Organic Waste to Biohydrogen 9789811919947, 9789811919954

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Clean Energy Production Technologies Series Editors: Neha Srivastava · P. K. Mishra

Mohammed Kuddus Ghazala Yunus Pramod W. Ramteke Gustavo Molina   Editors

Organic Waste to Biohydrogen

Clean Energy Production Technologies Series Editors Neha Srivastava, Department of Chemical Engineering and Technology, IIT (BHU) Varanasi, Varanasi, Uttar Pradesh, India P. K. Mishra, Department of Chemical Engineering and Technology, IIT (BHU) Varanasi, Varanasi, Uttar Pradesh, India

The consumption of fossil fuels has been continuously increasing around the globe and simultaneously becoming the primary cause of global warming as well as environmental pollution. Due to limited life span of fossil fuels and limited alternate energy options, energy crises is important concern faced by the world. Amidst these complex environmental and economic scenarios, renewable energy alternates such as biodiesel, hydrogen, wind, solar and bioenergy sources, which can produce energy with zero carbon residue are emerging as excellent clean energy source. For maximizing the efficiency and productivity of clean fuels via green & renewable methods, it’s crucial to understand the configuration, sustainability and technoeconomic feasibility of these promising energy alternates. The book series presents a comprehensive coverage combining the domains of exploring clean sources of energy and ensuring its production in an economical as well as ecologically feasible fashion. Series involves renowned experts and academicians as volume-editors and authors, from all the regions of the world. Series brings forth latest research, approaches and perspectives on clean energy production from both developed and developing parts of world under one umbrella. It is curated and developed by authoritative institutions and experts to serves global readership on this theme.

Mohammed Kuddus • Ghazala Yunus • Pramod W. Ramteke • Gustavo Molina Editors

Organic Waste to Biohydrogen

Editors Mohammed Kuddus Department of Biochemistry College of Medicine, University of Hail Hail, Saudi Arabia

Ghazala Yunus Department of Basic Science University of Hail Hail, Saudi Arabia

Pramod W. Ramteke Department of Biotechnology Dr Ambedkar College Nagpur, India

Gustavo Molina Laboratory of Food Biotechnology Institute of Science and Technology/ UFVJM Diamantina, Brazil

ISSN 2662-6861 ISSN 2662-687X (electronic) Clean Energy Production Technologies ISBN 978-981-19-1994-7 ISBN 978-981-19-1995-4 (eBook) https://doi.org/10.1007/978-981-19-1995-4 # 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

Preface

In the present era, the development of any country mainly depends on energy, and the global energy requirements are generally fulfilled by conventional fossil fuels such as oil, coal, and natural gas. However, limited fossil resources and increase in greenhouse gases have made it unsustainable. Therefore, to fulfill the global energy demand, there is an urgent need for an alternative energy research. Biohydrogen is one of the promising sources of sustainable and clean energy as it is harnessed by biological means. Biohydrogen may be produced by utilizing different waste materials as a substrate, which include agricultural waste, municipal waste, industrial waste, and other hazardous wastes. Biohydrogen production from waste materials opened a new opportunity for the widespread use of everlasting renewable energy sources. This book aims to provide an updated knowledge on the biohydrogen production from organic waste materials. The book will be a comprehensive reference in the most progressive field of biohydrogen production and will be of interest to professionals, scientists, and academics working on renewable energy sources. This book includes 12 chapters related to biohydrogen production from around the world that provides an updated knowledge of biohydrogen production from different waste materials. The chapters highlight the potential of low-cost waste material from various industries for production of biohydrogen. Some of the chapters explain the novel methods and technologies along with nanotechnological approaches adopted for the production of biohydrogen. One of the chapters also presents future perspectives of biohydrogen for the sustainable environment. In conclusion, this book is an updated reference regarding the potential impact of biohydrogen and its production methods from specific waste materials that will be useful for the professionals, scientists, and academics related to this field. We would like to thank all the authors who have eagerly contributed their chapter in this book. Finally, we also express our sincere gratitude to Springer Nature, Singapore, for providing this opportunity. Hail, Saudi Arabia Hail, Saudi Arabia Nagpur, India Diamantina, Brazil

Mohammed Kuddus Ghazala Yunus Pramod W. Ramteke Gustavo Molina

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Contents

Biohydrogen from the Organic Fraction of Municipal Solid Waste . . . . Karina J. Salazar-Batres, Guillermo Quijano, and Iván Moreno-Andrade

1

Biohydrogen from Food Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Iván Moreno-Andrade, Karina J. Salazar-Batres, Edith Villanueva-Galindo, Jonathan F. Cortez-Cervantes, Ulises Jimenez-Ocampo, Julián Carrillo-Reyes, and Alejandro Vargas

31

Biohydrogen from Fruit and Vegetable Industry Wastes . . . . . . . . . . . . Bhaskarjyoti Kalita and Nandan Sit

69

Biohydrogen from Distillery Wastewater: Opportunities and Feasibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anubha Kaushik, Sharma Mona, and Raman Preet

93

Biohydrogen from Pentose-Rich Lignocellulosic Biomass Hydrolysate . . 123 Franknairy Gomes Silva, Vitor da Silva Liduino, Viridiana Santana Ferreira-Leitão, and Magali Christe Cammarota Biohydrogen Production Using Cheese Industry Waste: Current Trends and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147 Jyoti S. Gokhale, Devendra P. Tekale, and Uday S. Annapure Methods of Biological Hydrogen Production from Industrial Waste . . . . 163 Rekha Unni, R. Reshmy, Aravind Madhavan, Parameswaran Binod, Ashok Pandey, and Raveendran Sindhu Innovative Technologies for Biohydrogen Production at Industrial Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181 Dolores Hidalgo, Jesús M. Martín-Marroquín, and David Díez Thermochemical Conversion of Lignocellulosic Biomass for Biohydrogen Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Hortência E. P. Santana, Brenda L. P. Santos, Daniel P. Silva, Isabelly P. Silva, and Denise S. Ruzene

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viii

Contents

Nanotechnological Approaches in Biohydrogen Production . . . . . . . . . . 229 Hayrunnisa Nadaroglu and Azize Alayli Microalgal Biomass as a Promising Feedstock for the Production of Biohydrogen: A Comprehensive Review . . . . . . . . . . . . . . . . . . . . . . . 251 Akansha Singh, Richa Das, Vijay Upadhye, and Esha Rami Biohydrogen: Future Energy Source for the Society . . . . . . . . . . . . . . . . 271 Dolores Hidalgo, Jesús M. Martín-Marroquín, and David Díez

About the Editors

Mohammed Kuddus is currently working at the Department of Biochemistry, College of Medicine, University of Hail, Kingdom of Saudi Arabia. His main research areas include molecular biology, enzyme technology, biopolymers, waste utilization, and food and microbial biotechnology. He has more than 15 years of integrated teaching and research experience and published more than 75 research articles in peerreviewed international journals along with 7 books and 22 book chapters. He has also published 40 abstracts at international/national conferences and symposia and received best presentation awards. He has supervised 6 PhD theses and 12 PG/UG dissertations. He has been serving as an editor/editorial board member for 20 and reviewer for more than 40 international peerreviewed journals. He has also been awarded SERC Young Scientist Project from the Department of Science and Technology, Govt. of India, and Young Scientist Project from the International Foundation for Science, Stockholm, Sweden. Ghazala Yunus completed her PhD in Biophysics from Integral University, Lucknow, India. At present, she is working as an Assistant Professor at the Department of Basic Science, University of Hail, Kingdom of Saudi Arabia. Dr. Yunus’s main research areas include biophysics, bioinformatics, and biosensors. She has published more than 15 research articles in reputed international journals along with one book and two book chapters. She has been serving as an Editorial Board Member and Reviewer of various international reputed journals.

ix

x

About the Editors

Pramod W. Ramteke is currently an Adjunct Professor at the Department of Biotechnology, Dr. Ambedkar College, Nagpur; Department of Life Sciences, Mandsaur University, Mandsaur; and Department of Molecular Biology and Genetic Engineering, RTM Nagpur University, Nagpur. He has 36 years of teaching and research experience and contributed significantly towards the development of microbe-based eco-friendly processes for sustainable crop production and utilization of agricultural wastes for production of biofuel and industrially important enzymes. He is an elected Fellow of National Academy of Agricultural Sciences, Biotech Research Society of India, Royal Society of Biology, Academy of Microbiological Sciences (AMI), National Academy of Biological Sciences, Mycological Society of India, and International Society of Environmental Botanists. He is the recipient of BHU Centennial Award, Excellence in Science Award (SCON), J. C. Bose Gold Medal, Dr. J. C. Edward Medal, Prof. K. S. Bilgrami Memorial Award, Prof. K. V. Shastri Gold Medal, and V. S. Chauhan Gold Medal. He has mentored 39 PhD and 72 PG students, and 4 patents, 2 technologies, 5 databases, and 323 gene sequences in NCBI are to his credit. He has authored/edited 10 books and over 250 research articles. He was a visiting scientist to the USA, the United Kingdom, Belgium, Hungary, Turkey, the Czech Republic, and South Korea and a member of the 18th Indian Scientific Expedition to Antarctica. Gustavo Molina is graduated in Food Engineering and earned his Master’s (2010) and PhD degrees (2014) from the University of Campinas, Unicamp (Campinas, Brazil), and part of his doctoral research was developed at Laboratoire de Génie Chimique et Biochimique, Université Blaise Pascal (Clermont-Ferrand, France). He is the head of the Laboratory of Food Biotechnology and is conducting scientific and technical research at the UFVJM (Diamantina, Brazil). In 2016–2017, he developed his postdoctoral research at the Institut Polytechnique de Grenoble (Grenoble, France) in the area of biorefinery and development of the enzymatic hydrolysis process of lignocellulosic materials. He has published more than 50 articles and book chapters in

About the Editors

xi

national and international journals and is a member of scientific committees and editorial boards. His research interests are focused on industrial biotechnology, aiming at the biotechnological production of ingredients, bioprocess development and optimization, and use and valorization of agro-industrial by-products into new biotechnological additives, among others.

Biohydrogen from the Organic Fraction of Municipal Solid Waste Karina J. Salazar-Batres, Guillermo Quijano, and Iván Moreno-Andrade

Abstract

The disposal of municipal solid waste is a global concern considering that more than two billion tons are annually generated worldwide, and at least 33% of these residues are not managed in an environmentally safe manner. The feasibility of generating hydrogen (H2) from the organic fraction of municipal solid waste (OFMSW) by means of biological processes has been demonstrated consistently. The characteristics of municipal solid waste, such as composition, moisture, and traces of inorganic residues, might vary depending on the regional economy and culture of each country and must be considered for the design and operation of H2 production from OFMSW. The use of biological fermentation of OFMSW has been demonstrated to be efficient for the production of H2 and other metabolites of industrial interest, such as long-chain fatty acids and volatile fatty acids. This chapter describes OFMSW as a substrate for biohydrogen production by applying a dark fermentation process, reviewing H2 production from lab-scale studies and recycling facilities. This work critically discusses the operating conditions of dark fermentation and their effects on the potential of hydrogen production, yield, and process inhibition. The effects of nonfermentable material (paper, inorganic fraction, etc.) and the use of pretreatment methods are also addressed. Keywords

Biohydrogen · Nonfermentable material · OFMSW · Organic solid waste · Waste separation

K. J. Salazar-Batres · G. Quijano · I. Moreno-Andrade (*) Laboratory for Research on Advanced Processes for Water Treatment, Unidad Académica Juriquilla, Instituto de Ingeniería, Universidad Nacional Autónoma de México, Querétaro, Mexico e-mail: [email protected] # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Kuddus et al. (eds.), Organic Waste to Biohydrogen, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-1995-4_1

1

2

1

K. J. Salazar-Batres et al.

Introduction

Worldwide approximately two billion tons of municipal solid waste is generated annually; it has been estimated that each person generates 0.74 kg of solid waste per day; however, this value can vary from 0.11 to 4.54 kg/day. Food and green waste account for 32% of the total solid waste generated in high-income countries, while these fractions account for 53% and 56% in middle- and low-income countries, respectively. The composition of urban solid waste in a location may vary depending on the consumption patterns of its population; however, as levels of economic development decline, the fraction of organic waste increases (Kaza et al. 2018). The organic fraction of municipal solid waste (OFMSW) mainly refers to a mixture of food waste, leaves, and yard waste (Cesaro 2021). OFMSW can contribute to greenhouse gas emissions since it represents a large amount of biomass that can be fermented into methane under ambient conditions during storage (Bonk et al. 2015). Two of the main management strategies for these wastes are landfilling and composting; however, when the infrastructure is not adequate, there may be methane emissions, malodors, pollution of land, and underground water (Escamilla-Alvarado et al. 2015; Ebrahimian et al. 2020). The OFMSW composition depends on the place and time of collection for a specific municipality or area, number of inhabitants, social condition, predominant economic activities, regional food habits, and season (Alibardi and Cossu 2015; Campuzano and González-Martínez 2016); nevertheless, it usually contains high lignocellulosic and fatty fractions due to the large amounts of food waste, kitchen waste, and leftovers from residences, cafeterias, and markets (Shah et al. 2016). Table 1 shows the origin, composition, and characteristics of the different samples of OFMSW that have been used for hydrogen (H2) production. Even though these wastes are classified as OFMSW, their compositions vary. The fraction that is found in the greatest quantity is food waste in a range of 52–95%, which includes fruit, waste rice, vegetables, beans, garden wastes, meat, fish, cheese, and bread. However, it is important to note that when OFMSW comes from mechanical separation plants, it contains traces of other nonbiodegradable materials. AlzateGaviria et al. (2007) characterized a sample of OFMSW where the highest proportion corresponds to waste food (51.89%) but also contains portions of plastic (9.6%), PET (2.7%), tins (3.1%), tetra-pack (0.3%), aluminum (0.3%), glass (3.0%), wood (3.6%), paper (22.5%), and cardboard (2.8%). Favaro et al. (2013) reported OFMSW containing fruit 24.8%, vegetable 18.2%, meat–fish 10.2%, bread–pasta–rice 12.3%, and undescribed 13.0%. The characterized OFMSW sample also contained 21.5% of rejected materials that correspond to plastics and shoppers, paper and cardboard, metals, and glass. In many cases, a simulated OFMSW has been prepared to be used as a substrate in biohydrogen production. Alavi-Borazjani et al. (2021) elaborated a complex OFMSW that contained 95% food waste (fruits and vegetables, cooked pasta and rice, cooked meat and fish, bread and bakery, cheese, and biscuits) and 5% paper. Similarly, Yeshanew et al. (2018) elaborated a substrate that contained 7% beef meat, 3.9% coffee, 4.3% rice, 20.9% potatoes, 5.1% bread, 5.1% garden waste, 1.9%

Origin Waste segregation plant

Waste segregation plant

Municipal solid waste treatment plant “Las Calandrias”

Rouen OFMSW

Synthetic bases on average composition of OFMSW collected in France

Location Padova, Italy

Padova, Italy

Andalusia, Spain

Rouen, France

France

Fermentable waste (57.0%), papers (24.6%), and cardboards (18.4%) Meat 7%, coffee grounds 3.9%, rice 4.3%, potatoes 20.9%, bread 5.1%, yogurt 2%, grass 5%, office paper 31.7%, moving cardboard 16.7%, paper folder 3.4%

Municipal waste

Fruit 24.8%, vegetable 18.2%, meat–fish 10.2%, bread– pasta–rice 12.3%, rejected materials 21.5%, undersieve 13.0%

Composition Meat–fish–cheese, fruit, vegetables, bread–pasta, undersieve

TS: 160
10 g TS/L, VS: 93
1% (dry weight), TOC: 49
1% (dry weight), TKN: 3200
100 mg N/L, ammonium: 590
50 mg N/L, total phosphorus: 320
20 mg P/L TS (%): 52.8 (4.8), TVS: (%): 33.0 (3.9), C/N ratio: 18.6 (2.9), sCOD (g/kg): 137 (18.4), TVFA (g/kg): 13.1 (0.8) TS: 51.4%, VS: 136 mg/g of TS, BOD: 83.2 mg/g of TS, COD: 149.6 mg/g of TS TS: 0.74
0.01 g TS/g, VS: 0.63
0.01 g VS/g

Physicochemical characterization VS: 93
1%, TOC: 50
1%, COD: 1829
5 mg O2/g TS, proteins: 15
1%, lipids: 11
1, carbohydrates: 67
3% (all data as % of TS)

The mixture was ground to a particle size below than 16 mm Heat-shock treatment was performed in a stirred bottle at 90  C for 30 min in order to inactivate methanogens

The samples were mechanically sieved by a 30 mm cylindrical trommel

Pretreatment Samples were manually sieved. Shoppers, plastics, paper and cardboard, metals, glass, bones, shells, and fruit kernels were considered as rejected materials N.A.

Table 1 Origin and composition of different organic fraction municipal solid waste used for biohydrogen production

(continued)

Paillet et al. (2021)

Bru et al. (2012)

AngerizCampoy et al. (2017)

Favaro et al. (2013)

Reference Alibardi and Cossu (2015)

Biohydrogen from the Organic Fraction of Municipal Solid Waste 3

Collected from a municipal landfill

Rudrapur, India

Fruits, vegetables, rice, bread, beans, paper, and some meat

Composition Fruit skins, waste rice, vegetables, beans, garden wastes, and stale bread

TS: 26.3
1.73%, VS: 20.2
1.9%, COD (particles): 115
1.5 g/L, COD soluble: 167
5.2 g/L, TKN: 4.3
1.2 g/L, ammonia: 391
7.5 g/L, lipids: 5.4
1.5 g/L, proteins: 20.2
2.2 g/L, C/N: 35.2
4.4, TVFA: 1.425
0.46 g/L

Physicochemical characterization 50.8% Lignocellulose, 42.5% starch, 6.7% other components (including trace amounts of proteins, pectin, and lipids) Pretreatment Ethanolic organosolv pretreatment with ethanol– water solution (85% v/v) containing acetic acid (1% w/w OF-MSW dry weight), as a pretreatment catalyst The OFMSW was ground using an electrical grinder which resulted in the particle size of less than 2 mm

Sharma and Melkania (2018c)

Reference Ebrahimian and Karimi (2020)

N.A. not available, VS volatile solids, TS total solids, TOC total organic carbon, TKN total Kjeldahl nitrogen, BOD biological oxygen demand, TVFAs total volatile fatty acids

Origin MSW compost site

Location Isfahan, Iran

Table 1 (continued)

4 K. J. Salazar-Batres et al.

Biohydrogen from the Organic Fraction of Municipal Solid Waste

5

yogurt, 31.7% white office paper, 16.7% packing cardboard, and 3.4% colored cardboard. The compositions of these substrates were similar to the OFMSW described in studies where the sample was taken directly from a waste separation plant (Alibardi and Cossu 2015; Shah et al. 2016; Ebrahimian et al. 2020; Cesaro 2021). However, in other studies, OFMSW was simulated using food waste from cafeterias or restaurants, and it was mixed with paper. Escamilla-Alvarado et al. (2013, 2015) mixed food waste and office paper at a 60:40 ratio. Gomez et al. (2006) and Redondas et al. (2015) prepared a substrate with 10% banana, 10% apple, 10% orange, 35% cabbage, 25% potatoes, 8% bread, and 2% paper. Muñoz-Páez et al. (2012) and Valdez-Vazquez et al. (2006) prepared a substrate that contained 40% paper and 60% food waste. The last examples corresponded to mixtures with a smaller variety of residues. According to Alibardi and Cossu (2015), the different origins and compositions of the organic waste samples, coupled with different process conditions, might affect the high variability of hydrogen production yields. Therefore, it must be emphasized that the high variation of the OFMSW composition will be reflected in differences of H2 production among the several studies thus far reported, as in the case of Tyagi et al. (2014) who reported a hydrogen content in the biogas of 36%, while Alzate-Gaviria et al. (2007) and Angeriz-Campoy et al. (2018) reported higher percentages of 51% and 52.4%, respectively. The physicochemical characterization of the different samples of OFMSW varies according to its composition since it has been reported that the total solid (TS) content ranges from 19.3% to 75%; the OFMSW samples with a higher percentage of moisture are those that contain more fruit and vegetable waste, whereas when it contains more than 40% paper and cardboard waste, the moisture is lower, as in the case of the OFMSW used by Paillet et al. (2020, 2021), which was reconstructed according to the characteristics of the OFMSW collected in France. The carbohydrate concentration in OFMSW varies from one location to another and throughout the time of the year, as reported by various authors such as Sharma and Melkania (2018b, c), who indicate that the OFMSW samples from a municipal landfill located in Uttarakhand, India, presented different carbohydrate contents, which ranged from 31.4
2.42 to 48.6
5.21 g/L even though the samples were collected at the same location. Carbohydrate content is crucial since it has been found that there is a linear correlation between its content and the production of hydrogen-rich biogas; additionally, the chemical composition of the substrates has an influence on the final products of dark fermentation (Alibardi and Cossu 2015).

2

Potential of OFMSW for Biohydrogen Production

Dark fermentation has been proposed as a promising technique to produce clean hydrogen due to its low chemical energy requirement and, therefore, is more environmentally friendly than conventional chemical processes (Jarunglumlert et al. 2018). This biological process is divided into two stages: hydrolysis and acidogenesis (Fig. 1). During hydrolysis, complex organic polymers are hydrolyzed into simple soluble organic compounds; subsequently, the generation of volatile

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K. J. Salazar-Batres et al.

Fig. 1 Dark fermentation steps and microbiological pathways

fatty acids, H2, CO2, and other intermediates occurs during acidogenesis (AngerizCampoy et al. 2018). H2 is the cleanest carbon-free fuel with the highest energy content (120 MJ/kg) compared to methane (50 MJ/kg), gasoline (44 MJ/kg), and ethanol (26.8 MJ/kg) (Ebrahimian and Karimi 2020); however, worldwide hydrogen production mainly comes from fossil fuel technologies (>95%), and only 1% of H2 is produced from biomass (Dauptain et al. 2021). Biological processes constitute a promising alternative for the production of hydrogen from low-cost, renewable, and environmentally friendly resources (Akhlaghi and Najafpour-Darzi 2020), such as OFMSW. Biological processes for H2 production from OFMSW simultaneously offer waste minimization, energy recovery, and valorization (Yeshanew et al. 2018), providing an ecological solution for managing organics (Sharma and Melkania 2018a). The high calorific power of OFMSW makes it a suitable substrate for bioenergy generation (Paillet et al. 2021) due to its rich content of carbohydrates and biodegradability (Sekoai and Gueguim Kana 2014) and because it is available all year and apparently free of cost (Escamilla-Alvarado et al. 2015). Hence, hydrogen production by the process of dark fermentation using OFMSW has promising potential for biofuel generation and high-value products, including volatile fatty acids and solvents (Kobayashi et al. 2012; Ebrahimian et al. 2020). Table 2 presents a summary of H2 production from OFMSW found in the literature, highlighting the experimental conditions tested, the H2 content in the gas produced, H2 yield, and productivity. When using OFMSW as a substrate, H2 yields and productivities vary widely due to differences in the operational parameters used in each experiment, such as the hydraulic retention time (HRT), substrate concentration, organic loading rate (OLR), temperature, and pH (Castelló et al. 2020). It has been reported that the optimum pH for H2 production is specific to each type of substrate (Ziara et al. 2019), e.g., associated with the substrate composition. According to Baldi et al. (2019), it is possible to increase hydrogen production if the pH value is maintained between 5 and 6.5 since the metabolic pathways of acetate and butyrate predominate under these conditions. On the other hand, strongly acidic or basic pH values negatively

T: 35
1  C

Batch

5.5–8.9 6.8 5.5 5.5 6.0 7.9 7.0

Stirred manually twice a day, T: 37  C

Stirred at 160 rpm, T: 37  C

I/S ratio: 1 g VS/g VS, T: 35
2  C

F/M ¼ 6, T: 35
1  C

S/ ratio 10, T: 37  C

HRT: 60 h, T: 30  C

T: 37  C

Batch

Batch

Batch

Batch

Batch

Batch

Batch

Batch

6
0.1

6.0
0.2

6.5

7.5

pH 7.0

I/S ratio: 10 g VS substrate/g VS inoculum, T: 37
1  C S0/X0: 20
5 (g VS/g VS), T: 37  C

Batch

Batch

I/S ratio: 5% w/w. OLR: 16 VS/kg/day, HRT: 24 h, T: 38
2  C T: 38.0  C
1.0  C

T: 34  C

Batch

Batch

Conditionsb I/S ratio: 0.5, T: 55  C

Reactor typea Batch

39%

46.7%

47%

22
0.9%

H2 in gas

H2 yield 62.5 L H2/kg VSadded 30 L H2/kg VSadded 85
3 BHP L H2/kg VS 99 mL H2/g VSremoved 104.5
0.7 L H2/kg TVS 31.6 L H2/kg VS 40
3 L H2/kg VSadded 119.7 L H2/kg VS 151 L H2/kg of substrate 70.1
4.1 L H2/kg VS 29.8 L H2/kg VS 40.8
0.5 L H2/kg VS 246.93 L H2/kg TVS 61 L H2/kg VSadded 0.28
0.01 L H2/Lreactor/day 0.23 L H2/g VS/day

Productivity

(continued)

Reference Alavi-Borazjani et al. (2021) Redondas et al. (2015) Alibardi and Cossu (2015) Alzate-Gaviria et al. (2007) Baldi et al. (2019) Cesaro et al. (2020) Dauptain et al. (2021) Dong et al. (2010) Ebrahimian and Karimi (2020) Favaro et al. (2013) Lavagnolo et al. (2018) Paillet et al. (2020) Sekoai and Kana (2014) Shah et al. (2016)

Table 2 Biohydrogen production potential by dark fermentation process using organic fraction municipal solid waste (OFMSW) as substrate

Biohydrogen from the Organic Fraction of Municipal Solid Waste 7

The UASB was filled with the inoculum suspension (2 g/L) and sucrose (3.85 L), HRT: 24 h, T: 38
2  C OLR: 36
2 g VS L/L day, HRT: 3 d, stirred at 100 rpm, T: 55
1  C OLR: 16 kg TVS/m3 day, HRT: 3d, T: 55  C

OLR: 75.6 (g TVS/L/day), HRT: 1.9 d, Stirred at 12 rpm, T: 55
0.5  C

UASB

Semicontinuous

5.5

5.5

OLR: 66 g TVS/L/d, Stirred at 12 rpm, T: 55  C

Semicontinuous

35
4%

5.7
0.1

HRT: 1.2 d, T: 55  C

51%

5.7
2

44%

52.40%

51.0
1.5%

50%

36%

H2 in gas 67.9%

9.0

Semicontinuous

CSTR

CSTR

Batch

The substrate-to-inoculum ratio (S/I) was 20 g VS substrate/g VS inoculum, T: 35
2  C Stirred at 50 rpm, T: 55  C

5.5

T: 55  C

Batch

Batch

pH 5.5

Conditionsb T: 37  C

Reactor typea Batch

Table 2 (continued)

38 L H2/kg VSadded

33.8 L H2/kg VSadded

60
4 L H2/kg VSadded 29
5 L H2/kg TVSadded 50.9 L H2/kg VSadded

51 L H2/kg VSremoved 41.7
2.3 mL H2/g VSadded 82.5 L H2/kg VS 127 L H2/kg VSremoved

H2 yield 43.68 L H2/kg Carbo

2.51 L H2/ Lreactor/day

3.67 L H2/ Lreactor/day

2.2
0.4 L H2/Lreactor/day 0.43
0.03 L H2/Lreactor/day 3.33 L H2/ Lreactor/day

Productivity

Tenca et al. (2011) Zahedi et al. (2016) AngerizCampoy et al. 2017 AngerizCampoy et al. (2018) AngerizCampoy et al. (2015)

Reference Sharma and Melkania (2018c) Tyagi et al. (2014) Yeshanew et al. (2018) Kvesitadze et al. (2012) Alzate-Gaviria et al. (2007)

8 K. J. Salazar-Batres et al.

24.70% 123 mL H2/kg/ day

N.A.

0.97 L H2/ Lreactor/day

Continuous stirred tank reactor (CSTR), Upflow anaerobic sludge bed (UASB) Inoculum/substrate ratio (I/S), temperature (T), initial substrate-to-microorganism ratio (S0/X0), hydraulic retention time (HRT)

6.3

OLR: 8.6 g SV/kg/d, T: 55  C

Semicontinuous

b

a

5.0–6.0

T: 34  C

Semicontinuous

Cuetos et al. (2007) EscamillaAlvarado et al. (2013)

Biohydrogen from the Organic Fraction of Municipal Solid Waste 9

10

K. J. Salazar-Batres et al.

affect the activity of hydrogen-producing bacteria since ATP would be used to ensure cell neutrality rather than to produce hydrogen. Additionally, hydrogen production could be affected by pH values lower than 5 since hydrogenase activity could be inhibited (Kapdan and Kargi 2006). Moreover, pH values in the range of 5.5–6.5 are favorable for Clostridium sp. to produce solvents such as ethanol, butanol, and acetone rather than hydrogen (Baldi et al. 2019; Akhlaghi and Najafpour-Darzi 2020). Owing to the importance of acidic pH in the fermentation process to produce biohydrogen, the value of pH 5.5 has been tested in numerous experiments (Gomez et al. 2006; Cuetos et al. 2007; Bru et al. 2012; Kumar Tyagi et al. 2014; Tyagi et al. 2014; Redondas et al. 2015; Lavagnolo et al. 2018; Sharma and Melkania 2018a, c). Favaro et al. (2013) reported that when batch reactors are operated at a pH of 5.5, a H2 yield of 70.1
4.1 mL H2/g VS is obtained, while at pH 7, a significantly lower yield of 23.4
2.9 mL H2/g VS was recorded. Baldi et al. (2019) achieved better control of the dark fermentation process in terms of kinetics and pH stability when using an automatic pH control strategy since adding a buffer solution at the beginning of the experiment was not enough to maintain adequate control of the alkalinity in the process. They obtained the highest H2 yields with maximum average values ranging from 68.5 to 88.5 L H2/kg TVS by maintaining the pH value in the range of 5.5 and 6.5. However, Kvesitadze et al. (2012) carried out the operation of a batch reactor with a pH of 9, which resulted in the inhibition of the undesirable methanogenic activity in the H2 production processes. The percentage values of hydrogen in the H2 produced at initial pH 9.0 and pH 5.5 were almost equal. Nonetheless, a significant difference was noted in the cumulative production of gas, which was 3.5 times higher in the operation with a pH of 9 than in the operation at a pH of 5.5. Hydrogen production from OFMSW may also be influenced by the temperature; the operational ranges are mesophilic (25–40  C), thermophilic (40–65  C), extremely thermophilic (65–80  C), or hyperthermophilic (>80  C) (Gopalakrishnan et al. 2019). The structure of the bacterial community and the metabolic pathways are affected by varying the temperature (Toledo-Alarcon et al. 2018). Most experiments on H2 production from OFMSW found in the literature have been carried out under mesophilic conditions with a prevailing temperature range of 35–37  C (Lay et al. 1999; Gomez et al. 2006; Alzate-Gaviria et al. 2007; Cuetos et al. 2007; Dong et al. 2010; Bru et al. 2012; Favaro et al. 2013; Sekoai and Gueguim Kana 2014; Alibardi and Cossu 2015; Redondas et al. 2015; Shah et al. 2016; Lavagnolo et al. 2018; Yeshanew et al. 2018; Sharma and Melkania 2018a, c; Baldi et al. 2019; Cesaro et al. 2020; Ebrahimian and Karimi 2020; Paillet et al. 2020, 2021; Dauptain et al. 2021). On the other hand, in experiments carried out under thermophilic conditions, a temperature of 55  C predominates (Tenca et al. 2011; Kvesitadze et al. 2012; Escamilla-Alvarado et al. 2013; Kumar Tyagi et al. 2014; Tyagi et al. 2014; Angeriz-Campoy et al. 2015, 2017, 2018; Zahedi et al. 2016; Alavi-Borazjani et al. 2021). Valdez-Vazquez et al. (2005) evaluated the effect of temperature in the mesophilic and thermophilic regimes of the semicontinuous dark fermentation of

Biohydrogen from the Organic Fraction of Municipal Solid Waste

11

OFMSW, obtaining hydrogen concentrations of 58% and 42%, respectively; likewise, a yield of 360 N mL H2/g VSremoved was recorded under mesophilic conditions, which was significantly higher than the yield of 165 N mL H2/g VSremoved observed under thermophilic conditions. In general, under thermophilic conditions, several benefits are obtained, such as higher process rates, better sanitation, higher degradation of persistent organics, and higher solubility of hydrophobic compounds (Tyagi et al. 2018). However, extreme thermophilic conditions may not be self-supported due to the amount of energy required to maintain the high temperature (Gopalakrishnan et al. 2019). Concerning agitation, some degree of stirring, slurry, or biogas recirculation is likely required to obtain adequate bioavailability of substrate for the microorganisms (Lindmark et al. 2014). The usual mixing conditions for batch tests range from 100 to 160 rpm; however, continuous vigorous mixing was found to be inhibitory for reactors operated at a high organic loading rate. This could be continuous and rapid mixing could promote faster hydrolysis and fermentation, which subsequently results in the accumulation of VFAs when they are not consumed, whereas when minimal agitation is applied, it could result in slower hydrolysis and fermentation, allowing syntrophs and methanogens to consume the fermentation products without the buildup of these compounds (Stroot et al. 2001; Tyagi et al. 2018). Stirring also prevents hydrogen from remaining dissolved in the mixed liquor, since H2 can be consumed in NADH2 generation, obtaining reduced by-products such as propionate or lactate, and direct uptake as molecular H2 and CO2, which can be used in acetate or methane by homoacetogenesis or methanogenesis (Buitrón et al. 2020). Different stirring rates in continuous systems have been used in experiments focused on H2 production from OFMSW. The minimum mixing rates correspond to those reported by Angeriz-Campoy et al. (2015 and 2017), who operated semicontinuous reactors with a stirring rate of 12 rpm, while Kvesitadze et al. (2012) operated a batch reactor with a stirring rate of 50 rpm. Ebrahimian and Karimi (2020) and Sharma and Melkania (2018a) operated batch reactors stirring at 160 and 150 rpm, respectively. The highest stirring rate was reported by Bru et al. (2012), who operated a batch reactor stirring at 300 rpm. When considering a fullscale process, it should be taken into account that the energy demand for mixing can be very significant, comprising 29–54% of the total electricity demand of the plant; nevertheless, lowering the mixing from 150 to 25 rpm represents an 83% reduction in rotation speed, which is reflected in a lower energy demand for mixing (Lindmark et al. 2014). A standardized protocol for the determination of biohydrogen potential was proposed by Carrillo-Reyes et al. (2019), who recommended the use of 150 rpm for batch tests. Another critical factor to consider in addition to the operational parameters is the inoculum used. It has been reported that inoculum characteristics greatly influence the hydrogen yields and stable microbial communities established in the reactor; therefore, it is necessary to add an inoculum to start the hydrogen production process, which is critical for the evaluation of the H2 potential test in the batch process, whose primary requirement for efficient H2 production is linked to the availability of mixed microbial consortia in which H2-consuming and

12

K. J. Salazar-Batres et al.

nonhydrogen-producing bacteria are suppressed (Favaro et al. 2013; Dauptain et al. 2021). The most common inoculum sources for H2 production from OFMSW reported in the literature include: • Anaerobic digestion sludge obtained from domestic wastewater treatment (Sekoai and Gueguim Kana 2014; Alavi-Borazjani et al. 2021) • Granular sludge collected from a full-scale upflow anaerobic sludge blanket (UASB) digester (Favaro et al. 2013; Alibardi and Cossu 2015; Lavagnolo et al. 2018) • Seed inoculum obtained from a laboratory-scale reactor for treating different types of waste from a OFMSW or slaughterhouse (Cuetos et al. 2007; Dong et al. 2010; Kumar Tyagi et al. 2014) • Mixed culture obtained from a full-scale landfill AD bioreactor (Yeshanew et al. 2018; Paillet et al. 2020, 2021) • Pure cultures including Clostridium acetobutylicum NRRL B-591 (Ebrahimian and Karimi 2020), Enterobacter aerogenes PTCC 1221 (Ebrahimian et al. 2020), Enterobacter aerogenes, and E. coli (Sharma and Melkania 2018a) Pecorini et al. (2019) evaluated biological hydrogen production in batch fermentation assays using four types of inocula collected from different sources; they concluded that the hydrogenic process was highly dependent on the type of inoculum. Using activated sludge collected from a municipal wastewater treatment plant was the inoculum that best valorized biodegradable waste via dark fermentation. To increase H2 production through proper inoculum selection, it is necessary to remove hydrogen-consuming and nonhydrogen-producing bacteria from mixed microbial consortia; for this, several pretreatment methods have been proposed, including heat treatment, acidification, basification, aeration, and freezing (Favaro et al. 2013). Heat-shock pretreatment predominates among the pretreatments used in hydrogen production from OFMSW since it is possible to gather hydrogen producers and inactivate methanogens (Akhlaghi and Najafpour-Darzi 2020).

3

Microbial Communities in Dark Fermentation from OFMSW

Microorganisms performing the dark fermentation process are obligate or facultative anaerobes (Mathews and Wang 2009). The substrate characteristics and environmental conditions influence the microbial composition and determine the dominance of some group of microorganisms (Zahedi et al. 2016). H2-producing bacteria can be divided into three groups: spore-forming obligate anaerobes (e.g., Clostridium spp.), nonspore-forming obligate anaerobes (belonging to the phyla Firmicutes and Bacteroidetes), and facultative anaerobes with fermentative metabolism, such as members of the Enterobacteriaceae and Bacillaceae families, corresponding to the Enterobacter and Bacillus genera, respectively (Cabrol et al. 2017).

Biohydrogen from the Organic Fraction of Municipal Solid Waste

13

The heterogeneous composition of OFMSW as a substrate promotes the growth of a complex microbial community in dark fermentation. A positive effect can be obtained from the interaction of H2 producers and microorganisms unable to generate H2 (or those having low H2 production efficiency). These beneficial effects include O2 depletion (e.g., Bacillus and Klebsiella), medium acidification (e.g., lactic acid bacteria), and oxidation of short-chain fatty acids, preventing their accumulation and achieving a buffering effect (e.g., Leuconostocaceae and Streptococcaceae) (Cabrol et al. 2017). When using OFMSW as the substrate, some unusual H2 producers have been reported as dominant species, including lactic and propionic acid consumers, such as Megasphaera and Syntrophobacter or Syntrophomonas, respectively (MorenoAndrade et al. 2015). It has also been reported that the presence of microorganisms favors the formation of granules (such as Prevotella and Klebsiella), increasing the biomass retention in the reactor, reducing the washout, and offering structural advantages against toxic or antagonistic environments for H2-producing bacteria (Liang et al. 2010; Cabrol et al. 2017). Most microbial studies for H2 production have studied wastewater, and simple substrates (e.g., glucose) resulting in the predominance of Clostridium spp. However, reports using OFMSW as a substrate confirmed the predominance of genera associated with the complexity of the substrate. For instance, Elsamadony et al. (2015a) reported a microbial consortium with 92–93% affiliated Enterobacter, Escherichia, Buttiauxella, and Pantoea as H2-producing bacteria. Shah et al. (2016) also highlighted the ability of Bacillus sp. to convert OFMSW into H2. In the case of reactors dealing with low-moisture OFMSW, phylogenetic analysis of samples revealed the dominance of Pseudomonas fulva (Elsamadony et al. 2015b). The presence/dominance of facultative anaerobes (e.g., Enterobacteriaceae, Bacillales, Shewanella, Pseudomonas), alone or in combination, has been reported to be preferable under full-scale conditions using OFMSW. The predominance of facultative anaerobes achieved H2 yields similar to or higher than those of the conventional process with a predominance of Clostridium spp., especially under very specific operating conditions, such as cases with the presence of recalcitrant substrates (Cabrol et al. 2017). The composition of the microbial community changes as the reactors acclimate to the substrate and environmental conditions, reducing microbial diversity. In this context, Paillet et al. (2021) reported microbial abundance reduction from the beginning of reactor operation using OFMSW. The initial predominance of Pseudomonadales, Clostridiales, Lactobacillales, and Bacillales shifted to Clostridiales and Lactobacillales, the latter two being bacterial orders passing from initial and final relative abundance values of 37–63% and 5–60%, respectively, showing good stability in the microbial community. Concerning the Lactobacillales order, all the species observed were related to the genus Lactobacillus (five species obtained, described all as lactate producers). The equilibrium observed between Lactobacillales and Clostridiales can be related to stable H2 production, since the Clostridium beijerinckii and Clostridium butyricum species are able to consume lactate and acetate to produce butyrate and hydrogen (García-Depraect et al. 2019).

14

K. J. Salazar-Batres et al.

The role of lactic acid bacteria in H2 production, especially OFMSW, needs to be studied to develop efficient processes.

4

Selection of OFMSW in Recycling Facilities for H2 Production

The separation of residual material is a usual practice in developed countries, where the segregation of glass, metals, paper, and OFMSW is usually done. However, in developing countries (mainly in Africa, Asia, and Latin America), waste separation rates are typically low. This can be due to a number of factors, such as the low awareness of populations of the benefits from segregating wastes and the low willingness to comply with segregation practices due to a lack of incentives or penalties by the local authorities (Schingnitz 2017). In these cases, separation facilities are the solution to obtain recycling valued subproducts, and refused OFMSW is usually used to produce compost or disposed of in landfills. This organic waste is suitable for use as a substrate for H2 production, but in these separation facilities, OFMSW usually includes a fraction of nonfermentable materials. The postselection of the organic fraction from municipal solid waste is necessary to reduce the nonfermentable material, avoiding a possible decrease in H2 production, and damage to the process equipment (pumps, engines, etc.) (Romero-Güiza et al. 2014). A recent case study from our laboratory, OFMSW obtained in an actual separation facility (600 tons/d) located in Querétaro, Mexico, demonstrated the effect of the presence of the remaining nonfermentable material (plastics, glass, and metals) on the potential of H2 production from the final recycling separation of municipal solid waste. A sample of OFMSW was collected from an urban solid waste separation plant (solid waste delivered to the plant as a source segregated at the household level in an area with a population of one million inhabitants). As a final nonrecyclable product, OFMSW contained 78.8% organic solid waste, and the rest consisted of paper 12.2%, plastic 5.1%, glass 3.0%, metal 0.2%, wood 0.5%, and bones 0.2%. The presence of these materials can decrease the potential of hydrogen production by dark fermentation processes and lead to damage to industrial equipment such as pumps and grinders. A selected OFMSW (eliminating nonfermentable material) was compared with the raw OFMSW. Figure 2 shows the cumulative hydrogen production curves from the batch tests. The highest production of H2 was obtained in the reactors with a concentration of 15 g TS/L where the selected OFMSW was used as substrate; the accumulated volume of 167.4
20.1 mL H2 was achieved, and a yield of 31.0
3.7 mL H2/g TS was added. When using raw OFMSW, a cumulative volume of 62.3
2.5 mL H2 was obtained, and a yield of 11.6
0.6 mL H2/g TS was added. Table 3 shows the following kinetic parameters obtained from fitting the experimental H2 production to the Gompertz model: maximum amount of hydrogen produced (Hmax), maximum hydrogen production rate (Rmax), and lag time before exponential hydrogen production (ʎ). The H2 yield, total H2 volume generated, and TS removal were also determined. The results obtained showed a superior performance when using selected OFMSW compared

Biohydrogen from the Organic Fraction of Municipal Solid Waste

15

Fig. 2 Cumulative hydrogen productions from Raw and Selected OFMSW Table 3 Results from hydrogen production and mathematical model parameters

Run 5 (g TS) Raw OFMSW 10 (g TS) Raw OFMSW 15 (g TS) Raw OFMSW 5 (g TS) OFMSW 10 (g TS) OFMSW 15 (g TS) OFMSW

Total volume (mL H2) 15
1.1

Yield (mL H2/g TSadded) 8.7
0.08

TSremoved (%) 20.7
1.3

Hmax (mL H2) 16.79

Rmax (mL H2/ L h) 3.16

47.5
1.6

13.5
0.96

21.6
0.5

52.35

2.25

0.74

62.3
2.5

11.6
0.6

25
0.4

63.44

2.34

1.17

32.1
1.1

36.3
0.2

12
0.3

27.87

4.96

2.26

92.2
10.9

27.3
3.0

14.5
0.7

103.74

15.30

4.45

167.4
20.1

31.0
3.7

14.9
2.1

208.13

14.46

3.26

ʎ (h) 0.68

with raw solid waste that included nonfermentable compounds. The performance with raw OFMSW was only superior in terms of ʎ, supporting a value of 1.17 h, which was shorter than that for the selected OFMSW, which supported a value of 3.26 h. These results indicate that significant differences in H2 production can be expected when nonfermentable compounds are present in the OFMSW. Therefore, the composition of the substrate has a distinct influence on the production of hydrogen. In reactors where raw OFMSW was used, the maximum H2 production and the yields were lower. When using a higher selection level of OFMSW, the maximum H2 production and yields increase. It was also observed that the selected

16

K. J. Salazar-Batres et al.

OFMSW supported a higher production of acetate and butyrate, increasing by 10% and 110%, compared with the raw OFMSW, respectively. The conclusion of this study was that a secondary separation of remaining nonfermentable materials can increase H2 and butyrate production.

5

Effect of Specific Slowly or Nonfermentable Matter

A type of slowly or nonfermentable material affects H2 production when mixed with OFMSW (e.g., the presence of lignin, paper waste, citric waste, etc.). In the case of lignocellulosic compounds, feedstock pretreatment (acid or alkali), saccharification strategies, and fermentation technologies can be applied (Cheng et al. 2011). Paper waste is the second-largest category in municipal solid waste (MSW) (Hoornweg and Bhada-Tata 2012; Qin et al. 2019), including toilet paper/tissue, office printing paper, newspaper, paper bags, and cardboard, which all tend to be mixed with MSW (Fonoll et al. 2016) and need to be separated for recycling. However, in the separation facilities paper waste can be commingled with OFMSW. The combination of food waste and paper waste is potentially beneficial to recover bioenergy in wastepaper products (Fonoll et al. 2016). Qin et al. (2019) investigated the effect of paper waste content on H2 and bioCH4 production from OFMSW, demonstrating that increasing the paper waste content slightly decreased the removal efficiency of volatile solids from 84.9% to 78.4%; the H2 yields increased from 50 to 79 NL H2/kg VSfed, while the CH4 yields decreased from 426 to 329 NL-CH4/kg VSfed. In contrast, a study by Gomes et al. (2020) reporting the effect of toilet paper on H2 production from OFMSW showed that H2 production decreases with increasing paper proportion in the mixture. The slow hydrolysis of the cellulose contained in the toilet paper produced only 31.6 mL H2 and a lag time of 16 h with low carbohydrate consumption (5.9%) in the test with only paper as the substrate, limiting H2 production (and overall results) when paper is contained in a mixture with OFMSW. This strongly suggested that the utilization of toilet paper waste for H2 production points to the need to adopt pretreatments or the possibility of increasing the OFMSW/paper toilet ratio (or using other cosubstrates such as food waste) to optimize hydrogen fermentation production. Citrus waste has been demonstrated to have an inhibitory effect on anaerobic processes. The world production of citrus is estimated to be 98 million metric tons. Oranges account for half of the production, followed by tangerines/mandarins, lemons/limes, and grapefruit (USDA 2021). The limonene and eugenol, present in citrus waste, are known as microbial inhibitory agents toward some fermentative microbes, including those that are responsible for transforming polysaccharides into H2 and CO2 through dark fermentation (Khamdan Cahyari et al. 2018). Some examples are those reported by Khamdan Cahyari et al. (2018) and Mizuki et al. (1990), using citrus (Citrus sinensis Osbeck) and melon (Cucumis melo L.) waste for H2 production that showed inhibitory problems. Based on the kinetics, the noncompetitive model most accurately fit the H2 production rate data, which implies that glucose as substrate and limonene/eugenol as inhibitor could interact with the

Biohydrogen from the Organic Fraction of Municipal Solid Waste

17

microbial cell as bioenzymes at the same time. The reduction of the quantity of waste containing the inhibitor can be obtained by using a combination of substrates in a codigestion or by diluting the substrate conducting the fermentation at a low organic dry matter concentration (Khamdan Cahyari et al. 2018). Another strategy to improve the dark fermentation of citrus waste is the use of immobilized cultures (in alginate and activated carbon), demonstrating an increase in H2 production from orange waste by two to sevenfold depending on the substrate composition (Damayanti et al. 2015). A case study from our lab (Mar-Alvarez et al. 2014) using organic waste from a seafood restaurant containing 15% Persian lime (Citrus latifolia) reported the effect of this citrus waste on H2 production. The effect of different lime percentages on H2 production was investigated. The organic waste was composed of fruits and vegetables 38
8% (including a percentage of 15% lime), meat 10
3%, flours 30
8, and other fermentative waste 22
7%. The H2 percentage in the biogas was between 25 and 60%. Higher H2 production was observed when 6.3% lime, a decrease in H2 production was observed. The presence of lime in OSW increased the lag time (96–180 h). These results agree with the proposal of reducing the percentage of citrus material in the process. In this case, the codigestion of noncitrus waste can be a solution for this type of waste.

6

Inhibition of H2 Production from OFMSW

H2 production through dark fermentation has proven to be a technology that offers numerous advantages. However, it has not yet been fully commercially applied. One of the crucial reasons is that H2 production could be limited by several inhibitory factors restricting its commercial potential (Chen et al. 2021a). According to Bundhoo and Mohee (2016), inhibitors of the dark fermentation process are classified as preprocessing and in-processing. Preprocess inhibitors include: • Inhibitors in the mixed microflora: H2-consuming bacteria (hydrogenotrophic methanogens, homoacetogens, propionate producers, sulfate-reducing bacteria, nitrate-reducing bacteria) and lactic acid bacteria • Inhibition by metal ions: High concentrations of light metal ions (magnesium, sodium, calcium) or heavy metal ions (iron, nickel, copper, manganese, zinc, chromium, cadmium, lead) • Inhibitors from substrate pretreatment: furan derivates (furfural, 5-hydroxymethylfurfural) and phenolic compounds (phenol, vanillin, syringaldehyde) In-process inhibitors include the accumulation of ammonia, H2 partial pressure, and end products, such as acetic acid/acetate, butyric acid/butyrate, propionic acid/ propionate, formic acid/formate, and ethanol. Elevated levels of volatile fatty acids and long-chain fatty acids inhibit hydrolytic bacteria, which can occur via activity

18

K. J. Salazar-Batres et al.

loss, reversible reduction of hydrolases, or irreversible impacts resulting from changes in the enzyme chemical structure (Amha et al. 2018). Siegert and Banks (2005) investigated the effect of VFAs in batch systems and found that VFAs caused the inhibition of cellulolytic activity at a concentration of 2 g/L and therefore the inhibition of the rate of cellulose hydrolysis; additionally, the fermentation of glucose was slightly inhibited at VFA concentrations above 4 g/L. Sharma and Melkania (2018d) evaluated the effect of the phenolic inhibitors m-cresol, pentachlorophenol, bisphenol-A, and catechol on H2 production from OFMSW, concluding that the presence of phenolic compounds considerably affected hydrogen production and yield. When concentrations from 0.5 to 25 mg/L of the inhibitory compounds were applied, hydrogen production decreased. Concerning the inhibition by heavy metals in the production of H2 from OFMSW, Sharma and Melkania (2018b) evaluated the effect of heavy metals (lead, mercury, copper, and chromium) and found that H2 production was affected by the heavy metal type and concentration. Lead, mercury, and chromium inhibited hydrogen production at concentrations of 0.5–100 mg/L, while copper caused an increase in H2 production at low concentrations (10 mg/L). Although inhibitory compounds can be regarded as a problem in the dark fermentation process, there are a few strategies to control or avoid inhibition, such as the dilution of inhibitors, since it is possible to reduce the concentration of inhibitors to under the inhibitory threshold by adding extra broth or even tap water. Additionally, another method to mitigate the inhibition caused by inhibitors is to inactivate inhibitor, such as phenolic compounds by detoxification processes such as enzymatic hydrolysis. In addition, adjusting some operational parameters can obtain different hydrogen production results, which can also mitigate inhibition (Chen et al. 2021a).

7

Effect of Trace Elements (TE) on Biohydrogen Production

In addition to essential macronutrients such as carbon (C), nitrogen (N), phosphorus (P), and sulfur (S), anaerobes also require trace elements at relatively lower concentrations (Pobeheim et al. 2010; Choong et al. 2016; Nordell et al. 2016). A trace element is a chemical element whose concentration is very low, and these elements are essential components of cofactors and enzymes (Thanh et al. 2016; Hijazi et al. 2020). Trace elements play an essential role in the metabolic physiology and reproduction of bacteria that produce hydrogen during fermentation. The H2 production reactions are catalyzed by [Fe-Fe]–hydrogenases, [Ni–Fe]-hydrogenases, and [Ni–Fe–Se]-hydrogenases (Chen et al. 2021b; Cieciura-Wloch et al. 2020; do Nascimento Junior et al. 2021). The lack of microelements (as Fe, Ni, and Se) in the OFMSW composition is not usual, but its characterization can determine the necessity of external supplementation to increase the H2 production. In this way, several investigations have been carried out to find the optimal doses of TE to enhance H2 production (Table 4). Both low and high TE concentrations may significantly affect the hydrogen yields and production rates because of nutrient limitations or inhibitory

Anaerobic digested sludge heat treated

Anaerobic sludge heattreated

Anaerobic sludge heat treated

Waste-activated sludge

The experiments were performed using sugar beet pulp

Sugar beet pulp

Fruit and vegetable wastes

Granular anaerobic heat treated

Inoculum Anaerobic sludge heat treated

Substrate Acid-hydrolyzed WPT hydrolysate was used as substrate Cheese whey wastewater

Fe, Co, Ni, Zn

Fe added as Fe2O3

Fe added as Fe2O3

Co, Ni, Zn, (1.3 mg/L) and Fe (50 mg/L) Ni2+

Trace element used Fe added as FeSO47H2O

Table 4 Effect of trace elements on biohydrogen production

Fe (7.5), Co (8.71), Ni (29.48), Zn (79.76) mg/L

1 g Fe2O3/dm3

0.1 g Fe2O3/dm3

5 mg/L

Concentrations C/N/P/Fe ratio: 100/5/9/0.278

Batch, initial pH: 7, temperature: 55
1  C

Semicontinuous, temperature: 35
1  C

Reactor type and operational conditions Batch pH: 6.7, temperature: 37  C Batch, pH ¼ 5.5, temperature: 36  C Batch, pH: 7.0, temperature: 35  C Batch, pH: 5.5, temperature: 35  C

31–76 mL H2/g VS

52.11 dm3 H2/kg VS

200 dm3 H2/kg VS

3.5 mol H2/mol lactose consumed and 218.6 ml H2/g lactose consumed 0.48 mL H2/mg COD

Yield 0.656 mol H2/mol glucose

(continued)

CieciuraWloch et al. (2020) CieciuraWloch et al. (2020) Keskin et al. (2018)

Reference Argun and Onaran (2017) Azbar et al. (2009) Chen et al. (2021a)

Biohydrogen from the Organic Fraction of Municipal Solid Waste 19

Glucose

Substrate OFMSW

Table 4 (continued)

Inoculum E. coli cell suspension and Enterobacter aerogenes cell suspension Sewage sludge heat treated Fe

Trace element used Fe as ferric oxide

200 mg Fe2+/L

Concentrations 100 mg/L

Batch, pH: 5.5, temperature: 37  C

Reactor type and operational conditions Batch, pH: 5.5, temperature: 37  C 217.4
4.2 mL H2/g glucose

Yield 872.5
10.1 mL and 58.7 mL H2/g carbohydratesinitial

Reference Sharma and Melkania (2018e) Zhang et al. (2017)

20 K. J. Salazar-Batres et al.

Biohydrogen from the Organic Fraction of Municipal Solid Waste

21

impacts at high concentrations. For this reason, optimization of TE dosing is an important issue (Argun and Onaran 2017). Different concentrations of TEs have been proposed to increase hydrogen production. As reported by Soltan et al. (2019), the optimum dosages of Ca2+, Fe2+, Mg2+, Zn2+, and Na+ are in the range of 3000, 100–300, 100–600, 12, and 350–1000 mg/L, respectively. Chen et al. (2021b) indicate that adding 5 mg Ni2+/ L to the dark fermentation process can increase cumulative H2 production by 29% when using sludge from wastewater treatment as the substrate. However, a 0.1 mg Ni2+/L concentration was found to be optimum for H2 generation from glucose (Wang and Wan 2008). There is no optimal dose of any of the TEs that can be applied in a general way since hydrogen production is strongly dependent on the types of microorganisms present in the inoculum and the substrate used. Therefore, experiments carried out with different TEs on different substrates and under different conditions are important to identify the doses that can increase hydrogen yields (Keskin et al. 2018), particularly in the case of complex substrates such as OFMSW.

8

Impact of Metallic Nanoparticles on Biohydrogen Production

Nanotechnology applies functional materials at the nanoscale (1–100 nm), opening attractive research niches in environmental engineering (Abdelsalam et al. 2017). Due to their larger surface area-to-volume ratios, better specificity, greater capability of self-assembly, and dispensability, nanoadditives have been reported to affect the performance of anaerobic digesters and other anaerobic processes, such as H2 production from OFMSW (Zhu et al. 2021). Table 5 summarizes the nanoparticles thus far studied in H2 production systems through dark fermentation processes. Cheng et al. (2020) demonstrated that the appropriate dose of magnetite nanoparticles (MNPs) could promote fermentative H2 and CH4 cogeneration from glucose; in the presence of 200 mg/L MNPs, hydrogen production increased by 21.1%. Additionally, during the hydrogen-producing stage, the hydrogen production pathway shifted from the butyrate pathway to the acetate pathway, leading to a high NADH/NAD+ ratio. Malik et al. (2021) investigated the effect of two different iron compounds (zero-valent iron nanoparticles: nZVI and iron oxide nanoparticles: nIO) and found that the addition of nZVI and nIO resulted in 71% and 69.4% enhancements in biohydrogen production, respectively. Nevertheless, a higher nanoparticle concentration can cause inhibition of hydrogen production. Zhang et al. (2021b) studied the effect of nickel ferrite nanoparticles (NiFe2O4 NPs) on increasing hydrogen yields and found that moderate amounts (50–200 mg/L) promoted H2 generation, while excessive amounts of NiFe2O4 NPs (over 400 mg/L) lowered H2 productivity. In the same study, they obtained the highest yields of 222 and 130 mL/ g glucose in the 100 mg/L (under mesophilic conditions at 37  C) and 200 mg/L NiFe2O4 NPs (under thermophilic conditions at 55  C), and the values were 38.6% and 28.3% higher than those in the control groups without nanoparticles, respectively. Several types of nanoadditives with trace elements have been proven to

Anaerobic sludge heat treated

E. aerogenes ATCC13408

Sewage sludge

Anaerobic inoculum

Glucose

Glucose

Glucose and peptone

Anaerobic mixed culture heat treated

Inoculum Consortium composed of 34.5% Ruminococcus, 64% Clostridium, and 1.5% from other genera

Sucrose

Substrate Soft drink wastewater and corn steep liquor Molassesbased distillery wastewater Sucrose

MNPs (magnetic Fe3O4 NPs; purity: 99.5%; diameter: ~20 nm) Fe-ferric oxide/ carbon nanoparticles (FOCNPs) Ni and Fe as NiFe2O4 NPs

Au/Zn–Mg–Al (hydrotalcites)

Wimonsong et al. (2014) Cheng et al. (2020)

Zhang et al. (2018)

2.74
0.14 mol H2/mol sucrose 260.9 (mL/g glucose)

218.63 mL H2/g glucose

222 mL H2/g glucose

Batch, pH: 6.9
0.1, temperature: 37  C.

100 mg/L

Zhang et al. (2021b)

Wimonsong et al. (2013)

2.30
0.37 mol H2/mol sucrose

251 mL H2

Reference do Nascimento Junior et al. (2021) Malik et al. (2021)

Yield H2 production: 17.67
0.54 mL

Batch, initial pH: 6.6, temperature: 37  C

Batch, initial pH ¼ 5.5, temperature ¼ 37  C Batch, initial pH: 5.5, temperature: 37  C Batch, pH ¼ 6.0
0.2, temperature ¼ 37  C

Batch, initial pH ¼ 6.0, temperature: 37  C

Reactor type and operational conditions Batch, pH ¼ 7.0, temperature: 37  C

200 mg/L FOCNPs

MNPs concentration (200 mg/L)

167 mg/L

Zero-valent iron nanoparticle: and iron oxide nanoparticles at 0.7 g/L, pH: 6 167 mg/L

Fe

Mg, Al hydrotalcite materials

Concentrations 200 mg/L of LMNP

TE used Fe-lignin magnetic nanoparticles

Table 5 Impact of metallic nanoparticles on biohydrogen production

22 K. J. Salazar-Batres et al.

Glucose and peptone Protein and glucose

Anaerobic sludge heat treated

Anaerobic inoculum

Ni and Fe as NiFe2O4 NPs Co and Fe as cobalt ferrate nanoparticles 0.4 g/L

200 mg/L

Batch, pH: 6.9
0.1, temperature: 55  C Batch, pH: 6.9
0.1, temperature: 37  C 130 mL H2/g glucose 205.24 mL/g glucose Zhang et al. (2021b) Zhang et al. (2021a)

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improve the dark fermentation performance at low concentrations. However, it is necessary to continue studying the effects of nanoparticles on complex substrates such as OFMSW to determine the necessary doses that help increase H2 yields. Acknowledgments The support granted by the PAPIIT projects IN102722 and TA100121 from DGAPA-UNAM is acknowledged.

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Biohydrogen from Food Waste Iván Moreno-Andrade, Karina J. Salazar-Batres, Edith Villanueva-Galindo, Jonathan F. Cortez-Cervantes, Ulises Jimenez-Ocampo, Julián Carrillo-Reyes, and Alejandro Vargas

Abstract

Food waste is inevitable in the global food system during production, processing, distribution, retail, and consumption; food waste has the potential to be used as a substrate for hydrogen production by the dark fermentation process. Its high carbon composition and easy-to-hydrolysate characteristics result in a higher hydrogen production potential than other organic wastes. Depending on the food waste’s composition, its characteristics can vary (e.g., differences in carbohydrates, protein ratio, total solids, pH, lignocellulosic content). Several factors affect the bioprocess for hydrogen production, such as temperature, origin of the food waste, and pretreatment of the waste and the inoculum. Volumetric hydrogen production and yield depend on the process parameters. In this chapter, the composition of food waste and its hydrogen potential, the microbial community in the process, the effect of lactic acid bacteria, and the possible interaction with other hydrogen fermentative microorganisms are discussed. Strategies for the optimization of hydrogen production are also presented: codigestion, bioaugmentation, and use of automatic control strategies. Keywords

Automatic control strategy · Biohydrogen · Biodigestion · Dark fermentation · Food waste · Lactic acid bacteria

I. Moreno-Andrade (*) · K. J. Salazar-Batres · E. Villanueva-Galindo · J. F. Cortez-Cervantes · U. Jimenez-Ocampo · J. Carrillo-Reyes · A. Vargas Laboratory for Research on Advanced Processes for Water Treatment, Unidad Académica Juriquilla, Instituto de Ingeniería, Universidad Nacional Autónoma de México, Querétaro, Mexico e-mail: [email protected] # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Kuddus et al. (eds.), Organic Waste to Biohydrogen, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-1995-4_2

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Introduction

Energy production from fossil fuels has led to large-scale industrial expansion that has entailed the gradual depletion of natural resources and an increase in greenhouse gas emissions, causing environmental pollution and adverse effects on the global climate (Noblecourt et al. 2018; Abreu et al. 2019). Energy from biomass is a promising alternative to reduce our dependency on fossil fuels. It has vast potential as a low-cost raw material for producing bioproducts, such as alcohols, carboxylic acids, methane (CH4), and hydrogen (H2) (Slezak et al. 2020). H2 is a clean and environmentally friendly form of energy; it has an energy yield of 122 kJ/g, which is 2.75-fold higher than hydrocarbon-based energy (Zhao et al. 2020; Dahiya et al. 2021). H2 produced by the dark fermentation process can help combat the soaring energy demand, environmental pollution, and fossil fuel dependence (Zhou et al. 2013). Worldwide, approximately 1.3
109 tons of food waste (FW) are discarded in landfills, contributing to 3.3
109 tons-CO2-eq/year of greenhouse gas emissions (GHG) (Karthikeyan et al. 2018). According to the Food and Agriculture Organization of the United Nations (FAO) (2019), significant food losses can be due to a variety of reasons, such as inadequate storage conditions, decisions made earlier in the supply chain predisposing products to shorter shelf-life losses as a result of inadequate facilities, technical malfunctions, or human error. Nevertheless, FW can be an economical source for fermentative H2 production since it has a relatively high total solid (TS) content (20%), with approximately 90% corresponding to volatile solids (VS) (Capson-Tojo et al. 2018a). H2 production via dark fermentation is performed by the Embden-Meyerhof pathway requiring specific substrates, with the degradation of monosaccharides and glycerol being the most common pathway for H2 production (Kobayashi et al. 2012). FW is mainly composed of carbohydrates, proteins, and lipids, and it has been proven that FW composition affects the efficiency of the process in terms of H2 production and other by-products (Alibardi and Cossu 2015). Carbohydrates are the preferred substrates for dark fermentative hydrogen-producing bacteria such as Clostridium species (Zhou et al. 2013). Numerous investigations have been carried out to determine the potential for H2 production from different types of FW. FW is a complex mixture of different wastes, including fruits, vegetables, meat, eggs, cereals, bread, pasta, and sometimes paper. This mixture has been used in several studies to produce H2 through dark fermentation (DF) processes (Li et al. 2008; Kim et al. 2009; Cappai et al. 2014; Gadhe et al. 2014; Guo et al. 2014; Alibardi and Cossu 2016). FW is variable in its composition, depending on the carbohydrate, protein, and lipid contents; for this reason, H2 production from FW has a wide range of H2 yields. Data from 12 different studies (reported as FW, fruits, meat, cereals, and vegetables) were analyzed as box plots (Fig. 1). The average H2 yield obtained from FW was 112 mL H2/g VSadded. However, if the FWs reported are separated into fruits, meat, cereals, and vegetables, the yield ranges from 56 to 196 mL H2/g VSadded. This differentiation needs to be considered when comparing H2 production from FW, since sometimes the variation in the composition will influence the results.

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Fig. 1 Hydrogen yields reported from food waste

Fruits are one of the most widely used residues in the production of H2 due to their high carbohydrate contents (50–70% VS) and because they have relatively large economic and environmental potentials; in addition, large amounts of waste are generated by the consumption of fruits and industrial processing, which corresponds to between 10% and 65% of raw fruit (Akinbomi and Taherzadeh 2015; CapsonTojo et al. 2018a). Regarding H2 production of the analyzed residues, fruits have the highest yields with values as high as 523 mL H2/g VS, averaging 308 mL H2/g VS. Guo et al. (2014) evaluated H2 production in different protein-rich wastes that included chicken meat, fish residues, and meat residues from restaurants; in these residues, proteins represented 14.3–19.2% of the total solids and 69–100% of the volatile solids. The results obtained showed lower yields of H2, with only 2–7 mL H2/g TS produced, suggesting that the protein composition was unfavorable to bioH2 production. Fermentative H2 production has also been tested using cereals as substrates; some examples include rice slurries, rice straws, maize stalks, sorghum, and oats (Okamoto et al. 2000; Fang et al. 2006; Dong et al. 2009; Kobayashi et al. 2012; Guo et al. 2014). The yields obtained vary from 24.3 to 346 mL H2/g VS, averaging 137.8 mL H2/g VS; these yields are lower than those achieved using fruits as substrates, even though they are also substrates rich in carbohydrates. Concerning vegetable waste, the range of H2 yields obtained varies from 14.9 to 173.3 mL H2/g VSadded, averaging 79 mL H2/g VSadded; these yields have been reported by various studies (Okamoto et al. 2000; Shin et al. 2004; Dong et al. 2009; Kobayashi et al. 2012; Guo et al. 2014; Ghimire et al. 2015; Alibardi and Cossu 2016). Kobayashi et al. (2012) classified these residues as cellulose-rich substrates; the H2 yields obtained when these materials were used as substrates were lower than those obtained by cereal waste and uneaten vegetable kitchen waste, which may be due to poor degradation. However, vegetables potentially have high H2 conversion

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efficiencies, but the hydrolysis of cellulosic material by microorganisms and enzymes without physicochemical hydrolytic pretreatments is generally tricky and has a low reaction rate. As mentioned above, the composition of the substrates can influence the yields of fermentative H2. Table 1 compiles examples of the sources of waste collection and its composition. In some cases, the waste was collected in kitchens, canteens, universities, or restaurants; in these locations the FW had a varied composition, and included fruits, vegetables, meat, bread, pasta, etc. Meanwhile, when the FW was simulated, ingredients that made up the substrate were purchased from local supermarkets; therefore, they were not necessarily waste products. An example of simulated FW is the study carried out by Akinbomi and Taherzadeh (2015), who acquired different fruits to prepare a substrate and operate a reactor under thermophilic conditions, obtaining yields of up to 513 mL H2/g VSadded. In addition to the composition of the substrate, operating conditions can affect bio-H2 production; one of the parameters that are usually varied is the temperature. Reactions that take place in the DF process can be conducted at mesophilic (25–40  C), thermophilic (40–65  C), extreme thermophilic (65–80  C), or hyperthermophilic (>80  C) temperatures (Fang et al. 2006). Temperature can influence the rate of hydrolysis and the production of volatile fatty acids (VFAs) and thus the final pH; for this reason, temperature is one of the factors that most affect the DF process. Abubackar et al. (2019) operated a fully automated stirred tank at 55  C using a mixture of different fruits and vegetables as the substrate, and obtained a yield of 27.19 NmL H2/g VSadded; on the other hand, Akinbomi and Taherzadeh (2015) operated a CSTR reactor at the same temperature using a mixture of fruits as the substrate, and obtained a yield greater than 513 mL H2/g VS. Most of the investigations that focused on the production of fermentative H2 have been carried out under mesophilic conditions (Moreno-Andrade and Buitron 2015; Han et al. 2017; Azadeh Alavi-Borazjani et al. 2019; Cieciura-Wloch et al. 2020; Santiago et al. 2020; Slezak et al. 2020; Hovorukha et al. 2021), and obtained yields ranging from 52 cm3/g VS to 169 mL H2/g VSadded. pH is another parameter that can influence the performance of the process; various values have been reported for FW, with a predominance of pH 5.5 (Moreno-Andrade and Buitron 2015; Santiago et al. 2020; Wang et al. 2020), although the operation has also been carried out at pH values between 4 and 4.6 (Han et al. 2017); however, Cappai et al. (2014) recommend a starting value of pH at 6.5 for food waste. H2 production from FW is also influenced by the presence of an effective hydrolyzing H2-producing microbial community, which depends on the inoculum source and the inoculum pretreatment method (Ghimire et al. 2016); Akinbomi and Taherzadeh (2015) carried out the operation of reactors under thermophilic conditions at 55  C to produce H2, and they used sludge obtained from a thermophilic biogas plant located in Borås, Sweden. Alibardi and Cossu (2016) used anaerobic sewage sludge collected from a large-scale anaerobic digester from a municipal wastewater treatment plant located in Padua, Italy, as inoculum. Liu et al. (2018) utilized anaerobically digested sludge obtained from the Gaobeidian Wastewater Treatment Plant located in Beijing, China. Thermal pretreatment has been used in various studies focusing on producing H2 by DF to select H2-producing

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Table 1 Yields and rates of bio-hydrogen production from different types of food waste Origin Izmir municipality bazar (Turkey)

Local shop (Borås, Sweden)

University canteen (University of Aveiro, Portugal) Collected in individual households of the authors (Lodz, Poland) Collected from the local supermarket (Hangzhou, China) Purchased from a local grocery store (Kyiv, Ukraine)

FW simulated FW from a university cafeteria (Universidad Nacional Autónoma de México, Mexico) Buffet-type restaurant

Composition Radish, pepper, pomegranate, pear, apple, pumpkin, mandarin, tomato, onion, potato, peach, lemon, eggplant, carrot, orange, cucumber, cabbage, and grape Apple, banana, grape, melon, and orange Mainly composed of vegetables and fruits, meat and, potato, bread, rice, and pasta Fruit and vegetable waste

Waste bread

Raw and cooked potatoes, tomatoes, zucchini, cucumbers, carrots, cabbage, apples, parsley, boiled chicken fillet, cooked macaroni, bread, and apple juice Rice, cabbage, pork, and tofu Residues from fruits and vegetables, protein residues (meat and egg), flour-derived residues (including bread) Fruits and vegetable, flour-

Operational conditions Fully automated stirred tank, temperature: 55  1  C, stirred at 250 rpm, pH: 5.5–6.5

Results 27 NmL H2/g VSadded

Reference Abubackar et al. (2019)

CSTR, temperature: 55  1  C, pH: 5

513 mL H2/ g VS

Batch temperature: 37  C, stirred at 120 rpm, pH: 7.5 (initial)

169 mL/g VSadded

Akinbomi and Taherzadeh (2015) Azadeh AlaviBorazjani et al. (2019)

Semicontinuous, temperature: 35  1  C, pH: 3.98  0.08, OLR: 17 g VS/m3-day Batch temperature: 37  C, stirred at 300 rpm, pH: 4–4.6

52 mL/g VS

CieciuraWloch et al. (2020)

103 mL H2/ g waste bread

Han et al. (2017)

Batch temperature: 30  C, stirred at 24 rpm, pH: 6–7

102 L H2/ kg of solid waste

Hovorukha et al. (2021)

Batch, stirred at 120 rpm, pH: 7 Batch temperature: 36  C, pH: 5.5

46 mL H2/g VSadded 15 gTS/L

Liu et al. (2018) MorenoAndrade and Buitron (2015)

SBR temperature: 35  1  C, pH:

Santiago et al. (2020) (continued)

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Table 1 (continued) Operational conditions

Origin

Composition

(Queretaro, Mexico)

derived residues (e.g., bread), meat residues, and other (mixed residues) Kitchen waste

5.5  0.1, hydraulic retention time: 16 h

Food waste

Selectively collected from the local households (Lodz, Poland) Periodically collected from a canteen in Beijing, China

Results 127 mL H2/ g CODremoved

Reference

Batch temperature: 37.0  0.5  C, stirred at 100 rpm

68 mL H2/g VS

Slezak et al. (2020)

CSTR temperature: 55  1  C, pH: 5.0–5.5, stirred at 120 rpm, OLR: 6 g VS/L-day

126 mL H2/ g VS day

Wang et al. (2020)

CSTR continuous stirred tank, OLR Organic loading rate, SBR Sequencing batch reactor

microorganisms and inhibit the growth of methanogenic archaea (Buitrón and Carvajal 2010; Moreno-Andrade and Buitron 2015). Ren et al. (2019) performed a thermal pretreatment at 100  C for 60 min, while Slezak et al. (2020) applied different conditions at 70  C for 30 min to deactivate vegetative and nonsporulating forms of bacteria. In other cases, pure cultures have also been used, as in the case of Kim et al. (2008) who used C. beijerinckii KCTC 1785 purchased from the Korean Collection for Type Culture.

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Hydrogen-Producing Microorganisms in Dark Fermentation of Food Waste

Research on microbial communities producing hydrogen by dark fermentation has attempted to understand the complexities of the process to improve its management, yield, and stability. Few comprehensive reviews have discussed ecosystem function or ecological perspective of dark fermentation (Navarro-Díaz et al. 2016; Cabrol et al. 2017). Furthermore, in the existing literature, a limited number of studies evaluate their communities using food waste as a substrate (Table 2). To characterize microbial communities, several ecological estimators have been evaluated to describe their richness, diversity, and evenness (Laothanachareon et al. 2014; Jang et al. 2015; Moreno-Andrade and Buitron 2015). During the fermentation of food wastes, the richness estimators (e.g., number of OTUs, Chao1, or ACE) reflects the selection of specific anaerobic microbial groups prompted by the anoxic environment (Laothanachareon et al. 2014). A diversity index, such as Shannon index, reflects species richness but also evenness, which could be affected by the inhibition of hydrogen producers and other bacteria (Jang et al. 2015). In terms of using pretreatments at alkaline or acidic pH values, low microbial diversity (Shannon index) was related to low substrate degradation and hydrogen

Inoculum origin UASB (brewery wastewater)

FW fermentation

Food waste fermentation

Food waste fermentation

Food waste fermentation

Substrate origin Food waste

Cafeteria

Cafeteria

Cafeteria

Cafeteria

Heat at 90  C/ 20 min

Heat at 90  C/ 20 min

Acid shock pH: 2/12 h

Alkali shock at pH: 12/6 h

Inoculum pretreatment Heat at 100  C/ overnight

454 pyrosequencing

DGGE

DGGE

158 mL H2/ g VSadded

1.92 mol H2/mol Hexoseadded 148.7 mL H2/g SVadded

Batch (30 h) T: 35  C, 30 g COD/L, pH: 6 Batch (100 h) T: 35  C, 30 g carbohydrate COD/L, pH: 5 Batch (100 h) T: 35  C, 30 g COD/L, pH: 5, DRO: 100 day

Analysis platform Illumina iSeq platform

454 pyrosequencing

H2 yield 28.1 L H2/ kg CODtotal

162 mL H2/ g VS

Batch T: 37  C, pH: 6

Conditions CSTR, HRT: 4.5 h, OLR: 75 g carbohydrate/Lday, pH: 5.5–6

Lactobacillus; Clostridium; Klebsiella

Microbial genera Clostridium; Lactococcus; Enterococcus; Bacteroides; Olsenella; Bifidobacterium; Raoultella; Dialister; Paraclostridium; Vagococcus; Dysgonomonas Clostridium; Lactobacillus; Lactococcus; Enterococcus; Streptococcus; Trichococcus; Solibacillus; Enterobacter Clostridium; Lactobacillus; Enterococcus; Streptococcus; Peptoniphilus; Weissella Lactococcus; Clostridium; Streptococcus

Table 2 Microbial characterization by molecular tools of bioreactors for the H2 production from food waste

(continued)

Kim et al. (2009)

Kim et al. (2011)

Kim et al. (2014)

Jang et al. (2015)

Reference Jung et al. (2021)

Biohydrogen from Food Waste 37

Inoculum origin NA

Food waste fermentation

UASB (brewery wastewater)

UASB (brewery wastewater)

UASB (brewery wastewater)

Anaerobic sewage treatment plant

Substrate origin Cafeteria

Cafeteria

Cafeteria

Buffet restaurant

Buffet restaurant

Dining halls

Table 2 (continued)

NA

Heat at 105  C/24 h

Heat at 100  C/24 h

Heat at 103–105  C/ 24 h

Acid pH: 2/12 h

Inoculum pretreatment NA

CSTR T: 55  C, HRT: 5 day, OLR: 6–10 g VS/L, pH: 5.5

ASBR T: 35  C, HRT: 24 h, SRT: 37 h, pH: 5.5 ASBR T: 37  C, HRT: 2 day, pH: 5.5

Conditions ASBR T: 35  C, HRT: 30 h, SRT: 90 h, pH: 5.3, DRO: 100 day Batch (26 h) T: 35  C, 60 g carbohydrates COD/L, pH: 6 ASBR T: 35  C, HRT: 24 h, pH: 5.5

qPCR

Illumina MiSeq

DGGE

19.7 mL H2/g VS

2.2 mol H2/ mol hexose

454 pyrosequencing

102.8 mL H2/g CODremoved

0.2 mmol H2/g VS-day

454 pyrosequencing

Analysis platform DGGE

142 mL H2/ g VSadded

H2 yield 87.5 mL/g VSadded

Lactobacillus; Megasphaera; Prevotella; Bifidobacterium; Anaeroglobus Thermoanaerobacterium

Megasphaera; Selenomonas; Veillonella; Bifidobacterium; Olsenella; Aminomonas; Parabacteroides; Lactobacillus Clostridium; Lactobacillus; Enterobacter

Clostridium; Lactococcus; Enterococcus; Leuconostoc; Citrobacter

Microbial genera Clostridium; Anaerotruncus

Shin and Youn (2005)

Santiago et al. (2019)

Moreno-Andrade et al. (2015)

Moon et al. (2015)

Reference Kim et al. (2012)

38 I. Moreno-Andrade et al.

Anaerobic bioreactor treating wastewater

Syntheticb

NA

Heat at 100  C/1 h

iMBR T: 37  C, HRT: 10 day, OLR: 8 g VS/L-day, pH: 5.5

Batch (70 h) T: 37  C, S0/X0 ¼ 2 g VS/g VS, pH: 6

14.7 mL H2/VSadded

2.68 molH2/mol hexoseadded

Illumina MiSeq

454-Life pyrosequencing

Clostridium; Lactococcus; Macrococcus; Sarcina; Spirochaeta; Streptococcus; Syntrophomonas; Aeromonas; Treponema Clostridium; Lactobacillus; Anaeromassilibacillus; Bifidobacterium; Atopobium; Collinsella Wainaina et al. (2020)

Laothanachareon et al. (2014)

NA not available, UASB upflow anaerobic sludge blanket, T temperature, ASBR anaerobic sequencing batch reactor, CSTR continuous stirred tank reactor, iMBR immersed membrane bioreactor, DRO duration of the reactor operation, 454 pyrosequencing 454 pyrosequencing genome sequencer FLX titanium, 454-life pyrosequencing 454-Life Sciences GS-FLX genome sequencer system, DGGE denaturing gradient gel electrophoresis, qPCR quantitative polymerase chain reaction a Synthetic: constituted by rice, vegetables, meat b Synthetic: fruits, vegetables, meat, fish, pasta, rice, bread, bakery, and dairy

UASB reactor

Synthetica

Biohydrogen from Food Waste 39

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production under extreme alkaline conditions (pH 13). In contrast, higher species richness and diversity resulted in the best hydrogen-producing performances at pHs of 11 and 12 (Jang et al. 2015). Similarly, acidic pretreatments (pH values 1–4) caused a higher diversity and hydrogen production rate than the control, but a lower species richness in terms of the number of OTUs (Kim et al. 2014). Another relevant parameter is the food/microorganism (F/M) ratio, which produces a lower biological diversity when the food waste relation increases to 50, 80, and 100%. Similarly, an F/M ratio of 2 and 5 decreased the community diversity (Shannon index) but increased their richness in comparison to the inoculum (Laothanachareon et al. 2014). Few studies have characterized microorganisms during food waste fermentation in continuous or semicontinuous processes (Table 2), where operational parameters such as the hydraulic retention time (HRT) and the organic loading rate (OLR) outline community selection (Jung et al. 2021; Moreno-Andrade and Buitron 2015; Santiago et al. 2019). The best hydrogen production productivity (9.82 L/L-d) at an OLR of 74.7 g carbohydrate/L-d showed a lower species richness (measured as out abundance) and an intermediate diversity (Shannon index) during the OLR variation (40–80 g carbohydrate/L-d) (Jung et al. 2021). A similar trend was observed with SBR, when a lower HRT, from 72 to 24 h, increased hydrogen productivity, decreasing community diversity and evenness, mainly selecting bacteria from the Megasphaera genus (Moreno-Andrade and Buitron 2015). Due to the complex composition of food wastes, it can be assumed that the selection of specific communities, especially in continuous and semicontinuous reactors, is prompted by indigenous flora. The evaluation of hydrogen-producing communities using different substrates showed usage of real waste streams, such as solid organic wastes and agro-industrial streams, in selected similar communities (Etchebehere et al. 2016). Moreover, the potential of indigenous microflora has been proven during the fermentation of only OFMSW and food wastes, which have hydrogen potentials and microbial communities similar to those of the inoculum (Dauptain et al. 2020). In this sense, the role of native microorganisms can explain the loss of diversity in continuous and semicontinuous systems (Jung et al. 2021; Moreno-Andrade and Buitron 2015), which may overcome the inoculum in certain fermentation conditions. In addition to richness and diversity analysis, microbial characterization is a useful tool for understanding the dynamics of the main bacterial groups or taxonomies (Table 2), inferring their role in engineering systems for hydrogen production. In most studies, the Clostridium genus is considered the main H2 producer, selected by pH shock-based pretreatments or pH control (Kim et al. 2009, 2014; Jang et al. 2015; Moon et al. 2015), heat pretreatments (Kim et al. 2009, 2011), OLR (Wainaina et al. 2020; Jung et al. 2021), and anoxic fermentation conditions (Laothanachareon et al. 2014). Another relevant role for hydrogen production from food wastes is hydrolytic activity, such as C. frigidicarnis strain, which has demonstrated saccharolytic and proteolytic activities (Laothanachareon et al. 2014); Pseudomonas, which has the potential to degrade proteins and fatty acids; Bacteroides, which is associated with hemicellulose degradation (VillanuevaGalindo and Moreno-Andrade 2021); and Bifidobacterium, which is involved in

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the breakdown of substrates such as oligosaccharides (Wainaina et al. 2020; Villanueva-Galindo and Moreno-Andrade 2021). Another important bacterial group related to the indigenous microflora of food wastes is lactic acid bacteria. In early studies, pretreatments aimed to reduce lactic acid bacteria abundance (Kim et al. 2009; Jang et al. 2015) to avoid the consumption of sugars for lactic acid production. However, recently, some authors have discussed the use of lactic acid as a substrate to produce hydrogen by genera such as Megasphaera (Moreno-Andrade and Buitron 2015) or Clostridium (Dauptain et al. 2020), using food wastes as substrates.

3

Effect of Lactic Acid Bacteria on Hydrogen Production from Food Waste

Due to the diverse FW characteristics, process parameters, and microbial community diversity, dark fermentation is a complex biochemical process (Adekunle and Okolie 2015). Therefore, H2 production and yield depend not only on physicochemical and operational parameters, but also on microorganisms and their interactions. With the emergence of molecular tools, a wide phylogenetic diversity has been revealed, in which H2-producing bacteria (HPB) are not the only bacteria that participate in DF but also the native community of the inoculum and the substrate (Pakarinen et al. 2008; Rafrafi et al. 2013; Cabrol et al. 2017). In addition to HPB, lactic acid bacteria (LAB) are usually detected in mesophilic H2-producing systems from carbohydrateand protein-rich substrates such as tequila vinasses (García-Depraect and LeónBecerril 2018), molasses (Chojnacka et al. 2011), and food waste (Santiago et al. 2019; Villanueva-Galindo and Moreno-Andrade 2021). LAB are described as grampositive bacteria, and some genera belong to the phyla Firmicutes (Lactobacillus, Lactococcus, Streptococcus, Leuconostoc, and Enterobacter) and Actinobacteria (Bifidobacterium) (De Vos 2011). Many of them are known for their ability to convert hexoses, pentoses, and disaccharides into lactate (homofermentation) or to lactate, CO2, and ethanol (heterofermentation) (Asunis et al. 2019). The natural occurrence of LAB in fermentative systems may be attributed to their use of a wide range of carbon sources, their considerable adaptation to harsh conditions (Ghimire et al. 2015; Rajesh Banu et al. 2020), and their ability to outcompete other fermenters, such as Clostridium, due to their higher growth rate and substrate assimilation (Rombouts et al. 2020). However, the role of LAB is not yet clear as H2-producing reactors have been reported under stable and unstable conditions as H2-producing (acetate + lactate ! butyrate + H2) and non-H2-producing bacteria (lactate ! propionate + acetate) (Fuess et al. 2018). Table 3 shows the reported role of LAB in H2-producing systems, reporting both positive and negative effects on H2 production. According to Jo et al. (2007) and Sreela-Or et al. (2011), deficiencies in H2 yield and productivity have been observed during DF of food waste, since this substrate can be a direct source of LAB (as part of the native community or their proliferation during storage or transportation), which would imply a high and continuous load of

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Table 3 Role of LAB in H2-producing systems Substrate Food waste

Operational conditions CSTR, 35  C, pH: 5.2–5.5, HRT: 3 day

Food waste

Batch tests, 30  C, pH: 5.5

Food waste

AnSBR, 35  C, pH: 5.5, HRT: 48 h

Food waste

AnSBR, 37  C, pH: 5.5–5.0, HRT: 48 h

Food waste

CSTR pilotscale, 25–30  C, pH: 5.0–6.0, HRT: 30 h

Vegetable/ kitchen waste

Batch tests, 55  C, pH: 6.0

Cofermentation of cheese whey and fruit-

Batch flask reactor, 37  C, pH: 5.5

Role of LAB () The sudden LAB overproliferation inhibited Clostridium and it caused substrate competition () Inhibitory activity of LAB (Lactobacillus sp. and Enterococcus sp.) decreased the number of HPB and LAB became dominant () Enterobacter and Lactobacillus showed a negative relation with HPR, acetate, and butyrate production. H2 production decreased due to substrate competition and antimicrobial peptide production (+) () Bioaugmentation with Bacillus subtilis might lead to a cooperation between LAB (Bifidobacterium and Lactobacillus) and Megasphaera (able to convert lactate into H2). Nevertheless, this cooperation was lost due to LAB proliferation that declined H2 production (+) Lactobacillus plantarum and Enterococcus sp. might have selected specific strains (including Clostridium) through bacteriocin production Role of LAB is not discussed but effective lactate degradation is emphasized (+) A symbiotic relation between Bifidobacterium sp. and HPB was suggested (Lactobacillus, Klebsiella, and

Performance 90% of sequences) at pH 4.0, while at lower pH values (1.0–3.0) LAB activity was null; instead, Clostridium was highly abundant in the microbial community (>70% of sequences). Other studies have reported LAB activity in an acidic environment (pH 4.0) (Calderon Santoyo et al. 2003), and they have been detected in many kinds of foodstuff, in which fermentation reactions were not related to H2 production (Stiles and Holzapfel 1997). Because LAB are microorganisms ubiquitous in the environment with versatile metabolisms, they

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are able to prosper in DF systems (Baghchehsaraee et al. 2011). Therefore, extensive disinfection methods have focused on avoiding LAB, but, as discussed before, have proven to be insufficient and even economically and technically unfeasible (Moon et al. 2015; Cabrol et al. 2017). Therefore, it is necessary to adopt a different approach to take advantage of the LAB potential in H2 production instead of avoiding them. Even though the lactate pathway has a zero H2 balance, recent studies have focused on H2 production from lactate fermentation, in which the lactate produced by LAB is used by HPB (which produces H2 and butyrate from lactate), a phenomenon known as a cross-feeding mechanism (Schwalm et al. 2019). These studies have reported benefits such as process stability, substrate hydrolysis, pH regulation, and oxygen depletion (Ohnishi et al. 2010; García-Depraect and León-Becerril 2018; Blanco et al. 2019). Thus, LAB might induce stable microbial associations by producing H2 (Sikora et al. 2013), as previously observed for human gastrointestinal microbiota, wherein a cross-feeding mechanism occurred between lactate-producing (Bifidobacterium and Lactobacillus) and lactate-consuming or butyrate-producing bacteria (Rivière et al. 2016; Moens et al. 2017). The cross-feeding mechanism in DF has been suggested by Chen et al. (2015) between Bifidobacterium sp., a wellknown LAB that hydrolyzes starch to produce lactate, and other metabolites, which were then utilized by HPB such as Clostridium spp., in a starch-fed reactor. Similarly, García-Depraect et al. (2020b) observed a favorable association between LAB and HPB during DF of tequila vinasses. Most importantly, the aforementioned study argued that this relationship between Clostridium and LAB could be key to maintaining viability and culturability after long-term preservation at 4  C (GarcíaDepraect et al. 2020a). This positive interaction between LAB and HPB was evaluated through a novel strategy (lactate-driven DF) that involved lactate production in the first stage, and then, the resulting effluent was used in a H2 reactor (second stage), in which the effluent was fed a methanogenic substance in the last stage. Tequila vinasses were used as the substrate for feeding lactate fermenters, which were operated as anaerobic sequencing batch reactors (AnSBRs), while hydrogenic and methanogenic bacteria were in continuously stirred tank reactors (CSTRs) and upflow anaerobic sludge blankets (UASBs), respectively. In the first stage, the resulting effluent was enriched with lactate (12 g/L), which was attributed to the inoculum (ATCC PTA-124566, mainly composed of LAB, acetic acid bacteria, and HPB), and operated at a low pH (5.5) with a short reaction time (12 h). These factors were crucial for decoupling lactate fermentation: one dominated by LAB and a second dominated by HPB proliferation. Furthermore, lactate effluent was used to feed the hydrogenic reactor (operation pH 5.8), in which different HRTs were evaluated (24, 18, 12, 9, 6, and 4 h). The highest H2 production rate (HPR) and YH2 were obtained at an HRT of 6 h, with 11.7 L H2/L d and 109.8 mL H2/g VS, respectively. This period was considered stable in terms of H2 production and was defined by lower concentrations of lactate and propionate. In contrast, the lowest HPR and YH2 were registered (0.12 L H2/L d and 4.7 mL H2/g VS, respectively) at an HRT of 24 h. Additionally, the robustness of the system was evaluated by shortened HRT (6–4 h).

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As a result, a sudden decrease in the reactor performance was noticed as the H2 production stability index (HPSI) declined (0.93–075). Nevertheless, when the HRT increased, the system performance was recovered with higher HPR and YH2 and was statistically similar to that obtained in the previous steady stage. On the other hand, the methanogenic reactor achieved a COD removal of 61.3%, a similar value reported by Buitrón et al. (2014). The aforementioned study achieved removal efficiencies of 65% and 67% in a mesophilic UASB digester that was operated at an HRT of 24 h, and tequila vinasses were used as the feedstock. Therefore, it is imperative to optimize methanogenic systems to increase conversion efficiencies. These results can be used as the basis in FW treatment to increase H2 production since in some cases the FW can include some inhibitory waste components. It is worth noting that the true potential of LAB is still unknown due to a lack of studies that evaluate the operational conditions in DF that allow us to determine a positive role of LAB when crucial interactions between them and HPB (cross-feeding mechanism) may occur. This could lead to the development of more efficient and stable systems such as lactate-driven DF, even if the used substrates (e.g., food waste) are inherent with LAB.

4

Strategies for Improving H2 Production

The biochemical complexity of the dark fermentation process of FW and the dynamics of its microbial communities are subject to scenarios that limit the production of H2, generating unwanted operating conditions (Jarunglumlert et al. 2018; Yun et al. 2018; Zhang et al. 2019). To improve H2 production, several strategies have been developed, including optimizing the process parameters, use of codigestion with FW and other organic substrates, bioaugmentation, and development of automatic control strategies. Several parameters have been studied to improve H2 production, such as the pH, temperature, hydrogen partial pressure, hydraulic retention time (HRT), and sludge retention time (SRT) (Moreno-Andrade and Cercado 2020). The HRT needs to be long enough to obtain food waste hydrolysis and H2 production, but short enough to avoid the growth of methanogens (slow growth speed of 0.016 h1), which would consume the H2 produced or displace the H2 producers (fast growth speed of 0.172 h1) (Moreno-Andrade and Cercado 2020). The HRT is the main parameter controlling the community composition in the process. The typical HRT for H2 production ranges between 6 and 36 h (Santiago et al. 2020). Optimization of the SRT is also important since high SRT values allow for adequate hydrolysis of the particulate substrate (increasing the soluble COD) and degradation of the substrate. The degradation of the substrate and the retention of biomass need to be compromised to maximize H2 production. Santiago et al. (2020) used FW from a cafeteria in a sequencing batch reactor (SBR), and optimized H2 production (high substrate hydrolysis and yield of 127.26 mL H2/g CODremoved) at an SRT of 60 h and an HRT of 16 h. For the different processes (continuous or discontinuous) with different food waste characteristics, the optimal values of HRT and SRT need to be determined to control the process and maximize

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H2 production. Using different SRTs and HRTs also influenced the subproducts of the process, since with an SRT of 20 h and an HRT of 16 h, acetic acid-like fatty acids were mainly obtained, while at a long SRT (60 h), the obtained fatty acid was butyrate (Santiago et al. 2020).

5

Codigestion/Cofermentation

It is crucial to develop and optimize technologies that allow for efficient FW treatment, integrating its disposal with its valorization and recycling (Capson-Tojo et al. 2018b). One way to achieve this purpose is through codigestion (cofermentation), which involves mixing wastes in different ratios and proportions, keeping the C/N ratio within the desired range of 25–30 (Das and Mondal 2016). Codigestion has been shown to be beneficial due to its economic viability and the possibility of obtaining higher yields and mitigating certain problems of monodigestion, such as nutrient imbalance and dilution of toxic materials or recalcitrant compounds in the raw material (Rabii et al. 2019). Frequently, FW may lack nitrogen, which is an essential nutrient for H2 producers; thus, various investigations have focused on testing different cosubstrates that complement FW to increase the yields and productivity of H2, to treat different types of waste at the same time, and to improve the carbon/nitrogen (C/N) ratio. Table 4 shows a compilation of the different investigations related to H2 production through the codigestion of FW and other substrates. Codigestion of waste results in increased H2 production, as in the case of Abreu et al. (2019), who performed codigestion of FW with garden waste under hyperthermophilic conditions, achieving the best results when using a 90:10 ratio, obtaining a cumulative production of 46.2  0.9 mL H2/g; this is higher than using a 100% FW substrate, which generated only 16.5  0.8 mL H2/g. The same type of waste was used by Bundhoo (2017), who performed a codigestion of lignocellulosic materials, which were difficult to hydrolyze, to investigate the potential of microwave and ultrasound irradiations on the pretreatment of food and yard wastes before dark fermentation that resulted in enhanced solids and organic matter solubilization. Another residue used as a cosubstrate for FW is cattle manure, as it is rich in alkali and nutrients, and may be suitable for codigestion with carbohydraterich substrates such as FW; thus, Liu et al. (2020) carried out a codigestion to produce H2 under mesophilic conditions, obtaining the optimal conditions at a mixing ratio of 47–51%, a substrate concentration of 76–86 g/L, and an HRT of 2 days, producing L H2/L day; they concluded that bioH2 production from the codigestion of cattle manure and food waste is feasible. The codigestion of sewage sludge with FW has been investigated to give added value to these residues when they are disposed. Jung et al. (2013) carried out H2 production through the codigestion of food waste with sewage sludge without adding inoculum and obtained a H2 yield of 2.15 mol H2/mol hexoseadded, corresponding to a biogas conversion efficiency of 8.67%. Similarly, Kim et al. (2004) performed a codigestion of food waste and sewage sludge, obtaining an H2 production potential that increased as the sewage sludge composition increased up to

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Table 4 Operating parameters and performances of codigestion of food waste and cosubstrates Substrates Garden waste-FW FW-yard wastes Dry lake algae-FW

FW-corn stover FW-cattle manure FWsewage sludge FWsewage sludge FW-PSWAS FWCCW

Mixing ratio 90:10

Operational conditions Batch T: 70  C, Stirred at 90 rpm Batch T: 35  0.5  C, pH (initial): 6.5 Batch T: 37  1  C, stirred at 60 rpm

Main results 46  1 L H2/kg

Batch T: 35  C, pH (initial): 7, stirred at 150 rpm Batch T: 35  1  C, HRT: 2 d, stirred at 150 rpm ASBR T: 35  1  C, pH: 6.0  0.1

223.8 mL H2/g glucose

87:13

Batch T: 35  C, pH: 5.0–6.0, stirred at 100 rpm

Kim et al. (2004)

80:15:5

Batch T: 37  C, pH (initial): 5.5  0.2 Batch T: 35  C, pH: 7, S0/ X0 10.6 g COD/g VS, C/N ratio of 26.8 Batch T: 35  1  C, S/I: 6 pH: 5 Batch T: 30–33  C, pH of 6.5

122:9 mL H2/g carbohydrateCOD 165  13 mL H2/ g VSadded 1.66 mMol H2/g CODinitial 93.1 m3 H2/t VS

Girotto et al. (2017) Cardenas and Zapata (2019)

50:50 40:10 (based on the total solids) 3:1

NA 10:1 on COD basis

NA

FW-CW

NA

FWcoffee mucilage OFMSWFW

83:17

FW-GLC

FWcardboard

50:50

95% FW-5% GLC NA

SSTR T: 55  C, HRT: 4.4 d, OLR: 38.4 g TVS/L/ day, pH: 5–6 Batch T: 35  1  C, pH (initial): 5.5, stirred at 150 rpm Batch T: 35  C, S0/X0 of 0.25 g VS/g VS

21.27  0.65 mL H2/g VS 30.99 mL H2/g VS

30 mL H2/g VS 165  13 mL/g VSS substrate

670 mL H2/g VSadded 53.8  0.9 mL H2/g VSadded 180 mL H2/g VS

46.9 g COD/g CODbiodegraded

Reference Abreu et al. (2019) Bundhoo (2017) Zhao et al. (2020)

RodriguezValderrama et al. (2020) Liu et al. (2020) Jung et al. (2013)

Zhou et al. (2013) Basak et al. (2018)

AngerizCampoy et al. (2017) Silva et al. (2017) Capson-Tojo et al. (2018b)

FW food waste, OFMSW organic fraction of municipal solid waste, GLC crude glycerol, CCW cottage cheese whey, CW cheese whey, PS primary sludge, WAS wastewater biosolids, ASBR anaerobic sequencing batch reactor, SSTR semicontinuous regime of feeding, NA not available, initial substrate-to-microorganism ratio (S0/X0), substrate/inoculum ratio (S/I)

13–19%. When a waste composition ratio of 87:13 was applied, a maximum specific H2 production potential of 122.9 ml H2/g carbohydrate-COD was obtained. Zhou et al. (2013) performed a codigestion with FW, primary sludge, and waste-activated sludge under mesophilic conditions, obtaining the highest H2 yield (165  13 mL/g VSSsubstrate) with the three residues mixed at a volumetric ratio of 80:15:5, observing

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that the measured H2 productions were higher in the mixed codigestion than for sums of the H2 production calculated from each individual fraction. Residues resulting from food products such as cheese have also been used as cosubstrates for FW H2 production. Basak et al. (2018) and Girotto et al. (2017) carried out the codigestion of these wastes taking advantage of their potential for biogas generation, obtaining yields of 1.66 mMol H2/g initial COD and 93.1 m3 H2/t VS, respectively. Another residue of great interest in the codigestion of residues is OFMSW. According to Angeriz-Campoy et al. (2017), OFMSW has a low content of biodegradable organic compounds such as carbohydrates, lipids, and proteins; thus, they performed codigestion of this residue with FW, which contains a high content of carbohydrates and easily hydrolyzable materials, and obtained yields of 53.8  0.9 mL H2/g VSadded when the mixing residues had a 50:50 ratio. Anaerobic codigestion offers several benefits over monodigestion of FW. Various waste materials have the potential to be used as cosubstrates, such as industrial wastewater, municipal solid waste, agricultural waste, sewage sludge, cardboard, and kitchen wastes. Codigestion appears to be an economic and viable option for generating alternative renewable energy sources (Das and Mondal 2016). The implementation of large-scale anaerobic codigestion systems can lead to the goals of integrated waste management since they include waste reduction and renewable energy generation (Rabii et al. 2019).

6

Bioaugmentation Strategies for Enhancing Hydrogen in Dark Fermentation

Many strategies focused on the substrate and the inoculum, and have been evaluated to increase H2 production and yield (YH2), such as heat shock (Elbeshbishy et al. 2011), acid pretreatments (pH 1.0–4.0, exposure period, 24 h) (Kim et al. 2014), alkaline pretreatments (pH 8.5–12, exposure period, 24 h) (Elbeshbishy et al. 2011), and ultrasonic pretreatments (Elbeshbishy et al. 2010). The aim of these strategies is to suppress the growth of H2-consuming microorganisms (Lay 2000). However, the presence of microorganisms that contribute to H2 production, such as facultative anaerobic fermentative bacteria (Bacillus sp., Lactobacillus sp., Enterobacter aerogenes, and Citrobacter sp.), is limited by these drastic pretreatments (Cabrol et al. 2017). Therefore, bioaugmentation (the process of adding selected strains or mixed cultures to the native microbial community of the reactor) has been used to improve the start-up conditions (Ma et al. 2009) and the performance of reactors and to protect the microbial community from adverse effects (Mohan et al. 2005). The aforementioned strategy has gained attention due to the possibility of using facultative microorganisms that are more attractive than obligate anaerobes (Clostridium sp.) for their lower sensitivity to oxygen, cellular aggregation, and metabolic versatility (Cabrol et al. 2017). Due to complications during dark fermentation of food waste, such as high concentrations of ammonia, volatile fatty acid (VFA) accumulation (Drennan and DiStefano 2014), and proliferation of lactic acid bacteria (LAB) in the native community (Sreela-Or et al. 2011), bioaugmentation using different

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strains has been applied with mixed success (Table 5). There are several factors that contribute to successful bioaugmentation, such as strain concentration, type of strain, and pH (Kim et al. 2008; Marone et al. 2012; Sharma and Melkania 2018; Villanueva-Galindo and Moreno-Andrade 2021). According to Sharma and Melkania (2018), H2 production declined at a higher bacteria/sludge ratio with all the bioaugmented strains. Similar results were obtained by Villanueva-Galindo and Moreno-Andrade (2021), and the lowest H2 production was observed during the bioaugmentation test when higher amounts of Clostridium strains were added compared with the other tests (less strain added). This could be attributed to a microbial community change that led to H2 consumption pathways (e.g., homoacetogenic or propionic producing pathways) as a result of the higher production of acetic acid (Valdez-Vazquez and Poggi-Varaldo 2009; Motte et al. 2013). Additionally, the type of strain in bioaugmentation tests affects H2 production. In addition to adding Clostridium strains, in recent years, this strategy has been applied using other microorganisms, such as facultative anaerobes, which may be more attractive than Clostridium due to the factors mentioned previously. As shown in Table 5, many facultative strains, such as Bacillus subtilis, Paenibacillus polymyxa, Buttiauxella, and Rahnella, obtained higher H2 production and yields in the bioaugmentation batch test than some Clostridium strains (C. beijerinckii ATCC 10132 and C. butyricum DSM 10702). B. subtilis and P. polymyxa have been reported as H2-producing bacteria and have hydrolytic abilities (Haggag 2007; Sharma and Melkania 2018); meanwhile, Buttiauxella and Rahnella are known for their versatile metabolism, and both are able to use a wide range of carbon sources (Garrity et al. 2001). Finally, pH is one of the most important parameters of dark fermentation due to its effect on hydrolysis, enzymatic activity, and microbial populations (Azwar et al. 2014). The optimal pH range differs for each microorganism. According to Kim et al. (2008), bioaugmentation with C. beijerinckii showed a better performance in H2 production with an initial pH of 7.0 than at a pH of 5.0 (250 mL H2/Lreactor and 1155 mL H2/Lreactor, respectively). The aforementioned study attributed this behavior to the inhibition of hydrogenase activity (enzyme that converts H+ cations to molecular H2) due to a low pH value caused by concomitant VFA production.

7

Control Strategies to Improve H2 Production

An interesting approach in applied technological innovation for the production of biogas from FW has been to improve the operation process through control strategies that include the optimization of state variables to increase biogas production while making the process insensitive to external and internal disturbances (JimenezOcampo et al. 2021). At an industrial level, automatic control is an option to enhance and minimize human intervention in the operation process. Automation can be divided into two main categories: open-loop control and closed-loop or feedback control strategies (FCS). Both integrate a set of electronic, mechanical, pneumatic, and hydraulic components to achieve a desired objective. Open-loop control refers to

Clostridium saccharobutylicum ATCC BAA-117

Paenibacillus polymyxa ATCC 842

Bacillus sp. RM1 Bacillus subtilis ATCC 6051a

Bacillus subtilis

Escherichia coli

Enterobacter aerogenes

Raoultella sp. 47

Rahnella sp. 10

Strain Buttiauxella sp. 4

Bioaugmentation Strain added at 10% v/v Strain added at 10% v/v Strain added at 10% v/v Bacteria/sludge ratio: 0.20 Bacteria/sludge ratio: 0.20 Bacteria/sludge ratio: 0.20 OD600 0.4 3.5
109 CFU/ mL/Lreactor 3.7
109 CFU/ mL/Lreactor 7.6
108 CFU/ mL/Lreactor FW

FW

BW FW

OFMSW

OFMSW

OFMSW

VW

VW

Substrate VW

7.5

7.5

7.0 7.5

5.5

5.5

5.5

6.7

6.7

pH 6.7

Hmax (mL H2/ Lreactor) 1344 896 1314 1389 1209 1612 NA 1836 928 476

YH2 (mL H2/ g VS) 71.27  6.24 47.54  2.14 69.7  1.81 37.1a 32.9a 43.6a 142.0a 84.0  9.1 43.0  2.9 42.3  7.9

Table 5 H2 production by bioaugmentation with different strains in mesophilic batch tests

Mazareli et al. (2019) Villanueva-Galindo and MorenoAndrade (2021) Villanueva-Galindo and MorenoAndrade (2021) Villanueva-Galindo and MorenoAndrade (2021)

Sharma and Melkania (2018)

Sharma and Melkania (2018)

Sharma and Melkania (2018)

Marone et al. (2012)

Marone et al. (2012)

Reference Marone et al. (2012)

50 I. Moreno-Andrade et al.

Strain added at 5% v/v

1.7
109 CFU/ mL/Lreactor NA 5.5

7.0

FW FW

7.5

FW

915 1155 1700

21.8  2.1 128.0b 35.6  1.4 Ortigueira et al. (2019)

Villanueva-Galindo and MorenoAndrade (2021) Kim et al. (2008)

VW vegetable waste, OFMSW organic fraction of municipal solid waste, BW banana waste, FW food waste, NA not available a mL/g carbohydrates b mL/g COD

Clostridium beijerinckii ATCC 10132 Clostridium beijerinckii KCTC 1785 Clostridium butyricum DSM 10702

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a series of preprogrammed steps that are executed automatically with a minimal amount of human intervention. It depends on careful calibration procedures of the controls used in the process, but in the presence of a disturbance, the control strategy takes no action. It can only be used if the relationship between the input and output variables is known and in processes where disturbances do not occur or in critical uncertainties (Franklin et al. 2010). Industrially, a programmable logic controller (PLC) is normally used to implement the strategy, but a centralized computer system with data acquisition devices and diverse actuators such as relays may also be employed. An example of such a system for biohydrogen production is the usual operation of an anaerobic sequencing batch reactor (AnSBR), where the duration of the fill, reactions, sedimentation, and draw times is fixed a priori and the controller only sends instructions to valves, pumps, and other actuators in a timely manner. In contrast, in an FCS (closed-loop control), there is continuous feedback from the online measurements of some process variables (Gaida et al. 2017), called outputs, and it combines with the possibility of manipulating other variables, called inputs, to achieve a desired objective in the performance of the system. In an FCS, some of the input variables are manipulated variables; they are measurable and associated with the process design (flow, HRT, etc.). On the other hand, some output variables are controlled variables and they must also be measurable; they allow online information about the current key variables of the system to be recovered and in some cases to be indirectly known by the global state of the system, as well as to quantify the performance of the final product (biogas production rate, effluent concentration, etc.) (García-Gen et al. 2015). The denomination of a variable as a manipulated or a controlled variable depends on the control system loop and where it participates. For example, temperature or pH may be controlled variables in a regulation control loop that maintains their value at a constant level at a given set point, but they may be considered manipulated variables in another higher level control loop. To use this terminology, an FCS aims at achieving a desired objective using controlled variables that change manipulated variables based on gathered information from the online measurement of output variables. In biotechnological processes, monitoring is sometimes confused with automatic control. Monitoring is the systematic process of measuring, collecting, and analyzing data obtained from a process to obtain valuable information about its current and past state. Monitoring may be a crucial part of an FCS since it implies the use of diverse sensors to gather data that may be used to make decisions about how to operate the process. However, not all sensors may deliver useful online data, which is one interest for an automatic controller: for example, a device that automatically takes a sample from the process every 2 h and, through complex measurement techniques (e.g., chromatography or spectrophotometry), provides information about the variable which may provide useful data for monitoring purposes (e.g., showing a chart of these data for the last 24 h), but these data are of no use in an FCS that needs it to modify a manipulated variable every 5 min. Thus, monitoring a variable may be important for a human to make decisions about the operation of a process, but only some types of online monitoring may be used in a closed-loop controller.

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In the latter sense, one of the difficulties, particularly with dark fermentation processes, is the lack of reliable online sensors to assess the state of the bioreactor (Jimenez et al. 2015). Nevertheless, there have been several efforts to alleviate this problem by using software sensors, which are algorithms that compute an online estimation of an unmeasured variable based on past and present information from measured variables. One type of software sensor is state observers, which numerically solve the equations that define the mathematical model of the system online, combined with a correction term that considers the online measurements of other variables. The task is not easy, as it entails obtaining a suitable mathematical model (including parameter values) and ensuring that the choice of measured variables indeed leads to reliable estimations of the unmeasured variables (this is related to the property of observability of the model). For dark fermentation processes, some observer design proposals have been made, of which the most common is the extended Kalman filter (Jimenez et al. 2015). However, other observer designs based on nonlinear system theory have been proposed; they have mostly been tested only in simulation models (Lara-Cisneros et al. 2016), but others have been validated with experimental data (Aceves-Lara et al. 2010; Torres-Zúñiga et al. 2018). A FCS may have several objectives in a system. A fundamental objective is ensuring the correct operation of a process despite the numerous disturbances that could affect it, even possibly destabilizing the system or bringing it to another, which may be an undesirable operating point. For a biohydrogen-producing dark fermenter, FCS can be used to avoid the production of methane by changing the operating conditions such that methanogenic archaea do not thrive and fermentative hydrogen-producing bacteria have little competition. Several strategies that involve FCS can be used, such as maintaining a desired temperature or a favorable pH value in the system or operating within certain bounds of the loading rate. In control system theory, this is called the regulation task. Usually, the controlled variable is measured online and is compared with a desired set point, and the error signal is used to establish the value of some manipulated variable that directly affects the controlled variable. For example, pH can be easily regulated by manipulating base or acid pumps. In such cases, simple controllers that do not explicitly require a mathematical model of the system, such as the proportional integral derivative (PID) controller, are usually implemented (Nguyen et al. 2015). A higher level task in feedback control involves disturbance rejection, overall desired operating point regulation, and optimization. Since anaerobic digestion is a complex system, it usually requires the use of elaborate mathematical modeling, controllers that are based on advanced control theory, or ad hoc control strategies. As an example, Fig. 2 shows three plausible feedback loops used in a dark fermenter. The regulation of pH to a desired set point (e.g., 5.5 for hydrogen production) is performed by a PID controller that establishes whether to add an acid or a base to the bioreactor by turning on or off the pumps with some periodicity. On the other hand, an internal loop in a cascaded control regulates the temperature at a value that is established by an external controller; this may be accomplished by setting the heater current using a PID algorithm. The same controller may be a complex controller that also manipulates the hydraulic retention time (HRT) to optimize the biogas

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pH setpoint

pH Controller Acid / Base

pH HRT Biogas producon Temperature setpoint

Temperature Controller

Temperature

Biorreactor Heater

Biogás controller (complex)

Fig. 2 An example of several feedback control loops in a dark fermenter

productivity rate. In this example, temperature is the controlled variable for the internal loop, but it may also be regarded as a manipulated variable for the external (complex) control loop. For biohydrogen production using dark fermenters, several FCSs have been proposed (Jimenez et al. 2015; Nguyen et al. 2015), but most of them consider the feed rate as the main manipulated variable in schemes similar to the one depicted in Fig. 2, where temperature and pH are assumed to be regulated by other (simpler) FCSs (Gaida et al. 2017). Most proposals have tested the control strategy using numerical simulations, using a mathematical model that is mostly an adaptation of the anaerobic digestion model 1 (ADM1) (Zhou et al. 2020). Using food waste as a carbon and energy source for the production of biohydrogen, as well as a variety of system configurations, entails several challenges that need to be addressed specifically for this type of substrate. Therefore, there is a multiplicity of models that range from those used for batch processes to those used for specific types of substrates (Asunis et al. 2019) or complex system configurations (Kaashyap Balaji et al. 2020). Other proposals aim at simpler universal models for single- and two-stage AD processes, including the systems analysis of fundamental properties (Aceves-Lara et al. 2010; Diop et al. 2017). The objective of most proposed controller designs for dark fermentation or other anaerobic digestion (AD) processes has been to optimize biogas production either by automatically establishing suitable operating conditions or by dynamically changing the feed rate to reach a previously unknown operating point where biogas production is maximized. The latter is the task of an extremum-seeking controller (ESC), and

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several attempts have been made in this direction (Lara-Cisneros et al. 2014, 2015; Vargas and Moreno 2015). In AD processes, the convergence to the optimal operating point is very slow, and they may require the online measurement of difficult-to-obtain process variables, so most have only been tested with simulations (Lara-Cisneros et al. 2015). Since the process is dynamic (Vargas and Moreno 2015), its optimal conditions vary over time, which is why adaptive control may be a suitable option since it can automatically compensate for uncertainties in the model (Nguyen et al. 2015; Jimenez et al. 2015). Fuzzy logic control (FLC) uses surrogate models that are based on the knowledge and experience of the process operators (Huang et al. 2012; Robles, et al. 2018; Borges et al. 2020); it allows tracking and suggests a set point. FLC can also be used for starting, commissioning, overloading, and inhibiting methanogenesis (Gaida et al. 2017). If a mathematical model of the system is available and reliable enough for making predictions, a model-based predictive control strategy can be used (Aceves-Lara et al. 2010). The model may also be used for verifying the operation of a proposed controller using numerical simulations, even if the controller is designed to be model free. The simplest controllers in this sense are on/off controllers that turn a system element on or off (e.g., a pump, a heater, or a valve), usually to regulate a controlled variable within a set point (Jimenez et al. 2015; Gaida et al. 2017). Other approaches are more heuristic, in the sense that they use expert knowledge of the system to define ad hoc FCS strategies, for example, controlling the ammonia concentration using the measurement of correlated variables (Micolucci et al. 2014, 2020), periodically adjusting the organic loading flow rate to the value that maximizes a polynomial describing an input-output relationship with the hydrogen production rate, using alkalinity or dissolved H2 measurements (García-Gen et al. 2015), controlling the ambient temperature and other operating conditions (Lu et al. 2020), or using modelbased PID control loops to set the feed rate (Zhou et al. 2020). Another approach used for an AnSBR continually fits an empirical model of the accumulated H2 volume over time and establishes the reaction time to establish suitable conditions for H2 production (Jimenez-Ocampo et al. 2021). Table 6 presents a summary of the control strategies applied in the treatment of various organic wastes, homogenous liquid substrates, and wastewater using the dark fermentation process. These processes use a variety of FCSs; however, in most cases, an implemented control strategy was carried out using numerical simulations. It is necessary to develop and validate a robust FCS that increases biogas production, independent of the initial substrate characteristics (Jimenez-Ocampo et al. 2021), and test the actual differences obtained in the bioprocess compared with the simulations.

8

Conclusion

The H2 production by dark fermentation from food waste varies depending on the waste characteristics (including the carbohydrate, protein, and lipid contents; total solids; pH; lignocellulosic content; moisture, etc.), the process parameters (temperature, HRT, OLR, etc.), and the microbial community. The optimization of H2

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Table 6 H2 production by bioaugmentation with different strains in mesophilic batch tests FCS Mathematical modeling

Substrate Lignocellulosic wastesa

Objective Propose a model and use it to estimate nonmeasurable variables

Mathematical modeling

Biomassa

Optimization of H2 production

Mathematical modeling

Cheese whey

Evaluate H2 production by modifying the process pH

Model-based predictive control

Molassesa

Optimization of H2 production

Heuristic controller for optimization

Glucose

Maximization of H2 production

Description Proposal of a model that estimates the specific growth rates of the bacteria involved in the AD process in two stages in continuous reactors for the production of H2 and CH4 Simulation of the H2 production from biomass by AD: dry reforming of biogas in a fixed-bed catalytic reactor and H2 separation using a hollow fiber membrane separator Fermentation tests (2 L reactors) were performed using pH as a manipulable variable, modeling the biochemical pathways. The operating pH affected the H2 and VFA production kinetics, varying the duration of the process designed reaction time Application of a modelbased predictive controller in combination with an observer, using online measurement of biogas production rate to estimate substrate concentrations in a 2 L CSTR reactor Online optimization strategy in a CSTR of 1.25 L developed from the relationship between the OLR and the H2 production rate, modifying the HRT and setting the effluent concentration using the

Reference Diop et al. (2017)

Kaashyap Balaji et al. (2020)

Asunis et al. (2019)

AcevesLara et al. (2010)

RamírezMorales et al. (2015)

(continued)

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Table 6 (continued) FCS

Substrate

Objective

Fuzzy logic control

Molassesa

Optimization of H2 production

Online monitoring

Glucose

Maximization of H2 production

Operating condition automation

Wastewater

Design and validation of an FCS in a pilot H2 production system

Heuristic controller for increasing productivity

Diluted food waste

Long-term evaluation of a controller

Description feed flow as control variable Optimization of H2 production in a continuous upflow reactor from the realtime monitoring and control of pH and temperature, highlighting an optimal microbial growth rate and a production rate of 13.4 LH2/day Validation of a state observer to estimate unmeasured variables during the operation of a 1.25 L CSTR with FCS, where H2 production was used as a control variable. The concentration of the feed flow was estimated and the ideal OLR was defined to improve the H2 production rate, reaching values of 20 LH2/L/day Automatic operation of an 11 m3 reactor for H2 production via dark and photo-fermentation. The production rate was 96.3 mol/m3/day. The maximum pH and oxidation-reduction potential values were 5.92 and 490 mV, respectively Use of ammonium regulation as a means to increase bio-hythane in a two-stage thermophilic anaerobic reactor. The recirculation rate was controlled to prevent ammonia inhibition, where the concentration of this variable was estimated by correlation

Reference

Huang et al. (2012)

Torres Zúñiga et al. (2018)

Lu et al. (2020)

Micolucci et al. (2014)

(continued)

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Table 6 (continued) FCS

a

Substrate

Objective

Heuristic two-level controller for increasing productivity

Organic fraction of municipal solid waste

Long-term evaluation of two-level controller

Reaction time optimization

Food waste

Optimization of H2 production

Description with other readily available measurements: conductivity, VFA, and alkalinity Use of controllers on two levels for the concurrent production of VFA, H2, and CH4. A controller using information from a pH probe in the fermentation reactor, and a pH and a conductivity probe in the methanization reactor. pH was regulated in the first reactor, while ammonia concentration was managed in the digester. The FCS was tested experimentally with success Proposal and lab-tested FCS for optimizing H2 production from FW in a AnSBR. The control strategy is based on recursively fitting a modified Gompertz model to gathered online data of the cumulative H2 volume; it could eventually stabilize the system at an optimal reaction time. Evaluation during 140 cycles maintained a stable H2 production

Reference

Micolucci et al. (2020)

JimenezOcampo et al. (2021)

Tested only with numerical simulations

production includes selecting and controlling specific process parameters to obtain a stable transformation of the substrate to H2. The use of such strategies as codigestion, bioaugmentation, and automatic control can increase the H2 yields and improve the process stability. Codigestion has been demonstrated to improve the nutrient balance (C/N) and dilution of toxic materials combining two or more wastes. The bioaugmentation of dark fermentation by hydrolytic or fermentative microorganisms increases the H2 production for a period of time (days). To maintain this improvement, a periodic supplement of the microbial strains is necessary. For

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the case of the automatic control, H2 can be produced in a stable, robust, and efficient process, using easy-to-measure parameters optimizing not only H2, but also the yield and metabolite production. In all the proposed strategies, further studies are necessary to evaluate the large-scale application, the long-term operation, and the cost/ benefits in the dark fermentation process. Acknowledgments The support granted by the PAPIIT projects IN102722, IN109119, and IA200922 from DGAPA-UNAM is acknowledged.

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Biohydrogen from Fruit and Vegetable Industry Wastes Bhaskarjyoti Kalita and Nandan Sit

Abstract

Hydrogen gas produced during the microbial fermentation of organic materials is termed biohydrogen. It possesses higher energy in comparison to fossil fuels which makes it a potential source of biofuel or energy, and hence has been the topic of study recently. Biohydrogen can be obtained from fruit and vegetable waste by the process of dark fermentation which occurs in the absence of oxygen and light. Almost 50% of the food wasted in the world consists of fruit and vegetable residue obtained from household or kitchen waste, restaurants, wholesale and retail markets, municipal solid waste, fruit juice vendors, fruit processing industries, etc. It has proved to be an ideal substrate for biohydrogen production by dark fermentation on account of its high carbohydrate content, easy availability, sustainability, low cost, biodegradability, moisture content, high volatile solids, presence of micronutrients, hydrolyzable organic components, etc. Besides, the fruit and vegetable waste dumping in landfills or incineration causes odor, toxic gas emission, water pollution, and other environmental concerns, and its use for biohydrogen production can reduce the issue of costly solid waste management. Fruit and vegetable wastes acting as substrates are basically lignocellulosic that prevents the activity of microbial enzymes upon them. Different pretreatments including grinding, acid/alkali treatment, heat treatment, etc. help to hydrolyze the substrate for better biohydrogen yield. Another concern for biohydrogen production is the formation of organic acids like lactic acid, propionic acid, acetic acid, and butyric acid during the fermentation process which can be overcome by co-digestion of fruit and vegetable waste with other substrates that helps to regulate the volatile organic acid concentration, besides

B. Kalita · N. Sit (*) Department of Food Engineering and Technology, Tezpur University, Tezpur, Assam, India e-mail: # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Kuddus et al. (eds.), Organic Waste to Biohydrogen, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-1995-4_3

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the addition of alkali. On the other hand, biohydrogen production is increased on using a mixed inoculum derived from an indigenous consortium, but it contains hydrogen-consuming bacteria like methanogens and lactic acid bacteria, which reduces the yield. This problem is solved by using a heat-shock treatment on the inoculum before being placed into the reactor. Hydrogen-producing bacteria can form spores and survive the harsh environment, while the hydrogen consumers are killed in the process. Biohydrogen yield is dependent on a number of factors including pH, temperature, fermentation time, inoculum concentration, pretreatment methods, percolation frequency, substrate composition, and trace metal elements. Further research to improve the biohydrogen yield and economic viability of the production process and the application in the industrial scale will ensure superior results. Keywords

Biohydrogen · Fruit and vegetable waste · Dark fermentation · Bacteria · Pretreatment

1

Introduction

The increasing energy requirement of the modern world has forced us to utilize fossil fuels as the most common source of energy. The combustion of fossil fuel leads to the emission of greenhouse gases that causes atmospheric pollution and is related to the phenomenon of global warming and climate change. These are serious concerns and therefore require finding alternative sources of energy and fuels to fulfill the global energy demand. Bio-based energy is a gradually emerging field that has the potential to provide a sustainable, environment-friendly, and alternative source to fossil fuel-based energy (Chandrasekhar et al. 2015). Fossil fuels are nonrenewable sources of energy. The increasing usage of fossil fuel causes a depletion in its global reserves (Saidi et al. 2018; Cahyari et al. 2019). With depleting reserves, the generation and utilization cost of energy are bound to rise (Saidi et al. 2018) which adversely affects the market economy. Other effects due to overdependence on fossil fuel include the increase of carbon dioxide (CO2) and particulate matter concentration in the air, water contamination, climate changes, and many others (Cahyari et al. 2019).

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Renewable Energy Sources

Renewable sources of energy or alternative fuel are the energy generated from cheap and readily available feedstocks that are auto-generating or potentially renewable and obtained from natural sources and therefore are non-exhaustible (Saidi et al. 2018). The various renewable energy sources include bioethanol, biodiesel, butanol, terpenoids, syngas, biohydrogen, etc. produced from different natural sources (Lee

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et al. 2013). Potential resources including biomass, agricultural waste by-products, wood waste, waste from food processing, aquatic plants and algae, sewage sludge, livestock effluents, and animal excreta if used under appropriate control can become the major sources of energy in the future (Show et al. 2012). Other sources of renewable energy such as solar energy, wind power, thermal energy, hydroelectricity, and biomass are gaining attention worldwide (Saidi et al. 2018; Abubackar et al. 2019a, b).

3

Biohydrogen as a Source of Clean Fuel

Hydrogen gas produced during the microbial fermentation of organic materials is termed biohydrogen, and it provides a sustainable and environment-friendly method for generating energy (Chandrasekhar et al. 2015; Ergal et al. 2020). Biohydrogen production is an emerging technology and requires lesser energy than other physicochemical methods (Yun et al. 2018; Keskin et al. 2019a, b) and shows characters of energy systems more similar to electricity than fossil fuels (Chandrasekhar et al. 2015). It provides a nonpolluting and renewable source of energy as its combustion produces water as the major by-product (Chandrasekhar et al. 2015; Keskin et al. 2019a, b; Pascualone 2019; Cieciura-Włoch et al. 2020) categorizing it as a clean and sustainable source of fuel having a positive impact on the ecosystem (Dwivedi et al. 2020). Biohydrogen also provides a high-energy yield of 122 kJ/g, which is 2.75-fold higher than that of any hydrocarbon fuels (Chandrasekhar et al. 2015, Pascualone 2019) besides having a high energy density of 142 kJ/g (Tenca et al. 2011; Cahyari et al. 2019). Also, H2 gas is safer to handle than domestic natural gas (Das and Veziroglu 2008). Biohydrogen is stored as an electrical current in fuel cells and it can be applied as transportation fuel, or electrical energy (Saidi et al. 2018). Hydrogen can be used either as the fuel for direct combustion in an internal combustion engine or as the fuel for a fuel cell. Biohydrogen has high conversion efficiency (45–60%), obtained with fuel cells as well as with new homogeneous charge compression ignition (HCCI) engines (approx. 45%) (Tenca et al. 2011). Molecular hydrogen (H2) acts as a suitable energy vector given its high energy capacity, diversity of applications, and low environmental impact (Moreno Cárdenas and Zapata Zapata 2019). Biohydrogen can be produced by various pathways including direct biophotolysis, indirect biophotolysis, photo-fermentation, and dark fermentation. Among them, dark fermentation has proved to be the most efficient method since it does not require external energy, and its H2 production rate is much faster than other processes (Yun et al. 2018). All the biological hydrogen production pathways are environmentally friendly methods and have low investment costs (Saidi et al. 2018). Hydrogen is presently used in the fertilizer and petroleum refining industries spanning about 50% and 37%, respectively, and it is growing 10% per year. At this pace, hydrogen is set to represent 8–10% of the total energy of the world by 2025 (Kapdan and Kargi 2006; Saidi et al. 2018). Hydrogen in addition to being a source of energy also serves as a feedstock for the production of chemicals, hydrogenation of fats and oils in the food industry, production of

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electronic devices, processing steel, and many other purposes (Kapdan and Kargi 2006).

4

Sources of Biohydrogen

The current global H2 production scenario consists of 48% from natural gas, 30% from heavy oils and naphtha, 18% from coal, and 4% from electrolysis (Keskin et al. 2019a, b). Hydrogen is not readily available in nature. Hence, new processes for the cost-effective production of hydrogen using naturally available sources need to be developed (Kapdan and Kargi 2006): (a) Biohydrogen is produced by a variety of organisms including bacteria, cyanobacteria, archaea, green algae, protists, etc. These organisms can be categorized into aerobic, anaerobic, facultatively aerobic, obligately anaerobic, heterotrophic, photoautotrophic, photosynthetic, mesophilic, thermophilic, etc. and are found as a single culture or working in a mixed consortium. (b) Biohydrogen is usually produced by bacteria containing hydrogen-producing enzymes, such as hydrogenase and nitrogenase, through the process of dark and photo-fermentation, respectively. (c) Algae and cyanobacteria are photosynthetic organisms, which can split water molecules using solar energy to produce hydrogen and oxygen (Yasin et al. 2013; Chandrasekhar et al. 2015). (d) Biohydrogen as a biofuel is also produced currently by the process of genetic engineering of metabolic pathways in microorganisms. Genes suitable for biohydrogen production from one organism can be engineered into another model organism to express its product in a modified environment and through simplified pathways. Not only this can increase the production and yield of biohydrogen but also the organism is able to withstand toxic biofuel intermediates (Zhang et al. 2011). (e) Hydrogen production by the fermentation of waste products rich in carbon provides an attractive approach that solves the dual purpose of waste disposal and clean energy generation. It also costs cheaper than chemical and electrolytic processes of H2 production and processing (Chandrasekhar et al. 2015). Substrates including agricultural and food industry waste, and wastewaters like cheese whey, olive mill, and baker’s yeast industry wastewaters are good examples of waste having a high carbohydrate content (Kapdan and Kargi 2006). (f) Fruit and vegetable wastes are rich in carbohydrates and essential nutrients suitable for efficient biohydrogen production. Food processing wastewater from fruit processing or juice industry fermented by a combined process of dark fermentation and microbial electrolysis cell produced 13 times more hydrogen than the process of dark fermentation alone, with energy yield thrice than the supplied energy (Yun et al. 2018).

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(g) Biohydrogen can also be obtained from vinasses, soybean oil extraction residues, mushroom wastes, wastewater from cheese processing, water hyacinth, etc. (Gomez-Romero et al. 2014). (h) Lignocellulosic biomass derived from plants and animals available abundantly in nature is another efficient source for hydrogen production (RodriguezValderrama et al. 2020). Common methods for biohydrogen production include algae- and cyanobacteriamediated direct and indirect biophotolysis, photosynthetic bacteria-induced photofermentation, dark fermentation effected by fermentative bacteria, and microbial electrolysis cells (Keskin et al. 2019a, b). The method widely used currently for the production of hydrogen is the steam reforming of natural gas or methane (CH4). Industries also use a water-gas shift reaction, specifically used for ammonia production to obtain pure H2 (Chandrasekhar et al. 2015). A recently formulated method for the generation of hydrogen used the combustion or gasification of coal (Abubackar et al. 2019a, b).

5

Fruit and Vegetable Wastes as a Source of Biohydrogen

5.1

Volume of Waste Generated

A large part of the food amounting to about one-third of that produced for human consumption is spoiled or wasted before its consumption as stated by the Food and Agriculture Organization (FAO), in its report in 2011 (Pascualone 2019). The Asian Institute of Technology in 2010 reports about 15–63% of the total municipal solid wastes worldwide to consist of organic solid food waste (FW) (Yun et al. 2018). It is predicted that with the expected rapid economic development, the food waste generation for municipal solid waste in Asian nations will increase by 50–55% from 278 to 416 million tons during 2005–2025 (Panin et al. 2021). The energy stored in such wasted or spoiled food is generally lost or left unutilized. This food waste includes a share consisting of about 40–50% of fruit and vegetable waste (Pascualone 2019).

5.2

Where and How Is the Waste Generated

Fruit and vegetable wastes are abundantly available around us and are produced daily in large quantities. Fruit-vegetable waste is mainly produced from the wholesale and retail markets, household kitchen waste, restaurants, etc. It is obtained as postharvest losses in the farm, as transport or handling losses in the supermarket, local market, or grocery store, and as inedible, spoilage, and cooking losses in the restaurant and kitchen. Fruit waste is in addition also produced by the fruit processing industries, fruit juice vendors, etc. Mostly fruit-vegetable waste finds its way into the municipal solid waste of the locality. The reason for the huge

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wastage (about 640 million metric tons) of fruits worldwide includes improper grade, transportation losses, inadequate preservation facilities, etc. besides the loss resulting from inedible portion of fruits or natural waste (Mahato et al. 2020).

5.3

Disposal of Fruit-Vegetable Waste

The most common practice around the world for the disposal of biodegradable waste generated is by landfilling or by open dumping (Yun et al. 2018; Keskin et al. 2019a, b; Cieciura-Włoch et al. 2020). Fruit waste being a part of municipal solid waste when dumped in the open causes unpleasant odor and leachate due to emission of methane and carbon dioxide gases into the atmosphere and potentially attracts rodents and infectious insects. Environmental commissions across the world are working on action plans to substantially reduce food wastage and utilize the waste from the food industry like fruit and vegetable waste (FVW) for valorization and other commercial applications (Dwivedi et al. 2020).

6

Type of Wastes Suitable for Biohydrogen Production

Substrates rich in carbohydrates have a higher hydrogen production potential than substrates composed of lipids and proteins (Yun et al. 2018); hence biohydrogen production generally utilizes carbohydrate-rich substrates to generate molecular H2. Complex substrates have not yielded desirable results for biohydrogen production (Chandrasekhar et al. 2015). The properties of fruit and vegetable waste (FVW) including organic composition, biodegradable nature, high quantity, easy availability, low cost, high carbohydrate content (Pascualone 2019), high volatile solid content, and easily hydrolyzable organic components (Keskin et al. 2019a, b) make it a feasible option for biohydrogen production as an alternative to costlier pure reducing sugars like glucose, sucrose, mannose, and fructose.

6.1

Characteristics and Properties

The characteristics required for a substrate for the sustainable production of biohydrogen include the following: 1. 2. 3. 4. 5.

It should be a rich source of carbohydrates. It should be produced from a renewable source. It should be easily available in nature. It should require no or negligible pretreatment. It should be inexpensive.

Fruit and vegetable waste contains high amounts of polysaccharide, is cheap and readily available throughout the year, is easily biodegradable, and has a low total

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solid, high volatile solid, and high moisture content, which makes it an ideal feedstock for dark fermentative hydrogen production (Keskin et al. 2019a, b; Cieciura-Włoch et al. 2020). Food waste consists of carbohydrates ranging between 30 and 70% (Yun et al. 2018). On average fruit and vegetable, waste shows a solid concentration of 148 g/kg, of which organic fraction constitutes around 85% total solids, nitrogen constitutes around 6.91% total solids, and phosphorus content consists of around 0.32% total solid (Cieciura-Włoch et al. 2020). Biological production of hydrogen reduces greenhouse gas emissions by 57–73%, and it can run on a cheap and less energy-intensive technology (Pascualone 2019).

7

Methods for Production of Biohydrogen

Molecular hydrogen throughout the years has primarily been produced from fossil fuels, through steam reforming of natural gas or methane (CH4). A method employing water-gas shift reaction generally used for ammonia production in industrial reactions has also been utilized for obtaining pure H2. Other processes utilized in accordance to the requirement for hydrogen production include thermal decomposition, autothermal reforming, catalytic oxidation, pyrolysis, and steam gasification (Kapdan and Kargi 2006; Chandrasekhar et al. 2015). Fossil fuels are nonrenewable resources and its combustion under extreme conditions causes pollution by emitting significant greenhouse gases which is an environmental contradiction. Therefore, an environmentally friendly way using renewable and sustainable resources to obtain H2 is necessary (Yun et al. 2018). Production of hydrogen using different organisms under different conditions and utilizing different substrates is a clean and environment-friendly method. Some of the biological methods for biohydrogen production are described below (Manish and Banerjee 2008):

7.1

Direct Biophotolysis

It is carried out by algae using the photosynthetic process. In the process solar energy is used to convert water to oxygen and hydrogen. The enzyme Fe-hydrogenase catalyzes the reaction: 2H2 O þ light energy ! 2H2 þ O2 But the process is feasible only in the absence of oxygen as Fe-hydrogenase is extremely sensitive to oxygen produced in the reaction.

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Indirect Biophotolysis

It is carried out by cyanobacteria using CO2 from air and solar energy to split water into H2 and O2. The evolution of H2 and O2 in the process takes place in separate stages, thus preventing inactivation of hydrogenase enzyme: 12 H2 O þ 6 CO2 þ light energy ! C6 H12 O6 þ 6O2 C6 H12 O6 þ 12 H2 O þ light energy ! 12 H2 þ 6 CO2

7.3

Photo-Fermentation

Photo-fermentation is the process of generating molecular hydrogen by photosynthetic bacteria catalyzed by the enzyme nitrogenase using solar energy and reduced compounds like organic acids. The process occurs under anaerobic conditions by donating electrons from simple organic acids like acetic acid to the nitrogenase. Ferredoxin acts as an electron carrier for the process. The oxidized protons are reduced by this nitrogenase enzyme into hydrogen gas using energy in the form of ATP. The overall reaction of hydrogen production can be given as C6 H12 O6 þ 6H2 O þ light energy ! 12H2 þ 6 CO2

7.4

Two-Stage Process

The two-stage process is a combination of two types of fermentation, viz. dark fermentation and photo-fermentation. Theoretically 12 moles of hydrogen should be produced from the fermentation of 1 mol of glucose; however the reaction is not thermodynamically feasible; hence an external energy supply is used to achieve the desired result. On the other hand, a mole glucose is oxidized into 4 mol of hydrogen along with acetate as a sole by-product by fermentative bacteria during dark fermentation. Acetate produced in dark fermentation can be further oxidized by photosynthetic bacteria to produce hydrogen: C6 H12 O6 þ 2H2 O ! 4H2 þ 2 CO2 þ 2CH3 COOH CH3 COOH þ 2H2 O þ light energy ! 4H2 þ 2 CO2 Hence a maximum yield of hydrogen can be obtained in a continuous production process by integrating dark and photo-fermentation methods.

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Microbial Electrolysis Cells

Microbial electrolysis cells (MECs) produce hydrogen using a single-stage dark fermentation to generate H2 with high efficiency. Microbial electrolysis cell oxidizes the acetate from dark fermentation with the help of external electrical energy to form hydrogen. In the electrolytic cell the anode is constituted of the acetate bioreactor while the cathode is made of a platinum electrode where the protons and electrons produced by bacteria are collected: Anode: CH3 COOH þ 2H2 O ! 2CO2 þ 8Hþ þ 8e ; Eo ¼ 0:098 V Cathode: 8Hþ þ 8e ! 4H2 ; Eo ¼ 0 V It is an emerging technology and the current knowledge for successful H2 production by MEC technology deals with mostly bench-scale reactors which do not allow predictions on its successful implementation on the industrial scale. Hence further research on the field will be required (Yun et al. 2018).

7.6

Self-Fermentation

Self-fermentation of fruit-vegetable waste is another method of generating biohydrogen where an external inoculum is not used but the fermentation occurs using the indigenous microorganisms present in the substrate. Fruit and vegetable organic waste contains abundant indigenous microflora including hydrogenproducing bacteria that decompose the raw material when left at ambient temperature for a long time. It did not produce methane but generated H2, CO2, and N2. Therefore, the waste undergoes self-fermentation leading to biohydrogen production. This can be due to the inoculum/substrate ratio being very low, thus enabling the development of hydrogen-producing bacteria with a rapid growth rate while inhibiting the methanogens characterized by a slow growth rate. However, very little research has been done on it (Marone et al. 2014).

7.7

Dark Fermentation

Anaerobic digestion is a process wherein the fermentation of a substrate leads to energy recovery from the solid wastes along with simultaneous treatment of the waste materials. It consists of three stages—hydrolysis, acidogenesis, and acetogenesis. Dark fermentation is a part of the anaerobic digestion that can produce biohydrogen from the hydrolysis and acidogenesis stages. Anaerobic organisms hydrolyze the macromolecular organic compounds rich in carbohydrate into volatile organic acids, alcohols, simple sugars, and hydrogen under anaerobic and dark conditions releasing electrons that are accepted by protons to generate molecular H2 (Pascualone 2019). Anaerobic metabolism of pyruvate, formed by catabolism of

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substrates, is the primary source of hydrogen in microbes. The metabolism takes place by any of the following pathways: 1. Pyruvate:formate lyase (PFL) Pyruvate þ CoA ! acetyl CoA þ formate 2. Pyruvate:ferredoxin oxidoreductase (PFOR) Pyruvate þ CoA þ 2Fd ðoxÞ ! acetyl CoA þ CO2 þ 2Fd ðredÞ Different organisms follow different pathways for H2 formation. Some cultures like Enterobacteriaceae use the pyruvate-formate-lyase (PFL) pathway while others like Clostridiaceae prefer the pyruvate-ferredoxin oxidoreductase (PFOR) pathway (Ergal et al. 2020). The different methods for production of biohydrogen are summarized in Table 1.

Advantages of Dark Fermentation The processes of photo-fermentation and dark fermentation are both efficient and popular biological methods for hydrogen generation but dark fermentation has been found favorable over photo-fermentation due to its simplicity, minor energy requirements, higher hydrogen production rates, use of waste as raw material, and no light requirements (Pascualone 2019; Cieciura-Włoch et al. 2020). Dark fermentation can also be carried out in non-sterile conditions (Moreno Cárdenas and Zapata Zapata 2019; Rodriguez-Valderrama et al. 2020). Majority of the carbon and hydrogen contents (about 2/3rd) obtained from partial oxidation of organic Table 1 Different methods for production of biohydrogen Process name Direct biophotolysis

Indirect biophotolysis Photofermentation Two-stage process Microbial electrolysis cell Selffermentation Dark fermentation

Organism involved Algae

Method involved Photosynthesis

Cyanobacteria

Photosynthesis

Photosynthetic bacteria

Electron transport, nitrogenase, organic acid Dark fermentation and photo-fermentation Dark fermentation, electrical energy

Bacteria Bacteria

Bacteria

Fermentation

Bacteria

Anaerobic fermentation

Production or yield 0.07 (mmol/h L)

0.355 (mmol/ h L) 145–160 (mmol/h L)

Reference Kosourov et al. (2002) Melis et al. 2000 Sveshnikov et al. (1997) Levin et al. (2004)

4.8–5.2 (mol H2/mol glucose) 2.9 (mol H2/mol acetate)

Nath et al. (2005) Liu et al. (2005)

24  2 (L H2/kg volatile solid) 77 (mmol/h L)

Marone et al. (2014) Kumar and Das (2001)

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substrates are converted to microbial metabolic by-products during dark fermentation (Yun et al. 2018). The theoretical yield from dark fermentation is 4 mol of H2 per mol of glucose; however, in practice the actual H2 yield is reduced to half of the theoretical maximum. Various metabolic constraints of dark fermentative H2-producing microorganisms also limit the yield of hydrogen in dark fermentation (Ergal et al. 2020) that may include (1) thermodynamic limitation, (2) existence of non-H2 producers in the broth, and (3) acetogenic H2-consuming reaction. These limitations apply to all substrates of dark fermentation (Yun et al. 2018). Different species of anaerobic bacteria are involved in the generation of hydrogen in dark fermentation during the acidogenesis and acetogenesis phases (CieciuraWłoch et al. 2020), out of which the dominant microorganism was found to be Clostridium spp. which produced hydrogen via the butyric acid fermentation pathway (Pascualone 2019). Anaerobic fermentative bacteria also produced by-products like carbon dioxide and volatile fatty acids (VFA) besides hydrogen, during the degradation of carbohydrate-rich organic wastes in dark fermentation (Keskin et al. 2018). Hydrogen production at the initial stages used pure cultures as biocatalyst selected based on the fermentable substrates. However, with the passage of time it was observed that a mixed population of cultures provided benefits like easier operation, high stability, multiple biochemical functions, and diversity of substrates (Chandrasekhar et al. 2015). Co-digestion of fruit-vegetable waste with organic substrates such as glucose, cottage cheese, crude cheese, fish waste, and anaerobically digested sludge provided good results for the sustainable production of hydrogen (Dwivedi et al. 2020). As FVW is low in nitrogen, the co-substrate can usually balance the C/N ratio of the fermentation system (Gomez-Romero et al. 2014; Basak et al. 2018). Thus, co-digestion provides a nutrient balance and increased buffering capacity.

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Production of Biohydrogen from Fruit and Vegetable Wastes

8.1

Pretreatment of the Substrate

Fruit and vegetable wastes consist of lignocellulosic biomass covering its outer surface which is insoluble or not digestible in its natural form. This insoluble substrate can be made available for hydrolysis and improves the rate of biohydrogen formation by subjecting the organic waste to pretreatments (Pascualone 2019; Cieciura-Włoch et al. 2020). Pretreatment converts the complex substrates to simple substances, and they are classified into four major groups: physical (e.g., mechanical pretreatment, thermal pretreatment), chemical (e.g., acid hydrolysis, alkaline hydrolysis, ozonolysis), physicochemical (e.g., steam explosion, liquid hot water, sonification, and microwave-based pretreatment), and biological (e.g., enzymatic hydrolysis) pretreatments. Out of all the methods applied, the chemical treatments and physicochemical treatments are found to be the most effective (Chandrasekhar et al. 2015).

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Physical Pretreatment 1. The mechanical treatment involves milling, chipping, and grinding that produce fine particles with increased surface area increasing the degradation, which results in improving biohydrogen production rates (Keskin et al. 2019a, b). 2. Thermal pretreatment has multiple effects on the fermentation process. Thermal pretreatment of substrates improved substrate digestibility. It was found that a mild heat pretreatment of fruit and vegetable waste substrates increased the volumetric hydrogen yield (Pascualone 2019). It decomposes complex macromolecules in waste using low or high temperatures. Thermal pretreatment also removes counterproductive microbes that reduce hydrogen production. The thermal treatment removes methanogenic bacteria while retaining the sporeforming hydrogen-producing bacteria in FW besides reducing the rapid accumulation of organic acids. Thermal treatment also has an effect on the solubility of substrate components like protein and carbohydrate, which increases hydrogen yield. Chemical Pretreatment Chemical pretreatment provided much higher availability compared to other pretreatments but chemical contamination and formation of recalcitrant compounds prove to be risk factors. Besides chemical treatment is harsh on simple compounds like FVW. Pretreatments with a controlled quantity of compounds like acid, alkali, H2O2, FeCl3, O3, and other oxidative agents improve the surface area and prevent any potential inhibition of the biological process due to high doses of chemicals. A steam explosion under pressure in combination with chemical methods such as acid and base treatment has shown better results (Keskin et al. 2019a, b). Acid pretreatments of FVW using hydrochloric acid gave a better reducing sugar yield from substrates although sulfuric acid being less corrosive is generally preferred for fruit-vegetable waste treatment. However, both acid treatments generated higher butyric acid compared to propionic acid, during dark fermentation, favoring hydrogen production as the butyrate pathway produced higher molecules of hydrogen as compared to the propionate pathway which consumes H2 (RodriguezValderrama et al. 2020). Acidogenic fermentation of vegetable waste is an emerging technology that can concurrently produce biohydrogen (H2) and short-chain carboxylic acids (SCA) by the degradation of waste products (Kumar and Mohan 2018). A combination of dilute acid (0.5–1% (v/v)) hydrolysis of the vegetable waste followed by pretreatment with autoclaving at moderate temperature (80–125  C) effectively degrades the polysaccharides (cellulose/starch) present in the VW generating more soluble reducing sugar than physical or chemical methods alone. It was found that a pH of 6 was optimum to generate greater amounts of short-chain carboxylic acid (SCA) together with higher biohydrogen yield (Kumar and Mohan 2018).

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Physicochemical Pretreatment Physicochemical pretreatments using radiation energy such as gamma rays, electron beams, microwaves, and ultrasounds enhance the liquification of FW, by breaking the bonds present in cell walls, thereby increasing the efficiency of the bioprocesses: 1. Microwave-based pretreatments had low energy requirements but induced the formation of recalcitrant compounds due to high-temperature treatments. Microwave treatment creates vibrations in the intracellular components generating heat within the substrate that rapidly disrupts the cell walls of biopolymers. However high temperature generated by microwave solubilizes both hemicelluloses and lignin components, releasing inhibitors such as phenolic compounds, ethanol, and propionic acid, which affect the rate of anaerobic digestion. 2. Ultrasound pretreatment did not form inhibitory compounds but consumed high amounts of energy. Mechanical treatment with ultrasound produces high-energy bubbles that collapse in the solution releasing the walls that break the cell walls and increases FW solubilization, resulting in better exposure to microbial activity. 3. The thermochemical treatment is an effective technique for removal of both degradable and nondegradable compounds. It generates manyfold more energy than a single-stage process. Hot compressed water (HCW) pretreatment maintains water at temperatures above 180  C under pressure. This pretreatment technique is suitable for hydrolyzing fractions of food wastes containing starch. These carbohydrate components are initially converted into organic acids and subsequently to hydrogen gas. 4. Hydrothermal pretreatment uses water at a higher temperature for processing the FW into liquid fuel through the liquefaction process. The hydrothermal technique generates acetic and formic acids which transform most of the FW into simple sugars and then the sugar is fermented to generate biohydrogen (Yun et al. 2018; Abubackar et al. 2019a, b).

Enzymatic Pretreatment Pretreatment with enzymes gave better hydrolysis; however high cost, enzyme selectivity, and process inefficiency prevented its use.

8.2

Co-digestion of Substrate

Microbial fermentation of organic compounds like fruit-vegetable waste results in the formation of organic acids like lactic acid, propionic acid, acetic acid, and butyric acid. Formation of volatile organic acid intermediates can decrease the pH of the media that can inhibit microbial growth and subsequently stop hydrogen formation. This phenomenon can be overcome by co-digestion of fruit and vegetable waste with other substrates that helps to regulate the volatile organic acid concentration, besides the addition of alkali. Co-digestion of substrates also has positive synergistic effects of the mixed materials with complementary characteristics like the supply of missing

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nutrients by the co-substrate, adequate carbon/nitrogen (C/N) ratio, and macro- and micronutrient concentration. A balanced C/N ratio allows enhancing the buffer capacity of the system. Additionally, the co-digestion process also reduces the possibility of inhibitory effects, which, in turn, increases biohydrogen production. In particular, mixed culture fermentation of substrates such as organic waste or biomass appears to be one of the most promising approaches able to bring in the next future the distributed production of renewable hydrogen, sustainable also on smaller scales (Tenca et al. 2011). Co-digestion of FVW with livestock manure which is an abundant source of nutrients for cell growth and alkali helps neutralize the VFA produced in fermentation and also represents an ideal fermentation with carbohydrate-rich and promptly degradable materials for biohydrogen production (Tenca et al. 2011).

8.3

Mixed Microbial Inoculum

During the biohydrogen fermentation, the introduction of a mixed microbial inoculum has proved advantageous taking into account their low cost, easy availability, easiness in control, high versatility, and a broader choice of the substrate. Mixed microbial inoculum consists of organisms belonging to the genera Clostridium, Enterobacter, and Bacillus that are widely present in natural habitats such as sludge, compost, soil, sediments, leachate, and organic wastes and are responsible for biohydrogen production during dark fermentation (Rafieenia et al. 2018). But mixed culture provided a limitation as they consist of H2 consumers alongside H2 producers; hence their pretreatment represents a key factor for hydrogen fermentation. Pretreatment of mixed cultures helps in the inhibition of hydrogen-consuming microorganisms, namely hydrogenotrophic methanogens, and allows biohydrogenproducing bacteria to survive by forming spores when exposed to harsh environmental conditions (Rafieenia et al. 2018; Pascualone 2019). The addition of a controlled inoculum into fruit and vegetable waste substrate provided a fair amount of biohydrogen production (Panin et al. 2021).

8.4

Two-Phase Digestion

Hybrid systems are the most used for improving biohydrogen yield from solid waste materials in recent years. Two-phase systems involve an anaerobic digestion process that is carried out in two different reactors. Solid organic waste is known to be a very efficient source for fermentative biohydrogen production since it is readily available in large amounts and is economically viable. The hydrolysate produced in the first stage is fed into the second treatment stage where it is further utilized to produce biohydrogen and other useful products. This process helps in substrate stabilization and increasing the conversion of energy (Abubackar et al. 2019a, b).

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Type of Bioreactors

Most of the studies conducted on biohydrogen production use the batch system of fermentation which is not reproducible in the industry and is non-sufficient for the huge amount of waste generated daily. Therefore, a trend towards using continuous system reactors that can degrade a high amount of solid waste continuously along with maintaining the community of active microorganisms at a high level is gaining pace. Anaerobic baffled reactor (ABR), anaerobic sequencing batch reactors (ASBRs), and dry fermentation system and hybrid system are some continuous system reactors used currently (Abubackar et al. 2019a, b). ABR systems separate the acidogenesis and methanogenesis reactions into two compartments. With this property, biohydrogen-producing bacteria can be confined to one compartment which can be used to produce a higher biohydrogen yield. The ASRB has a high capacity for solid material loading from the feed compartment. The system involves four sequencing steps—feed, react, settle, and decant—and solid removal from the system, including inoculum recovery, is much more practical. Dry fermentation is a technique used for substrates with solid content greater than 20%. The dry fermentation system provides some advantages, such as mixing of reactants in the system is not required, a low energy requirement, and a lower amount of digestate production, which is directly related to reducing the bacterial contamination risk. Since the need for the addition of water to the system is very low, this means that the need for digester volume is reduced.

9

Factors Affecting Biohydrogen Production from Fruit and Vegetable Wastes

Biohydrogen provides numerous advantages but the generation of biohydrogen is filled with an equal number of hindrances including low speed of production, reduced formation of product, generation of acidic intermediates, and its aggregation leading to decreased pH that affect the fermentation process. Numerous studies are in progress and many novel techniques have been utilized to remove the disparities and increase the production of biohydrogen. The most significant factors affecting the H2 production by the dark fermentation from FVW waste were found to be temperature, pH, and substrate-to-inoculum ratio (Dwivedi et al. 2020). Besides the above, other factors affecting the H2 production included the substrate’s volatile solid (VS) content, carbon-to-nitrogen (C/N) ratio, chemical oxygen demand (COD), and presence of inhibitory compounds (Abubackar et al. 2019a, b). Various properties of the biomass such as its physical, chemical, and biological properties along with other physicochemical parameters such as chemical nature of the waste, substrate concentration, inoculum sources, pretreatment, volatile fatty acids, and hydrogen partial pressure also significantly affect the success of the biohydrogen production process. The acidogenesis and acetogenesis stages during the carbohydrate degradation in biohydrogen production are highly sensitive to the above environmental conditions (Yasin et al. 2013).

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Composition of Substrate

Studies have proven carbohydrate-rich substrates to produce nearly 20 times more H2 than fat or protein-rich substrates (Yun et al. 2018). Among carbohydrates, starch-based carbohydrate waste possesses a comparatively better potential for hydrogen production than cellulose-based waste, based on the biodegradability of the former which is very easily dissolved (Abubackar et al. 2019a, b). Theoretically, lipid and protein degradation products are not suitable for the production of biohydrogen. Lipids consist of triacylglycerides whose hydrolysis results in free fatty acid chains and glycerol. However, during the β-oxidation pathway, glycerol and free fatty acids could be further hydrolyzed to acetyl-CoA and acetate resulting in hydrogen evolution from NADH oxidation (Yasin et al. 2013).

9.2

Temperature

A key factor during the fermentation process is the operating temperature that determines the microbial use of the substrate, specific growth rate, metabolic product formation, etc. besides the H2 production. The operating temperature is typically dependent on the type of microorganism and the type of substrate used during the process. Most laboratory-scale studies on hydrogen production are performed at mesophilic conditions (30–45  C) owing to the ease of operation and maximum specific growth rates. But thermophiles and elevated temperature (45–75  C) have proven to provide advantages for the process resulting in higher yields.

9.3

pH of the System

The pH of the system is another significant factor regulating the metabolic pathway and H2 production process. Several physiological parameters in cells are affected by changes in the external pH of the system including parameters like the internal pH, proton motive force, and membrane potential which affects the rate of the fermentation process involved. A pH between 5.5 and 6.0 is beneficial for H2 production by dark fermentation while a pH between 6.0 and 7.5 proves to be optimum for methanogenesis and solventogenesis. Acidic pH lower than 5.5 leads to the accumulation of volatile fatty acids that decreases the system pH, ultimately inhibiting H2 production (Tenca et al. 2011; Chandrasekhar et al. 2015).

9.4

Feed-to-Inoculum (F/I) Ratio

The hydrogen yield of a system is also dependent on the feed-to-inoculum (F/I) ratio fed into the process as a higher amount of inoculum (low F/I ratio) will produce VFA faster, eventually leading to instability of the system. A low F/I ratio implies a limited amount of substrate for a larger number of inoculated organisms which

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potentially prevents the optimum growth of microorganisms. On the contrary, a higher F/I ratio indicates an excessive substrate concentration compared to inoculum which may cause inhibition of substrates (Keskin et al. 2019a, b).

9.5

Nutrient Concentration

A significant role in the H2 production process is played by nutrient concentration of the medium which includes the following parameters.

Carbon-to-Nitrogen (C/N) Ratio The optimum concentration of carbon-to-nitrogen ratio is critical in mixed or pure cultures to increase H2 production as excess nitrogen induced ammonification which results in increase in the intracellular pH of the microorganism that inhibits microbial function reducing the H2 production capability. Phosphate Concentration High amounts of phosphate in the medium induce an excessive production of VFAs, resulting in a significant decrease in the H2 yield. Adenosine triphosphate (ATP) is a form of phosphate and it provides an optimum concentration of phosphate in the bacterial cell necessary for energy generation and regulation of the buffering capacity besides acting as an alternative to carbonate in the fermentative process. Concentration of Metal Ions Metal ions are necessary to activate various enzymes and co-enzymes related to microbial metabolism, cellular transport, and cell growth. Trace elements are required for H2 production as they have been found to be associated with the specific function of the metabolic pathway of anaerobic microorganisms. Iron is an essential component of the ferredoxin and hydrogenase present in bacterial cell and is required for the electron transport mechanism. Studies conducted on the concentration of metal ion report a positive effect of high concentration of iron on the H2 production system. Other findings also describe the Mg, Na, Zn, and Fe ions to positively impact the yields of H2. Another study stated Cu, Zn, and Fe as increasing biohydrogen production (Keskin et al. 2019a, b). Ni2+ and Fe2+ constitutes the active site of hydrogenases while Mg2+ is the active component of many enzymes and is also used by ATP in the cells for the transportation of energy. Zn2+ and Mn2+ have also found importance for their association with the growth and survival of the cells (Keskin et al. 2018).

9.6

Hydraulic Retention Time (HRT)

The hydraulic retention time or fermentation time of the substrate also affects the H2 production, as long fermentation results in the metabolic shift from acidogenesis to

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methanogenesis. A HRT of 8–14 h proved satisfactory for a wide variety of substrates.

9.7

Partial Pressure of Hydrogen (Hpp)

The partial pressure of hydrogen in the system is determined by the amount of hydrogen dissolved or accumulated in the medium. The partial pressure of hydrogen is another factor affecting the rate of H2 production. An increased H2 level leads to the reduction of oxidized Fd more favorably than its oxidation, causing the hydrogenase to be reversibly oxidized and the Fd to be reduced, resulting in decreased H2 production. The low partial pressure of hydrogen (Hpp) in the H2 bioreactor yields higher amounts of H2 (Chandrasekhar et al. 2015).

9.8

Pretreatment of Substrate

Pretreatment of the substrate for hydrolyzing the cellulose content is a limiting step in anaerobic digestion processes. Various pretreatment methods such as heat, alkali, and acid applied to food waste increased the H2 yield by 5–20 times compared to the control. Pretreatment of the substrate was postulated to select the microbes favorable for H2 production instead of just increasing the substrate hydrolysis. Autoclaving the substrate is another method of heat pretreatment. Autoclaving does not form inhibitory compounds and the autoclaved substrate can be used for fermentation without neutralization. Thus, it offers advantage as reduced steps of processing substrate compared to other physicochemical pretreatment methods (Keskin et al. 2019a, b). Gene sequencing of microbes present in the substrate medium before and after treatment also supported the fact (Yun et al. 2018). Substrate pretreatment eliminated the natural microbiota of FVW, thus providing a competition-free environment for the inoculated microorganisms (Pascualone 2019).

9.9

Pretreatment of Inoculum

Thermal treatment of inoculum was also responsible for the better activity of microorganisms. Heat treatment of the inoculum before being placed into reactors avoided the activity of methanogens leaving behind hydrogen-producing microflora which supplemented H2 production.

9.10

Acidogenic Pathway

It has been found that the butyric acid pathway was favorable for hydrogen production, while the propionic acid pathway did not produce hydrogen.

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Reducing Sugar Composition

Reducing the sugar composition of the hydrolysate also affected the biohydrogen process. Though different researchers have produced hydrogen from both hexoses and pentoses, hexoses were found more favorable as discussed by Berlowska et al. Fruit and vegetable waste showed higher hexose and lower pentose concentration, suggesting it to be a better substrate for hydrogen production (Cieciura-Włoch et al. 2020).

9.12

Addition of Inoculum and Removal of Bioproduct

The addition of active hydrogen producers at specific intervals and removal of the accumulated VFAs or leachates have shown positive results for hydrogen production. Besides, the removal of the accumulated hydrogen from the headspace of the fermenter maintains partial pressure of H2 and also prevents the consumption of produced hydrogen by homoacetogens (Keskin et al. 2019a, b).

9.13

Inhibitory Compounds

Acid pretreatment of biomass, in general, liberates potential growth inhibitors like furfural, hydroxymethylfurfural (HMF), and vanillin into hydrolysates. These compounds need to be removed by a detoxification step such as precipitation technique by adding calcium carbonate and/or passing the acid hydrolysate through activated charcoal. Thus, hydrogen production from acid hydrolysates of biomass consists of a three-step process, including acid pretreatment and detoxification of hydrolysate followed by clostridial fermentation. Secondary waste generated due to acid digestion includes phenolic compounds (e.g., coniferyl aldehyde, ferulic acid, and 4-hydroxy benzoic acid), aliphatic carboxylic acids (e.g., acetic acid, formic acid, and levulinic acid), and furans (e.g., 2-furoic acid, furfural, and HMF) which are also harmful to microbial growth (Mahato et al. 2020). Indigenous volatile compounds including limonene, eugenol, phloretin, and phytochemicals, along with volatile fatty acids produced from plant materials, inhibit the growth of microorganisms acting as antimicrobials. Therefore, the presence of such compounds decreases biohydrogen yield (Cahyari et al. 2019).

9.14

Co-digestion

Co-digestion of substrates has been found to provide different benefits essential for H2 production such as improvement of nutrient balance (carbohydrates/proteins), dilution of inhibitory product, and various other synergistic properties during microbial fermentation (Dwivedi et al. 2020; Rodriguez-Valderrama et al. 2020).

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Particle Size

Nature or particle size of raw material also affected the yield of hydrogen. Grinding of substrates to the size below 2 cm or above 5 cm has been reported to produce a greater amount of hydrogen compared to substrate size between 2 and 5 cm (Keskin et al. 2019a, b).

9.16

Total Suspended Solid Content

Increased growth of inoculum in the fruit waste hydrolysate substrate gave larger production of hydrogen. Similarly, presence of a high substrate concentration within a defined limit enhanced the cumulative hydrogen production, although a further increase in the concentration of substrate showed inhibitory effects on cell growth and biohydrogen production. Excessive concentration of suspended solids in the fermentation media brought about variability to the media, creating a resistance to the growth of organisms. It also reduces the water content in the media required for biological functions, which presumably inhibits the cell growth, and reduces hydrogen production (Mahato et al. 2020).

10

Advantages of Biohydrogen Produced from Fruit and Vegetable Industry Wastes

The utilization of fruit and vegetable waste and microorganisms for biohydrogen production provided several advantages including renewability, sustainability, and carbon neutralization over conventional chemical processes for the production of biofuels (Chandrasekhar et al. 2015). Biohydrogen production utilizes less energy during the production process than electrochemical and thermochemical processes and could be carried out at atmospheric pressure and ambient temperature (Panin et al. 2021). The most important advantage however remains that biohydrogen production utilizes waste products as substrates removing them from the environment in addition to the production of energy in the form of biohydrogen.

11

Conclusions

The research available to date on the production of hydrogen from fruit and vegetable waste mostly reported studies conducted in bench-scale reactors using simple batch tests, which are not reflections of real processes occurring in full-scale installations. Numerous opportunities for increasing the revenue and reducing the cost of biohydrogen production are available including further processing of the sub-products like organic acids and solvents from dark fermentation by photofermentation for hydrogen production, or methane production by anaerobic digestion. This process makes use of a two-stage fermentation reaction that uses the

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by-products obtained during biohydrogen fermentation. The insolubilized biomass obtained after fermentation is a good biofertilizer and can be marketed as soil enhancers. Other areas like equipment construction, operating costs for gas compression, salaries, and taxes can be managed economically. However, biohydrogen is still not compatible with existing fuel infrastructure, and production yields of biohydrogen presently cannot compete with or replace fossil fuel. Hence a thorough study into the properties of biohydrogen is required for increasing its quality and quantity together.

References Abubackar HN, Keskin T, Arslan K, Vural C, Aksu D, Yavuzyılmaz DK, Ozdemir G, Azbar N (2019a) Effects of size and autoclavation of fruit and vegetable wastes on biohydrogen production by dark dry anaerobic fermentation under mesophilic condition. Int J Hydrog Energy 44(33):17767–17780 Abubackar HN, Keskin T, Yazgin O, Gunay B, Arslan K, Azbar N (2019b) Biohydrogen production from autoclaved fruit and vegetable wastes by dry fermentation under thermophilic condition. Int J Hydrog Energy 44(34):18776–18784 Basak B, Fatima A, Jeon BH, Ganguly A, Chatterjee PK, Dey A (2018) Process kinetic studies of biohydrogen production by co-fermentation of fruit-vegetable wastes and cottage cheese whey. Energy Sustain Dev 47:39–52 Cahyari K, Hidayat M, Syamsiah S, Sarto (2019) Optimization of hydrogen production from fruit waste through mesophilic and thermophilic dark fermentation: effect of substrate-to-inoculum ratio. Malaysian J Anal Sci 23(1):116–123 Chandrasekhar K, Lee YJ, Lee DW (2015) Biohydrogen production: Strategies to improve process efficiency through microbial routes. Int J Mol Sci 16(4):8266–8293 Cieciura-Włoch W, Borowski S, Otlewska A (2020) Biohydrogen production from fruit and vegetable waste, sugar beet pulp and corn silage via dark fermentation. Renew Energy 153: 1226–1237 Das D, Veziroglu TN (2008) Advances in biological hydrogen production processes. Int J Hydrog Energy 33(21):6046–6057 Dwivedi AH, Gedam VV, Suresh KM (2020) Sustainable hydrogen production from fruit and vegetable waste (FVW) using mixed anaerobic cultures via dark fermentation: Kinetic aspects. Int J Energy Environ Eng 11(3):341–349 Ergal İ, Gräf O, Hasibar B, Steiner M, Vukotić S, Bochmann G, Fuchs W, Simon KMR (2020) Biohydrogen production beyond the Thauer limit by precision design of artificial microbial consortia. Commun Biol 3(1):1–12 Gomez-Romero J, Gonzalez-Garcia A, Chairez I, Torres L, Garcia-Peña EI (2014) Selective adaptation of an anaerobic microbial community: biohydrogen production by co-digestion of cheese whey and vegetables fruit waste. Int J Hydrog Energy 39(24):12541–12550 Kapdan IK, Kargi F (2006) Bio-hydrogen production from waste materials. Enzym Microb Technol 38(5):569–582 Keskin T, Arslan K, Abubackar HN, Vural C, Eroglu D, Karaalp D, Yanik J, Ozdemir G, Azbar N (2018) Determining the effect of trace elements on biohydrogen production from fruit and vegetable wastes. Int J Hydrog Energy 43(23):10666–10677 Keskin T, Abubackar HN, Arslan K, Azbar N (2019a) Biohydrogen production from solid wastes. In: Biohydrogen. Elsevier, Heidelberg, pp 321–346 Keskin T, Abubackar HN, Yazgin O, Gunay B, Azbar N (2019b) Effect of percolation frequency on biohydrogen production from fruit and vegetable wastes by dry fermentation. Int J Hydrog Energy 44(34):18767–18775

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Kosourov S, Tsygankov A, Seibert M, Ghirardi ML (2002) Sustained hydrogen photoproduction by Chlamydomonas reinhardtii: Effects of culture parameters. Biotechnol Bioeng 78(7):731–740 Kumar N, Das D (2001) Continuous hydrogen production by immobilized Enterobacter cloacae IIT-BT 08 using lignocellulosic materials as solid matrices. Enzym Microb Technol 29(4–5): 280–287 Kumar AN, Mohan SV (2018) Acidogenic valorization of vegetable waste for short chain carboxylic acids and biohydrogen production: influence of pretreatment and pH. J Clean Prod 203: 1055–1066 Lee SJ, Lee SJ, Lee DW (2013) Design and development of synthetic microbial platform cells for bioenergy. Front Microbiol 4:92 Levin DB, Pitt L, Love M (2004) Biohydrogen production: Prospects and limitations to practical application. Int J Hydrog Energy 29(2):173–185 Liu H, Grot S, Logan BE (2005) Electrochemically assisted microbial production of hydrogen from acetate. Environ Sci Technol 39(11):4317–4320 Mahato RK, Kumar D, Rajagopalan G (2020) Biohydrogen production from fruit waste by Clostridium strain BOH3. Renew Energy 153:1368–1377 Manish S, Banerjee R (2008) Comparison of biohydrogen production processes. Int J Hydrog Energy 33(1):279–286 Marone A, Izzo G, Mentuccia L, Massini G, Paganin P, Rosa S, Varrone C, Signorini A (2014) Vegetable waste as substrate and source of suitable microflora for bio-hydrogen production. Renew Energy 68:6–13 Melis A, Zhang L, Forestier M, Ghirardi ML, Seibert M (2000) Sustained photobiological hydrogen gas production upon reversible inactivation of oxygen evolution in the green alga Chlamydomonas reinhardtii. Plant Physiol 122(1):127–136 Moreno Cárdenas EL, Zapata Zapata AD (2019) Biohydrogen production by co-digestion of fruits and vegetable waste and coffee mucilage. Revista Facultad Nacional de Agronomía Medellín 72(3):9007–9018 Nath K, Kumar A, Das D (2005) Hydrogen production by Rhodobacter sphaeroides strain OU 001 using spent media of Enterobacter cloacae strain DM11. Appl Microbiol Biotechnol 68(4): 533–541 Panin S, Setthapun W, Sinsuw AAE, Sintuya H, Chu CY (2021) Biohydrogen and biogas production from mashed and powdered vegetable residues by an enriched microflora in dark fermentation. Int J Hydrog Energy 46(27):14073–14082 Pascualone MJ (2019) Fermentative biohydrogen production from a novel combination of vermicompost as inoculum and mild heat-pretreated fruit and vegetable waste. Biofuel Res J 6(3):1046 Rafieenia R, Lavagnolo MC, Pivato A (2018) Pre-treatment technologies for dark fermentative hydrogen production: Current advances and future directions. Waste Manag 71:734–748 Rodríguez-Valderrama S, Escamilla-Alvarado C, Magnin JP, Rivas-García P, Valdez-Vazquez I, Ríos-Leal E (2020) Batch biohydrogen production from dilute acid hydrolyzates of fruits-andvegetables wastes and corn stover as co-substrates. Biomass Bioenergy 140:105666 Saidi R, Liebgott PP, Gannoun H, Gaida LB, Miladi B, Hamdi M, Bouallagui H, Auria R (2018) Biohydrogen production from hyperthermophilic anaerobic digestion of fruit and vegetable wastes in seawater: simplification of the culture medium of Thermotoga maritima. Waste Manag 71:474–484 Show KY, Lee DJ, Tay JH, Lin CY, Chang JS (2012) Biohydrogen production: Current perspectives and the way forward. Int J Hydrog Energy 37(20):15616–15631 Sveshnikov DA, Sveshnikova NV, Rao KK, Hall DO (1997) Hydrogen metabolism of mutant forms of Anabaena variabilis in continuous cultures and under nutritional stress. FEMS Microbiol Lett 147(2):297–301

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Biohydrogen from Distillery Wastewater: Opportunities and Feasibility Anubha Kaushik, Sharma Mona, and Raman Preet

Abstract

Energy recovery from industrial wastewaters has gained focus in the recent years in the context of emerging concept of circular economy. Techniques involving recovery of energy during remediation of wastewaters using microorganisms are coming up as an advancement to the traditional wastewater treatment processes that themselves have high-energy demand. Production of hydrogen from organically rich wastewaters by using various microorganisms through light-dependent or dark fermentation pathways is of great interest since hydrogen is being viewed as the next-generation fuel that has high-energy content without carbon emissions. This chapter critically reviews the potential of biohydrogen production from distillery wastewater that is one of the most polluted industrial wastewaters due to its high organic load. Microbial cultures and mixed microbial consortia have shown the capability to produce hydrogen from the effluent by degrading the organic matter. The role of various operational parameters in the process efficiency in batch mode and continuous mode and adopting of inoculum pre-treatment strategies to eliminate other microbial competitors from mixed inoculum for enhanced hydrogen production are being discussed. The limitations and challenges faced by this technology, particularly in application at large scale, are discussed along with possible strategies to address the same.

A. Kaushik University School of Environment Management, GGS Indraprastha University, New Delhi, India S. Mona Department of Environmental Science and Engineering, Guru Jambheshwar University of Science and Technology, Hisar, Haryana, India R. Preet (*) Maitreyi College, University of Delhi, New Delhi, India # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Kuddus et al. (eds.), Organic Waste to Biohydrogen, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-1995-4_4

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Keywords

Biohydrogen · Fermentation pathways · Distillery wastewater · Inoculum pre-treatment · Process parameters

1

Introduction

With rapid industrial development, the challenge of treating wastewaters containing different types of organic and inorganic pollutants is increasing before discharging them safely into the environment. Various conventional wastewater treatment technologies, such as membrane filtration, adsorption, coagulation/flocculation, electrochemical processes, irradiation, and conventional oxidation methods, are all energy demanding, costly, and also known to generate toxic sludge (Maurya and Patil 2018). In tandem with the United Nations Sustainability Development Goals (SDGs) of providing clean water (SDG 6) and clean renewable energy (SDG 7), the surging approach is to look for technologies that can recover various important resources from the wastewaters while treating them, which will help in developing the circular economy and pave the way for sustainable growth (Venkatesh 2018). Various techniques and approaches to recover energy from wastewaters include anaerobic digestion and fermentation for biogas and bioethanol production, biofuel production, bioelectricity production using microbial fuel cell (MFC), and biohydrogen production using microbial electrolysis cell (MEC). Stafford et al. (2013) have critically analyzed the applicability of different methods of energy recovery from wastewaters and emphasized upon the need for matching the wastewater characteristics with the energy-recovering potential of the technology. Thus, different types of wastewaters need to be explored for harnessing the energy by using appropriate techniques.

1.1

Distillery Industry and Wastewater Treatment

Distilleries are considered as one of the most polluting industries due to the use of carbohydrate-rich feedstock in the process that results in the discharge of huge quantities of effluent with high organic load along with various co-pollutants (Mikuska and Zeleinska 2020). Distilleries are agro-based industries, which utilize sugarcane juice, sugarcane molasses, corn, wheat, cassava, rice, or barley as raw materials, depending upon their local availability, and the characteristics of the generated wastewater depend mainly on the nature of the feedstock. Around 10–15 L of wastewater is generated per liter of alcohol produced, and a typical distillery industry producing ethanol from cane molasses generates half million liters of wastewater daily (Tiwari et al. 2007). The waste bottom product of distillation called spent wash or stillage is one of the most complex and troublesome organic effluents (Jain et al. 2002). The distillery wastewater is very polluting in nature due to high biodegradable organic content, such as sugar, dextrose, lignin,

Biohydrogen from Distillery Wastewater: Opportunities and Feasibility Table 1 Physicochemical characteristics of distillery wastewater (extracted from Mikuska and Zeleinska 2020)

Characteristics pH Temperature Color Total solids Volatile solids Total suspended solids COD BOD VFA Nitrogen Phosphorus Potassium Iron Sulfates Calcium Magnesium

Units –
C – mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L

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Value 4.0–4.5 71–81 Dark brown 59,000–82,000 38,000–66,000 2400–5000 100,000–150,000 35,000–50,000 2300–2400 1660–4200 225–308 9600–15,475 1550–1800 2100–2300 2300–2500 220–250

hemicelluloses, and organic acids along with a dark-colored pigment melanoidin, a by-product of nonenzymatic reaction between sugars and amino compounds (Melamane et al. 2007). Besides causing anesthetic discoloration, melanoidin pigments are also toxic to microorganisms present in water and soil. The biochemical oxygen demand and chemical oxygen demand (COD) of distillery wastewater typically range between 40,000–50,000 and 80,000–100,000 mg/L, respectively (Venkata Mohan et al. 2007). Characteristics of distillery wastewater and level of pollutants vary based upon the type of raw material used as substrate. The range of various parameter values in distillery wastewater is shown in Table 1. Distillery spent wash contains high concentrations of nutrients, such as sulfate, nitrogen, phosphate, iron, and total dissolved and suspended solids, which, when used in diluted form in short-term or long-term irrigation with certain amendments, have been found to boost up crop growth by providing necessary nutrients to germinating seedlings of crop plants (Kaushik et al. 2005). Discharging distillery wastewater into inland water bodies or onto land without any treatment has high pollution impacts that may range from thermal pollution and toxicity to aquatic food chain, eutrophication problem, disturbed energy balance, and serious impacts on water use and land use (Patel and Jamaluddin 2018). Considering high nutrient and organic loading, the discharge of distillery wastewater into water courses is regulated through stringent environmental standards (Fito et al. 2019). Like many other countries, zero liquid discharge (ZLD) norms have also been implemented in India by the Central Pollution Control Board. Industries must adopt more effective, environment-friendly, and cheaper treatment technologies to reduce the pollution load of wastewater. Distillery stillage is characterized by low pH, and is composed of negatively charged dispersed colloids, which are formed due to the dissociation of carboxylic acids and phenolic groups that are difficult to degrade using conventional

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technologies of treatment. To comply with the standards set by environmental regulators, various treatment processes involving physicochemical, biological, and integrated methods have been developed for efficient degradation of organic matter (Ghosh et al. 2004). Conventional treatment methods for distillery effluents include membrane filtration, adsorption, irradiation, electrochemical processes, coagulation/ flocculation, and oxidation methods. Incineration, anaerobic digestion, and direct wet oxidation are also used for treating distillery effluents. However, high operational costs, use of chemicals in large quantities, challenges of sludge generation, and disposal are some of the major limitations of the physical and chemical methods (Maurya and Patil 2018; Boopathy and Santhilkumar 2014). Biological treatment of wastewater using microorganisms has been gaining attention in recent times across the world. The type of microbes, their growth, and bioremediation potential are crucial for effective treatment of distillery wastewater. Different microbial species have different capacities for utilizing various pollutants as source of nutrients for growth and they have shown tremendous potential to break down the organic substrates into simpler organic molecules and finally into inorganic nutrients. These degradation processes may be aerobic (in the presence of oxygen) or anaerobic (in the absence of oxygen), or may occur in sequential steps. Several species of bacteria, fungi, and algae and various microbial consortia have shown the capability of degrading organic pollutants present in various industrial effluents including that of distilleries (Bezuneh 2016). Several factors including pH, inoculum dose, temperature, nutrients, and overall composition of the wastewater influence the remediation potential of microorganisms for distillery wastewater treatment. Immobilization of the microbes in some inert matrices such as alginate, polystyrene, polyacrylamide, polyurethane, or agar has been reported to give better bioremediation of wastewater in comparison to free microbial cells (Sankaran et al. 2015; Raghukumar et al. 2004). Many aerobic and anaerobic bacterial strains have been found effective in the bioremediation of distillery wastewater. Aerobic microbial strains, however, have high energy consumption and therefore are economically not effective. Though there are numerous reports on microbial treatment of industrial effluents, most of these are confined to laboratory-scale experiments (Bezuneh 2016). Their industrial application is limited due to reduced stability of the system, requirement of exogenous nutrient supplement, spore formation, loss of extracellular enzymes, and lack of scaled-up reactors (Bezuneh 2016). Anaerobic digestion is a more attractive natural process in which anaerobic bacterial strains degrade the organic material present in wastewater and transform it into biogas (Mikuska and Zeleinska 2020). Anaerobic degradation is carried out by physiologically different groups of microorganisms, two major types being the acetogens and methanogens that form acetic acid and methane, respectively. Anaerobic treatment has been found to be more effective in treating high-strength distillery wastewater, which produces a small quantity of sludge, and at the same time it is more energy efficient. While there is no need of energy for aeration, it also produces methane that itself is an energy resource. Combination of aerobic and anaerobic processes for effective treatment of pollutants in wastewater is also popular, and it has been possible to remove 90–95% COD

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through combined aerobic and anaerobic treatments (Jimenez et al. 2003; Kapdan et al. 2003). High organic load, high nutrient concentration, and dark color of the distillery wastewater put forth challenges for treatment and management, particularly in the light of the ZLD norms (Nure et al. 2017). It is therefore extremely important for distilleries to adopt appropriate technologies that go a step ahead of treatment of the high-strength wastewater by trying to recover the nutrients and energy from the waste.

1.2

Energy Recovery from Distillery Wastewater

With rising energy demand worldwide, various methods are being developed for energy recovery from wastes and renewable energy generation in the form of methane, ethanol, bioelectricity, and hydrogen. Along with biogas and biodiesel, biohydrogen is considered as one of the most promising eco-friendly energy resources, mainly because it does not leave any residue after burning. In all these methods, microorganisms play a key role, and produce energy during the bioremediation process of wastewater treatment. The potential of microorganisms in wastewater treatment is highly dependent on the chemical composition of wastewater, nutrient concentration, pH, temperature, and inoculum size. While pure cultures of microbes have been found very useful for energy recovery in different forms, mixed cultures are easier to handle. One of the methods of harnessing energy from wastewaters is the anaerobic wastewater treatment technique producing methane, which is at present the most widely used technology to recover energy from wastewater. With the energy lost due to the heat dissipation during energy transfer from various reducing matters to methane, the energy consumption for maintaining the microbial activity, and the residual reducing matters in the wastewater after treatment, 80% of the chemical energy contained in the original reducing matters can be transferred to methane. Considering that only 35% of the chemical energy of methane can be converted into electricity through combustion process, the overall energy recovery efficiency is 28%. This number can potentially increase with the development of more effective CH4-driven chemical fuel cells (Karagiannidis et al. 2011). Microbial fuel cell (MFC) technology, which treats wastewaters and simultaneously produces electricity, is a bioelectrochemical system that converts chemical energy of the organic matter present in the wastewater into electrical energy through microbial catalysis at the electrode (Heijne et al. 2010). This technology helps in stabilizing various pollutants like carbon, nitrogen, and phosphorus. MFCs also find application in the removal and recovery of heavy metals along with energy production, which is largely influenced by deterministic operational factors, microbial inoculum, and biofilm (Kaushik and Singh 2020). The predominance of electroactive microbial communities including gamma-proteobacteria and firmicutes in the anodic biofilm, due to their exoelectrogenic activity, seems involved in power generation from distillery wastewater using MFC (Singh and Kaushik 2020). MFCs have a competitive advantage over other water treatment technologies due to their

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unique features such as energy benefits, less environmental impact, good operating stability, and varied applications. However, operation of MFCs also suffers from drawbacks such as short life span, high cost, low production rates, limited efficiencies, and membrane fouling (Li et al. 2014). Over the years, there has been tremendous progress in structural design, electrode material, inter-electrode spacing, anode modification, and parameter optimization to enhance the performance of MFCs in terms of COD removal and power generation from wastewaters (Sharma and Li 2010; Singh and Kaushik 2019, 2020, 2021). Another way of industrial wastewater treatment and their utilization for energy recovery is dark fermentation, which seems effective due to its simple operation and low energy requirement. Microbial communities in dark fermentation help in breaking down the carbohydrates to form hydrogen and other intermediates like volatile fatty acids and alcohols. Distillery wastewater being rich in carbohydrates is a suitable feedstock for hydrogen production.

2

Biohydrogen Production from Distillery Wastewater

High rise in the demand and price of fossil fuels, and the global climate change challenge associated with fossil fuel burning, has made it essential now to switch over from conventional fossil fuels to renewable resources. Hydrogen is being considered as a substitute to fossil fuels that can help in transforming the economy from carbon to hydrogen based. Hydrogen has high energy content (3042 cal/m3; 120 MJ/kg) and the highest gravimetric energy density. However, there are some challenges like removal of impurities, safe storage, and distribution that are associated with hydrogen energy (Ren et al. 2011). In the past decade, biohydrogen production has emerged as a new and exciting area of research that is heading its way for industrial and commercial application (Das and Veziroglu 2001). At present, hydrogen is mostly produced by electrolysis of water or by steam reformation of methane. Biohydrogen production using various approaches is still developing for greater efficiency so that it is applicable at field level. Cyanobacteria and green algae are photoautotrophic microorganisms that can produce hydrogen by direct or indirect bio-photolysis, or photo-fermentation mediated by the enzymes, and these pathways can be exploited for scaled-up production (Kaushik and Mona 2017). Techno-economic feasibility for commercialization of biohydrogen production has been critically analyzed by Mona and Kaushik (2016). Since hydrogen production is still an expensive option in comparison to other fuels, it is worthwhile to explore biohydrogen production using distillery effluent as substrate since it is rich in sugar and therefore it can produce biohydrogen at much reduced cost (Kotay and Das 2008). High carbohydrate content in food wastes and agro-wastes in the form of simple sugars, or complex carbohydrates such as starch, cellulose, and lignin, makes them potential feedstocks for biological hydrogen production (Kapdan and Kargi 2006). The major criteria for the selection of waste material to be used in biohydrogen production are their high carbohydrate content, biodegradability, easy availability,

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and low cost (Pandu and Joseph 2012). The only problem associated with the food waste is the variations in the types of carbohydrate and protein and their concentrations in the mixture. The rate of hydrolysis of carbohydrates is faster as compared to proteins and lipids. Wastewaters of food processing industries have rich carbohydrate content, which on microbial hydrolysis produce simple sugars (Kapdan and Kargi 2006). Biohydrogen production is reported to be 20 times greater from carbohydrate-rich substrates in comparison to lipid- or protein-rich substrates (Lay et al. 2003). The microorganisms used in the culture determine the type of product from carbohydrate hydrolysis during biohydrogen production (Antonopoulou et al. 2011). While simple sugars are the favored substrates for hydrogen production, polysaccharides like starch are also found to be useful in biological H2 production (Arooj et al. 2007). Though hydrogen production by dark fermentation using pure substrates (glucose, sucrose, starch) is more efficient, due to high cost their use is limited, and low-cost wastes have better potential for industrial scale hydrogen production such as kitchen waste (Jayalakshmi et al. 2009), sewage sludge (Massanet-Nicolau et al. 2010), solid municipal waste (Dong et al. 2009), molasses (Li et al. 2007), and wastewater originating from biodiesel production (Han et al. 2012), olive oil production (Ntaikou et al. 2009), or palm oil production (Vijayaraghavan and Ahmad 2006). Both simple and complex carbohydrates are present in large quantities in agro-wastes and food processing wastes (Pakarinen et al. 2008; Ginkel and Logan 2005). Due to high carbohydrate content and availability of nutrients in distillery spent wash, it can be utilized favorably for hydrogen production. Several studies have been conducted to obtain value-added products from distillery spent wash and to recover biogas to generate electricity. Anaerobic hydrogen production from distillery wastewater has also been gaining attention in recent years. Distillery wastewater with high-strength organic compounds has been reported to successfully produce biohydrogen (Wicher et al. 2013). Hydrogen production potential of sludge-based microbial consortium of distillery wastewater was examined under optimized conditions of various operating parameters in batch studies followed by continuous-mode operation of the bioreactor exploiting the dark fermentation metabolic pathway of the consortium by the present research group (Kaushik and Preet 2017). It was reported that 20% strength of distillery wastewater gave a maximum specific hydrogen production rate (SHPR) of 7.96  0.69 mL/g/d. This opens new avenues of energy generation from distillery effluent for the small to medium distillery units in developing countries, which produce large quantities of wastewater. The hydrogen energy produced from the wastewater by dark fermentation can be used appropriately, such as to feed fuel cells and be used in the industrial complex itself to supplement energy requirements in an economical and environmentally sound manner. Biohydrogen (H2) production from distillery wastewater in anaerobic sequencing batch biofilm reactor was studied by Venkata Mohan et al. (2008a) using selectively enriched anaerobic mixed consortia pre-treated with repeated heat shock (100
C; 2 h) and acid (pH 3.0; 24 h). Feasibility of hydrogen production from distillery wastewater (at pH 6) was demonstrated along with COD removal (specific H2

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production of 6.98 mol H2/kg CODR/day; H2 production rate of 26 mmol H2/day). Biohydrogen from distillery wastewater has also been produced using microbial electrolysis cell (MEC) technology. Production of cumulative hydrogen of 40.05 and 30.12 mL was reported at current density of 811.7 20 and 908.32 mA/m2, respectively, for conventional dual-chamber and modified single-chamber MEC system. The cathodic hydrogen recovery (CHR), which is the recovery of electrons as hydrogen, was 46.5 and 38.8 mL in conventional and modified MEC (Samsudeen et al. 2008). Feasibility of improving the scale of hydrogen (H2) production from distillery wastewater has been evaluated using co-cultures of various strains like Citrobacter freundii, Enterobacter aerogenes, and Rhodopseudomonas palustris at 100 m3 scale (Vatsala et al. 2008) and average yield of 2.76 mol of H2/mol glucose was obtained and the rate of H2 production was estimated as 0.53 kg/100 m3/h. All these studies indicate that distillery effluent can serve as a source of clean hydrogen energy that may serve as an alternative energy source by properly exploiting the potential.

2.1

Biohydrogen Production Pathways: Light and Dark Fermentation

Hydrogen production by biological processes seems economically and environmentally more practicable because these processes operate at moderately low temperature, which in turn leads to lower energy consumption. Major biohydrogen production processes are direct/indirect bio-photolysis and dark and photofermentations (Manish and Banerjee 2008). The hydrogen-producing pathways use the enzyme hydrogenase or nitrogenase for hydrogen production and regulate the hydrogen metabolism of many prokaryotes and some eukaryotes. The functioning of nitrogenase, as well as hydrogenase, is related to the utilization of the products of photosynthetic reactions that generate reductants from water in photoautotrophic microorganisms (Kaushik and Mona 2017). Figure 1 shows various biological processes and microbes involved in biohydrogen production.

Photo-Fermentation Pathway Photo-fermentation process involves decomposition of organic acids with the help of light-dependent, sulfur, and non-sulfur purple bacteria. Sulfur bacteria can perform photosynthesis while non-sulfur bacteria are a group of photoheterotrophic bacteria capable of degrading organic substrates to produce hydrogen. Purple non-sulfur bacteria are the best hydrogen-producing microorganisms exploiting photofermentative pathway, showing good substrate conversion efficiency. Photofermenter species of Rhodobacter capsulatus and Rhodobacter sphaeroids use sunlight to convert small organic molecules into biomass to release hydrogen and carbon dioxide as a bioproduct (Basak and Das 2007). The photo-fermentation process offers the ability to produce hydrogen using a wide variety of substrates (dairy wastewater, sugar industry wastewater, distillery wastewater) and wastes rich in organic acids like effluent of dark fermentation process and agriculture waste after

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Fig. 1 Biological processes of hydrogen production

hydrolysis (Tian et al. 2019). COD removal efficiency and hydrogen yield from the photo-fermentative processes are relatively higher than those from dark fermentation process. However, there are some limitations associated with hydrogen production from photo-fermentative pathway including a low hydrogen production rate (HPR), toxicity to purple non-sulfur-producing bacteria caused due to high substrate concentration, and light requirement for hydrogen production. The common photofermentation reaction is as below: C6 H12 O6 þ 6H2 O þ light ! 6CO2 þ 12H2

ð1Þ

In general, there are two different fermentation processes employed in photofermentative hydrogen production, viz. single-stage and two-stage processes. In a single-stage process of photo-fermentation, a range of substrates including organic acids, sugars, industrial waste, agricultural waste, and acidic effluents can be utilized for hydrogen formation. For a two-stage photo-fermentative process, dark fermentation is united with photo-fermentative production to obtain a higher hydrogen yield (HY). In sequential dark-photo-fermentation, volatile fatty acids (VFAs) present in the acidic effluent obtained from dark fermentation in the first stage are used as the substrates to produce hydrogen by non-sulfur bacteria in the second stage, which

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reduces the cost of operation. This approach could help achieve a maximum theoretical HY of 12 mol H2/mol glucose when acetic acid is the only by-product of dark fermentation (Chen et al. 2010; Su et al. 2010).

Dark Fermentation Pathway Dark fermentation of organic wastes by mesophilic and thermophilic bacteria is considered as an eco-friendly and promising method of hydrogen production (Sharma et al. 2020). The conversion of organic substrate to hydrogen may be achieved using pure or mixed microbial cultures. In dark fermentation, the bacteria involved are mostly acidogenic in nature, which convert the polymeric organic compounds to simple monomers, and then into a mixture of simple organic acids and alcohols with low molecular weight. At the end of the dark fermentation process, generally hydrogen and carbon dioxide are produced along with methane and hydrogen sulfide, determined mainly by substrates used and the reaction process (Fig. 2). Since dark fermentative hydrogen production process is simple, can be operated continuously without light, uses low-cost substrates as raw materials, is less energy POLYMERS Proteins, polysaccharides, lipids (i) Hydrolysis

MONOMERS & OLIGOMERS Amino acids, sugars, fatty acids

(ii) Fermentation

INTERMEDIATES Propionate, butyrate, alcohols

H2 + CO2

(iii) Acetogenesis

(iv) Methanogenesis

CH4 + CO2 Fig. 2 Schematic of biohydrogen production by dark fermentation pathway

ACETATE

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demanding, and has higher hydrogen production rate, it is found to be more advantageous than other biohydrogen production pathways like photosynthetic or photo-fermentative pathways (Levin et al. 2004; Chen et al. 2006). In the fermentative process, carbohydrates (mostly glucose) are used as carbon source by the microbes. It has been shown by Levin et al. (2004) that a maximum of 4 mol of H2 can be produced per mole of glucose during the fermentative process when acetic acid is the end product, as shown below: C6 H12 O6 ! CH3 CH2 CH2 COOH þ 2H2 þ 2CO2

ð2Þ

And when butyric acid is the end product, 2 mol of H2 may be produced: C6 H12 O6 þ 2H2 O ! CH3 CH2 CH2 COOH þ 2H2 þ 2CO2

ð3Þ

However, 4 mol of H2 production/mole glucose is practically not achieved since the end products are generally found to be a mixture of both the acids.

2.2

Dark Fermentation Technology

Anaerobic dark fermentation occurs naturally in the dark, mediated by several microbial actions that include processes like hydrolysis, acidogenesis, acetogenesis, and methanogenesis leading to hydrogen generation along with simultaneous wastewater treatment, thereby providing dual benefits. While both pure and mixed culture of microbes can perform dark fermentation, careful selection of the microbe is crucial along with the maintenance of optimal conditions suited to it (Wang and Wan 2009). Pure cultures of microbial strains show greater selectivity, more hydrogen yield, and lesser number of by-products. It is easier to manipulate the metabolism of microorganism in pure culture than in mixed culture. However, it is difficult to maintain the pure cultures as completely aseptic conditions are required to prevent any contamination, which increases the process cost (Ntaikou et al. 2010). Use of mixed bacterial cultures is more prevalent in fermentative hydrogen production method owing to simpler operation, greater range of usable feedstock, and cheaper as well as easier maintenance. Since several metabolic interactions occur in a coordinated way when mixed cultures are used, there is almost complete degradation and mineralization of the organic pollutants present in the effluents (Zhu and Beland 2006). A group of anaerobic, gram-positive, mesophilic bacteria Clostridium butyricum (Masset et al. 2010), C. beijerinckii (Seelert et al. 2015), and Bacillus coagulans (Kotay and Das 2007) are the most popular representatives for fermentative hydrogen production, which generally withstand the extreme thermal stress, survive in highly acidic or alkaline pH, and tolerate toxic chemicals. Two other fermentative species Enterobacter aerogenes (Jo et al. 2008) and Escherichia coli (Turcot et al. 2008) belong to gram-negative group, which can successfully tolerate oxygen stress, if any, in anaerobic environment. Microorganisms require enzymes like hydrogenase or nitrogenase to mediate the production of hydrogen.

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Hydrogenases are classified as nickel-iron (Ni–Fe) hydrogenase and iron-iron (Fe– Fe) hydrogenase, of which the former type is more abundantly present. In the Ni–Fe hydrogenase, the metal Ni is an integral part, which in low concentration favors fermentative hydrogen production, but high concentration of Ni may sometimes retard the activity of the enzyme and fermentative H2 production (Lin and Lay 2005).

2.3

Role of Operational Parameters in Enhanced Biohydrogen Production

Improvement of biohydrogen production in a bioreactor can be achieved through optimization of operating conditions. Various metabolic pathways involved in biohydrogen production, the reaction kinetics, and the hydrogen yield are influenced by various process parameters. The major parameters are temperature, pH, inoculum and its pre-treatment, substrate concentration, co-substrates, substrate retention time (SRT), configuration of the reactor, and its mode of operation.

pH Regulation of pH of mixed microbial cultures can play an important role in obtaining a culture enriched with hydrogen-producing microbes by suppressing the populations of methanogenic microbes that consume hydrogen. There are reports showing different effects of pH on hydrogen production depending upon the microbial species. Effect of pH is mainly on the enzyme activity, metabolic pathway, and hydrolysis of the substrate, which influence hydrogen production (Chu et al. 2008). A strong influence of pH has also been reported in energy recovery from the distillery wastewater using bioelectrochemical method due to variations in functional behavior of the microorganisms at different pH (Kaushik and Chetal 2013). In the presence of hydrogen, excess ATP is used to make the cell neutral rather than to produce hydrogen, thus making the role of H+ ion concentration quite important (NazlinaHaiza et al. 2011). Lots of studies have been conducted on the optimal pH range for fermentative hydrogen production, which show inconsistent results due to the use of different types of substrates and seed sludge, and different operating conditions (Luo et al. 2010; Wu et al. 2010). Most of the studies focus on the effect of initial pH, which is often more important affecting indirectly the hydrogen yield by fermentative hydrogen processes (Wang and Wan 2009). Mohammadi et al. (2012) also showed that initial pH affected both hydrogen production potential and hydrogen production rate. Specific hydrogen production rates from rice branbased distillery effluent were found to be favored at slightly acidic pH 6.5 and at neutral pH (Kaushik and Preet 2017). Feeding pH influences not only fermentative hydrogen production but also the substrate removal efficiency (Venkata Mohan 2008). Though neutral pH is suitable for wastewater treatment, slightly acidic pH is found useful in effective hydrogen production. Low pH is inhibitory for hydrogenase activity, thus decreasing fermentative hydrogen production. Hydrogenproducing butyrate-acetate pathways are favored at pH 4.5–6.0, while at neutral or

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higher pH, ethanol and propionate accumulate (Hawkes et al. 2007, Pakarinen et al. 2008). Production of intermediate volatile fatty acids (VFAs) is also influenced by pH. Butyrate production is favored more in comparison to acetate at low pH. The optimum pH for high hydrogen production and lower solvent production has been reported to be between 5.5 and 6.5 (Ma et al. 2015).

Temperature In mixed cultures, temperature has been found to directly affect microbial metabolism and hydrogen yield (Li and Fang 2007). Temperature plays an important role in the activity of microorganisms, substrate degradation, and conversion rate of fermentative products (Jung et al. 2011). Hydrogen-producing bacteria are mainly categorized into two groups: mesophilic and thermophilic bacteria with favorable range of temperature as 30–40
C and 45–55
C, respectively. Higher temperature generally improves hydrogen production rate, and enhanced HPR was observed through temperature increment mainly for thermophilic bacteria (Yokoyama et al. 2007; Chu et al. 2008). Mesophilic sludge microbial consortium also showed increase in SHPR from 3.03 to 4.11 mL/g/d when the temperature was raised from 30 to 40
C (Kaushik and Preet 2017). The optimum temperature for biohydrogen production depends on the inoculum, carbon source, and other operating conditions (Mohammadi et al. 2012). The mesophilic bacteria have been reported to show an optima at 30
C (Lee and Chung 2010), 40
C (Wang and Zhao 2009), and in the range of 35–37
C (Dong et al. 2009). According to Morino and Gomez (2012) an appropriate increase in operational temperature helps to increase H2 production. Thermodynamically, higher temperature is expected to favor hydrogen production due to increment in entropy of the system, but extreme thermophilic conditions are not economically feasible because of high energy requirements (Hallenbeck 2005). Quite contrary to this, for biohydrogen production high temperature may induce thermal denaturation of essential enzymes and proteins, which may negatively affect hydrogen-producing potential of the mesophilic bacteria (Lee et al. 2006). Therefore, it is very important to monitor the temperature requirements of the microbes involved in dark fermentation of the wastewater to get high biohydrogen yield. Amount of Substrate and Nutrients Substrate concentration is also reported to be important in determining hydrogen yield from the wastewater. Wicher et al. (2013) recommended dilution of wastewater before the start of fermentation for hydrogen production. Though substrate concentration is important, it also gets influenced by pH. When pH in the reactor falls, hydrogen yield tends to decrease with increasing substrate concentration (Jung et al.2011). Batch assays performed by Kumar and Lin (2013) at various substrate concentrations (40–240 g/L) showed that the optimal substrate concentration was 200 g/L for maximum HPR using Monod model. Additional increase of the substrate concentration, however, resulted in lower rate of hydrogen production due to buildup of inhibitory VFAs in the medium. A periodic sequence batch reactor containing distillery wastewater (142,000 mg/L COD) operated for six consecutive

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cycles, with a single cycle period of 24 h (30-min filling, 23-h reaction, 30-min decantation phase), showed that hydrogen production rate (HPR) declined from 238.16 to 18.5 mL/L/d. Since half of the degraded substrate was replaced by fresh substrate in each consecutive cycle, there was overacidification of the system due to formation of VFAs, which led to a fall in hydrogen production in later cycles (Preet and Kaushik 2014). This emphasizes on the importance of substrate concentration in biohydrogen production that varies in fixed batch mode and cyclic batch mode operations. Availability of some macro- and micronutrients in the feedstock also influences biohydrogen production. Na, Mg, Zn, Ni, and Fe are important trace metals that affect H2 metabolism in microbes being essential as enzyme cofactors, in transport processes and dehydrogenases. Fermentative hydrogen production, like other fermentation processes, requires nutrients for bacterial growth, metabolism, and activity of enzymes. While Fe2+ is required as an essential cofactor of the hydrogenase I of Clostridium, the presence of Ni2+ is also required, as both the metal ions are needed for the hydrogenase. Other metal ions, such as zinc, magnesium, and sodium, are cofactors for enzymes involved in membrane transport (Lin and Lay 2005). Since these ions are required in minimal concentrations, generally supplements of ions are not required when wastewaters are used as substrate that possess the required concentrations of these nutrients. Table 2 shows hydrogen production using different types of wastes at various operating conditions.

Configuration of Reactors and Influence of Organic Loading Rate and Retention Time Fermentative pathway is expected to provide a simple, less costly, and strong operation system giving high rate of H2 formation and yield. Configuration of bioreactor is most important in the biohydrogen production process. Batch reactors are commonly used in lab-scale experiments, whereas continuous stirred tank reactors are used for applications at commercial scale. For biological hydrogen production from wastewaters, various types of reactors have been used, such as continuous stirred tank reactor (CSTR), upflow anaerobic sludge blanket reactor (UASBR), anaerobic fluidized bed reactor (AFBR), and membrane bioreactor and packed bed reactor (PBR). Most used bioreactors are CSTR in which there is constant mixing of substrate inoculum and other reaction products. Show et al. (2011), however, reported that due to continuous mixing, the CSTRs are unable to hold large number of microbes responsible for fermentation and washout of microbes takes place at low HRT. Most widely used batch operations generally suffer from high concentrations of initial substrate and final product, causing inhibition of hydrogen production. Continuous operation provides stable quality product when operated at a steady state. The performance is however influenced by the hydraulic retention time (HRT) or substrate retention time (SRT), and organic loading rate (OLR), which affects the substrate conversion efficiency, type of active microbial population, and metabolic pathways in the reactor system. To overcome the inhibitions induced by a substrate or product, it is often useful to reduce the feeding rate of substrate and remove the products as and when formed.

Varied

8.12–15.44 g COD/L

20 g COD/L

Dairy wastewater

Palm oil mill effluent

3.11–85.57 kg COD/m3 reactor/d 2 g COD/L

Two-stage fermentation reactor

Anaerobic batch reactor Granule-based continuous stirred tank reactor Anaerobic fluidized bed reactor

Continuous-flow reactor

Upflow anaerobic sequencing batch reactor Two-stage fermentation process

45 g COD/L

142 g COD/L and 108 g VS/L

Batch

Pilot scale

20 g COD/L

5 g COD/L

Operation mode Batch

Conc. of feedstock 20 g COD/L

Brewery wastewater Glucose

Molasses

Pulp and paper mill effluent Alcohol industry wastewater Food waste

Feedstock Beverage wastewater Sucrose

Mixed culture

Fermented biomass

Anaerobic granulated mixed consortium Sewage from wastewater treatment plant

Anaerobic sludge from wastewater treatment plant Sludge from wastewater treatment plant

Sludge from sewage treatment plant

Anaerobic sludge

Beach sludge

Inoculum source Mixed culture

5.5–6.5

3.7–4.3

5.5

5.5

5–6.5

5.5

5.5

5.0

6

pH 5.5

Table 2 Comparison of biohydrogen production in different studies under various optimized conditions

Pachiega et al. (2018)

1.5 mol H2/ mol fructose 1.84 mol H2/ mol glucose

55

24–30

37

37

2.56 mol H2/ mol carbohydrate 2.9 mol H2/ mol sugar

Ren et al. (2006)

26.13 mol H2/ kg COD

35  1

Maaroff et al. (2018)

da Silva et al. (2019)

Show et al. (2007)

Chu et al. (2008)

205 mL H2/g VS added

37

Varied

Vaez et al. (2017)

Reference Sivagurunathan and Lin (2019) Lin et al. (2010)

Poontaweegeratigarn et al. (2012)

Maximum H2 production rate 3.76 mol H2/ mol sucrose 2.34 mol H2/ mol sucrose 55.4 mL H2/g COD 125 mL H2/g COD removal

37

37

Temp (
C) 37

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The influence of SRT and OLR on hydrogen production is not consistent in several studies. A low SRT may reduce the substrate utilization efficiency, especially for complex substrates, which need longer period of hydrolysis, and cause the washout of the active biomass, in turn impairing the conversion yield. On the contrary, long SRTs favor the buildup of hydrogen-consuming methanogens, and some competitors for substrate such as non-hydrogen-producing acetogens (Wang and Zhao 2009). The organic loading rate may affect accumulation of VFAs, pH changes, and modifications in associated metabolic pathways (De Gioannis et al. 2013). In continuous-mode studies, an OLR of 172 g/L/d was found to be most suitable for generating renewable H2 energy and for simultaneous removal of COD from distillery wastewater using the sludge microbial consortium at ambient 37.5
C temperature (Kaushik and Preet 2017). Very high organic loading rates tend to decrease hydrogen yield due to congestion of organic substrates (Kargi et al. 2012), which lead to high microbial diversity resulting in more competitors or hydrogen feeders (da Silva et al. 2019).

2.4

Pre-treatment of Inoculum

In anaerobic fermentation, pre-treatment of inoculum is a widely accepted approach to enrich the inoculum with hydrogen-producing acidogenic bacteria by inhibiting the activities of methanogenic bacteria in mixed culture. Hydrogen-producing bacteria develop in to spore with a layer surrounding their cell surface to protect them from environmental stresses, but the hydrogen-consuming methanogenic bacteria lack this capability (Zhu and Beland 2006), which facilitates selective killing of methanogens using different agents. Pre-treatment of the inoculum also helps to reduce substrate degradation and hydrogen-consuming bacteria (Srikanth et al. 2010).

Physical Pre-treatment Physical treatment involves thermal treatment and microwave treatment without any involvement of chemicals and enzymes (Zheng et al. 2014). Heat-shock treatment of seed sludge is the most common method to screen out hydrogen-producing bacteria. Heat treatment is a thermal treatment, which helps the enrichment of hydrogenproducing bacteria in the mixed culture by eliminating the methanogens, which are hydrogen consuming in nature. Spore formation indicates the presence of hydrogenproducing bacteria and in this heat treatment non-spore-forming methanogens get killed (Zhu and Beland 2006). Heat treatment using steam is very easy and inexpensive. Generally, temperature range from 75
C (Chang et al. 2002) to 121
C (Wang and Wan 2008) has been used for heat treatment and the duration of treatment varies from 15 min (Lay et al. 1999) to 2 h (Fan et al. 2004). However, heat pre-treatment of inoculum is a popular method to enrich the inoculum with desired hydrogenproducing bacteria, but there is overall decline in microbial community including some non-spore-forming hydrogen producers (Kraemer and Bagley 2007; Zhu and Beland 2006).

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Microwave pre-treatment inactivates the microorganisms through thermal as well as nonthermal effects (Zheng et al. 2014). Guo et al. (2015) reported that Clostridium species dominated when pre-treated with microwave. In comparison to thermal treatment, microwave treatment exhibits higher efficiency of cell solubilization (Kuglarz et al. 2013). Aeration, freezing, thawing, and infrared treatment have also been found to be proficient in enhancing hydrogen yield. During freezing and thawing, disruption of microbial cells takes place due to formation and breaking of ice crystals (Sawicka et al. 2010). Different pre-treatment methods have shown varying results in different studies. Maximum hydrogen yield of 1.96 mol/molglucose was observed on pre-treating the seed sludge with repeated-aeration method (Ren et al. 2008), while combination of heat shock–acid treatment gave higher hydrogen yield than that due to single pre-treatment (Venkata Mohan et al. 2008b). Combined pre-treatment was however found less effective in the study conducted by Boboescu and Gherman (2014).

Chemical Pre-treatment Chemical inhibitors like acetylene, iodopropane, and 2-bromoethanesulfonic acid (BESA) have the ability to inhibit methanogens. Methanogen inhibitors disturb the metabolic pathway of hydrogen-consuming bacteria (Valdez-Vazquez et al. 2006). BESA, an analog of the coenzyme M in methanogens, specifically facilitates the destruction of methanogens (Sparling and Daniels 1987). Iodopropane restricts the functioning of B12 enzyme, which carries methyl group (Zhu and Beland 2006). The pH is generally maintained at or near pH 7 in the conventional process of wastewater treatment. As the pH falls below 6.3 or goes above 7.8, methane production falls (Chen et al. 2002). Thus, maintaining pH of anaerobic sludge away from pH 7 effectively suppresses the activity of hydrogen-consuming methanogens, without affecting the activity of the endospore-forming hydrogenproducing Clostridia. Both acidic and base pre-treatments have been found to be useful. Higher biohydrogen production using acid-treated sludge is reported by Chang et al. (2011), whereas Yin et al. (2014) reported higher hydrogen yield using base-treated sludge.

2.5

Cell Immobilization

Cell immobilization has also been found to help in enhancing biohydrogen production rate and yield by addressing the main limitations associated with the suspended cells’ operations (Kamra and Kaushik 2014). While there are several studies on photoautotrophic microorganisms where immobilized cells are reported to be more effective for prolonged H2 production with improved production rate (Kaushik and Mona 2017; Zhang et al. 2017), studies on heterotrophs are limited. Recently anaerobic bacterium Clostridium has also been reported to give higher hydrogen yield (3.57 mol H2/mol cellobiose) in comparison to that (1.77 mol H2/mol cellobiose) produced by suspended cultures (Güngörmüşler et al. 2021). It was suggested that the cells remain viable within the hydrogels and proliferate at a slow rate during

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the fermentation process providing stability to the system giving higher hydrogen production and less soluble metabolites than suspended cells, suggesting immobilization as a feasible process for future biohydrogen production. Immobilization practices, such as entrapment, encapsulation, and adsorption, are found to immensely help in sustained hydrogen production via dark fermentation (Kumar et al. 2016). Recently, purple non-sulfur bacteria, which produce biohydrogen by photo-fermentation pathway, have also been critically assessed for their performance in photo-bioreactors in immobilized conditions and found to be greatly beneficial (Sagir and Alipour 2021). Cell immobilization technology, which has till now been widely used in the treatment of wastewater, food processing technology, and pharma industry, is proved to be of immense use in biohydrogen production owing to its intrinsic worth such as enhanced process yields, reduced microbial contamination, and improved homogeneity.

3

Upscaling the Process of Hydrogen Production During Treatment of Distillery Wastewater

There is an urgent need of improving wastewater treatment technology of distilleries that is more efficient and economically feasible (Patel and Jamaluddin 2018). Wastewater of distillery industry is dark brown in color mainly because of a compound organic in nature and having high molecular weight called melanoidin. This dark-color wastewater of distillery industries if directly mixed with inland surface water can reduce the sunlight penetration in that water body and thus leads to decrease in photosynthesis process and dissolved oxygen level. If this dark-color wastewater of distillery industries is applied on land, it can lead to decrease in soil alkalinity and it can also inhibit germination of seeds. So, this liquid waste is one of the critical issues in the environment (Ali et al. 2015). The treatment process of distillery wastewater generally includes two steps: first is partial treatment. This partial treatment is performed by primary biomethanation and second is secondary treatment aerobic in nature like activated sludge treatment (Patel and Jamaluddin 2018). Secondary aerobic treatment is also not much effective in the removal of certain pollutants like TDS and color; thus the effluent generated is not matched with the standards of pollutants (Ali et al. 2015). Recovery of hydrogen energy from the distillery wastewater seems to be a promising approach as discussed in the chapter. However, application of the method at industrial level needs careful and systematic scale-up efforts.

3.1

Challenges and Opportunities

Currently, about 99% of H2, used by industry, is produced by steam reformation of natural gas that has high-energy input requirements and produces huge amounts of greenhouse gas. Though hydrogen is considered as a future substitute of fossil fuels, the current methods of H2 production are not that efficient and sustainable. The

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ability of living microbes to produce “biohydrogen” offers the prospect of fully renewable hydrogen, free from any dependence on fossil fuel, and the scope for tapping into this resource is potentially enormous. However, in reality, the amounts of biological H2 produced are low and sporadic, occurring mainly under conditions of darkness, anoxia, or other stresses such as sulfur deprivation or nitrogen limitation. In theory, hydrolysis of one molecule of glucose can produce 12 molecules of H2, but in practice fermentative bacteria such as Clostridium acetylbutylicum or E. coli produce only 2–3 molecules of H2 per molecule of glucose metabolized. Some thermophilic organisms, such as Caldicellulosiruptor saccharolyticus, can produce slightly more biohydrogen, depending on the type of sugar fermented, but it remains an inefficient process (EAFO 2020). If biohydrogen has to become a viable commercial fuel, an increase in yield is required. Another alternative is the coupling of biohydrogen production directly to the production of some value-added biochemical. Future studies on bioprospecting may unravel some natural organism that has hydrogen yield from the wastewater feedstocks, but under the present scenario, it appears that lots of efforts will be needed to upscale the process giving high yields. Though various pathways of biohydrogen production are available for possible exploitation at a commercial level (Kaushik and Mona 2017), dark fermentation appears more feasible for biohydrogen production from distillery wastewater. The process of dark fermentation takes place at a faster rate than photolysis or photofermentation, but the yield of hydrogen is low due to the formation of several intermediate metabolites. In anaerobic respiration, varieties of organic and inorganic compounds are used as terminal electron acceptors, which get reduced to regenerate the reducing power (NADPH, FADH). Glycolysis is the key metabolic pathway in which a substrate is transformed into pyruvate. Under anaerobic conditions, the pyruvate enters the acidogenic pathway coupled with H2 production, which results in the formation of volatile fatty acids (VFAs) such as acetic acid, propionic acid, butyric acid, or malic acid along with hydrogen. While H2 is produced through acetic acid and butyric acid pathways, there is consumption of H2 in the propionic acid and malic acid pathways leading to the formation of the VFAs as intermediate metabolites as may be seen below: Acetic acid pathway:

C6 H12 O6 þ 2H2 O ! 2CH3 COOH þ 2CO2 þ 4H2 C6 H12 O6 þ 2H2 ! 2CH3 CH2 COOH þ 2H2 O

Propionic acid pathway: Butyric acid pathway:

C6 H12 O6 ! CH3 CH2 CH2 COOH þ 2CO2 þ 2H2

Malic acid pathway: C6 H12 O6 þ H2 þ 2HCO3 2 ! 2COOHCH2 CH2 OCOOH þ 2H2 O Ethanol pathway:

C6 H12 O6 ! 2CH3 CH2 OH þ 2CO2

There is a need to explore ways to reduce the formation of these intermediate metabolites that lead to loss of biohydrogen. Metabolic engineering to cause a shift

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in the metabolic flux towards hydrogen production or modified microbial response to express some non-native enzymes is being studied in depth to achieve this. H2 production rates and yield by E. coli have been significantly improved by such synthetic biological approaches. Developing of novel engineered enzyme systems based on the understanding of hydrogenase enzymes that may interconvert electricity and H2 gas is also being explored to make biohydrogen production an important component of renewable energy sector (Chandrasekhar et al. 2016). Though some CO2 is produced during fermentative biohydrogen production, that is considered carbon neutral as there is no net increase in carbon emissions. This is because microbial fermentation of the sugars during dark fermentation is basically plant driven, which was originally produced during photosynthesis. The biohydrogen is usually also free of impurities, and hence may be used directly in fuel cell to generate electricity. The microbial electrolysis cell (MEC) has come up as a rather newer technology in recent years, wherein microbial metabolism in combination with electrochemistry is exploited to give much higher hydrogen yield. At the anode biofilm, electrongenerating microbes are present where they oxidize organic acids and produce electrons, which are transferred through an external circuit to reach the cathode, where on reaction with water, hydrogen is produced. In microbial fuel cell (MFC), this voltage is used to generate electrical power, while in MEC, an additional external voltage is supplied to the cell, and the combined voltage helps reduce the protons to produce hydrogen. The MECs have high hydrogen capture efficiency from different substrates (67–91%) giving high hydrogen yield as reported by Mishra et al. (2019). The need for external energy supply is, however, the major challenge for commercial application of MEC. Also, energy losses occur at several points in the MECs, thus enhancing the cost on applied voltage, and the overall cost of hydrogen production. Storage and transportation of hydrogen have often been considered as big challenges. Methods used to transport hydrogen have issues like high cost, not able to maintain purity of hydrogen, and leakage of hydrogen. Hydrogen can attack the pipes and the storage container, a property called hydrogen embrittlement. When hydrogen is transported, it also faces some issues because of its physical and chemical properties as only a small fraction of energy is enough for ignition while its range of flammability is quite large. An ideal method for storage and transport of hydrogen is still under research. Most widely used method for storage of hydrogen is cryogenic vessels, which have high energy demand. Cryoadsorption and gas cylinders are also used for storage of hydrogen, but volumetric densities are not sufficient in this case. Hydrogen can also be stored in the form of metal hydride; power requirement of this method is not very high, and it is a safe method. Combination of various available methods is also being tried for improved storage. It has been shown that evaporated hydrogen can be trapped with the help of metal hydrides and after that can be stored in cryogenic vessels. At present, this procedure is applicable in the field of aerospace (Tarasov et al. 2007).

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Economic Feasibility

Wastewater treatment technologies with simultaneous biohydrogen production seem attractive, but these have to be economically sustainable. Economic evaluation of energy production in a bioenergy production facility was performed, which had biohydrogen production from two different pathways: the capacity from bio-oil gasification pathway (147 t/d) had 47% biomass-to-fuel energy efficiency, while that from bio-oil reforming (160 t/d) showed a much higher 84% efficiency. Total capital investment for the bio-oil gasification pathway was 435  106 dollars, which was more than the cost (333  106 dollars) of the bio-oil reforming pathway (Zhang et al. 2013). Thus, biohydrogen production from bio-oil reforming pathway was economically more feasible than production from bio-oil gasification. Detailed analysis of cost, efficiency, and environmental implications of the technologies must be made before the treatment of effluent by aerobic or anaerobic method, effluent’s dilution, and disposal of effluent on land or in inland water (Sengupta 2001). The bioreactor design is one of the major determinants of cost of biohydrogen production using microalgae. Besides this, the production system has to consider the energy requirements, ease of operation, and quantum of hydrogen yield while making assessment for the most suitable production method (Mona and Kaushik 2016). Assessment of the capital cost for a stand-alone pond system (110,000 m2 area, 10 cm depth, 300 kg1 biomass of the algal mutant having truncated antennae at 0.2 g/L cell concentration) in Arizona gave an estimated cost of 5.1680  106 USD (Amos 2004). Unit cost of energy content of hydrogen derived from photobiological as well as fermentative process has been estimated as 0.0106 USD/GJ, which is higher than that (0.00424 USD/GJ) produced by the pyrolysis of coal or biomass, but less than that (0.01166 USD/GJ) from the electrolysis of water. Hydrogen from thermal decomposition of steam is also slightly higher 0.01378 USD/GJ as per the estimates given by Karthic and Joseph (2012). Thus, economically, the unit cost of energy produced is competitive for both photobiological and fermentative process of hydrogen production. Amos (1998) provided estimated capital costs for liquid hydrogen facilities of varying sizes (170–1500 kg/h) as 25,600–118,000 USD/kg/h. Where metal hydrides are used, the cost of materials involved in heat exchange and adsorption-desorption is included as storage material, pressure container, and integrated heat exchangers (Schwarz and Amonkwah 1993). Interestingly, scale-up for metal hydride storage does not trigger the cost significantly since the major capital cost is for the hydride material. Two major issues in biohydrogen production are hydrogen molar yield and feedstock cost. The organic source conversion efficiency in fermentative hydrogen production is only 15% that could be recovered as biohydrogen energy (Logan 2004). To achieve the goal of obtaining 6 mol of H2 per mol of glucose through fermentative H2 production, the US Department of Energy has focused its research and development (Davila-Vazquez et al. 2008). Research in the area of metabolic engineering and gene manipulation focusing on endogenous gene disruption may be the ways ahead in achieving the goals (Datsenko and Wanner 2000).

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Conclusion and Future Prospects

Distillery wastewater, which due to high COD, several pollutants, and dark color is an environmental challenge, can be treated with the new approach of energy recovery along with its treatment. Individual treatment process is not enough for complete treatment of distillery wastewater. So, an ideal treatment process that is environmentally sustainable as well as cost effective is needed. For distillery wastewater, biomethanation process of treatment is to be followed by aerobic treatment (Chowdhary et al. 2018). The resultant wastewater can be fed into MFC or MEC to yield bioelectricity or hydrogen. Same MFC can be used for hydrogen synthesis, electricity generation, and wastewater treatment (Oh and Logan 2005). Treatment of wastewaters with the help of various enzymes is also one of the emerging fields, which have various advantages but need further investigations (Patel et al. 2018). All these alternatives can only be successful if specific programs of waste management and monitoring are done. Monitoring results must be checked regularly, and management plan can be changed or modified accordingly (Sengupta 2001). Economic considerations are important for the field application of various methods including assessment of costs for production, storage, and transportation on a large scale. For sustainability and economic feasibility, it is proposed to combine different methods and have multiple benefits of wastewater treatment, nutrient recovery, biohydrogen production, and bioproducts. In a recent study, hydrogen was produced efficiently by co-treatment of two types of wastes involving hydrolysis of Mg scraps from the magnesium industry using brewery wastewater amended with acetic acid. The method proved useful in removing some persistent pollutants, >62% removal of chemical oxygen demand (COD), complete removal of color, and a fast 0.99 NL/min hydrogen production (Akbarzadeh et al. 2020). Exploration of new microbial strains with high hydrogenase activity, metabolic engineering to guide the dark fermentation pathways towards hydrogen production rather than formation of VFAs, use of low-cost waste substrates like wastewaters from different industries, and integration of bioreactor-MFC-MEC are some of the future prospects to improve the efficiency of biohydrogen yields from wastewaters. Integrating the biohydrogen production process with some other processes such as production of other value-added biomaterials can further help in enhancing the sustainability by facilitating resource recovery and eco-efficiency of the whole process.

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Biohydrogen from Pentose-Rich Lignocellulosic Biomass Hydrolysate Franknairy Gomes Silva, Vitor da Silva Liduino, Viridiana Santana Ferreira-Leitão, and Magali Christe Cammarota

Abstract

Agricultural activity produces many lignocellulosic biomass wastes, which are renewable carbon sources for biochemical processes. In dark fermentation, acidogenic bacteria use hemicellulosic waste to produce biohydrogen as an alternative source of renewable energy that minimizes environmental problems. Few studies have used hemicellulose fraction for biohydrogen production in continuous or semicontinuous reactors despite the extensive research on dark fermentation. Continuous reactors allow evaluation of the stability and performance of biohydrogen production in large-scale applications. However, the recalcitrance of molecules that make up the lignocellulosic structure hinders its bioconversion, and pretreatment methods are necessary to improve the accessibility to the lignocellulosic materials. Hydrothermal pretreatment stands out for its high efficiency and lower energy consumption, releasing pentoses from hemicellulose fraction. Depending on metabolic pathways followed by microorganisms for biohydrogen production, the yield obtained from xylose with acetic or butyric acids as the primary metabolite can reach 3.33 or 1.67 mol H2/mol pentose, respectively. The pentose-rich hemicellulose fraction of the hydrothermally pretreated sugarcane straw (C5 fraction) was used to feed a bench reactor that operated at 35
C, in semicontinuous mode, with hydraulic retention time of 48 h, and thermally pretreated anaerobic sludge. The reactor

F. G. Silva · V. da Silva Liduino · M. C. Cammarota (*) School of Chemistry, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil e-mail: [email protected] V. S. Ferreira-Leitão Biocatalysis Laboratory, National Institute of Technology, Rio de Janeiro, RJ, Brazil Department of Biochemistry, Federal University of Rio de Janeiro, Rio de Janeiro, RJ, Brazil # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Kuddus et al. (eds.), Organic Waste to Biohydrogen, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-1995-4_5

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attained maximum yield with the C5 fraction fed at 5.0 g COD/L.d (1.53 mol H2/ mol total reducing sugar). This best performance may be due to the components of the C5 fraction, which acted as nutrients and buffering agents. However, analysis of the microbial community found a decrease in microbial diversity and richness throughout the reactor operation. Keywords

Biohydrogen · C5 fraction · Hemicellulose · Metagenomic · Semicontinuous reactor · Xylose · Xylooligosaccharides

1

Introduction

Agricultural activity is an important economic sector in many countries to obtain food, beverage, raw materials for the clothing and pharmaceutical industry, and energy. Agricultural development results from several factors including the available natural resources, investments and advances in technologies and technical-scientific training, public policies, entrepreneurship, and support for small, medium, and large farms. However, the development of agricultural activity also produces a large amount of lignocellulosic biomass waste that is discarded incorrectly. Such wastes are low-cost carbon sources whose correct management lies in their possible use within the concept of biorefinery, as well as within the agro-industries themselves. The use of lignocellulosic biomass as a renewable carbon source is growing in the global agro-industrial sector. The high availability of this biomass, added to environmental and economic issues, has driven research to obtain numerous products (Singhvi and Gokhale 2019). The use of lignocellulosic biomass can occur through a wide variety of available technologies, divided into groups according to the type of conversion: thermochemical (gasification, pyrolysis), physical-chemical (extraction), and biochemical (fermentation, photolysis) (Zech et al. 2015). The biochemical conversion of lignocellulosic raw materials can be employed to obtain many chemical products with high added value (Baruah et al. 2018) or energy vectors such as methane (Phuttaro et al. 2019) and hydrogen (Basak et al. 2020), with advantages such as lower generation of solids, lower consumption of energy, and higher production of biogas. Lignocellulosic residues are already used as fuel to produce heat and energy for the production plant and the electricity grid, and more recently, as a carbon source in biochemical processes. Sugarcane bagasse is used as a raw material for secondgeneration ethanol production because it is available in conventional first-generation plants (Neto et al. 2018). Some studies also evaluate the possibility of using sugarcane straw as a raw material to produce second-generation ethanol, integrating first- and second-generation production. This integration has the advantage of sharing infrastructure between the two processes, such as concentration, fermentation, distillation, storage, and cogeneration facilities. However, not all mills are

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efficient enough to generate energy, and second-generation ethanol production is not yet an economically competitive process (Carvalho et al. 2020). Second-generation ethanol plants generate a liquid stream called hemicellulosic hydrolysate (HH), obtained after pretreatment (acid or hydrothermal) of lignocellulosic biomass (de Sá et al. 2020). The HH or C5 fraction is mainly composed of pentoses, such as xylose and arabinose (Zhang et al. 2020), and is not directly fermentable by traditional microorganisms used in ethanol production. Saccharomyces cerevisiae yeast, the most commonly used yeast for ethanol production from sugar- and starch-based raw materials, does not completely use the sugars from the lignocellulosic materials due to the inability of this yeast to convert pentoses from the hemicellulose fraction (C5 fraction) into ethanol (Banerjee et al. 2010). Some challenges associated with bioethanol production by microorganisms capable of simultaneously fermenting C6 and C5 sugars include financial and environmental costs, pretreatment technologies, variability of biomass structure and composition, prevention of sugar degradation in inhibitors, and a long fermentation time (Brenelli et al. 2020). A lower cost alternative to adding value to HH is fermentation to recover energy as hydrogen (Adarme et al. 2019). Unlike yeast, acidogenic bacteria can use the hemicellulosic waste with high energy value to produce biological hydrogen, an alternative source of renewable energy that minimizes environmental problems by reducing greenhouse gases (de Sá et al. 2020). Hydrogen has high heating value (285.8 kJ/mol), maximum electricity production by a fuel cell (237.2 kJ/mol) (Harrison et al. 2010), and 2.75 times greater energy content (122 kJ/g) than hydrocarbon fuels (Wang et al. 2017). Anaerobic fermentation of the C5 fraction also produces organic acids (Poletto et al. 2020) that can be recovered and used. In addition, the use of hydrogen as an energy vector becomes attractive when combined with biological processes with lower energy consumption and reduced CO2 emissions (Sinha and Pandey 2014; Ghimire et al. 2015). Most studies on hydrogen production by dark fermentation are conducted in batch reactors using synthetic substrates. Few studies use hemicellulose fraction for hydrogen production in continuous or semicontinuous reactors. The batch mode provides incomplete information for the hydrogen production in continuous reactors, because it is essential to control the pH and temperature and optimize the C5 fraction load that leads to less inhibition in the reaction system. Studies in continuous reactors allow evaluation of the stability and performance of biological hydrogen production in large-scale applications (Li et al. 2014). Acidogenic anaerobic reactors operating in continuous regime favor higher H2 yields from complex sources and suppression of methanogenic activity under mesophilic temperatures (Mota et al. 2018; Kongjan et al. 2019). In addition, more economic and energy value can be added with the use of a two-phase process, in which the effluent from the first reactor (acidogenic), containing the products from the metabolism of H2-producing acidogenic fermentative bacteria (volatile organics acid-rich), is then used in a second reactor (methanogenic) to produce methane, as shown in Fig. 1. According to this proposal, the hemicellulose fraction obtained after the hydrothermal pretreatment of lignocellulosic biomass is then employed for anaerobic fermentation in the acidogenic

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Fig. 1 Proposed sequential production of H2 and CH4 from the hemicellulose fraction of lignocellulosic biomass

reactor. The solid fraction is used for the production of second-generation ethanol. Metabolites in the liquid phase of the acidogenic reactor, mainly volatile organic acids, can proceed to a second stage (methanogenic reactor) for chemical oxygen demand (COD) removal and biogas production, aiming at the full use of the raw material and more remarkable energy recovery.

2

Composition and Recalcitrance of Lignocellulosic Biomass

Plant cell walls are dynamic structures that are both rigid and flexible to fulfill varied functions such as resistance to osmotic variations (Bhatla and Lal 2018), control of cell growth that determines cell size and shape (Majda and Robert 2018), and high turgor pressures (Lambers and Oliveira 2019). Thus, despite the rigidity for support and protection, cell walls are also extensible, allowing cells to grow due to the high intracellular turgor pressure (Majda and Robert 2018). Cell walls are organized in multilayers that are generally divided into three regions: primary cell wall, secondary cell wall, and middle lamella. These regions contain three main compounds: cellulose (C6H10O5)n, hemicellulose (C5H8O4)m, and lignin [C9H10O3(OCH3)0,9-1,7]x, in addition to pectin and glycosylated proteins (Wu et al. 2017; Sheng and Chen 2020). About 90% of the dry lignocellulosic material is cellulose, hemicellulose, and lignin, while the rest is extractives and ash (Nanda et al. 2014). Cellulose, a polysaccharide of D-glucose units linked by β-1,4-glycosidic bonds, is the most abundant organic compound in plants and acts as an important structural substance in plant cell walls, constituting from 25% to 50% of its organic material (Nobel 1999). In the cell wall, this polymer is organized into microfibrils consisting of an inner core of about 50 parallel chains of cellulose incorporated in a matrix that contains noncellulosic polysaccharides such as pectin, lignin, protein, water, and ions (Nobel 1999). The cellulose chain has a complex and robust network of hydrogen bonds between the hydroxyl groups, which organize and stabilize the molecules in a crystalline package along the axis of the microfibrils. The cellulose

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chain also has disordered and less organized segments (amorphous domains), unstabilized by hydrogen bonds and readily available for bonding (Bilatto et al. 2020). Typically, hydrolysis of crystalline cellulose is more difficult than amorphous cellulose (Yu and Wu 2010). Hemicelluloses (e.g., xylans, mannans, glucans, and galactans) are long linear chains containing several different polysaccharides that generally comprise about 20–35% of dry lignocellulosic biomass (Sheng and Chen 2020). Xylan is the main noncellulosic polysaccharide in plant cell walls. Despite the different varieties of this vegetable hemicellulose, a common structural feature of xylan is its structure of xylopyranosyl residues linked by β-1,4 bonds (Arai et al. 2019). Xylose is the main monosaccharide of hemicelluloses, and xylooligosaccharides are oligomers with 2–7 xylose molecules connected by β-(1,4) linkages (Otieno and Ahring 2012). The amorphous property of hemicelluloses makes them vulnerable to hydrolysis (Yang et al. 2015). Pectins, such as hemicelluloses, also have different polysaccharides and play an essential role in regulating cell wall properties. They control porosity and hydration, which causes swelling and influences cell wall thickness (Majda and Robert 2018). Lignin is the second most abundant organic molecule in plants. It is a highly branched complex macromolecule composed of phenylpropanoids, which are natural phenolic compounds that contribute to many aspects of plant development with responses to biotic and abiotic stimuli (Lam et al. 2017; Tian et al. 2018). Three different phenylpropanoid alcohols—coniferyl, coumaryl, and sinapyl—constitute the p-hydroxyphenyl, guaiacyl, and syringyl units in the lignin polymers. Unlike the monomeric units of the cellulose, the lignin units are not linked in a simple and repeated way. Lignin is generally deposited in the secondary cell wall; however, it can also be present in the primary cell wall and the middle lamella, in contact with the cellulose and hemicelluloses (Taiz et al. 2015). One of the primary roles of lignin in the chemical composition of the plant structure is to increase the mechanical resistance and allow water transport (Crivellaro and Büntgen 2020). Lignin is considered the most recalcitrant component of the plant cell wall (Ruiz et al. 2020), and its linking with cellulose and proteins reduces the biodegradability of these substances. The recalcitrance of molecules that make up the lignocellulosic structure hinders its bioconversion (Batista et al. 2019).

3

Pretreatment Methods for Lignocellulosic Biomass and Composition of Hemicellulose Hydrolysate

The hydrolysis step of anaerobic degradation of lignocellulosic biomass, in which polymers and macromolecules are converted into smaller and simpler compounds, has several limiting factors, such as lignin content and crystallinity of cellulose (Akhtar et al. 2016). The complex structure of lignocellulose makes the biological conversion of lignocellulosic material difficult; therefore, pretreatment is used to reduce the crystallinity of the cellulose (Barbosa et al. 2020), solubilize the hemicellulose (Candido et al. 2019), increase the area surface of the material, and separate the cellulose and hemicellulose from the lignin (Nanda et al. 2014). Efficient

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pretreatment or hydrolysis can make the fermentable sugars contained in lignocellulosic biomass suitable substrates for biological processes (Mood et al. 2013; Zheng et al. 2014). The pretreatment methods most studied for the conversion of lignocellulose into simpler oligomers and sugars include physical (hydrothermal, steam explosion), chemical (acid, alkaline), and biological (enzymatic and fungal hydrolysis) methods (Akhtar et al. 2016). All methods have disadvantages in terms of efficiency, cost, and treatment time. For example, chemical pretreatment needs a corrosion-resistant reactor and tubing and requires neutralization/detoxification of the hydrolysate before its use in anaerobic digestion; the steam explosion technique consumes a lot of energy, requiring a specially designed high-pressure vessel, while the biological pretreatment is generally slower and less effective (Zheng et al. 2014; Seidl and Goulart 2016). Compared to other methods, hydrothermal pretreatment is highly efficient and consumes less energy (Kumar et al. 2018a). It uses only water at high temperatures, resulting in hemicellulose solubilization and structural changes in the lignocellulosic biomass, which favors the subsequent step of enzymatic hydrolysis (Santo et al. 2018). In addition, this pretreatment method does not employ acids, alkalis, or oxidants, reducing the risk of corrosion of the equipment, the consumption of chemicals to adjust the pH of the hydrolysate (Nakasu et al. 2016), and the formation of toxic and inhibitory compounds (Candido et al. 2019). Table 1 lists the content of sugars and inhibitory compounds in hydrolysates of several types of lignocellulosic biomass after hydrothermal pretreatment. As hemicellulose is the most easily hydrolyzed component, xylose is the primary sugar in hydrolysates. Concentrations of both xylose and other sugars (glucose, arabinose) vary according to lignocellulosic biomass. Hydrothermal pretreatment employs temperatures in the range of 150–230
C (Garrote et al. 1999), and its effectiveness depends on factors such as temperature, time, pressure, type of biomass, and solid-liquid ratio (Yousefifar et al. 2017). More severe conditions of temperature and time increase hemicellulose hydrolysis (Candido et al. 2019). About 80% of the soluble sugars in hemicellulosic fractions are typically oligosaccharides, as the conditions that lead to a high sugar recovery do not allow its complete hydrolysis (Nakasu et al. 2016; Zhang et al. 2020). Table 1 also shows the concentrations of inhibitors and some by-products formed in the hydrothermal pretreatment. Pretreatment of lignocellulosic biomass can produce hemicellulosic hydrolysates containing inhibitors such as furfural, hydroxymethylfurfural, and phenolic compounds, which are generated from the degradation of pentoses, hexoses, and lignin, respectively (Wang et al. 2018). The degradation of the acetyl group of the hemicellulose forms acetic acid (Yan et al. 2017). High concentrations of acetic acid can inhibit anaerobic digestion (Phuttaro et al. 2019), furan derivatives can significantly affect the methanogenic populations involved in anaerobic digestion (Wang et al. 2018), and phenolic compounds exhibit high inhibition of cell growth in the fermentation of hemicellulose hydrolysate for the production of solvents (Guan et al. 2018). The concentrations of furfural and hydroxymethylfurfural inhibitors and acetic and formic acids also vary according to the temperature and time used in the pretreatment (Candido et al. 2019). To enable the use of hemicellulose hydrolysates, some authors propose a detoxification step

10

15

190

180

205

175 166

Sugarcane bagasse

Sugarcane bagasseb Rice straw

Corn cob

Corn straw Sorghum

0.03 (0.04) 7.52 (35.75) 4.0 4.4

16.3

Xylose (g/L) 2.18 2.43 4.85 0.46 (9.91) 15.31

0.8 0.8

2.28 3.1 3.4

nd nd

0.75 (3.15) 1.29

0.15 (0.16) 0.03

–

0.01

1.33

FF (g/L) 0.04 0.59 1.08 0.11

2.60

2.65

Glucose (g/L) 0.45 0.21 2.21 (1.32)

1.20

0.86

Arabinose (g/L) 0.98 0.76 0.89 1.4

0.06 0.11

0.26

–

0.07

0.58

HMF (g/L) 0.10 0.03 0.04 –

nd nd

2.26

1.02

0.04

11.92

Acetic acid (g/L) 0.83 0.96 1.13 –

– –

–

0.24

0.23

10.49

Formic acid (g/L) nd 0.03 0.05 0.21

a

FF furfural, HMF hydroxymethylfurfural, nd not detected. Values in parentheses are concentrations in the form of oligomers Deacetylation 60
C/30 min 0.8% NaOH w/w followed by hydrothermal pretreatment b Values obtained were divided by three because the liquid fraction was concentrated three times c According to Garrote et al. (2008)

30 30

45–50c

41

183

Sugarcane strawa

Time (min) 50 10 50 20

T (
C) 170 180 180 190

Lignocellulosic biomass Sugarcane straw

Table 1 Composition of hemicellulosic fraction from hydrothermal pretreatment of different lignocellulosic biomass

Baptista et al. (2020) Eskicioglu et al. (2017)

Brenelli et al. (2020) Bittencourt et al. (2019) Nakasu et al. (2016) Wang et al. (2018)

References Candido et al. (2019)

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before fermentation. However, considering the costs of this additional step and that methanogenic microorganisms are susceptible to inhibitory/toxic compounds, hydrolysate from the hydrothermal pretreatment enables acidogenic fermentation by mixed culture to produce biohydrogen since the acidogenic bacteria are more tolerant to toxicity.

4

Biohydrogen Production by Dark Fermentation of Hexose and Pentose Sugars

The development of efficient methods for the biological production of hydrogen from biomass waste has driven recent research (Kumar et al. 2018b; Mishra et al. 2019). The main biological production processes are biophotolysis, dark fermentation, photofermentation, and sequential dark and photofermentation (Nikolaidis and Poullikkas 2017). These processes may be dependent (e.g., photofermentation of photosynthetic bacteria) or not dependent on light (e.g., dark fermentation of anaerobic bacteria). Among the biological processes for hydrogen production, the dark fermentation of lignocellulosic biomass stands out due to the possibility of producing hydrogen and other biofuels (methane, biohythane), as well as contributing to the correct management of lignocellulosic waste (Kumar et al. 2018b). Several studies have evaluated the production of biohydrogen by dark fermentation from fermentable sugars contained in hydrolysates of lignocellulosic biomasses. The use of these substrates is reasonably addressed in batch systems (He et al. 2014; Zhang et al. 2015; Eskicioglu et al. 2017; de Sá et al. 2020; Mechery et al. 2021). However, little information is available on the production of biohydrogen in continuous systems using hemicellulose fractions. The high variation in hydrogen yields verified in these studies is due to different operating conditions, inoculum types, and carbon sources, in addition to the presence of competing organisms. Inoculum pretreatments affect the structure and productivity of the active microbial community (mesophilic and thermophilic) of the inoculum differently. According to Dessì et al. (2018), bacteria of the genus Lactobacillus are the main competitors for the substrate with hydrogenproducing bacteria, and only pretreatments that repress the production of lactate would result in higher hydrogen yields. In anaerobic digestion, several facultative and obligatory anaerobic microorganisms help transform the organic matter available in the medium, obtaining methane as the final product and generating an effluent with low organic matter. Hydrolytic fermentative bacteria perform the initial step of anaerobic degradation. If these bacteria cannot hydrolyze the substrate, the subsequent phase of bioconversion (acidogenic fermentation) is limited. As the hemicellulose fraction from the hydrothermal pretreatment is rich in oligosaccharides, pentose and hexose sugars are not readily available for assimilation by acidogenic bacteria. Thus, hydrolytic bacteria secrete enzymes that hydrolyze oligomers to soluble monosaccharides available for cell transport and assimilation by fermentative bacteria (Wang and Yin 2019). Sugars are converted to hydrogen and soluble products

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defined by a metabolic route. The main soluble products are acetic, butyric, propionic, valeric, lactic acids, and solvents such as ethanol (Valdez-Vazquez et al. 2019). Identifying acids formed during dark fermentation is vital to indicate the metabolic pathways followed by microorganisms for hydrogen production. Theoretically, the hydrogen yield from xylose with acetic acid or butyric acid as the primary metabolite can reach 3.33 or 1.67 mol H2/mol pentose (Kongjan et al. 2019), as summarized in reactions (1) and (2): C5 H10 O5 þ 1:67H2 O ! 1:67CH3 COOH þ 1:67CO2 þ 3:33H2

ð1Þ

C5 H10 O5 ! 0:83CH3 CH2 CH2 COOH þ 1:67CO2 þ 1:67H2

ð2Þ

Although the hemicellulose fraction is mainly composed of pentose sugars, hexose sugars are also present in the fraction composition (Candido et al. 2019). Theoretically, 4 and 2 mol of H2/mol hexose is produced when the by-products are acetic acid and butyric acid, respectively (Li et al. 2019), as summarized in reactions (3) and (4). Thus, the sugars from the hemicellulose fraction can be consumed simultaneously by reactions (1)–(4). The production of lactic and propionic acids indicates pathways that do not involve hydrogen production (Kumar et al. 2018a; Mockaitis et al. 2020): C6 H12 O6 þ 2H2 O ! 2CH3 COOH þ 2CO2 þ 4H2

ð3Þ

C6 H12 O6 þ 2H2 O ! CH3 CH2 CH2 COOH þ 2CO2 þ 2H2

ð4Þ

However, in practice, hydrogen yields are lower than theoretical ones. Some operational parameters hinder the full-scale technology, such as substrate type and concentration, pH, inoculum microbial population (Castelló et al. 2020), reactor configuration, sludge retention in the reactor (Mota et al. 2018), hydraulic retention time, and organic loading rate (Wang et al. 2019). Such factors are grouped into abiotic (such as pH, substrate concentration, and inhibitors present in the substrate) or biotic. The biotic factors that cause instability in hydrogen production are associated with hydrogen-consuming organisms, substrate-competing organisms, and inhibition by fermentation products (Castelló et al. 2020).

5

Biohydrogen Production by Dark Fermentation of the Hemicellulose Fraction of the Hydrothermally Pretreated Sugarcane Straw (C5 Fraction)

The method of harvesting sugarcane is in transition from the manual harvesting of whole cane with prior burning of the cane field to the mechanized harvesting of chopped cane without burning the cane field. Even though it is a relatively new technology, the chopped cane system is becoming the standard process for sugarcane harvest throughout the world. This transition results in a more significant amount of

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sugarcane straw (green leaves, dried leaves, stalks, and plant tips) recovered with each sugarcane harvest (Menandro et al. 2017). As sugarcane is composed of approximately two-thirds of lignocellulosic biomass (bagasse and straw), this agricultural sector generates a large amount of residues that should increase considerably with the installation of new production units (Santos et al. 2020). Unlike bagasse, which is already at the industrial plant, recovery of straw requires high transportation costs. However, the transport of sugarcane and its residues has evolved a lot in recent years, with the main objective of reducing transport costs and adapting to changes in the harvesting system. Thus, even though some straw must remain in the field for soil nutrition, and the energy recovery industry (cogeneration) uses a large part of sugarcane residues (Camargo et al. 2020), in the coming years, the greater availability of sugarcane straw should allow its use as raw material for the production of fuels or chemicals (Santos et al. 2020).

5.1

Pretreatment of Sugarcane Straw and Characterization of the Hemicellulose Fraction

The major components of the hemicellulose fraction obtained from the hydrothermal pretreatment of sugarcane straw are xylooligomers (7.8 g/L) and xylose (3.8 g/L). Other sugars in smaller amounts are arabinose, glucose, and galactose monomers, and glucooligomers (Silva et al. 2019). Because oligosaccharides (65% of total sugars) are not directly assimilated by acidogenic fermentative bacteria, hydrolytic bacteria must hydrolyze them in monosaccharides (Nasr et al. 2014). After hydrolysis, pentose and hexose sugars can be used to produce H2, CO2, and other metabolites. Acetic acid is the third major component (3.6 g/L) in the hemicellulose fraction; however, hydrogen-producing bacteria can tolerate this concentration in the fermentation medium, since acetic acid is one of the main metabolites generated in acidogenic fermentations (de Sá et al. 2015; Ghimire et al. 2015). The furfural (0.3 g/ L), hydroxymethylfurfural (0.1 g/L), and polyphenol (2.6 g/L) concentrations are also tolerated by anaerobic microorganisms (Silva et al. 2019). The hemicellulose fraction has sufficient concentrations of phosphorus and sulfur macronutrients for anaerobic fermentation, requiring only nitrogen supplementation for an ideal chemical oxygen demand (COD):N:P ratio of 350:5:1 (Chernicharo 2007).

5.2

Biohydrogen Production with Xylose and C5 Fraction as Raw Material

Despite many studies on hydrogen production by dark fermentation, most have been conducted in batch reactors using synthetic substrates. Previous experiments of our research group verified that anaerobic sludge (collected in an upflow anaerobic sludge blanket reactor in operation at a brewery industry) submitted to thermal pretreatment (100
C for 60 min) generated a H2 volume ten times higher than raw sludge, without pretreatment. In the batch fermentations with this thermally

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pretreated sludge and xylose as substrate, higher H2 yields were obtained after successive contacts of the sludge with the substrate at 35
C, inoculum/substrate ratio of 2 (g volatile suspended solids—VSS/g chemical oxygen demand—COD), and initial pH 5.5. However, the H2 yield obtained (0.93 mmol H2/mmol xylose) was less than 28% of the maximum that hydrogen-producing bacteria can theoretically produce (Silva et al. 2019). Acidogenic anaerobic reactors operating in continuous or semicontinuous mode favor higher H2 yields from complex substrates (Nualsri et al. 2016). However, operational parameters, such as the substrate type and concentration, the bioreactor design, the microbial population used as inoculum (pure or mixed culture) and its retention in the system, the pH, the hydraulic retention time (HRT), and the organic loading rate (OLR), hamper the application of these reactors for biohydrogen production on an industrial scale (Castelló et al. 2020). Understanding the performance of reactors for hydrogen production from xylose is essential to develop fermentative hydrogen production from xylose-rich substrates. The operation of a reactor in semicontinuous regime with standard xylose as the substrate, at 35
C, 48-h HRT, and OLR of 2.5 g COD/L.d, resulted in greater stability (in terms of pH, production of volatile fatty acids, and consumption of xylose) and a yield of 1.37 mmol H2/mmol xylose, which is 41% of the maximum theoretical yield (Table 2). The following parameters led to greater stability of H2 production: adaptation of microorganisms to xylose through prolonged operation in a semicontinuous regime and constant mixing of the reactor contents to reduce gradients of pH, organic acids, and local inhibition of microorganisms. Additionally, an increase in influent alkalinity maintained the pH at levels suitable for H2 producers. Few studies have been published about the use of hemicellulose fraction for hydrogen production in reactors operated under continuous or semicontinuous mode. According to Nissila et al. (2014), the batch studies indicate a pH between 5.5 and 7.0 and substrate concentrations of 10–20 g/L. However, the batch mode provides incomplete information for the hydrogen production in continuous reactors, and the pH and temperature control must be evaluated, in addition to the optimization of the C5 fraction load that leads to the minimization of inhibition in the reaction system. The operation of the bench reactor in Table 2 continued with the C5 fraction diluted with distilled water to an initial COD of 5, 7.5, and 10 g/L supplemented with nitrogen only (COD: N ratio of 350:5) and micronutrient solution. The replacement of synthetic xylose medium by C5 fraction in the reactor feed, maintaining the same operating conditions (2.5 g COD/L.d, HRT 48 h, influent pH 12), did not alter the fermentation pH. It maintained an average of 5.45  0.48 considering the entire monitoring period with C5 fraction compared to the value obtained during the operation with xylose as substrate. However, throughout the steps with a load of 3.8 and 5.0 g COD/L.d, lower and stable pH values were found. The operation with synthetic xylose as substrate probably allowed the adaptation of acidogenic fermentative bacteria to the main sugar contained in the sugarcane straw hydrolysate. Substrate consumption during the steps with the C5 fraction (monitored as total reducing sugars—TRS) did not show a significant difference between the means. Considering the entire period with C5 fraction, the average consumption was

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Table 2 Parameters and results from the operation of bench reactor in semicontinuous mode with 48-h hydraulic retention time, and feeding with synthetic solution of xylose or hemicellulose fraction of the hydrothermally pretreated sugarcane straw Parameter Working volume (L) Operation mode Inoculum Temperature (
C) Feed COD (g/L) Duration (d) OLR (g COD/L.d)) Feed pH Final pH Fed substrate (g) Final substrate (g) Substrate consumption (%) VFA production (mg/L) H2 production (mL H2/d) H2 yield (mL H2/g TRS)

Feeding with standard xylose 0.5

Feeding with hemicellulose fraction of the hydrothermally pretreated sugarcane straw

16-h feeding cycles (for 48-h HRT), mixing until 19-h fermentation, 5-h sedimentation of sludge, and removal of supernatant Thermally pretreated sludge from the brewery industry (61.8 g VSS/L) for I/S 2 (g VSS/g COD) 35.0  1.0 5.0 5.0 7.5 10.0 12 8 7 9 2.5 2.5 3.8 5.0 11.68  0.37 12.01  0.02 12.00  0.00 12.02  0.01 5.43  0.44 6.10  0.45 5.33  0.15 5.11  0.06 0.68  0.22 (xylose) 0.45  0.05 0.70  0.02 0.90  0.04 (TRS) (TRS) (TRS) 0.16  0.03 (xylose) 0.12  0.02 0.17  0.03 0.20  0.02 (TRS) (TRS) (TRS) 76.5  5.5 (xylose) 74.5  3.5 75.7  3.9 77.8  1.9 (TRS) (TRS) (TRS) 2107  514 529  84 787  337 939  260 172.9  26.5

257.4  72.3

189.8  23.3

217.5  46.6

230.6  35.0

564.9  130.1

270.1  28.5

226.4  14.0

COD chemical oxygen demand, OLR organic loading rate, VFA volatile fatty acids (as acetic acid equivalents), TRS total reducing sugar

76.2  3.3%, which is similar to that obtained in the step with synthetic xylose solution. Maintenance of the TRS consumption can be attributed to acidogenic bacteria adapted to the sugars present in the C5 fraction (Arriaga et al. 2011) and tolerant to inhibitors such as furfural, hydroxymethylfurfural, and polyphenols. In addition to xylose, the C5 fraction contains arabinose (0.80 g/L), glucose (0.12 g/L), and galactose (0.35 g/L) (Silva et al. 2019) that can also be metabolized by acidogenic bacteria. The final concentration of volatile fatty acids (VFA), which in the period with xylose as substrate showed an average of 2107 mg/L (as acetic acid equivalents), increased linearly throughout the operation with C5 fraction (Fig. 2a) and reached values above 4000 mg/L in the period with the highest OLR. The replacement of synthetic xylose medium by C5 fraction in the reactor feed, maintaining the same operating conditions (2.5 g COD/L.d, HRT 48 h, influent pH 12), considerably increased the H2 yield, from 230.6  35.0 to almost 800 mL H2/g TRS applied (Fig. 2b). However, throughout the operation with C5 fraction, the

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Fig. 2 VFA concentrations (a) and H2 yield (b) during operation of the reactor in semicontinuous mode with HRT 48 h and xylose or C5 fraction in the feed, under continuous mixing and OLR of 2.5, 3.8, and 5.0 g COD/L.d

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yield decreased to 518.1  68.9 mL H2/g TRS applied. The increase of OLR to 3.8 and 5.0 g COD/L.d led to lower and more stable H2 yields. Acetyl groups in the raw material increased the concentration of VFA in the system (Table 1). Furthermore, the hydrogen production process resulted in the accumulation of VFA and lower buffering capacity in the medium, influencing the decrease in pH (Kotharia et al. 2012). A low pH can affect hydrogenase enzyme activity and metabolic pathway of microorganisms, which can follow routes of lower H2 yield. In the study conducted by Thungklin et al. (2018), H2 production from sugarcane bagasse hydrolysate by mixed anaerobic culture was evaluated with initial pH ranging from 4.5 to 7. The main soluble metabolic products in all fermentations were acetic and butyric acids, except in the medium with initial pH 4.5. At this lower pH, lactic acid was the main soluble metabolite and there was no H2 production due to the change in the microbial population from hydrogen-producing bacteria to non-hydrogen-producing bacteria, such as lactate producers. Exceptionally, H2 production at pH below 3 was reported by Mota et al. (2018) in continuous acidogenic reactors fed with sucrose at 30
C and without pH control. The authors attributed the result to the growth of acid-tolerant bacteria that were able to produce H2 under very acidic conditions, especially Clostridium sp. and Ethanoligenens sp. The obtained data indicate that the reactor operating under semicontinuous mode with HRT 48 h, at 35
C, and continuous mixing during fermentation, resulted in more stable H2 yield (226 mL H2/g TRS) at an OLR of 5.0 g COD/L.d (COD ¼ 10 g/L) of the C5 fraction. Considering the composition of the C5 fraction (12.4 and 2.1 g/L of pentose and hexose sugars, respectively), at an OLR of 10 g COD/L.d the reactor was fed with 5.85 mmol of sugar daily. Therefore, the H2 yield of 1.53 mmol H2/mmol TRS, a value slightly higher than that observed with synthetic xylose medium, indicates that the H2 production was not inhibited by the constituents of the C5 fraction and suggests the consumption of the other sugars that compose the raw material. In the same operating conditions with xylose as substrate, the best H2 yield (1.37 mmol H2/mmol xylose, equivalent to 230.6 mL H2/g TRS) was obtained under an OLR of 2.5 g COD/L.d. The H2 yield obtained in the semicontinuous reactor using the C5 fraction is 12% higher than that obtained for a similar substrate concentration of synthetic medium containing xylose. In addition to the hexoses present in the C5 fraction, other components also contributed to higher yields. De Sá et al. (2020) evaluated the H2 production in batch experiments with dilute hemicellulose fraction from acidpretreated sugarcane bagasse. After 24 h of fermentation of 9 mmol/L of sugars with acid-pretreated anaerobic sludge, sugar consumption and H2 yield were 100% and 258.5  1.7 mL H2/g carbohydrates. Under increasing substrate concentrations, the authors observed decreased sugar consumption and H2 yield and attributed this decrease to several factors, such as the inhibition by greater concentration of organic acids, higher substrate concentration, osmolarity of fermentative medium, or even presence of HMF and furfural in the fermentative medium. In the bench reactor fed with hemicellulose fraction from the hydrothermally pretreated sugarcane straw, a sugar consumption of 76% and a yield of 270.1  28.5 mL H2/g TRS were obtained for an initial concentration of 9.1 mmol/L of sugars (OLR of 3.8 g COD/L.d),

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proving that the inhibition of H2 production can be much less in semicontinuous systems. The literature includes few studies on the production of H2 in continuous and semicontinuous reactors, especially with xylose-rich lignocellulosic hydrolysates as substrate and mixed culture. Table 3 shows a comparison with data from this study and some works found in the literature. According to the results, a continuous hydrogen production from xylose-rich lignocellulosic hydrolysates would be possible by using various reactors and operating conditions, but the data do not allow any correlations between operating conditions (suspended or attached growth systems, temperature, OLR, mode of operation) and hydrogen production rate and yield values. This study obtained higher H2 yields than other studies (Arriaga et al. 2011; Veeravalli et al. 2014) conducted under very different conditions of reactor/ biomass type (suspended growth, granular sludge, and biofilm) and OLR (5, 12, and 70 g COD/L.d). The hydrogen production rate, which was very low in the present study, has higher values (2 L H2/L.d) under higher OLRs (Arriaga et al. 2011; Haroun et al. 2016; Lopes et al. 2020). Another explanation for the lower productivity obtained, in addition to the lower applied OLR, would be that the C5 fraction in this study has a much higher xylose/glucose ratio (6.8 g/g total sugars) than that verified in other studies (0.3–4.3 g/g total sugars) and was used without dilution and much more concentrated. Xylose is degraded with lower consumption rates and biomass yields than glucose (Arriaga et al. 2011; de Sá et al. 2020), resulting in lower hydrogen production rate.

5.3

Composition and Diversity of the Microbial Communities

To determine the structure of prokaryotic communities in the sludge before and after bioreactor operation with hydrogen production, metagenomic analysis by highthroughput sequencing was performed; all samples together yielded a total of 206,453 reads and 358 OTUs. Figure 3 also details the community composition at the levels of phyla and genera, showing the predominant groups in the unpretreated inoculum, the pretreated inoculum, and the final sludge after dark fermentation. The unpretreated inoculum had high dominance of the phylum Euryarchaeota with 65% of abundance, which is the most diverse phylum in the Archaea domain and at genus level was represented by Methanosaeta (41%) and Methanobacterium (25%). Comprising the domain Bacteria, Firmicutes (21%) was the second most abundant phylum followed by Proteobacteria phylum (9%) and minor fractions of the phyla Actinobacteria, Atribacteria, Chloroflexi, Spirochaetes, and Caldiserica (1–5%). After thermal pretreatment, the inoculum exhibited a clear divergence of the microbial community structure. The Euryarchaeota phylum significantly decreased to 19%, probably because the Methanosaeta genus was absent. This genus revealed high sensitivity to the inhibitory effects of thermal treatment at 100
C. On the other hand, Methanobacterium was resistant to heating and maintained its relative abundance (23%). At the expense of decreasing archaea members, the proportion of

AFBR 1.52 L

CSTR 7L

UASB 8.5 L

ASBR 4L

UASB 0.22 L

BF 2.3 L

Reactor CSTR 0.7 L

Feedstock/hydrolysatea/substrate Wheat strawVFA 0.7 g/L, xylose 1.1 (11.3) g/L, glucose 1.15 (2.9) g/L, furfural 0.25 g/L, HMF 0.14 g/LBA medium, and 20% (v/v) HTP hydrolysate Oat strawSCOD: 35 g/L, xylose: 5.8 g/L, glucose: 2.0 g/LDiluted acid hydrolysate (10% v/v) Wheat strawPentoses (12.6) g/L, hexoses (2.9) g/LBA medium and 30% (v/v) HTP hydrolysate Oat strawSCOD: 30 g/L, xylose: 1.3 g/L, glucose: 3.8 g/L, furfural and HMF not detectedDiluted enzymatic hydrolysate (17% v/v) SwitchgrassVFA: 1.2 g/L, xylose: 11.6 g/L, glucose: 9.1 g/L, furfural: 0.7 g/L, HMF: 0.2 g/LDiluted steamexplosion hydrolysate to 5 g COD/L Synthetic lignocellulose hydrolysate: mixture of xylose, arabinose, glucose, and cellobiose 2.5 g/L each Mixture of xylose and glucose 2 g/L each Anaerobic granular sludge from UASB reactor, thermally pretreated

Anaerobic digester sludge, thermally pretreated

Adapted granular sludge from UASB reactor

Anaerobic granular sludge from UASB reactor, thermally pretreated

Anaerobic granular sludge from UASB reactor, thermally pretreated Mixed culture

Inoculum Mixed culture from batches with 20% (v/v) hydrolysate

Semicontinuous HRT: 8 h, pH: 5.5, 37
C, OLR: 32.1 g COD/L.d Continuous HRT: 8 h, pH: 4.5–5.0, 55
C, OLR: 12 g COD/L.d

Continuous HRT: 10 h, pH: 5.0, 37
C, OLR: 12 g COD/L.d

Continuous HRT: 12 h, pH: 5.5, 28
C, OLR: 70 g COD/L.d Semicontinuous HRT 24 h, pH: 5.1, 70
C, OLR: 11.4 g COD/L.d Semicontinuous HRT: 8 h, pH: 4.5, 35
C, OLR: 15 g COD/L.d

Operation mode Semicontinuous HRT 72 h, pH 4.9, 70
C, OLR: 10.7 g COD/L.d

1.95b

0.88

0.70

nd

5.12

2.48

2.9 mol H2/mol hexose 190 mL H2/g sugar 0.8 mol H2/ molsugar

2.6 mol H2/mol hexose

1.1 mol H2/ molsugar 0.4 mol H2/mol sugar

H2 yield 178 mL H2/ gsugar

HPR (L H2/ L.d) 0.18

Table 3 Comparison of hydrogen production in continuous and semicontinuous reactors fed with xylose-rich lignocellulosic hydrolysates

Haroun et al. (2016) Lopes et al. (2020)

Arriaga et al. (2011) Kongjan et al. (2011) ArreolaVargas et al. (2013) Veeravalli et al. (2014)

Reference Kongjan et al. (2010)

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Sugarcane strawSCOD: 10 g/L, VFA: 3.6 g/L, xylose 3.8 (11.6) g/L, glucose: 0.12 (1.7) g/L, furfural: 0.34 g/L, HMF: 0.1 g/L HTP hydrolysate with 10 g COD/L Anaerobic digester sludge, thermally pretreated

Semicontinuous HRT: 48 h, pH: 5.1, 35
C, OLR: 5.0 g COD/L.d

226 mL H2/ gsugar1.5 mol H2/molsugar

0.23

This study

nd not determined, HPR hydrogen production rate, HTP hydrothermal pretreatment, AFBR anaerobic fluidized bed reactor, ASBR anaerobic sequencing batch reactor, BA medium basic anaerobic medium, BF biotrickling filter, CSTR continuous stirred tank reactor, UASB upflow anaerobic sludge blanket reactor a Values in parentheses are total sugars, after acid hydrolysis b With HRT 24 h, OLR 1.2 g COD/L.d

CSTR 0.5 L

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Fig. 3 Diversity profiling at phylum and genus levels for microbial communities in the unpretreated inoculum, pretreated inoculum, and final sludge after dark fermentation. The remaining phyla and genera with $891 million with 10% contingency for dark fermentation. Overall photo-fermentation exhibits higher yields but is reported to be relatively expensive (Bolatkhan et al. 2019). In recent literature, economic analysis of biohydrogen production from fermentative route is scanty especially using cheese whey as substrate. Economy aspect of feasibility study of biohydrogen production has been estimated more than 15 years ago, using state-of-the-art technologies. Besides, most of the recent studies are optimistic about biohydrogen production (Ahmed et al. 2021). Show et al. (2019) have estimated that biohydrogen can be produced at a cost ranging between 10 and 20 USD/GJ but it will be too high as compared to 0.33 USD/GJ of gasoline cost. Cost comparison of the biologically produced hydrogen production with gasoline, ethanol, biodiesel, and natural gas showed that most of the available processes showed higher cost in comparison with conventional methods of fuel production (Kartic and Joseph 2012). Thus, in an attempt to reduce the cost of biohydrogen production, two major factors are critical. First is the cost of the photobioreactor and second is the cost of storage system (Rao and Basak 2021).

7

Challenges in Biohydrogen Production Using Cheese Industry Waste

Fermentative production of biohydrogen using food processing waste has the potential to produce hydrogen gas on an industrial scale in an economically viable path. Cheese industry waste being high in carbohydrates and organic acids is a feasible substrate for biohydrogen substrate. Though numerous research studies have explored cheese whey as the substrate for biohydrogen production, there are many challenges for carrying out our industrial scale production in a commercial manner. The major challenges are lower production rate, incomplete utilization of sugar present in CW, lack of efficient bioreactors, etc. Cheese whey, a cheese industry waste, is an attractive and promising substrate for hydrogen production but the need of suitable bioreactor design for optimizing critical parameters and for scaling up the process from lab to pilot scale is important.

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Based on the literature, few major lacunae, which are hampering maximum production of biohydrogen, are resistance of photo-fermentative bacteria towards stress conditions and production of by-products such as propionic acid, alcohol, and lactic acid. Also, reported energy conversion efficiencies of dark and photofermentation are 4.3% and 5.11% (Zhang et al. 2017). Thus, for improving biohydrogen production, research focus should be on genetic engineering approach to inhibit the production of by-products or changing reaction parameters such as HRT/temperature during continuous DF process, developing bacterial strain which can work at higher temperature and can consume the lignocellulosic mass without pretreatment, kinetics of substrate utilization in DF using suitable mathematical models, designing bioreactors of appropriate dimensions and ratio, elevated resistance power of photo-fermentative bacteria against unfavorable conditions, application of mathematical tools for studying the interaction between bioprocess parameters, and use of different light sources with optimal intensity to produce more biohydrogen from lactic acid.

8

Conclusion

Fermentative production of biohydrogen using cheese industry waste is a promising method for permanent disposal as it can effectively utilize the organic content of the CW. For industrial scale biohydrogen production using CW-based dark fermentation, designing cost-effective and robust bioreactors, optimum ratios of inoculum to substrate, microorganism type, CW composition, and genetic engineering for eliminating the genes responsible for selective uptake of hydrogenase play an important role. Recently, integrated bioprocessing of cheese whey using photoand dark fermentation is gaining interest as it has the capability to produce maximum theoretical yield of biohydrogen from glucose. Though availability of cheap feedstock and developing strategies to treat complex biomass are challenges associated and can be addressed for biological production of hydrogen, cheese whey can be utilized for industrial production of biohydrogen.

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Methods of Biological Hydrogen Production from Industrial Waste Rekha Unni, R. Reshmy, Aravind Madhavan, Parameswaran Binod, Ashok Pandey, and Raveendran Sindhu

Abstract

Hydrogen fuel is an interesting fossil fuel substitute due to its rich energy content, sustainable nature, and fuel efficiency. However, it is difficult to obtain. As a result, the demand for hydrogen has gone up significantly. Water electrolysis, hydrocarbon steam reforming, coal gasification, and partial oxidation techniques are all familiar approaches for producing hydrogen; however, they are not profitable because of high energy necessities. Compared to chemical approaches, R. Unni Department of Chemistry, Christian College, Chengannur, Kerala, India R. Reshmy Post Graduate and Research Department of Chemistry, Bishop Moore College, Mavelikara, Kerala, India Department of Science and Humanities, Providence College of Engineering, Chengannur, Kerala, India A. Madhavan Rajiv Gandhi Center for Biotechnology, Jagathy, Thiruvananthapuram, Kerala, India P. Binod Microbial Processes and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Trivandrum, Kerala, India A. Pandey Centre for Innovation and Translational Research, CSIR—Indian Institute for Toxicology Research (CSIR-IITR), Lucknow, Uttar Pradesh, India Centre for Energy and Environmental Sustainability (CEES), Lucknow, Uttar Pradesh, India R. Sindhu (*) Microbial Processes and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (CSIR-NIIST), Trivandrum, Kerala, India Department of Food Technology, T K M Institute of Technology, Kollam, Kerala, India # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Kuddus et al. (eds.), Organic Waste to Biohydrogen, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-1995-4_7

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biohydrogen gas processing has various advantages. Biological photolysis of water by algae and photo- and dark fermentation of organic resources, normally starch and sugars, by microbes, are the main biological processes used to produce hydrogen gas. The consecutive photo- and dark fermentation procedures are a relatively novel method to produce biohydrogen. In the manufacture of photoand dark fermentative hydrogen, the prices of raw materials are a crucial concern. Carbohydrate-enriched substances like starch and cellulose-containing food and agricultural industry wastes, effluents from the olive mill, cheese whey, and baker’s yeast industry can be used for hydrogen making use of appropriate bioprocess techniques. Biological decomposable substances for hydrogen generation, as previously stated, provide low-cost energy generation including wastewater treatment. Keywords

Biohydrogen · Steam reforming · Coal gasification · Fermentation · Biocatalyzed electrolysis

1

Introduction

Hydrogen, the lightest gas, is viewed as a potentially significant energy source since it is renewable, does not emit the “greenhouse gas” CO2 during ignition, liberates significant amounts of energy per gram weight, and is effectively transformed to electric power using fuel cells. The expanding energy demand and required energy supply as a result of the growing human population have inspired a lot of interest in new biofuel research and production in recent years (Ueno et al. 1995). Hydrogen fuel has a power density of 122 kJ/g. It is approximately 2.75 times that of conventional hydrocarbon fuels (Argun and Kargi 2010). Bio-H2 is H2 that is created from biological sources. As a result, it is regarded as one of the energy sources with the potential to partially replace conventional fossil-based fuels. Steam reforming, partial oxidation of oil, and coal gasification are all methods for producing hydrogen from fossil fuels and biomass. Nonbiological ways for producing hydrogen from H2O include thermal and thermochemical reactions, electrolysis, and photolysis. H2 is a gas that can be created physiologically. Many microbes create hydrogen as a result of reactions related to their energy metabolism. Dark fermentation (anaerobe microorganisms), photofermentation (with photoheterotrophic bacteria), fusion system of both, biological photolysis of water employing cyanobacteria and green algae, and water–gas shift reaction are all key biological hydrogen production mechanisms. Enzymes that produce hydrogen such as hydrogenase and nitrogenase regulate all processes (Azwar et al. 2014). Figure 1 depicts various materials and common hydrogen generation techniques used.

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Fig. 1 Various organic and inorganic sources and techniques used for hydrogen generation

2

Industrial Waste Materials for Hydrogen Gas Synthesis

Coal, oil, natural gas, kerosene, propane, etc. are the sources of fossil fuels for hydrogen production. Steam reforming, partial oxidation, and coal gasification techniques are chemical methods used for hydrogen production (Higman and Tam 2014). All these chemical processes are described in Sect. 3.1. The obtainability, rate, carbohydrate content, and biodegradability of waste constituents utilized in biohydrogen generation are the most important factors to consider. Simple sugars like glucose, sucrose, and lactose are easily decomposable and are excellent hydrogen generation substrates. Clean carbohydrate sources, on the other hand, are costly raw materials for hydrogen synthesis. Agricultural and food processing trash comprising starch and cellulose, industrial wastewaters that are high in carbohydrates, and wastewater treatment plant sludge are the major raw materials utilized to produce hydrogen gas.

2.1

Agricultural and Food Industry Residues Comprising Starch and Cellulose

Agricultural and food leftovers are low-cost, carbohydrate-rich resources for the generation of biohydrogen. Biomass containing starch or cellulose can be subjected to enzymatic or acid hydrolysis to produce a highly concentrated sugar solution. It is then dark fermented by anaerobic acetogenic microbes for making H2, CO2, and volatile fatty acids. Volatile fatty acids formed by dark fermentation can be converted to H2 and CO2 by Rhodobacter sp. (photoheterotrophic bacteria). Starch

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and cellulose-containing substances are cheap raw materials found in agricultural wastes that seem to be the concentrated sources of carbohydrates (Yokoi et al. 2001; Liu and Shen 2004). For anaerobic fermentation of carbohydrates, pure cultures of bacteria such as Enterobacter sp., Clostridia sp., and a mixed culture of thermally treated anaerobic sludge can be adopted. Acid or enzymatic hydrolysis can convert starch to glucose and maltose, which can be transformed into hydrogen gas and organic acids (Han et al. 2016; Rangel et al. 2020). Agricultural wastes containing cellulose require additional pretreatment. Before fermentation, cultivated wastes must be crushed and delignified using mechanical or chemical methods. The hemicellulose and cellulose components can be hydrolyzed into carbohydrates after they can subsequently be treated into hydrogen gas and organic acids.

2.2

Carbohydrate-Enriched Industrial Effluents

Dairy manufacturing, baker’s yeast, brewery, and olive mill effluents are examples of decomposable carbohydrate-comprising and nontoxic industrial wastes used for biohydrogen generation. Pretreatment of these sewer water may be necessary to eliminate unwanted constituents and restore nutritive balance. Using precise bio-processing methods, carbohydrate-enriched food sector sewages can be additionally treated to change the carbohydrate constituents to biological acids, and subsequently to hydrogen (Hao et al. 2015; Konstandopoulos et al. 2015).

2.3

Wastewater Treatment Plant Sewage Sludge

Large amounts of carbohydrates and proteins are present in wastewater treatment sludge. These wastes can then be used to yield methane or hydrogen gas. Additional wastes can be digested anaerobically in two phases. The primary phase (acidogenic phase) includes biological materials that will be transformed into organic acids, which can then be used to produce hydrogen gas in the next phase utilizing photoheterotrophic bacteria (Preethi et al. 2019).

3

Hydrogen Gas Production Techniques

Chemical and biological processes are used for hydrogen production. Heat and chemical reactions are used in thermochemical processes to release hydrogen from organic sources like fossil fuels and biomass, as well as from inorganic materials like water. Electrolysis or solar energy can also be used to divide water (H2O) into hydrogen (H2) and oxygen (O2). Biological activities can produce hydrogen in microorganisms like bacteria and algae.

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Manufacture of Hydrogen Gas by Chemical Procedures

The majority of hydrogen (95%) is produced from fossil fuels through partial oxidation of methane, coal gasification, and natural gas steam reforming. Biomass gasification and water electrolysis are two other approaches to hydrogen making.

Natural Gas Reforming with Steam Steam reforming is an ordinary gas-based hydrogen generation technique. This process is currently the most cost-effective way to obtain industrial hydrogen. The gas is heated to 700–1100
C using nickel catalyst and steam. The heat-absorbing process breaks down methane gas into hydrogen (H2) and carbon monoxide (CO). The CO gas formed is then passed over iron oxide with steam to produce more H2 through a water–gas shift reaction. The disadvantage of this process is that it produces significant amounts of greenhouse gases like CO and CO2 in the atmosphere. Depending on the feedstock quality, 1 tonne of hydrogen creates CO2 of 9–12 tonnes (Collodi 2010). The steam reforming plant is divided into four components, namely removal of sulfur and other impurities from the feedstock; it then transforms the feedstock components and vapor into syngas (mostly carbon monoxide and hydrogen) at elevated temperatures and reasonable pressures and performs syngas heat recovery using CO shift reactors to boost hydrogen generation and raw hydrogen purification using a pressure compression swing adsorption (PSA) unit for obtaining the desired quality. The simplified chemical reactions are Cn H2nþ2 þ nH2 O ! nCO þ ð2n þ 1ÞH2 ðIn the case of saturated hydrocarbonsÞ CH4 þ H2 O ! CO þ 3H2 ðIn the case of methaneÞ In the adiabatic CO shift reactor vessel, the relatively exothermic water–gas shift reaction transforms carbon monoxide and steam to hydrogen and carbon dioxide: CO þ H2 O ! CO2 þ H2

Partial Oxidation Techniques Partial oxidation is an exothermic reaction in which a small amount of oxygen is combined with natural gas or a heavy hydrocarbon fuel (heating oil). It is used to produce hydrogen from natural gas or other hydrocarbons. When a fuel-oxygen combination is partly ignited, a hydrogen-enriched syngas is formed. The water–gas shift process produces hydrogen and carbon monoxide (Mangold 2009): Oil=Coal þ O2 ! CO þ H2

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Food-Processing Trash containing starch and cellulose

Industrial waste waters

Grinding

Pretreatment

Pretreatment

Dark fermentation /Photo fermentation

CO2, H2

Hydrogen Separation

Hydrolysis to Glucose syrup

Fig. 2 The flowchart for biohydrogen manufacture from starch/cellulose-containing food/decomposable resources and industry effluents

Coal Gasification The conversion of any carbonaceous fuel into a gaseous product with a desirable chemical heating value is known as gasification. The method of producing hydrogen from coal begins with partial oxidation, which involves adding some air to the coal, which produces carbon dioxide gas through typical burning. However, not enough heat is given to burn the coal—simply enough to heat the gasification reaction. Carbon dioxide can act as a gasifier. It becomes carbon monoxide when it reacts with the rest of the carbon in coal. The carbon monoxide in the gas stream now reacts with steam, resulting in hydrogen and carbon dioxide. Figure 2 displays the flowchart of the conversion of coal to hydrogen gas (Mangold 2009). Coal gasification can also be done on existing layers, with the coal being transformed to gaseous fuels without the need for intermediary mining activity. Underground gasification entails drilling boreholes that intersect the layers in at least two places and allowing gas to flow through the seam from one intersection to the next. A gasification agent, such as steam or compressed air, is introduced into the seam after the coal is fired at the bottom of one hole. Coal is converted to methane (synthetic natural gas) or other natural gases, and then gasified with carbon dioxide to generate liquid hydrocarbon fuels and carbon monoxide gas (Yu et al. 2015). Electrolysis of Water Electrolysis is a viable process for creating carbon-free fuel hydrogen using nuclear and renewable energy resources. It is a technique of dissociating water into oxygen and hydrogen using electricity. This reaction is carried out in electrolyzers. Electrolyzers have a cathode and an anode separated by an electrolytic solution,

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similar to fuel cells. Because of the various types of electrolyte materials and ionic species it conducts, different electrolyzers work in different ways: At anode: 2H2 O ! O2 þ 4Hþ At cathode: 4Hþ þ 4e ! 2H2 Different electrolytic cells/electrolyzers used for hydrogen generation are membrane electrolyzers using polymer electrolytes, electrolyzers for the alkaline solution, and electrolyzers with solid oxide. In the electrolyzer containing polymeric membrane, the electrolytic medium is a solid-specific soft substance. At the anodic compartment, water reacts to produce hydrogen ions (protons) and oxygen. Hydrogen ions selectively move through the electrolyzer to the cathode as electrons flow through an external connection. Hydrogen ions mix with electrons from the external circuit at the cathode to form hydrogen gas. Hydroxide ions (OH) are transmitted from the cathode compartment to the anode compartment via the alkaline electrolyte, with hydrogen created at the cathode. The electrolyte used in electrolyzers is liquid alkaline sodium or potassium hydroxide solution. Solid oxide electrolyzers contain ceramic as the electrolyte produces hydrogen by transferring oxygen ions (O2) at raised temperatures. Steam forms hydrogen gas and oxygen ions when it combines with electrons from the external circuit. Through the solid ceramic membrane, oxygen ions flow to the anode, where they react to form oxygen gas and produce electrons. The temperatures of solid oxide electrolyzers are high enough for solid oxide membranes to function properly (approximately 700–800
C, related to 70–90
C for PEM electrolyzers and less than 100
C for commercial alkaline electrolyzers) (Sengodan et al. 2018). However, these approaches are not sustainable because they rely on nonrenewable energy sources to manufacture hydrogen. As a result, it is critical to investigate hydrogen synthesis from renewable energy sources. Biological hydrogen generation processes are projected to consume a lesser amount of energy than thermochemical techniques of hydrogen manufacture since they work at ambient temperatures and pressures. Waste products can also be used on a carbon basis, making unused reutilizing more efficient. However, the proportion of hydrogen manufacture is modest, and this process’ technology has to be improved. Biological hydrogen generation is an innovative and promising technique to fulfill growing energy requirements as an alternative for fossil fuels because it produces a clean energy source and uses waste materials.

4

Biological Techniques for the Synthesis of Hydrogen Gas

Hydrogen formed from biological sources is called biohydrogen. Biohydrogen production methods are mainly classified as (1) photobiological methods, (2) fermentative methods, and (3) biocatalyzed electrolysis. Figure 2 shows a chart outline for producing biohydrogen from decomposable wastes and food sector effluents that contain cellulose/starch. It depicts biohydrogen production using an anaerobic photo- and dark fermentation (Kapdan and Kargi 2006).

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Photobiological Methods

Photobiological methods include direct and indirect biophotolysis. The process that occurs in the presence of light is called direct biophotolysis and in the absence of light it is called indirect biophotolysis.

Direct Biophotolysis In biological organisms, direct biophotolysis refers to the formation of hydrogen below the influence of sunlight. This mechanism is analogous to plant and algae photosynthesis. One of the most famous hydrogen-generating algae is Chlamydomonas reinhardtii. Marine green algae Chlorococcum littorale, Scenedesmus obliquus, and Chlorella fusca and green algae Platymonas subcordiformis all have the enzyme hydrogenase. Dunaliella salina and C. vulgaris showed no hydrogenase activity (Florin et al. 2001; Winkler et al. 2002; Guan et al. 2004). The thylakoid membranes in algae and cyanobacteria’s chloroplasts are made up of chlorophyll pigments from both photosystems (PSI and PSII). Light energy is absorbed by the pigments, which raises electron energy levels from water oxidation through PSII to PSI to ferredoxin, where a portion of the light energy is promptly stored as hydrogen gas. The photosynthetic hydrogen generation method in direct biophotolysis involves H2O oxidation and transference of electrons to the hydrogenase enzyme, resulting in the manufacture of hydrogen. Although the method produces a 14:1 ratio of O2 and H2, it does not necessitate the fixation of CO2 or the storing of energy in metabolites produced by cells: 2H2 O þ light energy ! 2H2 þ O2 The above mechanism, however, can only work for a short period without the elimination of oxygen since the enzymatic reaction is strongly inhibited by oxygen, and hydrogenase gene expression is suppressed by it (Nguyen et al. 2008; Ghiasian 2019). In most hydrogenases, oxygen functions as a transcriptional repressor, a hydrogenase growth inhibitor, and an unalterable inhibitor of hydrogenase enzyme activity. The overall reactions of aerobic and anaerobic phases of photobiological processes involve H2 O ! 2Hþ ! 1=2O2 2Hþ þ þ2e ! H2

ðAerobic phaseÞ

ðAnaerobic phaseÞ

The H2O is divided into oxygen, electrons (e), and protons (H+) in the aerobic phase (O2). As a result, the e and H+ made by water in PSII are deposited as a range of metabolic products in the form of carbohydrates and proteins, and the e and H+ are required for H2 manufacture. Hydrogenase produces H2 during the anaerobic phase. Under anaerobic conditions, [FeFe] hydrogenase is rapidly activated and catalyzes H+ to H2 reduction by using the e donor ferredoxin. PSII donates to

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both indirect and direct e-source channels. Electrons generated by PSII’s watersplitting activity are carried into the electron transport chain (ETC) during the phase transition and eventually reach [FeFe] hydrogenase, which converts H to H2 (Najafpour et al. 2016). Hydrogenase enzyme is found in green algae for hydrogen production, whereas enzymes (nitrogenase and hydrogenase) promote H2 synthesis in cyanobacteria. Both heterocystous nitrogen-fixing cyanobacteria and unicellular include the nitrogenase enzyme, which can be utilized to create hydrogen (Ghysels and Franck 2010).

Indirect Biophotolysis In microalgae and cyanobacteria, indirect biophotolysis refers to the production of H2 from intracellular energy reserves such as starch and glycogen. As a result, there are two stages to this process: carbohydrate production in the light and carbohydrate fermentation in the dark for H2 generation (Huesemann et al. 2010; Ghiasian 2019). In direct and indirect biophotolysis, the influence of an indirect and direct electron transference path is used to produce hydrogen. Reduced substrates (starch in microalgae and glycogen in cyanobacteria) are collected during the photosynthetic O2 production and carbon dioxide fixation stages of the indirect biophotolysis process, and these are then used in a second stage for H2 creation under anaerobic conditions with carbon dioxide elimination. A diagram showing indirect electron transfer of biohydrogen production is shown in Fig. 3. The overall hydrogen production can be shown as 6H2 O þ 6CO2 ! ðC6 H12 O6 Þn þ 6O2 ðC6 H12 O6 Þn þ 12H2 O ! 12H2 þ 6CO2

Stage II (-O2)

Stage I (+O2)

Photo system II

H2O

2e-/CO2

Cell Material

Cell Material

Fermentation

O2

Fig. 3 A diagram showing indirect electron transfer of biohydrogen manufacture

2H+→H2

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Fermentative Methods

Fermentative methods include dark fermentation, photofermentation, and a combination of dark and photofermentation processes.

Photofermentation Photofermentation can also yield hydrogen gas. In daylight, photosynthetic bacteria transform organic molecules such as lactic acids, butyric acids, and acetic acids into CO2 and H2. This procedure, however, needs anaerobic conditions. Depending on the physiological parameters of the microorganisms, both nitrogenase and hydrogenase enzymes are included in hydrogen generation during photofermentation. Four factors limit nitrogenase-facilitated photofermentation in purple non-sulfur bacteria: • • • •

The occurrence of an H2 absorption enzyme Small photofermentation productivity for H2 making The little turnover quantity of nitrogenase The obtainability of organic acids

Nitrogenase and hydrogenase enzymes are active in purple non-sulfur bacteria. PNS bacteria generate hydrogenase in nitrogen-deficient environments. In an anaerobic situation, hydrogenase converts H2 to H+, electrons, and ATP for energy absorption. For photosynthetic bacteria to produce photofermentative H2, several parameters such as the amount of light, duration of light intensity, temperature, and inoculum age are critical (Ghysels and Franck 2010; Assawamongkholsiri and Reungsang 2015; Anwar et al. 2019). Rhodobacter sp., for example, prefers temperatures between 31 and 36
C. Using nitrogenase enzymes, these bacteria can change biological acids into CO2 and H2 under anaerobic circumstances. Supplementing micronutrients including molybdenum (Mo) and iron (Fe) in a Rhodobacter growing medium boosted photofermentative H2 generation. A chart for photofermentation of biohydrogen manufacture is shown in Fig. 4. The reaction can be shown as CH3 COOH þ 2H2 O þ light energy ! 4H2 þ 2CO2

Dark Fermentation Anaerobic bacteria thrive in the dark on carbohydrate-rich, hydrogen-producing substrates. Enterobacter, Bacillus, and Clostridium species have all been found to Photons Photosystem I

Organic Acids

Nitrogenase

Fig. 4 A chart for photofermentation of biohydrogen production

H+ + e-→ H2

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create hydrogen. Carbohydrates, particularly glucose, are the favored carbon bases for fermentation procedures that produce butyric and acetic acids as well as hydrogen. The chemical reactions involved can be shown as C6 H12 O6 þ 2H2 O ! 2CH3 COOH þ 2CO2 þ 4H2 C6 H12 O6 þ 2H2 O ! 2CH3 CH2 COOH þ 2CO2 þ 2H2 Dark fermentation procedures create mixed biogas that contains mostly carbon dioxide (CO2) and hydrogen, which also comprise smaller quantities of hydrogen sulfide (H2S), methane (CH4), and carbon monoxide (CO). Dark fermentation outperforms photofermentation since it does not need light and produces more energy due to the fermentation of sugar and carbs. Organic polymers can be hydrolyzed to monomers, which is followed by acetogenic transformation of monomers to alcohols, hydrogen, and organic acids. Even though dark fermentation produces more biohydrogen than photofermentation, it is more promising and advantageous. However, because organic biomass is required as a fuel, this method is highly expensive (Rafa et al. 2018; Sarangi and Nanda 2020). A schematic diagram for dark fermentation of biohydrogen production is displayed in Fig. 5. Temperature, pH, hydraulic retention period, partial pressure of H2/CO2, volatile fatty acids, and inorganic content are all factors that influence hydrogen manufacture in dark fermentation. The temperature of the process has a direct impact on microbial Fig. 5 A flowchart for dark fermentation of biohydrogen production

Waste Biomass (Carbohydrate rich substrates)

Pretreatment, Hydrolysis, Fermentation

Pyruvic Acid

Organic Acids, Alcohols

CO2, H2

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growth, metabolic activity, and amount of hydrogen generation. The mesophilic region (25–40
C), thermophilic region (40–65
C), severe thermophilic region (65–80
C), and hyperthermophilic region (>80
C) are among the bacteria that produce hydrogen through dark fermentation. Dark fermentation techniques work best at temperatures between 35 and 55
C. When compared to mesophilic circumstances, bacterial species produce more biohydrogen under high thermophilic conditions. The limited pressure of hydrogen in the bioreactor is another factor that affects hydrogen generation. The transference of hydrogen from the fluid to the gas phase increases as the limited pressure in the bioreactor falls. The alternative significant limit for biohydrogen manufacture via the dark fermentation procedure is the hydraulic retention period. By selecting acid-producing bacteria, short HRTs are utilized to clean up methanogens in a characteristic continuous stirred-tank reactor (CSTR) (Rangel et al. 2020).

4.3

Integration of Photo- and Dark Fermentation Processes

Many of the secondary products formed by dark fermentation could be employed as probable substrates for future transformation to hydrogen using other procedures, resulting in total biomass conversion and increased hydrogen production. As a result, implementing combined procedures for using the secondary products, particularly organic biological acids, is a viable strategy for achieving near-complete transformation of organic biomass while lowering surplus formation. Dark fermentation changes carbohydrates to organic biological acids in the first step of such combined bioconversion procedures for hydrogen production, and the second step employs these organic biological acids to produce methane and hydrogen as final products using a variety of methods. Purple non-sulfur bacteria are thought to have potential because of their capacity to transform dark fermentation by-products such as organic acids into hydrogen. Many researchers have explored the products containing organic acids after dark fermentation utilizing an integrated method such as dark fermentation and photofermentation (DF-PF) (Toledo-Alarcón et al. 2018; Rangel et al. 2020). The overall integrated dark and photofermentation are depicted in Fig. 6. Throughout the combination of DF-PF routes, special attention is paid to the following factors: • Convenient acceptance of photofermentation over dark fermentation • Farming of various microbes for photo- and dark fermentation in a common reactor • Membrane segregation of mutual fermentation procedures Various products generated from the integrated pathway can be expressed as chemical equations given below: Dark fermentation: C6 H12 O6 þ 2H2 O ! 2CH3 COOH þ 2CO2 þ 4H2 Photofermentation:

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Waste (Biomass, Municipal waste, Animal manure)

Agricultural Residues

Wood

Pre-treatment

Dark Fermenta tion

Organic Acids

Photo fermenta tion

Fig. 6 Integration of dark fermentation with photofermentation for hydrogen production

CH3 COOH þ 2H2 O þ Light energy ! 4H2 þ 2CO2 Lactic acid: C3 H6 O3 þ 3H2 O ! 6H2 þ 3CO2 Propionic acid: C3 H6 O2 þ 4H2 O ! 7H2 þ 3CO2 Butyric acid: C4 H8 O2 þ 6H2 O ! 10H2 þ 4CO2

4.4

Biocatalyzed Electrolysis

Other energy forms such as electrical energy can be used to oxidize the effluent of the dark fermentation process to make hydrogen instead of solar energy. The anodic compartment of an electrolyzer cell is formed by a bioreactor containing acetate, while bacteria in the cathodic compartment create protons and electrons. The platinum electrode acts as the cathode and liberates hydrogen. The reactions at anode and cathode can be summarized as Anode: 2CH3 COOH þ 2H2 O ! 2CO2 þ 8Hþ þ 8e Cathode: 8Hþ þ 8e ! 4H2 Chemical and biological methods used for hydrogen generation have their advantages and disadvantages. Table 1 describes various processes involved,

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Table 1 Advantages and disadvantages of different hydrogen generation methods Process Direct biophotolysis

Organism Green algae Cyanobacteria

Indirect biophotolysis

Cyanobacteria

Photofermentation

Photoheterotrophic bacteria

Dark fermentation

Obligate or facultative anaerobic fermentative bacteria

Advantages H2 may be formed directly from sunlight and water. Maximum efficiency, direct conversion of solar energy to fuels, a single-stage process, a simpler facility, and an easy-to-operate system that makes use of existing metabolic equipment Has the ability to fix N2 from the atmosphere and can create H2 from water. The separation of incompatible oxygenand hydrogen-evolving reactions could lead to a reduction in the number of photobioreactors needed for the H2-producing step These bacteria can use a wide range of spectrum light energy and can use a variety of waste materials such as distillery effluents and garbage It can manufacture H2 all day long without the need for light, and it can employ a variety of carbon sources as a substrate. As a by-product, it generates useful metabolites such as acetic, lactic, and butyric acids. There is no O2 constraint because it is an anaerobic process

Disadvantages Photobioreactors with large hydrogenimpermeable surfaces are required. Explosive hydrogen/oxygen combinations could be created. In the proximity of oxygensensitive hydrogenase, oxygen developed

Pumping between stages may result in energy loss. Energy waste in the manufacturing process and the reuse of a stored energy carrier

The effectiveness of light conversion is relatively low, and only 1–5% O2 is a powerful hydrogenase inhibitor

H2 yields are feasible at a lower rate. H2 fermentation becomes thermodynamically unfavorable as yields rise. CO2 must be isolated from the product gas combination

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organisms used, and advantages and disadvantages of methods used for hydrogen generation.

5

Conclusion

Hydrogen fuel is recognized as the “energy source of the future” when compared to fossil fuels. It is a low-carbon, high-energy resource. Chemical processes using high temperatures, such as partial oxidation of fossil fuels and steam reforming of hydrocarbons, are energy consuming and costly. Biological approaches for hydrogen production have different advantages, such as functioning under moderate circumstances and precise conversions. However, the most significant restrictions of biohydrogen generation are the expense of raw materials. The use of starch or cellulose-containing solid wastes and carbohydrate-rich and food-industry effluents for biohydrogen production is an intriguing option. To improve the “recent advances” in biohydrogen production, much research and development are required. Acknowledgments Reshmy R and Raveendran Sindhu acknowledge the Department of Science and Technology for sanctioning projects under DST WOS-B scheme.

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Preethi et al (2019) Biohydrogen production from industrial wastewater: An overview. Bioresour Technol Rep 7(April):100287. https://doi.org/10.1016/j.biteb.2019.100287 Rafa Ł et al (2018) Hydrogen production from biomass using dark fermentation. Renew Sustain Energy Rev 91(April):665–694. https://doi.org/10.1016/j.rser.2018.04.043 Rangel C et al (2020) Hydrogen production by dark fermentation process: effect of initial organic load. Chem Eng Trans 79(February):133–138. https://doi.org/10.3303/CET2079023 Sarangi PK, Nanda S (2020) Biohydrogen production through dark fermentation. Chem Eng Technol 4:601–612. https://doi.org/10.1002/ceat.201900452 Sengodan S et al (2018) Advances in reforming and partial oxidation of hydrocarbons for hydrogen production and fuel cell applications. Renew Sust Energ Rev 82(July 2016):761–780. https:// doi.org/10.1016/j.rser.2017.09.071 Toledo-Alarcón J, Capson-Tojo G, Marone A (2018) Basics of bio-hydrogen production by dark fermentation. In: Bioreactors for microbial biomass and energy conversion 5(10), pp 199–220. doi:https://doi.org/10.1007/978-981-10-7677-0 Ueno Y et al (1995) Biological production of hydrogen from cellulose by natural anaerobic microflora. J Ferment Bioeng 79(4):395–397. https://doi.org/10.1016/0922-338X(95)94005-C Winkler M et al (2002) Isolation and molecular characterization of the [Fe]-hydrogenase from the unicellular green alga Chlorella fusca. Biochim Biophys Acta 1576(3):330–334. https://doi.org/ 10.1016/S0167-4781(02)00239-7 Yokoi H et al (2001) Microbial hydrogen production from sweet potato starch residue. J Biosci Bioeng 91(1):58–63. https://doi.org/10.1016/S1389-1723(01)80112-2 Yu HY et al (2015) A facile one-pot route for preparing cellulose nanocrystal/zinc oxide nanohybrids with high antibacterial and photocatalytic activity. Cellulose 22(1):261–273. https://doi.org/10.1007/s10570-014-0491-0

Innovative Technologies for Biohydrogen Production at Industrial Level Dolores Hidalgo, Jesús M. Martín-Marroquín, and David Díez

Abstract

The main step towards creating a carbon-neutral society is the deployment of renewable energy sources (RES) as a substitute of fossil fuels. Even before the effects of climate change were visible on everyday life and global warming was “just a scenario,” warnings from scientists about the world’s dependence on fossil fuels undoubtedly influenced the development of RES-based technology systems. Furthermore, this situation marked a turning point for the exponential increase in the number of works and studies exploring new ideas and potential solutions to growing concerns and, consequently, the research and development of RES-based technology. But since RES are not produced continuously, energy storage has a critical role to play in this transition. Hydrogen has been recognized, from the beginning, as one of the most promising options for energy storage, both by the scientific community and by international agencies and administrations. Hydrogen, in general, and biohydrogen in particular, as a leading energy storage technology option, with zero greenhouse gas and carbon dioxide emissions, sparked curiosity and hope for future implementation. Despite the long and expensive development path towards a usable and stable technology, biohydrogen proved to be a good choice. For this reason, biohydrogen is being attributed a fundamental role in the future of energy storage these days, alone or linked to biomethane. As multiple biohydrogen applications are nowadays investigated, the current development of its technology is not always at the level of large-scale implementation. But with the growing number of works and projects initiated or foreseen in the short term, it is expected that the immense ecological potential of biohydrogen will be utilized in the coming decades. This

D. Hidalgo (*) · J. M. Martín-Marroquín · D. Díez CARTIF Technology Centre—Area of Circular Economy, Valladolid, Spain e-mail: [email protected] # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Kuddus et al. (eds.), Organic Waste to Biohydrogen, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-1995-4_8

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chapter provides an overview of innovative technologies for biohydrogen production that are being developed around the world at industrial level, also making reference to their financial viability. The future prospects of biohydrogen in the industry are also discussed. Keywords

Biohydrogen · Green hydrogen · Renewable gases · Waste-to-energy

1

Introduction

The use of fossil fuels has irrefutably led to a significant technological advance in a short time, but one of the main associated disadvantages has been the increase in pollution on such a scale that the Earth could not bear any more without considering the consequences (Hassan et al. 2021). Currently, these negative effects are perceived in many ways, the most notable of which are related to climate change. For today’s society, climate change represents the main reason for an energy transition, which is often perceived as not profitable from a financial point of view, but a necessary change for the long-term benefit of humanity (Blazquez et al. 2020). In this scenario, it is absolutely necessary to promote research in the development of new sustainable energy sources, alternatives to fossil fuels, with the aim of reducing the emission of greenhouse gases. In this context, biofuel research is considered an essential piece to achieve this goal. Both biohydrogen and biomethane are two gases that are making a strong presence in the current energy scenario (Stern 2020). Biohydrogen is most commonly defined as the biofuel or energy carrier that uses microorganisms for its generation through biological processes such as photolysis or dark fermentation in specific equipment called bioreactors. An extended definition considers biohydrogen as hydrogen which is produced biochemically, biologically, chemically, thermochemically, and biophotolytically from all biomass materials (Demirbas 2009). Biomethane is a source of energy biologically obtained mainly by anaerobic digestion, although sometimes bioelectrochemical systems are also used (Buitrón et al. 2019). Both gases have a renewable origin and their formation can be associated with CO2 capture and storage processes, another of our society’s major objectives in the fight against global warming. Biohydrogen and biomethane can “green” the energy sector, and they can do it together, as biomethane has the capacity to be used directly as a substitute for natural gas, or it can be used as feedstock to produce green hydrogen through reforming processes. On the other hand, biohydrogen can, in turn, be used directly as an energy source or transformed into biomethane by reacting with surplus CO2 from many processes through methanation reactions. This opens up a two-way street that connects both renewable gases and can help to make the energy model more flexible. But it is water clear that the success in the use of green hydrogen and biomethane will be necessarily linked to favoring the production of both energy vectors using local resources, such as biomass

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(understood in the broad sense, which includes forestry and agricultural waste, agrifood waste, manure, sewage sludge, or municipal waste) or nonrecyclable waste (such as textiles or some plastics). The potential for biomethane/biohydrogen production in the EU28, only from biomass sources, is estimated at 1150 TWh/year, when in none of the scenarios considered for 2030 or 2050 does the total gas demand in Europe exceed 4100 TWh/year, which would indicate that 28% of the natural gas consumed could be replaced by renewable sources (IEA 2021). Biohydrogen and biomethane are at the crossroads between two fundamental challenges of today’s society: the imperative to treat the huge quantities of organic waste currently produced and the societal demand to reduce global greenhouse gas emissions. A detailed, bottom-up study of the worldwide availability of sustainable feedstocks for the production of these renewable gases shows that the technical potential to produce these gases is huge and largely untapped (IEA 2020). These raw materials include animal manure, urban solid waste, wastewater, and—for direct production via gasification of these two renewable gases—crop and forestry residues (Molino et al. 2018). This analysis does not consider those feedstocks that compete with food for agricultural land. In 2018, biomethane production was estimated at 35 million tons of oil equivalent (Mtoe), only a small fraction of the calculated overall potential. On the other hand, hydrogen, as a leading energy storage technology option, with zero greenhouse gas and carbon dioxide emissions, sparked curiosity and hope for future implementation. Despite the long and expensive development path towards a usable and stable technology, hydrogen proved to be a good choice (Hetland and Mulder 2007). For this reason, hydrogen is being attributed a fundamental role in the future of energy storage. Today, around 70 Mt. of dedicated hydrogen is produced, 76% from natural gas and almost all the rest (23%) from coal using conventional reforming and gasification methods (IEA 2019). Obviously, if society relies on hydrogen for the development of a sustainable energy model, both the feedstocks and the technological processes for the production of this energy vector have to swift to more sustainable alternatives. Furthermore, the International Renewable Energy Agency (IRENA) has defined the energy transition as “a pathway toward transformation of the global energy sector from fossil-based to zero-carbon” taking place by the second half of the twenty-first century (IRENA 2021). Electrification is proving to be the driving force in the energy transition, but as argued by international energy agencies, greening the gas infrastructure is just as critical (IEA 2020), and the only two main options to decarbonize the gas supply are just biomethane and low-carbon hydrogen (CIWM 2020). The efficient use of renewable raw materials derived from biomass and waste feedstocks as a source of fuel has become a concern and, at the same time, an opportunity for today’s society, with the challenge of achieving an increasingly sustainable planet. Biomass is one of the most abundant renewable resources in all continents and this is the main reason why research into alternative uses and valorization of biomass (in particular recalcitrant biomass) is receiving more and more attention, with a focus on the potential application of waste-to-energy conversion. Huge amounts of agricultural and forestry residues and industrial and municipal organic waste or exhaust digestate are available, ready for use, anywhere in the

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world. Furthermore, modern societies and economies produce increasing amounts of nonrecyclable waste. For instance, multicomponent and inseparable materials result from the municipal waste separation process. This residual stream still contains around 24% of organic matter, 28% of paper and cardboard, 25% of plastic materials, and 9% of textiles, among other elements, but the specific characteristics of such materials currently do not enable their reuse or recycling. As a consequence, such materials were deposited in landfills (38.7%), incinerated without energy recovery (0.7%), or disposed of otherwise (6.3%) in the EU-27 in 2018 (Eurostat 2021). However, with the proper process, these materials can also be used to produce renewable gases as clean sources of energy, with multiple potential benefits for sustainable development. Focusing on biohydrogen, it is clear that, among the four strategically important alternative fuel sources, namely biofuels, natural gas, hydrogen, and syngas (synthesis gas), biohydrogen is of greatest interest, as it does not emit greenhouse gases, is renewable, can be easily converted into electricity using a fuel cell, and releases a large amount of energy per unit weight in combustion. Biohydrogen production provides clean hydrogen in a sustainable way with simple technology and more attractive potential than current chemical production of this gas, as it is suitable for the conversion of a wide spectrum of substrates, such as organic waste, process by-products from industrial manufacturing, and biomass as a raw material, mostly available at low cost or even free (Wang and Yin 2018). This chapter aims to provide an overview of the most promising methods of biohydrogen production and their development potential in the new scenario of energy transition.

2

Feedstocks Suitable for Biohydrogen Production

The feedstocks usually considered for the production of biohydrogen can be classified according to their complexity, type of material, processes necessary for its pretreatment, energy consumption for the production of biohydrogen, or production costs. The alternative sources of hydrogen (instead of traditional sources related to fossil fuels) are surplus biomass and simple inorganic compounds containing hydrogen. The biohydrogen source selection method is presented here using the example of dark fermentation. As alternative forms of biohydrogen production are limited to laboratory or small pilot scale, the search for the most suitable feedstocks continues. Selection of hydrogen-producing materials focuses on their abundance, efficiency, or production conditions (Kumar et al. 2018; Usman et al. 2019; Soares et al. 2020). Suitable raw feedstocks for dark fermentation include residual streams containing a high fraction of carbohydrates, such as cultures containing sugar and starch, starch in wastewater, only starch, chitin, hemicelluloses, lignocelluloses, glucose, cellulose, sucrose, dairy product, municipal organic waste, wastewater from the food industries, fertilizers, and compost among others (Ghimire et al. 2015). According to Sołowski (2018) for feedstocks to be suitable for commercial hydrogen production by dark fermentation, they must be of minimal costs, there should be no or little need for pretreatment, and they should have high carbohydrate content, apart from

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sufficient degree of fermentation conversion and energy recovery from the process. Kargi et al. (2012) showed that the optimal feedstock should include a high content of fats, proteins, and carbohydrates, with the mixture of whey with a rich carbohydrate content substrate being a good example. Some materials such as sucrose or glucose were used for dark fermentation under experimental conditions (sweet sorghum and sugar beets rich in sugars, or corn and wheat containing starch) (Bundhoo 2019). Other fairly efficient feedstocks for hydrogen production appear to be those related to livestock waste (Yilmazel and Duran 2021). Substrates for biohydrogen production can be classified as direct or indirect. In direct processes, biohydrogen is produced in just one stage. In indirect processes, biohydrogen is produced as a result of two or more stages, and only eventually biohydrogen is produced. In the indirect process of biohydrogen generation there is frequently a mixture of simple compounds in the last stage. Various raw materials and biohydrogen production methods are presented in Table 1. Analyzing Table 1, it can be seen that almost all the different methods use substrates dissolved in water and it is observed that traditional complex feedstocks for hydrogen production, such as oil and coal, are being replaced by indirect sources of biohydrogen, as manure or lignocellulosic feedstocks, by dark fermentation, gasification, or pyrolysis, among other methods. Industrial solid waste and lignocellulose cultures (e.g., fodder grass) are unexpensive and usually available for dark fermentation. However, they may be problematic due to the presence of lignin. Lignocelluloses are mainly composed of biopolymers: cellulose (20–50% by weight), hemicelluloses (10–40% by weight), and lignin (10–50% by weight). Cellulose is a polymer in the form of D-glucose subunits forming chains called fibrils. Hemicellulose is a complex structure composed of hexose (mannose, glucose, and galactose) and pentose (arabinose, xylose) units. Xylan is the main hemicellulose compound in straw and grasses.

3

Main Methods of Biohydrogen Production

Biohydrogen production methods began to be developed in the 1980s. Among the biohydrogen production methods, thermochemical processes are perhaps the closest to a large-scale industrial implementation. Among biochemical processes, dark fermentation is the most studied method in the last years. Starting in 2013, ideas of microbial electrolysis also appeared. In the 1990s, the greatest interest in the development of hydrogen methods was observed in Japan, Canada, and Turkey, but since the 2000s, the most active countries in the world have been India, Japan, the Netherlands, and China in developing biohydrogen production processes such as photofermentation, dark fermentation, and biophotolysis (Azbar and Levin 2012). However, most biohydrogen production methods have not been extended yet from the laboratory to the industrial scale. Biological hydrogen production is determined by a wide variety of factors: the enzymes that act in metabolism, the microorganisms involved, the source of energy, whether light or organic matter, the presence or absence of oxygen, or other parameters of process such as pH or temperature (Domenech 2020). Sometimes these processes are carried out sequentially

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Table 1 Raw materials for biohydrogen production (Adapted from Sołowski (2018)) Substrate Water

Method MEC

Type D

Acetic acid Propionic acid Lactic acid Glycerol Butyric acid

PhF PhF PhF DF PhF

D D D D D

Malefic acid Succinic acid Xylose

PhF PhF DF

D D ID

Arabinose

DF

ID

Rhamnose

DF

ID

Galactose

DF

ID

Glucose

DF

ID

Glucose

PhF

D

Fructose

DF

ID

Mannose

DF

ID

Mannitol

DF

ID

Citric acid Sucrose

PhF DF

D D

Sucrose Maltose

PhF DF

D D

Lactose

DF

D

Starch Cellulose Cellulose Livestock: Bovine manure Swine manure Sewage sludge

DF DF Pyr DF

ID ID ID ID

DF DF

ID ID

Sewage sludge

BG

ID

Remarks Potential up to 0.3 V To cover it extra power is needed H2 yield: 69 mL H2/g acetic acid (R. sphaeroides RV) H2 yield: 60 mL H2/g propionic acid (R. palustris A7) H2 yield: 138 mL H2/g butyric acid (R. monas) H2 yield: 172.87 mL H2/g glycerol (E. aerogenes) H2 yield: 209 mL H2/g butyric acid (R. sphaeroides RV) H2 yield: 100 mL H2/g maleic acid (R. sphaeroides RV) Additive fermentation to low organic acids and sugars H2 yield: 117.97 mL H2/g glucose (E. aerogenes strain HO-39) H2 yield: 120.96 mL H2/g arabinose (E. aerogenes strain HO-39) H2 yield: 69.69 mL H2/g rhamnose (E. aerogenes strain HO- 39) H2 yield: 118.218 mL H2/g galactose (E. aerogenes strain HO-39) H2 yield: 124.45 mL H2/g glucose (E. aerogenes strain HO-39) H2 yield: 33 mL H2 per g of glucose (R. palustris ATCC RV) H2 yield: 121.952 mL H2/g fructose (E. aerogenes strain HO-39) H2 yield: 121.96 mL H2/g mannose (E. aerogenes strain HO-39) H2 yield: 206.76 mL H2/g mannitol (E. aerogenes strain HO-39) H2 yield: 36 mL H2/g citric acid (R. palustris AT7) H2 yield: 109.40 mL H2/g sucrose (E. aerogenes strain HO-39) H2 yield: 33 mL H2/g sucrose (R. palustris AT7) H2 yield: 140.65 mL H2/g maltose (E. aerogenes strain HO-39) H2 yield: 37.767 mL H2/g lactose (E. aerogenes strain HO-39) Common food waste component Promising material due to its abundance Dehydration is expensive H2 yield: 58.48 mL H2/g of manure H2 yield: 209 mL of H2/g manure Pretreatment needed to prevent inhibitors from passing to digester of manure Low-temperature process but costs of drying still high (continued)

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Table 1 (continued) Substrate Wood

Method BG

Type ID

Remarks High temperature is required

MEC microbial electrolysis cell, PhF photofermentation, DF dark fermentation, BG biomass gasification, Pyr pyrolysis, D direct, ID indirect

combining them according to the needs of the microorganisms in order to maximize the production of biohydrogen. The most notable example is the combination of both fermentative production routes: a first dark fermentation stage, producing biohydrogen and volatile fatty acids from biomass, followed by a second photofermentation stage, transforming said fatty acids into biohydrogen. On the other hand, hydrogen production by thermochemical processes is also a complex process influenced by a wide variety of factors, such as the type of substrate, the thermochemical process followed, and the type of reactor used. The different methods of hydrogen production by thermochemical means can be broadly grouped into gasification processes and pyrolysis processes depending on whether it is operated in deficit or in the absence of an oxidizing agent. In both cases, once the gas is generated, it is necessary to proceed to its cleaning and to the separation and purification of the hydrogen formed. The purification and separation stage usually requires water–gas shift units to maximize hydrogen conversion, scrubber washing with different liquids and adsorbents for the removal of tar and impurities, as well as PSA units for the final purification of the H2 (Salam et al. 2018). The different routes of biohydrogen production by biological and thermochemical methods are developed in detail below.

3.1

Dark Fermentation

Biohydrogen is obtained mainly by dark fermentation thanks to the action of various microorganisms present in sewage treatment sludge similar to those used to obtain biogas. The main advantage of this production method is the wide variety of substrates that it is capable of transforming into biohydrogen: from simple sugars to lignocellulosic biomass, food waste and residual streams, or glycerol (Soares et al. 2020). The use of these substrates provides a competitive advantage to this production route with added value in terms of its role in the recovery of waste, key to the transition to a circular economy. Biohydrogen production by dark fermentation follows different metabolic routes depending on the microorganism used. The theoretical maximum at the stoichiometric level that could be obtained from the degradation of 1 mol of glucose is 12 mol of hydrogen. However, at the microbiological level, biohydrogen is actually a by-product of the fermentation of organic matter into different fatty acids, according to the specific route. The microorganisms present in the consortia that carry out this process transform sugars, preferably glucose, into different fatty acids, producing (or in some cases consuming) hydrogen. Among the global reactions that occur simultaneously, the

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production of acetic acid, butyric acid, or propionic acid stands out. Other products obtained in these processes are lactic acid and ethanol, both without consumption or production of biohydrogen, so they are not of interest in this case. The consumption of 1 mol of glucose for the production of acetic acid leads to the co-production of 4 mol of hydrogen; butyric production entails the generation of 2 mol of hydrogen, while to produce propionic from 1 mol of glucose requires the consumption of 2 mol of hydrogen. Given the great variety of microorganisms capable of producing biohydrogen by dark fermentation, it is preferable to use microbial consortia instead of pure cultures: their use implies lower operational costs by not requiring sterilization, easier handling, and accepting a greater variety of substrates (Soares et al. 2020). However, the use of microbial consortia implies the presence of hydrogenconsuming agents (propionic acid producers) that are counterproductive for the overall process, so it is necessary to carry out microbial enrichment or bioaugmentation procedures. This procedure is an evolutionary strategy consisting of subjecting the microorganisms to certain process conditions that favor the growth of those species favorable to the objective of the process, in this case the production of biohydrogen. In this way, based on the production reactions of the different organic acids, it would seek to promote the growth of microorganisms that produce acetic or butyric acid, since they generate biohydrogen as a product, and inhibit the growth of propionic producers, whose consumption of hydrogen in their metabolism is undesirable, as well as that of lactic acid or ethanol producers since the non-production of hydrogen in their metabolisms would reduce the overall performance of the process. Dark fermentation is an anaerobic process: the presence of oxygen is inhibitory for the metabolism of the microorganisms involved. These can be strictly anaerobic (they do not tolerate the minimum presence of oxygen), like different species of the genus Clostridium, or facultative (they carry out anaerobic processes but tolerate the presence of oxygen), among which several species of the genus Enterobacter stand out (Mishra et al. 2019). The use of microbial consortia with the presence of agents of both classes allows less control of anaerobic conditions, since any small leakage of oxygen to the system will be accepted and tolerated thanks to the facultative microorganisms; however, it will always be preferable to maintain the strict absence of air. Process temperature is an important factor influencing performance and biohydrogen production (Drapcho et al. 2008). In the consortia commonly used in fermentation, there is a higher proportion of mesophilic microorganisms, so the start of the process is usually slower working under these temperature conditions, although a higher yield of hydrogen production is observed compared to substrate when it is carried out under thermophilic conditions (70
C) (Łukajtis et al. 2018). The choice of the process temperature is subject to various factors that must be studied: characterization of the substrate and microbial biomass, economic aspects (high temperatures require a higher energy input and therefore higher operational costs), type of operation (continuous or discontinuous), etc. The pH of the fermentation medium directly influences the metabolism as well as the activity of the enzymes involved in the process. In general, the proper pH range for dark fermentation is 5–7. Although the most acidic pHs (below 5) inhibit the growth of

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hydrogen-consuming methanogenic microorganisms, which would benefit performance, they have the disadvantage of limiting the ability of bacteria to control the intracellular environment, so they are not suitable for the production of biohydrogen (Łukajtis et al. 2018). The simultaneous production of organic acids during dark fermentation tends to decrease the pH as the process progresses, so it is necessary to maintain adequate control of it, either by means of regulating solutions at the beginning of the process or by adding base to measure lower the pH. In general, the most commonly used process conditions in dark fermentation are 37
C (mesophilic conditions) and an initial pH of 7.0. The partial pressure of hydrogen affects the performance of the process since, with a greater presence of hydrogen, the reactions in which it is produced are thermodynamically disadvantaged as the ferredoxin oxidizes and loses its capacity (Łukajtis et al. 2018); therefore the metabolism tends to turn to other routes such as the formation of ethanol or lactic acid. To solve this problem, it is necessary to extract hydrogen as it is formed, either by vacuum or by washing with another inert gas such as nitrogen, in order to reduce its partial pressure and reduce its inhibitory capacity (Soares et al. 2020). In recent years, dark fermentation has established itself as the most promising biological hydrogen production method, motivated on the one hand by its higher productivity and biohydrogen yield and on the other by its greater versatility when treating different substrates (Show et al. 2019). The simultaneous production of volatile fatty acids in the process, which are the main substrate of photofermentation, provides a new way of study: the sequential processing of biomass, first by dark fermentation obtaining biohydrogen and fatty acids, followed by a photofermentation stage ending to take advantage of the resulting organic matter to produce a greater quantity of the desired product (Mishra et al. 2019). Özgür et al. (2010) obtained a yield of 6.85 mol of biohydrogen per mol of hexose consumed (57% of the theoretical maximum) in a sequential process (dark fermentation followed by photofermentation) from beet molasses: this result is especially striking if it is compared with the theoretical maximum of 4 mol/mol obtainable by dark fermentation of sucrose with acetic acid as the main product. Zhang et al. (2018) carried out a pilot-scale trial (11 m3) of dark fermentation followed by photofermentation using agricultural residues from corn, obtaining promising results of 59.7 m3 H2/day, with a view to enhancing production by addressing problems of transfer of matter arising from working in larger reactors.

3.2

Photofermentation

Photofermentation is the process of transforming organic matter into biohydrogen using sunlight as a catalyst for metabolism, so it is an intermediate method between the photobiological pathway and the fermentation pathway. The microorganisms responsible for this process are mainly purple bacteria (PNS, from purple non-sulfur) of the Rhodopseudomonas and Rhodobacter genera, heterotrophic organisms that require a carbon source as an electron source in contrast to the autotrophs that carry out the biophotolysis (Drapcho et al. 2008). The main source of electrons used in this

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process is organic fatty acids following anaerobic mechanisms (absence of oxygen) (Show et al. 2019), which provides an advantage over previous methods in the versatility of raw materials, with these fatty acids being an abundant resource present in waste streams. It is a non-spontaneous reaction that requires an external energy supply, with the PNS bacteria resorting to light to obtain this energy. In this process, the enzyme responsible for the production of hydrogen gas is nitrogenase, which transforms nitrogen into ammonia by capturing protons and releasing molecular hydrogen in the process. Nitrogenase is inhibited by the presence of ammonia and oxygen. Contrary to the biophotolysis mechanisms, oxygen is not generated during the process, so its presence depends exclusively on the extracellular environment (anaerobic conditions are needed) and on cellular metabolism (having a robust respiratory system that allows oxygen to be rapidly reduced molecularly) (Rubio and Ludden 2008). On the other hand, ammonia is formed as a product of nitrogenase activity in the presence of nitrogen; therefore, in addition to anaerobic conditions, it is necessary to maintain nitrogen-limiting conditions throughout the process (Akkerman et al. 2002). The electrons necessary for the action of these enzymes come from the oxidation of the organic substrate through the Krebs cycle or from tricarboxylic acid cycle (TCA), where protons and CO2 are released along with electrons. These are transported to nitrogenase from consecutive oxidation/reduction reactions using electron carriers such as ferredoxin or nicotinamide adenine dinucleotide (NAD) (Hay et al. 2013). The organisms that carry out photofermentation are fundamentally mesophilic: their optimal growth temperatures range between 31 and 36
C (Hay et al. 2013). Photofermentative processes require a neutral or slightly basic pH between 6 and 9, using buffer solutions, and some studies point to an optimal microbial concentration of 0.5–0.7 g/L (dry matter) (Kayahan et al. 2017). By requiring a constant supply of light, photobiological reactors require large surfaces, resorting to both tubular and panel reactors, arranged in a way that takes maximum advantage of sunlight (Grabarczyk et al. 2019). The preferred substrate of PNS bacteria is volatile fatty acids such as acetic, butyric, or malic (Hay et al. 2013), although it has been possible to produce biohydrogen by photofermentation from other substrates rich in sugars such as molasses obtained during the production process of sugar (Kayahan et al. 2017; Grabarczyk et al. 2019). The main interest of photofermentation lies in its potential application as a continuation of dark fermentation. To date, biohydrogen production has not been scaled up beyond small pilot-scale experiments. James et al. (2016) carried out a study in 2016 evaluating the economic viability of the industrial production of biohydrogen from dark fermentation (among other methods): It is currently estimated that this route involves a cost of more than $50/kg H2, in light competitive disadvantage compared to other gaseous fuels such as natural gas ($0.134/kg), hydrogen produced by methane reforming ($1–2/kg), or hydrogen produced by electrolysis ($4–6/kg) (Soares et al. 2020). This study includes an estimate of the price of said biofuel if certain technological advances are carried out until 2025, reducing the cost to $5.65/kg, considering improvements in the biomass product conversion performance, concentration of biohydrogen in the broth of fermentation, or use of the by-products generated. Regarding sequential

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processes, in the study by Grabarczyk et al. (2019) where the economic viability of biohydrogen produced by thermophilic dark fermentation followed by photofermentation was analyzed, a price of €33/kg H2 was obtained, with forecasts of a potential reduction of €12/kg.

3.3

Biophotolysis

Biophotolysis is the biological process of hydrogen production from the hydrolysis of water by photosynthesis. It is, therefore, a mechanism analogous to the production of green hydrogen by electrolysis of water using sunlight instead of renewable energy as a catalyst for the reaction, and photosynthetic autotrophic microorganisms instead of electrodes as mediators. Depending on the mechanism that this reaction follows, a distinction is made between direct and indirect biophotolysis. Direct biophotolysis consists of the breakdown of water into oxygen and protons, carried out by photosynthetic microorganisms such as cyanobacteria (genus Anabaena, among others) and algae (the species Chlamydomonas reinhardtii stands out) (Yu and Takahashi 2007; Drapcho et al. 2008). The protons released in this reaction are reduced to hydrogen gas, thanks to the action of the iron-dependent hydrogenase enzyme (Fe-hydrogenase) present in the chloroplasts of these microorganisms. The biggest problem in this process arises due to the simultaneous production of oxygen, since Fe-hydrogenase is strongly inhibited by its presence (Drapcho et al. 2008), so it is necessary to extract oxygen from the system as it is being formed. The use of water as raw material for the process provides a great advantage given its abundance and low cost; however, there are certain drawbacks such as the high reactor surface required by these systems to take advantage of as much sunlight as possible, or the problems associated with the inhibition caused by the oxygen produced in the system (Show et al. 2019). As a solution to these problems, several options are currently under investigation (Khosravitabar 2020): (a) reduce the presence of sulfur (a common nutrient in biological processes) to inhibit oxygen-producing metabolism and enhance anaerobic metabolism; (b) genetic engineering to obtain microorganisms that are more resistant to the inhibitory effect of oxygen; and (c) the use of co-cultures to mitigate this effect by promoting the growth of H2-producing microorganisms. On the other hand, indirect biophotolysis consists of the breakdown of the water molecule to obtain biohydrogen in two stages: a first stage of production of organic matter in the form of carbohydrates capturing carbon dioxide as substrate (light phase of energy storage), and a second stage under oxygen-limiting conditions for the production of hydrogen from the sugars produced (dark phase of microbial growth) (Drapcho et al. 2008). Again these processes require large reactor surfaces to take full advantage of the incidence of light. It is estimated that, at maximum production, almost 20 kg of hydrogen gas can be produced per 1000 m2 of cultivation surface, although to date it has not been possible to use more than 10% of the photosynthetic capacity of algae for the production of biohydrogen by indirect biophotolysis (Show et al. 2019). Research in this line is focused on genetic

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engineering to enhance this ability, study of processes under nitrogen-limiting conditions to redirect metabolism to hydrogen production versus growth (Vargas et al. 2018), or glucose supplementation to increase production in the second stage of the process (Vargas et al. 2020). The greatest attraction of indirect biophotolysis is its ability to use carbon dioxide as a substrate, being able to function as a carbon capture stage in processes with such emissions, for example, in the production of gray hydrogen to revalue it as blue hydrogen (Khosravitabar 2020). However, the production of biohydrogen by photobiological route using autotrophic microorganisms results in lower production levels compared to the fermentation route, this line being the one that concentrates more studies given also its greater versatility in terms of the raw materials used in the process (Drapcho et al. 2008).

3.4

Biomass Gasification

Gasification is based on the partial oxidation of carbonaceous material into a mixture of gases such as methane, hydrogen, higher hydrocarbons, carbon dioxide, carbon monoxide, and nitrogen, also known as synthesis gas or “syngas” (Demirbas 2006). The gasification process generally shows low thermal efficiency, since part of the energy is devoted to vaporize the moisture contained in the biomass. It can be carried out with or without the use of catalysts, reaching higher efficiencies in the latter case (Asadullah et al. 2002). The types of reactors commonly employed are fixed bed, fluidized, and entrained. The addition of steam and/or oxygen in the gasification process results in the production of hydrogen-rich syngas whose content can be further increased with the use of a WGS reactor (Asadullah et al. 2002). However, the gasification process provides significant amounts of tars (a complex mixture of high-molecular-weight aromatic hydrocarbons) in the product gas even operated in the range of 800–1000
C (Kalamaras and Efstathiou 2013). Another type of gasification technology suitable for hydrogen production is plasma gasification. Plasma gasification is a thermal process in which the material is subjected to very high temperatures (approximately 2000–4000
C) of plasma. Plasma is the fourth state of matter, which is obtained by breaking down atoms and molecules into constituent ions and electrons after electrifying a gas (Ibrahimoglu and Yilmazoglu 2020). The main advantages of plasma gasification are that its temperature profile is high and it has high-intensity nonionizing radiation (Farzad et al. 2016), which favors tar removal by thermal cracking. In addition, it can gasify feed material containing high moisture content, and it is less demanding in relation to the size and structure of the material to be gasified (Ibrahimoglu and Yilmazoglu 2020). On the other hand, biomass with high moisture content and liquid biomass itself can be treated by supercritical water gasification (Sikarwar et al. 2016). Supercritical gasification is a process for biomass conversion that takes place at relatively lower temperatures (around 600
C) but significantly higher pressures (approximately 30 MPa) than conventional gasification without water (800–1200
C). Supercritical gasification has a high solid conversion efficiency (over 99%) and the gas produced

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contains a high concentration of hydrogen gas (up to 50%) with suppression of carbon and tar formation (Salam et al. 2018).

3.5

Biomass Pyrolysis

Pyrolysis is a thermochemical decomposition method that allows the transformation of biomass or carbonaceous waste into biochar, bio-oil, and syngas under oxygenfree conditions (Lam et al. 2018). It is an endothermic process that requires thermal energy to reach temperatures within the range of 300–900
C and thus favors the thermal decomposition of carbonaceous materials (Miandad et al. 2016). Advanced pyrolysis techniques have been developed to improve the existing pyrolysis process, allowing for example to reduce the reaction temperature with the aim to increase energy efficiency and decrease production cost, and increase yield and product quality, reducing for example the secondary reactions. These include catalytic pyrolysis, microwave pyrolysis, vacuum pyrolysis, solar pyrolysis, and plasma pyrolysis (Foong et al. 2020). Hydrogen production by pyrolysis is favored with increasing temperature above 500
C, especially for biomass with high lignin content. On the other hand, a particle size below 2.0 mm improves heat and mass transfer during biomass pyrolysis maximizing H2 yield (Weerachanchai et al. 2011). Secondary pyrolysis reactions are favored by long residence times at high temperatures, leading to higher gaseous products. The use of Ni-based catalysts and zeolite-based catalysts has been shown to possess great potential by allowing lower operating temperatures compared to non-catalytic processes which improves process efficiency (Uddin et al. 2013). However, the catalytic activity of most catalysts tested is halved in less than 3 h (Chen et al. 2021).

4

Combine Production of Biohydrogen and Biomethane

As mentioned in previous sections, biohydrogen and biomethane are different gaseous energy vectors with different applications, but they both can originate (simultaneously or in different processes) from a range of organic and nonrenewable feedstocks whose potential is underutilized today. The joint production and use of these gases point the way towards a more circular economy, bringing benefits from improved waste management, reduced emissions, and greater resource efficiency. Furthermore, this joint production and use of both biogases could be a good strategy to promote the industrial implementation of biohydrogen, which is still in a nascent state, taking advantage of the pull of a gas with greater deployment such as biomethane. Biomethane and biohydrogen also provide a way to integrate rural communities and industries into the transformation of the energy sector, contributing to social equality and fight against rural depopulation through a positive impact on employment and rural economy (Kleperis et al. 2021). Various thermochemical and biological processes, some of them well known and others emerging, can be applied individually to produce biohydrogen or biomethane from biomass and waste

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streams. However, in the current global energy scenario, those combinations of technologies that give versatility to the energy system, that allow moving from one energy model to another and that allow the use of current energy infrastructure, would be especially interesting. In the case of biohydrogen and biomethane this could be a reality. The two gases are not mutually exclusive; on the contrary, biomethane could be used as a raw material to generate biohydrogen. The main technique to achieve this goal is steam reforming, which splits the molecule of methane into hydrogen and carbon dioxide. However, new options not considered up to now with potential for an enhanced efficiency are possible. For example, using an iron oxide as a catalyst, biomethane could be converted into renewable hydrogen, separating the carbon generated as subproduct to produce graphite (ARENA 2019). In addition, dry reforming of methane (DRM) involving the catalytic reaction of CO2 and CH4 could be an interesting process that acts as a carbon dioxide sink while generating hydrogen (Ranjekar and Yadav 2021). Despite its inherent benefits, both economic and environmental, DRM is still in its nascent stage (Aramouni et al. 2018). The main obstacle to industry-wide application of DRM is coke formation and sintering, which rapidly deactivate catalysts (Arora and Prasad 2016). On the other hand, and in order to demonstrate that these two energy vectors are two sides of the same coin, hydrogen could also react with CO2 to produce methane via the Sabatier reaction (i.e., methanation). This process of producing methane by the reaction of H2 and CO2 is one of the most interesting and promising processes to overcome the major difficulties associated with the large-scale transport, storage, and use of hydrogen (Hidalgo and Martín-Marroquín 2020). Syngas biological or chemical methanation is also a possibility that could transform renewable energy into feasible transport and high-density energy (Ren et al. 2020). Biological methanation, also called biocatalytic methanation, is more tolerant to the presence of impurities in the feed gas and less energy demanding than chemical (catalyzed) methanation. Sulfur-containing materials are a well-known poison for nickel catalyst (Voelklein et al. 2019) and hydrocarbons can decompose at temperatures above 500
C leading to catalyst deactivation (Neubert et al. 2017). However, the microbial biomass of the biological reactors can adapt to the presence of impurities and adverse conditions, keeping their overall performance unchanged (Lecker et al. 2017). On the other hand, the performance of biocatalytic process is still limited by the very slow microbial kinetics, poor mass transfer of H2 and CO (with Henry’s law constants of 50 and 40 at 37
C), and limited knowledge of the optimal bioconversion pathways (Ghaib and Ben-Fares 2018). This requires the engineering of novel gas-phase bioreactor configurations and the optimization of operational conditions to sustain high-performance microbial communities capable of transforming syngas into high-quality biomethane. In turn, syngas can be produced from any hydrocarbon feedstock either by gasification of agricultural or forestry biomass, wet biomass hydrothermal carbonization, or pyrolysis of carbon-based waste streams (El-Nagar and Ghanem 2019). For low-moisture-content feedstock, the pyro-gasification process is especially interesting (GRDF 2020). Pyro-gasification involves the thermochemical processing of many different sources, such as wood waste, residues from waste management,

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and most organic waste. In a study from 2018, ADEME (the French Environment and Energy Management Agency) forecasts that the process will produce large quantities of zero-carbon renewable gas to replace natural gas, both in uses such as mobility and in the grids. The main products obtained from this process are biochar, bio-oil, and syngas (García Martín et al. 2020). The performance, compositions, and distributions of the products depend on the pyro-gasification conditions, mainly on factors such as the nature of the feedstock (starting raw material), previous treatments, size of the particle, type of reactor, highest temperature of reaction, heating rate, residence time, pressure, and use of a mineral catalyst (Bhattacharjee and Biswas 2019). The liquid part, bio-oil, is composed of a very complex mixture of oxygenated derivatives from hydrocarbons. Bio-oil has a heating potential to be used as a cogeneration energy source. Non-condensable gases, typically called syngas, are a mixture of carbon monoxide (CO) and hydrogen (H2). Syngas calorific values range from 4.37 to 5.68 MJ/m3 (Solarte-Toro et al. 2018). Finally, biochar is a nonvolatile solid waste rich in carbon, composed of the remaining biomass that is not hydrocarbons, mainly parts of lignin, heavy metals, and oxides (commonly metallic), depending on the composition of the raw material. It is used as fuel, as an alternative adsorbent to remove different kinds of contaminants, as amendment to improve soil fertility, and as raw material for preparing activated carbon (Hu et al. 2021). The hydrothermal carbonization (HTC) is considered particularly suitable for the treatment of biomasses with high moisture content, thus avoiding the cost of drying prior to thermal treatment. The HTC process is directed by hydrothermal parameters like temperature, residence time, pH, and water content, which control the reaction intensity and coalification degree of the raw biomass (Khan et al. 2019). Research has shown that the liquid products obtained as a by-product of HTC treatment are biodegradable due to their content in organic substances (Tommaso et al. 2015). Also, the acidic conditions during the HTC promote the solubilization of the N (up to 63.4%) and P (up to 100%) into the process water during the HTC process (Dai et al. 2017). Therefore, the treatment of the liquid fraction by an anaerobic digestion process could be a very promising option to increase the level of biogas generation. The charcoal or hydrochar obtained can be used as fuel or fertilizer, since it can retain up to 50% of the N and P content of the starting organic material. On the other hand, many works have found that co-HTC, compared to single feedstock, improves chemical and thermal performance, and final and proximate properties, produces higher heating value (HHV), and improves overall hydrochar quality (Bardhan et al. 2021). In this sense, the co-HTC of blends of digestate can be a very efficient technique that helps the treatment and promotes its biodegradability and carbonization. Furthermore, the produced syngas from thermochemical processes should be submitted to syngas conditioning and cleaning before methanation process (Ren et al. 2020). Except for H2-rich syngas, the synthetic gaseous products also contain traces of H2S or HCl, among other components, that should be removed to prevent microbiota inhibition in methanation reaction (Giwa et al. 2019). The variability of the feedstock used for producing the syngas has to be taken into account in order to

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identify the potential formation of contaminants. The energy efficiency can be increased by improving char conversion in the pyro-gasifier or by reducing the energy losses during the process of syngas conditioning. The capital investment can be reduced by eliminating some process units or reducing their size and complexity (Haro et al. 2016). In this line, using integrated cleaning and conditioning systems helps in the economy of the whole process, minimizing energy losses. But, apart from the need of more efficient processes, the main barriers to hydrogen adoption lie in the end-user equipment and distribution infrastructure, which will need to be adapted to make them suitable for hydrogen transport and use. On the other hand, biomethane is not distinguishable from natural gas, so it can directly replace fossil gas without the need to modify the existing gas infrastructure. In fact, biomethane is already injected into the gas grid. Therefore, it is not unreasonable to think of a biomethane production and transport model using the existing gas infrastructure and transforming this gas into hydrogen at its destination using new reformers that adapt to this model. Also the transportation of H2/CH4 mixtures is a possibility today, but the injection of renewable H2 into natural gas (or biomethane) grid might require a cost-effective H2/CH4 separation at the consumption site. In this context, gas separation membranes have emerged as an attractive technology based on its high efficiency, versatility, modular design, and absence of chemical reagents’ demand (Fernández-Castro et al. 2021). However, the performance of state-of-theart membranes fabricated from conventional commercial polymers is still lacking in meeting industrial demands. As a consequence, the design of new polymers with a superior performance in terms of permeability–selectivity compromise has become the focus of much research in order to improve hydrogen separation (Robeson 2008). In the last few years, polymer blending and formation of mixed matrix membranes (MMMs) have emerged as a strategy to manufacture high-performance membranes for gas separation (Yong and Zhang 2021). The polymer blending approach is based on the creation of an integrated membrane, combining the advantages of two compatible polymers, with new potential benefits. On the other hand, MMMs are heterogeneous materials comprising solid fillers uniformly dispersed in a continuous polymer matrix, exploiting in synergy the advantages of polymers in mechanical stability, selectivity, easy processability, and low cost, with the strength of dispersed porous networks in terms of gas separation performance, as they act as molecular sieves enhancing simultaneously the gas permeability and selectivity. Both strategies could lead to hybrid membranes that reveal outstanding properties, showing promising potential for their use in hydrogen recovery.

5

Assessing the Sustainability of Biohydrogen

The sustainability of biohydrogen production is driven by the production rate and its affordable cost, and depends on production efficiency and purity. Physicochemical methods stand out for their high efficiency, both in terms of productivity and hydrogen purity, but they are limited by their high energy demand, which makes production more expensive. Because of this, the use of biological methods for

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hydrogen production has received significant attention in recent decades, as these processes are carried out under mild conditions and their energy demand is much lower than that of physicochemical processes, making them potentially costeffective processes. There are other factors that also focus the attention of researchers and industry on biohydrogen production, such as the use of organic waste as a starting feedstock, but this material, given its heterogeneous nature, entails the use of advanced technologies to operate safely and convert it into biohydrogen in an environmentally acceptable way (Argun et al. 2017). The criteria for assessing the sustainability of biohydrogen refer to three aspects, including environmental issues, economics, and social performance (Rathore et al. 2019). Sustainability generally refers to the simultaneous achievement of economic prosperity, environmental cleanliness, and social parity (Ren et al. 2013). Several research studies have been conducted to improve biohydrogen production and develop an ad hoc economy. However, it is generally accepted that the development of biofuel industries is a complex process in which, in addition to experimental studies, other non-technological aspects have to be taken into account, such as the economic, social, and environmental aspects associated with the energy model of each country or region. The energy efficiency and GHG emissions of biohydrogen are favorable when compared to natural gas and other H2 production routes. What is clear is that the energy share of biohydrogen production models must be positive for sustainable substitution of fossil sources or conventional H2 production processes (as methane reforming) to become a reality. Consequently, biohydrogen is worth considering in planning and developing an H2 economy, not only from an energy but also from an environmental perspective (Djomo and Blumberga 2007).

5.1

Economic Feasibility of Biohydrogen

A detailed financial viability analysis developed by Lee (2016a) indicated that an attractive business plan or investment proposal for biohydrogen is essential to attract investment for the production of this renewable gas in the long term. All financial indexes revealed that the investment in biohydrogen is economically viable and bio-H2 will be commercialized successfully before the deadlines set in many official reports. Lee’s results showed that decision criteria should all include financial incentives. This author also showed that the levelized cost of energy of biohydrogen has low sensitivity to the cost of biomass feedstock, but high sensitivity to the cost of capital, as well as operation and maintenance costs. The same author in another study (Lee 2016b) showed that the economic feasibility of a potential scenario in which biohydrogen and biobutanol replace fossil fuels was high. Biohydrogen has the greatest flexibility of all the fuels studied in the face of variations in the cost of biomass feedstock production. However, biodiesel is less economically competitive than biohydrogen and is also uncompetitive with biobutanol, but all three biofuels are cost competitive when compared with fossil fuels. Lee and Chiu (2012) analyzed the evolution of the biohydrogen sector in four countries: Japan, the United States, India, and China. Their study pointed to China as the largest biohydrogen market

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with the highest total production multiplier by 2050, followed by the United States, Japan, and India (in that order). A high investment will contribute to the rapid development of the biohydrogen sector in all the countries studied. The authors estimated that investing $1 in the biohydrogen industry will generate a total output of $3.22, $3.50, $3.09, and $3.00 in the four economies, respectively, by 2050. The analysis also showed that investment in hydrogen infrastructure will provide benefits, but not as many as investment in biohydrogen technology development. Han et al. (2016) performed a techno-economic analysis for the production of fermentative hydrogen from food waste. The study showed a return on investment (ROI) of 26.75% and an internal rate of return (IRR) of 24.07% within the 5-year payback period.

5.2

Social Analysis of Biohydrogen

The social impact of production and use of biohydrogen is less quantified due to the complexity of the analysis, although some reports suggest an advantage of biohydrogen over other fuels (Rathore et al. 2019). Ren et al. (2013) proposed ten criteria for evaluating the social impact of biohydrogen in a sustainability study. The criteria were inherent safety index, occupational index, employee safety, social attractiveness, tax contribution, contribution to GDP per capita, political acceptability, cultural influence, contribution to energy sufficiency, and security of primary supply. Sun et al. (2010) quantified the potential social benefits of hydrogen fuel cell vehicles using the lifetime social cost. The study included the vehicle’s purchase cost, operation and maintenance costs, cost of energy use, externality costs of oil use, noise damage costs, and emissions of air pollutants and GHG. The results of the study showed that the cost difference between hydrogen vehicles and gasoline vehicles is initially very large, but the former become cost competitive with gasoline vehicles for life, as their production volume increases, even without taking into account externalities. The high valuation of externalities and the high price of oil could reduce the purchase cost. Ogden et al. (2004) analyzed the hydrogen fuel cell car and concluded that it has the lowest externality costs of all the options studied, and when mass produced, the valuation of externalities increases, making the projected life cycle cost even lower. These costs are estimated over the entire useful life of the vehicle and include adjustments, such as fees and taxes, and the producer’s overheads related to fuel and vehicles. In another work, Stanislaus et al. (2017) have investigated biohydrogen production using herbaceous plants as feedstock and digested sludge as inoculum. These authors observed that the energy produced in the fermentation process was higher than the energy consumed in the process, showing a positive net energy balance (NEB). The biohydrogen generation process therefore achieves, with very low net demand for nonrenewable energy, a negative impact on global warming. This indicates that biohydrogen can be produced with positive NEB, which could be a sustainable approach. Singh et al. (2016) reviewed published research papers and concluded that hydrogen is the safest fuel due to its dispersive and nontoxic nature and the lower hazard it creates in terms of fire risk.

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Environmental Safety Through Biohydrogen

Romangoli et al. (2011) conducted a life cycle analysis of biohydrogen production using the photosynthesis method, and the results of the study showed that the use of biohydrogen to produce electricity is environmentally preferable to the use of a nonrenewable source based on fossil fuels. Wulf and Kaltschmitt (2013) estimated that a total of 29.9 Mt. CO2-eq could be reduced by using a hydrogen fuel cell vehicle instead of a gasoline vehicle over the 15-year service life. The Djomo and Blumberga life cycle study (Djomo and Blumberga 2011) compared the energy and environmental hydrogen yields of sweet sorghum stalk, wheat straw, and steamed potato skins, and found comparable energy ratios between the three raw materials used, that is, 1.14, 1.08, and 1.17, respectively, and GHG savings of 52–56% compared to diesel and 54–57% compared to H2 production from steam methane reforming. Dadak et al. (2016) conducted an exergy study and concluded that the eco-exergy concept could provide a differentiating insight beyond the traditional exergy study, providing a useful decision tool for photobiological hydrogen production. Wulf et al. (2017) have conducted a life cycle analysis comparing the performance of different processes for biohydrogen production in order to examine environmental impacts, such as acidification, eutrophication, human toxicity, and anthropogenic climate change. They evaluated biohydrogen production from biomass sources derived from short rotation forestry and scrub, energy crops, herbaceous biomass, and biowaste. They reported that the origin of biomass has a significant influence on the overall environmental impact of different biohydrogen production pathways and demonstrated that biomass reforming and gasification have the potential to be climate neutral. They also reported that methane steam reforming technology is the most interesting, from the environmental point of view, at the current stage of technology development.

6

Research Trends in Biohydrogen Production

In 1979 the term “biohydrogen” began to be used to describe the hydrogen generation processes in which living organisms or biomass was involved (Barrón et al. 2019). During four decades, the proposals for technologies developed in order to produce biohydrogen for industrial and energy purposes have evolved through different scientific and technological trajectories. In the 1980s, photofermentation and dark fermentation processes were already considered as the technological options, in early development, that were listed as the most promising for the largescale production of biohydrogen by biological processes (Vatsala and Seshadri 1985). The biomass gasification process was also highlighted as one of the most promising (Lucchesi et al. 1988; Pérez 2007). Recently, a study of the analysis of scientific articles published from 1984 to 2018 was published, where the processes of photolysis, photofermentation, dark fermentation, and hybrid processes (dark fermentation + photolysis) are identified as the main processes under development, with the last two being the most efficient in the production of biohydrogen by

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biotechnological processes at the laboratory level (Rathore et al. 2019). It should be noted that to date no industrial scale prototypes have yet been reported; therefore, the production of biohydrogen is still an emerging technological issue, which requires strong efforts for its possible implementation and adoption (Barrón et al. 2019). Biohydrogen production process for energy purposes can be divided into five stages: (1) classification of the raw material, (2) pretreatment, (3) process, (4) conditioning or posttreatment, and (5) energy production. Barrón et al. (2019) analyzed the temporal evolution of the patent applications of each one of the indicated process stages. The type of process carried out to obtain biohydrogen turns out to be the most “relevant” stage in most of the patents identified, since 70.8% highlight and specify details about obtaining this gaseous biofuel. The classification of the raw material is in second position, that is, 68.41% of the patents specify and highlight the specific use of one or more types of raw material to obtain biohydrogen. However, in the remaining 31.59% it is not specified, which may be due to the generality of the process described in these patents. The type of final energy obtained from this biofuel is in the third position. From the patent analysis carried out, it can be observed that there is a similar number of patents that address the use of a raw material as those of the process. However, while there is a wide range of options in raw materials, process patents accumulate mainly in fermentation processes, where the main technological trajectories are divided into photofermentation and dark fermentation. However, photolysis and microbial electrolysis technologies are in earlier stages of development and there is less evidence that the concept could work on a larger scale. In relation to the raw material pretreatment processes, there are a very small number of patents that provide information in this regard. However, thermal and mechanical pretreatment processes are relatively easy to couple, while chemical processes involve acidic or alkaline treatments. Biological or enzymatic pretreatment processes are still in the early stages of development and would imply high operating costs in the coming decades, so they are still considered nonviable on a large scale. In relation to the systems for conditioning the gaseous streams derived from the biohydrogen production process, there is also a very small number of patents providing information in this issue. It is essential to remember that the processes of transition and technological adoption are long-term processes where communities interested in the scientific and technological development of a certain subject intervene and require the development of the necessary synergies to convince economic, political, and social actors that technological solutions require a process of scaling and maturing to be able to develop their capabilities in real environments and subsequently in the face of market conditions. Currently, an international situation is observed where the industrial demand for hydrogen has grown and is expected to continue growing, in addition to the search to become an energy alternative for transport, which would cause the need to significantly increase hydrogen production in the face of greater demand from the market (IEA 2019). In line with this scenario, the production of biohydrogen has the possibility of attracting resources for the scaling up of processes at an industrial level, to the extent that it can sustain a good cost/benefit ratio that includes the reduction of gas emissions with greenhouse effect and sustainability.

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Conclusion

Biohydrogen production can provide clean H2 by using simple technology, and its sustainability gives it more attractive potential than current H2 chemical production. Although current hydrogen production industries are based on chemical processing units, the research trend on biohydrogen production promises a booming potential for industrial biohydrogen production in the near future. Until now, the biohydrogen production systems are suitable for small-scale decentralized units, integrated with waste processing facilities from agriculture and industries. Hydrogen production using biological tools is the predominant challenge for researchers, in terms of process efficiency and production costs. The future of biohydrogen production is influenced not only by advances in research, including the genetic engineering of microorganisms to improve the efficiency and new reactor design, but also by the global fuel economy, social adaptation, and development of models and infrastructure for hydrogen use. Current strategies aimed at improving biohydrogen production include immobilization of microbial cultures, modifications of reactors, optimization of process conditions, selection and enrichment of cultures, choice of substrates, and metabolic engineering. The cost factor is a determinant aspect for the sustainability of biohydrogen production. The scientific community agrees that the profitable production of biological hydrogen with a positive net energy balance is the main characteristic to obtain a global deployment of biohydrogen. Acknowledgements The authors gratefully acknowledge support of this work by CYTED (IberoAmerican Program of Science and Technology for Development) in the frame of the H2TRANSEL network (Ref. 721RT0122) and by the CDTI-Spanish Ministry of Science and Innovation in the frame of the project H24NEWAGE (Ref. CER-20211002).

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Thermochemical Conversion of Lignocellulosic Biomass for Biohydrogen Production Hortência E. P. Santana, Brenda L. P. Santos, Daniel P. Silva, Isabelly P. Silva, and Denise S. Ruzene

Abstract

With the increase of global warming and the challenge to achieve net-zero carbon emissions, biohydrogen has been considered one of the most promising alternatives for replacing conventio1nal fossil fuels. The hydrogen from renewable resources plays an important role in the energy system transition, not only for providing a clean and decarbonized scenario but also because it can be produced by using several waste valorization pathways, which include the gasification of lignocellulosic biomass. Gasification is a thermochemical process performed at high temperatures to convert organic matter into syngas, a fuel gas mixture of higher added value, mainly composed of hydrogen and carbon monoxide, and very often carbon dioxide. This route, among others, has gained particular attention due to its flexibility in employing different types of biomass, especially the residues resulting from agricultural and agro-industrial activities worldwide. Besides the low environmental impact and the economic benefits of using an inexpensive and readily available feedstock, the gasification of these wastes also H. E. P. Santana Graduate Program in Biotechnology, Federal University of Sergipe, São Cristóvão, SE, Brazil B. L. P. Santos Northeastern Biotechnology Network, Federal University of Sergipe, São Cristóvão, SE, Brazil D. P. Silva (*) · D. S. Ruzene Graduate Program in Biotechnology, Federal University of Sergipe, São Cristóvão, SE, Brazil Northeastern Biotechnology Network, Federal University of Sergipe, São Cristóvão, SE, Brazil Center for Exact Sciences and Technology, Federal University of Sergipe, São Cristóvão, SE, Brazil I. P. Silva Center for Exact Sciences and Technology, Federal University of Sergipe, São Cristóvão, SE, Brazil # The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 M. Kuddus et al. (eds.), Organic Waste to Biohydrogen, Clean Energy Production Technologies, https://doi.org/10.1007/978-981-19-1995-4_9

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has the advantages of not necessarily requiring the pretreatment of the raw biomass and demanding less operating time. Thus, the current study provides a further understanding and prospect of hydrogen production via biomass gasification, focused on the valorization and potential of lignocellulosic residues within this scenario. Keywords

Biohydrogen · Lignocellulosic biomass · Thermochemical processes

1

Introduction

Hydrogen (H2) is expected to play a vital role in the future energy system. Nowadays, nearly all of the hydrogen produced is used in oil refining, and ammonia and methanol production; nonetheless its outstanding properties, such as high energy per unit of mass, non-toxicity, long-term storage, and transportation, besides no harmful gas emission on the environment, make it an ideal energy carrier and fuel (Rosen and Koohi-Fayegh 2016; IEA 2019). Although the pure form of H2 is not readily available in nature, it is the most abundant element on earth and can be extracted or produced at an industrial scale from various materials and compounds (DOE 2020a). Additionally to the already existing industrial applications, H2 has an excellent potential to supply or complement a wide range of markets, especially those dominated by fossil fuels (DOE 2020b). This element can be efficiently used as fuel, provided its patchable power and heat required in transportation, commercial, industrial, and residential sectors, creating opportunities for sustainable economic growth and allowing distribution of energy across sectors and regions (Hydrogen Council 2017; Pivovar et al. 2018; IEA 2020). Besides being versatile, abundant, and safe to handle, the only by-product generated from hydrogen consumption is water (Manoharan et al. 2019; DOE 2020a). Thus, given the severity of the faced climate crisis and global concern in reducing greenhouse emissions, the prospects of using hydrogen as a fuel and energy source have been recognized as a substantial tool to achieve the desired decarbonized world (BCBN 2019; IEA 2020). According to the International Energy Agency (IEA 2021), to reach the net-zero CO2 emissions by 2050 will require the massive deployment of all available clean energy technologies expecting an expansion in hydrogen use from 75 million ton (Mt) in 2020 to approximately 212 million ton (Mt) in 2030. Even so, the current consumed H2 cannot be considered completely clean as about 80% of it is produced through natural gas steam reforming, partial oxidation of methane, or coal gasification, which results in around 900 Mt of carbon dioxide per year (WNA 2021). To overcome the issues, several projects (Kurokawa et al. 2011; Cormos et al. 2014; Antonini et al. 2020; Chauvy et al. 2021; Riley et al. 2021) suggested coupling the conventional methods with carbon capture, use, and storage (CCUS), to provide means of removing CO2 from the atmosphere and injecting into deep geological formation (IRENA 2019). However, although these technologies offered a strategic

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and near- to medium-term option to mitigate CO2 emission and scale up low-carbon hydrogen, deployment has been slow and fossil fuels are still a limited source, which means that the economic system based on fossil energy has to be changed while these resources are still available (Dickel 2020; IEA 2021). There is an urgency in addressing barriers and creating conditions that would improve the development and implementation of less intense carbon energy, to provide not only environmental quality but also energy-security fuel (IEA 2020; Howarth and Jacobson 2021; IEA 2021). With the continual rise of hydrogen as an industrial feedstock, along with interest in accruing all the environmental benefit of its use as an energy carrier, efforts have been made by the scientific community with the support of government policies to boost the synthesis of H2 from clean and sustainable sources, using biological, water dissociation, and thermoconversion routes (Martinez-Burgos et al. 2021; Yukesh Kannah et al. 2021). Water molecules can be split into hydrogen and oxygen by passing electrical current in an electrolyzer (electrolysis), supplying direct heat (thermolysis), or absorbing sunlight in the presence of a semiconductor catalyzer (photolysis), wherein, when in a sustainable context, electricity or heat is provided from a renewable source (wind, solar, hydro) (Bhandari et al. 2014; Chi and Yu 2018; Kim et al. 2021). As long thermolysis requires high-cost resistant refractory materials due to the extremely high temperatures (up to 2500 K) (Baykara 2004) and photolysis has very low conversion efficiency (Penconi et al. 2015; Jianghao 2021), the electrolysis is placed at a superior maturity stage as the only renewable technology that produces hydrogen at a commercial level. However, competitive large-scale implementation of electrolysis is susceptible to the instability of electricity surplus, availability, and prices, and still requires improvements in current density and purity of gases (alkaline electrolysis) as well as reduction of component cost (PEM electrolysis) (Mergel and Stolten 2012; Grigoriev et al. 2020). In biological pathways, hydrogen is produced by employing microorganisms to provide enzymes necessary to the water-splitting process (biophotolysis) or to break down cellulosic organic biomass in an anaerobic environment through photo- or dark fermentation (Ferraren-De Cagalitan and Abundo 2021). This route has been considered the most eco-friendly and less energy-intensive due to the mild conditions of temperature and pressure used, besides the wide range of microorganisms and substrates eligible, including wastes and cellulosic and lignocellulosic material (Akhlaghi and Najafpour-Darzi 2020; Saravanan et al. 2021). The biological process is a promising technology to integrate waste management into energy production and it will be an important tool in long-term energy systems. At any case, biological methods still have lower levels of technological maturity and H2 yields when compared to other processes once they are more time consuming and several substrates need to be pretreated to achieve considerable efficiency conversion, which increases, even more, the final costs (Jose 2021; Martinez-Burgos et al. 2021; Singh et al. 2021). Regarding the thermochemical conversions (gasification, pyrolysis, and combustion), heat is used to convert carbonaceous biomass into hydrogen-rich gas (Parthasarathy and Narayanan 2014). These processes were earlier developed to

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use coal as feedstock but the use of waste in energy generation also proved to be efficient over the years (Park and Raju 2016). Although gasification, pyrolysis, or combustion is not used in commercial scaling to hydrogen production, they are wellestablished technologies in chemical and fuel production and being studied worldwide to accelerate the growth of clean hydrogen use in parallel with other renewable sources (IEA 2019; Foong et al. 2021). Among several advantages, thermochemical processes, especially gasification, have attracted considerable attention for enabling high-efficiency conversion of lignocellulosic biomass, including waste from agriculture, forestry, and energy crop systems. Considering that these residues are abundant and readily available to support sustainable and low-cost bioenergy production, this chapter focuses on outlining its future potential to obtain renewable hydrogen via gasification. It also discusses principles, advantages, challenges, and some recent developments in using this technology.

2

Lignocellulosic Biomass Potential

Besides environmental challenges, another common issue is ensuring reliable and constant delivery of energy and fuels in the transition towards a low-carbon system. In this way, to successfully expand the clean hydrogen production as well as replace the already vast quantities of fossil consumption, alternate and renewable options must combine cost-effectiveness and potential for industrial scale production while combating climate changes and protecting ecosystems (DOE 2014; Gross 2020). Hence, biomass is considered an indispensable element on the basis of the new energy matrix for offering benefits in terms of versatility and abundance. Contrary to other renewable sources, most countries have widely available biomass resources, with no need to import, a compatible pathway to attend the special report signed in the Paris Agreement entitled Global Warming of 1.5
C, which pointed biomass as the major supplier of future primary energy made up of a median of 27.29% of the total share (IEA 2007; DOE 2015; IPCC 2018). Biomass is any organic matter that contains stored chemical energy from the sun, for example, agricultural and forest residues; animal, municipal, and industrial wastes; and exclusive energy crops (DOE 2015). Through photosynthesis, plants absorb sunlight and use it to convert carbon dioxide and water into carbon-rich compounds (EIA 2020). Biomass can regrow relatively quickly and be processed by different technologies to a more useable form of energy or fuel production, depending on the type of feedstock (Patrizio et al. 2021). However, most of the bioenergy still comes from the traditional uses of biomass. Although CO2 released in biomass combustion has previously been sequestered from the atmosphere during a plant’s lifetime, burning this material has a negative impact on local air quality and public health besides being the less efficient way to access its energy content (IEA 2007; EIA 2020). Furthermore, while bioenergy system is majority wood bases or from dedicated energy crops, which create debate about how to design and implement policies to avoid land availability and socioeconomic constraints, there are large quantities of lignocellulosic residues, especially from the agricultural sector, with huge energy potential and limited

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Table 1 Biomass residue chemical characterization Biomass Walnut shell Banana leaves Wheat rusk Corn cob Corn stover Soybean stalk Rice straw Maize straw Rice husk

Hemicellulose (%) 21 25.8

Cellulose (%) 34.3 26.7

Lignin (%) 43 17

30.8

37.8

22.5

29 16.4 17.4 22.7 23.6 24.3

32.2 28.3 35.4 37 42.7 31.3

18.4 23.8 21.3 13.6 17.5 14.3

Reference Shah et al. (2018) Fernandes et al. (2013) Prakash and Sheeba (2016) Raj et al. (2015) Raj et al. (2015) Raj et al. (2015) Chaturvedi et al. (2021) Chaturvedi et al. (2021) Chaturvedi et al. (2021)

applications (Kampman et al. 2010; IEA 2017; REN21 2021). Therefore, instead of old inefficient uses, managing residues as feedstock in controlled processes can increase bioenergy contribution by producing more sustainable heat, power, and high-value fuels, like hydrogen. The feedstock from the agriculture sector comes in the form of residues as stalks, stubbles, husk, bagasse, and seed pods, and others that represent the nonedible plant parts (Lal 2005; Hiloidhari et al. 2014). The composition of lignocellulosic material mainly consists of cellulose, hemicellulose, lignin, and smaller fractions of protein, pectin, extractives, and ash. Cellulose is the primary building substance in the walls of plant cells and consists of a long and linear chain of beta-D-glucopyranose subunits. Due to the multiple reactive hydroxyl groups, cellulose molecules are interconnected by hydrogen bonds to form a fibrillar structure characterized for its rigidity, fibrous, and water-insoluble properties (Heinze et al. 2018). Hemicellulose is also a polysaccharide, but unlike cellulose, it contains varied C5 and C6 sugar units (xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan), branch and amorphous structure, and low degree of polymerization and is easily hydrolyzed (Bajpai 2016). The third constituent, lignin, is a three-dimensional polymer of phenylpropanoid units with amorphous, cross-linked, and hydrophobic structures. The presence of lignin reinforces rigidity and provides a defense mechanism to the plant (Bajpai 2016). Although macromolecules differ in chemical and physical properties as well as their percentage in plants can vary with species, type, and even mature or grow condition (Table 1), all lignocellulosic materials share the same basic architecture from which hydrogen can be extracted. Nevertheless, in the absence of adequate practices most of the agricultural residues are commonly burned in open field, being responsible for deteriorating soil health and releasing a massive amount of air pollutants that in the year 2019 was equivalent to 36.7 Mt of CO2 (Prasad et al. 2020). Analyzing FAOSTAT database of total crop production and considering residue production ratios (RPR) suggested by Lal (2005)) and Morato et al. (2019)) it is estimated that the six main crops (maize, potatoes, rice, soybeans, sugarcane, and wheat), that represent 58% of the total share, generated in 2019 a

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surplus fraction of 1187 Mt of residue, i.e., leftover material after any competing uses or postharvest losses which is available for bioenergy generation (FAO 2019). Hence, since the literature indicates that 1 kg of dry biomass yields 40–190 g of H2 (depending on operation conditions) ~47.8–222.5 Mt of biohydrogen could be produced if this biomass was gasified rather than burned (Lepage et al. 2021).

3

Gasification

Thermochemical pathways have emerged as great tools for generating energy mostly because of their benefits related to products and technology flexibility, and the ability to directly use a varied range of feedstocks with high heating value and low costs, such as refinery residuals, lignocellulosic biomass, and municipal solid waste. Pyrolysis, gasification, liquefaction, and combustion are the main established thermochemical approaches to convert biomass into a more useful form of energy (Alhazmi and Loy 2021; Jiang et al. 2021; Teh et al. 2021). While direct combustion is the most low-efficiency and straightforward way to use biomass to produce heat and electricity, the other mentioned technologies are complex and widely carried out to obtain fuels and chemicals with diverse applications, including biohydrogen production (Fatehi et al. 2021). Liquefaction operates in a liquid (water or organic solvent), hot, and pressurized environment to break down the biomass macromolecules and form new liquid compounds that undergo subsequent reforming into syngas. This process can handle biomass with high moisture content; however, higher pressure requirements and higher time duration make it a limited and expensive process (Huang et al. 2018; Patra and Sheth 2018; Kumar et al. 2019). On the other hand, in the pyrolysis process, biomass samples are decomposed under moderate temperatures (300-700
C) and in the absence of oxygen to produce solids (char), a dark brown liquid named bio-oil, and non-condensable volatiles gases (biogas), which is a combination of methane, CO, CO2, and small amounts of hydrogen, hydrogen sulfide and other gases (Wang et al. 2017b; Pang 2019). According to operational conditions (residence time, temperature, and heating rate), this process can be subdivided into slow, fast, or flash pyrolysis. Slow pyrolysis results in almost equal mass yields of the three products and it is mainly used to produce biochar, whereas the latter ones were developed to give higher amounts of bio-oil (Yu et al. 2019). To produce biohydrogen, the gas content from flash and fast pyrolysis can be improved by increasing temperature and residence times, since it promotes the secondary reactions of liquids. However, even with these operational changes, the concentration of hydrogen remains very low, being necessary to adopt other strategic conditions and appropriate catalysis to be a competitive route (Chen et al. 2021). Gasification technology consists of the partial oxidation of organic material at high temperatures (>700
C) to obtain combustible gases. The process can be performed by applying different gasifying agents (air, steam, carbon dioxide, or combination of these gases) and can be split into four principal steps, namely (1) drying, which precedes the reactions to evaporate water content; (2) pyrolysis,

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Table 2 Main reactions involved in the gasification process (Huang and Jin 2019; Mazhkoo et al. 2021) Step Drying Pyrolysis Combustion

Gasification

Chemical reaction Raw biomass + heat ! dry biomass + H2O Dry biomass + heat ! char + volatiles C + 0.5O2 ! CO C + O2 ! CO2 CO + 0.5O2 ! CO2 H2 + 0.5O2 ! H2O C + H2O $ CO + H2 C + CO2 $ 2CO C + 2H2 $ CH4 CO + H2O $ CO2 + H2 CH4 + H2O $ CO + 3H2

ΔH
(kJ/mol)

Carbon partial combustion Carbon combustion CO combustion Water formation Water–gas reaction Boudouard reaction Carbon methanation Water–gas shift reaction Methane steam reforming

111 393 283 242 +131 +172 75 41 +206

characterized by releasing gases, volatile tars, and char; (3) combustion, in which char reacts with limited oxygen to form CO and CO2 and provide heat to supply the further endothermic reactions; and (4) gasification (or reduction), in which endothermic reactions take place to form mainly CO, H2, and methane (Table 2). In this way, the producer gas is mainly composed of carbon monoxide and hydrogen, small amounts of carbon dioxide, water, nitrogen, methane, and light hydrocarbons, as well as some content of remaining solid char, ash, and tars, from incomplete conversion reactions. In addition, sulfur and nitrogen contaminant gases may also be produced (Balat 2008). The further utilization of syngas to obtain value-added products depends on its quality since different applications in the power, chemical, and fuel sectors require specific H2/CO ratio, LHV, and other parameters. Moreover, one of the major problems in utilizing syngas from biomass gasification is the content of tar. Thus, although much of the process and final product depend on feedstock chemical properties, there are in the literature some general basic and wellestablished operational conditions to adjust and improve cleaning steps to maximize syngas quality and make it more suitable for biohydrogen production (Samiran et al. 2016). Using steam, steam-oxygen, air-oxygen, or air-steam rich as a gasifying agent instead of the conventional air enhances syngas (Islam 2020). Increasing the oxygen volume in the air causes the promotion of oxidation reaction while the presence of steam favors the steam-methane reactions that enhance H2, carbon conversion efficiency, and also LHV, considering that less nitrogen in the final product leads to a higher concentration of the combustible gas (Guo et al. 2021). Keeping the same equivalent ratio, Barisano et al. (2016) showed a slight increase in H2 content by altering the volume of O2 from 35 to 50% and doubling the yield by using a mixture

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of steam and O2. Cerone and Zimbardi (2021) adding steam in the air and Gallucci et al. (2019) replacing pure air with pure steam noticed a positive effect in H2/CO ratio that ranged from 0.46 to 0.77 and 0.39 to 1.11, respectively, as improvements in terms of overall syngas concentration and cold gas efficiency. Gasification with supercritical water as a reaction medium has also been investigated as an efficient method to obtain hydrogen, mostly because it allows direct use of wet biomass (Rodriguez Correa and Kruse 2018). Nevertheless, experiment with real biomass is scarce and still presents serious barriers to scale up due to the biomass pumpability and energy consumption, which requires high temperature and pressure levels (De Blasio and Järvinen 2017; García-Jarana et al. 2020; Lee et al. 2021). In this sense, even pure oxygen and supercritical water gasification provide the best quality gas; cost-effective steam or steam/air gasification has attracted the highest interest in hydrogen production (Kalinci et al. 2012; Okolie et al. 2020). Raising operating temperature (>800
C), residence time, and steam-to-biomass ratio (S/B) until certain point is suggested in order to achieve high carbon regeneration of the biomass and change the gasification efficiency to a great extent (Hernández et al. 2010; Singh Siwal et al. 2020). At higher temperatures endothermic reactions dominate gasification processes; therefore, the degree of thermal cracking is favored, causing a positive effect on the total gas production and composition, leading to higher amounts of CO and H and lowering solid and liquid products (Cerón et al. 2021; Tsekos et al. 2021). As similar increase in S/B and residence times enhance water–gas shift reaction, methane, and tar reforming moving towards H2 production and consumption of other species, which is also beneficial to better H2/CO ratio (Hernández et al. 2010; Huang and Jin 2019; Sun et al. 2020). On the contrary, when a large biomass particle size is employed there is a smaller specific surface area available to react and a higher heat transfer resistance, resulting in a lower conversion and gas quality (Hernández et al. 2010). Regarding reactor type, the most common gasifiers are divided into fixed/moving bed (downdraft or updraft), fluidized bed (circulating or bubbling), and entrained flow. In fixed-bed reactors, solid fuel particles are top loaded, through which the oxidation medium moves in countercurrently (upflow) or concurrently (downflow) with feed. Updraft gasifier is suited for small scale and has higher thermal efficiency, but produces relatively higher tar content, once the formed tar quickly exits the gasifier without encountering elevated temperatures. On the other hand, in downdraft the formed tar passes through a hotbed of chars in the combustion zone, reducing its content. Both fluidized bed types, generally used in intermediate units, are easier to scale up, offer extremely good mixing between feed and oxidant, and are flexible in terms of biomass type, but can promote ash agglomerate due to the restricted operation temperature. Entrained flow produces low tar and is indicated for big/large scale; however, feed materials must be pulverized since it requires small particle sizes