Biotechnology for Zero Waste E-Book

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 1152

Veröffentlichungsjahr: 2022

Beschreibung

Biotechnology for Zero Waste

The use of biotechnology to minimize waste and maximize resource valorization

In Biotechnology for Zero Waste: Emerging Waste Management Techniques, accomplished environmental researchers Drs. Chaudhery Mustansar Hussain and Ravi Kumar Kadeppagari deliver a robust exploration of the role of biotechnology in reducing waste and creating a zero-waste environment. The editors provide resources covering perspectives in waste management like anaerobic co-digestion, integrated biosystems, immobilized enzymes, zero waste biorefineries, microbial fuel cell technology, membrane bioreactors, nano biomaterials, and more.

Ideal for sustainability professionals, this book comprehensively sums up the state-of-the-art biotechnologies powering the latest advances in zero-waste strategies. The renowned contributors address topics like bioconversion and biotransformation and detail the concept of the circular economy. Biotechnology for Zero Waste effectively guides readers on the path to creating sustainable products from waste. The book also includes:

• A thorough introduction to modern perspectives on zero waste drives, including anaerobic co-digestion as a smart approach for enhancing biogas production
• Comprehensive explorations of bioremediation for zero waste, biological degradation systems, and bioleaching and biosorption of waste
• Practical discussions of bioreactors for zero waste and waste2energy with biotechnology
• An in-depth examination of emerging technologies, including nanobiotechnology for zero waste and the economics and commercialization of zero waste biotechnologies

Perfect for process engineers, natural products, environmental, soil, and inorganic chemists, Biotechnology for Zero Waste: Emerging Waste Management Techniques will also earn a place in the libraries of food technologists, biotechnologists, agricultural scientists, and microbiologists.

Leseprobe

Table of Contents

Cover

Title Page

Copyright

Foreword

Preface

Part I: Modern Perspective of Zero Waste Drives

1 Anaerobic Co‐digestion as a Smart Approach for Enhanced Biogas Production and Simultaneous Treatment of Different Wastes

1.1 Introduction

1.2 Anaerobic Co‐digestion (AcD)

1.3 Digester Designs

1.4 Digestate/Spent Slurry

1.5 Conclusion

References

2 Integrated Approaches for the Production of Biodegradable Plastics and Bioenergy from Waste

2.1 Introduction

2.2 Food Waste for the Production of Biodegradable Plastics and Biogas

2.3 Dairy and Milk Waste for the Production of Biodegradable Plastics and Biogas

2.4 Sugar and Starch Waste for the Production of Biodegradable Plastics and Biogas

2.5 Wastewater for the Production of Biodegradable Plastics and Bioenergy

2.6 Integrated Approaches for the Production of Biodegradable Plastics and Bioenergy from Waste

2.7 Conclusions

References

3 Immobilized Enzymes for Bioconversion of Waste to Wealth

3.1 Introduction

3.2 Enzymes as Biocatalysts

3.3 Immobilization of Enzymes

3.4 Bioconversion of Waste to Useful Products by Immobilized Enzymes

3.5 Applications of Nanotechnology for the Immobilization of Enzymes and Bioconversion

3.6 Challenges and Opportunities

Acknowledgments

References

Part II: Bioremediation for Zero Waste

4 Bioremediation of Toxic Dyes for Zero Waste

4.1 Introduction

4.2 Background to Dye(s)

4.3 The Toxicity of Dye(s)

4.4 Bioremediation Methods

4.5 Conclusion

References

5 Bioremediation of Heavy Metals

5.1 Introduction

5.2 Ubiquitous Heavy Metal Contamination – The Global Scenario

5.3 Health Hazards from Heavy Metal Pollution

5.4 Decontaminating Heavy Metals – The Conventional Strategies

5.5 Bioremediation – The Emerging Sustainable Strategy

5.6 Conclusion

References

6 Bioremediation of Pesticides Containing Soil and Water

6.1 Introduction

6.2 Pesticide Biomagnification and Consequences

6.3 Ill Effects of Biomagnification

6.4 Bioremediation

6.5 Methods Used in Bioremediation Process

6.6 Bioremediation Process Using Biological Mediators

6.7 Factors Affecting Bioremediation

6.8 Future Perspectives

References

7 Bioremediation of Plastics and Polythene in Marine Water

7.1 Introduction

7.2 Plastic Pollution: A Threat to the Marine Ecosystem

7.3 Micro‐ and Nanoplastics

7.4 Microbes Involved in the Degradation of Plastic and Related Polymers

7.5 Enzymes Responsible for Biodegradation

7.6 Mechanism of Biodegradation

7.7 Biotechnology in Plastic Bioremediation

7.8 Future Perspectives: Development of More Refined Bioremediation Technologies as a Step Toward Zero Waste Strategy

Acknowledgment

Conflict of Interest

References

Note

Part III: Biological Degradation Systems

8 Microbes and their Consortia as Essential Additives for the Composting of Solid Waste

8.1 Introduction

8.2 Classification of Solid Waste

8.3 Role of Microbes in Composting

8.4 Effect of Microbial Consortia on Solid Waste Composting

8.5 Benefits of Microbe‐Amended Compost

References

9 Biodegradation of Plastics by Microorganisms

9.1 Introduction

9.2 Definition and Classification of Plastics

9.3 Biodegradation of Plastics

9.4 Current Trends and Future Prospects

References

10 Enzyme Technology for the Degradation of Lignocellulosic Waste

10.1 Introduction

10.2 Enzymes Required for the Degradation of Lignocellulosic Waste

10.3 Utilizing Enzymes for the Degradation of Lignocellulosic Waste

10.4 Conclusion

References

11 Usage of Microalgae: A Sustainable Approach to Wastewater Treatment

11.1 Introduction

11.2 Microalgae for Wastewater Treatment

11.3 Cultivation of Microalgae in Wastewater

11.4 Algae as a Source of Bioenergy

11.5 Conclusion

References

Part IV: Bioleaching and Biosorption of Waste: Approaches and Utilization

12 Microbes and Agri‐Food Waste as Novel Sources of Biosorbents

12.1 Introduction

12.2 Conventional Methods for Agri‐Food Waste Treatment

12.3 Application of the Biosorption Processes

12.4 Use of Genetically Engineered Microorganisms and Agri‐Food Waste

12.5 Biosorption Potential of Microbes and Agri‐Food Waste

12.6 Modification, Parameter Optimization, and Recovery

12.7 Immobilization of Biosorbent

12.8 Conclusions

References

Note

13 Biosorption of Heavy Metals and Metal‐Complexed Dyes Under the Influence of Various Physicochemical Parameters

13.1 Introduction

13.2 Mechanisms Involved in Biosorption of Toxic Heavy Metal Ions and Dyes

13.3 Chemistry of Heavy Metals in Water

13.4 Chemistry of Metal‐Complexed Dyes

13.5 Microbial Species Used for the Removal of Metals and Metal‐Complexed Dyes

13.6 Industrial Application on the Biosorption of Heavy Metals

13.7 Biosorption of Reactive Dyes

13.8 Metal‐Complexed Dyes

13.9 Biosorption of Metal‐Complexed Dyes

13.10 Conclusion

References

14 Recovery of Precious Metals from Electronic and Other Secondary Solid Waste by Bioleaching Approach

14.1 Introduction

14.2 What Is Bioleaching?

14.3 E‐Waste, What Are They?

14.4 Role of Microbes in Bioleaching of E‐Waste

14.5 Application of Bioleaching for Recovery of Individual Metals

14.6 Large‐Scale Bioleaching of E‐Waste

14.7 Future Aspects

References

Part V: Bioreactors for Zero Waste

15 Photobiological Reactors for the Degradation of Harmful Compounds in Wastewaters

15.1 Introduction

15.2 Photobiological Agents and Methods Used in PhotoBiological Reactors

15.3 Conclusion

Acknowledgment

References

16 Bioreactors for the Production of Industrial Chemicals and Bioenergy Recovery from Waste

16.1 Introduction

16.2 Basic Biohydrogen‐Manufacturing Technologies and their Deficiency

16.3 Overview of Anaerobic Membrane Bioreactors

16.4 Factors Affecting Biohydrogen Production in AnMBRs

16.5 Techniques to Improve Biohydrogen Production

16.6 Environmental and Economic Assessment of BioHydrogen Production in AnMBRs

16.7 Future Perspectives of Biohydrogen Production

16.8 Products Based on Solid‐State Fermenter

16.9 Koji Fermenters for SSF for Production of Different Chemicals

16.10 Recent Research on Biofuel Manufacturing in Bioreactors Other than Biohydrogen

References

Part VI: Waste2Energy with Biotechnology: Feasibilities and Challenges

17 Utilization of Microbial Potential for Bioethanol Production from Lignocellulosic Waste

17.1 Introduction

17.2 Processing of Lignocellulosic Biomass to Ethanol

17.3 Biological Pretreatment

17.4 Enzymatic Hydrolysis

17.5 Fermentation

17.6 Conclusion and Future Prospects

References

18 Advancements in Bio‐hydrogen Production from Waste Biomass

18.1 Introduction

18.2 Routes of Production

18.3 Biomass as Feedstock for Biohydrogen

18.4 Factors Affecting Biohydrogen

18.5 Strategies to Enhance Microbial Hydrogen Production

18.6 Future Perspectives and Conclusion

References

19 Reaping of Bio‐Energy from Waste Using Microbial Fuel Cell Technology

19.1 Introduction

19.2 Microbial Fuel Cell Components and Process

19.3 Application of Microbial Fuel Cell to the Social Relevance

19.4 Conclusion and Future Perspectives

References

20 Application of Sustainable Micro‐Algal Species in the Production of Bioenergy for Environmental Sustainability

20.1 Introduction

20.2 Cultivation and Processing of Microalgae

20.3 Genetic Engineering for the Improvement of Microalgae

20.4 Conclusion and Challenges in Commercializing Microalgae

References

Part VII: Emerging Technologies (Nano Biotechnology) for Zero Waste

21 Nanomaterials and Biopolymers for the Remediation of Polluted Sites

21.1 Introduction

21.2 Water Remediation

21.3 Soil Remediation

References

22 Biofunctionalized Nanomaterials for Sensing and Bioremediation of Pollutants

22.1 Introduction

22.2 Synthesis and Surface Modification Strategies for Nanoparticles

22.3 Binding Techniques for Biofunctionalization of Nanoparticles

22.4 Commonly Functionalized Biomaterials and Their Role in Remediation

22.5 Biofunctionalized Nanoparticle‐Based Sensors for Environmental Application

22.6 Limitation of Biofunctionalized Nanoparticles for Environmental Application

22.7 Future Perspective

22.8 Conclusion

Acknowledgment

References

23 Biogeneration of Valuable Nanomaterials from Food and Other Wastes

23.1 Introduction

23.2 Green Synthesis of Nanomaterials by Using Food and Agricultural Waste

23.3 Synthesis of Bionanoparticles from Food and Agricultural Waste

23.4 Conclusion

Acknowledgments

References

24 Biosynthesis of Nanoparticles Using Agriculture and Horticulture Waste

24.1 Introduction

24.2 Agricultural and Horticultural Waste

24.3 Biosynthesis of Nanoparticle

24.4 Characterization of Biosynthesized Nanoparticles

24.5 Applications of Biosynthesized Nanoparticles

References

25 Nanobiotechnology – A Green Solution

25.1 Introduction

25.2 Nanotechnology and Nanobiotechnology – The Green Processes and Technologies

25.3 The Versatile Role of Nanotechnology and Nanobiotechnology

25.3 Nanotechnologies in Waste Reduction and Management

25.5 Conclusion

References

26 Novel Biotechnological Approaches for Removal of Emerging Contaminants

26.1 Introduction

26.2 Classification of Emerging Contaminants

26.3 Various Sources of ECs

26.4 Need of Removal of ECs

26.5 Methods of Treatment of EC

26.6 Biotechnological Approaches for the Removal of ECs

26.7 Conclusion

References

Part VIII: Economics and Commercialization of Zero Waste Biotechnologies

27 Bioconversion of Waste to Wealth as Circular Bioeconomy Approach

27.1 Introduction

27.2 Biovalorization of Organic Waste

27.3 Bioeconomy Waste Production and Management

27.4 Concerns About Managing Food Waste in Achieving Circular Bioeconomy Policies

27.5 Economics of Bioeconomy

27.6 Entrepreneurship in Bioeconomy

27.7 Conclusion

References

28 Bioconversion of Food Waste to Wealth – Circular Bioeconomy Approach

28.1 Introduction

28.2 Circular Bioeconomy

28.3 Food Waste Management Current Practices

28.4 Techniques for Bioconversion of Food Waste Toward Circular Bioeconomy Approach

28.5 Conclusion

References

29 Zero‐Waste Biorefineries for Circular Economy

29.1 Introduction

29.2 Bioenergy, Bioeconomy, and Biorefineries

29.3 Bioeconomic Strategies Around the World

29.4 Challenging Factors and Impact on Bioeconomy

29.5 Effect of Increased CO2 Concentration, Sequestration, and Circular Economy

29.6 Carbon Sequestration in India

29.7 Methods for CO2 Capture

29.8 Conclusion and Future Approach

References

30 Feasibility and Economics of Biobutanol from Lignocellulosic and Starchy Residues

30.1 Introduction

30.2 Opportunities and Future of Zero Waste Biobutanol

30.3 Generation of Lignocellulosic and Starchy Wastes

30.4 Value Added Products from Lignocellulose and Starchy Residues

30.5 Conclusion

References

31 Critical Issues That Can Underpin the Drive for Sustainable Anaerobic Biorefinery

31.1 Introduction

31.2 Biogas – An Energy Vector

31.3 Anaerobic Biorefinery Approach

31.4 Technological Trends and Challenges in the Anaerobic Biorefinery

31.5 Perspectives Toward the Revitalization of the Anaerobic Biorefineries

31.6 Conclusion

Conflict of Interest

References

32 Microbiology of Biogas Production from Food Waste: Current Status, Challenges, and Future Needs

32.1 Introduction

32.2 Fundamentals for Accomplishing National Biofuel Policy

32.3 Significances of Anaerobic Microbiology in Biogas Process

32.4 Microbiology and Physico‐Chemical Process in AD

32.5 Pretreatment

32.6 Variations in Anaerobic Digestion

32.7 Factors Influencing Biogas Production

32.8 Application of Metagenomics

32.9 Conclusions and Future Needs

References

Note

Part IX: Green and Sustainable future (Zero Waste and Zero Emissions)

33 Valorization of Waste Cooking Oil into Biodiesel, Biolubricants, and Other Products

33.1 Introduction

33.2 Treatment

33.3 Evaluation of Waste Cooking Oil and Valorized Cooking Oil

33.4 Versatile Products as an Outcome of Valorized Waste Cooking Oil

33.5 Conclusion

References

34 Agri and Food Waste Valorization Through the Production of Biochemicals and Packaging Materials

34.1 Introduction

34.2 Importance

34.3 Worldwide Initiatives

34.4 Composition‐Based Solutions and Approaches

34.5 Biochemicals

34.6 Biofuels

34.7 Packaging Materials and Bioplastics

34.8 Green Valorization

34.9 Conclusion

References

35 Edible Coatings and Films from Agricultural and Marine Food Wastes

35.1 Introduction

35.2 Sources of Food Waste

35.3 Film/Coating Made from Agri‐Food Waste

35.4 Film/Coating Materials from Marine Biowaste

35.5 Film/Coating Formation Methods

35.6 Conclusion

References

36 Valorization of By‐Products of Milk Fat Processing

36.1 Introduction

36.2 Processing of Milk Fat and Its By‐Products

36.3 Valorization of Buttermilk

36.4 Valorization of Ghee Residue

36.5 Conclusion

References

Index

End User License Agreement

List of Tables

Chapter 4

Table 4.1 Some examples of azo dyes and their toxic effects.

Table 4.2 Bacterial species reported as dye degraders.

Table 4.3 Competitive advantages of bacteria for the degradation of azo dyes...

Table 4.4 Phytoremediation performances of various indigenous/wild plants fo...

Table 4.5 Application of integrated approaches for textile dye wastewater tr...

Chapter 5

Table 5.1 Health hazards caused by environmental heavy metal pollution.

Table 5.2 Various conventional metal decontamination strategies.

Table 5.3 Adaptive mechanisms in microorganisms resulting in metal resistanc...

Table 5.4 The environmental heavy metal pollution and the responsible genes ...

Table 5.5 Glimpses of different categories of phytoremediation strategies to...

Chapter 7

Table 7.1 Microorganisms involved in the bioremediation of different types o...

Table 7.2 Plastic‐degrading enzyme with source.

Chapter 8

Table 8.1 Composting studies with microbial addition to various solid waste ...

Chapter 9

Table 9.1 Different types of microbial enzymes responsible for the degradati...

Chapter 10

Table 10.1 Types of substrates used by fungi for cellulase production.

Table 10.2 Types of enzymes used to cleave side chains in hemicellulose [3].

Chapter 12

Table 12.1 Biosorption mechanism of various microbes and agri‐food waste.

Chapter 15

Table 15.1 Various photobiological agents used in photobiological reactors w...

Chapter 16

Table 16.1 Bioreactors used for production of different products.

Chapter 17

Table 17.1 Summary of commonly used pretreatment methods with their advantag...

Table 17.2 Different yeasts used for sustainable bio ethanol production alon...

Table 17.3 Biological pretreatment techniques for lignocellulosic biomass an...

Table 17.4 Fungi and bacteria participating in biological pretreatment of LC...

Table 17.5 List of microorganisms, biomass, and ethanol yielded.

Chapter 18

Table 18.1 Comparison of the catalytic and biological route of hydrogen prod...

Table 18.2 Pure and mixed cultures for biological hydrogen production.

Table 18.3 Various genetic engineering approaches used to enhance the biohyd...

Chapter 19

Table 19.1 Various designs of MFC and its power densities.

Table 19.2 Microbes used in MFC.

Chapter 20

Table 20.1 Cultivation and processing of microalgae.

Chapter 22

Table 22.1 Various biofunctionalized nanomaterials used for bioremediation.

Chapter 23

Table 23.1 Biogeneration of nanomaterials using agro‐waste.

Table 23.2 Cellulose‐based bionanomaterials.

Chapter 24

Table 24.1 Waste material used in the biosynthesis of nanoparticles.

Table 24.2 Procedure for preparation of metal salt solutions, extract and st...

Chapter 25

Table 25.1 Twelve principles of green chemistry.

Chapter 26

Table 26.1 Enzymatic treatment of EC and its efficiency.

Table 26.2 Efficiency of removal of EC using biofiltration.

Chapter 29

Table 29.1 Sources, processes, and product recovery of waste biorefineries‐b...

Table 29.2 Biorefinery types and their sustainable assessment for circular e...

Chapter 30

Table 30.1 Structural composition of lignocellulosic residues (dry basis).

Table 30.2 ABE and butanol yield comparison from different substrates.

Table 30.3 Butanol production cost from lignocellulose and starchy residues.

Table 30.4 Economics of butanol production from corn and glycerol.

Chapter 31

Table 31.1 General properties of gaseous fuels [10, 11].

Chapter 32

Table 32.1 Action of microbial enzymes on feedstock polysaccharides.

Table 32.2 Direction of process occurring in a bioreactor and simultaneously...

Table 32.3 Metagenomics insights into microbial ecology during anaerobic dig...

Chapter 34

Table 34.1 Agri‐food wastes (AFWs) used for synthesis of biopolymers and its...

Chapter 35

Table 35.1 Edible films made from agri‐waste residues.

Table 35.2 Edible films made from marine biowaste.

List of Illustrations

Chapter 1

Figure 1.1 Applications of anaerobic co‐digestion.

Figure 1.2 Real‐time monitoring of anaerobic digesters. * Daily tests.

FOG

–...

Chapter 2

Figure 2.1 Schematic representation of deriving biodegradable plastics and b...

Figure 2.2 Pathway that leads to ethanol from sucrose.

Chapter 3

Figure 3.1 Schematic representation of waste to wealth.

Figure 3.2 Diagrammatic representation of various enzyme immobilization meth...

Chapter 7

Figure 7.1 Effect of micro‐ and nanoplastics on marine life.

Chapter 8

Figure 8.1 Process flow of a SW composting with microbial additives.

Chapter 9

Figure 9.1 Classification of plastics (based on biodegradability).

Figure 9.2 Reaction pathways of polymer biodegradation.

Figure 9.3 Mechanism for the biodegradation of plastics.

Chapter 12

Figure 12.1 Schematic representation of the different mechanisms of microbia...

Chapter 13

Figure 13.1 Schematic representation of a fluidized bed.

Figure 13.2 Schematic representation of fluidized bed for the removal of Cu,...

Figure 13.3 Schematic diagram of a fixed bed reactor supported with luffa pu...

Figure 13.4 Schematic representation of packed bed reactor[32].

Figure 13.5 Formazan dye.

Figure 13.6 Irgalan dye.

Chapter 15

Figure 15.1 Sludge management in wastewater treatment.

Figure 15.2 Cyanobacteria in treatment of wastewater.

Figure 15.3 Various photobiological agents used in photobiological reactors ...

Chapter 16

Figure 16.1 Potential feedstock, conversion technologies, and products.

Figure 16.2 Biogas production and the potential applications.

Figure 16.3 Schematic diagram of AnMBR configurations (a) submerged and (b) ...

Figure 16.4 (a) Cake layer and (b) corresponding TMP development phenomenon ...

Chapter 17

Figure 17.1 The sources of different lignocellulosic biomass.

Figure 17.2 Overview of bioethanol production from lignocellulosic biomass....

Chapter 18

Figure 18.1 Mechanism of biological routes of hydrogen production. (a) Bioph...

Chapter 19

Figure 19.1 Schematic representation of a MFC for bioelectricity production....

Figure 19.2 Schematic representation of single chambered microbial fuel cell...

Chapter 20

Figure 20.1 Classification of biofuels.

Figure 20.2 Biofuel production pathway.

Chapter 21

Figure 21.1 Application of polymeric nanocomposites for the removal of vario...

Chapter 22

Figure 22.1 Components of biofunctionalized nanosensor.

Chapter 24

Figure 24.1 Overview of biosynthesis of nanoparticles, characterization, and...

Chapter 28

Figure 28.1 Simplified model for generation of food waste through the food s...

Figure 28.2 Strategies applied to reduce the foaming in 327 biogas plants....

Figure 28.3 Bioenergy recovery through enzymatic pretreatment process.

Chapter 29

Figure 29.1 Schematic representation of the process involved in circular eco...

Figure 29.2 Schematic representation of process involved in life cycle of bi...

Figure 29.3 The schematic representation of zero‐waste management concept.

Figure 29.4 Schematic representation of algae‐based biorefinery.

Chapter 30

Figure 30.1 World butanol utilization.

Figure 30.2 Worldwide availability of lignocellulosic feedstock.

Figure 30.3 Biobutanol process descriptions.

Chapter 31

Figure 31.1 Biogas production in continents and worldwide..

Figure 31.2 Biogas production in North America, Europe, China, and India. Al...

Figure 31.3 Aspects for a sustainable anaerobic biorefinery.

Figure 31.4 Valorization process in biorefineries.

Figure 31.5 Pros and cons of lignocellulosic waste pretreatment methods.

.

...

Figure 31.6 The relation between academia, industry, and government.

Figure 31.7 Multi‐beneficial system context of the AD.

Chapter 32

Figure 32.1 Microorganisms in anaerobic digestion of plant biomass under mes...

Figure 32.2 Microbial community in a typical biogas digestion system treatin...

Chapter 34

Figure 34.1 Global production capacities of bioplastics in 2017 (by market s...

Guide

Cover Page

Table of Contents

Title Page

Copyrigt

Foreword

Preface

Begin Reading

Index

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Biotechnology for Zero Waste

Emerging Waste Management Techniques

Edited by Chaudhery Mustansar Hussain and Ravi Kumar Kadeppagari

The Editors

Professor Chaudhery Mustansar HussainDepartment of Chemistry & Environmental ScienceNew Jersey Institute of TechnologyNewarkNew Jersey, 07102USA

Professor Ravi Kumar KadeppagariCentre for Incubation Innovation Research and ConsultancyDepartment of Food Technology, Jyothy Institute of TechnologyTataguni EstateBengaluru, Karnataka, 560082India

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Foreword

This book reveals innovative biotechnology tools for Zero Waste Drives, providing an integrated approach for biotechnology tools, methodology, and indicators for waste management practices and evaluating the advanced biotechnology and other transformational options. The new concept of Zero Waste is a sustainable approach to minimize the waste and making the world better and currently is being adopted in various sectors like mining, urbanization, manufacturing, agriculture, etc. Zero waste approach looks wastes as salvageable resources, which contain valuable nutrients, bioactives, industrial chemicals, and precious metals. Most of the zero waste drives are nowadays focused on optimum recycling, reuse, and resource recovery, ideally leading to the zero waste manufacturing as a futuristic approach. Among them, biotechnological approaches for reaching zero waste are more eco‐friendly and sustainable, being based on the recovery of energy and biofuels from agricultural, urban, and food wastes. In whole, bioconversion technologies like bioleaching, biosorption, and bioremediation can be used to obtain valuable products from different wastes and these technologies use different organisms and enzymes. Classic examples are the enzyme‐based technology for the recovery of ethanol from lignocellulosic waste, bio‐H2 production by dark fermentation process and recycling of used cooking oil as fuel, microbial‐enzymatic degradation of plastic, creation of biodegradable polymers or bioremediation of pesticides, energy generation from biowastes, among many others, described in this book. Economic aspects and commercialization of zero waste biotechnologies are also discussed.

I consider this monograph as “all‐in‐one” handbook in the area of zero waste approach, discussing emerging biotechnological and nanobiotechnological approaches for futuristic greener and sustainable future with zero emissions and production of marketable products from wastes.

Universidad Autónoma de Nuevo León

Boris I. Kharisov

Monterrey City, México

Preface

Zero waste should be a sustainable approach to minimize or nullify the waste and making the world better. This concept is being adopted in various sectors like mining, urbanization, manufacturing, agriculture, etc. Though zero waste manufacturing is believed to be the best and futuristic approach, most of the zero waste drives are currently focused on optimum recycling, reuse, and resource recovery. Manufacturing scrap, e‐waste, discarded constructional materials, plastics, domestic, agri‐food waste, and sewage have been haunting because their disposal affects the environment. Different physical and chemical methods to tackle these wastes by recycling and resource recovery in turn generate hazardous chemicals, emissions, and accessory wastes which are not eco‐friendly.

Biotechnological approaches for reaching zero waste are more eco‐friendly and sustainable. Research has been conducted on the recovery of energy and biofuels from agricultural, urban, and food wastes since long, and it has been practiced quite well, though enzyme‐based technology was developed recently for the recovery ethanol from lignocellulosic waste. Bio‐H2 was produced by dark fermentation process, and recycling of used cooking oil as fuel is gaining momentum. Zero waste approach should look wastes as salvageable resources, which contain valuable nutrients, bioactives, industrial chemicals, and precious metals. Bioconversion technologies like bioleaching, biosorption, and bioremediation were used to obtain above valuable products from different wastes, and these technologies use different organisms and enzymes. However, composting has been used for converting agro‐food waste into biofertilizers since long time. Submerged and solid‐state fermentation technologies were used for the biotransformation of agro‐food wastes into useful biochemicals and biopolymers which can be used for making biodegradable packaging materials. Plastic waste is one among the major current threatening problems to environment. Recently, Microbes and their enzymes were explored for the degradation of plastics, and microbes were used for the production of biodegradable plastics, though it was not economical. Microbes were also used in the bioremediation of pesticides which originate as accessory contaminants of agricultural practices. Biopulping and biofiltration were also applied for processing agro wastes. In this book, biotechnological approaches for reaching zero waste will be discussed in detail.

This book was divided into several parts focusing on recent advancements in biotechnology for zero waste drives. Biotechnological approaches like anaerobic co‐digestion, integrated biosystems, immobilized enzymes, zero waste biorefineries for circular economy, membrane bioreactors, microbial fuel cell technology for energetic valorization, biosorbents, bio‐diesel, biofunctionalized nanomaterials for bioremediation, etc. for zero waste drive were brought in.

Part IModern Perspective of Zero Waste Drives

1Anaerobic Co‐digestion as a Smart Approach for Enhanced Biogas Production and Simultaneous Treatment of Different Wastes

S. Bharathi and B. J. Yogesh

Bangalore University, The Oxford College of Science, Department of Microbiology, Sector 4, H.S.R Layout, Bangalore, 560102, India

1.1 Introduction

The world has witnessed tremendous growth over the past hundred years fueled by richness of earth's natural resources, but now we stare at the bleak prospects of exhaustion due to overutilization. With future economies balanced precariously on cost of fuel, with increasing demand for energy, ever‐increasing annual fuel consumption, limited natural resources, volatility and disruption in fossil energy supplies, need of clean technologies has certainly driven us toward a pragmatic approach for optimized and proper use of natural resource for a sustainable ecosystem. Insightful planning and innovative methods are essential to enhance energy production in order to meet surge in future energy demands. Another scourge of the modern society is waste management; especially in the developing economies punctuated by improvement in individual purchase parity, it has led to tripling of waste generation per person just over the last one decade. An attempt is made in this chapter to link these two possible issues of fuel generation and waste management through a biotechnological intervention. The era of biotechnology as a futuristic technology strives to tap the service of the potential saprophytic microbes, which not only hastens the recycling of dead organic matter but can provide the fuel for running the future economy.

1.1.1 Biodegradation – Nature's Art of Recycling

The elemental components of our periodic table have finely blended the earth into molecules of infinite diversity. The organic forms of molecules are the basis of life existence in which the principal elements carbon, hydrogen, nitrogen, and oxygen have a subtle role in the formation of living system. The photosynthetic forms of life are one of the biggest producers of the organic matter, and it comes with an inherent clause of undergoing natural degradation over a period of time. This biodegradation is a very important invention of the nature, for, without recycling, a continuous existence of new life over millions of years would have been impossible. Microorganisms play a pivotal role in this process of biodegradation, without it recycling would have been unimaginably slower.

1.1.2 Anaerobic Digestion (AD)

Naturally existing anaerobic ecosystems such as paddy fields, swamps, lakes, ponds, intestine of ruminants, and ocean sediments rich in dead organic matter have paved way for microbes especially the archaeal obligate anaerobes‐methanogens, mutual togetherness with other prokaryotic anaerobe leading to the production of methane. Though it can be attributed as a natural process, it leads to release of methane, a potential greenhouse gas capable of global warming far many times higher than carbon dioxide (CO2). Anaerobic digestion (AD) as a technology refers to a provision of a closed condition for efficient digestion of the organic waste and to collect the by‐product, methane.

The benefits of AD are immense for both the economy and ecosystem:

Firstly, the digestion takes place in a closed environment, thereby preventing air pollution from obnoxious gases or disease‐spreading germs.

There is no issue of leachate escaping into water bodies and thus prevents open water body pollution.

No underground seepage and pollution of groundwater.

Faster degradation of organic matter compared with composting (aerobic).

The AD process can be easily monitored circumventing the problems, for example, seasonal variation in temperatures.

A microbial consortium can be developed, and it would aid in continuous and efficient digestion of waste.

Biogas production with a range of fuel applications.

Downstream processing is not required as biogas collects in the head space and is siphoned off for clarification and usage.

Further effluent treatment would not be necessary as the slurry can be used as organic manure.

Pathogens are inactivated, thus rendering the digestate harmless and safe.

The drawbacks are few, but critical enough to be highlighted:

Limited access to high‐quality feedstock that is free of contamination

Non‐perennial aspects of feedstock

Transportation costs

Long‐term sustainable biomethanation

Unexpected digester failures

Maintenance of high fuel quality

Issues of multistakeholders (in case of co‐digestion)

The first four issues are related to feedstocks and its management, while the last three issues are related to lack of good microbial inoculum. Thus in this chapter, these two aspects of feedstock and real‐time monitoring of operational parameters are dealt in detail.

Technical issues could be overcome by reliable public–private partnership, government initiatives, financial supports followed by technological advancement.

1.1.3 Sustainable Biomethanation

Sewage water treatment plants mandatorily follow AD for sludge treatment, and the ensuing methane‐based gas is used for running wastewater treatment plants (WWTPs), though this is in principle, but the scenario is that many WWTPs struggle to maintain sustainable digesters, which are progressively jeopardized by frequent reactor failures. Biogas plants were ideally found to be an alternate source for renewable energy and were operated widely in rural areas of India; however, over the last few decades, it has taken a back seat, partially attributed to:

digester operational instability,

nonhomogeneous substrate,

lack of good microbial inoculum,

promotion and easier availability of LPG,

deeper reach of electricity to remote rural areas,

dip in active promotion of AD and their significance, especially in rural areas.

Renewed interest in AD stems from the problems of rapid urbanization and urgent need of waste management. Running successful biogas digesters depends mainly on two important factors: nature of substrate and the quality of inoculum. Real‐time monitoring emphasizes on the following factors:

balanced micro‐ and macronutrients,

efficient microbial inoculum,

digester design optimization,

optimized

organic loading rate

(

OLR

),

efficient monitoring of critical parameters (pH fluctuations, temperature range,

total solid

s (

TS

s) utilization rate,

volatile solid

s (

VS

s) accumulation and dispersal rates, microbial profiling: that is, eubacterial versus archaeal load ratio),

continuous evaluation of digester performance [rate of biogas production, methane percentage, reduction in total solids, reduction in

chemical oxygen demand

(

COD

)],

Reducing inhibitor concentrations.

1.2 Anaerobic Co‐digestion (AcD)

Biogas technology is a perfect example to emphasize on zero waste concept, conversion of waste into fuel, and even the final digested remnant slurry's immense value as organic manure, which is potentially free of pathogens. Mono‐digestion refers to the classical way for biogas production from a single type of feedstock while a co‐digestion refers to mixing of two different feedstocks in a digester for biogas production. Co‐digestion was initially planned to balance a carbon‐to‐nitrogen (C/N ratio) content of the feedstocks, as few feedstocks are either rich in carbon (agricultural) or found to be rich in nitrogen (animal waste). High C/N ratio of feedstock will ultimately lead to reduction in microbial load due to overall nitrogen deficiency while lower C/N can result in ammonia poisoning that could particularly affect methanogens leading to lower biogas production. Excess of carbohydrates in feedstocks needs shorter retention time (RT) in digesters attributed by its quick oxidation, while excess protein content leads to lesser biogas production ascribed to accumulation of toxic levels of ammonia; on the other hand, excess lipids though results in higher biogas production but RT nearly doubles [1] further characterized by high concentrations of volatile fatty acids (VFAs) and low pH, thus leading to a consensus that excess of any nutrient cannot be beneficial for biogas production [2]. The anaerobic co‐digestion (AcD) thus offers an opportunity to modify the composition of the waste to our need that suits our microbial consortium very well, and in this regard, C/N ratio can be altered to the optimum range. WWTPs around the world have increasingly opted for co‐digestion to increase biogas output, and a WWTP in Mesa, USA, has successfully evaluated co‐digestion of commercial solid food waste with sewage sludge in pilot‐scale anaerobic digesters [3]. Lipid‐rich restaurant waste has been co‐digested with sewage sludge [4].

1.2.1 Zero Waste to Zero Carbon Emission Technology

The biogas as renewable energy can contribute in a big way to meet an overzealous future goal of zero emission economy by supplying fuel to major contributors of greenhouse gas emissions such as transportation and heavy industries (power plants, steel and cement industry, to name a few). Presently the biogas, which is rich in methane, burns clean and helps in the cutdown of carbon emissions at a domestic level. It is evident now as many countries have taken initiatives in setting goals for tapping the renewal energy resources, the Australian water industry is said to have generated 187 GW/year of electricity from biogas via WWTPs and an additional 5.5 GW/year through AcD [5]. Channeling of organic wastes from land fill, restaurants, other urban wastes toward existing and time‐tested WWTPs is advocated by many countries and has envisioned zero carbon emission by the year 2040. Figure 1.1 summarizes the scope of AD.

1.2.2 Alternative Feedstocks

Feedstock refers to the particular form of organic waste available for AD but if left unattended can lead to environmental pollution. United State Environmental Protection Agency (USEPA) has assigned each feedstock a unique RIN (renewable identification number) that helps to rate how much of greenhouse gas it can emit in comparison to fossil fuel [3]. Cattle dung has been traditionally preferred as the typical substrate for AD; however, in terms of substrate quality it represents the semi‐digested material excreted by ruminants. However, the advantage of cattle dung as a substrate is that it has inherent microbes catered from intestines of ruminants specialized in AD and biogas production. Any substrate for AD is basically referred to as organic wastes generated at its source; it can be available in many forms and its characteristic depends on the source. It can be available from a single crop agricultural waste to a blended form as municipal solid waste (MSW/urban waste) categorized in terms of complexity in defining the exact composition of waste. Emphasis has been laid on alternative feedstock such as:

agricultural residues (energy crops),

commercial food waste (canteen/mess/restaurant),

retail wastes/fruits and vegetable wastes (peels, press cake),

animal waste (ranch waste/poultry waste/livestocks processing wastes),

effluent treatment in industries (dairy wastes, bioprocess industry, sugar industry),

garbage waste (MSW),

sewage sludge (WWTP), etc.

Figure 1.1 Applications of anaerobic co‐digestion.

It is still contradictory to classify based on source/origin because some untreated waste such as food waste may ultimately end up in land fill or may be diverted to WWTP. The wastes are characterized based on principal nutrient content for microbes, namely carbohydrates, proteins, and fats. Animal wastes are protein‐rich, while agricultural wastes are carbon‐rich with cellulose, hemicelluloses, lignin, etc. Dairy‐industry‐generated wastes are fats and protein‐rich. Thus each type of feedstock is unique in composition and based on that requires different approach for digestion. Feedstock composition should be assessed for certain inhibitors of methanogenesis, such as nitrates, sulfates as they could support growth of denitrifiers and sulfate reducers at the expense of methanogens [6, 7]; this tends to have a drastic effect on hydrogen foraging methanogen population leading to suboptimum biogas production. Though the organic waste is abundant in nature, its availability at a particular location could vary on a daily basis. Moreover, substrate heterogeneity, seasonal variation, and feasibility of transportation of waste from source are also to be coordinated. The idea of setting up the AD at the source of waste generation is a viable option; still the supplies could be erratic or inconsistent. The opportunity to go for co‐digestion not only helps in circumventing the problem of nonavailability of single substrate but also helps in managing different wastes generated at source efficiently.

1.2.3 Microbiological Aspects

The emphasis of the role of microbes is well documented in every successful biogas digester. There is a systematic and sequential breakdown of complex organic waste into methane carried out by four metabolically distinct bacterial groups:

hydrolyzing bacteria: complex carbohydrates, fats, and proteins converted to simple sugars,

long‐chain fatty acid

s (

LCFA

s) and amino acids;

acidogens: lead to the accumulation of VFAs, alcohols, and carbonic acids;

acetogens: further degradation results in acetic acid, hydrogen, carbon dioxide with trace amount of ammonia, H

2

S, etc.; and

methanogens: scavenge on H

2

and C1 and C2 carbon compounds for energy leading to production of methane.

Each of the aforementioned groups plays a pivotal role in AD and inactivation of any one group could possibly lead to accumulation of intermediate compounds impacting the outcome of the digester performance, while methanogen biomass ratio is miniscule in comparison to other groups [8]; still their influence is immense and found to be critical for sustainable biomethanation [9].

1.2.4 Strategies for Inoculum Development

It is highly impossible to define the exact microbial composition of any anaerobic digester, culturing techniques in coordination with molecular diagnostics can aid in identification, but never have we deduced the true potential population of AD. Inoculum for any biogas digester is usually sourced from ruminant fluid, municipal WWTPs, landfill leachate, or sludge collected from any preexisting active biogas digester. It is primarily important to relate inoculum with its role in biogas digesters, for example, an inoculum collected from WWTP may have few cellulolytic bacteria and thus may not lead to a sustainable biomethanation of agricultural wastes. Ruminant intestines harbor a natural population of methanogens, hydrolytic and other fermentative anaerobes, which cater to efficient biogas production and general success only for cattle‐dung‐based digesters; the same success is difficult to reproduce when inoculum from cattle‐dung‐based digester is added to digest poultry waste or dairy‐waste‐based digesters. Microbial population may vary even between sample inoculum and digester, for example, fresh cattle dung is rich in hydrogenotrophs (93–80%) [10] compared with acetoclastic methanogens (6–20%) [10] (Reasons being nonavailability of acetates, which are being reabsorbed by ruminant intestines along with other VFAs leading to the formation of animal fat) [10] while active digesters exhibit higher load of acetoclastic methanogens in comparison to hydrogenotrophs.

Even within digesters the microbial population may change, which can be attributed to the complex metabolic processes leading to accumulation of various intermediates that continuously influence the dynamics of microbial population. Hence, there is need for inoculum development, which involves acclimatizing a set of microbes to the digester environment; this could be done by pooling in a set of potential dominant anaerobes isolated from successfully running digesters to form a working consortium. Such microbial consortium had proven to give higher yield of biogas and better degradation of biological waste [11].

Consortium development is mostly targeted on methanogens as they are found to be the sole reason for biogas digester failure. The consortium has to be tested under lab‐scale digesters for their efficiency before implementing in larger‐scale biogas digesters. Care should be taken while developing consortium to select potential strains capable of withstanding digester environment fluctuations in pH and temperature, resistance to inhibitors, nutritionally diverse, and can syntrophically coexist. Potential strains of methanogens have been mostly identified to be hydrogenotrophic methanogens, acetoclastic and methylotrophic methanogens. The most abundant species among hydrogenotrophic methanogens are Methanobacterium, an hydrogen foraging methanogen that is known to dominate rumen intestinal environment while its role in a typical biogas digester is overshadowed by acetate utilizing methanogens (Methanosaeta, Methanosarcina, and Methanospirillum) that represent nearly 75% of the methane produced in digesters, still hydrogenotrophs are crucial for interspecies hydrogen transfer between syntrophic bacteria that could help diminish the concentrations of fatty acids in digesters [1], especially propionic acid as its presence can upset digester performance.

As mentioned earlier, there are four groups of bacteria in a synergetic action in digesters, each group of bacteria have their own physiological requirements and show varying degree of growth efficiency and wide range of sensitivity to environmental parameters. Acidogenic bacteria are among the fastest‐growing organisms, generally leading to quick accumulation of acid end products. While acetogenic bacteria and methanogens are slow‐growing organisms, to further complicate the matter, the methanogens are found to be very sensitive to changes in environmental parameters, which is detrimental for sustained biomethanation. Hence, inoculum is a critical parameter for determining the efficiency of anaerobic digesters. There is still diverse population of microbes that could not be cultivated and assessed from AD, and hence, any potential microbial consortium that is developed in laboratory should be considered as an supplementary feed and cannot by itself regarded as sole group of organisms that could digest waste in a digester [12].

1.2.5 Real‐Time Monitoring of AcD

Real‐time monitoring is essential for sustainable biogas production, will help us to continuously evaluate the digester performance, and help us to take immediate remedial action to circumvent the problem and prevent digester failures (Figure 1.2). Direct monitoring of microbial growth is not always a feasible option, as it requires an equipped anaerobic laboratory for studies, further the problems are compounded by slower growth rate of methanogens as it takes days to evaluate the exact microbial content of the digester. Molecular techniques such as fluorescence in situ hybridization (FISH), 16S rRNA, real‐time polymerase chain reaction (RT‐PCR), and denaturing gradient gel electrophoresis (DGGE) aid in assessment of microbial load feasible mostly for laboratory studies and applicable to large‐scale biogas digesters.

Figure 1.2 Real‐time monitoring of anaerobic digesters. * Daily tests. FOG – fat, oil, and grease; P – phosphorus.

1.2.5.1 The pH Fluctuations

There are other ways of monitoring bioreactor performance; these parameters are simple and can efficiently diagnose the current status of the working reactors. pH is one such factor that can be readily checked at regular intervals; neutral pH is preferred for sustainable biomethanation; and any variation in pH can drastically cut down methane production. Fluctuations in pH are one of the biggest problems associated with AD and mostly shift toward lower pH, which is directly attributed to accumulation of VFAs. Sometimes pH may shift toward alkalinity contributed by accumulation of ammonia. This pH problem is due to microbial metabolism, especially by higher growth activity of acid‐producing bacteria, compounded by the absence of buffering agents. Simultaneous degradation of proteins can lead to formation of ammonia that could help in balancing of pH in a digester averting shift toward acidic range. As mentioned earlier, too much of protein degradation in digesters can lead to excessive ammonia shifting pH toward 8.0 that shuts down microbial activity. The pH fluctuations should be seriously dealt with and a delay could permanently alter the microbial population of the digesters and sometimes cause irreversible damage to digester performance. Either way the methanogens are said to very sensitive to pH change and the problem can be overcome by neutralizing the pH with an alkali or a weak acid, but could turn to costlier affair to invest on alkali treatment, which is not generally recommended. A robust and an efficient microbial population of VFA converters are essential, while few digesters have adopted for dual digesters/two‐stage digestion for circumventing the pH problem.

1.2.5.2 Carbon–Nitrogen Content

It is essential to know the total carbon (TC) and nitrogen (N) content of the feedstock while the optimum C/N ratio for AD should preferably be in a range of 20–30. And increase in the value signifies the problem of nitrogen shortage leading to lesser load of microbes and process of AD getting delayed while lower ratio could imply higher microbial growth but the biogas could abruptly stop due to problems associated with by‐products of protein degradation significantly changing the digester balance toward inactivity. The AcD thus plays a crucial role as we can finely balance the carbon–nitrogen ratio for optimum biogas production.

Anaerobic digesters can work in a wide range of temperature; however, it been noted that temperatures below 20 °C can affect the efficiency of digesters by considerably slowing down the process; still in natural habitats, methanogenesis is found to happen significantly at low temperatures and over a period of time has contributed to global warming [13].

1.2.5.3 Temperature

Eightfold reductions in COD can be observed with mesophilic and thermophilic digestion at hydraulic retention time (HRT) of 35 days, while digesters at lower temperature are stable for a longer period of time more than 45 days [12]. Digesters around the globe are mostly operated in mesophilic conditions with recommended temperatures of around 35 °C, while faster digestion is generally reported at thermophilic temperatures of 55 °C but that comes with an inherent need of heat exchangers for temperature maintenance that can either shoot up or drastically fall reflecting microbial metabolism. Here biogas can be self‐employed for heating the digesters, and thus it could be self‐sustained process without much investment. It has been noted that the microbial population dynamics vary greatly between mesophilic and thermophilic digesters, for example, at 55 °C, hydrogenotrophs are found to dominate and if properly supplemented by syntrophic acetate‐oxidizing bacteria [14] could even lead to sustainable biogas production in complete absence of acetoclastic methanogens.

1.2.5.4 Volatile Fatty Acids

Efficient monitoring of digesters can also be carried out by constant evaluation of VFA content of the digesters. Though VFA accumulation above 2000 mg/l leads to digester failures, still it should be kept in mind that the same VFA gets finally converted to methane, in fact carbon atom of VFA is the principal source for methane production. The answer lies in the nature of VFA that accumulates in the digesters; most preferred form of VFA is acetic acid as it is the essential substrate for methanogens.

Fatty acid oxidizing bacteria breakdown LCFA to acetic acid, and these bacteria are inherently resistant to the toxic effects of accumulated LCFA. It has been noted that microbial load of fatty acid oxidizing bacteria fluctuates within the digesters directly influencing LCFA conversion rate, and their total absence in digesters leads to digester failures. Fatty acids oxidizing bacteria have been identified to be either producer of hydrogen (obligate hydrogen‐producing acetogens [OHPAs]) or hydrogen consumer (homoacetogens) but certainly lead to the formation of acetic acid. Not all VFA contributes to methane, certain volatile acids have a deleterious effect on the overall process especially propionic acid, and its accumulation decreases the pH to an extent of inhibiting the growth of methanogens, leading to fall in biogas production.

1.2.5.5 Ammonia

High protein content‐based feedstocks on AD can trigger an alkaline shock with accumulation of ammonia or ammonium ions, at about pH 8.0 the drastic reduction in microbial activity can be noted and with pH reaching 8.5 can completely deactivate methanogens thereby completely stopping methane production. The problem can be circumvented by balancing C/N ratio of the feedstock; immediate actions would be to reduce loading rate and further diluting the digester content. This corrective action can quickly adjust the pH to optimum range, it is imperative that the microbial consortia play a significant role in AcD.

Both ammonia and VFA thus play a crucial role and are intricately related to pH fluctuations; a VFA/ammonia ratio of 0.1 is preferred for a balanced sustainable digesters and increase to 0.5 indicates that the digesters could fail and further rise can completely stop biogas production.

1.2.5.6 Organic Loading Rate

Continuously operated digesters require balanced input of feedstock, (feedstocks/organic) loading rate (OLR) refers to the rate at which the feedstocks are fed into the digesters. OLR depends on the waste composition and is directly correlated to microbial growth rate, substrate conversion rate and evaluated by the rate of methane production. Excess OLR can dilute the microbial load, reduce digestion, foaming, and lesser yield of methane. OLR is further related to HRT, which implies the time taken by the digester for maximum gasification of the feedstocks. Shorter RT is preferable to avoid accumulation of fatty acids and toxins but way less than shorter RT can lead to microbial washout. Minimum one day RT is enough for stable buildup of fermentation bacteria especially for protein and nonfiber carbohydrates‐based feedstocks; cellulose and hemicelluloses may require two to three days to establish the process, while fat‐based feedstock may require longer RT of five days.

Complete gasification of waste can be achieved in a digester by increasing RT to 35 days (in case of batch digestion); the process is influenced by temperature: higher the temperature, shorter the RT, and RT of more than 35 days is required for psychrophilic temperature. Longer RT leads to improvement in quality of biogas in terms of methane concentration, shorter RT may generally exhibit 70% methane content while the percentage of methane tends to increase with longer RT. Total solid (TS) of more than 30% is not preferred for AcD as it leads to the problem of mixing concentrated pockets of temperature and pH burst in a continuously operated digesters depends on feedstock composition. The volatile solid (VS), which is a part of TS, is generally preferred in a range of 60–90% for efficient biogas production and for optimum microbial growth.

Pretreatment of feedstock is essential to minimize the natural flora on the surface of substrate as it will hinder the role of potential consortium developed for the purpose that is already active inside the digesters.

1.3 Digester Designs

The earliest digesters were simple in design with a digestion chamber, an inlet for feedstocks, and two outlets, one for spent slurry and one for biogas. The appropriate modeling of anaerobic digesters is imperative for biogas production. Digesters are designed with the view of maintaining strict anaerobic conditions and for collection and retrieval of biogas. The digesters can be operated in batch or continuous phase. Anaerobic biogas digester such as the one used in WWTP is distinct as it is continuously fed with heterogeneous liquid wastes, microbes agglomerate to form the granules (sludge) that set in to form a layer/blanket with a constant upflow hydraulic regime [15]. WWTPs around the world have opted for upflow anaerobic sludge blanket (UASB) digester for anaerobic treatment, which has been found to be cost‐effective and emphasizes the role of microbial granules (solid phase) that knit into a group of specialized agglomerated bacterial biofilm [16].

Expanded granular sludge beds (EGSBs) are a modified version and next‐generation biogas digesters with enhanced flow rate of liquid waste that could result in mixing of sludge particles establishing contact with nutrient for the purpose of breakdown. Further efforts have been taken to make thin, lighter‐weight biofilm of uniform thickness (granular sludge) for better fluidization and at lower energy expenses in the form of inverse fluidized bed reactors (IFBR), which would reduce HRT at a higher OLR that was initially carried out for distillery effluent [17]. Digesters with constant mixing can take up higher OLR, and it has been reported that OLR increased up to 300 kg COD/m3/d using super high rate anaerobic bioreactor (SAB) that works on a principle of spiraling baffle running through the middle of the digester body [15].

Mixing helps in uniform distribution of feedstocks during AcD and provides access of metabolic intermediates, microbial interaction; prevents stratification and release of trapped methane that has been observed with completely stirred/mixed tank reactors (CSTRs) [4]. Mixing of digester content can occur naturally to some extent by rise of methane bubbles, which is by itself not sufficient for optimum biogas production, hence auxiliary mixing is essential. It has been reported that intermittent mixing leads to better biogas production in comparison to continuous mixing [4].

As we know that four groups of microbes are responsible for biogas production, an attempt has been made to build two‐stage digesters basically dividing microbial role of hydrolysis/acidogenesis and acetogenesis/methanogenesis [18]. The first‐stage hydrogenic reactor (HR) and the second‐stage methanogenic reactor (MR) are linked but operated at different pH [19] and only recommended for digesting sugar‐rich feedstocks [20]

1.4 Digestate/Spent Slurry

The effectiveness of AcD can be evaluated based on the quality of the digestate/spent slurry of the digester. The composition of the digestate will naturally differ from initial feedstock, there should have been a drastic reduction in total solids content and COD. With richness in nitrogen and potassium and low on carbon content, the digestate can be an excellent source for organic manure for crop production, could support by minimizing usage of chemical fertilizers, and bedding can prevent soil erosion and help to retain soil fertility [21]. There have been few concerns on long‐term impact on usage of manure as fertilizer:

chances of altering preexisting and natural soil microflora,

impact of excessive nitrogen emissions from manure applied farm lands,

presence of recalcitrant compounds, and

slow degrading remnant organic matter contributed by manure.

There has been considerable research over the aforesaid drawback, and we have conclusive results with reports stating minimal or of minor relevance with no major impaction on overall soil fertility [22]. Manure can be packed and stored over of period of few months without much loss in nitrogen content and has been evaluated for storage during different seasons for their efficacy [23