Centre for Innovation and Translational Research, CSIR-Indian Institute of Toxicology, Lucknow, India
BIOMASS, BIOFUELS, BIOCHEMICALS
Microbial Fermentation of Biowastes
Edited by Le Zhang
NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
Yen Wah Tong
NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore; Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
Jingxin Zhang
China-UK Low Carbon College, Shanghai Jiao Tong University, Shanghai, China
Ashok Pandey
Centre for Innovation and Translational Research, CSIR-Indian Institute of Toxicology, Lucknow, India
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Contents
Contributors ix
Preface xiii
1. Strategies for enhanced microbial fermentation processes
Le Zhang, Jonathan T.E. Lee, Kai-Chee Loh, Yanjun Dai, Yen Wah Tong
1.1 Introduction 1
1.2 Characteristics of common biowastes 2
1.3 Strategies for enhancing microbial fermentation 6
1.4 Global prospects 10
1.5 Conclusions and perspectives 15 Acknowledgments 16 References 16
2. Conversion of food waste to bioenergy and biochemicals via anaerobic digestion
Liwen Luo, Nicholas Cheuk Him Ng, Jun Zhao, Dongyi Li, Zhiqiang Shi, Mi Zhou
2.1 Introduction 25
2.2 Conversion of food waste to bioenergy and biochemicals 27
2.3 Integrated bioprocesses with food waste anaerobic digestion for bioresource recovery 35
2.4 Challenges and limitations in bioconversion of food waste to bioresources via anaerobic digestion 37
2.5 Conclusions and perspectives 38 References 39
3. Conversion of agricultural wastes to bioenergy and biochemicals via anaerobic digestion
Chenjun He, Tao Luo, Hairong Yuan, Fei Shen
3.1 Introduction 45
3.2 Biomass densification to promote feedstock supply efficiency 47
3.3 Pretreatment of agricultural residues for anaerobic digestion 51
3.4 Enhancement techniques for anaerobic digestion 54
3.5 Utilization of the anaerobic digestion products 61
3.6 Conclusions and perspectives 63 References 64
4. Conversion of manure to bioenergy and biochemicals via anaerobic digestion
Qigui Niu, Liuying Song, Jingyi Li
4.1 Introduction 69
4.2 Biogas and biomethane production from manures through AD 70
4.3 Additional value from AD 84
4.4 Conclusions and perspectives 86 References 86
5. Conversion of wastewater to bioenergy and biochemicals via anaerobic digestion
Yang Li
5.1 Introduction 91
5.2 Bioenergy production 92
5.3 Biochemicals production 101
5.4 Conclusions and perspectives 107 References 107
6. Algal cultivation and algal residue conversion to bioenergy and valuable chemicals
Pengfei Cheng, Chengxu Zhou, Yanzhang Feng, Ruirui Chu, Haixia Wang, Yahui Bo, Yandu Lu, Roger Ruan, Xiaojun Yan
6.1 Introduction 115
6.2 Advantages and development of energy from microalgae 116
6.3 Microalgae sewage treatment and resource engineering technology 121
6.4 Application of microalgae culture in wastewater treatment 124
6.5 Conclusions and perspectives 127 Acknowledgments 127 References 128
7. Additive strategies for enhanced anaerobic digestion for bioenergy and biochemicals
7.2 Additive strategies for enhanced AD for bioenergy 132
7.3 Additive strategies for enhanced AD for biochemicals 147
7.4 Conclusions and perspectives 149 References 150
8. Bioreactors for enhanced anaerobic digestion for bioenergy and biochemicals
Tengyu Zhang, Endashaw Workie, Jingxin Zhang
8.1 Introduction 159
8.2 Bioreactors for enhanced AD for bioenergy 161
8.3 Bioreactors for enhanced AD for biochemicals 169
8.4 Conclusions and perspectives 174 References 175
9. Bioaugmentation strategies via acclimatized microbial consortia for bioenergy production
Le Zhang, Hailin Tian, Jonathan T.E. Lee, Jun Wei Lim, Kai-Chee Loh, Yanjun Dai, Yen Wah Tong
9.1 Introduction 179
9.2 Theoretical basis and operational procedures of bioaugmentation strategies 181
9.3 Key findings in bioaugmentation strategies for enhancing AD 185
9.4 Challenges and opportunities of bioaugmentation to enhance AD for biofuel production 205
9.5 Conclusions and perspectives 207
Acknowledgments 207
References 207
10. Microbial fermentation via genetically engineered microorganisms for production of bioenergy and biochemicals
Kang Zhou, Jie Fu J. Zhou
10.1 Introduction 215
10.2 Engineering E. coli to produce 1,4-butanediol 216
10.3 Engineering S. cerevisiae to utilize xylose 222
10.4 Conclusions and perspectives 230
Acknowledgment 231
References 231
11. Anaerobic digestion via codigestion strategies for production of bioenergy
Wangliang Li
11.1 Introduction 233
11.2 Composition of organic wastes and their monodigestion performances 234
11.3 Anaerobic codigestion 237
11.4 Microbial community in codigestion system 240
11.5 Life-cycle assessment of codigestion process 243
11.6 Conclusions and perspectives 247
Acknowledgements 247
References 248
12. Feedstock pretreatment for enhanced anaerobic digestion of lignocellulosic residues for bioenergy production
Xihui Kang, Chao Xu, Richen Lin, Bing Song, David Wall, Jerry D Murphy
12.1 Introduction 253
12.2 Biomass pretreatment for enhanced AD 257
12.3 Opportunities for AD in a circular bioeconomy 272
12.4 Conclusions and perspectives 274
References 275
13. Application of enzymes in microbial fermentation of biomass wastes for biofuels and biochemicals production
Luciana Porto de Souza Vandenberghe, Gustavo Amaro Bittencourt, Kim Kley Valladares-Diestra, Nelson Libardi Junior, Luiz Alberto Junior Letti, Zulma Sarmiento Vásquez, Ariane Fátima Murawski de Mello, Susan Grace Karp, Maria Giovana Binder Pagnoncelli, Cristine Rodrigues, Adenise Lorenci Woiciechowski, Júlio César de Carvalho, Carlos Ricardo Soccol
13.1 Introduction 283
13.2 Biomass-degrading enzymes 284
13.3 Separated enzymatic hydrolysis and fermentation processes 291
13.4 Simultaneous enzymatic hydrolysis and fermentation processes 296
13.5 Commercial enzymes and enzymes’ costs 304
13.6 Advancements and innovation in biomassdegrading enzymes 304
13.7 Conclusions and perspectives 308
References 308
14. Hybrid technologies for enhanced microbial fermentation process for production of bioenergy and biochemicals
Lingkan Ding, Bo Hu
14.1 Introduction 317
14.2 Mechanisms in hybrid MEC-AD systems 318
14.3 Performances of hybrid MEC-AD systems 328
14.4 Conclusions and perspectives 336
References 337
15. Acidogenic fermentation of organic wastes for production of volatile fatty acids
Le Zhang, To-Hung Tsui, Kai-Chee Loh, Yanjun Dai, Yen Wah Tong
15.1 Introduction 343
15.2 Substrates for volatile fatty acids production through acidogenic fermentation 346
15.3 Inocula for acidogenic fermentation 346
15.4 Bioreactors and operation modes for VFAs production via acidogenic fermentation 349
15.5 Enhancing strategies for elevated VFAs yield from acidogenic fermentation 350
15.6 Separation/recovery of VFAs from fermentation broth 354
15.7 Subsequent applications of VFAs 356
15.8 Conclusions and perspectives 358
Acknowledgments 359 References 359
16. Functional microbial characteristics in acidogenic fermenters of organic wastes for production of volatile fatty acids
Le Zhang, Miao Yan, To-Hung Tsui, Jonathan T.E. Lee, Kai-Chee Loh, Yanjun Dai, Yen Wah Tong
16.1 Introduction 367
16.2 Procedures for bacterial community analysis 369
16.3 Microbial characteristics in acidogenic fermenters 370
16.4 Conclusions and perspectives 388
Acknowledgments 388 References 389
17. Microbial fermentation for biodegradation and biotransformation of waste plastics into high value-added chemicals
Haojie Liu, Lijie Xu, Xinhui Bao, Jie Zhou, Xiujuan Qian, Weiliang Dong, Min Jiang
17.1 Introduction 395
17.2 Classification of plastics 396
17.3 Biodepolymerization and biotransformation of hydrolyzed plastics 398
17.4 Biodepolymerization and biotransformation of nonhydrolyzed plastics 406
17.5 Conclusions and perspectives 408 References 409
Index 413
Contributors
Gustavo Amaro Bittencourt Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Xinhui Bao State Key Laboratory of MaterialsOriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, PR China
Maria Giovana Binder Pagnoncelli Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Yahui Bo College of Food and Pharmaceutical, Sciences, Ningbo University, Ningbo, Zhejiang, China
Pengfei Cheng College of Food and Pharmaceutical, Sciences, Ningbo University, Ningbo, Zhejiang, China; Center for Biorefining and Department of Bioproducts and Biosystems, Engineering, University of Minnesota-Twin Cities, Saint Paul, MN, USA
Ruirui Chu College of Food and Pharmaceutical, Sciences, Ningbo University, Ningbo, Zhejiang, China
Chicaiza-Ortiz Cristhian China-UK Low Carbon College, Shanghai Jiao Tong University, Shanghai, China; School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, China; Faculty of Life Sciences, Amazon State University (UEA), Puyo, Pastaza, Ecuador
Yanjun Dai Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore; School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China
Júlio César de Carvalho Department of Bioprocess Engineering and Biotechnology, Federal
University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Lingkan Ding Department of Bioproducts and Biosystems Engineering, University of Minnesota, MN, USA
Weiliang Dong State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, PR China; Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, PR China
Yanzhang Feng College of Food and Pharmaceutical, Sciences, Ningbo University, Ningbo, Zhejiang, China
Chenjun He Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, Sichuan, P. R. China; Rural Environment Protection Engineering & Technology Center of Sichuan Province, Sichuan Agricultural University, Chengdu, Sichuan, P. R. China
Bo Hu Department of Bioproducts and Biosystems Engineering, University of Minnesota, MN, USA
Min Jiang State Key Laboratory of MaterialsOriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, PR China; Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, PR China
Luiz Alberto Junior Letti Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Xihui Kang MaREI Centre, Environmental Research Institute, University College Cork, Cork, Ireland; Civil, Structural and Environmental Engineering, School of Engineering and
Architecture, University College Cork, Cork, Ireland
Susan Grace Karp Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Kim Kley Valladares-Diestra Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Jonathan T.E. Lee NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
Dongyi Li Department of Biology, Institute of Bioresource and Agriculture, Sino-Forest Applied Research Centre for Pearl River Delta Environment, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China
Jingyi Li School of Environmental Science and Engineering, China−America CRC for Environment & Health of Shandong Province, Shandong University, Qingdao, Shandong, China
Wangliang Li CAS Key Laboratory of Green Process and Engineering, Institute of Process Engineering, University of Chinese Academy of Sciences, Beijing, China
Yang Li School of Ocean Science and Technology, Dalian University of Technology, Panjin, Liaoning, China
Nelson Libardi Junior Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Jun Wei Lim NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
Richen Lin MaREI Centre, Environmental Research Institute, University College Cork, Cork, Ireland; Civil, Structural and Environmental Engineering, School of Engineering and
Architecture, University College Cork, Cork, Ireland
Haojie Liu State Key Laboratory of MaterialsOriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, PR China
Kai-Chee Loh Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore; Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
Adenise Lorenci Woiciechowski Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Yandu Lu State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan University, Hainan, China
Liwen Luo Department of Biology, Institute of Bioresource and Agriculture, Sino-Forest Applied Research Centre for Pearl River Delta Environment, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China
Tao Luo Biogas Institute of Ministry of Agriculture (BIOMA), Chengdu, P. R. China
Ariane Fátima Murawski de Mello Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Jerry D Murphy MaREI Centre, Environmental Research Institute, University College Cork, Cork, Ireland; Civil, Structural and Environmental Engineering, School of Engineering and Architecture, University College Cork, Cork, Ireland
Nicholas Cheuk Him Ng Department of Biology, Institute of Bioresource and Agriculture, SinoForest Applied Research Centre for Pearl River Delta Environment, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China
Qigui Niu School of Environmental Science and Engineering, China−America CRC for Environment & Health of Shandong Province, Shandong University, Qingdao, Shandong, China
Luciana Porto de Souza Vandenberghe Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Xiujuan Qian State Key Laboratory of MaterialsOriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, PR China
Cristine Rodrigues Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Roger Ruan Center for Biorefining and Department of Bioproducts and Biosystems, Engineering, University of Minnesota-Twin Cities, Saint Paul, MN, USA
Zulma Sarmiento Vásquez Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Fei Shen Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, Sichuan, P. R. China; Rural Environment Protection Engineering & Technology Center of Sichuan Province, Sichuan Agricultural University, Chengdu, Sichuan, P. R. China
Zhiqiang Shi Department of Chemical Engineering, Tianjin University Renai College, Tianjin, China
Carlos Ricardo Soccol Department of Bioprocess Engineering and Biotechnology, Federal University of Paraná, Centro Politécnico, Curitiba, Paraná, Brazil
Liuying Song School of Environmental Science and Engineering, China−America CRC for Environment & Health of Shandong Province, Shandong University, Qingdao, Shandong, China
Bing Song Scion, Te Papa Tipu Innovation Park, Rotorua, New Zealand
Hailin Tian NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
Yen Wah Tong NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore; Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
To-Hung Tsui NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
David Wall MaREI Centre, Environmental Research Institute, University College Cork, Cork, Ireland; Civil, Structural and Environmental Engineering, School of Engineering and Architecture, University College Cork, Cork, Ireland
Haixia Wang College of Food and Pharmaceutical, Sciences, Ningbo University, Ningbo, Zhejiang, China
Chao Xu Institute of Bast Fiber Crops, Chinese Academy of Agricultural Sciences, Changsha, China
Lijie Xu State Key Laboratory of MaterialsOriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, PR China
Xiaojun Yan Key Laboratory of Marine Biotechnology of Zhejiang Province, Ningbo University, Ningbo, Zhejiang, China
Miao Yan NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
Hairong Yuan Centre for Resource and Environmental Research, Beijing University of Chemical Technology, Beijing, P. R. China
Le Zhang NUS Environmental Research Institute, National University of Singapore, Singapore; Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
Pengshuai Zhang China-UK Low Carbon College, Shanghai Jiao Tong University, Shanghai, China; School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai, China
Tengyu Zhang China-UK Low Carbon College, Shanghai Jiao Tong University, Shanghai, China; Department of Chemical and Biological Engineering, University of Sheffield, Sheffield, UK
Jun Zhao Department of Biology, Institute of Bioresource and Agriculture, Sino-Forest Applied Research Centre for Pearl River Delta
Environment, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China
Mi Zhou Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, AB, Canada
Chengxu Zhou College of Food and Pharmaceutical, Sciences, Ningbo University, Ningbo, Zhejiang, China
Kang Zhou Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
Jie Fu J. Zhou Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
Jie Zhou State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing, PR China; Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing, PR China
Preface
The book titled Microbial Fermentation of Biowastes is a part of the Elsevier comprehensive book series on BIOMASS, BIOFUELS, BIOCHEMICALS (Editor-in-Chief: Ashok Pandey). This book intends to cover different aspects of microbial fermentation of biowastes, providing the state-of-art information on various aspects and perspectives for future developments. As modernization, industrialization, and urbanization continue to increase, substantial organic wastes (e.g., food waste, agricultural waste, manure, sludge, wastewater, and plastics, etc.) are being generated, which pose a growing threat to the environment and sustainable development of mankind. To address these challenges, microbial fermentation is seen as a useful technology that can convert the abundant organic wastes to various biofuels and biochemicals in many forms and applications, especially in the global context of the carbon cycle. In the past decade, extensive studies have focused on enhanced microbial fermentation for better biowaste management and higher production of renewable bioenergy or biochemicals. Hitherto, great progress has been achieved in this area via various strategies, including the use of additives, multistage bioreactors, microbial bioaugmentation, genetically engineered microorganisms, codigestion, feedstock pretreatment, enzyme technologies, and hybrid technologies, etc. For instance, enhanced anaerobic digestion is used for biowastes conversion to biomethane and biohydrogen while the enhanced acidogenic fermentation is used for the production of volatile fatty acids from biowastes. In addition, microbial
biodegradation of plastics is increasingly becoming a significant topic in recent years due to the fact that “white pollution” caused by the plastics has become a pressing problem that needs to be solved within the shortest timespan. In recent years, many microbes have been identified as candidates for microbial degradation of plastic debris, but further research remains needed. However, the aforementioned advances have not been consolidated and are quite diverse, and the state-of-the-art has yet to be established.
Motivated by the above gaps, this book aims to provide a solution, using microbial fermentation technology, to address the problem of how to efficiently and economically manage various organic wastes and recycle simultaneously energy/resources from the wastes. To achieve this target, this book summarizes the latest research achievements on the development of various strategies for enhanced microbial fermentation for organic wastes conversion to bioenergy and biochemicals and for the biodegradation of plastic waste. The technical principles and practical applications of the enhanced microbial fermentation processes via various strategies can be a reference for the researchers, engineers, investors, policy makers, and students in the fields of waste management with energy and resource recovery.
The chapter 1 of the book presents an overall introduction to the enhancing strategies for microbial fermentation processes, where particular focus is set on the technical principles of additive strategies, multistage bioreactors, microbial bioaugmentation
strategies, genetically engineered microorganisms, codigestion strategies, feedstock pretreatment strategies, enzyme technologies, and hybrid technologies. Chapters 2–6 present progress on the conversion of common wastes such as food waste, agricultural waste, manure, wastewater, and algal residues to bioenergy and biochemicals via enhanced anaerobic digestion. Chapters 7–14 discuss the significant progress achieved on enhancing microbial fermentation via additive strategy, multistage bioreactor strategy, microbial bioaugmentation strategy, genetic engineering approach, codigestion strategy, feedstock pretreatment strategy, enzyme applications, and hybrid technologies. Chapter 15 provides details on the recent advances on enhanced acidogenic fermentation of organic wastes for the production of volatile fatty acids; Chapter 16 presents functional microbial characteristics in acidogenic fermenters of organic wastes for the production of volatile fatty acids. Finally, for the plastic waste management, Chapter 17 presents the recent advances on microbial fermentation for biodegradation and
biotransformation of plastics into high value–added chemicals.
The editors gratefully acknowledge the authors for their contributions in this book. Thanks are due to the reviewers who provided valuable suggestions to improve the quality of the chapters. We greatly appreciate Dr Kostas Marinakis, Former Senior Book Acquisition Manager and Ms Katie Hammon, Senior Book Acquisition Manager, Ms Andrea Dulberger, Editorial Project Manager, and others in the Elsevier team associated with this book for their support toward publishing this book.
We are confident that this book would be of great value for the researchers broadly working in the areas of waste to wealth, resource recovery, and production of chemicals and fuels from renewable resources. The book will be of equal value to industry persons as well as policy planners.
Editors Le Zhang Yen Wah Tong Jingxin Zhang Ashok Pandey
1 Strategies for enhanced microbial fermentation processes
Le Zhanga,b, Jonathan T.E. Leea,b, Kai-Chee Lohb,c, Yanjun Daib,d, Yen Wah Tonga,b,c
aNUS Environmental Research Institute, National University of Singapore, Singapore bEnergy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
cDepartment of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
dSchool of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China
1.1 Introduction
Ever-increasing global population and anthropogenic activities lead to higher energy demands and rampant environmental deterioration (De Sanctis et al., 2019; Saud et al., 2020). To deal with the latter, the management of various biomass wastes remains a critical challenge (De Schouwer et al., 2019; Foong et al., 2020). Microbial fermentation technologies such as anaerobic digestion (AD) have been extensively adopted to generate bioenergy and biochemicals from diverse biowastes (Chen et al., 2020; Kougias & Angelidaki, 2018; Rawoof et al., 2020). Such biowastes comprise food waste, agricultural waste, horticultural waste, animal manure, wastewater/sludge, and algal residues. After upgrading (Angelidaki et al., 2018), methane-rich biogas derived from AD is frequently converted to heat and electricity using combined heat and power (CHP) units (Akkouche et al., 2020; Di Maria et al., 2019). As a result, methane production efficiency plays an important role in the commercial viability of AD technology in biowaste treatment. Nevertheless, as a multistep (i.e., hydrolysis, acidogenesis, acetogenesis, and methanogenesis) biological process governed by a spectrum of microbiota, AD processes are usually affected by many factors (Chen et al., 2014; Chen et al., 2008; Yenigün & Demirel, 2013), which lead to a relatively low conversion efficiency in some AD plants (Zhang et al., 2019c), particularly for nonhomogeneous substrates. At the heart of AD process, microbial communities are also significantly different for different types of substrates (Fernandez-Bayo et al., 2020; Narihiro & Sekiguchi, 2007; Ting et al., 2020). Indeed,
the microbial community structures in a given AD system have intrinsic link to bioreactor performance. Hence, it is becoming increasingly essential to comprehensively understand the microbiome in AD systems. Fortunately, bioinformatic analysis of biogas-producing microbial communities in anaerobic bioreactors has become a widely used tool to analyze microbiomes and diverse metabolic pathways (Lim et al., 2020; Zhang et al., 2019b).
In the past decades, many studies have been conducted to develop various strategies to enhance the microbial fermentation of organic matters for higher productivity of biofuels and biochemical; these include additive supplementation (Chiappero et al., 2020; Zhang et al., 2019f), multistage bioreactors (García‐Ruíz et al., 2020; Zhang et al., 2020a), microbial bioaugmentation (Lee et al., 2020a; Tsapekos et al., 2017; Yan et al., 2020), genetically engineered microorganisms (Llamas et al., 2020; Sáez-Sáez et al., 2020), codigestion (Gao et al., 2020; Oladejo et al., 2020), feedstock pretreatments (Gunes et al., 2019; Millati et al., 2020), enzyme technologies (Agabo-Garcia et al., 2019; Garcia et al., 2019), and hybrid technologies (Cui et al., 2020; Rezaee et al., 2020). While these studies are diverse, there was not good identification of the state-of-the-art, though slowly appearing in the literature. For instance, additional insights in many new aspects such as nanotechnology, engineered enzymes, multistage anaerobic bioreactor, bioaugmentation via microbial consortia, metagenomics-based data mining, precision fermentation platform via artificial intelligence, and industrial application progress have been reported in recent years.
This book focuses on the most recent progress in various enhancing strategies for microbial fermentation of various biowastes for the production of bioenergy and value-added biochemicals. In this introductory chapter, a brief introduction to the various biowastes and the respective enhancing strategies are the main focus. The advantages and limitations, industrial applications, and future perspectives of various performance-enhancing strategies are also highlighted.
1.2 Characteristics of common biowastes
1.2.1
Food waste
Food waste represents the food discarded during the processes of production, transportation, transaction, processing, and consumption (Tong et al., 2018). Due to the differences in the eating habits of people in various countries and regions, the derived food wastes vary in the major components such as vegetables, meat, rice, eggs, noodles, etc. As such, the heterogeneous compositions of proteins, lipids, carbohydrates, and lignocellulosic components in different food waste result in relatively unstable biodegradation rate and methane production rate. Table 1.1 shows the typical characteristics of common biowastes. The volatile solids (VS) and total solids (TS) contents of food waste are in ranges of approximately 12%–26% and 12%–31%, respectively, which indicates that water accounts for around 69%–88% of the mass in food waste. It does contain a fair amount of proteins, lipids, carbohydrates, and lignocellulosic components that are promising substrates for anaerobic bioreactors to produce high value-added products (Ravindran & Jaiswal, 2016). Notably, the pH value of food waste is rather low (i.e., pH 4.0–5.0) and therefore requires appropriate monitoring and control to maintain an efficient AD process.
Biomass, biofuels, biochemicals
TABLE 1.1 Typical characteristics of common biowastes.
Waste activated sludge ( Ebenezer et al., 2015 ; Yu et al., 2014 ; Zhang et al., 2019e ) Algal residues ( Suganya et al., 2016 ; Zhang et al., 2020b )
Animal manure ( Dechrugsa et al., 2013 ; Zarkadas et al., 2015 ; Zhang et al., 2019d )
Yard waste ( Brown & Li, 2013 ; Yao et al., 2017 )
Agricultural waste ( Hoornweg & BhadaTata, 2012 ; Hu et al., 2015 )
Food waste ( Chen et al., 2016 ; Zhang et al., 2020c ; Zhou et al., 2015 )
Characteristics
Note: w.b., wet basis.
1.2.2 Agricultural waste and yard waste
Agricultural waste is one of the most abundant biomass resources, and therefore has great potential for the generation of renewable energy and value-added biochemicals (Yu et al., 2019). Essentially, the bulk of agricultural waste is lignocellulosic biomass, which is composed of three polymers (i.e., cellulose, hemicellulose, and lignin) entangled together into a superstructure and is thereby resistant to microbial degradation. Hence, hydrolysis is regarded as the rate-limiting step during AD of lignocellulosic material. Many current studies therefore are focused on enhancing hydrolysis of lignocellulosic biomass through strategies such as microbial bioaugmentation (Lee et al., 2020a; Shetty et al., 2020), feedstock pretreatment (Lee et al., 2020b; Vieira et al., 2020), and enzymatic hydrolysis (da Silva et al., 2020).
Yard waste consists mainly of plant leaves, wood chips, and grasses, which is an inherent component of municipal solid waste, especially in the green cities and towns (Lee et al., 2018). It has been reported that the contents of cellulose, hemicellulose, and lignin of common yard waste are approximately 24%–29%, 10%–32%, and 8%–23%, respectively (Table 1.1). Similar to agricultural waste, a major impediment to industrial AD of yard waste lies in the need for various pretreatments (e.g., enzymatic hydrolysis, physical, chemical, and biological techniques) to facilitate the breakdown of lignocellulosic components so that sugar polymers become bioavailable. Notably, the C:N ratio of yard waste is relatively high (e.g., 55.3), compared to the recommended C:N ratios between 20:1 to 30:1 (Uçkun Kiran et al., 2016). Ergo, yard waste is frequently codigested with other wastes (e.g. food waste) with a lower C:N ratio to balance the nutrition requirement of carbon and nitrogen for microbial growth and metabolic reactions.
1.2.3 Animal manure
Animal manure is mainly derived from animal feces, which usually contain liquid farm manure slurry and a solid farmyard manure. The latter is composed mainly of plant materials such as crop straw that are utilized as bedding materials for various farm animals. Due to the fact that different animals have different digestive systems and diets, the manure derived from them displays diverse qualities, which leads to the necessity of tailored optimization of different AD operating conditions (Li et al., 2015). The digestate of the AD operations using animal manure is usually utilized as fertilizer. Nevertheless, the high abundance of antibiotic resistance genes (ARGs) in animal manure caused by overused antibiotics in livestock feed is a pertinent issue. To mitigate the ARGs issue during AD of manure, the effectiveness of feedstock pretreatment with microwave pretreatment has been evaluated and validated (Zhang et al., 2019d). Another critical issue associated with AD of manure is the inhibition caused by high amounts of ammonia (Fuchs et al., 2018; Sun et al., 2016; Wang et al., 2016). To tackle ammonia inhibition for efficient methane production from manure, several strategies such as substrate dilution (Bujoczek et al., 2000), codigestion (Li et al., 2017), microbial bioaugmentation (Yan et al., 2020), and additives (e.g., biochar (Pan et al., 2019) and trace elements (Molaey et al., 2018)) have been investigated. In addition, animal manure was recently found promising as a feedstock to produce volatile fatty acids (VFAs) using a membrane system (Jomnonkhaow et al., 2020).
Biomass, biofuels, biochemicals
1.2.4 Wastewater and waste activated sludge
There has been a paradigm shift from regarding wastewater as a waste requiring treatment, to that of a substrate for recovery of energy and resource in the context of a circular economy (Jiang et al., 2012; Robles et al., 2020). Indeed, wastewater treatment plants in many countries and areas have employed water resource recovery facilities (Diaz-Elsayed et al., 2019; Meena et al., 2019). The traditional wastewater treatment focuses mainly on the pollutant removal and safe discharge of treated wastewater (Kanaujiya et al., 2019), whereas the water resource recovery facilities focus on recycling of nutrient elements (e.g., phosphorus (Chrispim et al., 2019)), energy (Goswami et al., 2019), and clean water (Li et al., 2020) from a wide range of wastewater. Essentially, the organic compounds in wastewater are consumed by microbes in anaerobic reactors. The process efficiency of AD of wastewater can be further enhanced by integrating with other technologies such as microbial electrolysis cells (Hassanein et al., 2017).
Waste activated sludge (WAS) is derived from the secondary aerobic process of municipal wastewater treatment (Shin et al., 2019). WAS commonly contains an array of biodegradable organics, nutrients, pathogens, and heavy metals (Jeong et al., 2019), and therefore is frequently treated by AD for energy recovery prior to disposal (Ruffino et al., 2019). Nevertheless, from the perspective of industrial applications, AD of WAS for biogas production generally presents a relatively low economic feasibility due to the high water content in WAS. To enhance biomethane production rate, high-solids AD of dewatered activated sludge with a TS content of over 15 wt% has been suggested (Guo et al., 2017). Meanwhile, the C:N ratio in WAS is relatively low (e.g., 5.0–5.2), thereby relegating it as unsuitable for an effective anaerobic monodigestion, but a good cosubstrate to anaerobic codigestion. Hitherto, codigestion of WAS with other organic wastes that contain very high C:N ratios such as agricultural waste (Zhang et al., 2019a), yard waste (Lee et al., 2019), and food waste (Du et al., 2020b) have been investigated. In addition, it is noteworthy that sludge drying followed by incineration was recently reported as a sustainable disposal method of excess sludge (Hao et al., 2020). However, more efficient dewatering technologies have to be developed before incineration becomes a feasible alternative option.
1.2.5 Algal residues
Microalgae have been utilized as a potential feedstock for generation of biofuels such as biodiesel (Hossain et al., 2019). However, large amounts of algal residues remain after algaebased biodiesel production (Zhu et al., 2018). Algal residues contain mainly carbohydrate, protein, lipid, and a small amount of hemicellulose, and therefore they can be further converted to value-added products (e.g., biomethane, biohydrogen, bioethanol, and biolipid) using AD or microbial fermentation (Chandra et al., 2019; Kumar et al., 2020; Zhang et al., 2020b). AD of dry algal residues plays an essential role in biorefinery of algal biomass toward zero waste targets. In AD of lipid-extracted algal residues, potential solvent toxicity and unbalanced C:N ratio remain challenges (Bohutskyi et al., 2019). Accordingly, codigestion and feedstock pretreatments (Kumar et al., 2020) could be promising strategies to alleviate solvent inhibition and enhance methane yield.
1.3.1
1.3 Strategies for enhancing microbial fermentation
Additive strategies
The additive strategies for enhancing AD and microbial fermentation processes developed rapidly in the past decade, as there are several advantages such as being able to use infrastructure already in place, easy application, and relatively economical operation costs (Arif et al., 2018; Paritosh et al., 2020; Romero-Güiza et al., 2016; Zhang et al., 2018). These features make additive strategies suitable for potential industrial scale application. Hitherto, various additives have been explored in AD in the form of carbon-rich additives (e.g., activated carbon, biochar and graphene) (Chiappero et al., 2020; Lin et al., 2017; Zhang et al., 2020c), trace metals (Thanh et al., 2016), nanoparticles (Lee & Lee, 2019; Zhu et al., 2020), and zeolite (Montalvo et al., 2020), etc. Carbon-rich additives are capable of simultaneously providing an immobilization substrate for enhanced microbial growth (Fagbohungbe et al., 2016; Weber et al., 1978), promoting direct interspecies electron transfer among microorganisms (Lin et al., 2018; Park et al., 2018), and mitigating acid stress (Luo et al., 2015) and ammonia inhibition (Lü et al., 2016). The technology readiness level for using biochar addition strategy to enhance AD performance was at least 1000 L pilot-scale demonstration, on the way to early commercialization (Hu et al., 2020; Zhang et al., 2020c). Trace metals play an essential role in the growth of anaerobic microorganisms and their metabolic functions, so supplementations of vital trace metals such as Fe, Ni, Co, Mo, Ca, Mg, Mn, W, Se, and Zn have been widely studied (Hijazi et al., 2020; Mancini et al., 2018; Matheri et al., 2016). Nonetheless, in practice they could be added to anaerobic digesters in excessive amounts, leading to potential process inhibition (Thanh et al., 2016). The key research required to avoid potential metal inhibition is an in-depth understanding of chemical speciation and bioavailability of trace metals in anaerobic digesters (Thanh et al., 2016). Bioavailability of trace metals was reported to be increased in anaerobic digesters through appropriate supplementation of metal chelating agents (e.g., nickel-chelator complexes (Zhang et al., 2020d)) and encapsulated metal additive (Zhang et al., 2019f). However, the environmental and human health risk of chelating agents as carriers for metal supplementation greatly restricts their extensive application (Serrano et al., 2017). The two most common nanoparticles added into anaerobic digesters are zerovalent metals (Aguilar-Moreno et al., 2020; Hassanein et al., 2020) and metal oxides (Faisal et al., 2020; Ghofrani-Isfahani et al., 2020). Conductive metal nanoparticles could simultaneously provide key nutrients, aid synthesis of key enzymes and co-enzymes, as well as facilitate interspecies electron transfer in anaerobic digesters. In-depth effects on dominant metabolic pathways of microbial communities, economic feasibility, environmental friendliness, and pilot-scale tests of the aforementioned additives could be areas for further research.
1.3.2 Multistage bioreactors
The idea of enhancing AD via multistage bioreactors stems from the intrinsic feature of AD, namely, multiple sequential steps of hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The different processes are spatially separated in a multistage bioreactor. The most commonly investigated and utilized two-stage bioreactor (Srisowmeya et al., 2020) is such an example. In a two-stage bioreactor, hydrolysis, acidogenesis, and acetogenesis are carried
out in the first stage with a pH range of 5.0–7.0, while the methanogenesis is conducted in the second stage with a pH between 7.0 and 8.0 (Li et al., 2018). Compared to the traditional single-stage bioreactor, the two-stage bioreactor is superior due to better control of process parameters, providing better functioning environmental conditions for microbial communities and a resultant improved process performance (De Gioannis et al., 2017; Nguyen et al., 2020). Two-stage bioreactors have been utilized in AD of food waste (Srisowmeya et al., 2020), horticultural waste (Li et al., 2018), vinasse (Fu et al., 2017), sewage sludge and glycerol (Silva et al., 2018), landfill leachate (Begum et al., 2018), vegetable waste (Chatterjee & Mazumder, 2020), organic fraction of municipal solid waste (Lavagnolo et al., 2018), algal residue (Lunprom et al., 2019) for the production of biomethane (García‐Ruíz et al., 2020), biohydrogen (Ding et al., 2020), and biohythane (Promnuan et al., 2020). As an enhancement, threestage anaerobic bioreactors with three separate chambers in a single reactor have been proposed to provide more diverse environment for various functional microorganisms responsible for stabilizing the AD process. Hitherto, three-stage AD of food waste (Zhang et al., 2017b), corn stover (Liu et al., 2019), cornstalks (Cheng & Liu, 2012), tofu wastewater (Vistanti & Malik, 2019), ethanol wastewater (Intanoo et al., 2020), food waste + horse manure (Zhang et al., 2017a), food waste + sludge (Zhang et al., 2019e), food waste + horticultural waste (Zhang et al., 2020a), and fruit waste + vegetable waste (Chatterjee & Mazumder, 2018) has been investigated. Compared to the traditional single-stage bioreactor, the three-stage anaerobic bioreactor can simultaneously allow three available pH windows, including pH ∼ 4–5 for substrate hydrolysis, pH ∼ 5–7 for acidification, and pH ∼ 7–8 for methanogenesis in the first, second, and third stage, respectively. The biggest advantage of a multistage bioreactor is that independent optimization of each process can be conducted without interference from other processes. However, the pH control will further increase the operating cost, which may hinder the large-scale application of multistage bioreactors. Accordingly, a study of the economic feasibility of AD using three-stage bioreactors can be conducted in the near future. Governmental subsidies and technical upgrading (e.g., biochar amendment) could be promising methods to increase the feasibility of the three-stage thermophilic bioreactor at an industrial scale (Zhang et al., 2020a).
1.3.3 Bioaugmentation strategies
Bioaugmentation strategies refer to supplementation of exogenous pure culture, preadapted consortium, or genetically modified microbes harboring specific metabolic activities into the bioreactor to facilitate the microbe-mediated multistep AD process (Herrero & Stuckey, 2015). Hitherto, bioaugmentation strategies with microbial consortia have been successfully utilized to enhance biodegradation of lignocellulosic components (Lee et al., 2020a; Tsapekos et al., 2017), mitigate inhibition caused by organic acids and ammonia (Yan et al., 2020), as well as improve production of biogas, hydrogen, and VFA (Atasoy & Cetecioglu, 2020; Jiang et al., 2020). The main target of bioaugmentation in AD of lignocellulosic biomass is to increase the bottleneck hydrolysis rate of cellulose and hemicellulose. In bioaugmentation operations for mitigating ammonia inhibition or acid inhibition, ammonia-tolerant or acid-tolerant microbial strains are supplemented into the respective anaerobic bioreactors. In addition, methanogens or hydrogen-producing strains can also be bioaugmented to promote methanogenesis or enhance metabolic pathways leading to hydrogen production. Common operational
Biomass, biofuels, biochemicals
procedures include obtaining microbial cultures, cultivation of microbial cultures, acclimatization of microbes, and bioaugmentation of acclimated microbes. Additionally, long-term storage of acclimated microbes is a key for potential industrial application of bioaugmentation strategies employing microbial consortia. Key findings in bioaugmentation strategies for enhancing AD in recent years (see Chapter 9) are numerous; however, the reported bioaugmentation studies are frequently limited to lab-scale, which remain inadequate for potential industrial application. More large-scale studies should be conducted to assess the technical and economic feasibility.
1.3.4 Genetically engineered microorganisms
A genetically engineered microorganism refers to a microorganism whose genetic material has been altered by genetic engineering techniques or recombinant DNA technologies. Molecular biology techniques have been developed that can overcome the technical limitations of cultivation-based methods and allowed the identification of noncultivable microorganisms involved in AD (Lee et al., 2017b; Lim et al., 2020). These microbial genomics resources have shown great potential for novel insights and new applications in enhanced biofuel productivity. Subsequent to the identification of the predominant bacteria and methanogens (Lim et al., 2018; Zhang et al., 2019b), genetic engineering tools can be utilized to manipulate specific enzymes by altering its DNA sequence for targeted enhancement of biogas production. Notably, the cost effectiveness could be a critical factor to affect the industrial-scale application of the genetic engineering approach. Providentially, the operation costs of genetic engineering and synthetic biology have been decreasing. Thus, the biggest challenge for large-scale application of engineered microorganisms for biowaste fermentation and biofuel production might not be the cost, but the available standardized genetic units and modules in the area of synthetic biology. Furthermore, synthetic biology approaches allows the building of genetically engineered microorganisms with entirely new sequences of DNA, which can make AD more efficient for biogas production. To do so, a considerable understanding of functional enzymes and genes associated with the acetate oxidation, hydrogenotrophic and aceticlastic methanogenesis pathways must be attained. Presently, genetically engineered microorganisms have been utilized in fermentation of organic wastes for the production of biochemicals such as 2-phenylethanol (Wang et al., 2019), single-cell oils (Bandhu et al., 2019), and limonene (Pang et al., 2019).
1.3.5 Codigestion strategies
Codigestion refers to the simultaneous digestion of two or more substrates in the same reactor, a well-established strategy to overcome the limitations of monodigestion (Solé-Bundó et al., 2019). Codigestion possesses advantages such as the balancing of C:N ratio and dilution of inhibitory/toxic compounds in the bioreactor (Brown & Li, 2013). For instance, monodigestion of food waste is challenging due to high C:N ratio and low pH, while the monodigestion of yard waste is slower due to the high lignocellulosic content, and monodigestion of algal residues is associated with the risk of ammonia inhibition and low biodegradability. By combining these biowastes appropriately, codigestion can effectively enhance the rate of substrate digestion, as well as the rate of methane generation. For instance, food waste has been
Biomass, biofuels, biochemicals
successfully codigested with a wide variety of wastes, including WAS (Du et al., 2020b), animal manure (Oladejo et al., 2020), yard waste (Panigrahi et al., 2020), blackwater (Gao et al., 2020), cymbopogon citratus (Owamah, 2020), rice straw (Kainthola et al., 2020a), algal biomass (Zhang et al., 2020b), etc. Additionally, in some cases, more than two kinds of wastes have been combined for effective codigestion. For instance, a feedstock comprising wheat straw, chicken manure, and sheep manure demonstrated effective AD and enhanced methane production under a fermentative environment of appropriate VFA concentrations and low ammonia (Liu et al., 2015). In addition to AD, the codigestion strategy has also been applied to aerobic fermentation for the production of chemicals such as bioethanol (Lee et al., 2017a), microbial lipids (Chen & Wan, 2017), organic acids (Ong et al., 2019), etc. Codigestion is therefore a promising approach for intensification of production of bioenergy and biochemicals, and could be easily implemented in pilot- and full-scale biorefinery factories.
1.3.6 Feedstock pretreatment strategies
In some recalcitrant substrates, particularly for feedstock with a relatively high lignocellulosic content such as agricultural waste and horticultural waste, pretreatments are essential to improve subsequent fermentation efficiency. In the literature, numerous substrate pretreatment methods have been suggested, including physical, chemical, biological, and combined pretreatments (Atelge et al., 2020; Yu et al., 2019; Zabed et al., 2019; Zhen et al., 2017). Chemical pretreatment can provide better performance in a short period of time, which is a substantial advantage for practical industrial application (Xu et al., 2020). However, the use of strong alkalis or acids could cause potential hazards to the environment. A possible solution is development of more environmentally friendly solvents for chemical pretreatments. In terms of physical pretreatments, hydrothermal pretreatment seems a promising method because it avoids requirement of chemicals (Lee et al., 2020b; Thompson et al., 2020). However, a high energy input is required. Biological pretreatments using microbial species or biological agents have attracted attention as they can avoid the disadvantages of chemicals usage and high energy requirement (Kainthola et al., 2020b). Notwithstanding lower operating costs and being environmentally benign, the main limitation of biological pretreatments would be the relative longer treatment time. A combination of two or more individual pretreatment methods could offer synergistic effects of multiple pretreatment strategies, at the cost of higher pretreatment expenditure (Siami et al., 2020). In the context of the development of a circular economy, the energy performance, economic feasibility, and environmental impact of the aforementioned pretreatment technologies have to be taken into account before adoption.
1.3.7 Application of enzymes
As mentioned previously, hydrolysis is regarded as the rate-limiting step of recalcitrant substrates such as lignocellulosic biomass and sludge (Kainthola et al., 2019; Park et al., 2020). During hydrolysis, complex polymers are converted into corresponding soluble monomers by a suite of extracellular microbial enzymes (Rajin, 2018). Specifically, cellulose polymer is hydrolyzed to cellobiose and glucose monomers, while hemicellulose polymer is hydrolyzed to hexose and pentose monomers. Carbohydrates, lipids, and proteins are hydrolyzed to
Biomass, biofuels,
corresponding monomers, namely, sugars, fatty acids, and amino acids. The concentration of hydrolytic enzymes that are naturally secreted by the microbes in the bioreactor is relatively low. Thus, additional enzymes can be supplemented to facilitate the hydrolysis process of recalcitrant substrates. Essentially, appropriate enzymes as catalysts can decrease activation energy of complex biochemical reactions, thus accelerating the rates of reactions. The advantages of augmentation of enzymes in microbial fermentation processes include high efficiency, high selectivity, environmental friendliness, and operational convenience. A disadvantage of enzyme supplementation is the relatively high cost, which may hinder their industrial application. Thus far, many enzymes have been applied to enhance the performance of the microbial fermentation systems for simultaneous waste degradation and resource recovery (Nigam, 2013; Singh et al., 2016; Singh et al., 2019).
1.3.8 Hybrid technologies
Among the above-mentioned enhancing strategies, various approaches may have different mechanisms by which the enhancement is affected. For instance, some strategies such as pretreatments can enhance the conversion efficiency from the perspective of feedstock properties (Sanchez et al., 2020), while other strategies such as the supplementation of additives (e.g., trace elements) can improve the process from the perspective of nutrition supply (FitzGerald et al., 2019). Strategies such as two-stage (Deng et al., 2019; Liu et al., 2020) and multistage bioreactors (Rabii et al., 2019) can boost the fermentation from the microbial growth and enrichment. By coupling two or more techniques, synergistic effects could be achieved to significantly promote the microbial fermentation processes. So far researchers have sought to combine a variety of the aforementioned strategies. Many of the hybrid technologies or approaches integrate substrate pretreatment with codigestion, additive supplementation, bioreactor optimization, etc. For instance, an increase in methane yield from AD has been reported by integrating alkali-hydrodynamic pretreatment of WAS with two-stage AD process (Grübel & Suschka, 2015); in coupling the codigestion of food waste and animal manure with the operation of a three-stage anaerobic bioreactor (Zhang et al., 2017a); and combining microwave pretreatment with activated carbon supplementation (Zhang et al., 2019d). In addition to AD, cofermentation has also been carried out in two-stage bioreactor systems. Compared with using a single enhancing strategy, hybrid technologies could exhibit higher conversion efficiency and higher productivity. Based on the aforementioned summary, a comprehensive research framework, as shown in Fig. 1.1, is established for enhanced microbial fermentation of common biowastes for production of biofuels and biochemicals. For each strategy, the economic feasibility, environmental sustainability, carbon neutrality, management policies, and system availability should be taken into consideration before implementation.
1.4 Global prospects
Rather than being relegated to the laboratory, several practical industrial applications employing such enhanced microbial fermentation technologies have been operationalized globally. In the biogas industry for example, great efforts have been made by many countries to
Biomass, biofuels, biochemicals
research and develop practical applications of biogas, including Germany, Denmark, United Kingdom, Italy, Sweden, United States, France, China, Singapore, etc. Industrial applications of biogas technology in several countries are listed in Table 1.2
Due to the benefits on reducing the issues of environmental pollution and power energy, many commercial biogas plants have been installed globally. Brazil, Germany, China, the United States, and India are currently leaders of bioenergy production in the world (Tasmaganbetov et al., 2020). As one of the leaders in bioenergy, simultaneous support from different sectors including wastes/water treatment, electricity, agriculture, and natural gas have been given to further decrease technology prices and create a more favorable biogas market in Brazil (Borges et al., 2020). In Germany, the United States, Denmark, and the United Kingdom, at least 9545, 1497, 114, and 265 commissioned AD plants have been installed, respectively (Edwards et al., 2015). Among these countries, the development of Germany’s biogas industry has long enjoyed a good reputation, which is due to advanced biogas technology and equipment, strong support from the federal government (e.g., adaptive Renewable Energy Act), and substantial biogas generation and purification companies (Table 1.2). Sustainable development of biogas industry requires adaptive legislation, national regulation, and healthy market order (Thrän et al., 2020). In Germany, over 40,000 new jobs were created and 37,470 m3/h biogas was produced and utilized in the natural gas pipeline network. Howbeit, some limitations remain, including high investment and operational costs, the complicated structure of the equipment, and the substantial government subsidies needed (Lajdova et al., 2016; Xue et al., 2020). Similarly, the growth of the biogas industry in the United States also requires implementation of some new policies and practices. The US Environmental Protection Agency has adopted biogas from anaerobic digesters as a transportation biofuel under the expanded Renewable Fuel Standard. In addition to regulatory issues, other critical issues in deploying biogas production technology in the United States
FIG. 1.1 A comprehensive research framework for enhanced microbial fermentation of biowastes for production of biofuels and biochemicals.
