Biomethane Production from Vegetable and Water Hyacinth Waste

By Zhao Youcai and Wei Ran

Biomethane Production from Vegetable and Water Hyacinth Waste explores the production of biomethane from vegetable waste and water hyacinth via anaerobic fermentation, focusing on effect factors, control methods and optimization. The book introduces principles and key technologies before proceeding into a deeper exploration of the fundamentals of municipal vegetable waste, such as composition, main indicators, experiment devices, and pretreatment. Processes to produce volatile fatty acids and resource recovery from anaerobic fermentation liquid and residue are described in detail. Other topics covered include the utilization of fermentation products, engineering design of a reaction tank, process parameters, and environmental and economic aspects.

The practical, application-oriented approach of this book allows engineering researchers, PhD students, and industry practitioners in the field of biogas and biomethane production, biomass conversion, and waste management to immediately utilize the information it provides.

• Covers the fundamentals and applications of the use of food waste and water hyacinth for biomethane production through anaerobic digestion
• Explores core challenges of a biomethane production operation, including details on process optimization and control
• Includes multiple case studies grounded in current industrial practice
• Offers practical examples and numerical calculations for large-scale operation
• Provides a representative treatment of a PPP plant considered from a process systems design perspective

• Language: English
• Publisher: Elsevier Science
• Release date: Oct 6, 2020
• ISBN: 9780128217641

Zhao Youcai

Zhao Youcai, is currently a professor of environmental engineering at School of Environmental Science and Engineering, Tongji University. He got bachelor degree from Sichuan University (1984) and Ph.D. from Institute of Chemical Metallurgy (now Institute of Process Engineering), Chinese Academy of Sciences, Beijing (1989). After finished Post-doctoral research work at Fudan University, Shanghai, he joined in Tongji University in 1991. Meanwhile, he had ever worked at Aristotle University, Greece, National University of Singapore, Tulane University, USA, and Paul Scherrer Institute, Switzerland, for 4 years as research fellow or visiting professor. He had authored or co-authored 200 publications published in peer-reviewed internationally recognized journals, 480 publications in China journals, authored 9 English books (at Elsevier and Springer) and authored or co-authored 98 Chinese books (as an author or Editor-in-chief), 4 textbooks for undergraduate, graduate and PhD students with a fourth edition of undergraduate textbook (in Chinese). Currently, his research interests include treatment and recycling of municipal and rural solid waste, construction and demolition waste, hazardous waste, industrial waste, electric and electronic waste, and sewage sludge, and polluted soil.

Book preview

Biomethane Production from Vegetable and Water Hyacinth Waste

Zhao Youcai

State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai, P.R. China

Wei Ran

State Key Laboratory of Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai, P.R. China

Table of Contents

Cover image

Title page

Copyright

List of contributors

Biography

Preface

Summary

Abbreviations

Chapter 1. Anaerobic fermentation process for biomethane production from vegetable waste

Abstract

1.1 Vegetable waste

1.2 Treatment and disposal of vegetable waste

1.3 Methods of improving methane productivity by vegetable waste anaerobic digestion

1.4 Anaerobic fermentation reactor

1.5 Principle of anaerobic fermentation of organic waste

1.6 Equipment and technology of anaerobic fermentation

1.7 Wet and dry fermentation

1.8 Comprehensive utilization of biogas and residue

1.9 Biological hydrogen and methane production in the fermentation of organic waste

Chapter 2. Bioproduction of volatile fatty acids from vegetable waste

Abstract

2.1 Sources of vegetable waste

2.2 Analysis indicators and methods

2.3 Fermentation reactors and methodology

2.4 Analysis of the modified Gompertz equation

2.5 Gas tracking test device of batch anaerobic fermentation hydrogen production

2.6 VFAs production from vegetable waste via anaerobic fermentation with inoculating yeast and acetic acid bacteria

2.7 Microaerobic fermentation by adding yeast and acetic acid bacteria in vegetable waste with different pretreatment methods

2.8 Optimization of fermentation conditions for VFA production from vegetable waste using yeast and acetic acid bacteria

2.9 Degradation of vegetable waste using a primary microelectrode coupled with microbial stimulation

2.10 Antimicrobial degradation of organic waste in the presence of acetic acid bacteria

2.11 Conceptual landfill for organic waste

2.12 Fe–C microelectrolysis on VFA production enhancement of organic waste by fermentation

2.13 Iron powder with different diameters on anaerobic fermentation of vegetable waste

2.14 74 µm Fe effect on vegetable waste anaerobic fermentation

2.15 18 μm iron powder effect on acid production of vegetable waste (TS 10%) anaerobic fermentation

2.16 74 μm iron powder effect on acid production of anaerobic fermentation of vegetable waste

2.17 Fe powder addition effect on anaerobic acidogenic fermentation of kitchen wastes

2.18 Fe²+ addition effect on anaerobic fermentation of vegetable waste

2.19 Iron–carbon microelectrolysis effect on excess sludge anaerobic fermentation

Chapter 3. Methane production by two-phase anaerobic digestion for vegetable waste

Abstract

3.1 Methanation based on residue after biohydrogen production

3.2 Pilot-scale test of methane production by two-phase anaerobic digestion

3.3 Treatment of anaerobic fermentation residue by earthworms

3.4 Bottleneck of methane production by two-phase anaerobic digestion

Chapter 4. Recovery of resources from anaerobic fermentation liquid and residues of vegetable waste

Abstract

4.1 Characteristics of volatile fatty acids produced by microaerobic fermentation in vegetable waste under semicontinuous model

4.2 Proportional relation of components in VFAs produced by microaerobic fermentation of vegetable waste under a semicontinuous model

4.3 Variation of oxidation-reduction potential with time in anaerobic fermentation in vegetable waste under semicontinuous model

4.4 Separation and extraction of VFAs from fermentation mixed liquid of kitchen waste and utilization of residual residue

4.5 Extraction of VFAs in mixed fermentation liquid of vegetable waste by tributyl phosphate

4.6 Suggestion for preparing RDF-5 waste-derived fuel from the residue of vegetable waste fermentation

4.7 C and N recovery and application for anaerobic fermentation liquid from vegetable waste

4.8 Product properties and application performances

4.9 Making coal water slurry with vegetable waste fermentation residue

Chapter 5. Technical development and a pilot trial for anaerobic digestion of water hyacinth

Abstract

5.1 Single-phase fermentation of water hyacinth under constant temperature

5.2 Two-phase fermentation of water hyacinth under constant temperature

5.3 Comparison between two- and single-phase digestion

5.4 Fragmentation of water hyacinth for anaerobic digestion

5.5 Acidification of water hyacinth at room temperature

5.6 Water hyacinth compost pretreatment

5.7 Engineering design of water hyacinth anaerobic reaction tank

5.8 Anaerobic fermentation equipment and process engineering design of water hyacinth

5.9 Anaerobic reactor start-up for engineering facilities

5.10 Water hyacinth anaerobic reaction engineering tank operation

5.11 Comprehensive utilization of water hyacinth anaerobic fermentation products

5.12 Compared with other anaerobic treatments of organic waste

5.13 Kinetic model of the digestion process

5.14 Economic analysis on anaerobic engineering of water hyacinth

5.15 Prospects for application of engineering

Chapter 6. Anaerobic fermentation engineering design for a vegetable waste treatment plant public-private partnership project

Abstract

6.1 Project overview

6.2 Process of overall scheme selection and determination

6.3 Selection of the production process

6.4 Odor treatment process comparison

6.5 Overall design thought

6.6 Recycling products instructions

6.7 Process system design

6.8 Illustrations of the plant’s arrangement

6.9 Design characteristics of the system and pertinent measures

6.10 Anaerobic fermentation system

6.11 Methane cleansing and refining system

6.12 Biogas residues composting system

6.13 Sewage treatment system

6.14 Odor treatment system

6.15 List of process control indicators

6.16 Process system composition and production line configuration

6.17 Environmental education center

6.18 General drawings and transport

Bibliography

Index

Copyright

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List of contributors

Zhao Aihua,     Shanghai Chengtou Group Company Ltd, Shanghai, P.R. China

Chen Bin,     Tongji University, Shanghai, P.R. China

Niu Dongjie,     Tongji University, Shanghai, P.R. China

Chai Fuliang,     Chongqing Environmental Sanitation Engineering Group, Chongqing, P.R. China

Zhou Haiyan,     Shanghai Laogang Waste Disposal Company Ltd, Shanghai, P.R. China

Tai Jun,     Shanghai Design Institute of Environmental Sanitation Engineering, Shanghai, P.R. China

Song Lijie,     Shanghai Design Institute of Environmental Sanitation Engineering, Shanghai, P.R. China

Zhang Meilan,     Shanghai Laogang Waste Disposal Company Ltd, Shanghai, P.R. China

Wu Na,     Tongji University, Shanghai, P.R. China

Kuang Qian,     Chongqing Environmental Sanitation Engineering Group, Chongqing, P.R. China

Wei Ran,     Tongji University, Shanghai, P.R. China

Huang Renhua,     Shanghai Chengtou Environment Company Ltd, Shanghai, P.R. China

Zhang Ruina,     Shanghai Design Institute of Environmental Sanitation Engineering, Shanghai, P.R. China

Chen Shanping,     Shanghai Design Institute of Environmental Sanitation Engineering, Shanghai, P.R. China

Dai Shijin,     Tongji University, Shanghai, P.R. China

Lin Shucan,     Shanghai Laogang Waste Disposal Company Ltd, Shanghai, P.R. China

Wu Shuya,     Tongji University, Shanghai, P.R. China

Zhou Tao,     Tongji University, Shanghai, P.R. China

Li Tian,     Tongji University, Shanghai, P.R. China

Cao Weihua,     Tongji University, Shanghai, P.R. China

Geng Xiaomeng,     Tongji University, Shanghai, P.R. China

Wang Xing,     Shanghai Design Institute of Environmental Sanitation Engineering, Shanghai, P.R. China

Zhang Xingqing,     Chongqing Environmental Sanitation Engineering Group, Chongqing, P.R. China

Wang Yan,     Tongji University, Shanghai, P.R. China

Li Yang,     Tongji University, Shanghai, P.R. China

Guo Yanyan,     Tongji University, Shanghai, P.R. China

Yu Yi,     Shanghai Design Institute of Environmental Sanitation Engineering, Shanghai, P.R. China

Zheng Yilin,     Tongji University, Shanghai, P.R. China

Zhao Youcai,     Tongji University, Shanghai, P.R. China

Chen Yu,     Tongji University, Shanghai, P.R. China

Wang Zhengyu,     Tongji University, Shanghai, P.R. China

Zhu Zihan,     Tongji University, Shanghai, P.R. China

Biography

Zhao Youcai is a professor of environmental engineering at the School of Environmental Science and Engineering, Tongji University, China. He got bachelor degree from Sichuan University (1984) and PhD from Institute of Chemical Metallurgy (now Institute of Process Engineering), Chinese Academy of Sciences, Beijing, China (1989). After finishing his postdoctoral research work at Fudan University, Shanghai, China, he joined in Tongji University in 1991. He has worked at Aristotle University, Greece; National University of Singapore, Singapore; Tulane University, United States; and Paul Scherrer Institute, Switzerland, for 4 years as research fellow or visiting professor. He has authored or coauthored 158 publications in peer-reviewed, internationally recognized journals, 420 publications in Chinese journals, 85 books in Chinese, and 6 books in English. His research interests include treatment of municipal solid wastes, sewage and industrial sludge, hazardous wastes, polluted construction wastes, and industrial wastes.

Wei Ran received his bachelor degree from Chongqing University, China, and is a researcher at the School of Environmental Science and Engineering, Tongji University, China. His research interests include anaerobic fermentation of organic waste, dehydration and utilization of fermentation residue, garbage classification, prevention and control of environmental health, polymerization and chemical decomposition of food waste, and recycling of aged refuse from landfills.

Preface

Vegetable waste, alternatively described as, organic waste, kitchen waste, or in some cases, fresh food waste (probably mixed with some cooked waste), and also called wet organic refuse in China, refers to the wastes that are produced in food processing by householders, canteens, restaurants, shops, and urban markets, and is favorable to biological treatment, such as anaerobic fermentation for biomethane production. The components and composition of vegetable waste vary with location, climate, and even from day to day.

Water hyacinth (Eichhornia crassipes), an aquatic plant of the genus Eichhornia of the pickerelweed family (Pontederiaceae), native primarily to tropical America, and introduced into China in the 1930s, has spread very quickly in recent years because of water eutrophication in watershed areas. It has become a social problem as it occupies waterways, disturbs the aquatic ecosystem equilibrium, and impacts local economies. Its fantastic growth pattern and rapid spreading mechanism in the field, as well as anaerobic fermentation for biogas production, are very important for the control and recycling of water hyacinth. It can be collected and treated anaerobically to produce biomethane.

There are many processing methods for the treatment of vegetable and water hyacinth wastes. According to the treatment principle, the treatments can be divided into thermochemical and biochemical methods. Thermochemical treatment is the process of decomposing organic waste by oxidation. The most frequently used treatment methods include incineration, pyrolysis, and wet oxidation. Biochemical treatment utilizes microorganisms to degrade organic waste, and can be divided into aerobic composting, semiaerobic fermentation, and anaerobic fermentation, depending on the oxygen content.

Anaerobic digestion is considered to be a favorable option for the effective treatment of waste, enabling methane gas and fertilizer to be produced while the wastes are stabilized. Traditionally, only methane has been obtained. Fermentation technology has become the mainstream for energy utilization of vegetable waste. Compared with aerobic treatment, anaerobic fermentation with low power consumption does not need artificial aeration to provide oxygen; in addition, it can also produce large amounts of clean energy. Anaerobic fermentation to dispose of organic wastes has broad prospects for the development of renewable energy utilization and environmental protection.

Whether using composting or fermentation, some metabolites will be produced by microorganisms, which can be utilized as a resource. During the metabolism process the main products are methane gas, volatile fatty acids (VFAs), and residue. The methane gas can be used for incineration power generation as clean energy, VFAs can be used as chemical raw materials after separation, and the residue can be used as a fertilizer in agriculture and forestry.

In this book, biomethane production from vegetable and water hyacinth waste via anaerobic fermentation technology, principles, process development and engineering design, and practical applications are discussed in depth. In Chapter 1, Anaerobic Fermentation Process for Biomethane Production From Vegetable Waste, the anaerobic fermentation process for biomethane production from vegetable waste is fully introduced, including crushing technology, the making of protein feed and organic acids, mixed anaerobic/microaerobic fermentation, vermicomposting, methods of improving methane productivity such as the use of additives, substrate reflux, raw material pretreatment, anaerobic fermentation reactors such as plug flow reactor, upflow solids reactor, upflow anaerobic sludge bed, upflow anaerobic sludge blanket, plug flow reactor, internal circulation reactor, anaerobic sequence batch reactor, expanded granular sludge bed, and anaerobic digestion reactor construction are illustrated. The principles of anaerobic fermentation of organic waste, that is, the definition of anaerobic fermentation, microbial communities in anaerobic fermentation, theory and calculation of methane formation, and the thermodynamics and kinetics of anaerobic fermentation are described. Equipment and technology used in anaerobic fermentation, such as traditional biogas fermentation equipment, modern large-scale industrial biogas fermentation equipment, wet fermentation and dry fermentation processes, and comprehensive utilization of biogas and residue are discussed. Biological hydrogen and methane production in the fermentation of organic waste, biological hydrogen production, integrated equipment combining hydrogen and methane production with a daily processing capacity of 10 t of kitchen waste are also described.

In Chapter 2, Bioproduction of Volatile Fatty Acids From Vegetable Waste, bioproduction of VFAs from vegetable waste is introduced, including pretreatment, analysis of the modified Gompertz equation, VFA production via anaerobic fermentation using inoculating yeast and acetic acid bacteria, pH and yield of acetic acid in microaerobic fermentation, hydrolysis of microaerobic fermentation by adding yeast and acetic acid bacteria, and the temperature effect on the pH of vegetable waste inoculated with yeast and acetic acid bacteria are discussed. The mechanism of acid production by microaerobic fermentation inoculated with yeast and acetic acid bacteria, degradation of vegetable waste using primary microelectrodes coupled with microbial stimulation, antimicrobial degradation of organic waste in the presence of acetic acid bacteria, and a conceptual landfill for organic waste are illustrated. Fe–C microelectrolysis on VFA production enhancement of organic waste by fermentation, iron powder with different diameters on anaerobic fermentation of vegetable waste, 18 μm iron powder effect on acid production of vegetable waste (total solid 10%) anaerobic fermentation, 74 μm iron powder effect on acid production of anaerobic fermentation of vegetable waste, Fe powder addition effect on anaerobic acidogenic fermentation of kitchen wastes, and iron–carbon microelectrolysis effect on fermentation of excess sludge anaerobic fermentation are elucidated.

In Chapter 3, Methane Production by Two-phase Anaerobic Digestion for Vegetable Waste, methane production by two-phase anaerobic digestion is fully described, including biomethane based on residue after biohydrogen production, a pilot-scale test of methane production by two-phase anaerobic digestion for biohydrogen and biomethane production, design of a methane production pilot test, and variations in methane and hydrogen production and metabolites composition and content. Determination of the maximum gas production rate and first-order kinetic parameters of biohydrogen and methane, biodegradability at different organic load rates in the hydrogen and methane fermentation step, treatment of anaerobic fermentation residues by earthworms, the bottleneck of methane production by two-phase anaerobic digestion, and methane production based on the residue after biohydrogen production are reviewed.

In Chapter 4, Recovery of Resources From Anaerobic Fermentation Liquid and Residues of Vegetable Waste, recovery of resources from anaerobic fermentation liquids and residues is introduced in detail, including the characteristics of VFAs produced by microaerobic fermentation in vegetable waste under a semicontinuous model, proportional relation of components in VFAs produced by microaerobic fermentation of vegetable waste under a semicontinuous model, and variation of oxidation–reduction potential with time in anaerobic fermentation in vegetable waste under a semicontinuous model. The separation and extraction of VFAs from fermentation mixed liquid from kitchen waste and utilization of residual residues, extraction of VFAs in mixed fermentation liquid of vegetable waste by tributyl phosphate, with a suggestion for preparing refuse derived fuel-5 waste-derived fuel from residues of vegetable waste fermentation, C&N recovery and application for anaerobic fermentation liquid from vegetable waste, product properties and application performance, and making coal water slurry with vegetable waste fermentation residue are examined.

In Chapter 5, Technical Development and a Pilot Trial for Anaerobic Digestion of Water Hyacinth, technical and pilot-trail anaerobic digestion of water hyacinth is introduced, including single-phase fermentation of water hyacinth under constant temperature, two-phase fermentation of water hyacinth under constant temperature, a comparison between two-phase and single-phase digestion, fragmentation of water hyacinth for anaerobic digestion, acidification of water hyacinth at room temperature, and water hyacinth compost pretreatment. The engineering design of a water hyacinth anaerobic reaction tank, anaerobic fermentation equipment and process engineering design of water hyacinth, anaerobic reactor start-up for engineering facilities, and water hyacinth anaerobic reaction engineering tank operation are explored. Comprehensive utilization of water hyacinth anaerobic fermentation products, comparison with other anaerobic treatments of organic waste, a kinetic model of digestion process, economic analysis of the anaerobic engineering of water hyacinth, and prospects for application in engineering are given.

In Chapter 6, Anaerobic Fermentation Engineering Design for a Vegetable Waste Treatment Plant PPP Project, the anaerobic fermentation engineering design for a vegetable waste treatment plant Public-Private Partnership (PPP) project is designed and practiced in full, including the process of overall scheme selection and determination, selection of the production process, overall design thoughts, process route, technological process, material balance, heat equilibrium, and water equilibrium. Process system design, anaerobic fermentation system layout, methane cleansing and refining system, biogas residues composting system, sewage treatment system, odor treatment system, process system composition and production line configuration, and environmental education center are discussed in detail.

This book is aimed at a readership including solid waste engineers, managers, technicians and maintenance staff, recycling coordinators and government officials, undergraduates and graduates, and researchers.

This book is financially supported by the National Key R&D Program of China (no. 2018YFC1901400), Social Development Programs of Science and Technology Committee Foundation of Shanghai (nos. 19DZ1204600, 19DZ1204703, and 18DZ1202604), National Natural Science Foundations of China (nos. 51878470 and 51678419), and the Fundamental Research Funds for the Central Universities (no. 22120190232).

Summary

Vegetable waste, alternatively described as, organic waste, kitchen waste, or in some cases, fresh food waste (probably mixed with some cooked waste), and also called wet organic refuse in China, refers to the wastes that are produced in food processing by homeowners, canteens, restaurants, shops, and urban markets, with complex components and composition. Water hyacinth (Eichhornia crassipes) grows in rivers and lakes and has a high organic component content. Both vegetable and water hyacinth wastes are favorable to biological treatment, such as anaerobic fermentation for biomethane production. In this book, biomethane production from vegetable and water hyacinth waste using anaerobic fermentation technology, principles, process development and engineering design, and practical application is discussed in depth. This includes an anaerobic fermentation process for biomethane production from vegetable waste, bioproduction of volatile fatty acids from vegetable waste, methane production by two-phase anaerobic digestion, technical and pilot-trial anaerobic digestion of water hyacinth, and anaerobic fermentation engineering design for a vegetable waste treatment plant PPP project.

Abbreviations

AMP adenosine monophosphate

AOP advanced oxidation process

ASBR anaerobic sequence batch reactor

ASTM American Society of Testing Materials

ATP adenosine triphosphate

BOD5 biochemical oxygen demand for 5 days

CEC cation-exchange capacity

CFWFRS coal–food waste fermentation residue slurry

COD chemical oxygen demand

CSTR continuous stirred-tank reactor

DC direct current

EGSB expanded granular sludge bed

EIA environmental impact assessment

FID flame ionization detector

FTIR Fourier-transform infrared spectroscopy

FWFR food waste fermentation residue

HRT hydraulic retention time

IC internal circulation reactor

LDH lactic dehydrogenase

MFC microbial fuel cell

MLSS mixed liquid suspended solid

MLVSS mixed liquid volatile suspended solid

MRT microbial retention time

MWD molecular weight distribution

OCV open-circuit voltage

OLR organic load rate

ORP oxidation–reduction potential

PCR polymerase chain reaction

PD power density

PE polyethylene

PET polyethylene terephthalate

PLC programmable logic controller

PSA pressure swing adsorption

RDF refuse-derived fuel

RID refractive index detector

RSM response surface methodology

SBR sequence batch reactor

SCOD soluble chemical oxygen demand

SEM scanning electron microscope

SPFRD screw plug-flow rotation drum anaerobic bioreactor

SRT sludge retention time

SS suspended solid

TBP tributyl phosphate

TCD thermal conductivity detector

TN total nitrogen

TOC total organic carbon

TS total solid

UASB upflow anaerobic sludge bed

UBF upflow sludge bed-filter

USR upflow solid reactor

UV ultraviolet

VFA volatile fatty acid

VS volatile solid

VSS volatile suspended solid

XRD X-ray diffraction

Chapter 1

Anaerobic fermentation process for biomethane production from vegetable waste

Abstract

In this chapter, the anaerobic fermentation process for biomethane production from vegetable waste is fully introduced, including crushing technology, making of protein feed and organic acids, mixed anaerobic/microaerobic fermentation, vermicomposting, methods of improving methane productivity such as the use of additives, substrate reflux, raw material pretreatment, anaerobic fermentation reactors such as a plug flow reactor, upflow solids reactor, upflow anaerobic sludge bed, upflow anaerobic sludge blanket, plug flow reactor, internal circulation reactor, anaerobic sequence batch reactor, expanded granular sludge bed, and anaerobic digestion reactor construction, are illustrated. The principles of anaerobic fermentation of organic waste, that is, definition of anaerobic fermentation, microbial communities in anaerobic fermentation, theory and calculation of methane formation, and thermodynamics and kinetics of anaerobic fermentation are described. Equipment and technology used in anaerobic fermentation, such as traditional biogas fermentation equipment, modern large-scale industrial biogas fermentation equipment, wet fermentation and dry fermentation process, and comprehensive utilization of biogas and residue are also discussed. Biological hydrogen and methane production in the fermentation of organic waste, biological hydrogen production, and integrated equipment combining hydrogen and methane production with daily processing capacity of 10 t of kitchen waste are outlined.

Keywords

Organic waste; water hyacinth; vegetable waste; food waste; anaerobic fermentation process; fermentation reactor; biogas and residues utilization

1.1 Vegetable waste

Vegetable waste, also called kitchen waste or organic waste, with a predominant component of uncooked food origin waste, sometimes mixed with cooked fractions, may include fruit peel, rotten vegetable leaves, eggshells, rice, flour, vegetables, vegetable oil, animal oil, meat and bone, starch, cellulose, protein, lipids and inorganic salts, nitrogen, phosphorus, potassium, calcium, sodium, magnesium, iron and other trace elements, utensils, toothpicks, and paper. Hence, vegetable waste is a solid–liquid mixture, and quite viscous, with complex chemical composition, and rich in organic matters, which may provide a good environment for the growth and reproduction of a variety of pathogenic microorganisms and flies carrying pathogens, resulting in the spreading of diseases, if it does not undergo proper treatment and disposal.

Around 30%–40% of the municipal solid waste in China consists of vegetable waste, mainly the uncooked waste from households and food shops, it is also called wet waste in the sorting sector, with an annual quantity of around 110 million, and possibly several billion of tons globally.

Vegetable waste may be characterized by its high moisture content, and is highly perishable, rich in organic matter and trace elements, and can be harmful. It can rot quickly and is likely to breed bacteria, resulting in the spread of diseases in summer. The use of this waste as animal feed can also cause and spread illnesses such as foot-and-mouth disease and form a pollution chain from animals to humans.

1.2 Treatment and disposal of vegetable waste

In addition to discharge to sewer directly, the vegetable waste treatment and disposal technologies include anaerobic digestion and aerobic composting, which rely on the fermentation of microorganisms. This digestion transforms the majority of vegetable wastes into carbon dioxide and water vapor that can be removed by volatilization. Composting can turn vegetable wastes into organic fertilizer which can be recycled into the environment via soil.

1.2.1 Crushing technology

Direct discharge of scattered vegetable waste after crushing on site into sewer systems was never been practiced in Europe and the USA. For example, a small amount of vegetable waste and cooked waste generated at home is cut by a crusher installed in the kitchen and then goes to the municipal sewer network, where it is then treated with municipal wastewater in the wastewater treatment plant.

The crushing method is a cheap and simple treatment for a small amount of vegetable waste. It can reduce the moisture content and raise the calorific value of urban organic waste. However, this method also has its deficiencies, such as a remarkable increase in the organic load of the municipal wastewater treatment plant and blockage of pipes. For a large city, the organic waste should be collected separately and anaerobically or aerobically treated independently.

1.2.2 Physical and chemical technology

By heating the organic waste below or above the boiling point of water, part of the organic compounds can be hydrolyzed into small molecules, while they are rapidly decompressed and expanded, and the water in the product is instantaneously spilled out of the waste particles, and finally a loose fine granular product is formed after filtering and drying. High-temperature hydrothermal treatment can promote the production of water-soluble proteins and low-calorie fats, with the passivation and inactivation of lipase and peroxidase, and hence the taste of the product is improved for use as a feed material.

1.2.3 Biological treatment technology

Vegetable waste is mainly composed of organic components, which have high biodegradability. Biological treatment technologies mainly consist of aerobic composting and anaerobic fermentation. Aerobic composting uses aerobic microorganisms to transform waste into fertilizer in the presence of air. Anaerobic fermentation degrades organic wastes with anaerobic bacteria into methane, which can be used as energy, and the substrate can be used as fertilizer or greenery soil after further dehydrating and composting. Over 4 weeks may be required at the primary stage for aerobic composting and anaerobic fermentation. Malodor and greenhouse gas emissions are the main drawbacks of these biological treatment processes.

1.2.4 Making protein feed from vegetable waste

Vegetable waste contains a lot of organic matter and can produce protein feed through biochemical treatment by microbial fermentation and physical drying after adding an appropriate nitrogen source and trace elements, using the protein, cellulose, carbohydrates, and other organic ingredients present in the vegetable waste as a carbon source. This product is rich in bioactive peptides, amino acids, nucleic acid degradation products, nucleotides, and nucleosides.

Vegetable waste must be pretreated, and virus pollution eliminated, before it can be made into animal feed. The pretreatment methods are aimed at controlling the bacteria, virus, and other pollution in the vegetable waste. The common methods include high-temperature drying sterilization and high-temperature pressing. High-temperature pressing and other means of pretreatment for killing bacteria and viruses have a significant effect, but there remain some security risks. This method can significantly reduce the number of pathogens such as coliform bacteria in vegetable waste but cannot completely eliminate pathogens and other remaining microorganisms. There exist many traces of toxic and hazardous substances in vegetable waste, such as crop pesticide residues and food additives, and some of these have a significant effect on environmental stability and bioaccumulation.

1.2.5 Anaerobic/aerobic fermentation for the production of organic acids

In recent years, anaerobic digestion has been widely used as an ideal method for waste disposal. It is also called biogas fermentation or methane fermentation, and uses anaerobic fermentative bacteria, hydrogen-producing acetic acid bacteria, hydrogen-consuming of acetic acid bacteria, hydrogen-producing methanogens, methane-producing bacteria, etc. to degrade the complex organic matter into N and P inorganic compounds, methane, carbon dioxide, and other gases under anaerobic conditions.

As a result of the different core technologies, vegetable waste disposal in China is formed into the so-called four models: the "Beijing model" mostly uses anaerobic digestion as the central technology (such as Biomax Wet Anaerobic Digestion Process of Beijing Dongcun Domestic Waste Comprehensive Treatment Plant). The "Xining model mostly adopts feeding technology, and the Shanghai model adopts dynamic aerobic digestion, which is mostly used for sewage treatment. Meanwhile, the Ningbo model" produces bacterial protein, feed additives, and industrial oils and fats. It can be seen that the current domestic vegetable waste disposal technologies are mainly concentrated in anaerobic digestion, feed processing, aerobic composting, and industrial grease and bacterial protein production. At present, the economic value of most vegetable waste treatment technologies is not high. According to the types and technological level of large- and medium-sized processing technology of Shanghai food and organic waste, there are three representative processing plants for organic fertilizer, feed, and refined fertilizer, and the cost of food and organic waste is analyzed here. It is found that processing 1 t on of vegetable waste can only make a small amount of feed which creates a profit, the rest is processed at a loss, and in view of the risk to the food chain, the use of vegetable waste to produce feed needs to be carefully considered, especially when it contains animal meat and oils. Applying the life cycle model to evaluate the vegetable waste disposal technology, the unit processing costs of anaerobic fermentation, aerobic composting, feed technology, and small biochemical treatment machines vary, and vegetable waste treatment only has some of its income coming from its product, with subsidies primarily supporting the development of vegetable waste treatment technology. Moreover, most vegetable waste treatments are not completed in vegetable waste disposal enterprises, the liquid from vegetable waste is usually left for processing by the city sewage treatment system, and there is high salinity that is not conducive to microbial growth and can corrode equipment easily, increasing the burden on the sewage treatment system.

Research into the influencing factors of volatile fatty acids (VFAs) caused by microoxygen fermentation of vegetable waste has mainly focused on the temperature, pH, level of sludge inoculation, and the addition of other substances in the process of acid production. The results show that the acidification efficiency of the hydrolysis phase is affected by the hydraulic retention time (HRT), pH, and content rate of feed solids during the acidification stage of the vegetable waste in a batch anaerobic reactor. In addition, it is proposed that the substrate has a role in the production of VFAs, and experiments have shown that carbohydrates play a more important role in the production of VFAs than protein. The temperature has a great influence on both the hydrolysis and acidification process of vegetable waste anaerobic fermentation, and the optimum temperature condition of the hydrolytic acidification process of vegetable waste is 37°C. The level of sludge inoculation is also a factor affecting the anaerobic fermentation of vegetable waste, and the flora of the inoculum and substrate composition will affect its the lactic acid production.

The digestive temperature and level of sludge inoculation are not the dominant factors in determining the type of acid-producing fermentation. pH has an important effect on the acid metabolism of fermentation bacteria and the distribution of fermentation products. The hydrolytic acidification process of vegetable waste does not adversely affect the digestion process at pH=4, but the lower pH will seriously affect the activity of the microorganisms. When the pH of the reaction system is 4–5, it tends to produce acetic acid-type fermentation. The initial pH of the reaction is not the same as the pH after the reaction is stabilized, and the initial pHs are not the same under different reaction conditions. It is found that acetic acid is the main product at different pH conditions, and the optimum initial pH is 7 for the anaerobic fermentation of vegetable waste. When the pH value is 6, hydrolysis and acidification of the vegetable waste are the best, and the concentration of VFA in the fermentation liquid is at its highest. The hydrolysis rate, production and efficiency of organic acids, composition of the organic acid, and distribution of the hydrolyzed acidification products of vegetable waste are under four pH conditions. And when pH is 7, the hydrolysis and acidification rate of vegetable waste are higher. Under the condition of initial pH 9.0, the VFA yield is at its highest, and the pH is stable at about 4.5 and 3.1, respectively, in pH=5.0 and a blank reactor, and the proportion of acetic acid in the product is at its highest. The campus food junk breakfast is quite different from the lunch/dinner vegetable waste, the total amounts of Na+, Ca²+, and Cl− are higher than those of the latter two, and the cumulative methane production is 212.2, 331.6, and 362.4 mL/g, respectively.

A vegetable waste system with 12% of the initial solid content has a higher efficiency of waste disposal and reactor operation, and the system with a dilution rate of 0.33 d−1 has significant advantages in pH stability, total solid (TS) removal rate, and the efficiency of hydrolytic acidification production. In addition, the production of VFAs can be promoted by the addition of other substances to the anaerobic fermentation reactor. The addition of zerovalent iron promotes the process of homoacetogenesis. With the increase in the urea dosage, the concentration of VFAs in the reaction system increases. The rate of hydrolysis of vegetable waste is significantly improved by adding yeast to the anaerobic fermentation in the vegetable waste, and the acid production rate and acid production are significantly higher than those without inoculation of yeast. In a study on the effect of trace metal element cobalt in sludge by mixed anaerobic fermentation with soy waste and sludge, the yield of VFAs increased from 367 [chemical oxygen demand (COD)] to 432 mL/g COD when 2 μmol/L trace cobalt–serine was added.

Thermo-alkaline pretreatment also increases the rate of acidified product accumulation. The vegetable waste is treated by anaerobic digestion to produce a high concentration of VFAs after thermo-alkaline pretreatment filling with oxygen and nitrogen. The optimum conditions for the production of anaerobic fermentation are as follows: the inoculation ratio is 4:1, the pH value is 6, and the temperature is 37°C. The order of influence of each factor is temperature>pH>inoculation ratio. The maximum mass concentration of total VFA is 31.56 g/L and the maximum mass concentration of acetic acid is 19.46 g/L when the pH is 6.5 and the TS is 7% (mass fraction) and C/N is 16:1. Under the conditions of a subcritical experiment, starch and cellulose are used as model compounds and H2O2 as an oxidant. When the carbon conversion rate is used as the evaluation index. The starch and cellulose are treated at 1–1.5 min, 70%–100% of oxygen content, and temperature 280°C. When the treatment time is 1–2 min, the oxygen content is 70%–100%, and the temperature is 280°C–300°C, the organic acid concentration is the largest. When the quality ratio of vegetable waste to water is 1:3, the pH of the vegetable waste anaerobic acidification solution is 6.5, and the acidification temperature is 30°C, which is beneficial in the production of organic acid in an acidified waste solution. The maximum yield of organic acids is 25–35 g/L. When the temperature is 18°C±2°C, HRT is 5 days, the adjusted pH is 5.2–6.7, and the maximum VFA yield of cooking waste is 10% (mass fraction), the VFA yield is 0.318 g/t.

1.2.6 Mixed anaerobic/microaerobic fermentation of organic waste

Household vegetable waste and pig manure are mixed with mesophilic anaerobic digestion, and the VFA concentration increases with the increase in the influent volatility solid (VS) organic loading. In the anaerobic digestion test, when vegetable waste has not been added, the concentration of VFA is very low, at only 120 mg/L. When 0.59 VS/(L d) of vegetable waste material was added, the VFA began to increase from the starting point of 138 mg/L. When the organic loading of vegetable waste was 0.5–1.259 VS/(L d), the concentration of VFA in the reaction system started to fluctuate. The VFA concentration increases significantly when the organic load is 1.59 VS/(L d) and reaches a maximum of 1630 mg/L on the 14th day. Mixtures of (1) kitchen waste and chicken manure, (2) fruits and vegetables, and (3) fruits and vegetable waste and chicken manure were used separately as raw materials. In the systems with low levels of addition of chicken manure (4:1 and 3:1) serious acidification took place. During the acidification stage, the inhibition of VFAs can be avoided with pH=7.2, thereby ensuring the digestive stability and improving the digestive performance.

The main production is of short-chain fatty acids that are acetic acid dominated, followed by propionic acid, butyric acid, iso-butyric acid, and other forms of VFA during sludge acid fermentation. Anaerobic digestion in wastewater and sludge treatment has been widely used, and the current method of solid-waste anaerobic digestion has been applied widely. Compared with other treatments, anaerobic digestion reduces greenhouse gas emissions. The residue after the digestion of organic waste (residue, biogas slurry) is rich in N, P, and other nutrients and active substances, and can be used as high-quality organic fertilizer and soil conditioner. There is less synthetic biomass and content of residual carbon in the process of organic matter conversion into methane, which can achieve a reduction in the amount of waste. Compared with the aerobic process, on the one hand, anaerobic digestion does not require artificial aeration to provide oxygen, therefore reducing power consumption.

1.2.7 Vermicomposting

Vermicomposting is a biological treatment technology developed in recent years based on the function of earthworms to promote the decomposition and transformation of organic matter in a natural ecosystem. Earthworms swallow a large amount of organic matter during their metabolic process and mix it with soil, which can be decomposed and transformed through mechanical grinding of the sachet and the biochemical effects of the intestinal tract.

The inhibitory effect of earthworm compost on pathogenic bacteria was tested in a shallow pond with a length of 6 m, width of 1.5 m, and depth of 0.2 m, in which the organic waste was divided into two piles in the pool, each weighing about 1361 kg, and each pile was inoculated with four pathogenic bacteria (fecal coliforms, Enterovirus, roundworm eggs), with an adding ratio of 1:1.5 (earthworms: organic waste). The test continued for a week. The earthworms were added on the 7th day, but not as a control. The results showed that the removal rates of Escherichia coli, Salmonella, Enterovirus, and Ascaris eggs in earthworm compost are, respectively, 99.99%, 99.99%, 98.92%, and 47.54%; and 71.60%, 93.18%, 53.85%, and 0.00% in the control group.

Using earthworms to deal with a vegetable waste and cow manure mixture, the test used plastic containers with a height of 20 cm and diameter of 26 cm as the earthworm bed. In this experiment, two kinds of earthworms were selected, and 50 earthworms were added to the container. The moisture content of the materials was 40%–60%, pH=6.5–8.5, and the experimental time was 150 d, and the control group had no earthworms added. Compared with the control group, the study found that the N, P, and K contents of the earthworm compost finished product significantly increased, while C/N and C/P were significantly reduced. Compared with the control group, the increase in Org-C, N, P, and K in the earthworm-treated condition was 0.27%, 156%, 41%, and 38%, and the C/N and C/P decline was 61% and 29%. The increase in Org-C, N, P, and K for the earthworms treated with the bottom activity was 14%, 102%, 33%, and 42%, respectively, while C/N and C/P decreased by 43% and 14%, respectively.

On the application of vermicomposting products, using food and paper waste composting as the field test after the earthworm bed treatment, the experiment started in a greenhouse and with strawberries selected as the plants. Vermicomposting was applied to the plot after nutrient testing, supplementing fertilizer, and the total nutrient content of nitrogen and phosphorus were 15.5 and 12.5 g/m², respectively. The results showed that, compared with paper waste compost, the product of vegetable waste compost had higher contents of C, N, Ca, Fe, K, and S, and compared with the other control plot, using vermicomposting, the highest leaf area of strawberry plants was increased by 37%, biomass increased by 37%, flower number increased by 40%, stolon increased by 36%, and commercial strawberry yield increased by 35%.

1.3 Methods of improving methane productivity by vegetable waste anaerobic digestion

There are many ways to improve organic waste anaerobic digestion biogas productivity. Most originate from the anaerobic treatment of industrial high-concentration organic wastewater sludge anaerobic digestion or anaerobic digestion of agricultural organic wastes.

1.3.1 Use of additives

Suitable natural plant additives can stimulate the physiological activities of microorganisms, increasing the local concentration of the fermentation substrate, creating an environment more suitable for microbiological activities, thereby increasing the production of biogas. In the anaerobic digestion process of mango products, 1500 ppm of bean curd, black hummer, and geranium mixed extract can increase biogas production by two to three times. A 1:1 mixed bean plant Pistia stratiotes and cow dung mixture can raise biogas production to 0.62 m³/(m³ d), with 76.8% methane content, and the HRT is 15 d. When using a 400 L drum bioreactor for anaerobic digestion of organic waste, the biogas production increased by 40%–80% by adding 1% onion residue. Biological additives are generally microorganisms, such as bacteria and fungi. They can increase the activity of certain enzymes, thereby increasing biogas production. Using cellulose degradation bacteria, Phanerochaete chrysosporium, to treat 3-week-old bagasse can improve the productivity of biogas from anaerobic digestion.

Adding metal cations can promote the enrichment of microbial populations, thereby increasing the residence time and microbial concentration of the microorganisms in the substrate, and eventually increasing biogas production. Adding an Ni ion solution to the substrate, when the Ni concentration reaches 2.5 or 5 ppm, can increase biogas production by 54%. The relevant study deems that adding Ni ions can promote the activity of Ni-based specific enzymes and the full methanation of the substrate and improve biogas production. Adding Ca²+ and Mg²+ can not only improve biogas production, but it is also possible to avoid foaming when the substrate is digested.

Adding 10 g/L of commercial pectin to anaerobic digestion substrate can improve biogas production by 150%, with a methane content of 65%. The commercial charcoal Darco G-60 is used in anaerobic digestion of organic waste and the gas production and corresponding blank sample have, respectively, 17% and 34.7% increases. Zeolite has an effect on organic waste anaerobic digestion with a high NH4–N content. Selected zeolites such as mordenite, clinoptilolite, 3A zeolite, and 4A zeolite have an obvious removal effect on NH4–N. The release of Ca²+ from the zeolite promotes gas production.

1.3.2 Improvement of gas production using substrate reflux

The return of the digestive substrate into the bioreactor can prolong the residence time of microorganisms, promote full degradation of the substrate, and improve biogas production. Using a continuous-feed drum reactor of 1 m³, when the substrate reflux ratio is controlled to 0.3 m³/(m³ d), HRT is controlled for 30 days, and gas production increases by 18.8%. Another test used a bioreactor with a capacity of 70 L to carry out the anaerobic digestion of vegetable waste. Two different substrate reflux ratios with 9 and 21 L/d, respectively, and a treatment with no substrate reflux as a control group, have been designed. As the time of digestion reached the 50th day, the methane contents in the biogas comprised 30% (control group), 50% (9 L/d), and 40% (21 L/d), respectively. It is believed that the appropriate reflux ratio can promote the proliferation of methanogens, thereby increasing gas production. The high reflux ratio will cause the substrate to acidify and inhibit the activity of methanogens. The appropriate substrate reflux can promote biogas production, and the bioreactor gas delivered with the substrate is 12 times greater than that of the control group.

1.3.3 Raw material pretreatment

Pretreatment methods can be divided into mechanical, chemical, and biological pretreatments. Mechanical and chemical pretreatments can transform the complex organic structure of the raw material into a biodegradable small-molecule material, destroy the long chain or net structure of the raw material, and increase the specific surface area of the substrate and the opportunity for the microorganisms contacting the substrate to improve the gas production efficiency. Biological pretreatment mainly includes adding high-purity biological species, using microbial secretion of extracellular enzymes and other substances to prehydrolysis of the substrate to improve the anaerobic digestion process of substrate hydrolysis stage efficiency, and thereby increasing gas production. Cooked vegetable waste has been treated by the cooking process, where protein, starch, and carbohydrates have been partially hydrolyzed, so that the fiber material is more suitable for mechanical and biological methods of pretreatment.

As for solid waste with a high cellulose content, crushing can not only significantly improve the biogas production and degradation rate of organic matter, which shortens the digestion time, but also reduce the effective volume of the reactor. The contents of cellulose, hemicellulose, fat, protein, lignin,