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- Serie: Wiley Series in Renewable Resources
- Sprache: Englisch
Biogas Plants Comprehensive resource highlighting the global significance of biogas and reviewing the current status of biogas production. Biogas Plants presents an overview of biogas production, starting from the substrates (characteristics, pretreatment, and storage), addressing technical and technological aspects of fermentation processes, and covering the environmental and agricultural significance of obtained digestate. Written by a team of experts with extensive theoretical and practical experience in the areas of bio-waste, biogas plants, and reduction of greenhouse gas emissions, Biogas Plants discusses keys topics including: * Anaerobic digestion, including discussion of substrates and products * Advantages of biogas plants, with emphasis on their future potential for stable and controlled renewable energy * Global significance of the biogas sector, including its importance in electro-energy system stabilization, biogas plants for energy storage, bio-waste utilization, and biomethane production A thorough and complete resource on the subject, Biogas Plants will appeal to academic researchers and industry scientists and engineers working in the fields of biogas, bio-waste, bioenergy, renewable resources, waste management and carbon reduction, along with process engineers, environmental engineers, biotechnologists, and agricultural scientists. For more information on the Wiley Series in Renewable Resources, visit www.wiley.com/go/rrs
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Veröffentlichungsjahr: 2023
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Table of Contents
Cover
Table of Contents
Wiley Series in Renewable Resources
Title Page
Copyright
Dedication
List of Contributors
Series Preface
1 Anaerobic Digestion Process and Biogas Production
1.1 Introduction
1.2 Basic Knowledges of AD Processes and Operations
1.3 Current Challenges of AD Process and Biogas Production
1.4 Proposed Strategies for Enhanced Biogas Production
1.5 Techno‐Economic and Environmental Assessment of Anaerobic Digestion for Biogas Production
References
2 Pretreatment of Lignocellulosic Materials to Enhance Biogas Recovery
2.1 Introduction
2.2 Available Pretreatment Technologies for Lignocellulosic Materials and the Corresponding Biogas Recovery Associated
2.3 Pertinent Perspectives
2.4 Conclusions
Acknowledgments
References
Note
3 Biogas Technology and the Application for Agricultural and Food Waste Treatment
3.1 Development of Biogas Plants
3.2 Anaerobic Digestion Process
3.3 Biogas Production from Livestock and Poultry Manure
3.4 Food Waste Anaerobic Digestion
References
Note
4 Biogas Production from High‐solid Anaerobic Digestion of Food Waste and Its Co‐digestion with Other Organic Wastes
4.1 Introduction
4.2 Reactor Systems for HSAD
4.3 Intensification Strategies for HSAD
4.4 Microbial Communities for HSAD
4.5 Digestate Management for HSAD
4.6 Conclusions and Perspectives
Acknowledgments
References
5 Biomethane – Production and Management
5.1 Introduction
5.2 Purification and Usage of Biogas
5.3 Opportunities for Biogas Upgrading
5.4 Possibilities of Using Biomethane
5.5 Profitability of Biomethane Production and Recommended Support Systems
5.6 Conclusion
References
6 The Biogas Use
6.1 Introduction
6.2 Biogas Utilization Technologies
6.3 Use of Biogas as Trigeneration
6.4 Biogas as a Transportation Fuels
6.5 Use of Biogas in Reciprocating Engine
6.6 Spark Ignition Gas Engine
6.7 Use of Biogas in Generator
6.8 Use of Biogas in Gas Turbines
6.9 Usage of Biogas in Fuel Cell
6.10 Hydrogen Production from Biogas
6.11 Biogas Cleaning for its Utilization
6.12 Different Approaches for H
2
S Removal
6.13 Different Approaches for Moisture Reduction
6.14 Siloxane Removal
6.15 CO
2
Separation
6.16 Conclusion
References
7 Digestate from Agricultural Biogas Plant – Properties and Management
7.1 Introduction
7.2 Digestate from Agricultural Biogas Plant – Production, Properties, and Processing
7.3 Digestate from Agricultural Biogas Plant – Management
7.4 Conclusion
References
8 Environmental Aspects of Biogas Production
8.1 Introduction
8.2 Impact of Farms and Livestock Complexes on the Environment
8.3 The Environmental Benefits of Biogas Production
8.4 Environmental Safety of the Integrated Model of Bioprocesses of Hydrogen Production and Methane Generation in the Stages of Anaerobic Fermentation of Waste
8.5 Life Cycle Assessment for Biogas Production
8.6 Environmental Issue of Biogas Market in Ukraine – Case Study
8.7 Conclusion
References
9 Hybrid Environmental and Economic Assessment of Biogas Plants in Integrated Organic Waste Management Strategies
9.1 Introduction
9.2 Methodology
9.3 Results and Discussion
9.4 Conclusion
References
10 Reduction of the Carbon Footprint in Terms of Agricultural Biogas Plants
10.1 Introduction
10.2 Methodology of CF
10.3 Life Cycle CO
2
Footprints of Various Biogas Projects – Comparison with Literature Results
10.4 Conclusions
References
11 Financial Sustainability and Stakeholder Partnerships of Biogas Plants
11.1 Introduction
11.2 Basic Technological Factors
11.3 Economic Evaluation and Failures
11.4 Stakeholders Partnership and Co‐governance
11.5 Summary and Outlooks
Acknowledgments
References
12 Measuring the Resilience of Supply Critical Systems: The Case of the Biogas Value Chain
12.1 Introduction
12.2 Background
12.3 Methodology
12.4 Measurement Scheme
12.5 Conclusion and Recommendations
References
13 Theory and Practice in Strategic Niche Planning: The Polish Biogas Case
13.1 Introduction
13.2 Main Conceptual Frameworks for Studying Sustainability Transitions
13.3 Studying Biogas from a Sustainability Transitions Perspective
13.4 Strategic Niche Planning for Sustainable Transitions
13.5 Strategic Propositions and Concluding Comments
13.6 Conclusion
References
14 Social Aspects of Agricultural Biogas Plants
14.1 Introduction
14.2 The Benefits of Agricultural Biogas Plants for Society
14.3 Social Acceptability of Agricultural Biogas Plants
14.4 Conclusion
References
15 Practices in Biogas Plant Operation: A Case Study from Poland
15.1 Introduction
15.2 Legal Aspects Related to Running a Business in the Field of Biogas Production and Waste Management
15.3 Biogas Plant Components: A Case Study from Poland
15.4 Functioning of a Biogas Plant Processing Problematic Waste: A Case Study from Poland
15.5 Summary
References
Note
Index
End User License Agreement
List of Tables
Chapter 1
Table 1.1 Summarization of the threshold values of ammonia inhibition in dif...
Table 1.2 Summary of related research on acid inhibition in wastewater anaer...
Table 1.3 Performances of methane production in AD system with conductive ma...
Chapter 2
Table 2.1 Integrated Pretreatment Approaches.
Chapter 3
Table 3.1 Scale classification of biogas plants (NY/T 667‐2022).
Table 3.2 Properties of livestock manure and methane production potential.
Table 3.3 Characteristics of straw and methane production potential.
Table 3.4 Positive effect of trace metals on chicken manure AD.
Chapter 5
Table 5.1 Composition of biogas from different generation sources.
Chapter 6
Table 6.1 Requirements for treating biogas impurities [4, 10].
Chapter 8
Table 8.1 Characteristics of the life cycle phases of biogas production.
Table 8.2 Generation and management of waste of the I–IV hazard classes by c...
Chapter 9
Table 9.1 Input LCC parameters and assumptions of different waste management...
Chapter 10
Table 10.1 Types of emissions.
Table 10.2 Global Warming Potential (GWP) values relative to CO
2
.
Table 10.3 Examples of available tools for calculating agricultural GHG flux...
Table 10.4 Types of activity data that may be needed to calculate GHG fluxes...
Table 10.5 CF (kgCO2eq MW−1 hel−1) of the renewable energy sources (RES)....
Table 10.6 CF (kg CO2eq MW−1 help−1) of various biogas projects literature d...
Table 10.7 Biogas plant as a practice that can reduce GHG emissions and impr...
Chapter 12
Table 12.1 Production of biogas (GWh) and distribution by plant type in Swed...
Table 12.2 Amount of digestate produced (kton wet weight) and its use as fer...
Table 12.3 The resilience functions of the management system and physical re...
Table 12.4 Impacts and their probability.
Table 12.5 Production and efficiency data for the biogas‐producing waste man...
Table 12.6 Statistical risk for loss of biogas production from impact under ...
Table 12.7 Actual loss of biogas production from impact under low system res...
Table 12.8 Statistical risk for loss of biogas production from impact under ...
Table 12.9 Actual loss of biogas production from impact under high system re...
Table 12.10 Statistical risk for loss of biogas production from impact under...
Table 12.11 Loss of biogas production from actual impact under moderate syst...
Chapter 13
Table 13.1 Current Polish and EU‐level laws that influence the biogas sector...
Table 13.2 Network of actors in the biogas sector linked with different rela...
Table 13.3 Sequence of steps for strategy design and elaboration.
Table 13.4 Matching classification based on TIS with foresight dimensions an...
Table 13.5 Questions to be discussed using the TOWS structure.
Table 13.6 Interaction grid Strengths‐Opportunities: illustrative example....
Table 13.7 Driving factors and barriers for biogas development.
Table 13.8 Internal factors derived from the drivers/barriers list.
Table 13.9 External factors derived from the drivers/barriers list.
Table A.1 Populated TOWS with exhaustive strategic actions.
Table 13.10 Strategic propositions suggested in the TOWS grid (Table A.1) gr...
List of Illustrations
Chapter 1
Figure 1.1 General biochemical process involved in anaerobic digestion.
Figure 1.2 Mechanisms of ammonia inhibition occurred in anaerobic digestion ...
Figure 1.3 Inhibition mechanisms of volatile fatty acids on anaerobic digest...
Figure 1.4 Schematic diagram of methane formation during anaerobic digestion...
Figure 1.5 Digestion mechanism of methanogen (a) volatile fatty acid to acet...
Figure 1.6 Co‐digestion of different substrates to enhance methane productio...
Figure 1.7 Improvement of biogas production via bioaugmentation [137, 138, 1...
Figure 1.8 Schematic diagram of methane formation during bioelectrochemical ...
Figure 1.9 Social, economic, and environmental impacts of renewable energy r...
Chapter 2
Figure 2.1 Structure of lignocellulosic material (G, H, and S refer to guaia...
Figure 2.2 Schematic of anaerobic digestion process.
Figure 2.3 List of pretreatments according to type.
Figure 2.4 Mechanism of action of physical pretreatments (G, H, and S refer ...
Figure 2.5 Mechanism of action of chemical pretreatments (G, H, and S refer ...
Figure 2.6 Mechanism of action of biological pretreatments (G, H, and S refe...
Figure 2.7 Summary of biogas production enhancement in percentages for indiv...
Chapter 4
Figure 4.1 Schematic diagram of high‐solid anaerobic membrane bioreactor....
Figure 4.2 Schematic diagram of two‐stage HSAD reactor system.
Figure 4.3 Schematic diagram of high‐solid plug‐flow bioreactor.
Chapter 5
Figure 5.1 Production potential for biogas or biomethane by feedstock source...
Figure 5.2 Section of a biological desulfurization plant with a blower feedi...
Figure 5.3 Desulfurization plant consisting of two filters filled with iron ...
Figure 5.4 Filter made of HDPE plastic containing an activated carbon bed.
Figure 5.5 Schematic of biogas treatment plant including desulfurization pla...
Figure 5.6 Membrane technology (own study based on [16]). Legend: 1 – raw bi...
Figure 5.7 CO2 separation by water scrubbers. Legend: 1 – raw biogas, 2 – co...
Figure 5.8 Pressure swing adsorption scheme (own study based on [18]). Legen...
Figure 5.9 Cryogenic separation (own study based on [17]). Legend: 1 – raw b...
Figure 5.10 Percentage of biomethane support systems in Europe (own study ba...
Chapter 6
Figure 6.1 Schematic of cleaning of biogas.
Figure 6.2 Schematic of biogas digester module.
Figure 6.3 Flow chart of process involved in cryogenic technique.
Figure 6.4 Schematic of water scrubber [57, 58].
Figure 6.5 Schematic of membrane‐based CO
2
removal technique.
Figure 6.6 Overview of biogas cleaning techniques.
Chapter 7
Figure 7.1 Small agriculture biogas plant in Switzerland.
Figure 7.2 Liquid fraction of digestate.
Figure 7.3 Solid fraction of digestate.
Chapter 8
Figure 8.1 Analysis of the Scopus and Web of Science database search with th...
Figure 8.2 Greenhouse gas emissions in the agricultural sector.
Figure 8.3 Comparison diagram of the ecological effect of anaerobic digestio...
Figure 8.4 Factors of ecological safety of anaerobic fermentation processes ...
Figure 8.5 Model of the combination of bioprocesses for hydrogen and methane...
Figure 8.6 Model of the life cycle phases of biogas production (author's dev...
Figure 8.7 The amount of waste generation in Ukraine by type of economic act...
Figure 8.8 Structure of emissions of the agro‐industrial complex of Ukraine,...
Chapter 9
Figure 9.1 Proposed methodology framework of the present chapter.
Figure 9.2 Schematic flow diagram of the proposed BAU and WTE‐based waste ma...
Figure 9.3 General schematic of the LCA system boundary for the examined str...
Figure 9.4 Amounts of landfilled biodegradable and nonbiodegradable waste fo...
Figure 9.5 Energy recovered in the examined waste management strategies.
Figure 9.6 Environmental impact categories of the proposed waste management ...
Figure 9.7 Potential energy production and revenues of the examined waste ma...
Figure 9.8 Annual net present values for the examined waste management strat...
Figure 9.9 Eco‐efficiency assessment for the examined waste management strat...
Chapter 10
Figure 10.1 The potential of biogas production from swine manure and slurry ...
Figure 10.2 The potential of biogas production from cattle manure and slurry...
Figure 10.3 Map chart of farm manure produced in EU‐28.
Figure 10.4 Substrates used for the production of agricultural biogas in 201...
Figure 10.5 Substrates used for the production of agricultural biogas in 201...
Figure 10.6 Greenhouse gases emission from agriculture in the EU countries i...
Figure 10.7 The general process for calculating GHG fluxes (own study).
Figure 10.8 Anaerobic digestion process in agricultural biogas plant (own st...
Figure 10.9 Example of GHG fluxes emissions during AD in five manure managem...
Chapter 12
Figure 12.1 The measurement concept of resilience (own study).
Figure 12.2 Relations between standards supporting the management of value c...
Figure 12.3 Model of a supply chain to exemplify calculation of total system...
Figure 12.4 Two alternative separated biogas value chains agricultural and i...
Chapter 13
Figure 13.1 Contexts (axes), specific drivers (in bold), and transition path...
Chapter 15
Figure 15.1 Functioning of an agricultural biogas plant (own study).
Figure 15.2 Scheme of handling waste requiring hygienization process (own st...
Guide
Cover Page
Wiley Series in Renewable Resources
Title Page
Copyright
Dedication
List of Contributors
Series Preface
Table of Contents
Begin Reading
Index
Wiley End User License Agreement
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Wiley Series in Renewable Resources
Series Editor:
Christian V. Stevens, Faculty of Bioscience Engineering, Ghent University, Belgium
Titles in the Series:
Wood Modification: Chemical, Thermal and Other Processes
Callum A. S. Hill
Renewables‐Based Technology: Sustainability Assessment
Jo Dewulf, Herman Van Langenhove
Biofuels
Wim Soetaert, Erik Vandamme
Handbook of Natural Colorants
Thomas Bechtold, Rita Mussak
Surfactants from Renewable Resources
Mikael Kjellin, Ingegärd Johansson
Industrial Applications of Natural Fibres: Structure, Properties and Technical Applications
Jörg Müssig
Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power
Robert C. Brown
Biorefinery Co‐Products: Phytochemicals, Primary Metabolites and Value‐Added Biomass Processing
Chantal Bergeron, Danielle Julie Carrier, Shri Ramaswamy
Aqueous Pretreatment of Plant Biomass for Biological and Chemical Conversion to Fuels and Chemicals
Charles E. Wyman
Bio‐Based Plastics: Materials and Applications
Stephan Kabasci
Introduction to Wood and Natural Fiber Composites
Douglas D. Stokke, Qinglin Wu, Guangping Han
Cellulosic Energy Cropping Systems
Douglas L. Karlen
Introduction to Chemicals from Biomass, 2nd Edition
James H. Clark, Fabien Deswarte
Lignin and Lignans as Renewable Raw Materials: Chemistry, Technology and Applications
Francisco G. Calvo‐Flores, Jose A. Dobado, Joaquín Isac‐García, Francisco J. Martín‐Martínez
Sustainability Assessment of Renewables‐Based Products: Methods and Case Studies
Jo Dewulf, Steven De Meester, Rodrigo A. F. Alvarenga
Cellulose Nanocrystals: Properties, Production and Applications
Wadood Hamad
Fuels, Chemicals and Materials from the Oceans and Aquatic Sources
Francesca M. Kerton, Ning Yan
Bio‐Based Solvents
François Jérôme and Rafael Luque
Nanoporous Catalysts for Biomass Conversion
Feng‐Shou Xiao and Liang Wang
Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power, 2nd Edition
Robert Brown
Chitin and Chitosan: Properties and Applications
Lambertus A.M. van den Broek and Carmen G. Boeriu
The Chemical Biology of Plant Biostimulants
Danny Geelen, Lin Xu
Biorefinery of Inorganics: Recovering Mineral Nutrients from Biomass and Organic Waste
Erik Meers, Evi Michels, René Rietra, Gerard Velthof
Process Systems Engineering for Biofuels Development
Adrián Bonilla‐Petriciolet, Gade P. Rangaiah
Waste Valorisation: Waste Streams in a Circular Economy
Carol Sze Ki Lin, Chong Li, Guneet Kaur, Xiaofeng Yang
High‐Performance Materials from Bio‐based Feedstocks
Andrew J. Hunt, Nontipa Supanchaiyamat, Kaewta Jetsrisuparb, Jesper T. Knijnenburg
Handbook of Natural Colorants, 2nd Edition
Thomas Bechtold, Avinash P. Manian and Tung Pham
Biogas Plants: Waste Management, Energy Production and Carbon Footprint Reduction
Wojciech Czekała
Biogas Plants
Waste Management, Energy Production and Carbon Footprint Reduction
Edited by
Wojciech Czekała
Poznań University of Life Sciences
This edition first published 2024
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Library of Congress Cataloging‐in‐Publication Data
Names: Czekała, Wojciech, editor. | Stevens, Christian V., editor.
Title: Biogas plants : waste management, energy production and carbon footprint reduction / edited by Wojciech Czekała, Christian V Stevens.
Description: Hoboken, NJ : Wiley, 2024. | Series: Wiley series in renewable resources | Includes index.
Identifiers: LCCN 2023046450 (print) | LCCN 2023046451 (ebook) | ISBN 9781119863786 (hardback) | ISBN 9781119863779 (adobe pdf) | ISBN 9781119863922 (epub) | ISBN 9781119863946 (oBook)
Subjects: LCSH: Biogas. | Renewable energy sources.
Classification: LCC TP359.B48 B537 2024 (print) | LCC TP359.B48 (ebook) | DDC 665.7/76–dc23/eng/20231107
LC record available at https://lccn.loc.gov/2023046450
LC ebook record available at https://lccn.loc.gov/2023046451
Cover Design: Wiley
Cover Image: © Lulub/Shutterstock
To my mother and father, who never stopped believing in me.
To my wife for understanding me better than everyone.
To my sons, who fill my heart with joy each and every day.
List of Contributors
Mohamed Abdallah
Department of Civil and Environmental Engineering, University of Sharjah, Sharjah, United Arab Emirates
Muhammad Arslan
Department of Energy Systems Engineering, University of Agriculture, Faisalabad, Pakistan
Raul Carlsson
Certification Development Unit, RISE Research Institutes of Sweden, Jönköping, Sweden
Yelizaveta Chernysh
Faculty of Tropical AgriSciences, Department of Sustainable Technologies, Czech University of Life Sciences Prague, Suchdol, Czechia
Department of Ecology and Environmental Protection Technologies, Faculty of Technical Systems and Energy Efficient Technologies, Sumy State University, Sumy, Ukraine
Viktoriia Chubur
Faculty of Tropical AgriSciences, Department of Sustainable Technologies, Czech University of Life Sciences Prague, Suchdol, Czechia
Department of Ecology and Environmental Protection Technologies, Faculty of Technical Systems and Energy Efficient Technologies, Sumy State University, Sumy, Ukraine
Wojciech Czekała
Department of Biosystems Engineering, Poznań University of Life Sciences, Poznań, Poland
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
Renjie Dong
College of Engineering, China Agricultural University, Beijing, China
Nalok Dutta
Department of Biochemical Engineering, University College London, London, UK
Bioproducts Sciences and Engineering Laboratory, Washington State University, USA
Amal Elfeky
Department of Civil and Environmental Engineering, University of Sharjah, Sharjah, United Arab Emirates
Kazi Fattah
Department of Civil, Environmental, and Architectural Engineering, University of Kansas, Kansas, United States of America
Likui Feng
State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, China
Jan Jasiński
Department of Biosystems Engineering, Poznań University of Life Sciences, Poznań, Poland
Tomasz Jasiński
Tomasz Jasiński Biogas Consulting, Nowe, Poland
Mengmeng Jiang
College of Engineering, China Agricultural University, Beijing, China
Piotr Jurga
Department of Bioeconomy and Systems Analysis, Institute of Soil Science and Plant Cultivation, Pulawy, Poland
Muhammad U. Khan
Department of Energy Systems Engineering, University of Agriculture, Faisalabad, Pakistan
Martyna Kulińska
Department of Biosystems Engineering, Poznań University of Life Sciences, Poznań, Poland
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
Jianju Li
State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, China
Ee Y. Lim
Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
Yu Liu
State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, 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
Aleksandra Łukomska
Department of Biosystems Engineering, Poznań University of Life Sciences, Poznań, Poland
Tatiana Nevzorova
Certification Development Unit, RISE Research Institutes of Sweden, Stockholm, Sweden
Wei Qiao
College of Engineering, China Agricultural University, Beijing, China
Hynek Roubík
Faculty of Tropical AgriSciences, Department of Sustainable Technologies, Czech University of Life Sciences Prague, Suchdol, Czechia
Stelios Rozakis
BiBELab, Department of Chemical and Environmental Engineering, Technical University of Crete, Chania, Greece
Abid Sarwar
Department of Irrigation and Drainage, University of Agriculture, Faisalabad, Pakistan
Yapeng Song
College of Engineering, China Agricultural University, Beijing, China
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
Katerina Troullaki
BiBELab, Department of Chemical and Environmental Engineering, Technical University of Crete, Chania, Greece
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
Department of Engineering Science, University of Oxford, Oxford, UK
Simon M. Wandera
Department of Civil, Construction & Environmental Engineering, Jomo Kenyatta University of Agriculture & Technology, Nairobi, Kenya
Agnieszka Wawrzyniak
Department of Biosystems Engineering, Poznań University of Life Sciences, Poznań, Poland
Liangliang Wei
State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, China
Xinhui Xia
State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, China
Hang Yu
State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, China
Jingxin Zhang
Energy and Environmental Sustainability for Megacities (E2S2) Phase II, Campus for Research Excellence and Technological Enterprise (CREATE), Singapore
China‐UK Low Carbon College, Shanghai Jiao Tong University, Shanghai, 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
Department of Resources and Environment, School of Agriculture and Biology, Shanghai Jiao Tong University, Shanghai, China
Weixin Zhao
State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, China
Series Preface
Renewable resources, their use and modification, are involved in a multitude of important processes with a major influence on our everyday lives. Applications can be found in the energy sector, paints and coatings, and the chemical, pharmaceutical, and textile industries, to name but a few.
The area interconnects several scientific disciplines (agriculture, biochemistry, chemistry, technology, environmental sciences, forestry, etc.), which makes it very difficult to have an expert view on the complicated interactions. Therefore, the idea to create a series of scientific books, focusing on specific topics concerning renewable resources, has been very opportune and can help to clarify some of the underlying connections in this area.
In a very fast‐changing world, trends are not only characteristic of fashion and political standpoints; science too is not free from hypes and buzzwords. The use of renewable resources is again more important nowadays; however, it is not part of a hype or a fashion. As the lively discussions among scientists continue about how many years we will still be able to use fossil fuels – opinions ranging from 50 to 500 years – they do agree that the reserve is limited and that it is essential not only to search for new energy carriers but also for new material sources.
In this respect, the field of renewable resources is a crucial area in the search for alternatives for fossil‐based raw materials and energy. In the field of energy supply, biomass‐ and renewables‐based resources will be part of the solution alongside other alternatives such as solar energy, wind energy, hydraulic power, hydrogen technology, and nuclear energy. In the field of material sciences, the impact of renewable resources will probably be even bigger. Integral utilization of crops and the use of waste streams in certain industries will grow in importance, leading to a more sustainable way of producing materials. Although our society was much more (almost exclusively) based on renewable resources centuries ago, this disappeared in the Western world in the nineteenth century. Now it is time to focus again on this field of research. However, it should not mean a “retour à la nature”, but should be a multidisciplinary effort on a highly technological level to perform research towards new opportunities, and to develop new crops and products from renewable resources. This will be essential to guarantee an acceptable level of comfort for the growing number of people living on our planet. It is “the” challenge for the coming generations of scientists to develop more sustainable ways to create prosperity and to fight poverty and hunger in the world. A global approach is certainly favored.
This challenge can only be dealt with if scientists are attracted to this area and are recognized for their efforts in this interdisciplinary field. It is, therefore, also essential that consumers recognize the fate of renewable resources in a number of products. Furthermore, scientists do need to communicate and discuss the relevance of their work. The use and modification of renewable resources may not follow the path of the genetic engineering concept in view of consumer acceptance in Europe. Related to this aspect, the series will certainly help to increase the visibility of the importance of renewable resources. Being convinced of the value of the renewables approach for the industrial world, as well as for developing countries, I was myself delighted to collaborate on this series of books focusing on the different aspects of renewable resources. I hope that readers become aware of the complexity, the interaction, and interconnections, and the challenges of this field, and that they will help to communicate on the importance of renewable resources.
I certainly want to thank the people of Wiley's Chichester office, especially David Hughes, Jenny Cossham, and Lyn Roberts, in seeing the need for such a series of books on renewable resources, for initiating and supporting it, and for helping to carry the project to the end.
Last, but not least, I want to thank my family, especially my wife Hilde and children Paulien and Pieter‐Jan, for their patience, and for giving me the time to work on the series when other activities seemed to be more inviting.
Christian V. Stevens
Faculty of Bioscience Engineering, Ghent University, Belgium
Series Editor, “Renewable Resources”
June 2005
1Anaerobic Digestion Process and Biogas Production
Liangliang Wei, Weixin Zhao, Likui Feng, Jianju Li, Xinhui Xia, Hang Yu, and Yu Liu
State Key Laboratory of Urban Water Resources and Environment (SKLUWRE), School of Environment, Harbin Institute of Technology, Harbin, China
1.1 Introduction
The increasing amount of organic wastes worldwide has become problematic for most countries due to the continuous deterioration of land and water conditions, which poses serious risks to the safety of our community [1]. Moreover, the improper treatment of these organic wastes might lead to the undesired release of huge greenhouse gases (GHGs) into the atmosphere [2, 3]. It was estimated by the Intergovernmental Panel on Climate Change (IPCC) and US Environmental Protection Agency (US EPA) that the global anthropogenic methane emission from municipal solid wastes (MSWs) reached 1077 million metric ton of CO2 equivalent in 2020 and is expected to increase by 17% in the year 2030. Mitigation practices have forced global action to adopt a technology that can address anthropogenic methane emissions [4]. Numerous available mitigation opportunities currently include the treatment of the organic portion of MSW in a controlled facility and recovering methane as a fuel for on‐site or off‐site electricity generation [5].
Energy generation from the MSW and the other alternative sources will benefit climate change mitigation and minimize the alarms posed to the environment [6]. There has been a high uptake of renewable energy technologies (RETs) worldwide to deal with the detrimental effects paused by fossil‐related energy generation technologies. For a purpose of increasing the energy accessibility while simultaneously restricting the worldwide temperature increased within 2 °C before 2050, adoption of RETs should be highly encouraged and raised significantly. This growing impetus for alternative avenues for renewable energy demands the consideration of different feedstocks, exploring of novel techniques, and improvements of existing technologies.
Bioenergy has been regarded as the most substantial renewable energy source due to its cost‐effective advantages and great potential for substituting nonrenewable fuels. Bioenergy derived from biomass materials, such as biological organic matter obtained from plants or animals, is renewable and green. Generally, those biomass energy sources include but are not limited to terrestrial plants, aquatic plants, timber processing residues, MSWs, animal dung, sewage sludge, agricultural crop residues, and forestry residues. Undoubtedly, bioenergy is one of the most versatile renewable energies because it can be made available in solid, liquid, and/or gaseous forms. Different avenues can be explored to harvest energy from biomass materials. Biomethane has a high heating value ranging between 50 and 55 MJ m−3 and a low heating value ranging between 30 and 35 MJ m−3[7].
Anaerobic digestion (AD) is practiced extensively for the treatment of biodegradable waste for biomethane generation [8]. This technology has the capability of managing the typical organic wastes such as food waste, lignocellulosic biomass and residues, energy crops, and the organic fraction of municipal solid waste (OFMSW) [9], and its environmentally sound features attracted worldwide attention for biogas production. AD is a microbe‐driven, multiphase, and complex biochemical process, and four typical biochemical phases such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis are involved in its whole process. Organic matter could be efficiently metabolized by bacteria and archaea and finally converted into methane and carbon dioxide [10, 11]. However, AD processes are always limited by three main factors: (i) hydrolysis of substrates is the rate‐limiting factor for the bioconversion phase; (ii) inefficient utilization of key intermediates such as propionic and butyric acid; (iii) slow growth of anaerobes of methanogenesis [12], and finally lead to a low biomethane recovery rate during their practical operation [13]. Thus, the advancements in the AD process are largely aimed toward one goal: improving biogas production and recovery.
There is currently considerable potential for biogas technology to be developed as a RET that addresses energy and environmental issues. Biogas is a critical technology that provides renewable energy from processing a variety of digestible biomass types. Substrates such as straw, forestry residues, animal and poultry manure, and other organic wastes can be treated within AD systems. The purified biomethane can be integrated into conventional fossil energy supply systems and guarantee the AD technology in energy transformation and ecological civilization construction. However, the biogas industry faces many challenges, including low gas productivity, short biogas tank life, high deterioration rates of digesters, difficulty in digestion residue utilization, and limited economic benefits [14, 15]. To improve the biogas and highlight its role in energy and environmental problem‐solving, it is necessary to develop new approaches for the purpose of extending the industrial chain and further exploring new models that can promote the commercialization.
1.2 Basic Knowledges of AD Processes and Operations
1.2.1 Fundamental Mechanisms and Typical Processes of AD
AD, full microbiological degradation process under anaerobic conditions, represents one of the most promising processes to convert diverse organic substrates (animal manure, food waste, MSW, and lignocellulosic biomass as agricultural waste) into energy carriers (produced biogas mainly 55–75% CH4 and 25–45% CO2) [16].
Figure 1.1General biochemical process involved in anaerobic digestion.
Source: D'Silva et al. [17]/with permission of Elsevier.
Microbial ecology in anaerobic digesters is quite complex, and different bacterial and archaeal communities are involved in the digestion process. The AD process is composed of four main steps, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Figure 1.1). The hydrolysis process is the primary step (stage I) in AD where organic polymers (i.e. cellulose, lipids, carbohydrates, polysaccharides, proteins, and nucleic acids) are hydrolyzed into monomers, simple sugars, saccharides, peptides, glycerol, amino acids, and other higher fatty acids, which could be summarized in Eq. (1.1):
Hydrolytic bacteria, also known as primary fermenting bacteria, are facultative anaerobes that hydrolyze the substrate with extracellular enzymes. A wide range of enzymes, i.e. cellulases, hemicellulases, proteases, amylases, and lipases, were generated in this stage and played a great role in the substrate degradation [18]. Undoubtedly, the generation of the aforementioned enzymes enhanced the whole hydrolysis. By contrast, the lack of the suitable enzymes would negatively affect the biogas generation, for instance, the hydrolyzation of lignocellulosic substrates becomes the rate‐limiting step of the AD process [18]. During acidogenesis (stage II), primary fermentative bacteria convert hydrolysis products into volatile fatty acids (VFAs), including acetate, propionate, butyrate, valerate, and other acids (i.e. lactate, succinate, and alcohols). Acidogenic bacteria are able to metabolize organic compounds at a very low pH around 4. Methanogenic microorganisms cannot directly use all products from the acidogenic step. Except for acetate, H2 and CO2 and some other micromolecular organic acids were abundantly generated during the so‐called acetogenic phase (stage III) by secondary fermenting bacteria, also called obligate hydrogen‐producing bacteria (OHPB). However, the thermodynamics of these reactions are unfavorable, and these microorganisms can only live in syntrophy with end‐product users, i.e. methanogens.
The methanogenic step (stage IV) corresponds to the final conversion of acetate, carbon dioxide (CO2), and hydrogen (H2) into biogas, and the obligate anaerobic archaea of hydrogenotrophic and acetoclastic methanogens abundantly exist in the digesters and could transform the mixture of CO2/H2 and acetate into methane. Specifically, hydrogenotrophic microorganisms convert H2 and CO2, produced by fermentative bacteria, into CH4 and keep the reactor under a low hydrogen partial pressure and thus enhanced the growth of acetogenic bacteria. The relative abundance of hydrogenotrophic and acetotrophic is variable according to environmental factors (i.e. acetate, ammonia, hydrogen, and hydrogen sulfide concentrations), and operational conditions (i.e. hydraulic retention time [HRT], pH, type of substrate, and source of inoculum), as well as solid contents [19]. It has been reported that the hydrogenotrophic methanogens (i.e. Methanoculleus and Methanobacterium) are predominated during the start‐up of anaerobic digesters and lead to a subsequent decline of the H2 concentration; Then, a shift of the methanogens into the acetoclastic methanogens (i.e. Methanosarcina and Methanosaeta) were observed after the stabilization of the reactor [20]. In addition, a high concentration of ammonia of the anaerobic digester benefited for the growth of hydrogenotrophic methanogens in mesophilic anaerobic digestors [21], and approximately 65–70% of the methane generation was closely related to the degradation of acetate; otherwise, the oxidation of acetate to H2 and CO2 is the main pathway in the absence of acetoclastic methanogens (such as Methanosaeta sp.) [22].
1.2.2 Factors Affecting the AD Process of Biogas Production
1.2.2.1 Temperature
Three different temperature regimes, namely psychrophilic, mesophilic, and thermophilic conditions, with varied optimum temperature ranges for the domination of different strains of methane‐forming bacteria, were traditionally used in anaerobic digesters [23]. Specifically, psychrophilic digesters usually operate at about 25 °C, whereas mesophilic ones operate at around 35 °C and thermophilic ones at around 55 °C. Generally, the metabolic activity and bioconversion rate of microorganisms at higher temperature are usually higher than that at lower temperature. However, the much more energy is required for maintaining a high temperature in the fermenter, which increases cost in practical operation [23]. For instance, a much higher degradation rate of fatty acids was observed for the digester operated under 55 °C with a 11 HRT than that operated under 38 °C condition with a 27 day HRT [23]. Similarly, an increase of 54–61% in CH4 yield from algal remnants was observed when the temperature increased from 25 to 35 °C [24]. In addition, some of the recent works also revealed that reported that the variation of operational temperature, even under a very small range, would decline the biogas production rate of the digesters [25], and the fluctuation of the temperature even 1 °C per day would deteriorate the operation [26].
1.2.2.2 pH
Operational pH might be another main factor that would significantly affect the performance of the digesters, and the most favorable range of pH to achieve maximal biogas yield in AD is 6.8 to 7.2 [23]. Specifically, the methanogenic bacteria are extremely sensitive to pH fluctuations, and their preferred pH was around 7.0, and the growth rate of methanogens was seriously inhibited once the pH declined to <6.6 [27]. Acid‐forming bacteria are less pH‐sensitive, and the optimal pH for hydrolysis and acidogenesis is between 5.5 and 6.5, despite their tolerated pH ranged from 4.0 to 8.5 [26, 27]. Therefore, some designers prefer the isolation of the hydrolysis/acidification and acetogenesis/methanogenesis processes into two separate stages [27]. At the beginning of the fermentation, the significant accumulation of acids and CO2, as a consequence of the growth of acidogens and acetogens, leads to a significant decline in the pH. Afterward, the consumption of these acids by the methane‐producing bacteria would maintain the digester under a stable condition [23] Excessive fatty acids, hydrogen sulfide, and ammonia are toxic only in their nonionized forms (FA and H2S–pH below 7, NH3–pH above 7); thus, the proportional distribution of ionized and nonionized forms of inhibitors of methanogenesis was essential for the stable operation of the digesters.
1.2.2.3 Organic Loading Rate (OLR)
Organic loading rate (), generally defined as kilograms of VS loaded per volume of digester per day, is hence considered as one of the main parameters for stable operation of AD systems [28]. The production of biogas and methane in continuous systems is highly dependent on the OLR value (related to the TS in the digester and the composition of feedstock), and the variation of the OLR would lead to significant variation of the methane yields and system stability. The recent work of Nizami and Murphy (2010) [29] stated that the optimum OLR of the anaerobic digesters ranged from 12 to 15 kg VS m−3 d−1 for corn silage, while 8.5 kg VS m−3 d−1 for other substrates [30] clearly demonstrated that the OLR values are highly dependent on the feedstock compositions. Practically, the accumulation of inhibitory compounds, such as VFA or ammonia, negatively affected the increasing operational OLR values of the digesters [31]. Many authors highlight the need for understanding. Thus, OLR needs to be carefully selected by simultaneously considering the feedstock characteristics, inhibitory compound existences, and co‐digestion opportunities, to maximize waste treatment capacity and enhance the renewable energy productivity.
1.2.2.4 Carbon–Nitrogen Ratio
Feedstock total organic carbon (TOC), total nitrogen (TN) and their ratio are also critical for the stable operation of the AD systems. The addition of co‐substrates, for the purpose of element balance, has been regarded as one of the most common practices for a purpose of achieving stable co‐digestion [32], and the optimal C:N ratio of digesters was always ranged from 20 to 30 [33]. The nitrogen in the AD reactor is mainly derived from proteins, and it plays a key role in microbial growth. However, a low C:N ratio in the digesters system (high amount of nitrogen) can produce an ammonia accumulation, subsequently affecting the biogas and methane yields and eventually causing the system to deteriorate [34]. Thus, the additive paper waste or agricultural waste has been traditionally applied to increase the feedstock's carbon content [35].
1.2.2.5 Inoculum‐to‐Substrate Ratio (ISR)
Inoculum‐to‐substrate ratio (ISR), which determines the initial ratio between microbial populations, is an important parameter for starting up of anaerobic digesters [36]. The more the inoculum, the higher the number of methanogens in the anaerobic digesters and the better the buffering capacity. Raposo et al. used sunflower oil cake as the substrate to explore the effect of different inoculation rates on AD [37], and they found that the volatile acids were not accumulated under the operational conditions of ISR 1.0–3.0, whereas significant volatile acid accumulation occurs when the inoculation rate is less than 1.0. For instance, the ratio of total volatile acid to total alkalinity was much higher than other experimental groups once the inoculation ratio declined to 0.5.
1.2.2.6 Solids Concentration
The reduced water content of the organic wastes within the digesters is generally regarded as the main reason for the difficulty in the gas and liquid diffusion and the accumulation of inhibitors and in turn reduces the substrate availability and affects their metabolism [38]. A number of studies reported that an increase in the water content of substrate increases the methane yielding and also leads to an excellent homogenization of the AD systems, efficient element diffusion, and effective interaction between microorganisms and nutrients. In addition, the recent work of Le Hyaric et al. (2012) [39] reported that there was a linear increase in the specific methanogenic activity with the increase in water content, ascribing to the improvement of the homogeneity of the digestion reactors [40].
1.2.2.7 Hydraulic Retention Time (HRT)
Retention time of the digesters refers to both HRT and solid retention time (SRT) and was an another important parameter used for designing and optimization of anaerobic digesters (represented in Eqs. (1.2) and (1.3)) [41]. Specifically, HRT represents the retention time of the liquid phase, whereas SRT denotes the retention time of the microbial culture in the digester. Assuming that the feedstock and microbial mixed cultures existed in the same phase in the anaerobic digester, the HRT value of the digestion system equals to SRT. For example, in the AD systems, using food waste, kitchen waste, and MSW as the substrates, the HRT of the system is essentially SRT. In contrast, the interaction between solids and microbial cultures is biphasic for the digesters using waste‐activated sludge and primary sludge as substrates and leads to quite different distribution of HRT and SRT:
where V refers to the individual reactor volume (m3), Q is the influent flow rate (m3 d−1), X presents the mixed liquid suspended solids in an individual reactor (mg L−1), Qx denotes the excess biosolids removal rate (m3 d−1), and Xx is the mixed liquid suspended solids in excess biosolids flow (mg L−1).
In general, the chosen HRT during the AD systems operation closely depended on the feedstock compositions, reactor volumes, operational parameters, and biomass activities. For example, those substrates with simple structure (e.g. starch and sucrose) can be easily hydrolyzed and digested, which only needed a much shorter retention time compared to the digesters using complex substrates (e.g. lignin and cellulose). In addition, a high operational temperature increases the decomposition rate of substrates and benefits the declining of the HRT that might be the main reason why majority of the thermophilic reactors are operated under a lower HRT than mesophilic reactors. Generally, a shorter HRT poses serious threat to the bacterial mobilization and consequently elevates the stress of the methanogens [42]. Therefore, the optimization of the operational HRT is usually neither too long nor too short (majority cases lie between 10 and 25 days), although a very high HRT in the order of 50–100 or more days may be needed for digesters operated in colder climates.
1.3 Current Challenges of AD Process and Biogas Production
1.3.1 Ammonia Inhibition
Due to the sensitivity of AD, the accumulation of certain substances in digesters can result in their performance inhibition or process failure. Ammonia is an essential nutrient for bacterial metabolisms, and an optimal ammonia level guarantees sufficient buffer capacity of methanogenic medium in AD systems and subsequentially improves the stability of the digestion [43]. On the other hand, it may also inhibit the methane production if the concentration of ammonia keeps at a high concentration, especially when treating the mixed substrates such as manure or MSW [44]. During the AD, the majority of the nitrogen organic compounds, principally in the form of proteins and urea, are finally converted to ammonia ion (NH3) and free ammonia (NH4+‐N) [45]. Therefore, NH3 and NH4+‐N are the two major forms of inorganic ammonia nitrogen, which can directly or indirectly cause the inhibition in AD. Particularly, free ammonia (FAN) and total ammonium nitrogen (TAN) exhibit a stronger effect of inhibition in AD when their concentrations reach the threshold value.
The knowledge of how ammonia inhibition occurs is limited, and few studies using pure culture have confirmed that ammonia may affect methanogenic bacteria in two ways [43], mainly including (i) direct suppression of methane‐producing enzymes by ammonium ion and (ii) diffusion of the hydrophobic ammonia molecule into the cell and the occurrence of the proton imbalance, as well as the potassium deficiency [34, 46, 47] (Figure 1.2). Specially, partial of NH3 will convert into NH4+ by absorbing protons once they enter the cells and negatively leading to a significant variation in pH. To keep balance in proton, the cells must then use a potassium (K+) pump to maintain the intracellular pH, thereby increasing the energy consumption and further causing inhibition of the activities of the specific enzymes [46]. The previous studies suggested that the methanogens are the least tolerant and are most likely to be inhibited at higher ammonia concentrations among four types of anaerobic microorganism [34, 49]. Thus, the most recent work of the inhibition of AD process by ammonia is mainly focused on the evolution of methanogenic populations with increasing TAN concentrations.
Figure 1.2Mechanisms of ammonia inhibition occurred in anaerobic digestion systems [43, 46, 48].
Tremendous researches have been focused on clarifying the threshold values of ammonia inhibition with different substrates for optimizing AD performance. For example, the recent work of J. Prochazka et al. revealed that the AD bioreactors with different substrates (pig slurry, primary/excess activated sludge, and maize) exhibited a high methane productivity at TAN concentrations of 600–800 mg L−1, whereas low buffering capacity and the subsequent lack of nitrogen as nutrient were observed for the reactor with a lower TAN concentration [48]. For comparison, Abouelenien et al. found that a TAN concentration of 8000–14,000 mg L−1 would suppress the AD system fed with chicken manure [50], while 800–1400 mg L−1 TAN for the AD of swine slurry [51]. In another study using piggery wastes as the substrate, the concentration of 3000 mg L−1 TAN would partially inhibit the digesters [44]. This discrepancy in ammonia concentrations on the inhibition of the digesters is probably caused by the differences in chemical characteristics of the substrates, inocula, and environmental conditions (temperature or pH) [45].
Notably, AD is a biochemical process with multiple phases, and its stability and efficiency closely depend on external and multiple syntrophic interactions among different taxa [52]. For instance, a stable AD reactor with synthetic acetic acid substrate was inhibited at TAN levels of >5000 mg L−1 (the corresponding FAN was 256 mg L−1), under an operational pH of 8.0 [43]. For the substrates of 9–10% sewage sludge, as high as a TAN concentration of 2500 mg L−1 would cause noteworthy inhibition of the microorganisms under thermophilic conditions [50]. Moreover, a much higher tolerance of TAN concentration even under 5000 mg L−1 was observed for the digester operated under a long 40‐day SRT, in comparison with the reactor operated under 25‐day SRT [53], implying that the ammonia inhibition also correlated with the SRT of the digesters fed with sludge. The recent work of Yenigun and Demirel summarized the ammonia inhibition occurred in AD [44], and the relevant results are cited in Table 1.1.
For the purpose of alleviating ammonia inhibition, tremendous approaches including air stripping, bioaugmentation, and ammonia binding have been widely applied to counteract ammonia inhibition in AD process [60]. However, high operational costs and technical challenges associated with these approaches further hinder their full‐scale practical application [61]. Exploring low‐cost input, easy maintenance, and practical method for alleviating ammonia inhibition will be still the main stream in AD field.
Table 1.1Summarization of the threshold values of ammonia inhibition in different anaerobic digestion processes.
Substrates
Digester type
Organic loading rate
Temperature (°C)
pH
TAN
FAN
Acclimation
References
Sludge
Laboratory scale
—
30
7.2–7.4
>5000 mg L
−1
—
Yes
[44]
Piggery mature
Laboratory scale
—
30
7.2–7.4
>3075 mg L
−1
—
Yes
[44]
Cattle mature
Continuously
—
55
7.9
>4000 mg L
−1
900 mg L
−1
No
[54]
Slaughterhouse wastes
Laboratory scale
2–3 kg COD m
3
d
−1
38
8.1
>6000 mg L
−1
—
Yes
[55]
Cattle mature
Continuous stirred tank reactor
(
CSTR
)
2.5 and 6.0 g N L
−1
40–64
7.4–7.9
—
>700 mg L
−1
Yes
[56]
Slaughterhouse wastes
Semi‐batch mode
4.2 kg COD m
3
d
−1
38
8.0
>6000 mg L
−1
—
Yes
[57]
Cattle manure
CSTR
—
45
7.4–7.9
6000 mg L
−1
700 mg L
−1
Yes
[56]
Food waste
Anaerobic batch
—
35
7.7
>6 g L
−1
Yes
[58]
Piggery manure
CSTR
9.4 g COD L
−1
d
−1
51
8.0
11 g L
−1
1450 mg L
−1
Yes
[59]
TAN concentration is the start of inhibition concentration.
MSW, municipal solid waste.
1.3.2 Volatile Fatty Acid Inhibition
VFAs, which mainly include acetic acid, propionic acid, butyric acid, and valeric acid, usually act as the most important intermediate products in the acidogenesis and acetogenesis steps and play a key role in the overall performance of AD systems. During these four steps of AD processes, VFA acts as an important intermediate metabolite that connects acid formers (e.g. acidogenic bacteria and acetogens) and utilizers (e.g. methanogens). As is known to us all, more than 72% of the methane production is derived from acetate, and the majority of the acetic acid in the AD reactors is converted from ethanol, propionate, and butyrate.
Despite VFAs being essential nutrients for methanogens, excessive VFA generation, especially that of propionic acid and butyric acid, under some special cases (e.g. organic overload, nutrient deficiency, toxicant exposure, or other factors) might be the essential reason for the deterioration of AD due to their inhibition of methanogen activity (Figure 1.3). From the perspective of thermodynamics, the degradation process of VFAs usually belongs to the thermodynamic nonspontaneous reaction due to the high Gibbs free energy as described in Eqs. (1.4) and (1.5):
Generally, the degradation of VFA would occur under the digestion condition with a low hydrogen partial pressure, as well as a low concentration of degradation products. However, a large amount of H2 could not be easily consumed in a short time due to the tightness of the anaerobic fermentation tank, and the accumulated degradation products are also not easily consumed due to the complex microbial relationships; thus, those digesters should be carefully operated.
The inhibition ability of VFAs during the operation of AD reactors correlated well with the operational pH and unionized VFAs concentration. For instance, the activated sludge microorganisms are easily inhibited by unionized VFAs when pH declines to 6.0 and severely inhibited at a lower pH [62]. Furthermore, the continuous accumulation of VFAs will lead to a further declining of system pH and enhance the conservation of VFAs from ionized phase to unionized one [63]. Theoretically, the ionized VFAs could not penetrate the membrane due to the lipid‐bilayer base structure of the bacterial plasma membrane; thus, the damage of those ionized VFAs to the cell is negligible. By contrast, the unionized VFA could penetrate the membrane freely due to its smaller size and nonpolar characteristics [65] and undoubtedly cause serious damage to DNA and proteins, ascribing to its lipophilic characteristics for passing through the cell membrane freely [64]. This suggests that the dissociation state of the organic acids is more decisive for microbial activity than the total concentration of VFAs. In addition, the dissociated H+ can acidify the cytoplasm of the biomass within the digester; thus, the cell needs to export H+ via a proton ATPase pump mechanism, which is energy demanding and may result in energy depletion [64]. With the gradual accumulation of H+, the gradual decline of the intracellular pH would finally lead to the cessation of the cell growth once the pH drops to the limit value of the biomass [65] (Table 1.2).
Figure 1.3Inhibition mechanisms of volatile fatty acids on anaerobic digestion [62–64].
Aforementioned experimental results revealed the inhibition mechanism of different VFAs on individual digesters. In practical AD systems, those microorganisms, such as hydrolytic acid‐producing bacteria, acetic acid‐producing bacteria, and methanogenic archaea, exhibit different tolerances to VFA and pH. Specifically, hydrolytic bacteria and fermentation acid‐producing bacteria have a wider pH tolerance range (4.0–8.5) and a stronger tolerance to VFAs, while majority of the methanogenic archaea are susceptible to the VFAs inhibition and could grew only under neutral pH conditions (6.8–7.2). From the aforementioned analysis, it is clear that the fundamental mechanism of acid inhibition is that when the digestive system has a higher VFA concentration due to high organic loading or imbalance of substrates, the imbalance between the hydrolytic acid production (upstream of the digestive system) and the methanogenic process (downstream of digestion) occurs due to the different tolerance capacity of microorganisms, and the higher generation rate of hydrolytic acid than that of the methanogenic consumption leads to the VFA accumulation and negative feedback, which eventually leads to the collapse of the digestive system.
1.3.3 Psychrophilic Temperature Inhibition
Temperature is a critical factor affecting AD performance because of the influence of both system heating requirements and methane production. Although AD can successfully operate under psychrophilic (15–25 °C), mesophilic (35–40 °C), and thermophilic (50–60 °C) conditions, mesophilic and thermophilic digestions have been typically recommended for CH4 generation [75]. It was reported that thermophilic process shows tremendous advantages in achieving high rates of digestion, excellent waste organics conversion, fast solid liquid separation, and insignificant accumulation of viral pathogens [44]. For comparison, mesophilic temperatures can relieve the fast FAN accumulation and are more resistant to higher FAN concentration as compared to mesophilic digestion [43]. Adversely, psychrophilic digestion attracted less attention due to the weak performance and single substrate demanding.
As for psychrophilic anaerobic digesters, the performance of CH4 production and solid removal is closely dependent on the ambient conditions [76]. The main inhibition mechanism of the psychrophilic temperature on AD operation is the slow hydrolysis, the first step of digestion where the complex compounds are converted into soluble simple structures [77]. Low temperatures will inhibit the activity of hydrolytic microbials, especially for those methanogenic biomasses, undoubtedly a notable decline in digestion performance [78]. In addition, low temperatures also lead to a poor mixing of the sludge bed due to the considerable decreases in the biogas production [79] that might be the main reason why those cellulose‐rich substrates, such as grass, were traditionally digested under the mesophilic condition instead of psychrophilic condition, for their complex chemical structures and difficult hydrolysis characteristics. Cysneiros et al. [77]
