Emerging Technologies and Biological Systems for Biogas Upgrading

Emerging Technologies and Biological
Systems for Biogas Upgrading

Emerging Technologies
and Biological Systems
for Biogas Upgrading
Edited by
Nabin Aryal
Department of Biological and Chemical Engineering,
Aarhus University, Aarhus, Denmark
Lars Ditlev Mørck Ottosen
Michael Vedel Wegener Kofoed
Deepak Pant
Separation and Conversion Technology, Flemish Institute for
Technological Research (VITO), Mol, Belgium

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Contents
List of contributors xv
Foreword xix
Preface xxi
Part I
Introduction 1
1. Status of biogas production and biogas upgrading:
A global scenario 3
J. Shanthi Sravan, Athmakuri Tharak and S. Venkata Mohan
1.1 Introduction 3
1.2 State-of-the-art of biogas production and upgradation 4
1.3 Recent trends in biogas utilization: A global prospective 6
1.4 Anaerobic digestion 8
1.4.1 Mechanism of anaerobic digestion 9
1.4.2 Factors affecting biogas production 11
1.5 Biohythane 14
1.6 Electrochemically induced biogas upgradation 16
1.6.1 Conductive materials in biogas upgradation 17
1.7 Challenges and way forward 18
Acknowledgments 19
References 19
Part II
Physiochemical upgrading systems 27
2. Chemical absorption—amine absorption/stripping
technology for biogas upgrading 29
Alastair James Ward, Lu Feng and Henrik Bjarne Møller
2.1 Introduction 29
2.2 Process fundamentals 31
2.2.1 Amine chemistry 31
2.2.2 Amine selection 32
2.2.3 Process description and technology 34
2.2.4 Energy consumption 42
v

2.2.5 Operational problems and emissions 43
2.2.6 Economic considerations 45
2.3 Research and development directions 47
2.3.1 Novel liquid absorbents 47
2.3.2 Water-lean solvents/nonaqueous amine solvents 48
2.3.3 Amine-functionalized solid sorbents 48
2.3.4 Process optimization 49
2.4 Conclusions and future perspectives 50
References 50
Further reading 55
3. Water scrubbing for biogas upgrading:
developments and innovations 57
Valerio Paolini, Patrizio Tratzi, Marco Torre, Laura Tomassetti,
Marco Segreto and Francesco Petracchini
3.1 Introduction 57
3.2 Absorption methodologies 58
3.2.1 Absorption in water (water scrubbing) 58
3.2.2 Absorption in NaOH solutions (alkaline scrubbing) 60
3.2.3 Absorption in K2CO3 solutions (hot potassium
carbonate) 61
3.3 Absorption configurations 62
3.3.1 Packed column reactors 62
3.3.2 Hollow fiber membrane contactors 63
3.4 Chemical promoters in water absorption 64
3.5 Energy consumption 66
3.6 Methane slip and efficiency 67
3.7 Conclusions 68
References 68
4. Factors affecting CO2 and CH4 separation during
biogas upgrading in a water scrubbing process 73
Rimika Kapoor, Pooja Ghosh and Virendra Kumar Vijay
4.1 Introduction 73
4.2 Approaches for CO2 removal from biogas 74
4.3 Water scrubbing technology 75
4.4 Water as a solvent for gases 76
4.5 Solubility of biogas components in water 77
4.6 Factors affecting biogas upgrading in water scrubbing
process 78
4.6.1 Effects of operating parameters on CO2 removal
in water scrubber 78
4.6.2 Effect of packed-bed design parameters 80
vi Contents

4.7 Scrubbing column internals 83
4.7.1 Packing support and gas distributor 83
4.7.2 Liquid distribution and redistribution 86
4.7.3 Demister or entrainment eliminator or mist eliminator 86
4.8 Major challenges and future directions 86
4.9 Conclusion 87
Acknowledgments 88
References 88
5. Recent developments in pressure swing adsorption
for biomethane production 93
Goldy Shah, Shivali Sahota, Virendra Kumar Vijay,
Kamal K. Pant and Pooja Ghosh
5.1 Introduction 93
5.2 Types of swing adsorption technologies 95
5.2.1 Temperature swing adsorption 95
5.2.2 Electric swing adsorption 97
5.2.3 Vacuum swing adsorption 97
5.2.4 Pressure swing adsorption 98
5.3 Parameters influencing pressure swing adsorption 99
5.3.1 Process performance indicators 100
5.3.2 Design parameters 101
5.3.3 Adsorbents 103
5.4 Adsorption isotherm 107
5.5 Adsorption kinetics 109
5.5.1 Molecular diffusion 110
5.5.2 Knudsen diffusivity 110
5.5.3 Poiseuille diffusion or viscous diffusion 110
5.6 Mathematical modeling 112
5.7 Conclusion and future perspectives 113
References 113
6. Membrane-based technology for methane
separation from biogas 117
Birgir Norddahl, M.C. Roda-Serrat, M. Errico and K.V. Christensen
6.1 Introduction: how the basic membrane processes for gas
separation have evolved 117
6.2 Basic terms of gas separation on membranes 120
6.3 Membrane materials and structures 127
6.3.1 Polymer structures and their influence in permeation 127
6.3.2 Inorganic membranes for gas separation 132
Contents vii

6.3.3 Carbon molecular sieve membranes 132
6.3.4 Mixed-matrix membranes 134
6.3.5 Results of membrane operations with different
materials 135
6.4 Theory of transport in gas separation on membranes 135
6.4.1 Transport through rubbery polymers 135
6.4.2 Transport equations through glassy polymers 137
6.5 Membrane configurations and plant design for upgrading
biogas 140
6.6 Recent developments in membrane-based CO2/CH4
separation 142
6.6.1 Biogas upgrading by cryogenic and hybrid
cryogenic-membrane separation 143
6.6.2 Biogas upgrading by absorption and hybrid
absorption-membrane 145
6.6.3 Microbial conversion of CO2 to CH4 on a membrane
diffuser 148
6.7 Summary and outlook 150
6.8 Future developments 151
References 152
7. Cryogenic techniques: an innovative approach
for biogas upgrading 159
Francisco Manuel Baena-Moreno, Luz M. Gallego,
Fernando Vega and Benito Navarrete
7.1 Introduction 159
7.2 Cryogenic biogas upgrading 160
7.2.1 Cryogenic distillation 162
7.2.2 Cryogenic packed-bed technology 163
7.3 Cryogenic hybrid systems 166
7.3.1 Cryogenic-absorption combination process 167
7.3.2 Cryogenic-adsorption synergized process 168
7.3.3 Potential combination of cryogenic and
membrane processes 170
7.3.4 Cryogenic-hydrate processes 170
7.4 Cryogenic-membrane processes 171
7.5 Full-scale experiences and technoeconomic studies 176
7.6 Comparison of documented technologies 177
7.7 Conclusions and future perspectives 179
Appendix I: Conversion factor for unit transformations 180
Appendix II: State forms for CO2 and CH4 as a function of
temperature and pressure 180
Acknowledgments 181
References 181
viii Contents

8. Power-to-gas for methanation 187
Anirudh Bhanu Teja Nelabhotla, Deepak Pant and
Carlos Dinamarca
8.2 Electrocatalytic methanation 189
8.2.1 Alkaline electrolyzers 190
8.2.2 Polymer electrolyte membrane electrolyzers 193
8.2.3 Solid oxide electrolyzers 196
8.2.4 Fixed-bed methanation reactors 198
8.2.5 Fluidized bed methanation reactors 200
8.2.6 Three-phase reactor 202
8.2.7 Micro(channel) reactors 203
8.3 Bioelectrochemical methanation 205
8.3.1 Direct electron transfer 207
8.3.2 Biocathodes 209
8.3.3 Reactor configurations 210
8.4 Challenges and future prospects 211
References 213
9. Electrochemical approach for biogas upgrading 223
Grzegorz Pasternak
9.2 Faradaic and energy efficiency 226
9.3 Electroreduction of CO2 226
9.3.1 Basic considerations 227
9.3.2 Reactor and process design 230
9.4 Electrochemical oxidation of H2S 236
9.4.1 Basic considerations 237
9.4.2 Reactor and process design 237
9.5 Biogas upgrading approach and its challenges 241
9.5.1 CO2 electroreduction 241
9.5.2 H2S oxidation 243
9.5.3 Biogas and scale-up approaches 244
9.6 Concluding remarks and perspectives 246
Acknowledgments 247
References 247
10. Siloxanes removal from biogas and emerging
biological techniques 255
Kazimierz Gaj
Contents ix

10.2 Methods for reducing the content of volatile
organic silicon compounds in biogas 258
10.2.1 Pretreatment methods 258
10.2.2 Refrigeration and freezing methods 259
10.2.3 Adsorption methods 260
10.2.4 Absorption methods 276
10.2.5 Membrane techniques 277
10.2.6 Biological methods 278
10.3 Combined methods for volatile organic silicon
compounds removal from biogas 281
10.4 Comparison of the methods for reducing the
content of volatile organic silicon compounds in biogas 281
10.5 Conclusions and future perspective 286
References 288
Part III
Biological upgrading systems 293
11. Technologies for removal of hydrogen sulfide
(H2S) from biogas 295
Anish Ghimire, Raju Gyawali, Piet N.L. Lens and
Sunil Prasad Lohani
11.2 Technologies for removal of biogas contaminants 297
11.3 Physicochemical removal technologies 298
11.3.1 Absorption process 298
11.3.2 Adsorption process 300
11.3.3 Membrane separation 304
11.4 Ex situ removal using sulfur-oxidizing microorganisms 306
11.4.1 Biological air filtration 306
11.4.2 Microalgal removal of H2S 311
11.5 In situ H2S removal 312
11.5.1 In situ microaeration 312
11.5.2 Dosing iron salts/oxides into the digester 313
11.6 Combined chemical-biological processes 314
11.7 Comparison of H2S removal techniques 314
11.8 Conclusions 316
References 317
12. Biological upgrading of biogas through CO2
conversion to CH4 321
Michael Vedel Wegener Kofoed, Mads Borgbjerg Jensen
and Lars Ditlev Mørck Ottosen
12.1 Biogas upgrading 321
12.2 Hydrogen generation and utilization 324
x Contents

12.3 Methanation 326
12.4 Microbial basis for biomethanation 327
12.4.1 Methanogens 327
12.4.2 Processes in anaerobic digestion 328
12.5 Reactor configurations 330
12.5.1 In situ biomethanation 331
12.5.2 Ex situ biomethanation 332
12.6 Factors controlling biomethanation 337
12.6.1 Mass transfer of H2 337
12.6.2 Temperature 343
12.6.3 Growth requirements 344
12.6.4 pH and CO2 346
12.6.5 Bacterial interaction and competition 348
12.7 Reactor design for biological methanation 350
12.7.1 Continuous stirred tank reactor 350
12.7.2 Trickle-bed reactors 352
12.8 Future perspectives and applications 354
Abbreviations list 357
References 357
13. Bioelectrochemical systems for biogas upgrading
and biomethane production 363
Nabin Aryal, Lars Ditlev Mørck Ottosen, Anders Bentien,
Deepak Pant and Michael Vedel Wegener Kofoed
13.1 Background 363
13.2 Fundamentals of bioelectrochemical biogas upgrading 364
13.3 Methane enrichment of biogas 368
13.3.1 Electron transfer mechanism 368
13.3.2 Microbial communities in biocathode for methane
enrichment 370
13.3.3 State-of-the-art bioelectrochemical biogas upgrading 370
13.4 Economical insights 375
13.5 Prospective and challenges 376
13.6 Conclusion 378
Acknowledgments 378
References 378
14. Photosynthetic biogas upgrading: an attractive
biological technology for biogas upgrading 383
Vijay Kumar Garlapati, Swati Sharma and Surajbhan Sevda
14.2 Positive attributes of photosynthetic “microalgae”
toward biogas upgradation 385
Contents xi

14.3 CO2 and H2S removal through photosynthetic-bacterial
associated biogas upgradation 386
14.4 Microalgae-based biogas upgrading and concomitant
wastewater treatment 387
14.5 Photobioreactor designs for biogas upgradation 388
14.6 Impact of different process variables in biogas upgradation 392
14.6.1 Light intensity 392
14.6.2 Media pH 394
14.6.3 Temperature 394
14.6.4 Biogas composition 395
14.6.5 Gas flow rate 395
14.7 The future prospects 396
14.8 Conclusion 401
References 401
Part IV
Policy implications for biogas upgrading 411
15. Biogas upgrading and life cycle assessment of
different biogas upgrading technologies 413
Moonmoon Hiloidhari and Shilpi Kumari
15.2 Biomethanation 415
15.2.1 Cleaning of biogas 416
15.2.2 Upgrading of biogas into biomethane 417
15.3 Brief overview of life cycle assessment 421
15.4 Life cycle assessment of biogas upgrading technologies 423
Acknowledgment 441
References 441
Further reading 445
16. The role of techno-economic implications and
governmental policies in accelerating the
promotion of biomethane technologies 447
Dhamodharan Kondusamy, Mehak Kaushal, Saumya Ahlawat and
Karthik Rajendran
16.2 Role of techno-economic studies in anaerobic digestion 449
16.2.1 Feedstocks 450
16.2.2 Gas purification technology 450
16.2.3 Biogas utilization 451
16.2.4 Subsidies 452
xii Contents

16.3 Successful policies in anaerobic digestion implementation 452
16.3.1 Policies and regulations 452
16.3.2 Renewable energy-related policies and regulations 453
16.3.3 Agriculture policies and regulations 454
16.3.4 Waste management policies 454
16.3.5 Incentives 455
16.3.6 Policy instruments introduced in various countries
as a support to AD industry growth 456
16.4 Decision-support system for biomethane implantation
with techno-economic analysis and policies 461
16.5 Conclusion 462
References 463
17. Large-scale biogas upgrading plants: future
prospective and technical challenges 467
Ram Chandra Poudel, Dilip Khatiwada, Prakash Aryal and
Manju Sapkota
17.2 Biogas composition and feedstock types 468
17.3 Biogas upgrading for natural gas grid injection and
transport fuel 468
17.4 State-of-the-art of large-scale biogas upgrading
technologies 472
17.4.1 Physicochemical upgrading technologies 472
17.4.2 Power-to-gas technology for methanation 474
17.4.3 Bioelectrochemical system (Cambrian Innovation) 482
17.4.4 Photosynthetic biogas upgrading system 484
17.5 Conclusion and future perspective 485
References 486
Index 493
Contents xiii

List of contributors
Saumya Ahlawat Department of Biosciences and Bioengineering, Indian Institute of
Technology Guwahati, Guwahati, India
Nabin Aryal Department of Biological and Chemical Engineering, Aarhus
University, Aarhus, Denmark
Prakash Aryal Department of Chemical Engineering, Monash University, Clayton,
VIC, Australia
Francisco Manuel Baena-Moreno Chemical and Environmental Engineering
Department, Technical School of Engineering, University of Seville, Sevilla,
Spain
Anders Bentien Department of Biological and Chemical Engineering, Aarhus
K.V. Christensen Department of Green Technology, Faculty of Engineering,
University of Southern Denmark, Odense M, Denmark
Carlos Dinamarca Department of Process Energy and Environmental Technology,
University of South-Eastern Norway, Porsgrunn, Norway
M. Errico Department of Green Technology, Faculty of Engineering, University of
Southern Denmark, Odense M, Denmark
Lu Feng Department of Biological and Chemical Engineering, Aarhus University,
Aarhus, Denmark
Kazimierz Gaj Department of Environment Protection Engineering, Wroclaw
University of Science and Technology, Wrocław, Poland
Luz M. Gallego Chemical and Environmental Engineering Department, Technical
School of Engineering, University of Seville, Sevilla, Spain
Vijay Kumar Garlapati Department of Biotechnology and Bioinformatics, Jaypee
University of Information Technology, Waknaghat, India
Anish Ghimire Department of Environmental Science and Engineering, Kathmandu
University, Dhulikhel, Nepal
Pooja Ghosh Centre for Rural Development and Technology, Indian Institute of
Technology Delhi, New Delhi, India
Raju Gyawali Nepal Electricity Authority, Government of Nepal, Kathmandu,
Nepal
Moonmoon Hiloidhari IDP in Climate Studies, Indian Institute of Technology
Bombay, Mumbai, India
xv

Mads Borgbjerg Jensen Department of Biological and Chemical Engineering,
Rimika Kapoor Centre for Rural Development and Technology, Indian Institute of
Mehak Kaushal System Biology for Biofuel Group, International Centre for Genetic
Engineering and Biotechnology, New Delhi, India
Dilip Khatiwada Division of Energy Systems, Department of Energy Technology,
KTH Royal Institute of Technology, Stockholm, Sweden
Michael Vedel Wegener Kofoed Department of Biological and Chemical
Engineering, Aarhus University, Aarhus, Denmark
Dhamodharan Kondusamy Department of Civil Engineering, Indian Institute of
Technology Guwahati, Guwahati, India; Institute of Soil, Water and
Environmental Science, Agricultural Research Organization, Israel
Shilpi Kumari Centre for Energy Studies, Indian Institute of Technology Delhi,
New Delhi, India
Piet N.L. Lens UNESCO—IHE Institute for Water Education, Delft, The Netherlands
Sunil Prasad Lohani Department of Mechanical Engineering, Kathmandu
University, Dhulikhel, Nepal
S. Venkata Mohan Bioengineering and Environmental Sciences Lab, Department of
Energy and Environmental Engineering, CSIR-Indian Institute of Chemical
Technology (CSIR-IICT), Hyderabad, India; Academy of Scientific and Innovative
Research (AcSIR), Ghaziabad, India
Henrik Bjarne Møller Department of Biological and Chemical Engineering, Aarhus
Benito Navarrete Chemical and Environmental Engineering Department, Technical
Anirudh Bhanu Teja Nelabhotla Department of Process Energy and Environmental
Technology, University of South-Eastern Norway, Porsgrunn, Norway
Birgir Norddahl Department of Green Technology, Faculty of Engineering,
Lars Ditlev Mørck Ottosen Department of Biological and Chemical Engineering,
Deepak Pant Separation and Conversion Technology, Flemish Institute for
Technological Research (VITO), Mol, Belgium
Kamal K. Pant Department of Chemical Engineering, Indian Institute of
Valerio Paolini National Research Council of Italy, Institute of Atmospheric
Pollution Research, Monterotondo, Italy
Grzegorz Pasternak Laboratory of Microbial Electrochemical Systems, Department
of Process Engineering and Technology of Polymer and Carbon Materials,
Wroclaw University of Science and Technology, Wrocław, Poland
xvi List of contributors

Francesco Petracchini National Research Council of Italy, Institute of Atmospheric
Ram Chandra Poudel Department of Biological Sciences, University of Bergen,
Bergen, Norway
Karthik Rajendran Department of Environmental Science, SRM University-AP,
Mangalagiri, India
M.C. Roda-Serrat Department of Green Technology, Faculty of Engineering,
Shivali Sahota Centre for Rural Development and Technology, Indian Institute of
Manju Sapkota Institute of Chemistry, Bioscience and Environmental Engineering,
Faculty of Science and Technology, University of Stavanger, Stavanger, Norway
Marco Segreto National Research Council of Italy, Institute of Atmospheric
Surajbhan Sevda Department of Biotechnology, National Institute of Technology
Warangal, Warangal, India
Goldy Shah Centre for Rural Development and Technology, Indian Institute of
Swati Sharma Department of Biotechnology and Bioinformatics, Jaypee University
of Information Technology, Waknaghat, India
J. Shanthi Sravan Bioengineering and Environmental Sciences Lab, Department of
Technology (CSIR-IICT), Hyderabad, India; Academy of Scientific and
Innovative Research (AcSIR), Ghaziabad, India
Athmakuri Tharak Bioengineering and Environmental Sciences Lab, Department of
Technology (CSIR-IICT), Hyderabad, India
Laura Tomassetti National Research Council of Italy, Institute of Atmospheric
Marco Torre National Research Council of Italy, Institute of Atmospheric Pollution
Research, Monterotondo, Italy
Patrizio Tratzi National Research Council of Italy, Institute of Atmospheric
Fernando Vega Chemical and Environmental Engineering Department, Technical
Virendra Kumar Vijay Centre for Rural Development and Technology, Indian
Institute of Technology Delhi, New Delhi, India
Alastair James Ward Department of Biological and Chemical Engineering, Aarhus
List of contributors xvii

Foreword
Global energy demand is increasing to fulfill the
growing human population needs, with fossil fuels
being the most dominating source. One of the most
significant environmental problems associated with
fossil fuel use is the emission of greenhouse gases
(GHGs), leading to global warming and creating pro-
blems related to climate change. Increasing the supply
of renewable energy sources would replace fossil
sources and significantly limit the dominating carbon-
intensive fossil fuels in the future energy system.
Therefore the development and utilization of renew-
able energy sources such as solar, bioenergy, wind, hydro, and geothermal are
essential to mitigate the environmental problems associated with GHG emis-
sions. Bioenergy, where biomass produced via photosynthesis can be con-
verted to biofuel (biogas), heat, and electricity, is the most widely used form
of renewable energy.
Biogas can be produced from the anaerobic digestion process and can be
utilized as a fuel for cooking, industrial processes, and transportation fuels.
Nevertheless, biogas contains significant amounts of carbon dioxide and other
constituents such as hydrogen sulfide that have to be removed prior to applica-
tion as a natural gas substitute. By upgrading the biogas and thereby increas-
ing its methane content, the resulting biomethane can replace natural gas
obtained from fossil sources. Recently, emerging technologies for biogas
upgrading, such as microbial-based and cryogenic-based technologies, have
been developed. However, available information about these technologies is
limited. This book represents a milestone by providing the technical knowl-
edge and information on emerging biogas upgrading technologies.
This book on Emerging Technologies and Biological Systems for Biogas
Upgrading provides fundamental knowledge on anaerobic digestion, the
global scenario of biogas production, state-of-the-art information on upgrad-
ing, and policy implications for promoting the utilization of upgraded biogas.
The book deals with physiochemical upgrading systems with great insight into
absorption, scrubbing, membrane separation, electrochemical, and cryogenic
techniques. The book furthermore presents technologies currently under devel-
opment, including biological and bioelectrochemical power-to-gas technologies
xix

employed for biogas upgrading. The editors have put together a host of
highly relevant topics and experts in their respective fields to contribute with
thoroughly described chapters.
What I like about this book is the information about the current state-of-the-
art, the practical information, and highly qualified consortium of authors, who
have contributed with their knowledge in each chapter, which will be highly
beneficial for researchers, university students, biogas developers. and practi-
tioners who are entering into the biogas production and biogas-upgrading field.
Ashok Pandey
Editor-in-Chief, Bioresource Technology, Elsevier
xx Foreword

Preface
Biogas is a methane-rich gas produced from biological degradation of bio-
mass. The anaerobic digestion (AD) process has been commercially initiated
to produce methane (CH4) from organic waste degradation that significantly
contributes to global renewable energy production and consumption. Biogas
has an important role in the global carbon cycle and has traditionally been
used as an alternative renewable energy source. Especially in developing
countries, a large part of the rural population relies on decentralized small-
scale biogas digestors for meeting their household energy needs and further-
more for utilizing the digestate from such plants as a source of nutrient-rich
fertilizer for their soils. Worldwide uncontrolled solid waste production leads
to greenhouse gas (GHG) emissions in the form of carbon dioxide (CO2) and
CH4, contributing significantly to climate change. Hence, harvesting biogas
from the organic waste stream results in an environmentally sustainable
source of renewable energy while reducing GHG emissions. According to the
World Biogas Association, the annual biogas production in 2018 exceeded
60.8 billion m3
, of which almost 54% was in Europe. The global production is
projected to increase further worldwide, illustrating the worth of biogas, espe-
cially in a scenario with dramatic reductions in the consumption of fossil
fuels.
Biogas predominantly consists of 40% 60% CH4, 60% 40% CO2, and
traces of hydrogen sulfide (H2S), ammonia (NH3), hydrogen (H2), oxygen (O2),
nitrogen (N2), siloxanes, carbon monoxide (CO), hydrocarbons, and volatile
organic compounds. Primarily, the CO2 content in biogas lowers its heating
value compared to natural gas, and the presence of other constituents may cause
corrosion and salt accumulation on the associated appliances such as boilers,
burners, and gas engines.
Biogas therefore has to be treated and conditioned to improve its gas heating
value and downstream applicability. The gas can ultimately reach gas grid qual-
ity by upgrading it even further through removal or conversion of CO2.
Recently, biogas upgrading has gained intense attention due to national targets
for renewable energy production, environmental concerns, and the need to
replace fossil fuels with sustainable fuel alternatives. Several technologies are
today commercially available and implemented for biogas upgrading at com-
mercial biogas plants, including water scrubbing, amine scrubbing, pressure
swing adsorption, and membrane-based technologies. Among the implemented
xxi

biogas upgrading technologies, water scrubbing is a widely applied technology
that accounts for almost 40% of the total upgrading. These scrubbers upgrade
the biogas by removing CO2 from biogas and emitting it to the atmosphere.
The focus on reducing emissions from the biogas industry and utilizing
biogas as a source of CO2 has spawned the development of alternative tech-
nologies for biological, bioelectrochemical, and chemical biogas upgrading.
These technologies are at different technological readiness levels but all rep-
resent promising solutions for reducing the carbon footprint of the biogas
industry and at the same time increasing its importance as a supplier of
renewable energy and chemicals. As an example, biological methanation
today constitutes a promising technology for converting biogas CO2 to CH4
by the use of electricity from renewable sources through a process that com-
bines carbon capture and utilization with energy conversion (Power-to-X).
The development and demonstration of new technologies is not only an aca-
demic exercise but also includes heavy industrial involvement. Nonetheless,
comprehensive access to technical information on biogas upgrading technol-
ogy remains limited. To overcome such a gap, this book intends to provide
complete technical details on biogas upgrading. Each chapter of the book is
designed to give a fully comprehensive and most recent state-of-the-art on
different technologies currently in use or under development for biogas
cleaning and upgrading.
In this book, fundamental principles, state-of-the-art, biogas cleaning, and
upgrading technologies for CO2, H2S, and siloxane removal or conversion
have been elaborated. The book begins by outlining the global scenario of
biogas production and upgrading processes, followed by an insight into phys-
icochemical upgrading systems that have been implemented at an industrial
scale. The critical process parameter optimization of absorption/stripping
technology, and optimization of cryogenic, membrane, and power-to-gas are
discussed. The subsequent chapters describe biological cleaning and conver-
sion of H2S, H2-mediated CO2 conversion, bioelectrochemical conversion of
CO2 to CH4, and algal-based photosynthetic biogas upgrading. Finally, the
implications of policies for biogas upgrading are provided. In a nutshell, the
information in this book brings insights into technologies and processes for
biogas treatment and upgrading. This book is aimed at a broad audience,
mainly researchers, biogas specialists, academics, entrepreneurs, industrial-
ists, policymakers, and others who wish to know the latest developments and
future perspectives of biogas upgradation approaches for the enhancement of
the existing digestors, and also discusses the bottlenecks of the various tech-
nologies that currently limit scale-up and commercialization.
The chapters are written by experts in the field from all parts of the
world. Consolidation of the most recent state-of-the-art into an independent
chapter for each type of physical, chemical, or biotechnological upgrading
system is the main aim of this book. A key point of the book is that it also
provides guidance on which procedures should be followed under what
xxii Preface

conditions to get the best results in terms of upgrading. It is our sincere hope
that this book will contribute to the necessary transition to environmentally
benign and sustainable adoption of biogas in general and biogas upgrading
approaches in particular. Though we have tried to be objective in our choice
of topics to be covered in this book, some not so common themes which
may become important in the future may have been missed out, we will try
to cover them in the second edition of the book.
This book is intended to have three roles and to serve three associated
audiences, namely, the students and research community who will benefit
from the lucid explanation of the possible applications of biogas upgradation
for the betterment of environment, the policymakers who will find it easier
to identify the pros and cons of different upgradation systems, and finally,
the industries involved, as it will give them a feeling about the current loop-
holes (technological possibilities and possibilities for optimization) and ways
to fix them. Each chapter begins with a fundamental explanation for general
readers and ends with in-depth scientific details suitable for expert readers.
The text in all the chapters is supported by numerous clear, illustrative, and
informative diagrams, flowcharts, and comprehensive tables detailing the sci-
entific advancements, providing an opportunity to understand the process
thoroughly and meticulously. Written in an eloquent style, the book compre-
hensively covers each point to give the reader a holistic picture of biogas
treatment technologies and the future perspective of their use. The book may
even be adopted as a textbook for university courses that deal with such
courses related to both energy and the environment.
Despite the best efforts of the authors and editors, along with extensive
checks conducted by many experts in the field of biogas and AD, mistakes
may have crept in inadvertently. We would appreciate if readers could high-
light these and make comments or suggestions to improve and update the
book contents for future editions.
Nabin Aryal1
, Lars Ditlev Mørck Ottosen1
,
Michael Vedel Wegener Kofoed1
and Deepak Pant2
1
Department of Biological and Chemical Engineering, Aarhus University, Aarhus, Denmark,
2
Separation and Conversion Technology, Flemish Institute for Technological Research (VITO),
Mol, Belgium
Preface xxiii

Chapter 1
Status of biogas production and
biogas upgrading: A global
scenario
J. Shanthi Sravan1,2
, Athmakuri Tharak1
and S. Venkata Mohan1,2
1
Bioengineering and Environmental Sciences Lab, Department of Energy and Environmental
Engineering, CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad, India,
2
Academy of Scientific and Innovative Research (AcSIR), Ghaziabad, India
Chapter outline
1.1 Introduction 3
1.2 State-of-the-art of biogas production
and upgradation 4
1.3 Recent trends in biogas utilization:
A global prospective 6
1.4 Anaerobic digestion 8
1.4.1 Mechanism of anaerobic
digestion 9
1.4.2 Factors affecting biogas
production 11
1.5 Biohythane 14
1.6 Electrochemically induced biogas
upgradation 16
1.7 Challenges and way forward 18
Acknowledgments 19
References 19
1.1 Introduction
Biogas is considered one of the most sought-after bioenergy resources across
the world to overcome the environmental and energy challenges. It has
numerous applications in the household, domestic, transportation, gas grid
areas, and as a substrate for platform chemicals generation. Conventionally,
biogas upgradation (BU) is performed by physico-chemical (absorption,
adsorption, membrane seperation, and cryogenic) and biological (in situ and
ex situ) processes which are site/case specific (Baena-Moreno et al., 2020;
Kapoor et al., 2019; Vrbova and Ciahotny, 2017; Munoz et al., 2015).
Limitations need to be addressed for managing of energy and carbon flux,
which could essentially benefit the process efficiency (Aryal et al., 2018;
Salihu and Alam, 2015). BU technologies need to focus mainly on the reuse
of impurities such as CO2, H2S, and other gases generated from individual
3
Emerging Technologies and Biological Systems for Biogas Upgrading.
DOI: https://doi.org/10.1016/B978-0-12-822808-1.00002-7
© 2021 Elsevier Inc. All rights reserved.

processes in the presence of electron donors which are essential to produce
methane (CH4) with above 90% productivity (Nelabhotla et al., 2019; Sahota
et al., 2018; Scholz et al., 2013).
BU can potentially enhance the conversion rate/efficiency of organic substrates
with further use of impurities such as CO2, H2S, etc. in the presence of
electron donors like H2 and volatile fatty acids (VFAs) towards CH4 production
and its applications (Sravan et al., 2020; Nelabhotla and Dinamarca, 2018; Venkata
Mohan et al., 2016b; Jiang et al., 2013). Electrochemical interference towards CH4
production, called electromethanogenesis (EM), can influence the direct interspe-
cies electron transfer (DIET) with a focus on creating a microbeelectrode
synergy that neutralizes the disruptors influence (from side reactions) on enhancing
the biogas production (Zhou et al., 2017; Lovley, 2017; Simon, 2015). BU for cen-
tralized and decentralized applications needs to provide specific interventions and
strategies that could beneficially influence the microbial electrometabolism and its
energy dynamics, while focusing on the techno-economics toward increasing the
overall CH4 yields at an industrial scale.
1.2 State-of-the-art of biogas production and upgradation
Biogas is an important source of renewable energy that contributes signifi-
cantly in terms of overall calorific value (Baena-Moreno et al., 2020;
Koonaphapdeelert et al., 2020; Curto and Martin, 2019). Biogenic solid and
liquid wastes are degraded in anaerobic digestors for CH4 generation along
with other gases and heat generation (Gotz et al., 2016; Prasad et al., 2017;
Singhal et al., 2017; Subbarao, 2018; Venkata Mohan et al., 2017). The break-
down of organic matter contributes to various biogas constituents, namely,
CH4 (5070%), CO2 (3050%), and other trace gases(0.13%) (Baena-
Moreno et al., 2019; Bharathiraja et al., 2018; Kulkarni and Ghanegaonar,
2020; Maurya et al., 2019). Biogas utilization in the gas grid as energy
requires a CH4 content of at least 90%, with decreased CO2, H2, H2S, etc.,
composition (Angelidaki et al., 2018; Kadam and Panwar, 2017). Usually pro-
cesses like absorption, membrane seperation, scrubbing, and water washing
are used to remove the excess CO2 generated (Adnan et al., 2019; Angelidaki
et al., 2018). In situ integrated processes that could potentially upgrade the
biogas to CH4 within the anaerobic digestion (AD) system could benefit the
energy sector (Sarker et al., 2018; Luo and Angelidaki, 2012). Alternative pro-
cesses to upgrade biogas to CH4 include utilizing the excessive CO2 in the
presence of H2 in the AD systems. This upgraded CH4 could be used to meet
the requirements of the gas grid and other energy systems (Fig. 1.1).
Globally, biogas is being exploited with great interest as a substitute for natural
gas. Hence, increasing the calorific value of biogas by removing CO2 and other
trace gases for CH4 upgradation is essential (Kapoor et al., 2019; Sun et al.,
2015a). BU by fixation of CO2 and H2 in the presence of redox intermediates to
4 PART | I Introduction

CH4 remains undeveloped, but is gaining interest in the context of renewable
energy utilization (Alvarez-Gutierrez et al., 2016; Angelidaki et al., 2019).
Commercial biogas upgrading systems using conventional processes can perform
only the separation of CH4 from CO2 and require additional integration of individ-
ual processes to increase the efficiency of CH4 conversion and to also avoid carbon
emissions (Baena-Moreno et al., 2020; Xu et al., 2018; Rodero et al., 2018;
Vrbova and Ciahotny, 2017; Yuan et al., 2013). The lower density of H2 requires
higher storage capacities, while its transportation and direct utilization as a technol-
ogy is still under development. Hence, the transformation of H2 to CH4 is consid-
ered appropriate and could be considered beneficial for its utilization as a natural
gas (Fig. 1.2).
Methane is advantageous over H2 due to its higher volumetric energy density
and its readily available existant infrastructure for its utilization/storage towards
application feasibility (Luo et al., 2012). H2 is readily utilized in lab-scale sys-
tems for converting CO2 to CH4 with increased conversion efficiency (Luo et al.,
2012; Maegaard et al., 2019; Sun et al., 2015b). The increased H2 utilization influ-
ences VFA accumulation because of homoacetogenesis leading to higher
acidification (Liu et al., 2016). The major limitations to BU are pH regulation and
CO2 utilization, where the pH range needs to be maintained between 6.58.5 to
increase the CH4 production (Bassani et al., 2015; Luo and Angelidaki, 2012).
A biological method of BU is considered as a potential alternative for
CH4 production using various microbial genera such as hydrogenotrophic
methanogens, acetoclastic methanogens, and microalgae (Meier et al., 2017;
Muha et al., 2012). In this process, CO2 and H2 are biologically converted to
Biogas upgradaon Biomethane and biogas upgradaon Anaerobic digeson and power grid
Waste to biomethane and power grid Biomethane and power grid Biogas and power grid
FIGURE 1.1 Scientometric analysis of the current state-of-the-art on biogas upgradation.
Status of biogas production and biogas upgrading: A global scenario Chapter | 1 5

CH4 involving the action of hydrogenotrophic methanogens without any
additional energy inputs [Eq. (1.1)].
4H2
1 CO2
-CH4
1 2H2O ΔGO
5 2 130:7 kJ=mol: ð1:1Þ
H2 injection with a stoichiometric ratio of 4:1 between H2 and CO2 during
hydrogenotrophic methanogenesis increases CO2 utilization thus leading to a
pH increase, which is one of the main influencing parameters for efficient
performance of the methanogenic population (Liu et al., 2016;
Luo and Angelidaki, 2012; Maegaard et al., 2019; Siegert et al., 2015). Higher
alkaline pH values usually limit the methanogenic activity, while CO2
utilization helps overcome the substrate inhibition for autotrophic hydrogeno-
trophic methanogens towards CH4 production. The methanogenic population
in AD systems usually consists of acetoclastic and hydrogenotrophic methano-
gens, majorly contributing to the CH4 production (Liu et al., 2018; Christy
et al., 2014; Luo et al., 2012; Sarkar and Venkata Mohan, 2020). A higher H2
presence helps towards the enrichment of hydrogenotrophic methanogens like
Methanomicrobium, Methanoculleus, and Methanobacterium, which relatively
increases the rate of methanogenesis (Luo and Angelidaki, 2012; Bassani
et al., 2015).
1.3 Recent trends in biogas utilization: A global prospective
Biogas is one of the most important sources of renewable energy produced
from anaerobic digestors and could contribute significantly in terms of
energy value (Curto and Martin, 2019). Biological CH4 production is pro-
duced by dark fermentation, where the organic substrate is converted into
In situ
(liquid gas
interacon)
Ex situ
(removal of
CO impuries
from biogas)
Cryogenic
separaon
Hydrogenotrophic
methanogenesis
Absorpon
technology
Biological
technique
Absorpon
technology
Membrane
technology
Water
scrubbing
Biogas
upgrading
FIGURE 1.2 Various biogas upgradation techniques/processes.

biogas, biofuels, and other value-added intermediate products. Electrodes
and a polarizing microenvironment with applied potential could be beneficial
in regulating the microbial metabolism and increasing the substrate conver-
sion rate towards BU (Castellano-Hinojosa et al., 2018; Dou et al., 2018; Liu
et al., 2016; Nikhil et al., 2015; Schroder et al., 2015; Zhao et al., 2016).
Apart from these, redox intermediates (activated carbon/biochar/magnetic
field) act as redox shuttles for electron acceptance, influencing the microbial
electrogenic activity towards decreased losses and increasing CH4 recovery.
Integration of a polarized microenvironment with dark fermentation, called
electromethanogenesis (EM) could be innovative in increasing the CH4 con-
version rate, productivity, and calorific value.
EM regulates the electron flux with the endogenous or applied potential
establishing synergistic redox microbeelectrode and microbemicrobe
interactions (Sravan et al., 2020; Modestra et al., 2015a,b). Microbial
activity with deprived electrons needs an increased energy conversion rate
and therefore could utilize the applied potential to regulate their metabo-
lism, resulting in higher CH4 production (Ren et al., 2019; Villano et al.,
2017; Jin et al., 2017). BU/EM systems emphasize electrode material
placement, biocatalyst, system design, and operation. Dark fermentation is
mainly focused on short-chain carboxylic acids (C2C6) and alcohol pro-
duction, with respect to methanogenic microbial suppression losing a
highly significant amount of energy in the form of biogas. However,
EM stresses streamlining towards higher CH4 production to meet the
increasing demands with regulated microbial metabolism, microenviron-
ment, and energetics.
Biogas production has recently focused specifically on CO2 capture.
Electrochemical CO2 reduction to CH4 selectively orients for BU using
bioelectrochemical systems (BES), and is described as power to gas technol-
ogy (Collet et al., 2017; Stangeland et al., 2017; Zhao et al., 2016; Xu et al.,
2014). Overcoming the limitations of DIET and cathode development shows
a marked effect on CO2 reduction and CH4 production, involving electroac-
tive microbial catalysis with polarized electrodes and applied potential
(Dykstra and Pavlostathis, 2017; Fu et al., 2015; Sravan et al., 2020). An
optimum pH of 7.07.5 helps to increase the biogas productivity and also
favors enrichment of electroactive methanogens.
Several studies have approached in the direction of integrating BES with
AD for biogas upgradation with increased purity. A polarized microbial envi-
ronment with the interference of electrodes in the microenvironment favors
the conversion of raw biogas components with the support of intermediate
acceptors to CH4 (Jiang et al., 2019; Sravan et al., 2020; Venkata Mohan
et al., 2014; Xu et al., 2014). In situ developed or applied potential regulates
the charge transfer kinetics between the electroactive microbial populations
and inert electrode surfaces, enhancing the CH4 production (Ren et al., 2019;
Paiano et al., 2019). Applied potential drives the methanogenic metabolic

pathways in order to increase the substrate conversion rates along with pro-
ductivity (Sarkar and Venkata Mohan, 2020; Sravan et al., 2020; Meier
et al., 2017; Liu et al., 2016). Hydrogenotrophic methanogens were studied
to increase the CH4 production from endogenous H2 produced by reutilizing it
as an additional electron donor, whereas acetoclastic methanogens were
involved in the inhibition of H2 towards CH4 production. Different types of
feedstocks like municipal solid waste, spent wash, domestic food waste, and
C1 gases like CO2 and CO were used in BES to achieve higher BU rates
(Jiang et al., 2013; Zhen et al., 2017) (Fig. 1.3).
1.4 Anaerobic digestion
AD is a conventional process for the biological conversion of organic sub-
strate to biogas, mainly CH4 and CO2, with other trace gases along with
other value-added products. AD requires a longer operation time to achieve
effective substrate removal and value-addition yields. The lower conversion
efficiencies during AD (upto 60%) are considered as a disadvantage for the
process performance. Recently, BU has gained significance for the improve-
ment of CH4 content in total biogas yields, along with value-addition with
ADBES integration. The biological conversion efficiency of organic sub-
strate in the presence of H2 and CO2 to CH4 occurs at the anode/cathode in
BES, and is also called the EM process (Siegert et al., 2015).
Biogenic waste
(substrate) inlet
Gas outlet
Biogas
CH
CO
H , H S
Effluent oulow
Substrate
Anaerobic bacteria
Raw biogas
Acid/base inflow
Motor
Baffles
Impeller
CH 50%–75%
20%–40%
CO
H , H S
Membrane
separaon of CO
CO reducon to
methane
Biomethane
(methane 90%–95%)
Gas grid
Raw biogas
Phase II : Biogas upgradaon
Household supply Industrial supply
Phase I : Biogas producon
5%–10%
FIGURE 1.3 Integration of anaerobic digestors with biogas upgradation to increase methane
production.

Single-chambered AD and BES have shown improved CH4 efficiency
with the presence/enrichment of hydrogenotrophic methanogens that are sig-
nificantly involved in H2 and CO2 conversion to CH4 (Sarkar and Venkata
Mohan, 2020; Sravan et al., 2020). CH4 production was also improved in a
single-chambered BES, involving H2 gas recycling at the anode towards high-
er waste utilization and process intensification. Membraneless single-cham-
bered systems are robust for CH4 production rather than being confined to
hydrogen at a slightly acidified pH. ADBES integration showed a significant
increase in CH4 composition in the biogas (Sravan et al., 2020; Liu et al.,
2018; Chen et al., 2016).
CO2 1 4H2-CH4 1 2H2O ð1:2Þ
2CH3COO2
1 6H1
1 4e2
-C4H7O2
2 1 H1
1 2H2O ð1:3Þ
1.4.1 Mechanism of anaerobic digestion
The organic substrate during AD is catabolized through anaerobic fermenta-
tion and anaerobic respiration under dark conditions in the absence of oxy-
gen as an electron acceptor. Microorganisms conserve energy through an
internally balanced oxidationreduction reaction (ORR) in anaerobic fer-
mentation, whereas in anaerobic respiration it uses nitrates, sulfates, fuma-
rates, etc., as the electron acceptor rather than oxygen. AD involves four
steps, i.e., hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The
multi-molecular organic substrates are converted into simple, chemically sta-
bilized molecules using H2 and acid as intermediate metabolites, and finally
to CH4 and CO2 (Sarkar and Venkata Mohan, 2020).
1.4.1.1 Hydrolysis and acidogenesis
During hydrolysis, the microorganisms initially hydrolyze the complex
organic polymers to monomers and further ferment them to a mixture of
organic acids and alcohols, mainly through the EmbdenMeyerhofParnas
or EntnerDoudoroff pathways (Angelidaki et al., 2018). The hydrolysis
rate mainly depends on the particle size, pH, gas diffusion/production, and
enzyme adsorption on the waste particles in AD (Appels et al., 2008).
Extracellular enzymes belonging to the hydrolases group such as amylase,
protease, and lipase produced by specific hydrolytic bacteria are used during
hydrolysis. Biochemical pathways end with pyruvate as a key intermediate
which is utilized as an internal electron acceptor for the NADH reoxidation
for production of VFAs (C2C6) such as acetate, propionate, butyrate, lac-
tate, valerate, and caproate, along with H2 and formate. In acidogenesis, the
acidifying bacteria involving hydrogenation and dehydrogenation convert
organic substrates and hydrolysis products to organic acids, alcohols,

aldehydes, CO2, and H2. Pyruvate during anaerobic respiration can be fur-
ther oxidized to acetate using acetogenic bacteria for comparatively higher
production of oxidized end products (acetate and CO2), which inherently
increases the overall ATP yield.
1.4.1.2 Acetogenesis
Acetogenesis is performed by a WoodLjungdahl pathway involving a
phylogenetically diverse bacterial group (acetogens) which is specifically
characterized by CO2 reduction to the acetyl-co-enzyme A. The acetyl-
CoA pathway serves towards electron acceptance and carbon assimilation
with energy conservation, where the carbon sources are used effectively
as electron donors/acceptors for enrichment of autotrophic/heterotrophic
bacteria. In acetogens, the electron donors are CO, H2, formate, methyl
chloride, pyruvate, lactate, oxalate, etc., and the electron acceptors are
CO2, fumarate, nitrate, thiosulfate, dimethyl sulfoxide, pyruvate acetalde-
hyde, and H1
. Acetogenesis directly illustrates the biogas production effi-
ciency in the system, with approximately 70% of CH4 coming from
acetate reduction, in which approximately 25% of acetates and 11% of H2
are produced. Hence, it is crucial to target the acetogenic phase for higher
CH4 output with an integrated approach towards increasing its energy and
calorific value.
1.4.1.3 Methanogenesis
Methanogenesis is the final process for CH4 production by methanogenic
microorganisms, which convert acidogenesis/acetogenesis products
(VFAs, H2, CO2, H2S, and alcohols) into CH4. Methanogens are classified
into two groups based on the substrate availability. Hydrogenotrophic
methanogens use H2/formate as an energy source in the presence of cer-
tain electron donor like alcohols and CO2 to reduce to CH4. Methanogens
are either obligate that use only H2/formate as the sole electron donor or
more flexible and use other forms of energy sources. Methylotrophic
methanogens are a more versatile group that include organic substrates
such as VFA, H2, CO2, CO, and alcohols for methanogenesis using the
methyl-S-CoM pathway. Hydrogen and interspecies formate transfer hap-
pens during methanogenic syntropy where acetate acts as an efficient
electron carrier between the syntrophic partners. The electron transfer
with direct contact of microbes without production of H2 occurs in the
presence of conductive pili and is called DIET (Sravan et al., 2020,
Lovley, 2017; Raghavulu et al., 2012). H2 utilization provides optimized
conditions for the enrichment of acidogenic bacteria, which produce
short-chain carboxylic acids (VFAs) in the acidification phase followed
by low H2 production in the acetogenic phase. These conversions produce

CO2-rich gas which can be effectively converted to CH4. Microbial com-
munities show significant variations in diversity in a short period of time
by varying different reactions in single process. Microbial diversity is
directly influenced by the operating parameters, namely, substrate and its
composition, pH, temperature, pretreatment, and retention time (Sunny
and Joseph, 2018; Venkata Mohan et al., 2008).
1.4.2 Factors affecting biogas production
1.4.2.1 Hydrolysis
Hydrolysis is a key rate-limiting factor during AD that influences the conversion
efficiency of the waste into CH4 and associated products. Efficient pretreatment
strategies (physical and chemical) have been mostly focused on overcoming
sludge hydrolysis (Venkata Mohan et al., 2008). The various pretreatment strate-
gies directly influence the specificity of products generated by influencing the
granular size of the sludge that leads to the enrichment/inactivation of biocata-
lyst (Sarkar and Venkata Mohan, 2016). Hence, the microbes involve a higher
rate of degradation by recycling the carbon and nutrients in the digesters.
Anaerobic digesters when integrated with BES influence on increasing the fer-
mentation efficiency by cascading the individual processes towards higher utili-
zation of redox equivalents. The increase in the electrocatalytic rates and
electrometabolism shows a significant impact on the microbeelectrode interac-
tion and biofilm formation.
1.4.2.2 pH
The optimal pH range is a critical parameter that influences the AD pro-
cess, thereby affecting the CH4 output and product synthesis. pH repre-
sents the hydrogen ion concentration in the digestion medium and its
variations have a direct influence on the growth rate and metabolism of
the microbial community. A near neutral pH (6.87.4) is considered as
the ideal pH for the enrichment, growth, and relative abundance of metha-
nogenic microbial community towards increasing the CH4 production (Liu
et al., 2018; Sravan et al., 2020). Studies on acidogenesis have confirmed
the variations in relative abundance of a particular species at certain pH
ranges. pH 6 was found to be more suitable for the growth of Clostridium
butyricum and pH 8 for Propionibacterium sp. to dominate and perpetuate
their communities during AD. Methanogenesis is another important pro-
cess during AD that is regulated at an optimal pH range of 6.58.2, but in
most cases pH 7 is considered as ideal for its production (Liu et al., 2018;
Cavinato et al., 2013). The development of these specific microbial com-
munities has greatly influenced the VFA composition, where at the opti-
mal pH range the enzymatic activity of the microorganisms is higher,

leading to the production of higher amounts of fatty acids (Hajji et al.,
2016). The production of VFA in relation to the optimal pH also needs to
consider the COD conversion efficiency of the microorganisms which
reflects their metabolic activity. A positive correlation between the pH
and hydrolysis was also established and is known to influence the conver-
sion efficiency of the organic substrates to products (Venkata Mohan
et al., 2016a). Integration of AD-BES processes is by an electrocatalysis
mechanism for supplementation of additional electrons to the existing pro-
cess and decreasing the energy requirements for BU and the synthesis of
other products. Therefore, pH needs to be considered as the most influen-
tial factor that controls the rate-limiting factors involved during the AD
process for higher product synthesis.
1.4.2.3 Temperature
Ambient temperature is a requirement of the AD process, where it exhibits
faster reaction rates with stability at higher organic loading of substrate for
increased biogas production. Variations in temperature result in variations
in the profile of the microorganisms, giving them stability to tolerate
adverse conditions (Sunny and Joseph, 2018). Thermophilic (5570
C)
and mesophilic (37
C) conditions are the most prevalent conditions
involved during AD (Liu et al., 2018; Dobre et al., 2014). Thermophilic
conditions have the advantage of higher biodegradability with the provision
of heat energy resulting in higher product synthesis. It is more suitable for
acidification while inhibiting biogas production. It can influence the efflu-
ents from the process which are environmentally susceptible, while lower
biogas output and energy inputs affect the overall economics of the pro-
cess. Mesophilic conditions provide comparatively better stability and
microbial abundance, with the capacity to produce higher CH4 yields.
Hence, thermophilic conditions are mostly suitable for acidogenesis, and
mesophilic conditions for the methanogenesis process. AD microorganisms
are directly influenced by optimal temperature changes that can vary the
biogas production and product synthesis quantities. The integration of elec-
trochemical processes with conventional fermentation could increase the
rate of product synthesis.
1.4.2.4 Substrate load
A higher substrate load is a parameter that leads to bacterial inhibition
decreases the productivity of the AD process (Jiang et al., 2013; Babaee and
Shayegan, 2011). A higher organic loading rate (OLR) increases the rate of
hydrolysis/acidification compared with methanogenesis, which eventually
leads to increased VFA production and bacterial inhibition (Pasupuleti et al.,
2014). The increased VFAs lower the pH, making the microenvironment
acidic and thereby negatively influencing the methanogenic microorganisms

which cannot further convert the VFAs to CH4. Hence effluent recirculation
and integration of AD-BES have great potential to decrease the overloading
inhibition. Microbial community profiling varies with organic load with
Firmicutes being predominant at lower OLR and Gammaproteobacteria,
Actinobacteria, Bacteroidetes, and Deferribacteres being observed at higher
substrate loads.
1.4.2.5 C/N ratio
An optimal substrate load also needs to reflect specific levels of nutrients
in the form of carbon to nitrogen (C/N) ratio. Provision of sufficient
amounts of nutrients to the microbial community helps in the maintenance
of biomass and faster utilization of nitrogen, resulting in higher biogas pro-
duction. Lower C/N ratios decrease nitrogen inhibition, which is toxic to
methanogens and leads to reduced utilization of carbon sources. The pres-
ence of nitrogen in the organic substrate benefits as an important element
for the synthesis of amino acids and proteins, while proteins are further
converted to ammonia, which helps in maintaining a favorable pH microen-
vironment for microorganisms. A higher nitrogen content causes toxic
effects, while lower quantities of nitrogen cause nutrient limitation (Khalid
et al., 2011). The C/N ratio range of 20:1 to 35:1 is considered optimum,
and the ratio of 25:1 is considered ideal for the AD process (Christy et al.,
2014; Ellabban et al., 2014). The C/N ratio of 25:1 resulted in three-fold
higher biogas production when compared to a C/N ratio of 15:1. Hence, an
ideal substrate load with a specific C/N ratio would help in higher biogas
production by influencing the metabolism of microorganisms involved in
the AD process.
1.4.2.6 Hydraulic retention time
The hydraulic retention time (HRT) critically influences an increase in bio-
gas production/upgradation. It indicates the period of time at which the pro-
ductivity could start to decline, while the organic fermentable substrate
remains in the anaerobic digester. Increased HRT will require a large
digester volume, increasing the overall operational cost, while a shorter
HRT will remove the active bacterial population (Sreekrishnan et al.,
2004). Maximum CH4 production and its upgradation essentially occur at
optimized HRTs. The optimized HRT mainly depends on the type of bio-
catalyst (mixed or pure culture) and the OLR. A shorter retention period
leads to VFA accumulation that causes severe fouling, resulting in
decreased biogas production, whereas if the retention time is longer, the
biogas components are not utilized effectively, resulting in decreased bio-
gas production (Chen et al., 2016; Dobre et al., 2014). HRT also depends
on the reactor size and volume (L/D ratio), where in lab-scale operations,
the HRT is much less because of the small reactor size, but in contrast the

HRT in centralized biogas systems is high to due pilot-level operations.
Hence, HRT needs to be considered for BU for increased CH4 production
during an integrated process.
1.5 Biohythane
Biohydrogen (H2) is a clean and sustainable energy-dense fuel which is bio-
logically produced during anaerobic fermentation (AF)/AD, photofermenta-
tion, biophotolysis, and integration of these individual processes (Pasupuleti
et al., 2014; Venkata Mohan et al., 2009). The global economy is expected
to rely on H2 as a primary source of energy with zero carbon emissions and
high energy-carrying capacity (Venkata Mohan and Sarkar, 2017; Sharma
and Ghoshal, 2015). H2 is produced by obligatory acetogenic bacteria using
renewable organic sources (Roy and Das, 2016; Sarkar and Venkata Mohan,
2020; Venkata Mohan et al., 2009). Biogenic waste with organic fraction act
as a carbon and energy source for the microorganisms for H2 production.
The production of H2 by the acidogenic/dark-fermentation process is at a
higher rate and is a versatile process which is light independent, and converts
biogenic organic wastes predominantly to VFAs (acetic, propionic, and
butyric acids) along with simultaneous H2 production (Sarkar and Venkata
Mohan, 2017; Dahiya et al., 2018). H2 production through dark fermentation
as an individual process has certain limitations. The gaseous energy recovery
in terms of only H2 is not sufficient for its commercial viability and applica-
tion, where only 2030% of total gaseous energy is recovered through H2
production (Sarkar and Venkata Mohan, 2016; Edison, 2014; Bauer et al.,
2013). Integrated processes need to be commercialized for the economic
feasibility of H2 production via dark fermentation which is worthy of
commercialization, where it could be essentially integrated with AD. AD
processes are easy to scale up, and the integration of these two processes
can lead to .5060% gaseous energy recovery (Sen et al., 2016). The
integration of AD and dark fermentation processes would also help to
decrease the operational cost. The development of such processes would
lead to decentralized use of both H2 and CH4. Hence, integrated processes
with varied microorganisms and individual capabilities need to be
exploited to overcome the disadvantages of individual processes and to
enhance the system energetics.
Biohythane, an alternate renewable biofuel, can be potentially pro-
duced when H2 and CH4 are mixed in appropriate ratios with a blend of
75 6 90% (v/v) CH4 and 10 6 25% (v/v) H2 to make an alternative to
fossil-based fuels (Pasupuleti and Venkata Mohan, 2015; Sarkar and
Venkata Mohan, 2016). H2 and CH4 are the most widely used biofuels
due to their high calorific values of 143 and 55 kJ/g, respectively
(Pasupuleti et al., 2014; Sharma and Ghoshal, 2015; Edison, 2014).
Biohythane is H2-enriched CH4 that has the scope to be a good alternative

to the increasing demands for compressed natural gas (CNG) as an engine
fuel. H2 is considered as a clean energy fuel since it does not release even
a small fraction of CO2 into the atmosphere during combustion (Roy and
Das, 2016). On the other hand, CH4 combustion generates greenhouse
gases such as CO2. Also, utilization and combustion of both CH4 and H2
do not show any evidence for the release of NOX (nitrous oxide) or
SOX (sulfur oxide). The lower ignition power of CH4 and highly flamma-
ble nature of H2 are usually considered as drawbacks while individually
using them as vehicle fuels. The individual limitations of H2 and CH4 can
be overcome with this blending in optimized proportions to form bio-
hythane (Sarkar and Venkata Mohan, 2016; Dahiya et al., 2018). Its
appropriate blending makes it a fuel that is clean with a good calorific
efficiency (Pasupuleti and Venkata Mohan, 2015; Sarkar and Venkata
Mohan, 2017; Sen et al., 2016).
Biohythane has numerous practical applications as a vehicular fuel and
is comparatively advantageous over CNG. Its high H2-reducing power
increases the combustion rate and burning capacity of CH4 (Roy and Das,
2016; Moreno et al., 2012). It is an eco-friendly fuel due to its advantage
of reducing the impact of greenhouse gas emissions on the environment, while
the H2 presence helps to decrease the carbon in the gas mixture. Biohythane
production was evaluated from the lab to semi-pilot scale, while more recently
the blending of flammable H2 gas with CH4 grid injections as a technology
was established on a large scale for biohythane production in vehicle fuel
plants. Biohythane mimics the hydrogen-enriched compressed natural gas
(H-CNG) composition. H-CNG supplemented with emission-free hydrogen
(H2) has application feasibility in both residential (heating and cooking) and
transport sectors (vehicle engines) as fuel with lower emissions being the
prime advantage (Talibi et al., 2017). H-CNG helps in increasing the flamma-
bility limits, speed propagation, pressure rise, and deflagration index when
compared to CNG. European Union project “NATURALHY” studied the
blending of H2 to natural gas for clean combustion of CNG with efficient calo-
rific value and lower ignition energy requirement (Cinti et al., 2019). H-CNG
in recent times is receiving significant prominence as an energy carrier, due to
its application flexibility with the existing engines (Khab et al., 2019; Miao
et al., 2011). This application flexibility could help in injecting H2 in the exist-
ing gas pipelines/natural gas grids for both industrial and household purposes
related to the transition toward hydrogen economy and would be a beneficial
factor for commercial application of the technology at the root level. The
molecular H2 as a blend (25–50%) in CNG is currently being derived from
fossils in the market which could be environment-impacting with significant
emissions being produced (Cetinkaya et al., 2012). Hence, the alternate biological
(AD)/bioelectrochemical (BES) and their integrated processes producing green/
low-carbon H2 need to be considered to further suit the advantages of H-CNG.
Regulation of process parameters such as pretreatment, pH, microenvironment,

and specific bacterial enrichment needs to be considered to substantially produce
higher H2 than CH4.
1.6 Electrochemically induced biogas upgradation
BU involves a synergy of microbial interactions that show a regulatory influ-
ence on electron flux, resulting in the conversion/utilization/reduction of CO2
for CH4 production (Deng et al., 2020; Sarkar and Venkata Mohan, 2020;
Sravan et al., 2020). Fermentation redox intermediates (H2, CO2, VFAs, etc.)
from the microbial metabolic side reactions counter the targeted end-product
due to endogenous losses. BU depends on the syntrophic interactions between
fermentative and methanogenic microorganisms to increase electron transfer
via mediated/direct interspecies electron transfer (MIET/DIET) to increase the
H2 utilization and other electron carriers and redox intermediates towards
enhanced CH4 production (Sravan et al., 2020; Deng et al., 2020; Yang et al.,
2019). Microbial interactions for increased electrogenic activity could be trig-
gered for increased performance during AD, with the polarized potential
developed due to electrode placement or by the external supplementation of
potential towards higher CH4 production, described as electromethanogenesis
(EM). Electrode placement or applied potential to a microenvironment influ-
ences on increasing the reaction/electron transfer rates with respect to conven-
tional fermentations towards increasing the CH4 content in total biogas
(Sravan et al., 2020; Deng et al., 2020; Meier et al., 2017). It influences an
increase in the microbial electrocatalysis while controlling electron flux,
energy utilization, and system buffering for CO2 conversion in the presence of
H2 and VFA to CH4. The EM strategy in the presence of electrodes or applied
potential helps in efficiently neutralizing/reducing the overpotentials and elec-
trochemical losses to overcome the limitations of BU.
Hydrogenotrophic methanogenesis aids in the in situ H2 utilization or
reducing equivalents (e2
and H1
) for CO2 reduction to enhance additional
CH4 production (Sravan et al., 2018, 2020). Hydrogenotrophic methanogens
directly utilize H1
and e2
with the use of lower activation energy as an elec-
trocatalytic activity for CO2 reduction to form CH4. The use of lower activa-
tion energy aids in establishing efficient microbeelectrode interactions to
increase the CO2 reduction.
Homoacetogens directly involve CO2 and H2 reduction to acetic acid,
which is utilized further for CH4 production (Villano et al., 2010). Syntrophic
interactions of homoacetogenic and hydrogenotrophic bacteria towards CH4
utilize H2 as an electron donor, while inhibiting medium/long-chain fatty acid
formation with regulated microbial metabolism (Rader and Logan, 2010).
4H2 1 2CO2-CH3COOH 1 2H2O ð1:4Þ
CH3COOH 1 2H2O-CH4 1 CO2 ð1:5Þ

EM also depends on the metabolic microenvironment which is vital to
understand the metabolic pathways for the targeted products (Sarkar and
Venkata Mohan, 2020; Jiang et al., 2019; Paiano et al., 2019). The anodic
metabolic function of the BES effectively contributes to energy generation
with respect to substrate oxidation (Sravan et al., 2020; Moscoviz et al.,
2016; Nealson and Rowe, 2016). Enriched microbes under the specified
microenvironment increase the process efficiency by creating equilibrium
between substrate oxidation and oxygen utilization (Nealson and Rowe,
2016; Raghavulu et al., 2012). EM in synergy with microbeelectrode
interactions and the specific microenvironment helps in regulating metabo-
lite biosynthesis for CH4 production and could be considered as an essen-
tial unit operation in the waste biorefinery.
1.6.1 Conductive materials in biogas upgradation
Direct interspecies electrons transfer (DIET), a syntrophic metabolism
where free electrons flow from one cell to another through shared physical
(microbe-microbe/microbe-electrode) and electrical connections (via con-
ductive pili) without the requirement of reduced electron carriers (redox
mediators) like molecular hydrogen or formate. Biogas production from the
conventional AD has several rate-limiting factors such as a) accumulation
of intermediate compounds like VFA that affects the process forward b)
development H2 partial pressure in the digester leads to inhibition of spe-
cific co-enzymes of methanogenic bacteria c) inhibition of ammonia traces
developed d) washout of the methanogenic biofilm during feeding (Baek
et al., 2018; Fagbohungbe et al., 2015). Conducting materials act as effi-
cient redox mediators by shuttling electrons between the syntrophic micro-
organisms. Conductive materials provide the regulatory influence of
electron transfer between the microbe-microbe and microbe-electrode in a
specified redox environment in AF/AD for BU. Parent inoculum, pretreat-
ment, pH, microenvironment, etc., critically influence DIET with conduc-
tive materials for BU in AD, BES, and integrated systems (Baek et al.,
2018; Zhao et al., 2017). The conductive materials mediated DIET with
varied concentrations have shown to be highly efficient in the enhancement
of CH4 yield. In AD, the transfer of electrons between two different syn-
trophic microbial communities such as bacteria and archaea is a vital pro-
cess for methanogens to get control of energy barriers and catabolized
complex organics. Amendment of the several conductive materials to AD,
accelerate DIET between microbes leading to increased process efficiencies
(Cheng et al., 2020). Conductive materials viz., granular activated carbon
(GAC), carbon cloth, biochar, reduced graphene, iron conducting materials
like magnetite (Fe3O4), carbon nanotubes are being successfully implemen-
ted as an approach to improve the AD process for improved CH4 produc-
tion (Tan et al., 2021; Zhao et al., 2017; Cheng et al., 2020). The higher

surface area of the conductive materials favours the growth of the methano-
gens with tendency to form dense aggregates of biofilm. Higher conductiv-
ity and biocompatibility of conductive materials positively influences the
DIET between the microbes leading towards process intensification result-
ing in increased CH4 content.
1.7 Challenges and way forward
BU has the scope and possibility for practical applications with an inte-
grated electrochemical strategy. BU increases the CH4 yields by enhancing
the reduction of CO2 during the process (Sravan et al., 2020; Chen and
Liu, 2017; Zhao et al., 2016; Beese-Vasbender et al., 2015). The calorific
value of biogas is directly proportional to its CH4 content. Hence, increas-
ing the CH4 composition in the biogas increases the energy and economic
value, while also decreasing the transportation and storage costs. The
increase in CH4 composition in biogas also significantly improves the fea-
sibility and compatibility of its utilization in natural gas distribution
(Mamun et al., 2016). CH4 can be used effectively as a vehicle fuel or
directly injected into the gas grid for storage due to its high calorific
energy content (Persson et al., 2006). The biogas can also be used directly
for diverse applications such as for heating using gas-based boilers
and cooking using gas stoves and ovens (Harasimowicz et al., 2007).
Household Industries
Biogenic waste
Anaerobic digeson unit
Biogas upgrading unit
Gas transmission line Gas transmission line
Industrial biogas
supply head
Local biogas head
unit
Household supply Industrial supply
Biomethane
distribuon
Biomethane
distribuon
Biogas grid injuncon Household biogas supply line
Industrial biogas supply line
FIGURE 1.4 Biogas grid supply schematic for industrial and household purposes.

BU applications are also associated with the removal of organic substrates
and harmful trace components as an additional benefit during the process.
BU with integrated bioelectrochemical strategy for the conversion of CO2
to CH4 has potential feasibility for application in replacing the stripping
tower in a water scrubbing unit (Vijayanand and Singaravelu, 2017;
Cheng et al., 2009). The integration of electrochemical energy with AD
possibly regulates the rate of CO2 utilization with specificity towards
product synthesis by nongenetically regulating the microbial metabolism.
It also influences the direct electron transfer with effective
microbeelectrode interactions during the operation. The syntrophic inter-
actions between the microbes and electrode, through extracellular electron
transfer, catalyze the anodic and/or cathodic reactions. The electrochemi-
cally driven BU provides a solution for excessive CO2 produced in the
process to be directly transformed into CH4 rather than its separation from
the biogas, significantly increasing the productivity and energy value of
biogas plants. The integrated bioprocesses with energy and waste compo-
nents require a thorough life-cycle and techno-economic analysis to assess
the environmental impact and economic feasibility under different condi-
tions of operation. The mechanisms and variations in microbial diversity
with respect to the microbeelectrode interactions (anode/cathode) need
to be further understood to enable improved CH4 production for industrial
scale applications. These studies would benefit from a transfer of knowl-
edge to large-scale operations with overall process understanding, while
overcoming several technology related challenges. Symbiotic integration
of multiple processes as a single unit can efficiently contribute to the cost
economics and environmental sustainability along with specified products
generation from the systems (Fig. 1.4).
Acknowledgments
The Department of Biotechnology (DBT), Government of India, supported this research
(BT/HRD/35/01/02/2018) in the form of Tata Innovation Fellowship to SVM. JSS
acknowledges CSIR for providing research fellowship. The authors wish to thank CSIR-
IICT for supporting the research (Manuscript No. IICT/Pubs./2021/008).
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