digestate showed lower dewaterability, high nutrient concentration and organic matter due to incomplete digestion. Polychlorinated biphenyls (PCBs) are mainly used as oil plasticizers in various plastic products such as PVC pipes. The occurrence of PCBs is a huge concern during sludge digestion since they hinder volatile solid destruction, sludge dewaterability and methane production [71]. For instance, there was a decrease in cumulative methane yield by 26.6 ± 0.1% when the anaerobic digester contained 100 mg/kg DS of PCBs. The low yield of methane in the presence of PCBs was linked to significant suppression of methane-forming enzymes (e.g., cellulase coenzyme F420 , acetate kinase and protease). For instance, the relative activities of F420 , protease

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and cellulose decreased to 78 ± 3, 87 ± 1, 90 ± 2%, respectively, at 100 mg/kg DS (double strength) [71]. The volatile solid removal from waste-activated sludge during AD reduced from 26.2 ± 0.5–21.7 ± 0.6%, when the concentration of PCBs in the digester increased from 50 to 100 mg/kgDS. Thus, the presence of PCBs invariably increases the sludge volume and would result in a higher cost of transport and disposal [71]. According to Wei et al. [19], polyethylene terephthalate (PET) microplastics hindered hydrogen production by suppressing hydrolysis, acidogenesis and acetogenesis during alkaline anaerobic stabilization (fermentation) of waste activated sludge. It is thought that leaching of toxic components such as di-n-butyl phthalate from PET microplastics contributed to the process inhibition. It was demonstrated that the nanoparticles inhibited the growth and metabolism of the microorganisms by attaching to the surface of the cell membrane. Given the negative impacts of microplastics or their additives (e.g., heavy metals, bisphenol-A) on AD of waste-activated sludge, controlling the release of toxic compounds into anaerobic digester necessary important to prevent digester inhibition [72]. It has been opined that the use of thermophilic systems during the AD processes has potentials in the biodegradation of not only the organic matter in the sludge but also the recalcitrant microplastics [64, 73].

5.2 Effects of Microplastics on the Methane-Forming Microbial Community During AD The toxicity of microplastics on microbes during AD stems from the ease of migration of chemical additives from the microplastics [19]. Thus, the exposure of the anaerobic digesters to microplastics or their additives results in microbial community shifts [23, 74]. As demonstrated by Fu et al. [39], the presence of nano-sized PS microplastics has been shown to reduce methane production [39]. The interference with methane yield could be attributed to the inhibition of enzyme formation by microbes AD processes [18]. During anaerobic sludge digestion, high level of microplastics (mostly that of PE and PET), suppresses the activities of hydrolytic organisms such as Proteobacteria, Rhodobacter sp. but had no pronounced variation on Bacteroides sp. [18, 70]. There was also a decrease in the population of Proteiniclasticum sp., Fonticella sp. and Proteiniborus sp., that are linked to acidogenesis and acetogenesis [18, 70]. This action inhibits hydrogenotrophic methanogens (e.g., Acetoanaerobium sp., Methanobacterium sp., Methanospirillium sp., etc.) and acetoclastic methanogens (e.g., Methanosaeta sp.), leading to incomplete digestion, high nutrient and organic matter concentration at the end of AD [12, 19, 75, 76]. Besides the toxic additives from the microplastics, another source of inhibition of anaerobic microorganisms is the generation reactive oxygen species (ROS) in the anaerobic digesters [18]. According to Mu and Chen [77], toxic oxidative stress in microbial cells is stimulated in the presence of ROS (i.e., the hydroxyl radical (OH• ),

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superoxide (O2 – ) and hydrogen peroxide (H2 O2 )). The cytotoxicity and ultimately cellular apoptosis are direct results of ROS disruption of electron transfer in the inner membrane and/or stimulation of redox-sensitive signaling pathways (e.g., NFκB cascades and Mitogen-activated protein (MAP) kinases). Bhattacharya et al. [78] and Jeong et al. [79] have demonstrated that loss of cell viability could be associated with up-regulation of ROS generation when microbial cells are exposed to microplastics. It has been widely reported that anaerobic systems (conditions) could activate ROS production. Given the large surface area of microplastics due to their miniature size, there is a large pool of reactive groups on the surface. Thus, in an anaerobic system (e.g., anaerobic sludge digester) the reaction between reduced groups on the microplastics and the negligible amount of oxygen remaining in the digester generates minute concentration of H2 O2 and (O2 – ) via dismutation [19]. Through Fenton chemistry, an additional ROS (i.e., OH• ) could be produced by interaction between H2 O2 and Fe2+ [19].

5.3 Effect of Microplastics on Biofilm Formation During AD Under extreme conditions, microorganisms form biofilm by attaching to a solid surface and then attaching to form congregate surrounded by extracellular polymeric substances (EPS) [80]. The EPS is made up of DNA matrix and proteins [81]. During AD, microplastics present in sludge provide platform for microbial biofilm formation as well as acting as a carbon source for growth sustenance [82, 83]. The biofilm develops within 72 h of the AD initiation [80]. Microplastics enhance the formation of biofilm due to its large surface area and absorption capacity thus promoting corrosion and degradation by microorganisms in anaerobic digesters [64]. Other changes induced by biofilm formation include crystallinity, stiffness and maximum compression [84]. The alteration in chemical orientation of microplastics could also be attributed to the interaction of microbial exo-enzymes with polymers that make up microplastics [83]. The short-chain fragments of microplastics are then absorbed as a carbon source to the cell membrane of the microorganisms where it is converted into carbon dioxide, water and methane by the process of mineralization [80]. The utilization of microplastics as carbon source by microorganisms within the anaerobic digester inhibits the metabolic activities due to formation of ROS [85]. Other organic pollutants such as antibiotic-resistant genes (ARGs), antibiotic residues and mobile genetic elements (MGEs) and heavy metals can also attach to the layers of films formed by microorganisms [11].

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5.4 Effect of Microplastics on the Proliferation of Antibiotic-Resistant Genes During AD The amount of veterinary antibiotics utilized in animal feed for disease control has soared in the past decade [86]. Due to poor metabolism by the livestock, a considerable amount of antibiotics are excreted into the wastewater generated from the farm [86]. Apart from animal wastes, dewatered sludge from wastewater especially those from hospitals have been identified as hotspots for antibiotic-resistant microorganisms (ARMs) and ARGs [87, 88]. The presence of residual antibiotics in wastewater creates environmental-driven stress that results in the acquisition of ARGs by antibiotic-sensitive microorganisms from the resistant-bacterial communities (RBC) [89]. It has been demonstrated that ARGs and RBC are hardly degraded by WWTPs (WWTPs) [89]. In particular, the microplastics in the sludge are potential vectors for the proliferation of ARGs, MGEs and metal-resistant genes (MRGs) as well as heavy metal/antibiotics residue [90–92]. This is because the robust absorption capacity of microplastics provides a platform for microbial colonization and biofilm formation (Fig. 2). For instance, compared to water samples which accounted for 23.9% of the dominant multi-antibiotic resistant profile (TET-SFX-ERY-PEN), a higher amount of the profiles (25.4%) was reported for microplastics samples [92]. Also, the abundance of MGEs such as class 1 integrons (intI1) detected in microplastics samples was more prominent than those associated with water samples [92]. The absorption of heavy metals (zinc, iron, nickel, lead, mercury, arsenic, copper, cobalt, etc.) by microplastics during AD varies among the different types of microplastics [93]. The co-existence of ARGs and MRGs follow a specific pattern on the microbiome of microplastics because they have the same mechanism for the regulation of their efflux

Fig. 2 Microplastics in bioreactor during AD of dewatered sludge

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pumps and they can be linked and transferred by one mobile genetic element [93]. This co-existence also promotes horizontal gene transfer (HGT) among the anaerobic plastisphere (environmental niche for microbes in the plastic surface) through conjugation [94]. Although the resistant strain from microplastics biofilm helps in the degradation of microplastics during AD, it also enhances the spread of ARGs to other species of microorganisms during and after the treatment process [24, 80]. It has been confirmed that there is a faster rate of HGT among the phylogenetically distinct groups of microorganisms attached to microplastics than the free-living microbes [90]. Taken together, this can be explained by the fact that ROS-driven oxidative damage on microbial cells when exposed to microplastics in anaerobic systems increases cellular membrane permeability. These result in a faster rate of transfer of MGEs among the microbial community in the anaerobic digester as the higher number of bacteria become receptors for ARGs [95]. AD is an effective technique for the degradation of antibiotic residues found in wastewater-generated sludge [86]. Reduction in the abundance of ARGs and ARMs can be achieved by opting for thermophilic rather than mesophilic digestion. For instance, the abundance of the tet(O) and tet(W) ratios appeared consistently low in the thermophilic digesters than the mesophilic counterparts [96]. During anaerobic treatment of sludge, it has been hinted that mesophilic digestion is more prone to ARGs proliferation than thermophilic digestion. This could be attributed to either a higher differential survival rate of ARGs or HGT between digester microbial community and raw sludge bacteria in the former than the latter [96, 97]. Again, it is suggested that operating the anaerobic digester thermophilic condition helps to prevent the rebounding of ARGs [98]. About 94% reduction in the abundance of ARGs and MGEs was accomplished by effective pre-treatment steps such as thermal hydrolysis during AD of sludge [98]. Tong et al. [99] also supported the view that AD with thermal pretreatment, not only and reduced ARGs and MGEs, but also improved methane formation. However, Huang et al. [100] have argued that AD of sludge under thermophilic temperature does not always result in ARGs reduction unless there was a suppression of the pathogenic organisms as well as the gene transfer. Overall, the influent antibiotic-resistant bacteria and ARGs composition together with the sludge digestion conditions essentially determine the anaerobic digester effluent ARGs content [96].

6 Recommendations for Further Studies A large amount of microplastics from sewage/wastewater pass through the WWTPs along with the sludge into the bioreactor for AD. To minimize the concentration of microplastics in the anaerobic digesters, density separation should be carried out on sewage/wastewaters before they are introduced into the WWTPs. WWTPs should be upgraded with recent and more effective units for the reduction of nano/microplastics [3]. For adequate modification and upgrade, the characteristics (stability, size,

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concentration and surface chemistry) of nano/microplastics in water and their negative effects on microorganisms required for AD should be known [8, 101]. The filter membranes should be modified and made smoother to limit interactions with the rough edges of nano/microplastics. Negatively charged filter membranes should be used as nano/microplastics are negatively charged, thereby inducing repulsion to avoid membrane fouling. The surface charge of the filter membrane can be altered by creating an electrical double layer or by plasma polymerization [102–104]. In particular, microbial degradation of microplastics within the anaerobic digesters should be evaluated as a potential bioremediation option. To understand the attenuation route for the microplastics, the evaluation of the essential factors for the mobilization and transport within the freshwater and terrestrial systems should be considered [2]. Therefore, strain improvement should be conducted to develop microorganisms with ability to degrade microplastcs during the anaerobic stabilization of wastewater sludge. The use of periphytic biofilm by adding carbon sources such as glucose, peptone or a combination of both, improves the degradation process of microplastics by boosting the population of cyanobacteria, firmicutes, proteobacteria and bacteroidetes which are the dominant bacteria in biofilm [105]. There should be additional policy adjustments or technology upgrades to achieve high-rate sludge digestion for improved resource recovery [12]. Microbial degradation of organic pollutants during the wastewater treatment protocols yields polyhydroxyalkanoate (PHA). Besides being an active ingredient in the production of biodegradable plastics, PHA could also find useful applications in boosting energy yield during anaerobic stabilization of sludge. For instance, Wang et al. [106] hinted that increasing the PHA levels of sludge from 21 to 184 mg−1 of volatile suspended solids (VSS) improved the methane yield by 46%. On a unit volatile suspended solids (VSS) basis, PHA showed a higher biochemical methane potential than the major components of the microbial cell such as carbohydrate and protein. The presence of PHA in the digester activated the methanogens along with methane-yielding enzymes such as coenzyme F420 acetate kinase and protease [106]. Based on an integrated economic and environmental standpoint, a novel approach such as PHA enrichment of sludge has been suggested. In the approach, the operation of WWTPs would be designed to remove organic compounds through PHA accumulation rather than cell growth and CO2 formation [106]. Should the suggested operation of the WWTPs be properly implemented, the costs of aeration would be reduced, sludge for dewatering and final disposal would be minimized, and ultimately there would be an enhancement of bioenergy (methane) yield.

7 Conclusion Demand for plastic products has soared in the recent past due to their durability, low-cost, portability and versatile applications. The increased use of plastics has resulted in abundance of microplastics in the environment. Microplastics have become emerging issue of global concern due to their associated human health

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risks (e.g., they are vectors of pathogenic microorganisms and antimicrobial-resistant genes) and environmental pollution (e.g., they leach toxic chemicals into the environment). Sludge generated during the wastewater treatment protocols is the major route through which microplastics gain entry into the soil and water bodies. Anaerobic digestion is the preferred option for the stabilization of sludge as it reduces the organic load and produces methane-rich biogas. However, the occurrence of microplastics sludge hinders the anaerobic digester performance. Improving anaerobic treatment process (e.g., running the digester under thermophilic conditions) would aid in the reduction of microplastics in the sludge.

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Role of Microplastics as Attachment Media for the Growth of Microorganisms Megha Ukil, Srinjoy Roy, Atun Roy Choudhury, and P. Sankar Ganesh

Abstract Owing to their size (