Food Waste Valorisation: Food, Feed, Fertiliser, Fuel And Value-added Products 1800612885, 9781800612884

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Published by World Scientific Publishing Europe Ltd. 57 Shelton Street, Covent Garden, London WC2H 9HE Head office: 5 Toh Tuck Link, Singapore 596224 USA office: 27 Warren Street, Suite 401-402, Hackensack, NJ 07601

Library of Congress Cataloging-in-Publication Data Names: Wong, Ming H., editor. | Purchase, Diane, editor. | Dickinson, Nicholas, editor. Title: Food waste valorisation : food, feed, fertiliser, fuel and value-added products / edited by Ming Hung Wong, The Education University of Hong Kong, China, Diane Purchase, Middlesex University, UK, Nicholas Dickinson, Lincoln University, New Zealand. Description: New Jersey : World Scientific, [2023] | Includes bibliographical references and index. Identifiers: LCCN 2022028859 | ISBN 9781800612884 (hardcover) | ISBN 9781800612891 (ebook for institutions) | ISBN 9781800612907 (ebook for individuals) Subjects: LCSH: Food industry and trade--By-products. | Agricultural wastes--Recycling. | Food industry and trade--Waste minimization. Classification: LCC TP373.8 .F67 2023 | DDC 664/.08--dc23/eng/20220809 LC record available at https://lccn.loc.gov/2022028859 British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

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Preface Food insecurity, hunger, and malnutrition are serious issues affecting all countries worldwide, with one in nine people suffering from starvation and malnourishment. About 811 million people suffer from hunger, and two billion from micronutrient deficiencies, stunting the growth of millions of children who are at greater risk of dying from common infectious diseases. Although the United Nations has set a target as part of the Sustainable Development Goals to end hunger by 2030 globally, we are far from reaching this goal. Contributing to this, the amount of food wasted is a vast and emergent global problem, with about 17% of food wasted amounting to one billion tonnes per year. Food waste is happening at home, in restaurants, and in stores, with food wasted at about the same rate in all but the poorest countries. Throwing away date-expired or best-before food even though it is still safe to consume in supermarkets and households creates part of the problem in more affluent countries. Lack of refrigeration contributes to the situation in poorer countries. Portions of food such as bones and shells, pits, seeds, and peels from fruits are thrown away, even though they contain residual energy, nutrients, and chemical compounds that could be utilised. Food is lost or wasted because of bad weather, processing problems, overproduction, unstable markets, overbuying, and confusion over labels at homes and in stores. It means up to 31% of the materials, energy, water, etc., used to grow, harvest, package, distribute, and store our food are all used in vain. There are also environmental problems related to this emerging issue of food waste. The problems start at the beginning of the food production

v

vi  Preface

process. Farming crops and livestock generate between 10 and 30% of greenhouse gas emissions, exacerbating climate change. When food is unnecessarily disposed of or wasted, it creates severe environmental problems. Improper waste disposal affects ecosystems, human health, aquatic life, terrestrial plants, agriculture, land use, and global warming. Food waste destined for anaerobic decomposition in landfills emits harmful greenhouse gases and leachate. In the last decade, the emergence of food waste as a global issue stimulates over 1,000 academic papers with food waste in the title every year. Early texts that aimed at a more popular readership include Stuart (2009, Waste: Uncovering the Global Food Scandal, Penguin) drew attention to the actual cost of what the global food industry throws away. A few recent textbooks have flowed from this increased interest. Galanakis (2020, Food Waste Recovery, 2nd Edn., Elsevier) deals with processing technologies and industrial applications. Two multiauthor volumes provide current perspectives: Kosseva & Webb (Eds., 2020, Food Industry Wastes, Academic Press) focus on the assessment of food waste commodities, and Reynolds et al. (Eds., 2021, Routledge Handbook of Food Waste, Taylor & Francis) draw together the many different perspectives of food waste and its management. The current textbook provides a collection of contemporary futurefocused reviews from invited experts in the field, who were considered by the editors to be involved in some of the most innovative and exciting possibilities for food waste management in the future. The 18 chapters are concerned with critical aspects of treating food waste as a resource, particularly by converting food waste into food, feed, fertiliser, fuel, and other value-added products. Chapter 1 introduces the essential background and sets the scene for the book. Food wastage includes “food loss” during earlier stages of food production and “food waste” at the consumer level. Together, they are commonly referred to as “Food waste and loss”, which adversely impact our environment and human health. Although avoidance of food waste would make a huge difference, substantial wastage remains inevitable. We explain why an integrated strategy should be adopted for reducing and redistributing FLW that involves a combination of treatment, disposal, and recycling. As part of the future circular economy, we must reduce food loss, avoid surplus food, and convert the remaining food waste

Preface  vii

resource to value-added products. Investigating the scientific and technological options and possibilities to achieve this is the objective of this book. Chapter 2 describes the importance of retaining food waste in urban centres, the places where most of the global population now lives. Large amounts of food waste and other organic waste could be locally recycled using composting to meet urban demands for soil-forming materials, soil conditioners, and fertilisers. Additionally, thermal treatment (pyrolysis) can produce biochar/hydrochar with similar uses for urban farming and landscaping. There are ample opportunities to use these products to reactivate and restore marginal, vacant, and derelict land and for green walls, rooftop farming, vertical farming, and community gardening. Closing the urban food cycle would ease the food waste disposal problem, improve food security, and contribute to socio-economic and health issues. It would support a circular economy and innovative, sustainable lifestyles contributing to environmental awareness and societal engagement with nature in the urban context. Chapter 3 compares the conventional treatments techniques used for treating food waste: (1) Anaerobic digestion and pyrolysis to produce pyro-oil and syngas for the enhancement of biomethanation; (2) hydrothermal carbonation for food waste with a high moisture content to generate hydrochar and other products such as hydro-oil; (3) gasification techniques for creating hydrogen syngas for bioenergy. The review provides valuable information on the techniques for optimal resource recovery. Chapter 4 presents a case study from the modern, forward-looking city of Shenzhen (in mainland China, close to Hong Kong) and the management of food waste generated in the most developed city in China. Food waste accounts for over half of municipal solid waste (MSW), and it is not effectively separated from MSW. Three industrial-scale projects involving anaerobic digestion, aerobic composting, and insect-based conversion in treating food waste are described, analysing and comparing these projects’ technologies, investment, treatment processes, and challenges. Chapter 5 provides essential information and modern techniques for turning organic waste into soil conditioners and organic fertilisers. In addition to explaining optimal environmental and treatment conditions, emphasis

viii  Preface

is given to the microbial communities involved in the different phases of the composting process. This is revealed using next-generation sequencing (NGS) and microbial DNA sequencing to examine microbial abundance and diversity. Chapter 6 transforms food waste into more stable humus-like substances. This chapter discusses the limitations and challenges to composting food waste and describes how microbial communities in the compositing process can also resolve contamination and safety issues. The authors explain that even diesel and dioxin/furan pollutants can be degraded by over 75% in as little as 3–6 weeks, simultaneously generating heat and combustible gases. Optimising management can deal with residual issues that otherwise reduce the value of food waste resources. Chapter 7 deals with the pretreatment and technology required to prepare food waste for recycling. The primary purpose of pretreatment is to organise food waste for anaerobic digestion and remove contaminants that may adversely affect digestion efficiency. The author explains the processes involved in industrial-scale operations, suggesting different ways to improve yields and overcome limitations associated with processing and recycling. Chapter 8 evaluates single-cell protein (SCP) production using different types of food wastes. This could significantly resolve worldwide demands for protein and ease disposal pressures of food waste. The use of submerged fermentation, semisolid fermentation, and solid-state fermentation and the preparation of substrates and cultivation techniques vary with the assemblages of microbial species (yeast, bacteria, fungi, and algae). This provides new opportunities, but realistic limitations of the technology are also discussed. Chapter 9 explores the feasibility of using food and agricultural waste for animal production to replace maize and soybean grains, aiming to produce high-quality feed to meet growing demands for high-quality nutrition. The authors address the major challenge of re-channelling nutrients and bioactive compounds from the waste into food production systems. They show that microbes and enzymes from fruit residues could enhance the feed quality of waste materials. This requires adequate characterisation of the

Preface  ix

chemical composition of wastes to develop optimum dietary levels and avoid antinutritional factors in animal feeds. Chapter 10 provides a concise review on converting food waste into vermicompost using specific species of earthworms that thrive in food waste and composting systems. Vermicompost can offer a high-quality soil amendment with outstanding stimulatory effects on seed germination and crop growth. This technology is low cost and requires minimal inputs and maintenance, producing a product already conveniently tested on living organisms providing safety assurance. Vermicompost has an immediate and significant role in urban agriculture that is particularly suitable for community-led, offering immediate buy-in from individuals and households and direct involvement in citizen science. Vermicomposting can also be expanded to a large scale for full integration into overall food waste management. Chapter 11 documents the techniques involved in treating and recovering resources from anaerobic digestate food waste (DFW) rich in nutrients and derived from bacteria decomposition. Properties of DFW and treatment methodologies (including pyrolysis and composting) are reviewed. Opportunities to improve the quality of biochar made from DFW that has better pollutant adsorption capability are discussed, also considering this technology’s limitations. Chapter 12 covers the broad topic on converting food wastes into valueadded chemicals, including chemicals, fuels, and materials derived from different biorefinery processes. This chapter reviews the integrated biorefineries, the most efficient for the food waste transformation, comparing the methods adopted and final products with those from petrochemicals. Several processes in integrated biorefineries that effectively act on gaseous, liquid, and solid raw food waste materials are evaluated. Chapter 13 deals with the feasibility of transforming nutrient-rich food waste into different products. Various additives could be added to increase the values of products, e.g., the colour and texture of synthetic fabrics and materials and the nutraceutical benefits of feed supplements and even functional food for human consumption. There is a wide range of applications in food and beverage, cosmetics, domestic products, etc.

x  Preface

The techno-economic barriers and the challenges posed for commercialisation (such as scale-up and assessment of environmental impacts) are discussed. Chapter 14 focuses on converting food waste into nanocellulose, with attractive properties and numerous applications. This chapter provides evidence of the excellent performance of these products, describing their morphology, chemical properties, and thermal stability. It is described how different chemical, mechanical, and biological processes can be used to generate nanocrystals and cellulose nanofibres from various food wastes. The authors provide a review of the use of these products in nanocomposite development and other sectors, such as textile, food, biomedical, and healthcare, with additional environmental applications. Chapter 15 deals with the potential use of food waste to produce construction materials (such as biocement, biobricks and agents for soil strengthening and stabilisation) via biocementation process. Compared with traditional construction materials, biocement is a novel and sustainable alternative involving renewable, environmentally friendly, and safe microbes. This chapter reviews the usage of food waste in biocement technology; emphasis is given to screening food wastes that could be used as raw materials for culture media, cementing agents, biopolymers, and enzymes to produce biocement. Chapter 16 is a timely review on producing biodegradable non-woven fabrics, biopolymers from food waste and their potential medical usages, including their contemporary application for COVID-19 PPE, facemasks, and medical gowns. Bacterial processes can make green, biodegradable, non-woven fabrics using different fermentation techniques. This chapter examines the electrospinning process that converts food waste into medical textiles such as personal protective equipment. Chapter 17 is a specific application for collecting and recycling Traditional Chinese Medicine (TCM) residues. The authors describe the abundance of bioactive compounds with antibacterial properties in this waste and how they could be transformed into value-added products in agriculture, animal farming, energy production, environmental remediation, food, and drug industries. The efficiency of re-extracting active ingredients from

Preface  xi

mixed TCM residues and the potential adverse health effects of residual heavy metals and pesticides are evaluated. Chapter 18 provides a critical account of the by-products generated by the global brewery industry and potential solutions to the disposal problem. The chemical properties of brewery wastes and the state-of-the-art advances in biotechnology in their valorisation are presented. A circular bioeconomy conceptual model is proposed. The author provides essential information for maximising the socio-economic potential of brewery wastes and minimising its environmental impact, contributing to a more sustainable brewery industry. Besides raising public awareness of the necessity and opportunities for recycling food waste, this book will be a valuable reference source for scientists and technologists. It also hopes to inform and encourage policymakers and regulators to formulate efficient integrated waste resources management strategies suitable for specific regions. This book contains 18 chapters with contributions worldwide. We appreciate their enthusiasm and dedication during the current pandemic environment, and we want to thank all the authors. The editors are also glad that the book project has provided an excellent opportunity for old colleagues who share the same interests to work together. Ming Hung Wong Diane Purchase Nicholas Dickinson February 15, 2023

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© 2023 World Scientific Publishing Europe Ltd. https://doi.org/10.1142/9781800612891_fmatter

About the Editors Ming Hung Wong is currently Advisor (Environmental Science) of The Education University of Hong Kong, Member of the European Academy of Sciences and Arts, Emeritus Professor of Hong Kong Baptist University, Chang Jiang Chair Professor of the Ministry of Education, China, and Editor-in-Chief of Environmental Geochemistry and Health (Springer Nature). His research interests include “environmental health and toxicology, ecological restoration, and resource reuse”. According to the World’s Top 2% Scientists (Stanford University) list, he is ranked No. 6 (career-long) and the top Chinese scientist globally in 2020 and 2021 under Environmental Sciences. Please see his complete profile at https://repository.eduhk.hk/en/persons/ming-hung%E9%BB% 83%E9%8A%98%E6%B4%AA-wong. Diane Purchase is a Professor of Environmental Biotechnology at Middlesex University, UK. Her research interests are the roles of micro-organisms in providing sustainable solutions to environmental and health challenges — “To heal, to feed and to fuel the world”. She is the editor of two academic volumes on microbial and fungal biotechnology and serves as editor and guest editor in several academic journals.

xiii

xiv  About the Editors

Nicholas Dickinson joined the Faculty of Agri­ culture and Life Sciences at Lincoln University in New Zealand as Professor of Ecology in 2010. His earlier career was spent in East Africa and the UK. He is a research leader interested in the remediation of degraded environments and ameliorating the impacts of pollution through ecological restoration to improve the functionality and sustainability of soils. His main current focus is on agroecology and sustainable agricultural systems. Please see his ­complete profile at https://researchers.lincoln.ac.nz/nicholas.dickinson.

© 2023 World Scientific Publishing Europe Ltd. https://doi.org/10.1142/9781800612891_fmatter

Contents Prefacev About the Editorsxiii Part I  Food Waste: Global Impact and Management

1

Chapter 1

Impacts, Management, and Recycling of Food Waste: Global Emerging Issues Ming Hung Wong, Diane Purchase, and Nicholas Dickinson

Chapter 2

Towards a Circular Economy: Integration of Food Waste into Urban Agriculture and Landscaping Xun Wen Chen, Ming Hung Wong, and Nicholas Dickinson

Chapter 3

Trends of Food Waste Treatment/Resources Recovery with the Integration of Biochemical and Thermochemical Processes Abdulmoseen Segun Giwa, Xiaoqian Zhang, and Kaijun Wang

55

Food Waste Management Practices in Shenzhen, China Qiyong Xu and Chao Zhang

81

Chapter 4

xv

3

33

xvi  Contents

Part II  Composting and Digestion of Food Waste

103

Chapter 5

The Microbiology of Food Waste Composting Hong-Giang Hoang, Chitsan Lin, Minh-Ky Nguyen, Xuan-Thanh Bui, Dai-Viet N. Vo, and Huu-Tuan Tran

105

Chapter 6

Food Waste Composting: Current Status, Challenges, and Opportunities Chitsan Lin, Nicholas Kiprotich Cheruiyot, Thi-Hieu Le, Adnan Hussain, Duy-Hieu Nguyen, and Chia-Hung Kuo

Chapter 7

Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion R. A. K. Szmidt

125

149

Part III  Food and Feed from Food Waste

185

Chapter 8

Food Waste and Single-Cell Proteins Yu Bon Man, Tsun Man Lee, Md Faysal Hossain, Yiu Fai Tsang, Chun Fung Wong, and Ka Lai Chow

187

Chapter 9

Exploiting Agro-Wastes for Sustainable Animal Production Systems and Food Security Caven Mguvane Mnisi, Victor Mlambo, Godfrey Mhlongo, and Mveleli Marareni

221

Part IV  Fertiliser and Fuel from Food Waste

251

Chapter 10 Vermicomposting Food and Organic Wastes Ye Yuan and Nicholas Dickinson

253

Chapter 11 Treatment and Resource Recovery from Anaerobic Digestate of Food Waste Chao Zhang, Ning Wang, and Qiyong Xu

285

Part V  Value-Added Products from Food Waste

303

Chapter 12 Food Wastes for Value-Added Chemicals Miguel Ladero, Jesús Esteban, Juan Manuel Bolívar, Víctor Martín-Domínguez, Alberto García-Martín, Álvaro Lorente, Jorge García-Montalvo, and Itziar A. Escanciano

305

Contents  xvii

Chapter 13 Opportunities for Food Waste Products as Sustainable Synthetic Alternatives Sarah Hunter and Aiduan Borrion Chapter 14 Nanocellulose from Food Industry Waste Dileswar Pradhan, Kalpani Y. Perera, Semiu Rasaq, Swarna Jaiswal, and Amit K. Jaiswal Chapter 15 The Potential Use of Food Waste in Biocementation Process for Eco-Efficient Construction Materials Wilson Mwandira, Maria Mavroulidou, Michael Gunn, Hemda Garelick, and Diane Purchase

331 363

397

Chapter 16 Production of Biodegradable Fibres from Food Waste through Electrospinning and Their Prospective Medical Applications: An Emerging Method for Combating the COVID-19 Pandemic 419 Md Ariful Haque, Sik Chun Johnny Lo, Jin-Hua Mou, Anshu Priya, Zi-Hao Qin, Zubeen Jyotiwadan Hathi, Chrysanthi Pateraki, Dimitris Ladakis, Apostolis Koutinas, Chenyu Du, and Carol Sze Ki Lin Part VI  Opportunities to Recycle Specific Food Wastes

443

Chapter 17 Application of Traditional Chinese Medicine Residues in Animal Farming, Agriculture, Biofuel, Food, and Pharmaceutical Industries Feng Zhang and Ming Hung Wong

445

Chapter 18 Valorisation of Brewery Wastes in a Circular Bioeconomy — from Low-Cost Animal Feed to High-Value Products Diane Purchase

471

Index503

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Part I

Food Waste: Global Impact and Management

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© 2023 World Scientific Publishing Europe Ltd. https://doi.org/10.1142/9781800612891_0001

Chapter 1

Impacts, Management, and Recycling of Food Waste: Global Emerging Issues Ming Hung Wong*,§, Diane Purchase†,¶, and Nicholas Dickinson‡,|| *

Consortium on Health, Environment, Education and Research (CHEER), Department of Science & Environmental Studies, The Education University of Hong Kong, Hong Kong, China Department of Natural Sciences, Faculty of Science and Technology, Middlesex University, London, UK †

‡

Faculty of Agriculture and Life Sciences, Lincoln University, Christchurch, New Zealand [email protected] [email protected] ||[email protected] §

¶

Abstract Food waste is an emerging environmental issue of unprecedented importance. Food loss and waste (FLW) refers to the wastage of food, which includes “food loss” during the early stages of food production and “food waste” at the consumer level. According to the Food and Agriculture Organization (FAO) of the United Nations, about one-third of the food produced for human consumption (1.3 billion tonnes) is lost or wasted yearly. If global dietary trends, food losses, and waste patterns 3

4  M. H. Wong et al.

are not changed, food production will need to increase by 50% by 2050. Sustainable intensification of agri-food systems is the only realistic option for the future. There are numerous environmental impacts related to FLW: clearance of natural landscapes for food production, loss of habitats for wildlife leading to less biodiversity, water requirements of food production, demands for other resources, and emissions of greenhouse gases contributing to climate change. Food packaging materials also impose severe environmental and health impacts. Reducing food loss, donating surplus food, and converting food waste to animal feeds and value-added products will substantially benefit both the environment and the socio-economy. An integrated strategy catering to local and regional needs is essential for reducing and redistribution of FLW and treatment, disposal, and recycling of food waste into the future circular economy. Keywords: climate change; food loss and waste; food system; singlecell protein

1. Introduction Food waste has become a topic of increasing global concern. Entering the search topic of “food waste” as the title of publications in Web of Science listings reveals over 3,000 papers in the last five years, double the number in the previous five years, and 1,000 of these papers were published over the previous 12 months (website accessed on February 2022). Food waste has substantial negative impacts on society, the economy, and the environment. Food waste is an emerging global issue strongly linked to other environmental agendas, such as global warming, agricultural and environmental pollution, and threats to biodiversity. “Food loss and waste (FLW)” is related to food weight and associated inedible parts removed from the food supply chain (Hanson et al., 2016). “Food loss” commonly refers to food lost during the early stages of food production, including growing crops, farming livestock, harvest, storage, and transportation. “Food waste” refers to consumer-level wastage, such as that thrown away at supermarkets, restaurants, and leftovers at home, though they may still be safe for human consumption. A global FLW Standard (FLW Standard) has been devised to compare data generated from different countries more accurately and transparently (WRAP, 2016). Widespread use of the standard will enable us to minimise FLW loss, which will, in turn, provide economic benefits, accompanied by enhanced food security, efficient use of natural resources, and mitigation of environmental impacts.



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  5

Our “Food system” comprises of all the contributory activities: food production, transport, manufacturing, retailing, consumption, and waste, exerting impacts on nutrition, human health, and well-being, as well as the environment. Land used for or converted to crop production is considerably disturbed, affecting soils, water, biodiversity, and local microclimates. Processing, transport, and retail need energy, water, infrastructure (roads), and other inputs (notably packaging). Emissions and overspills cause environmental pollution due to chemicals (fertilisers and pesticides), wastes, and various industrial processes. Furthermore, food waste management involving collection, disposal, and reuse/recycling will also create environmental problems if not adequately addressed. These include greenhouse gas and odour emissions, generations of packaging materials, particularly plastics (for packaging and chemicals associated with plastics, such as bisphenol A and phthalates), pending the choice of methods for treatment, disposal, and reuse/recycling. This introductory chapter provides an overview of the current food waste situation worldwide. The environmental impacts of FLW and potential strategies for solving the problems are also discussed. It is hoped this will stimulate interest in different aspects of food waste, including management, treatment, and disposal. The potential of recycling food waste into value-added products is also briefly discussed, which leads to more in-depth discussion in various chapters of this book.

2.  The Absurd Waste of Food Waste The global population of 7.7 billion will reach 9.7 billion in 2050, peaking at nearly 11 billion around 2100 (United Nations, 2022). Feeding so many more people will require a strategy of sustainable intensification: growing more food more efficiently, on less land, and in a more environmentally sensitive manner. It is estimated that in 2020 up to 811 million people faced hunger, 118 million more than in 2019 (Food and Agricultural Organization, 2022). Currently, vast amounts of food are wasted in the food chain, estimated to cost US$2.6 trillion. It is estimated that US$1 trillion is the economic loss of the food product, US$700 billion due to the social impacts of food waste, and US$900 billion due to environmental effects (O’Connor et al., 2021). These costs exclude the hidden economic value of food waste if used as a resource to reclaim or manufacture composts, fertilisers, foods, fuels, bioplastics, chemicals, and many other value-added products: the topic of this textbook.

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Currently, about 30% of the world’s food is squandered (Rosenberg, 2021). EU consumers waste 88 Mt of food each year, excluding that used as animal feed (Scherhaufer et al., 2018), throwing away almost 20% of edible food, excluding bones, eggshells, etc. (Usubiaga et al., 2018). More than 30% of the food supply in the US is unconsumed, with food scraps accounting for the largest share of post-recycling municipal solid waste (Landry et al., 2018); up to two-thirds are from households, with the rest from retail outlets, during processing and shipping. There is significant geographical variation in the amounts of food waste generated, representing a direct loss of food resources. Megacities in some developing countries produce the highest percentage of food waste. Fresco (2016) states that more than half of the food entering megacities in India and Africa remains uneaten, mainly due to poor storage and distribution standards. Still, more food is thrown away in rich countries: 100–150 kg per person per year, compared to 6 kg in Africa. Among more wealthy countries, for example, 85 kg of food per person is wasted annually in Canadian households  (von Massow et al., 2019). Global estimates of 65 kg of food wasted per year per person have been published (Chen et al., 2020). In the US, food waste accounts for 30% of daily calories available for consumption and 25% of the food by weight (Conrad et al., 2018). In China, 27% of food annually produced for human consumption is lost or wasted, more than half of this in homes or through out-of-home consumption activities (Xue et al., 2021). Of 24 essential nutrients embedded in food waste, a human dietary intake of eight is inadequate (Chen et al., 2020). This worldwide waste of food impacts food security, defined as reliable access to affordable and nutritious food (International Food Policy Research Institute, 2022).

3.  Food Loss and Waste (FLW) Using an FLW Standard can aid authorities and regulators to find out how much, where, and why FLW is happening to have better measurement and subsequently more effective management to address deficiencies. Different quantification methods have been used, and the level of expertise required differs. Choosing a more suitable method or more than one will largely depend on the scope, goals, and available resources. Several ways essentially involved measuring weight: (a) direct weighing, (b) counting (counting items to obtain weight), (c) assessing volume (physical space



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  7

occupied to obtain weight), (d) waste composition analysis (separating FLW from other materials to obtain weight), (e) records (based on data, such as receipts of waste transfer, or similar records), (f ) mass balance (based on inputs and outputs), (g) modelling (based on the interaction of multiple factors which affect generations of FLW), and (h) proxy data (based on data obtained elsewhere) (WRAP, 2016). However, a cautious approach should be adopted when choosing datasets to avoid double counting and overestimation (Tonini et al., 2018). The wastage of food described above amounts to 1.3 billion tonnes of food produced for human consumption being lost or wasted across the entire supply chain worldwide (FAO, 2019). Low- and high-income countries generate similar total amounts of food waste; 630 and 670 million tonnes, respectively. In low-income countries, food is mostly lost during the early (production) and middle (post-harvest) stages of the food supply chain, mainly due to losses during handling and storage phases (FAO, 2011; Kummu et al., 2012). Conversely, food is primarily wasted at the consumption stage; food is commonly discarded even when it is still suitable for human consumption (Chaboud and Benoit, 2017). It is particularly alarming, with 1 out of 9 people starving or malnourished (estimated to be 690 million people starving in 2019). These numbers are likely to increase during and post-COVID (Marchant, 2021). Redistribution of food appears to be an apparent global requirement. Asia is now generating over half the global food waste, with countries such as China, Singapore, Indonesia, and the Philippines leading the increase in FLW. Table 1 shows the combined data for countries with current estimates for 2019, according to a first “food waste index report” (UNEP, 2021) and food waste assessment, based on the standardised methodology (FLW Standard). The information is intended to support Sustainable Development Goal 12.3, halving food waste, and reducing food loss by 2030. It allows Table 1.   Estimates of global food waste by sector (UNEP, 2021). Global food waste (kg person–1 year–1)

2019 total (million tonnes)

Household

74

569

Food service

32

244

Retail

15

118

Total

121

931

8  M. H. Wong et al.

meaningful comparisons of FLW data generated from different countries. These countries could choose their method(s) to make more accurate estimations at levels of households, food service providers, and retailers to guide their strategies to prevent and reduce the impacts of food waste. These were different from earlier estimations that food waste at the consumer level only happened in developed countries. Food loss due to production, storage, and transportation was associated with developing countries (Marchant, 2021). The key findings were as follows: (1) Approximately 17% of global food production is wasted, with about 931 million tonnes of food waste generated across three sectors (households 61%, food service 26%, and retail 13%). (2) The global average of 74 kg of food per capita wasted every year is very similar in lower-middle-income and high-income countries (advocating that most countries in the world must reduce the amount of food wasted). (3) Food waste at the consumer level, including household and food service, appears to be more than double that of previous estimates by FAO (Gustavsson et al., 2011). (4) There is sufficient total food waste even in lower-income countries for circular approaches or other food waste diversion strategies to be significant (UNEP, 2021). The overall results indicated that a large amount of food is wasted. Reducing waste at all levels could undoubtedly induce social, economic, and environmental benefits in all countries included in the report. Households contribute the most considerable amount of food waste, and therefore reducing at source is essential. More widely, reducing food waste can help mitigate food security, climate change, loss of biodiversity, and environmental pollution.

4. The Environmental Footprint of Food Waste Numerous resources and processes are involved in the life cycle that brings food from farms to dinner tables. Consequently, food loss and waste (FLW) involves substantial loss and wastage of resources and significant environmental impacts. The various impacts include (1) wastage of freshwater resources, (2) demands on land, which contributes to lower biodiversity, (3) demand for resources, emission of greenhouse gases, which affects climate change, (4) environmental pollution due to FLW, and (5) packaging materials and environmental/health impacts (Fig. 1). Integrated management is required (Tonini et al., 2018).



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  9

Fig. 1.   Integrated management of food waste (Tonini et al., 2018).

Food security is impacted by the inefficient use of water, energy, and food within agri-food systems (Santeramo and Lamonaca, 2021), which are indirectly affected by food waste. It has been estimated, for example, that food waste leads to 25–30% of water wastage (Capone et al., 2020; Cattaneo et al., 2021). Reducing food waste would efficiently improve land and water resources and make food systems more sustainable (Cattaneo et al., 2021). Food waste indirectly contributes to environmental degradation (Hamilton and Richards, 2019), deforestation, biodiversity loss, water pollution, and land degradation due to intensive production and high application rates of fertilisers and pesticides (Capone et al., 2020). Food waste indirectly impacts global biodiversity due to its consumption of freshwater and as a cause of eutrophication (Scherer and Pfister, 2016). More than 90% of the potential environmental benefits of reducing food waste would be attributable to eliminating household food waste (Usubiaga et al., 2018). Reducing food processing waste would have the greatest effect on reducing the impacts on land use and eutrophication, while reducing household waste would significantly impact water consumption. However, reducing food waste from retail, institutional food service, and farms averts less environmental impact (Read et al., 2020). 

10  M. H. Wong et al.

4.1.  Food waste and water resources There is an urgency to establish and implement an FLW policy to moderate water consumption and reduce water scarcity. Lack of clean water is already a critical global issue; more than two-thirds (4.0 billion) of the world’s population live under water scarcity. This has been especially true during the last few decades, due to the steady increase in demand: population growth, income growth, improved living standards, dietary shifts, and expansion of irrigated agriculture (Mekonnen and Hoekstra, 2016). The consumption of fresh surface water and groundwater (the blue water footprint) for unconsumed plant-based food is estimated to be 174 km3/year (Kummu et al., 2012). If the waste of meat products is also included, 250 km3 of blue water was wasted in 2007 due to FLW (FAO, 2013). The global loss of freshwater associated with FLW is about 21 m3 person–1 annum–1, 43 m3 in high-income countries, and slightly over 4 m3 in low-income countries (Chen et al., 2020). In 2020, nearly 3,000 km3 of freshwater resources will be needed; by 2050, the figure will be 65% higher due to a growing and more affluent population (Springmann et al., 2018). Reduction of FLW will yield positive nutritional and environmental benefits (Chen et al., 2020). Therefore, it is vital to raise water productivity, limit the water footprint, change diets to low-water requirement food, and reduce food waste (Mekonnen and Gerbens-Leenes, 2020). Bear in mind that FLW, when added to this, further exacerbates the water shortage problem. While international food trade (Chapagain et al., 2006) and diets less reliant on water-intensive foods (Vanham et al., 2018) can lower our water footprint, reducing FLW is much more effective (Mekonnen and Fulto, 2018).

4.2. Demand for land, destruction of natural habitats, and loss of biodiversity Most cities are established on fertile land in areas with good rainfall or other water supplies. Urban expansion encroaches on adjacent farmland. Urbanisation has an increasingly pervasive role in food production; most people live in cities, demanding, paying for, and consuming food grown in the countryside. Nutrients (mainly nitrogen and phosphorus) from the countryside are transferred in food to the city, producing wastes that are largely not returned to the land (Fresco, 2016). The amount of food waste



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  11

in Ukraine has been calculated to amount to the equivalent of 7% of the total area of its arable land (Kotykova et al., 2021). Food waste accounts for 12 million hectares, 7% of cropland in the US (Conrad et al., 2018). Higher diet quality is associated with a huge increase in food waste, but less cropland waste and a more significant waste of agricultural irrigation water and pesticides. This is mainly due to the much larger amount of fruits and vegetables in higher-quality diets, higher waste rates, lower cropland needs, and higher application rates of agricultural inputs than other crops. Food waste is also a major indirect cause of biodiversity loss, compounding unsustainable agriculture practices and agricultural expansion into rural and more natural areas, as well as through the impact of industrial-scale fishing and aquaculture practices (Pateman et al., 2020). Food production is the main reason for the loss of biodiversity. Natural ecosystems and natural habitats are destroyed to make way for the demand for agricultural land. The agricultural land area (cropping and animal husbandry combined) currently occupies about 50% of the world’s habitable land (Ritchie and Roser, 2019). A significant number of domesticated plant and animal species, which have been food sources historically, have become less widely consumed and crop and livestock breeding has modified productivity traits. Genetic diversity has subsequently been lost, rendering food systems less resilient to threats, such as pests, pathogens, and climate change, thereby threatening global food security (IPBES, 2019). Monoculture cropping and heavy tillage may be unsustainable practices, further reducing the variety of landscapes and habitats. Habitats for wildlife are destroyed, lost, or threatened. Invertebrates, birds, and mammals are adversely affected. Most biodiversity is below ground, in the soil, and modern analytical techniques also reveal the impact on microbial biodiversity and soil health. According to the International Union for Conservation of Nature (IUCN), agriculture is an identified threat to 24,000 of the 28,000 species at risk of extinction (Ritchie and Roser, 2019), while fishing is the most significant driver of biodiversity loss in marine ecosystems. Agriculture and soil degradation and erosion are destroying soils (Scholten and Seitz, 2019). The food system also causes biodiversity loss indirectly through its influence on climate change. It emits more greenhouse gas (GHG) (including emissions during food production, land-use change, and transport and energy use along the food supply chain) than any other aspect of our lives (Benton et al., 2021). Climate change will also affect biodiversity by changing habitat suitability for wildlife, causing the loss of

12  M. H. Wong et al.

sensitive species (Peel et al., 2017). A “cheaper food” paradigm has led to intensified agricultural production, potentially depleting the future productive capacity of the land. This imposed adverse effects on GHG emissions and the quality of air and water (IPCC, 2019).

4.3.  Food waste and climate change Food waste accounts for 8% of global GHG emissions and increases the amount of water used in agriculture by 25% (Cattaneo et al., 2021). Food wasted in the European Union (excluding food waste used as animal feed ) causes 186 Mt CO2-eq. GHG emissions (Scherhaufer et al., 2018). FLW accounts for about 8% of total anthropogenic greenhouse gas (GHG) emissions (Mariam et al., 2020). Using UK data, it has been calculated that halving consumer food waste would result in a 2–7% annual reduction of the overall environmental footprint of the European Union (Usubiaga et al., 2018). In the UK, the global warming impact of avoidable food waste has been calculated to be between 2000 and 3600 kg CO2-eq. t−1, with this range of emissions dependent on food waste composition (Tonini et al., 2018). Roughly double this amount was estimated in another study of Ireland, where it was considered reducing food waste could feed an additional 1.46 million people (Oldfield et al., 2016). Australian food waste accounts for 57,500 t. CO2-eq. per year, representing 9% of the total water use and 6% of greenhouse gas (GHG) emissions (Reutter et al., 2017). When landfilling was replaced with anaerobic digestion and incineration, significant savings could be made. Anaerobic rotting of organic material in landfills is the third-largest producer of methane in the atmosphere (Landry et al., 2018). Crops are processed into specific foods or fed to livestock during the manufacturing lifecycle consuming substantial amounts of energy and resources. Food is discarded as waste, accompanied by emissions of harmful gases. Intensified agricultural production heavily relies on fertiliser, pesticides, and fuel that further contribute to climate change. It has been estimated that about one-quarter (24%) of greenhouse gas (GHG) emission from food is due to losses in supply chains or consumer wastage (Searchinger et al., 2018). Around two-thirds of this (15% of food emissions) derives from lack of refrigeration, poor storage and handling, and spoilage during transportation and processing. Food discarded by retailers



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  13

and consumers accounts for the other 9%. Overall, amounts of food wastage are responsible for about 6% of global GHG emissions. The real figure may be higher as food losses on the farm during production and harvesting were not accounted for in this estimate (Searchinger et al., 2018). The figure of 6% is about three times the global emissions from aviation. It is the world’s third-largest source of greenhouse gas emissions (Poore and Nemecek, 2018). The global food system is a significant driver of climate change that has increased pressure to intensify production further and clear more land for farming alongside the need to sequester carbon in the ground to mitigate climate change. However, “Bioenergy with carbon capture and storage (BECCS)” seems to compete with “Afforestation and reforestation” for the same land (IPCC, 2019).

4.4.  Culture and food waste Most food waste in developed economies occurs at the household level, but public campaigns to improve household food utilisation has provided little practical improvement (Hamilton and Richards, 2019). A developing urban counterculture raises concerns about food miles, food quality, food processing and packaging, and criticism of supermarkets’ business. More interest in healthy fresh food, buying local organic produce, and farmers’ markets led to more interest in urban farming. Population-level shifts toward healthier and more plant-based diets are needed to improve human and environmental health. It has been suggested that a singular focus on reducing food waste may come at the expense of obesity-related public health outcomes. Some food waste may be necessary to ease obesity rates and improve dietary variety and quality. In developed countries, an emphasis on households right-sizing their food purchases to prevent over-purchasing, overconsumption, and waste is needed (Ellison and Prescott, 2021). Many governments, organisations, and campaigns have focused on making consumers aware of food waste and fostering more sustainable food consumption (Stöckli et al., 2018). Various interventions against consumer food waste have been introduced, but informational interventions have been largely ineffective in changing human behaviours. Complex behaviours and people’s ambivalence towards expired food products or leftovers because of safety concerns have made them more willing to

14  M. H. Wong et al.

waste them, promoting domestic food waste (Buttlar et al., 2021). It has been suggested that citizen science could bring about change through influencing action, from individual behaviour to policymaking, rather than relying on educational programmes (Pateman et al., 2020). Changing people’s mindsets may work better through their direct involvement in the management of food waste.

4.5. Impacts of food loss and waste (FLW ) on environmental pollution Fertilisers, pesticides, veterinary medicines, and growth-promoting substances harm the environment during crops and livestock farming. Grizzetti et al. (2013) attempted to estimate the global loss of nitrogen associated with food waste at the consumption level. They estimated that 2.7 Tg of N annum–1 is lost from this source, amounting to 9% of global consumption. The amount of N discharged to the environment associated with the worldwide waste was estimated at 6.3 Tg N annum–1. Of the total N emissions, 35% and 65% enter the atmosphere and water, respectively (Grizzetti et al., 2013). A significant component is nitrous oxide (N2O), a large contributor to climate change. Nitrogen dissolved in water as nitrate (NO3– ) and phosphate from fertiliser losses causes eutrophication and deterioration of water bodies. Landfilling and incineration are the most common food waste disposal methods worldwide, exerting harmful environmental effects. Unless they are managed carefully, both contribute to greenhouse gas emissions (CH4 and CO2) and leachate discharges that directly or indirectly lead to high biological oxygen demand (BOD) in water bodies. Aerobic and anaerobic digestion of food waste has gained popularity in some countries. Composting diverts food waste from landfills and can be recycled as a soil conditioner but has also been associated with air, surface, and underground water pollution when not appropriately managed (e.g., Huang et al., 2015).

4.6. Impact of food packaging materials on biota and human health Food packaging materials include ceramics, glass, metal, paperboard, cardboard, wood, and plastic. Food packaging makes the most



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  15

considerable demand (two-thirds) of the packaging industry (Shin and Selke, 2014). Most of these packaging materials are single-use and not reused or recycled. In 2014, more than 63% of municipal solid waste (MSW) generated (258 million tonnes of MSW) was packaging materials, and only 35% (89 million tonnes) was recycled or composted (USEPA, 2017). Resources’ demands for packaging include energy, water, chemicals, petroleum, wood, and plant fibres, further responsible for emitting greenhouse gases, toxic gases, and fine particulate discharges. Plastics used for food packaging have presented substantial environmental problems. There are demands for resources (fossil fuels are used to produce plastic polymers such as polypropylene, polyethylene, and polystyrene) and methods to tackle air, soil, and water pollution during production and disposal. Microplastics derived from the breakdown of larger fragments of plastics and used as additives in cosmetic products have become an emergent environmental pollutant of concern. Physical damage caused by plastic litter to wildlife is well known. Of similar concern are the health hazards of chemical components of plastics (bisphenol A and phthalates that leached into the packaged food items and to the broader environment). Bisphenol A and phthalates are known hormone analogues and endocrine disrupters that disrupt mammals’ growth, development, and reproductive identity, including humans (Wang et al., 2018). Microplastics can also adsorb chemicals, such as heavy metals, PCBs, and pesticides, potentially impacting soil chemistry and finding their way through the food chain to humans. A direct route via seafood consumption is now widely recognised (Zhang et al., 2020).

5. Management Issues and Potential Solutions Strategies are required to promote the circular economy keeping resources in use for as long as possible to reduce food wastage (Santeramo and Lamonaca, 2021). One improvement might be to bring food production into the cities. Urban Farming and Urban Agriculture have been the topic of more than 6,000 publications listed on the Web of Science for the last five years, many of which include food waste management as part of the rationale. Using food-waste-derived composts to grow food crops in urban gardens, on marginal land, rooftops, living walls, and other outdoor and indoor spaces makes a lot of sense. Unfortunately, the amount of space available to grow food in cities is minimal, and realistically urban

16  M. H. Wong et al.

farming in cities will always make only a minor contribution to overall food requirements. Carbohydrate and protein production in large quantities requires a lot of space. Urban agriculture will always be primarily concerned with vegetable and fruit production, but this could play a significant role in the cultural adjustments required to keep food waste resources in cities, avoiding economic and environmental costs of transport and disposal. Urban farming and landscaping create substantial requirements for growth substrates, composts, and fertilisers currently primarily imported into cities. The growth of research interest described above is well justified. The United Nations declared an aim to reduce domestic food waste by 50% by 2030 in their sustainable development goals in 2015 (SDG 12.3) (Buttlar et al., 2021). The evidence presented in this chapter and elsewhere in this textbook clearly illustrates the importance of this strategy. Even if this is successful, there is much common sense in finding different uses for the remaining 50% of food waste. In compiling the authors’ work in this book, our intention is to advance efforts towards more sensible management and valorisation of the food waste resource.

5.1. Mitigations of environmental impacts caused by food production Despite food shortages, one-third of its production is discarded as FLW, directly threatening food security and environmental sustainability. The World Resource Institute (Searchinger et al., 2018) has investigated potential solutions to feed 10 billion people by 2050. To achieve and deliver future food sustainability, they consider that a “menu of solutions” is required to target and address 22 supplies and demand measures. It will need current food production to be doubled, assuming recent wastage losses. Their focuses were on the technical opportunities to meet various goals to alleviate starvation and hunger without exacerbating water usage challenges. This should not include expansion of the agricultural land area to protect natural ecosystems, also bearing in mind that agriculture is a significant and growing source of emissions of greenhouse gases. At least 585 million hectares of agricultural land must be reforested to stay under a 1.5°C temperature rise. Despite a growing demand to produce more food, we must lower the growth rate of need, especially for resourceintensive foods, including crops with high water demands and our consumption of animal protein.



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  17

5.2.  Integrated food waste management Reduction of FLW is essential to achieve global Sustainable Development Goals (SDGs), to “End Hunger” (SDG 2), and to “Ensure sustainable consumption and production patterns” (SDG 12). Collaboration between many international organisations and government bodies will be necessary to develop policies and promote awareness among farmers, handlers, processors, traders, and others involved in civil society’s food supply chain and public and private sectors. At the consumer level, FAO focuses on changing individual attitudes and behaviour, especially shopping habits and food consumption (FAO, 2021). Food waste management has become a major environmental issue across societies as the world wakes up to the profound effect of food waste on the global environment, community, and economy (Papargyropoulou et al., 2014). An integrated management strategy is required for waste treatment and disposal, especially in densely populated cities and urban areas. Using Hong Kong’s example, minimal options for treatment of MSW mean that 63% of MSW (including a large portion of food waste) is disposed of in three landfills; only 37% is reused or recycled. Other economies, including Japan, Germany, and Denmark, depend primarily on incineration and recycling for MSW management. Initial sorting of MSW and separation of food waste by household are becoming well advanced in some countries (such as South Korea), which have reduced demands on its modern sanitary landfills. Figure 2 compares MSW treatment options adopted in different places (Legislative Council Secretariat, 2016).

Germany (2014) Japan (2013) Denmark (2014) Austria (2014) Korea (2013) United Kingdom ( 2014) Hong Kong (2014) 0%

20% Landfilling

40%

60%

Recycling/compost

80%

100%

Incineration

Fig. 2.  Comparison of municipal solid waste treatment options in selected places (Legislative Council Secretariat, 2016).

18  M. H. Wong et al.

5.3.  Food recovery hierarchy A “Food Recovery Hierarchy” is considered the best strategy for sustainable food and food waste (USEPA, 2021). Wasted food must be avoided at the source, and the volume of surplus food generated must be reduced. Redistribution and donation of surplus food, using food waste as animal feed, fuel, and compost, are two obvious early solutions; the last option should be landfill and incineration. Developing a hierarchy of avoidance and mitigation would prevent pollution during food production, preparation, and transport, by using less energy, pesticides, and fertilisers. These will also include reducing landfill gas emissions, saving costs by purchasing only food that will be used, less wasted food for disposal, and more efficient preparation, handling, and storage of food.

5.3.1.  Reduction of food loss and waste Reducing food loss and waste requires education at all levels and improving infrastructures. New refrigeration technologies and renewable energy sources can reduce food waste. Other new technologies can provide suppliers with more time to deliver goods while giving consumers more time to consume purchased food by extending the time it takes for food to spoil. FLW could be reduced during handling, storage, processing, and transport. More proactive and science-based actions are also needed, requiring the buy-in of food producers, suppliers, and consumers, as well as policymakers, along the food supply chain (Shafiee-Jood and Cai, 2016).

5.3.2.  Donation of surplus food A study of 858 food manufacturers/distributors and 1,532 chain convenience stores in Hong Kong revealed that most food-related companies do not want to donate surplus food for fear of product liability, which has already led to a US$7.7 million loss annually (OXFAM, 2014). Government intervention is probably required to solve this problem. In supermarkets in France, discarding or destroying new foods has become illegal, and regular donations are needed through Food Waste Prevention Legislation (Zero Waste Europe Factsheet, 2016). Supermarkets now exist that sell excess food that other markets have discarded. Customers can buy food items which are still considered safe for consumption at a lower cost. Food companies’ donation of surplus food, which is still edible, is



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  19

encouraged in some countries to provide food banks, charities, and soup kitchens.

5.3.3. Strengthening public environmental awareness and government policies Education programmes in various countries have been designed to disseminate the concept that food waste is an ethical and economic issue that depletes valuable natural resources. This raises awareness of why and how to prevent food waste and generates local and regional consensus towards food waste reduction. Feijoo and Moreira (2020) employed carbon and water footprints as major environmental indicators to assess the extent of the impacts associated with food waste. Another study has identified the factors that determine behaviour changes related to food waste and how to prevent food waste, for example, by having better shopping plans (Principato et al., 2015). These studies have provided important information for policymakers and social marketers in dealing with the food waste problem. In short, education can start from the beginning of the food processes, through the entire chain, until it reaches the consumers. It is essential to raise consumer awareness that some foods are safe to consume beyond the dates stated on the packaging under correct storage conditions.

6. Transforming Food Waste into Usable Products More sustainable methods should be adopted for treating food waste, and food waste should be regarded as a valuable resource rather than a waste. Different types of food waste can be transformed into food, feed, fertiliser, fuel, and other value-added products, according to the properties and substances they contain and the best available technologies in different cities/ regions/countries. The following section outlines the feasibility of converting food waste into different recyclables, which will provide a general introduction to more detailed information in other chapters of this book.

6.1.  Mushrooms and single-cell protein (SCP ) Nutrient circularity is essential in the circular economy, including converting waste nutrients to food and feed (Girotto and Piazza, 2021).

20  M. H. Wong et al.

Various agro-industrial waste and food wastes can be used to grow mushrooms, which is also one of the most economical food sources that could be used to tackle malnutrition. Mushrooms contain low calories and high protein, fibre, and other substances beneficial to human health. They have also gained popularity in the pharmaceutical industry (e.g., antioxidants and immunomodulating agents). Different types of agricultural waste, such as rice and wheat straw, banana leaves, cotton waste, and sugarcane leaves, have been used as substrates for culturing different species of mushroom: straw mushroom, button mushroom, Shitake mushroom, etc., commercially (Niazi and Ghafoor, 2021). Pre-consumed food waste (vegetable and fruit) and postconsumer food waste, mixed with suitable waste such as rice/wheat straw, are ideal substrates for growing mushrooms. O’Brien et al. (2019) showed that the digestate of cow manure mixed with food waste (from anaerobic digesters) could be used to grow oyster mushrooms, thereby converting the digestate nutrients into safe and quality food. The spent mushroom substrate can also be used for land application to improve soil fertility and produce animal feed, fertiliser, and biofuel. This is an excellent example of zero waste management, diverting food waste from landfills and converting residual nutrients into valuable products (Umor et al., 2021). Fungi, bacteria, yeasts, and algae can provide excellent sources of single-cell protein with relatively high growth rates. They can utilise many substrates and can be grown in any season to provide continuous production. They also have lower requirements for land, water, and climate. Suitable growth substrates for SCP include bagasse, molasses, whey, citrus waste, fruit and vegetable waste rich in cellulose and hemicellulose waste, petroleum by-products, sewage, and even animal and human excreta (Nasseri et al., 2011). These microbes can convert agricultural and food waste into protein sources such as food (human), feed (animal), or nutrient supplements (for both humans and animals). The technology can combat the challenge of protein shortage worldwide and provide a waste management option (Ukaegbu-Obl, 2016). Being rich in protein, with essential amino acids (including lysine, methionine, and threonine), lipids, and vitamins, SCP can replace plant and animal protein for human and animal consumption without adverse effects (Sharif et al., 2021).

6.2.  Fish food and insect protein Food waste derivatives can be transformed into safe and value-added animal feeds, a long-term practice in many parts of the world, turning food



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  21

waste into fish, meat, eggs, and milk (Westendorf, 2000). There are lessons to be learned from the past. In Hong Kong up until the 1990s, food waste from restaurants was in high demand. It was regularly collected from urban centres in Hong Kong Island and the Kowloon Peninsula to be transported to farms in the New Territories. It primarily served as the primary feed ingredients for pigs and poultry when the farming industry was more vibrant. Currently, South Korea and Japan convert about 45% and 40% of their food waste to animal feed in this way (Dou et al., 2018). By law, Japan has now prioritised turning food waste into animal feeds above other options of composting and incineration (Takata et al., 2012). The leftover school meals in the UK and elsewhere were routinely collected and transferred to pig farms earlier. Due to an outbreak of foot and mouth disease, which occurred in the UK in 2001, tighter control on food waste as animal feeds has been stipulated, while the regulations vary in different states of the USA (Mo et al., 2018). Risks of this nature must be ruled out before resuming these practices. In parts of Asia, including Hong Kong, until the 1980s, wastes such as composted pig manure were added to fishponds to enrich the water with nitrogen and phosphorus to culture freshwater fish. This polyculture system involved rearing different carp species, mainly in the same pond. These fish species could fully utilise all the materials derived from the added waste with varying feeding modes (e.g., bighead carp as a filter feeder, grass carp as a herbivore, grey mullet as a detritus feeder, and tilapia as an omnivore). Expired bread, noodles, soybean waste, and peanut bran have also been popular items for fish culture (Wong et al., 2004). It has been proved feasible to turn food waste into quality fish feed pellets by adding different feed supplements, such as baker’s yeast, enzymes, and Chinese Medicinal Herbs, to culture freshwater and marine fish species. Studies have shown that the cultured fish had improved growth and immunity and contained lower concentrations of undesirable chemicals (such as DDT and mercury) than those fed with the commercial feed pellets at a 30% average lower cost (Wong et al., 2016). Dzepe et al. (2020) fed fruit waste (orange, pineapple, and papaya) to the larvae of the black soldier fly (BSF) (Hermetia illucens) and houseflies (Musca domestica), using the larvae as the sole diet for broiler chickens. No adverse effects were found on growth, health, or mortality. The larval biomass provided an alternative protein source in broilers’ diets. It has been indicated that the black soldier fly larvae are efficient in converting different types of organic waste (livestock manure and fruit and vegetable waste) into high-value biomass with high protein (40%) and fat

22  M. H. Wong et al.

(30%) content. This could have potential in countries that lack the technology for treating organic waste (Kim et al., 2021). Furthermore, frass (excreta) of black soldier fly larvae reared in the urban solid waste can be turned into compost for growing vegetables, reducing global warming potential compared with incineration (Song et al., 2021).

6.3.  Fertilisers, soil conditioner, and compost Plants growing in soils amended with compost benefit from improved physical and chemical properties that provide a better growth medium. Conversion of waste materials depends on a suitable ratio of carbon (C) to nitrogen (N). Mixtures such as rice straw and pig manure are commonly used to achieve an appropriate C:N ratio to improve the soil by serving as a soil conditioner or organic fertiliser. It generally achieves improved fertility and better water retention and soil drainage, leading to improved plant growth. Due to its higher concentrations of heavy metals and other contaminants, composts made from sludge and slurries often are not recommended on crops grown for food production. In contrast, food waste can be an ideal feeding material for converting to compost with high organic matter and plant nutrient content and low concentrations of heavy metals. However, high salt concentration from food additives sometimes requires attention. To achieve a higher C:N ratio, food waste requires the essential addition of bulking agents (such as wood chips, sawdust, or straw) to facilitate a healthy microbial community of fungi and bacteria. Many studies have shown that the application of food waste compost results in healthier soils and higher yields (e.g., Lee et al., 2004). There are fewer problems composting pre-consumer waste because post-consumer food waste requires efficient separation to avoid contaminants of the composted products. Odour production, due to emission of ammonia, and leachate generation are common problems that are frequently encountered but quickly addressed. Vermicomposting involves selected earthworm species that promote converting biodegradable waste into a valuable end-product, called vermicompost. Earthworms consume organic waste (such as fruit and vegetable scraps) and produce castings rich in nutrients that are readily available to plants (e.g., N, P, K, and Mg) with valuable growth-promoting organic compounds (including plant hormones). Earthworms play an essential



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  23

natural role in nutrient cycling and improving soil’s chemical and physical properties. Non-soil burrowing species of earthworm that naturally thrive in compost on the soil surface (especially Eisenia fetida, tiger worms) are preferred (Gupta and Garg, 2011). Kitchen waste such as fruit and vegetable waste (apple, potato, carrot, and beetroot), and other food waste (pasta, potato, and bread), can be easily turned into vermicompost (Kostecha et al., 2018). Vermicompost is added directly to the soil or mixed with water to form a liquid fertiliser called compost tea. In many countries, institutional processing of food waste is typical in hospitals, shopping malls, universities, prisons, and domestic homes (Clive, 2010). Not all earthworms can be used for this purpose, and common garden earthworms are often unsuitable. After screening 3,000 earthworm species, four species have been identified as appropriate for producing vermicompost: E. fetida, Eisenia andrei, Eudrilus eugeniae, and Perionyx excavatus (Ahmad et al., 2021).

6.4. Fuel Bioconversion of organic waste (including food waste) via anaerobic digestion has become a viable and popular option worldwide. Converting waste into a renewable energy source can reduce waste volume, contribute to the decarbonisation of the economy through mitigating gaseous emissions, and conserve resources (Uddin et al., 2021). Associated technologies have become sophisticated, with anaerobic reactors catering to various organic waste, such as sewage sludge and food waste. The Environmental Protection Department adopted Waste-to-Energy and Resources Facilities for generating electricity, where sewage sludge mixed with food waste is used to generate electricity and produce compost (HKEPD, 2021). The biogas generated can be used for heat and power generation and fuel for the transport sector, replacing fossil fuels. Cathay Pacific, the Hong Kong-based airline, has committed to purchasing 1.1 million Mt of sustainable aviation fuel for 10 years from Fulcrum Bioenergy (USA), which is equivalent to 2% of the airline’s current operations (Heard, 2018). Tor maximises the use of digestate for the generation of biofuel and microalgae, and progress is being made to employ emerging technologies such as thermal conversion, including conversion of anaerobic digestate into biochar/hydrochar, with a potentially wide application (Dutta et al., 2021).

24  M. H. Wong et al.

6.5.  Other value-added products Food waste is a reservoir of various valuable substances, including proteins, lipids, carbohydrates, and nutraceuticals, which could provide raw materials for manufacturing multiple metabolites with commercial importance. Recent investigations have focused on the production of enzymes, bioactive substances, biodegradable plastics, nanoparticles, and different molecules (Ravindran and Jaiswal, 2015), depending on the biochemical composition of food waste (Kannah et al., 2020). Various valuable substances can be simultaneously produced from ginseng (Panax ginseng) residue, a common industrial herbal waste: polysaccharides and ginsenosides by physio-chemical separation, and succinic acid by enzymatic hydrolysis and fermentation (Escherichia coli-ZW33) (Su et al., 2021).

6.6.  An integrated approach The award-winning technology, “Food Waste Total Recycling” developed by the Hong Kong Productivity Council (HKPC, 2020), uses decentralised pretreatment and more centralised facilities. After pretreatment (aerobic decomposition, grinding, and mixing in relatively more minor tanks) on-site, the food waste slurry is transported to centralised facilities equipped with a bioreactor (anaerobic decomposition). Different valuedadded products are generated: with waste oil converted to biodiesel, solid residue to animal feed, and nutrient solution to biogas, after further treatment in the wastewater treatment system. This is a robust and compact system for decentralised food waste recycling, set up at clusters of food waste sources (such as industrial parks, university campuses, and outer islands). The use of food waste for feeding animals also addresses issues related to food security, environmental pollution, waste and resource management, and climate change (Dou et al., 2018). Furthermore, using food waste to produce fuel and chemicals will significantly contribute to energy and environmental sustainability. Current legislation and policies of different countries tend to emphasise the reduction of food waste at the source and more minor on disposal and treatment, reuse, and recycling. It is hoped that the examples provided in detail in the chapters of this book will raise interest in further exploring the feasibility of turning food waste into valuable products.



Impacts, Management, and Recycling of Food Waste: Global Emerging Issues  25

7. Conclusion Food waste management is clearly an emerging global issue, concerned with the sustainability of resources and the direct and indirect impacts of food waste on the environment. Future agri-food systems must be based on sustainable intensification of farming to produce sufficient food for a burgeoning global population. Substantially better management of food waste is a fundamental prerequisite to a circular economy for food production and consumption involving reusing and recycling as much as possible. Various international organisations are attempting to tackle the problems associated with food waste, using standardised methodologies to calculate food waste across the whole food supply chain so that tracking and comparison can be made across and between different countries. A more sustainable resolution to addressing this pressing issue is to adopt a sustainable food production and consumption approach and then deal with surplus food and waste through the global food supply chain. The food waste hierarchy shows that prevention (reducing avoidable food waste and food surplus) is undoubtedly the best option. This must be coupled with redistributing surplus food and then converting food waste to animal feed and other value-added products, substantially benefiting the environment, society, and the economy.

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Chapter 2

Towards a Circular Economy: Integration of Food Waste into Urban Agriculture and Landscaping Xun Wen Chen*,§, Ming Hung Wong†,¶, and Nicholas Dickinson‡,|| Guangdong Provincial Research Centre for Environment Pollution Control and Remediation Materials, Department of Ecology, College of Life Science and Technology, Jinan University, Guangzhou, China *

Consortium on Health, Environment, Education and Research (CHEER), Department of Science and Environmental Studies, The Education University of Hong Kong, Hong Kong, China

†

‡

Faculty of Agriculture and Life Sciences, Lincoln University, New Zealand [email protected] [email protected] ||[email protected] §

¶

Abstract Organic wastes associated with food in urban areas are of critical concern due to the economic costs and environmental impacts associated

33

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with transport and disposal. Food wastes could be converted into soil amendments for urban agriculture and landscaping, recycling nutrients, and improving urban resilience and sustainability. Organic wastes can be composted, to produce valuable soil conditioners and fertilisers, or bioconverted, to produce a wide range of products, as evidenced in later chapters. Alternatively, they can be recovered using thermal treatment converting biomass into biochar to amend problematic soils, improve water conservation and purity, decrease crop pollutant accumulation, and increase crop yield. Retaining proportion of organic wastes, potentially supplemented with inert wastes such as paper waste, will encourage and enhance the development of urban agriculture, as well as stimulate other contemporary urban initiatives such as green walls, rooftop farming, vertical farming, and community gardening. Benefits would also include both short- and longer-term utilisation, restoration, and re-activation of marginal, vacant, and derelict land. A more closed urban food cycle would improve food security, socio-economic, and health systems while also enhancing education and societal engagement with nature. Better food waste recycling in urban areas will support and build smart and sustainable urban living. Keywords: food waste; reuse; soil amendment; urban ecology; manmade ecosystem; closed-loop management

1.  Food Security and Urban Agriculture The current world population of 7.7 billion will increase to 10 billion in 2050, when two-thirds will be living in urban areas (United Nations et al., 2019a, 2019b). Alongside urban expansion, agricultural land area will be substantially decreased and food shortages will increase (FAO, 2020; UNEP, 2021). Food quality and contamination also raise concerns; food production on contaminated land (FAO, 2018) and using contaminated fertilisers (Gupta et al., 2014) have a high risk to human health (Norton et al., 2015; Song et al., 2017; Rai et al., 2019; Liu et al., 2021). Smart management of expanding urban areas through integrating emerging technologies is pivotal (Bibri, 2020). Urban agriculture (urban farming and urban gardening) could be a viable alternative and addition to the current reliance on rural production systems. This would reduce bidirectional transport of food and wastes, helping to mitigate the problem of food insecurity, especially under climate extremes (Chapter 1). Huge amounts of organic wastes are created (United Nations, 2021), while



Towards a Circular Economy  35

Fig. 1.  Typical locations suitable for urban agriculture (Illustration courtesy of Yarui Chen). Urban agriculture and landscaping are integrated into the urban areas.

800 million people face hunger (FAO, 2021). In response to this, the FAO (2019) Vision for the Urban Food Agenda promotes resilient, integrated, sustainable, and inclusive food systems, also highlighting the essential role of urban agriculture in coming decades. Urban agriculture is the practice of cultivating, processing, and distributing food produced in urban, suburban, and/or peri-urban areas (Ravetz et al., 2013). Typical locations suitable for urban agriculture include but are not limited to school gardens, urban farms, community gardens, backyard gardens, edible landscapes (integration of landscape with edible gardening), rooftop greenhouses, open-air rooftops, vertical farming, indoor farming, edible walls (Integration of green wall with edible gardening), hydroponics, and aquaculture (Fig. 1) (Nogeire-McRae et al., 2018). However, urban agriculture is largely a fringe activity that needs to be developed and better integrated into urban life and to build our future cities (Fig. 2). This chapter discusses the use of organic wastes, particularly food wastes, for urban agriculture and landscaping. A case study and proposal from Hong Kong are described illustrating the generation, management, and potential of incorporating organic waste recycling into sustainable urban development.

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Fig. 2.   Schematic overview of the key components of this chapter, involving converted organic wastes that are used to promote urban agriculture and urban landscaping, which subsequently contribute to urban ecosystem functions and services. Food security and human well-being are thus improved.

2.  Urban Ecology Urban ecology is the biodiversity and related ecological functions and services in urban areas (Lepczyk et al., 2017). Both urban agriculture and landscaping can improve biodiversity in cities (Lin et al., 2015; Clucas et al., 2018) and integrated to offer more habitats for plants and animals, with more opportunities for nature to improve the multifunctionality of urban ecosystems (Piana et al., 2019). Anthropogenic activities have generally excluded or been to the detriment of biodiversity, apart from through introduction of invasive species or pests (IPBES, 2019; Douglas et al., 2020; SCBD, 2020), but urban areas are now becoming hot spots for biodiversity, which is generally now integrated into urban planning (Nilon et al., 2017). Urban ecology has substantial benefits to cities, such as improving air and water quality, moderating temperature and conserving energy, reducing soil erosion, and connecting communities. Soils used for urban agriculture and landscaping are generally artificially manufactured or disturbed soils, the latter often with poor structure, inadequate nutrients and organic matter, and elevated concentrations of heavy metals that support mainly stress-tolerant invasive species (Chen et al., 2016b, 2017). Organic waste-derived soil amendments in urban locations could significantly improve soil conditions to favour both crop production and nature.



Towards a Circular Economy  37

3.  Urban Organic Wastes as a Resource Organic wastes associated with food primarily contain water, carbohydrates (e.g., cellulose and sugars), lipids (e.g., oil, phospholipid, and steroids), proteins, and minerals (e.g., nitrogen, phosphorus, and potassium) (Lopez et al., 2016; Nelson and Cox, 2017; Pagliaccia et al., 2019). The variability of these components may determine their suitability for either bioconversion or thermal treatment and the desired end-product. For example, food waste-derived products most suitable to be converted to soil amendments are summarised in Table 1. Bioconversion includes composting, protein harvesting using insects, vermicomposting, and fermentation, with examples of each described in the following chapter of this book. Thermal treatment, also discussed in more detail in later chapters, includes incineration (400–540°C), pyrolysis (250–750°C, limited supply of oxygen), gasification (350–1800°C), and hydrothermal carbonisation (180–350°C, high pressure under wet conditions), depending on the processing temperature and conditions. The products after heating are heat energy (via incineration), biochar, syngas, and oil (via pyrolysis and gasification), and hydrochar (via hydrothermal carbonisation) (Ahmed and Gupta, 2010; Pham et al., 2015; Grycová et al., 2016). Later chapters describe technical details of these options, including conversion to Table 1.   Preferred uses of typical organic waste-derived products in urban as soil amendments. Feedstock

Conversion method

Potential application*

Bread waste

P, V

Fertiliser

Eggshell

P

Liming agent

Fishbone

P

Fertiliser and liming agent

Expired frozen meat

V

Fertiliser and pollutant immobiliser

Coffee grounds

P

Fertiliser and liming agent

Vegetables

P

Fertiliser and pollutant immobiliser

Boiled rice

P, V

Fertiliser

Shrimp shell

P

Fertiliser and liming agent

Yard waste

P, C

Pollutant immobiliser

Mixed food waste

C, V

Fertiliser and liming agent

Note: P, pyrolysis; C, composting; V, vermicomposting. *Actual application can be identified according to experimental results. For pyrolysis, the temperatures could be set at 350, 500, or 750°C.

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fertilisers (e.g., compost and vermicompost) (e.g., Chapters 5, 6 and 10), protein and animal feed (e.g., Chapters 8 and 9), energy through anaerobic digestion (e.g., fermentation) (e.g., Chapters 7 and 11), and other valueadded materials, such as synthetic alternatives and construction materials (e.g., Chapters 12–16).

4.  Case Studies As a developed and highly populated city, Hong Kong faces three major problems: it is too reliant on food imports, rural land for local agriculture is scarce, and large amounts of organic wastes are produced and sent to landfills. Re-activating and promoting urban agriculture and landscaping by using recycled food waste could have great economic and environmental benefits to the sustainability of the city. This example illustrates the concepts and a future direction for using recycled organic wastes (including food wastes) to promote urban agriculture and landscaping within existing urban spaces.

4.1. Current status of organic waste recycling in Hong Kong Hong Kong has extensive policy and management strategies to treat organic wastes. For example, avoidance and minimisation, “3Rs” principle (reducing, reusing, and recycling), bulk reduction and disposal, and charging scheme are the critical components of the government policy (ENB, 2014, 2021). However, there is still a significant amount of organic waste disposed to landfills. Due to the limitation of land area, it is difficult to further expand the landfilling area. There is room for considerable improvements in the recovery of organic wastes. Over 11,000 tonnes of municipal solid waste (MSW) are generated daily in Hong Kong (2019 data). Three major waste components made up 78.6% of the total: paper (24.5%), plastics (21.0%), and putrescibles (33.1%) (EPD, 2020). The paper wastes contain cardboard, newsprint, office paper, tetrapak, tissue paper, paper bags, paper dining wares, etc., but recycling is not straightforward due to ink or water/oil-proof coatings (Pivnenko et al., 2015). The putrescibles mainly consist of food waste and yard waste. Of the daily food waste, 1,143 tonnes came from commercial and industrial (C&I) sources, such as restaurants, hotels, wet markets, food production, and processing industries. The domestic sector generated another 2,286 tonnes (ENB, 2014, 2021). The amount of yard waste was



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only 8% of the amount of food waste: 59 and 225 tonnes from C&I and domestic sectors, respectively. The wastes may be initially homogeneous (i.e., a single type), such as bread waste, eggshells, coffee grounds, frozen meats, fishbone, boiled rice, and individual yard wastes, or may be mixed and complicated or impossible to sort. Recycling companies can easily collect these wastes from various commercial and domestic sectors throughout the city. The Hong Kong Government issued “A Food Waste and Yard Waste Plan for Hong Kong 2014–2022” (“Food Waste Plan”) (ENB, 2014) and “Waste Blueprint for Hong Kong 2035” (“Blueprint”) (ENB, 2021) to provide strategies for organic waste management and has launched an anaerobic digestion technology that converts food waste into biogas for electricity generation: OPARK1 (utilising 200 t daily) and OPARK2 (expected to use 300 t daily). The output residues can be further processed to produce compost for agriculture and landscaping.

4.2. Protecting food quality and revitalising local agriculture Hong Kong citizens consumed nearly 1 million tonnes of vegetables in 2013, with local production accounting for only 2% of the market share (FHB and AFCD, 2014). Most of the agricultural products are directly imported from neighbouring regions of mainland China. There is some concern about contamination of imported food products, with reports of contamination by pesticides, heavy metals, and persistent organic pollutants (POPs), potentially raising problems with food security (MEE and MNR, 2014; Song et al., 2017; Zeng et al., 2019; Sun et al., 2019; Qin et al., 2021). There may also be associated health risks due to long-term exposure to low or moderate levels of environmental pollutants such as heavy metals and certain emerging chemicals of concern such as flame retardants through food intake (Wong et al., 2016b; Wong, 2017). Sources of these contaminants in food products have been cited to include sewage irrigation and agricultural chemicals with contamination of leaf vegetables cultivated in areas of the Pearl River Delta (PRD) region (Chang et al., 2014); up to 20% of vegetables produced around the PRD were found to contained excessive concentrations of arsenic, cadmium, copper, and mercury. Pollution emanating from industrial development in Guangdong Province in recent decades, combined with increased acidification of soils, may have increased the mobility of heavy metals (Zhao, 2016). In terms of quality control, it is possible for the Centre for Food Safety (CFS) of the Food and Environmental Hygiene Department to

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analyse samples of only a tiny proportion of imported food for a limited range of potential contaminants before the point of sale. For these reasons, safeguarding the food supply has become a serious public issue (Jing and Nip, 2013). However, in the last 10 years, mainland China has issued several key environmental protection policies and remediation measures, already significantly mitigating the food safety problems. Nevertheless, this does illustrate the potential benefits to be gained by recycling organic wastes locally to build a more resilient and sustainable city. Agricultural production is currently insignificant within the Hong Kong Special Administrative Region, both in terms of its value-added contribution to GDP and its contribution to the employment sector. This is a marked change since the 1960s, before which agriculture played an important role in the economy. Since then, agricultural activities have been in gradual but continual decline; only 7.29 out of a total of 1107 km2 of the land area is currently actively farmed (FHB and AFCD, 2014). In 2013, the local crop farming industry (including vegetables, fruits, flowers, and field crops) produced US$256 million worth of production, less than 0.1% of GDP (FHB and AFCD, 2014). There is an opportunity to redress this loss by rehabilitating the agriculture sector and improving the supply of high-value untainted and safe food. Local production would be easier to manage, with a more reliable monitoring system for quality control than relying on less-controlled large-scale import of food products. In Hong Kong, land areas used for vegetables, flowers, field crops, and orchards are 347 ha, 128 ha, 7 ha, and 273 ha, respectively (AFCD, 2021). The development of local agriculture would also provide more employment opportunities; improved local production of crops has created at least 4300 jobs for farmers and workers (AFCD, 2021). The Hong Kong Government has planned a more proactive role in promoting local agriculture (HKFHB, 2014). The Agriculture, Fisheries, and Conservation Department (AFCD) assists potential farmers in initiating agricultural rehabilitation through the Agricultural Land Rehabilitation Scheme (HKAFCD, 2015). The Food and Health Bureau (FHB) and the AFCD conducted a public consultation to collect opinions regarding the new policy to support agricultural rehabilitation and subsequently released a consultation document: The New Agricultural Policy: Sustainable Agricultural Development in Hong Kong. This provides goals to: (i) investigate the feasibility of establishing an Agrotechnology Park to stimulate new agricultural practices, (ii) establish a Sustainable Agricultural Development Fund (SADF) to financially support research and



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development (R&D) activities related to agricultural development, and (iii) strengthen support for farmers, with a focus on marketing their crops and vegetables. Thus, government policy clearly encourages agricultural rehabilitation through innovation in sustainable agricultural development.

4.3.  Residual land quality issues Some agricultural soils within the North East New Territories (NENT) of Hong Kong contain high concentrations of arsenic due to natural geochemistry, not associated with human activities (HKCEDD, 2013). However, the soil is also a major sink of historic pollutants from industrial and urban fallout, such as heavy metals (Cd, Cr, Cu, Hg, Pb, and Zn) as well as organochlorine pesticides (including DDT) from agriculture, PAHs from petroleum products, and dioxins (PCDD/Fs) from waste disposal and spillages (Lee et al., 2006; Zhang et al., 2006; Man et al., 2010, 2011; Lopez et al., 2011). There are also contemporary sources of pollution on abandoned farm soils, for example, through storage of e-waste in the New Territories, leading to higher metal concentrations in soils, with associated environmental and health risks. Our previous study indicated such abandoned farms’ soils contained up to (As) 27.5 ± 34.7 mg/kg As, 0.27 ± 0.53 mg/kg Cd, 13.8 ± 26.7 mg/kg Cr, 14.6 ± 19.1 mg/kg Cu, 129 ± 59.3 mg/kg Pb, and 119 ± 77.9 mg/kg Zn (Man et al., 2010). Furthermore, rock phosphate contains trace amounts of heavy metals, notably Cd (a common impurity in phosphate fertilisers), and repeated application of such fertilisers has resulted in elevated concentrations of Cd, As, and Pb in farm soils, which could pose health hazards through uptake by crops (Jiao et al., 2012). There is a reduced area of agricultural land suitable for growing safe crops in Hong Kong. Since local agricultural production has been declining for the last 50 years, the quality of previously farmed soils has received less attention. However, soil degradation has led to negative impacts on physical, chemical, and biological properties that determine soil health. These include changes in mechanical and hydrological properties, reduced fertility properties, reduced organic matter and carbon, and lower soil biodiversity (Lal, 2016). Vegetables grown in these soils may be substandard in growth, quality, and contaminant content and may present a health risk to consumers. This jeopardises the suitability of the land for a return to agriculture. Remediation is needed by restoring soil fertility prior to crop cultivation. Restoration of soil structure, soil texture, and application of organic matter and basic nutrients are needed. In addition,

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the growth of plants also depends on their interactions with various soil organisms, including microbes, such as fungi and bacteria, and also earthworms (Wong, 2003).

4.4.  The way forward The range of food and associated organic wastes are likely to require sorting and pre-processing to align with the most suitable treatment process. Leaching with water to reduce salt and some degree of wetting or drying may be necessary. Using different facilities, wastes can be converted into a range of products including fish feed, mushroom growth substrates, and biochar. In this section, opportunities to use food waste-derived compost products within urban and built environments are explored in more detail. Existing, new, and future treatment technologies and opportunities to exploit food waste resources and to manufacture valuable and novel products are described in the later chapter of this book. It has been proven that using organic waste-derived products can significantly benefit soil health. Compost and vermicompost can be used to restore soil structure, soil nutrients, organic matter, and soil water holding capacity (Kalantari et al., 2010; Schulz and Glaser, 2012; Schmidt et al., 2014; Gravuer et al., 2019). Once these essential properties are restored, plant growth can be greatly improved. In contaminated soils, food wastederived biochar could also be useful to immobilise pollutants in soils and decrease uptake into food crops (Jeffery et al., 2011; Bian et al., 2013; Khan et al., 2013; Akhtar et al., 2014; Hu et al., 2015). The way forward with food waste undoubtedly involves valorisation and utilisation of food waste-derived products within urban environments.

4.4.1.  Utilisation of food wastes Organic waste recycling has also made significant advances across the world. For example, in Taiwan, 20–30% of food waste has been recycled, with 70% of this converted into animal feeds and more than 20% were converted into fertilisers (Thi et al., 2015; Pour and Makkawi, 2021). The 70–80% that was not recycled was incinerated. In Singapore, most of the recycled food waste is a homogenous mix that includes spent yeast/grains, soya bean residues, and bread waste, from food manufacturers (NEA, 2021). On-site treatment systems are located in hotels, shopping malls,



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Fig. 3.  The organic waste cycle in urban environment (Hong Kong as an example). Others in putrescibles are wastes such as hair and cotton.

and schools that are used to produce compost for landscaping purposes (NEA, 2021). Singapore is also developing urban agriculture and integrating farming into urban environments (URA, 2021). Currently, less than 20% of food waste and 80% of horticultural/yard waste are recycled (NEA, 2021b), but both animal feed and compost products have been derived from food waste. A conceptual proposal for a food waste cycle that could be adopted in urban centres in Hong Kong is shown in Fig. 3. Output products can be applied to fishponds or to land, and the Innovation Technology Fund and Sustainable Fisheries Development Fund has already supported the conversion of organic waste into fish feed to support freshwater and marine fish culture in former industrial estates (Wong et al., 2016b, 2016c). A series of related studies have demonstrated the potential to converting food wastes into fish feed (Mo et al., 2015, 2016a, 2016b; Cheng et al., 2016b; Choi et al., 2016; Chen et al., 2019). We have also proposed a hypothetical plan for a demonstration unit in Hong Kong (Fig. 4) that could also act as a community and education centre to share the concept, processes, and technologies. Essential

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Fig. 4.   Schematic diagram of the demonstration unit located in New Territories, Hong Kong, showing the integrated application of the organic wastes.

involvement of the public would include collection and provision of wastes, availability and provision of high-quality food, and employment opportunities. This is presented as a local closed cycle of organic wastes that integrates sustainable measures for waste management.

4.4.2.  Recovery of traditional agriculture and aquaculture In earlier times, organic waste was more extensively recycled in Hong Kong and southern China by integrating agriculture and aquaculture. A mulberry-fish pond model incorporated cultivation of fruit crops (mulberry, with other species including longan), silkworm rearing, and polyculture fish farming, which represented closed-loop recycling of resources. Food wastes and other human and animal wastes were disposed into the ponds to feed fish, and pond mud was later dredged on to the crops as a fertiliser. This traditional system demonstrated that polyculture of different fish species, with varying modes of feeding, meant that nutrients derived from the organic wastes could be optimally utilised. Similar integrated approaches were also practised more widely in Asia, where wetlands have been transformed into ponds (for fish culture), separated by dykes (for growing crops, including Napier grass for feeding grass carp). Pond mud is excavated regularly, serving as fertiliser for crop cultivation. Different species of freshwater fish (mainly carp) are cultured in the same pond (polyculture), using digested animal manure from nearby pig or poultry farms as pond fertiliser for enriching the concentrations of nutrients to facilitate algal growth and stimulate the pond’s food



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chains. These closed-loop systems are labour intensive, and they raised unfortunate issues with hygiene, chemical and biological contamination, and human health; for example, manure was replaced by artificial fertilisers, the systems sometimes harboured diseases of livestock or poultry, and harmful pesticides entered the food chain. However, with more advanced contemporary knowledge, there is certainly a case to be made to resurrect some of the traditions, with some modifications. Processed food wastes, such as feeds and biochar, can be utilised to replace manure for the agriculture–aquaculture systems. Other possibilities include the use of food waste-derived biochar to enrich nutrients or to adsorb pollutants such as heavy metals and POPs in pond water. It is feasible to use food waste-based pellets which contain satisfactory levels of essential amino acids, crude proteins, crude carbohydrates, crude lipids, and phosphates for culturing low trophic level freshwater fish. These included grass carp, grey mullet, bighead carp, and tilapia (Cheng et al., 2016a; Mo et al., 2016b). Furthermore, fish that are fed with food waste-based pellets have been shown to contain significantly lower levels of tissue contaminants (including DDT and mercury) than those fed commercial feed pellets (Cheng et al., 2014, 2015). Food waste-based biochar can also be used to stabilise, degrade, or entrap pollutants including the most recalcitrant heavy metals and Persistent Organic Pollutants (POPs).

4.4.3. Urban agriculture and the built environment: Catching the wave Clearly, food waste-derived products and waste streams must be utilised and internalised within urban centres. The opportunities outlined in Fig. 1 reflect initiatives that are already underway in most countries and most cities. Incorporating food waste management and food waste-derived products into existing developments would appear to be merely a case of catching a wave of popularity. The positive emotions associated with recycling are already well embedded in urban populations. Subsequent chapters of this book provide ample evidence that safe, healthy, fertile, and uncontaminated composts can be derived from food wastes, using composting processes that can be activated most simply by community groups but also offering business, commercial, and industrial opportunities. There is a wide demand for plant growth substrates, soil ameliorants, mulches,

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and fertilisers in the built environment. Demand for a reliable, substantial, and sustainable internal supply of product already exists. In Hong Kong, which offers a typical example rather than an exception, more than 60 urban farms have been established since 2015, with some located at the top of 150 m tall buildings in the Central area. Other current urban farming types that have been piloted are vertical farming integrated with aquaculture, school gardens, and reactivated abandoned lands. Other initiatives flourish, such as green walls and edible landscapes, increasingly combined with digital technologies such as using programed unmanned aerial vehicles for regular maintenance and harvest (Khoo and Wee, 2019). Ecological restoration of degraded habitats, including slopes, is important for maintaining urban ecosystem functions and services. Existing landscape methods combining exotic and native plant species (GLTMS, 2016) are often insufficient to sustain a functional vegetation cover (Chen et al., 2016b; Wong et al., 2016a). It has been demonstrated that the use of waste-derived products as soil amendments can improve soil condition, microbial activities, carbon sequestration, as well as the performance of plants (Lehmann et al., 2011; Chen et al., 2016a). Studies of the restored area of the South East New Territory landfill in Hong Kong (Chen et al., 2016b, 2017) have demonstrated that restored ecosystems with low biodiversity are sustainable in terms of species maintenance and protection from soil erosion. There is considerable scope for the use of food waste-derived products in landscaping, protection of nature, and enhancement of biodiversity.

5. Conclusions Continued disposal of organic wastes to landfills is a huge waste of valuable resources and will be untenable in the future. Food wastes can be converted to composts, vermicomposts, and other plant growth products or to biochar, animal feeds, biofuel, bio-oil, gum, biopolymers, and a wide range of other value-added materials. Compost, vermicompost, biochar, and some biopolymers are safe to be used as fertilisers, contaminant immobilisers, and physical soil stabilisers. Incorporating food wastederived products as soil amendments to resources and improving the productivity and quality of urban agriculture will be cost-effective and will meet existing and rapidly expanding demand. Additional or excess



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production could be extensively used to upscale urban landscapes in all of their forms with better quality soils and improved biodiversity. Closing the urban food cycle by recycling food waste indisputably would provide substantial economic, ecological, and environmental gains.

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© 2023 World Scientific Publishing Europe Ltd. https://doi.org/10.1142/9781800612891_0003

Chapter 3

Trends of Food Waste Treatment/Resources Recovery with the Integration of Biochemical and Thermochemical Processes Abdulmoseen Segun Giwa*,†,§, Xiaoqian Zhang†,‡,¶, and Kaijun Wang†,|| *

School of Environment and Civil Engineering, Nanchang Institute of Science and Technology, Nanchang, China †

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China

‡

§

[email protected] ¶[email protected] ||[email protected]

Abstract Urgent efforts and appropriate measures to utilise food waste (FW) for holistic exploitation of resources are needed. This chapter briefly reviews the global FW generation scenario among several selected

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high economic and low developing countries. It further highlights the common globally adopted treatment techniques, such as landfilling, composting, heat–moisture reaction, and anaerobic digestion, their challenges, and associated merits for FW treatment. This review discusses considerations for optimal resource generation from FW and highlights the selection of three conceptual routes. The first route proposes anaerobic digestion and pyrolysis, where valuable products such as biochar can serve as additives in anaerobic digestion for optimal biogas production and stabilised digestates. In addition, in the upstream coupling section, pyro-oil and syngas can be used in anaerobic digestion for biomethanation enhancement. The performance of the syngas biomethanation in anaerobic digestion reflects on hydrogen and carbon monoxide concentrations. A higher concentration of hydrogen could accelerate the carbon monoxide degradation rate and vice versa. The second route offers a direction for high water content FW valorisation via hydrothermal carbonation (HTC) combined with anaerobic digestion and vice versa to obtain hydrochar and valuable products, such as hydro-oil, high biogas, and enriched digestates. Previous studies showed that hydrochar could mitigate ammonia in anaerobic digestion and enhance methane generation compared with pyrochar. For high hydrogen syngas generation, the gasification technique coupled with anaerobic digestion was proposed for FW and associated residues in the third route, thus offering sustainability towards increased bioenergy production. This review stimulates the possibility for the development of the baseline approach on FW and associated residues in different technology (anaerobic digestion, pyrolysis, HTC, and gasification) combinations for optimal resource recovery. Keywords: anaerobic digestion; food waste; gasification; hydrothermal carbonation; integration; pyrolysis

1. Introduction Food waste (FW) and its associated product are annually generated via manufacturing, preparation, and food consumption (Giwa et al., 2021b). A rise in the FW production in 2005–2025 by 44% is envisaged due to the increase in industrial and population growth, notably in developing nations (Giwa et al., 2021b). A large amount of FW is generated from food manufacturing, production, service industry, canteens, and university or institution cafeterias. The United Nation’s Food and Agriculture



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Organisation approximated that about 1/3 of globally produced food for human feeding is wasted or lost (European Commission, 2018). Nevertheless, FW can be a treasured source when reasonably and effectively utilised, offering to alleviate environmental impacts (Gao et al., 2017) and produce renewable energy sources (Giwa et al., 2019a; Lin and Xu, 2019). Various techniques have been adopted to treat FW to attain valuable products (Giwa et al., 2021a). Considering the complex physicochemical properties of FW, such as high moisture, oil, organic contents, and salinity, seeking efficient treatment and recycling methods is deemed necessary. Generally, the most commonly adopted treatments are landfilling, composting, heat–moisture reaction, and anaerobic digestion. Anaerobic digestion generates biogas with methane as the core product and digestates for fertiliser (Giwa et al., 2020b). Demerits of the technology include long-duration start-up for methanogens and cost implications (Gao et al., 2017; Xu et al., 2017; Giwa et al., 2019a). However, landfill is a conventional FW treatment approach with a complex ecosystem. Biological, physical, and chemical processes interact during biodecomposition, emitting gaseous products and liquid materials such as leachate. The landfill has shortcomings due to the requirements for a vast land area and a substantial amount of volatile organic compound emission. Incineration is a thermal process that remarkably minimises waste generation and produces thermal energy (Dong et al., 2019). Incineration performance is inhibited due to the high water content (80% dry weight) of Chinese FW (Gao et al., 2017), but it is enhanced when mixing with other household organic materials (Giwa et al., 2019a). Composting is known for its economic value (Gao et al., 2017), simple operational management, termination of microbes, and odour reduction (Giwa et al., 2018). Previous studies mainly applied a single treatment method for FW valorisation (Opatokun et al., 2016). Several studies focused on characterisation, whereas several studied potential valuable products from FW (Sindhu et al., 2019; Giwa et al., 2021b). However, very few studies have broadly harnessed the integration of technological routes to valorise FW, notably via anaerobic digestion coupled with pyrolysis, hydrothermal carbonation (HTC), and gasification. In this vein, this chapter focuses on reporting conventional treatment techniques, challenges, and associated merits for treating FW and the amplification of resource recovery via coupling of different technologies. Emphases were laid on FW treatment by integrating biochemical process (anaerobic digestion) with thermal

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techniques (pyrolysis, HTC, and gasification) for prospect to improve energy yield, hydrochar/biochar properties, and other by-products. The integration’s main possibilities are that the anaerobic digester could be installed at the downstream and/or upstream section, depending on the FW characteristics and the core objectives to promote the high yield of specific by-products. The production of biochar from pyrolysis or other thermal process and the insertion as additives in an anaerobic digester promote the mineral content of the digestates and stabilise the digester for enhanced biogas production. Digestates produced from the FW anaerobic digestion at the downstream can be dried or converted with thermal process to treasured by-products such as bio-oil, char, and syngas. Aqueous HTC liquids and bioliquids are thermal process by-products considered a challenging waste stream. They can be injected as feedstock or stabilisers in anaerobic digestion to improve performance and product yield. The integration cases for FW anaerobic digestion with HTC, pyrolysis, gasification, and vice versa for enhanced resource recovery are grouped and summarised in Section 4 (Conceptual Treatment and Resource Recovery Approach).

2.  Global Food Waste Generation Globally, in 2019, the municipal solid waste (MSW) generation exceeded 2 × 109 tonnes (t) and is forecast to rise to 70% by 2050 with 44% FW constituents as the organic fraction (The World Bank, 2018; Sarrion et al., 2021). FW generation worldwide has posed several challenges that are unsustainable for economic and environmental development. Greenhouse gas emission from food losses and wastage constitutes approximately 8–10% of the global emission (United Nations Environment Programme, 2021). The generation of FW and loss varies in different countries. Therefore, adequately evaluating and comprehending the source generation of FW and holistic treatment to accomplish the sustainable development goal is paramount.

2.1.  Developed countries’ scenario Different proportions of FW and loss are generated according to each country’s circumstances. The globally productive land of 28% from 1.4 billion hectares of land was used in food production lost and wasted



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(Paritosh et al., 2017). Most Asian advanced cities’ annual FW generation was projected to increase from 278 t to 416 million t from 2005 to 2025. China’s FW production amounts to approximately 17–18 million t annually; this waste is sufficient to provide food for an estimated 30–50 million individuals. The prevalent quantity of FW was reported to occur at the post-harvest and preservation phase totalling approximately 159 ± 3 Mt (45%) in China (Xue et al., 2021). Additional study on food loss advocates that above 35 million t of food equal to 6% of all the food produced in China vanishes in warehouse storage and household, processing, and transportation. Different categories of FW production percentage can be classified for cereals (28%), roots and tubers (54%), fruits and vegetables (50%), and meat (56%) at the post-harvest level. Comparing China’s FW production at the post-harvest stage with that of other advanced countries such as Japan (10.5%) and the United Kingdom (4.8%) reflects that it generates a vast amount of FW. In most developed countries, the production of FW per capita is still on the rise due to the low conversion techniques for FW; however, virgin waste production is still on the high side. More than half of the food loss and waste route occurred via postharvesting, storage, and production in several upcoming economic countries such as South Africa and China (Xue et al., 2021). Highly developed countries are exempt from these routes as they witnessed more FW generation via consumption, for instance, countries such as Finland (50%) and the United Kingdom (53%). In addition, FW production from industrialised countries could emerge from their eating habits, higher economics, and shopping nature (Lemaire and Limbourg, 2019; Xue et al., 2021). In the European Union, approximately 88 million t of FW is generated yearly, amounting to an estimated 143 billion euros (European Commission, 2018).

2.2.  Underdeveloped and developing countries’ scenario Several challenges are associated with the prevention of FW and loss in most developing and underdeveloped countries. These problems are limited to cultural habits or poverty, but the effects of technology deficiency, warmer climate, infrastructure, and technological know-how are more intense. Approximately 40% of fresh foods produced in developing countries are wasted (Sand, 2017). After harvest, fresh fruits and vegetables are often spoilt due to storage and logistic challenges. Occasionally, fresh

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farm products harvested could not be kept for a more extended period before being demanded by consumers; sometimes, the qualities are depreciated (Li and Pan, 2021). In these logistic, storage, and retailing instances, associated challenges are peculiar to the food loss. It exists in the logistic and supply chain before final consumption, which could also generate waste referred to as the FW at the end of the food assessment chain. Underdeveloped and developing countries are often linked to the severity of food loss and FW. Moreover, purchasing, eating habits, and improper food storage result in FW production. However, another investigation showed that FW production is connected with the standard of living associated with a particular country; for instance, the amount of FW generation for developing countries is 56 kg/year, whereas that for developed countries is 107 kg/year. Unlike developed countries, in developing and underdeveloped nations, FW production drift can chart trends in line with wealthier countries with higher GDP per capita, further associated with a rise in the FW generation.

3. Conventional Food Waste Treatment Techniques 3.1.  Animal feeds Several countries’ environmental and local laws permit FW utilisation for farming animals, including Japan, South Korea, and Taiwan, under China’s jurisdiction (Thi et al., 2015; Giwa et al., 2019b). The respective amount of FW generated by these countries is enormous and comprises 33% (Japan), 81% (South Korea), and 72.1% (Taiwan). Collection and separation often pose high challenges in developing countries because FW in MSW is not purified appropriately before use as animal feed. China and a few other countries, such as South Korea and Japan, use FW to feed animals (Giwa et al., 2020b). In several developed countries such as Norway, FW is used as soil conditioners and fertilisers. The quality of meat from animals fed with FW is still of concern because of the possibility of pathogenic infection. However, research has shown that the addition of food supplements could improve the poor quality of FW when utilised as animal feed. Moreover, a vast amount of crop residues emerging from non-food parts of crops as agro-industrial and by-products and crop residues could add value to environmental mitigation and economic improvement if harmonised correctly in the food supply and production chain. Harnessing



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the nutrient value in the FW and the non-food part of the crops could be recycled into the food chain for production as feed for animals. Irrespective of the advantages of using FW as animal feed, proper caution in food safety and animal health standards should be established to prevent the spread of zoonoses and animal diseases. The establishment can be attained through a legislative framework that incorporates feed business owners, enlightenment, and strengthening programs such as critical point and hazards analysis enforcement to oblige to policy and standard of operations, management control, and auditing. The Japan FW recycling policy for animal feed was introduced in 2001 and subsequently revised in 2007, offering feed production a priority over other FW treatment techniques, such as incineration, landfilling, and composting (Dou et al., 2018). Currently, 40% of Japan’s FW is reportedly recycled as feed for animals such as fish, pigs, and others. By contrast, in Europe, proper treatment of FW and sufficient monitoring and management measures are of principal importance. The hazard of animals feeding with FW devoid of adequate treatment could be high, as proven by the eruption of foot and mouth diseases. On this ground, the ban on FW utilisation for animal feeding was implemented in the UK in 2001 following the outbreak of foot and mouth diseases equally applied to other countries in the European Union in 2002 (Dou et al., 2018).

3.2.  Thermal processes for food waste treatment 3.2.1. Pyrolysis Pyrolysis is an environmental, energy-efficient thermal process for the decomposition of organic matter such as FW in the absence of atmospheric air to generate solid char, non-condensable gaseous product, and oil tar (Giwa et al., 2020b, 2020d). The application of pyrolysis in the decomposition of biomass, FW, and other organic waste materials can simultaneously and individually produce power and heat. Each pyrolysis product has found remarkable application in various areas such as wastewater treatment, energy optimisation, carbon sequestrations, agronomy, and soil conditioners, and as an adsorbent precursor (Giwa et al., 2020b, 2021b). The exact dispersal of yields hinges on mostly numerous pyrolysis conditions such as temperature, heating rate, residence times of the vapours, operating pressure, state of mixing, and sizes of the converted biomass/residues/FW. Pyrolysis can be viewed from three main

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operational standpoints, and these are operated as fast, slow/conventional/ traditional, and flash pyrolysis. Slow/traditional/conventional pyrolysis is the same irrespective of the adopted title. Traditional pyrolysis often occurs within several hours and mainly yields char products at a temperature range of 400–600°C (Giwa et al., 2019b). Fast and flash pyrolysis are often operated via fast heating rates of 1 to >1000°C/s and short residence time of 80%) and a low C/N ratio (90%), leading to cost-effectiveness than first-generation sequencing (e.g., Sanger sequencing) (Baudhuin et al., 2015). In recent years, high throughput sequencing technologies, including Illumina sequencing, 454 pyrosequencing, and Ion Torrent sequencing, have been used to gain an in-depth view of microbial community characteristics during the composting process (Schuster, 2008).

3.2.  Microbial communities 3.2.1. Bacteria Aerobic and anaerobic bacteria are involved in the degradation of food waste, in which aerobic bacteria are dominant with higher abundance and diversity (Nakasaki et al., 2019; Yamada and Kawase, 2006). Various indigenous bacteria were identified in organic wastes, such as Bacillus sp., Brevibacillus brevis, and Pseudomonas sp. (Tang et al., 2007). Several other species have also been isolated from bulking agents, such as spent mushroom wastes, sawdust, and wood chips (Gong et al., 2017; Zhong et al., 2020). Bacteria species produce specific enzymes, such as monooxygenase, protease, and alkane hydroxylases, that significantly enhance the biodegradation of organic substrate during the composting process (Tran et al., 2020). Monooxygenase can oxidise simple carbon substrate (C2-C4), whereas hydroxylases play an essential role in breaking down large organic compounds such as alkaloids and vitamins (Al Hosni et al., 2019). Protease performs catalytic to disintegrate proteins through metabolism mechanisms (Bouzaiane et al., 2008).

3.2.2. Fungi Fungal species in composts originate from organic waste and bulking agents (Liao et al., 2019). For instance, Mucor sp. and Aspergillus sp.



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used in composting were isolated from green waste (e.g., cow and poultry manure), whereas Trichoderma sp. used in composting is a fungus naturally found in woodchips and sawdust (Qin et al., 2021; Tran et al., 2021c). Fungi can decompose organic matters through extracellular enzymes, including lignin peroxidases (LiP), manganese peroxidases (MnP), and laccases (Lac) (Tran et al., 2021b). These enzymes play a crucial role in the biodegradation process (Duan et al., 2019). For example, LiP directly oxidises organic substrates, and MnP indirectly co-oxidise them with a significant catalyse for oxidation from Lac (Wei et al., 2018). In addition, fungi can produce biosurfactants (e.g., sophorolipids and rhamnolipids) with specific functional groups (Gong et al., 2017). Most biosurfactants are synthesised extracellularly, leading to enhanced biodegradation of organic matters by increasing their solubility and bioavailability (De Clercq et al., 2017).

3.3.  Microbial community abundance and diversity The microbial community is essential in decomposing organic waste during the composting process (Awasthi et al., 2017; Nakasaki et al., 2019). The characteristic of microbial community structure could be defined through the community members’ abundance (richness) and diversity (Zhang and Sun, 2016). The abundance and diversity of the microbial community changed during the composting phase (Table 1). The microbial community structure depends significantly on the physio-chemical parameters such as oxygen content, pH, moisture content, temperature, organic matters, and other factors such as initial compost material and bulking agent (Holman et al., 2016; Tsai et al., 2007). In the mesophilic phase (55℃), the abundance of the microbial community decreases significantly due to the gradual decrease of the carbon-rich substrate, and several mesophile species may not survive at high temperatures (Awasthi et al., 2017). During this phase, the bacteria community significantly changed, in which the phyla Firmicutes and Proteobacteria were replaced by Actinobacteria and Bacteroidetes (Partanen et al., 2010). Bacillus sp. and species of the phylum Bacteroidetes were the dominant bacteria strains in this phase (Zhu et al., 2019). Similarly, phylum Ascomycota was the most dominant in the fungal community, favouring high temperatures (60–65℃) (Wei et al., 2018). Microascus, Agaricus, and Geomyces prevailed in the fungal community (Cai et al., 2018). In this phase, most organic matters were degraded through these bacterial and fungal activities, leading the carbon source to deplete at the end of the thermophilic phase (Awasthi et al., 2018). The temperature gradually decreases, indicating that composting process is entering the cooling and maturation phase. In the cooling and maturation phase, mesophilic species prevailed in the community, implying the increasing diversity (Wu et al., 2017). The phyla Firmicutes, Proteobacteria, and Actinobacteria were represented in the community, with Bacillus sp., Brevibacillus brevis, and Pseudomonas sp., the dominant bacteria strains (Tran et al., 2021c; Zhong et al., 2020). Thermomyces and Microascus detected an abundance of fungi community in this phase (Yang et al., 2019). These species take place to degrade lignocellulose in the biomass, leading to carbon available substrates becoming empty at the final maturation phase (Sun et al., 2019). The temperature is going down to ambient temperature (25–30℃), indicating the composting process is completed and matured (Liao et al., 2019).

3.4.  Microbial enzymes Microbial enzymes contain several specific enzymes for biodegradation of organic matter, in which extracellular enzyme plays a significant role and is significantly involved in the hydrolysis of organic pollutants (Angnes et al., 2013; Lee et al., 2004). Extracellular enzymes include hydrolase, lipases, lignocelluloses, proteases, and other enzymes (Tiquia et al., 2002). Lipase hydrolase plays an essential role in the degradation of macromolecules of organic wastes (e.g., fats, cellulose, and hemicelluloses) (Adams and Umapathy, 2011). In the initial step of decomposing organic matter, the hydrolase enzyme converts large molecules into simpler molecules and transports them to the cell membrane via heterotrophic



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metabolism (Crecchio et al., 2001). Polysaccharides with large molecules are converted into oligosaccharides and monosaccharides. Peptides and amino acids are then produced through protein hydrolysis reactions (Adams and Umapathy, 2011). Similarly, lipids or fat was broken into fatty acids and glycerol with the support of the hydrolase enzyme (Yamada and Kawase, 2006). The lipase enzyme breaks down fatty acids in the second step through several catalysis reactions such as hydrolysis and esterification to produce carboxylic acid and alcohol (Vargas-Garcia et al., 2005). These products can convert to carbon dioxide (CO2) and water (H2O) under aerobic conditions and methane (CH4) and hydrosulphide (H2S) formations under anaerobic conditions (Lee et al., 2004). Several factors (e.g., temperature, oxygen, and moisture content) significantly affected the enzyme activities during the composting (Zhu, 2007). The temperature may shift the metabolic needs of microbial, leading to an effect on the production of the enzymes (Chang et al., 2006). Low temperatures (50℃) accelerated the microbial activity, leading to higher enzyme production (Tiquia et al., 2002). For instance, Boon et al. (2000) evaluated the effect of temperature (20, 30, 40, and 50℃) on the enzymatic synthesis of oligosaccharides as an extracellular enzyme using batch experiments. These results indicated that the extracellular enzyme production at 50℃ was 122 µmol h–1 four times higher than at 20℃ (26 µmol h–1). On the other hand, oxygen content and moisture are key players, significantly affecting enzyme activity during the composting process (Crecchio et al., 2001). Therefore, the optimal conditions of oxygen content (15–20%) and moisture content range of 50–60% could enhance enzyme production and secretion due to accelerated microbial activity (Tran et al., 2021b). Zhang and Sun (2015) reported that at the thermophilic phase, the enzyme activities of ­dehydrogenase (730 TPF/g), protease (250 IU/g), and urease (355 IU/g) were observed highest and then significantly decreased to 160 (TPF/g), 27 (IU/g), and 12 (IU/g) at the maturation phase during the green waste composting, respectively. These trends were similar to the biodegradation rate of organic matter and the microbial community structure.

4.  Conclusions and Future Opportunities Food waste has become a global concern due to its link to critical challenges facing society such as climate change and environmental sustainability. This chapter provided an overview of the current status of food

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waste worldwide. Western Asia has the highest food waste with over 118 kg capital per year, followed by sub-Saharan Africa and Southern Europe with nearly 115 and 89 kg capital per year. Composting is robust and efficient for food waste degradation. The specific enzymes of bacteria (monooxygenase and cytochromes P450) and fungi (lignin peroxidases (LiP), manganese peroxidases (MnP), and laccases (Lac)) play important roles in the biodegradation of food waste during composting. The structure and dynamics of the microbial community were observed to change significantly at each stage of the composting process using modern analytical techniques such as Illumina sequencing, 454 pyrosequencing, and Ion Torrent sequencing. The abundance and richness of the microbial communities are highest at the mesophilic stage and then gradually decrease in the thermophilic stage because several mesophiles cannot survive at high temperatures and low oxygen contents. Modern sequencing techniques are powerful tools to determine the microbial communities due to increased capacity for sequencing hundreds to thousands of genes at one time and high sensitivity and accuracy. In further studies, adjusting and maintaining the optimal conditions should be conducted and investigated to enhance the performance of the composting process. Moreover, specific microbial strains should be added to the compost mixture as bioaugmentation to accelerate the biodegradation rate and improve the mature compost quality. Microbial ecology and the interactions between specific strains with their environment (e.g., pH, temperature, moisture content, and oxygen content) must be elucidated in the future.

Acknowledgements The authors gratefully acknowledged the time and facility support from Van Lang University.

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Chapter 6

Food Waste Composting: Current Status, Challenges, and Opportunities Chitsan Lin*,†,**, Nicholas Kiprotich Cheruiyot‡,§,††, Thi-Hieu Le*,‡‡, Adnan Hussain¶,§§, Duy-Hieu Nguyen*,¶¶, and Chia-Hung Kuo‖,*** Maritime Science and Technology, College of Maritime, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan *

Department of Marine Environmental Engineering, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan

†

‡

Super Micro Mass Research and Technology Center, Cheng Shiu University, Kaohsiung, Taiwan

Center for Environmental Toxin and Emerging-Contaminant Research, Cheng Shiu University, Kaohsiung, Taiwan

§

Aquatic Science and Technology, College of Hydrosphere, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan ¶

Department of Seafood Science, College of Hydrosphere, National Kaohsiung University of Science and Technology, Kaohsiung, Taiwan ‖

[email protected] [email protected] ‡‡[email protected] §§[email protected] ¶¶[email protected] ***[email protected] **

††

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Abstract Composting is a green solution for food waste management, transforming food waste into a stable humus-like substance that can be used as an organic fertiliser. This chapter provides an overview of: (i) the environmental factors such as moisture content, oxygen content, carbon/ nitrogen ratio, temperature, and pH that affect composting processes, (ii) challenges of food waste composting, and (iii) spin-off applications of the process. Food waste supply varies with time of year, climate, and regional socio-economics. Despite this variability, diverse microbial communities in the composting process make the technology robust enough to degrade food waste and remediate non-food impurities and residues. We use the example of recalcitrant organic contaminants that include diesel and polychlorinated dibenzo-p-dioxins/dibenzofurans that have demonstrated high removal efficiencies of >75% in as little as 3–6 weeks. Furthermore, heat and combustible gases generated during the process can be recovered. This chapter collates knowledge of dealing with the environmental variables associated with food waste technology. It is shown that sustainable waste management approaches can have spin-off applications in bioremediation and energy production. Keywords: food waste composting; composting phase changes; composting process control; compost parameter monitoring; aerobic composting; compost aeration; bioremediation

1. Introduction The United Nations Environmental Programme (UNEP) estimates that global food waste amounted to 931 million tonnes in 2019 (UNEP, 2021). This represented 17% of the total global food production and is estimated to have contributed to 8–10% of global greenhouse emissions. Aligning with the UN Sustainable Development Goal 12 on responsible consumption and production, food waste can be significantly reduced. However, food waste production is inevitable, and green technology solutions are required. Composting has been traditionally used to treat and transform organic waste into a stable humus-like substance used as a soil amendment and organic fertiliser. The process can also degrade recalcitrant organic contaminants, including plasticisers, pesticides, and petroleum. Furthermore, the heat from this exothermic biological process can also be recovered and used in heating applications. Therefore, composting of food waste can be



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Fig. 1.  Overview of the key parameters and application of food waste composting process.

a promising technology for addressing the food waste problem. Figure 1 shows an overview of food waste composting, key monitored parameters, and applications. This chapter presents the fundamentals of food waste composting, the key parameters influencing the process, the challenges, and the applications of food waste composting. This chapter aims to interest readers in the potential of composting as an effective food waste management solution and robust bioremediation technology.

2.  Food Waste Composting 2.1.  Composting phases Composting involves the aerobic degradation of organic matter such as food waste, plant residues, and animal manure by microorganisms (Diaz et al., 2011). The organic matter provides nutrients for energy and growth for the organisms. The end product is a stable solid humus-like substrate called compost. The composting process is commonly divided into four phases: mesophilic, thermophilic, cooling, and maturation phases, based on metabolic activities of the microbial community and the resultant heat generated. The mesophilic phase is the initial phase of the composting process. The readily degradable and soluble organic matter decomposes into

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simple organic acids, e.g., acetic acid, lowering the pH into the acidic range (Makan et al., 2013). The mesophilic phase is characterised by high microbial diversity that primarily originates from the initial compost materials. Microbial species that thrive in this phase, such as Acidovorax sp. and Alcaligenes sp., are dominant (Ryckeboer et al., 2003). As the microbial community utilises the readily available food waste and acclimates in the new environment, the temperature of the ­compost mixture increases from the ambient temperature to as high as 40°C in a few days or even hours depending on the size and operating conditions of the compost mixture (Waqas et al., 2018b; Meena et al., 2021; Xiao et al., 2009). The heat is generated from the metabolic activity of the microbes as they utilise organic matter for growth, reproduction, and energy (Meena et al., 2021). The thermophilic phase follows the mesophilic phase and is characterised by an increase in temperature >50°C. Since temperature is an indicator of microbial activity, the thermophilic phase is considered to have the highest microbial activity. As the name implies, this stage is dominated by microorganisms, such as Clostridium thermocellum and Thermus thermophilus, that thrive under high temperatures (Westerman and Bicudo, 2005; Xiao et al., 2009). Species that cannot tolerate high temperatures go inactive during this phase. Complex organic matter, such as proteins, fats, and carbohydrates such as cellulose and hemicellulose, is degraded during this stage. The ammonification and mineralisation of organic nitrogen increase the pH to the alkaline range (Waqas et al., 2018b; Wang et al., 2016). Pathogenic microorganisms, e.g., Escherichia coli and Salmonella, are inactivated during this phase due to the high temperature, making composting a sanitising technology. However, the high temperatures lead to moisture loss, which might negatively affect microbial activity. The cooling phase is the second mesophilic phase characterised by a continuous decrease in temperatures from around 70°C to the ambient temperature. This decrease in temperature is triggered by a reduction in microbial activity caused by easily digestible organic matter depletion. Lignocellulosic materials, which are more difficult to decompose, are the primary nutrient source during this stage (Cao et al., 2021). Mesophilic microorganisms recolonise the environment, including several fungal species, which prefer to consume lignocellulosic materials. The compost pH decreases slightly but remains slightly alkaline (Keng et al., 2020).



Food Waste Composting: Current Status, Challenges, and Opportunities  129

Table 1.   Agronomic indices and guidelines used to assess compost maturity and stability in selected regions. Parameter

Hong Kong Canada Ireland (HKORC, 2021) (MOE, 2018) (Foster and Prasad, 2021)

Carbon/nitrogen ratio

≤25

20

>30

>20

Electrical conductivity (mS cm–1)

≤4

≤4

Declare

≤0.4

≤0.4

70% (Zhang et al., 2021; Adhikari et al., 2008). Bulking agents such as sawdust, tree barks, and rice husks are added to adjust the initial moisture content of the composting feedstock (Zhou et al., 2014). During the composting process, especially the thermophilic phase, the moisture content is monitored and controlled to ensure enough moisture for stable microbial activities (Hemidat et al., 2018). The high temperatures during the thermophilic phase lead to dehydration of the compost mixture. Therefore, water is usually added during this stage to maintain the acceptable moisture content range (Hemidat et al., 2018). However, high moisture content at this phase reduces the compost pile’s temperature, thus lengthening the incubation period and boosting the survival of pathogenic bacteria (Guo et al., 2012).

2.2.2.  Oxygen content The aerobic microorganisms in the composting mixture require oxygen for their survival. Oxygen is needed to grow, reproduce, and convert organic matter into energy (Hou et al., 2017). The oxygen content of the compost mixture is influenced by the porosity of the food waste (Cerda et al., 2018) and microbial oxygen demand (Tran et al., 2021b). Porosity is governed by characteristics of the food waste, including the particle size and moisture content (Cerda et al., 2018). For instance, the water content



Initial composting mixture Materials

Variation during the composting process

pH

Moisture (%)

C/N

pH

Temp. (℃)

Moisture (%)

Fresh FW + inoculant (8 t, industrial scale)

5.2–6.5

~75

—

7.5–8.5

25–>65

75–18

Zhang et al. (2021)

FW (potatoes/vegetables/rice = 2:2:1) + biochar/sawdust + inoculant (0.16 m3 electric composter 4 kg of FW day–1)

5.0–6.5

—

58–62

4.2–6.5

20–56

79–20

Voběrková et al. (2020)

Municipal biowaste + sugarcane filter cake + star grass (150 kg conical piles)

5.1–7.5

65–77

19–30

7.8–9.9

30–>65

—

Soto-Paz et al. (2019)

FW, biochar

5.3–7.5

≤70

—

4.9–9.3

27.6–60

60–30

Waqas et al. (2018a)

11.0–11.6

55–60

25–26

8.5–8.9

20–65

60–55

Zhou et al. (2018)

FW, sawdust, herbal residues, lime

Note: “—” means the parameter values were not mentioned.

Refs.

Food Waste Composting: Current Status, Challenges, and Opportunities  131

Table 2.   Summary of operational parameters and ranges in food waste (FW) composting processes.

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of food waste is typically high (Adhikari et al., 2008); thus, the pore spaces may be saturated with water leading to the formation of anaerobic zones in the compost mixture. Microbial oxygen demand varies at each stage of the composting process. Microorganisms consume considerable amounts of oxygen to biodegrade the easily decomposable organic material during the first mesophilic phase (Lin et al., 2012; Ge et al., 2020). When microbial activity is at its peak, the thermophilic phase sees the most oxygen consumption, and the maturation phase has the least oxygen demand (Tran et al., 2021b). A 5–10% oxygen content is recommended for a successful composting operation (Bertran et al., 2004; Walling and Vaneeckhaute, 2021). Therefore, oxygen content needs to be monitored and controlled effectively to ensure optimal microbial activity during the composting process.

2.2.3. Temperature Temperature is a critical parameter used to monitor the performance and rate of biological processes during composting (Waqas et al., 2018b). Temperature variation affects the succession and evolution of microbiological communities during the biodegradation process (Ermolaev et al., 2015). The effects of temperature are from both the ambient temperature and the heat produced during the composting process. The biologically produced heat during the composting process is directly related to microbial activity (Tran et al., 2021b). Thus, it can effectively reflect the process. From the first phase, heat is generated during the microbial degradation of the organic matter. In the thermophilic phase, due to the highest microbial activities, the temperature could go as high as 80°C (Palaniveloo et al., 2020). The generated temperature affects the microbial structure. Temperature-sensitive organisms go inactive when the temperature is no longer hospitable. For instance, mesophiles, including pathogenic microorganisms, cannot thrive in the thermophilic stage (El Haggar, 2005; Waqas et al., 2018b; Hou et al., 2017). The high temperature also causes moisture loss through evaporation and the volatilisation of volatile organic compounds. The ambient temperatures also play a vital role in promoting efficient composting. The low ambient temperature in winter makes the composting process slower than in spring and summer. Low ambient temperatures may considerably prolong the mesophilic phase and shorten the



Food Waste Composting: Current Status, Challenges, and Opportunities  133

thermophilic stage. These psychotropic conditions could also enable the survival of pathogenic microorganisms. Furthermore, these conditions might result in the failure of the composting process (Das et al., 2002). In-vessel composting and heating the aeration air have been used in cold regions to ensure successful composting (Elving et al., 2010; Wang et al., 2013).

2.2.4. pH The initial pH of food waste is slightly acidic, unlike other organic waste like animal manure which is slightly alkaline (Ghinea et al., 2019). After adding bulking agents and other substances, the pH slightly rises, as shown in Table 2. Some studies have investigated the adjustment of initial pH to the neutral range and showed improvement in the composting process (Adhikari et al., 2008; Pandey et al., 2016). However, the composting process consists of a diverse microbial community under different pH conditions. For instance, bacteria prefer near-neutral pH, while fungi prefer slightly acidic conditions. This allows the composting process to proceed successfully under a wider pH range (5.5–8.0). Furthermore, the pH changes significantly at each phase during the composting process. At the initial mesophilic phase, microbial activity decomposes organic material producing organic acids that lower the pH value (Waqas et al., 2018b). The pH increases steadily at the thermophilic phase because of ammonification (Jindo et al., 2016). The pH of the matured compost is usually slightly alkaline.

2.2.5. Carbon/Nitrogen (C/N) ratio Carbon and nitrogen are the essential elements required for microbial decomposition. Carbon is utilised as an energy source as well as the basic building block making up to 50% of microbial biomass. Similarly, nitrogen is an essential part of the amino acids, proteins, enzymes, and DNA required to grow and function. C/N ratio has been used to evaluate the available nutrients for a successful composting process. C/N ratios between 25 and 30 have been considered optimum for composting (Kumar et al., 2010; Guo et al., 2012). Nevertheless, these optimum C/N ratios assume that all carbon will be completely mineralised. Therefore, C/N ratios as low as 20 are adequate.

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Low C/N ratios allow rapid decomposition and microbial growth but cause undesirable odours due to the loss of nitrogen in the form of ammonia (Voběrková et al., 2020). Contrarily, high C/N ratios result in inadequate nitrogen for microbial growth and lower decomposition rates, and longer composting processes. The C/N ratio of food waste reported in the literature range from 14 to 40 (Shah et al., 2014; Lin et al., 2021; Tran et al., 2021b). The addition of fertilisers, e.g., ammonium sulphate, has been used to decrease the C/N ratio (Yu et al., 2019). However, this increases the cost of composting. Measures of conserving and preventing nitrogen loss during composting by adding adsorbents, e.g., biochar, can also be useful to ensure a healthy microbial community.

2.2.6.  Type of food waste The type of food waste can affect the composting process in several crucial ways. First, there are types of food waste that will take longer to degrade than others, for example, fats versus carbohydrates. Second, some food waste might consist of ingredients that make it difficult for microorganisms to thrive, e.g., excessive salt content. Chang and Hsu (2008) showed that the temperature, pH, and CO2 evolution of synthesised food waste rich in carbohydrate, fat, and protein were starkly different. Fat-rich food waste had the lowest temperature and the longest acidification time. Conversely, the protein-rich food waste had the highest temperature and the shortest acidification time. The challenge with food waste is that it varies based on cultural habits, time of year, climate, and the economic level of the region. The ­so-called “western diet”, characterised by highly processed carbohydrates, high sugar, fat, and sodium content, has gained popularity globally because of its convenience and affordability (Conrad et al., 2018). Fast food restaurants like McDonald’s, KFC, Pizza Hut, and Subway have become ubiquitous in almost all countries. Surprisingly, composting of fast-food waste is quite rare. Two similar articles (Hayat et al., 2015; Azeem et al., 2014) compared composting different ratios of fast food waste from KFC and Pizza Hut and poultry litter. They showed that the temperature of 100% fast food waste was lower than all other treatments, while the moisture content was the highest. The C/N ratio was the highest at 26 after 105 days of composting. This could imply that the compost was still immature after the composting period. Although they did not offer an explanation for these observations, it would appear that the high oil, fat,



Food Waste Composting: Current Status, Challenges, and Opportunities  135

and sodium content might negatively impact the composting process. However, further investigation should be carried out to provide clear insights. Other diets like Korean, Chinese, and Japanese cuisines are quite high in salt and water content which also present unique challenges during composting and even the final matured compost application (Lee et al., 2020; Lee et al., 2017). Spicy food, like Indian cuisine, might also influence the composting process. Although there are currently no publications on the effect of spices on aerobic processes such as the composting process, spices such as cinnamon, black pepper, cardamom, and clove have been reported to inhibit anaerobic digestion (Sahu et al., 2017), with cumulative biogas yield reduced by as much as 85%. This variation in food waste composition could challenge the engineering and design aspects of composting (Chang and Hsu, 2008), limiting a one-size-fits-all approach in food waste composting technology.

3.  Challenges During FW Composting 3.1.  Aeration monitoring and control Aeration control is an important factor in designing an efficient composting system since it is directly related to pore oxygen concentration. Additionally, aeration is required to meet the oxygen demand for organic decomposition, excess water removal, and heat dissipation (Haug, 1993). Forced aeration and physical turnover are the standard aeration methods in composting. Forced aeration is performed via aeration pump systems, while physical turnover is conducted through manual or mechanical turning. The choice of the aeration technique will depend on the composting scale and cost. Manual and mechanical turning is used in aerated or turned windrow composting, which is suited for vast quantities of food waste from entire communities. These turning techniques can be applied at low or no cost and are suitable for both pilot and commercial scales. However, a continual labour supply is required to maintain and operate the facilities. Generally, the turning frequency is dictated by the composting stages. Several authors have studied the effect of turning frequency on moisture content and nutrient loss, recommending turning every 2, 4, 6, and 15 days during the active phase of the process and once or twice per week during the maturation phase (Ogunwande et al., 2008; Onwosi et al., 2017; Soto-Paz et al., 2019).

136  C. Lin et al.

Aeration pump is often used in aerated static pile composting, which produces compost relatively quickly. The composting materials can be placed over a network of pipes that deliver air into or draw air out of the pile, referred to as positive and negative pressures, respectively. A timer or temperature sensor activates the aeration pump. A biofilter made from finished compost is usually layered on top to filter the air during positivepressure aeration and reduce odours. This technique requires high initial and operational cost and technical assistance, and it is better suited for commercial scales. Setting up an optimal aeration rate for the aeration pump is essential to ensure stable composting systems. For instance, Guo et al. (2012) showed that a low aeration rate, 0.0

14.0

20.0

35.0

Max’

12%

20.0

40.0

40.0

Notes: *Values relate to combined material after pretreatment and blending. †Includes systems suitable for effluent and wastewater treatment.

food waste and yard waste to maintain consistency of the digestion matrix material. When solid and semisolid feedstocks are handled in predigestion bunkers, not only is DS content important in respect of the downstream AD process but the bulk density of feedstock also becomes a critical postpretreatment design parameter (Wedwitschka et al., 2016). Pretreatment such as chopping and shredding will increase bulk density, which will substantially impact system throughput capacity. Wet AD systems tend only to be suited to processing low-DS inputs, while plug-flow HSAD systems are relatively flexible in the range of input DS values. Batch systems require high DS input material to allow appropriate dry materials’ handling, such as wheeled loaders and conveyors. The relevance of this is that the DS content of feedstock must be matched to the design capabilities of the AD system employed (Table 2). Definition of food waste is inconsistent in the literature and in common usage (Anon, 2021; Bagherzadeh et al., 2014; Kelleher and Robins, 2013). It spans kerbside-collected domestic food residues, supermarket waste, hospitality and events’ residues, commercial and industrial food waste, and, even in some cases, organic fraction of unsegregated waste. Most researchers focus on food in the supply chain up to a final product, which is of good quality and fit for consumption, but that does not get consumed. Such a definition ignores other aspects of food waste which, from a processing and AD perspective, may be mono-streams or mixed organic material, food-only or include packaging and/or additional organic waste such as biodegradable bags, paper bags, and newspaper. Food waste may be putrescible or difficult to process such as seeds, nuts, and eggshells. Campuzano and González-Martínez (2016) reviewed food waste data from 22 countries, reporting a range of total solids’ (TS) content with the majority reporting values >25% TS: range 15.0–50.3% (average 27.2 + 7.6%). Selvam et al. (2021) reviewed food waste

156  R. A. K. Szmidt

properties reported in the literature, including 58 comparative reports of TS or DM% with an average of 23.2% + 7.7%. This agrees closely with data reviewed by Fisgativa et al. (2016) who considered data from 70 reports (102 samples) with an average of 22.8 + 10.0% DM. However, the data range reported by Selvam et al. (2016) was 5.4–51.5% TS or DM. Food waste properties are also potentially seasonal (Slorach et al. (2019)), even varying weekly, reflecting changes such as holidays and events in society. Overall, this underlines the importance of understanding actual waste streams and their variability and not relying on average data when designing pretreatment systems. The range of food waste and potential co-digestion feedstock DS values have been variously reported in the literature. These are summarised in Table 3. Dilution of food waste-based feedstock to reduce DS content is relatively easy at pretreatment. However, an increase of DS content to match the needs of AD technology is not, other than by adding relatively dry feedstock to the mix or by using heat, and therefore energy, for drying. Increasing the DS content of feedstock, e.g., by belt-pressing makes little economic or technical sense as this would waste the biogas potential of the supernatant and would create liquid that may otherwise be difficult to dispose of, with the potential to reduce the efficiency of subsequent aerobic wastewater treatment (Macintosh et al., 2019). On a relatively small scale, drying of food waste before AD may be attractive as it reduces odour, improves temporary storage without loss of energy potential, improves handleability, and reduces transit mass and volume, thereby reducing waste collection costs (Goldstein and Dreizen, 2017; Sotiropoulos et al., 2019). Drying food waste at the source is a technique most commonly employed in high population-density cities, particularly in Asia, where food waste collection is relatively difficult and costly. In most European and North American cases, the preferred strategy is to manage liquid with too low a %DS for the available AD technology by co-digestion with other, appropriate feedstock. In turn, this may require a mixing or blending step in pretreatment. Food waste may physically behave differently than expected. DS content alone may not be sufficient to define its material characteristics, e.g., in terms of viscosity or flowability. Baroutian et al. (2017) noted that few studies of food waste rheology had been made. They noted that food waste of different origins (carbohydrates, vegetables, fruits, and meats) exhibits shear-thinning flow behaviour. Food semisolids, such as yoghurt and other dairy-based products, may exhibit thixotropic characteristics,



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  157 Table 3.  Typical dry solids’ (%DS) content of food waste and other biodegradable material suitable for co-mingled AD feedstock with food waste (reported and range values). Breakfast cereal and snacks Catering waste

90–99 9.5–51.5

Corn fodder (stover)

91.1

Corn silage

45.8

Feedlot manure

54.1

Food waste (unspecified)

4–88

Food waste (supermarket)

15–19

Fruit and vegetable waste

5–10

Horse manure (hay)

19.9

Horse manure (silage)

25.9

Horse manure (dung)

20.1–37.0

Domestic kitchen waste

12–40

OFMSW*

15–53

Sawdust

80–85

Straw (various)

72–95

Peat

40–45

Waste paper

90–94

Wood chips

79.8

Wood pellets

85–90

Yard waste (garden waste)

80–95

Note: *Organic fraction of municipal solid waste. Source: Bong et al. (2018), Brown and Li (2013), Campuzano and González-Martínez (2016), Chang et al. (2006), Forster-Carneiro (2008), Hadin and Erikson (2016), Karthikeyan et al. (2018), Miller and Coker (2021a), Sotiropoulos et al. (2019), Selvam et al. (2021), Tampio et al. (2016), Tas and Shah (2021), Wedwitschka (2016), Zhang et al. (2020).

changing viscosity depending on materials’ handling. Accumulation of food thickeners such as xanthan gums may cause problems of unexpected sedimentation after shearing (Barnes, 1997). This means that pretreatment equipment may not only have an intended effect, e.g., particle size

158  R. A. K. Szmidt

reduction and mixing, but also have a seemingly disproportionate effect on flowability and, therefore, downstream system requirements such as type and capacity of pumps. A key factor in AD performance, particularly biogas yield, is the input material’s volatile solids’ (VS) content. However, regarding pretreatment steps as a sub-set of feedstock total solids, this has little direct relevance to the choice of pretreatment.

3. Food Waste Contamination and Waste Separation Food waste is rarely available as a mono-stream suitable for AD without pretreatment. It typically requires removal of contaminants, particle size reduction, and blending different materials to optimise chemical characteristics, including supplementation with additional chemicals or nutrients. The literature is surprisingly limited regarding the assessment of food waste contamination (Echavarri-Bravo, 2017; Kenny, 2021). Most scientific and technical research has focused on processing food waste, for instance, by composting or AD, rather than the nature and impact of the whole waste stream on food waste reduction measures (Filho et al., 2021). Zhang et al. (2007) considered daily and weekly variation in food waste characterisation in northern California but did not comment on the regional variation in food waste arisings: a factor that may have a significant impact on the efficiency of management and pretreatment of that material. This leaves general assumptions and the task of managing contamination to industry. Miller and Coker (2021b) summarised the rejects recoverable from food waste processing as film plastic, high-density polyethylene (HDP), polyethylene terephthalate (PET), polypropylene (PP), cardboard, paperboard, aluminium, and steel. The proportion of waste contamination varies considerably, ranging from nil, e.g., bulk material such as spoiled grain, to heavily contaminated where source-segregated kerbsidecollected food waste may contain c. 5–10% of non-processable material. The organic fraction of unsegregated municipal solid waste (OFMSW), frequently classified as food waste in the literature, may have significantly higher levels of non-processable material such as soils and grits and inert domestic waste residues such that the organic fraction may be 50% or less of the material. However, even most factory food waste is contaminated, including processed or part-processed packaged food. For instance, food



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  159

waste may itself be uniform, e.g., spoiled breakfast cereal, but it may be boxed, palletised and wrapped or packaged in plastic. Similarly, canned products may appear simple, but the can may be steel or aluminium, paper labels may separate, and the primary content, such as fruit, meat, or vegetables, may separate on de-packaging from liquids also within the can. Categories of material typically contaminating food waste are as follows: · plastics, particularly packaging, — polystyrene, — rigid, — semirigid, — film, and — bioplastics and biodegradable film. · metals, particularly cans, cutlery, and kitchenware, · crockery, and · glass. Although these main categories represent the majority of contamination by weight, small items may have a significant impact on contamination in terms of visibility/quality of post-AD digestate and represent a risk in terms of environmental impact, particularly the presence of microplastics. Material of this general nature, including tea bags, fruit label stickers, “paper” towels, mixed material bread bags, coffee filters, and nappies (biodegradable and non-biodegradable), was identified in food waste by Aspray and Tomkins (2019). While most contamination is relatively small, e.g., food cartons, this is not always the case, and deliveries may include pallets or bulk bins and large containers which need to be removed from the waste stream either manually or by pretreatment equipment. In this regard, it is vital that educational programmes related to recycling and energy recovery of food waste emphasise the difference between kitchen waste (all) and the organic fraction that is food waste suitable for AD. Equipment commonly used for waste separation comprises one or more machines in series, having the potential for more than one of these tasks. For instance, a feed hopper in front of a separator also provides the opportunity to load and blend different materials at the point of separation from others. Therefore, good system design is more than just the sum of

160  R. A. K. Szmidt

machine capabilities and throughput capacity but also provides flexibility to the operator. Kenny (2021) considered that there remains a significant need for research in this respect, including the following: · determination of the level of plastic contamination and associated particle sizes (e.g., microplastics or nanoplastics) generated from food waste, · impacts of technologies (e.g., shredders, grinders and de-packagers) commonly used by processors on the level of plastic contamination and size of plastic particles, and · identification of the most effective strategies to prevent plastic contamination in food waste streams. The use of compostable or biodegradable bags in the collection of food waste is common. This is often linked to a recommendation to use food-caddy liners to make emptying containers easier and less messy. Wrapping material may include paper bags or newspapers. While such materials may be considered organic and are generally denatured in an aerobic composting process, they are mostly less well suited to AD. It is essential that waste collection strategies for food waste are holistic in this regard and that waste collection is carried out in a way that matches the available technology. In the case of biodegradable bags, if aerobically degradable bags are used for food waste that is then processed using AD, the bags themselves become a contaminant rather than a carrier that will contribute to the process. In Europe, standard EN 13432 defines the nature of such material to enable matching of material to process (Anon, 2007). Using an inappropriate degradable material may mean that pretreatment equipment must remove it. Failure to do so may result in shredding of bags, which results in strips and particles of partially degraded material (bioplastics). In turn, these risks coalesce to clog AD equipment and result in the accumulation of material in digesters as floating or settling layers (Garaffa and Yepsen, 2012). All such degradable material that transits AD generally will degrade quickly once recovered digestate becomes aerobic, e.g., in post-AD composting (Kern et al., 2018), although it may nonetheless appear unsightly. Paper fibre generally does not sufficiently degrade within the residence time and the conditions of mesophilic AD, requiring typically up to 70 days (Coker, 2018). However, it will contribute positively to the mix of materials suitable for thermophilic HSAD and may increase methane yield under such conditions (Gonzalez-Estrella et al., 2017).



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  161

3.1.  The theory of waste separation Coker (2019) considered successful waste separation to be dependent on the steps recognised by Vesilind (1984) as linking materials’ characteristics such as colour or size (the code) and the ability to perform some function such as route diversion (the switch) (Vesilind et al., 2002; Vesilind and Morgan, 2004). Typically, each separation step is binary, with in-series machines achieving further separation and refining of the material. For mixed material such as food waste (x) contaminated with, e.g., metal drink cans (y), a separator would be expected to separate x and y into two streams (Fig. 1). If all material is partitioned, then the rate of recovery would be 100%, but in most real-life situations, some of x goes to product stream y and vice versa, expressed as Rx1 = x1/x0 × 100 where R is the recovery rate, x1 is the material flow in output stream 1, and x0 is the material flow in the material input stream. Similarly, material y would also reflect the recovery rate in stream 2. In addition to the recovery rate (R), the degree of contamination with unseparated material defines the purity (P) of each stream. In the examples shown in Fig. 1, the purity of x in stream 1 is expressed as Px1 = x1/(x1+y1) × 100 Different AD systems require different levels of purity to function. For instance, many wet AD systems demand a low level of contamination

1 x0y0

x1y1

0

2

x2y2

Fig. 1.   A black box binary separator (Vesilind and Morgan, 2004).

162  R. A. K. Szmidt

in the input stream. Therefore, the pretreatment system must be designed to ensure a high level of purity. Consequently, this may mean a compromise in terms of recovery of material from the input stream. In the case of AD, this may mean failure to recover all of the organic fraction in food waste to avoid undesirable material such as sand, grit, or plastics entering the AD system. In contrast, HSAD, which is more tolerant of lignin-rich and non-putrescible material, may be able to receive feedstock with a relatively high degree of impurities from pretreatment equipment, to maximise food waste energy recovery, but at the expense of the purity of the input to the digester. In such a case, the requirement for the removal of contaminants may be shifted from the use of pretreatment equipment to post-digestion screening and recovery (Szmidt, 2018). Waste separation as a binary process is readily applicable to many waste-industry scenarios such as separating ferrous and non-ferrous metals, e.g., aluminium and steel drink cans. However, the food waste industry and the organics industry generally have an added complication. Food waste and other organic materials are readily influenced by their environment, including handling and storage conditions. Factors such as compression and delays in processing may result in significant changes, e.g., to physical properties, including effluent production, and partial biological degradation, respectively. In this case, the pretreatment system design should take into account potential transformation products (TP). This is illustrated in Fig. 2.

1 x0y0

(x1-x3)y1

0

3

2

TP

(x 2-x4)y2

y3

Fig. 2.   A separator with transformation product (TP) derived from input material x.



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  163

3.2.  Particle size reduction techniques As biological material may be physically transformed by waste separation and pretreatment, this may also affect biogas yield in downstream AD. Panico et al. (2014) noted in a comparison of two types of food waste having an initial similarity in particle size that they reacted differently to particle size reduction. In validating a mathematical model, Esposito et al. (2011) noted that biomethane production increased with a decrease in particle size of some samples. This may be due simply to a higher degree of solubilisation of some material, e.g., pasta, in post-pretreatment AD, where porous organic solids with a chemical composition mainly made of simple carbohydrates can be easily disintegrated in water. This results in the rapid availability of substrate to microorganisms active in the AD process. If the rates of hydrolysis and solubilisation are underestimated, the downstream AD process is potentially at risk due to acid accumulation in a situation where the hydrolysis rate does not limit volatile acid formation during acidogenesis. As this is generally faster than the following methanogenesis step, acid accumulation risks exceeding consumption to produce methane (Panico et al., 2014). Izumi et al. (2010) noted that particle size reduction increased food waste solubilisation by up to 30% and the yield of biomethane by up to 28%. However, excessive particle size reduction of substrate resulted in VFA accumulation, decreased methane production, and decreased solubilisation in the anaerobic digestion process. This agrees with the findings of Szlachta et al. (2018), who found that excessive particle size reduction did not increase biogas yield. These results indicate that there is an optimum balance in respect of particle size reduction that should be determined. These authors also suggested that the energy balance of the whole system needs to take into account the parasitic energy demand for particle size reduction compared to the potential yield advantage that may result.

4.  Depackaging Techniques A wide range of machines are available for waste separation and mixing as part of commercial-scale AD pretreatment. These were summarised by Coker (2020) as being primarily variations on applying a force to the food waste and material to be separated. This may be either in a tangential shearing action, a vertically oriented hammering action, horizontal, vertical, or rotational pressure through a sieve or screen. Most de-packaging

164  R. A. K. Szmidt

equipment is fixed although some mobile equipment, such as tractormounted screens, is available for small-scale operations. Most systems employed in the AD sector are mechanical and operated under ambient conditions. When considering which method of pretreatment de-packaging is appropriate, it is important to take into account the matching of feedstock characteristics, output quality, and downstream AD requirements. Some de-packaging equipment requires substantial amounts of water, e.g., c. 600 kgt−1, while others operate on an as-received basis. Power usage varies in the range c. 2.5–8.5 kWht−1 with an installed power of up to 170 kW per unit, depending on the manufacturer (Coker, 2021).

4.1.  Mechanical handling On a commercial scale, most food waste AD is preceded by one or more machines for feedstock pretreatment. These range from simple bag openers to sophisticated sequential arrays of machines for the following: · particle size reduction, — shredding, — hammer milling, and — conveying. · sorting, — screening, — pressing, — air separation, and — density separation. · intermediate storage, and · materials’ handling. Although intermediate storage such as buffer storage and bunkering of waste and handling, e.g., conveying, may generally not be considered part of pretreatment, they can have a significant impact. Handling may result in changes to the physical nature of organic material, including putrescible food waste. Bulk waste reception procedures may result in layering and compaction, increasing bulk density, which will in turn influence in-storage air-porosity and biodegradation, and can also be a locus of partition, e.g., of liquid or effluent runoff (Ahn et al., 2007). On an industrial scale,



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  165

reception area and bunkering design should be matched to daily throughput capacity. Food waste should be pretreated and transferred to the downstream AD facility as quickly as possible on a “first-come-first-served” basis. There is a risk, particularly with crane-operated deep bunkers, that material remains in the reception and pretreatment zone and its energy value is lost through aerobic biodegradation and putrefaction.

4.1.1.  Mechanical shredding and mill separation In the simplest sense, particle size reduction of AD feedstock increases the effective surface area of the waste, making it more available to microbes and, therefore, the biochemical processes involved in biogas production. Such techniques also improve handleability and uniformity of the waste. Particle size reduction is arguably the most widespread form of AD feedstock pretreatment. Bong et al. (2018) reviewed the literature in this regard and considered that particle size reduction contributes to increased substrate utilisation and higher COD solubilisation. However, they observed that excessive particle size reduction can lead to subsequent AD instability where small particle size may result in early AD acidification. Mechanical pretreatment may not result in increased biogas production from easily degraded food waste but was considered to increase biogas yield from material with high cellulosic content, such as straw, vegetable waste, or co-mingled green waste (Bong et al., 2018). A wide range of knife shredders and crushers are available for this purpose. While particle size reduction is itself a form of pretreatment prior to AD, it can drastically reduce the quality of both input material and postAD digestate through the creation of small-particle contaminants, particularly plastic fragments. These, and other inert materials, may accumulate in the digester as sinking or floating layers, thus reducing effective digester volume, resulting in loss of throughput capacity and potential risk to the facility. Shredding and crushing of food waste, unless free of contaminants is therefore, generally, to be discouraged. Mechanical mill separators both reduce input particle size and at the same time separate digestible material in the incoming food waste from non-processable material. Large-scale food waste separators include hammer mills: essentially a rigid drum containing a horizontal rotating shaft on which hinged hammers are mounted directly to the shaft or shaft spurs. The hammers are free to swing on their mountings. The shaft rotates longitudinally inside

166  R. A. K. Szmidt Mixed waste including food waste Variable-speed rotating paddle-shaft with hammers

Residual waste

Separated food waste

Fig. 3.   Principles of mill separation.

the drum while the material is fed into a feed hopper. The material is impacted by the hammers and is thereby particle size reduced or shredded and expelled through size-specific screens at the drum base. The shaft typically has variable speed, e.g., 300–600 rpm, and should be adjusted to avoid material destruction. Such separators can be adjusted to separate a known type of input material, e.g., separation of drink cans and their contents or breakfast cereal from cardboard containers. Feed rate, shaft rotational speed, and choice of hammerhead can all be optimised. This can give an extremely high degree of separation, over >99%. For mixed wastes, such as kerbside collected food waste, the degree of separation will be less and will depend on operator skill and the variability of the input material (Coker, 2021). A schematic of a mill separator is shown in Fig. 3.

4.1.2. Screening Success of sorting equipment depends greatly on the matching of mechanical equipment to the nature of the material to be separated. Most designs of screening equipment are not suited to the separation of wet and semiliquid material due to clogging of moving parts and blocking of apertures through which material may otherwise be sorted. The most common types of screens are rotary screens (trommels) and star screens. Where food



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  167

waste treatment is regulated, e.g., in regard to hygiene, it is generally the case that operators are required to evidence that material is treated below a particle size limit value at screening, even if other separations and pretreatment steps are employed (Anon, 2009).

4.1.2.1.  Star screening Star screens consist of one or more screen decks over which the material to be separated is passed. Rotating shafts equipped with an array of closely spaced “stars” at right angles to the direction of material flow convey the material away from the input point. Materials that cannot pass through the stars’ gaps are separated as coarse-grain materials. Typically, material that passes through the screen drops onto a conveyor and then onto a fine screen deck equipped with smaller stars. This then sub-divides the balance of material into a fine and a medium fraction using the same principle (Fig. 4).

Fig. 4.   The sequence of a typical two-stage star screen.

168  R. A. K. Szmidt

Particle size of separation fractions can be changed by adjusting the rotary speed of the star shafts. Sophisticated installations allow control, via frequency converters, of the grain size, which is determined by the star geometry. Star screens are less well suited to sorting of wet and putrescible materials than, for instance, trommel screens due to clogging and the difficulty of cleaning.

4.1.2.2.  Drum and trommel screens Drum screens, also known as trommel screens, are widely used for mechanical sorting in the waste and recycling industry. Application ranges from screening and particle size separation of sands, gravel, and soil to separation of inert and semisolid organic fractions. The mechanism is that of an inclined cylinder mounted on rollers. The cylinder has holes of a specific size punched out across the side of the drum. The hole size determines the maximum size of material that may fall through and be collected below, usually onto a conveyor for onward transfer. Mixed material to be separated is fed to the upper inner area of the rotating drum. As the material migrates along the length of the drum, small particles fall through the holes while larger materials transit along the drum. The drum may be in two parts, with the second part having larger holes than the first part, allowing two-stage separation according to particle size. The drum may be internally fitted with ridges and spikes to facilitate bag-opening and moderate throughput. The drum is rolled relatively slowly, e.g., 10–15 rpm, to ensure close contact between the waste to be sorted and the drum and its apertures. A rolling motion of waste ensues either as a cascading motion or as cataracting whereby the waste free falls across the drum. Cataracting is preferred for materials containing food waste as it is most likely to ensure the break up of clumps. Such tromelling of waste has a highly abrasive effect both through collision with the drum interior and the movement of organic material against inert material in the waste stream. Excess rotational speed of the drum will result in centrifuging, resulting in little or no waste separation (Vesilind et al., 2002). Drum screens used to separate relatively wet organic material such as food waste from other, inert, materials risk adherence and clogging of the drum holes. Therefore, they should be equipped with a hole-clearing



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  169

Mixed waste Fine screen Coarse screen

Rotation

Recovered fines including food waste

Coarse fraction

Oversize

Fig. 5.   Principles of a drum or trommel screen.

mechanism such as a rotary brush positioned at the top side of the drum. Drum screening can also be combined with air separation as light materials, such as paper and film plastics, are most easily air-sifted through cataracting movement. A two-stage drum screen (trommel) is shown diagrammatically in Fig. 5.

4.1.2.3. Pressing Food waste may be separated into fine particle material and rejects by actively pressing material to be sorted against a separation sieve or grid. In these cases, the material is not actively tumbled as with drum screening but is slowly forced either by rotational compression of a screw against a sieve (screw press) or by direct in-line compaction (hydraulic). These configurations are shown in Figs. 6 and 7. In both cases, presses function best if preceded by a bag opener or other device to remove large material, particularly solid and plastic film, which may blank filter screens. Conversely, prescreening equipment should not be so vigorous, e.g., high-speed shredding, so as to create large numbers of small particles that risk transiting the filter screen. Screw presses are operated continuously while hydraulic compaction operates in batch mode. In some cases, hydraulic compaction may be combined with a downstream screw press to remove undesirable material able to pass through the hydraulic press plate.

170  R. A. K. Szmidt Partially sorted or pre-screened mixed waste Slow-rotating screw-press within a filter basket

Solid residual waste

Recovered liquids and liquified food waste

Fig. 6.   Schematic of a screw press for separation of liquids from food waste. Partially sorted or pre-screened mixed waste

Input flap

Ram compression when waste is present

Hydraulic ram

Output flap Solid fraction

Liquid fraction

Fig. 7.   Schematic of a batch hydraulic food waste press.

4.1.2.4.  Air separation Air separation is usually employed as a primary separation method of inert material from food waste. It is typically used as an adjunct to other in-line separators such as screens and used to remove lightweight materials such as paper and plastic film. They are only likely to be successful if



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  171

the material to be separated is relatively dry and without adhesion films of moisture. Conversely, air separation is unlikely to succeed if the predominant food waste is relatively dry and low density, e.g., breakfast cereal flakes.

4.2.  Manual sorting Although mechanical depackaging and waste-sorting equipment are available globally, manual sorting may still be employed in some situations. This approach for putrescible food waste may be cost-effective depending on labour and energy costs but it is problematic, particularly in terms of the following: · · · · · ·

throughput rate, hygiene and biohazards for operators, mechanical hazards and operator safety, shift operations and timing of attendance, lack of odour containment, and maintenance and cleaning, e.g., of conveyors.

Apart from the most elementary and small-scale operations, manual sorting of food waste is likely to suffer mechanical problems of maintenance and breakdown, most commonly due to conveyor system operation when handling putrescible and semiliquid materials. Simple manual sorting of food waste may be combined with basic mechanical systems such as overband magnets for ferrous metals’ removal.

5.  Thermal Pretreatment and Hygienisation Since food waste is generally putrescible and highly biodegradable, it represents a significant potential biohazard to humans, livestock, and wild animals. It may also contain plant pathogens and other undesirable microbes (Gajdos, 1998; Böhm, 2002; Insam et al., 2002; Smith et al., 2004). In Europe, the potential risks of disseminating pathogens via organic waste were highlighted by the economic consequences of an outbreak of Foot-and-Mouth Disease (FMD), particularly in the UK in 2001. Knight-Jones and Rushton (2013) considered the potential impact of FMD alone to be colossal. With this background, European rules were established to govern the treatment of animal by-products (ABPs) and

172  R. A. K. Szmidt

biowastes, including ABPs. This includes handling and pasteurisation requirements: commonly known as the Animal By-Product Regulations (ABPR) (Anon, 2009). Under this regime, waste-derived organic products must not exceed specific threshold values for microbial pathogens. They must be treated in ways that reflect the level of risk and the nature and source of input material. In some cases, commercial operators may achieve the required standard by managing the AD process, e.g., in digester thermophilic treatment or post-AD pasteurisation. However, a preferred approach is to pretreat AD feedstock to the standard needed in most cases. Pretreatment has the advantages of ensuring biosecurity at an early stage, reducing operator risk, and providing flexibility in the downstream AD process. Thermal pretreatment of AD feedstock to attain regulatory hygiene standards also provides an opportunity to influence the subsequent AD process. From the perspective of food waste AD, pasteurisation standards applied most commonly worldwide are based on the European standard (Anon, 2009), summarised as follows: · treat low-risk ABPs at 70°C for 1 hour with a maximum particle size of 20 minutes before AD. Other treatment strategies have also been proposed (Thwaites et al., 2013). In some countries outside Europe, only end-product hygiene requirements may be adopted. This includes the USA, where the EPA part 503 (biosolids rule) applies (Anon, 1994; Salter and Cuyler, 2003). Nonetheless, such objectives are likely to be achieved by consistent application of hygiene and pasteurisation measures, including pretreatment. Although there is a general belief in the industry that thermal pretreatment increases the yield of biogas in any subsequent AD process, this is not necessarily the case. Zhang et al. (2020) found that thermal pretreatment hygienisation, broadly in line with the requirements stated above, did not generally increase biogas yield. Bong et al. (2018) observed that thermal pretreatment at higher temperatures (>120°C) increased solubilisation of feedstock, but not necessarily biodegradation, and therefore considered it appropriate as a pretreatment suitable, particularly, for material with a high cellulosic content.



Pretreatment Equipment and Food Waste Preparation for Anaerobic Digestion  173

In reviewing a range of pretreatment options, Jain et al. (2015) revealed that low-temperature pretreatment results are not consistent. In most cases considered by these authors, biogas yield was enhanced by pretreatment at c. 170oC may result in a reduction in biogas production. The combined effect of temperature and pressure was not clear in this review. Forster-Carneiro et al. (2012) considered that for activated sludge digestion, temperature in the range from 160°C to 180°C and treatment times from 30 min to 60 min were optimal, with best results in the range 120–180°C and hydraulic retention time 0–30 minutes. Pressure at these temperatures was not accurately specified, varying from no set pressure to 4 bar (4000 kPa). In reviewing the literature, these authors considered that temperatures in the range 70–121°C typically led to a 20–30% increase in biogas production while treatments at 160–180°C led to a 40–100% biogas production increase. They concluded that pretreatment at 160– 180°C was thus most efficient to enhance sludge AD. Temperatures above these values tend to depress biogas yield due to the formation of inhibitory xylose and lignin breakdown compounds, including heterocyclic and phenolic compounds (Hendriks and Zeeman 2009). In industrial terms, thermal pretreatment in the lower temperature range described for regulatory compliance is most readily applied in the liquid phase. Examples include various designs of batch treatment of particle-reduced food waste “soups” using closed vessels with steam or hot water jacket heating. Such batch heaters are typically in the range 2–25 m3 with larger installations installing an array (Fig. 8). Higher temperatures and pressurised systems typically require a purpose-built pressure vessel, generally with steam injection to achieve a thermal pressure process similar to autoclaving, also referred to as thermo-pressure hydrolysis (TPH). Industrial-scale TPH vessels may be relatively large (>50 m3) and require to be installed with a comprehensive suite of monitoring, control, and safety devices (Fig. 9) (Montgomery and Bochman, 2014). Thermal treatment may have other beneficial effects such as agglomeration of soft plastics during thermal pressure treatment, facilitating subsequent screening and waste separation. Yin et al. (2020) showed that thermal pretreatment (70oC for 1 hour) of AD feedstock contaminated with antibiotics resulted in a significant reduction in antibiotic levels

174  R. A. K. Szmidt

Valved material input with particle size reduction

Treatment vessel with stirrer

Control loop

Heat circuit

Valved output after timed treatment

Fig. 8.   Basis of batch heat treatment of food waste 200°C)

Average width 15 ± 8 nm

40%

Diameter: 8–20 nm

76.5%

–48.4 ± 1.3 mV

Sai Prasanna and Mitra (2020)

Onset decomposition temperature close to 227°C

—

Zhang et al. (2016)

Onset decomposition temperature of 277°C

—

Zhang et al. (2016)

368  D. Pradhan et al.

Table 1.   Characteristics of CNC produced from different food industry wastes.



Garlic skin

Sulphuric acid hydrolysis + sonication

58–96 nm

63%

—

Prasad Reddy and Rhim (2014)

Lychee peel

Sulphuric acid hydrolysis

5–10 nm

—

TGA temperature: 380–500°C

—

Thulasisingh et al. (2021)

Orange peel

Sulphuric acid hydrolysis

10–20 nm

—

TGA temperature: 380–490°C

—

Thulasisingh et al. (2021)

Cocoa pod husk Sulphuric acid hydrolysis + sonication

Average diameter 10–60 nm

67.6%

Maximum degradation temperature of 332°C

—

Akinjokun et al. (2021)

Maximum degradation temperature of 310°C

—

Evans et al. (2019)

Sugarcane bagasse

Sulphuric acid hydrolysis + sonication

38 nm

76.89%

Apple pomace

Sulphuric acid hydrolysis + sonication

20.65 ± 3.66 nm

78%

Pineapple peel

Sulphuric acid hydrolysis + sonication

Average diameter 15 ± 5 nm

61.19%

Main degradation between 150 and 300°C

–30.40 ± 5.25 mV

Melikoğlu et al. (2019)

Maximum degradation temperature of 370°C

–36.7 ± 0.2 mV

Dai et al. (2018)

Nanocellulose from Food Industry Waste  369

Lower thermal stability than cellulose microfibre and native fibre

Food waste

CNF Production method

Lignocellulosic Enzymatic hydrolysis biomass of lemongrass Cassava root Chemical treatment bagasse and peelings

Particle size 105.7 nm

2.4–5.4 nm (peelings) 2.3–3.2 nm (bagasse)

Banana peel

Chemical treatment 2.89–4.65 nm (alkaline treatment and acid hydrolysis) and mechanical treatment (highpressure homogeniser)

Culinary banana peel

High-intensity ultrasonication combined with chemical treatment

20–35 nm

Crystallinity index

Thermal properties

Low and diffuse peaks Thermal stability at 15.2°, a sharp peak is slightly at 20.8°, and broad lower, compared diffraction peak at 27° to cellulose  46% (bagasse) and 56.3% (peelings)

—

 63.1–66.4%

—

30.50–63.64%

Initial degradation temperature 260.81– 295.33°C

Zeta potential −22.4 mV

−69 to −48 mV (peelings) −91 to −51 mV (bagasse)

Refs. Kumari et al. (2019)

Leite et al. (2017)

−37.60 to −67.37 Tibolla et al. mV (2018)

—

Khawas and Deka (2016)

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Table 2.   Characteristics of CNF produced from different food industry wastes.



Banana peel

Enzymatic treatment

3.7–8.8 nm

61.5–66.2%

—

−29.1 to −31.5 mV

Cassava peel

Chemical and physical treatment

5−16 nm

46.67–53.47%

—

−30 mV

Czaikoski et al. (2020)

Bagasse

Enzyme-assisted 23.58–25.59 nm mechanical grinding

—

Liu et al., (2018)

Banana peels

Enzymatic hydrolysis and mechanical treatment

3.3–3.5 nm

Banana peel and bract

Ball milling assisted ultrasonication method

73 nm (banana peel) and 89 nm (banana bract)

Sugarcane bagasse waste

Water-based steam explosion and high-pressure homogenisation

Cassava peels

Mechanical disruption (homogenisation and ultrasonication)

45.95–61.85%

51.0–66.7%

Tonset and Tmax respectively 314.6–327.3°C and 350.4–361.2°C —

Costa et al. (2018)

—

Harini et al. (2018)

3–7 nm

4.27–81.42%

Thermal degradation temperature 313.92–326.34°C

—

Hongrattanavichit and Aht-Ong, (2020)

8.2–6.7 nm

—

Maximum degradation rate temperature 338.2°C

—

Widiarto et al. (2019)

Nanocellulose from Food Industry Waste  371

77.2% (banana peel) Higher thermal and 75.8% (banana stability than bract) native microcellulose

−24.3 to −55.5 mV

Tibolla et al. (2019)

372  D. Pradhan et al.

3.1. Cellulose nanocrystals (CNCs) from food waste 3.1.1. Preparation techniques of CNCs CNCs can be produced from food waste using various methods, including acid hydrolysis and oxidation, where the amorphous portion of the cellulose fibres is dissolved and the crystalline portion is retained. However, the cellulose fibres must be separated from food industry wastes prior to producing CNCs. Therefore, the production of CNCs involves several operations: (1) purification, (2) drying/grinding/dewaxing, (3) delignification, (4) bleaching, and (5) filtration/washing/drying (Mali and Sherje, 2021). Acid hydrolysis is a well-established method for producing CNCs from food industry wastes. In this method, acid molecules are diffused into the cellulose fibres cleaving the glycosidic bonds. Following the acid hydrolysis process, centrifugation, dialysis, and ultrasonication are generally carried out to obtain CNCs (Mariano et al., 2014). To obtain a high yield, hydrolysis is usually performed using strong acids, such as hydrochloric acid or sulphuric acid under a controlled reaction temperature, hydrolysis time, and acid concentration (Ng et al., 2021). However, the utilisation of organic acids or weak acids, such as maleic acid, oxalic acid, acetic acid, and phosphotungstic acid, for CNC production has also been reported (Long et al., 2021). Nevertheless, sulphuric acid hydrolysis has been widely investigated for CNC preparation and seems to be the most efficient method (Mariano et al., 2014). Most studies related to food waste have been carried out using specific food crops. Gupta et al. (2021) carried out acid hydrolysis of amla or Indian gooseberry (Phyllanthus emblica) pomace-derived cellulose fibres (APCF) for 40 min at 50°C using 64% sulphuric acid and a fibre to acid solution ratio of 1:20 obtained a 45% CNCs yield. Although the sulphuric acid hydrolysis method produces a high yield of CNCs, it has various drawbacks, including the probable degradation of cellulose, a reduction in crystal’s heat resistance, high corrosivity, and environmental pollution (Long et al., 2021). Ammonium persulphate (APS) oxidation has also been investigated as a novel technique to produce CNCs from food industry wastes. Due to its low toxicity and high solubility in water, APS has been recognised as a viable candidate to produce H2O2 and SO42− free radicals at an acidic medium and a high operating temperature, which are effective in solubilising the amorphous portion of cellulose and lignin content (Ng et al.,



Nanocellulose from Food Industry Waste  373

2021). Zhang et al. (2016) used a one-step APS oxidation method to isolate CNCs from bleached sugarcane bagasse pulp (BSBP). The reported method involved the hydrolysis of pulp using different concentrations of APS (1, 1.5, and 2 mol/L) at 60°C for 16 h, followed by washing, centrifugation, and lyophilisation. They observed a decrease in the yield of CNCs with the increase in APS concentration. A maximum CNC yield of 53.5% was obtained using 1 mol/L APS solution; however, the yield reduced to 41.7% and 26.6% when 1.5 mol/L and 2 mol/L APS were used. These results indicated that the yield of CNCs using the APS oxidation method is dependent on the raw material, APS concentration, and other processing conditions, such as temperature and processing time. However, the long processing time is a major disadvantage of this method, limiting its utilisation in the industrial-scale production of CNCs. TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) oxidation is another method for producing CNCs from food industry wastes. The main goal of utilising TEMPO is to reduce the energy required for mechanical disintegration. It is achieved by diminishing the negative or positive charge on the surfaces of fibre and improving the colloidal suspension’s stability of the produced CNCs (Rana et al., 2021). Zhang et al. (2020) obtained a CNC yield of 52.01% using TEMPO hydrolysis of lemon seeds using a TEMPO/ NaBr/NaClO system in an aqueous medium at pH 10, followed by centrifugation, dialysis, and sonication. Although a high yield of CNCs can be obtained using this method, it has several drawbacks, such as limited oxidation position, tedious steps, and toxic reagents (Zhang et al., 2020).

3.1.2. Properties of CNC 3.1.2.1. Morphology The crucial parameters of CNC morphology are fibrillation, smoothness, and size. The morphological properties of CNC are primarily investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) (Rajinipriya et al., 2018). In a study, Evans et al. (2019) used TEM to examine the morphological characteristics of the CNCs derived from chemically purified cellulose of sugarcane bagasse. The obtained CNCs were spherical (Fig. 3(a)). In addition, there was a small amount of agglomeration due to the surface ionic charge that caused the crystallites to stack together because of acid hydrolysis. Similarly, Akinjokun et al. (2021) used TEM

374  D. Pradhan et al.

(a)

(b)

(c)

Fig. 3.  (a) TEM image of sugarcane bagasse-derived cellulose nanocrystals (Evans et al., 2019), (b) TEM image, and (c) SEM image of cellulose nanocrystals produced from cocoa pod husk (Akinjokun et al., 2021).

and SEM to analyse the morphological characteristics of CNCs obtained from cocoa pod husk using the acid hydrolysis method. Rod-like CNCs are noticed in the TEM image (Fig. 3(b)). Furthermore, the cellulose’s amorphous fraction was degraded during the acid hydrolysis process, while the fibre bundles were broken into individual cells, as seen in the SEM image (Fig. 3(c)).

3.1.2.2. Chemical properties The source material and production method influence the chemical structure of CNCs, which can be characterised using Fourier transform infrared spectroscopy (FTIR). In a study, Zhang et al. (2020) carried out FTIR analysis of CNCs prepared from lemon seeds using sulphuric acid hydrolysis, APS oxidation, and TEMPO oxidation methods. FTIR spectra of the CNCs were identical to that of lemon seed cellulose, indicating that the original chemical structure of cellulose was not destroyed during production. However, in the spectrum of CNCs obtained using TEMPO oxidation, the peak at 1600 cm−1 became stronger and sharper, primarily because of the introduction of carboxylic groups into the primary alcohol groups on cellulose’s surface after TEMPO oxidation. Furthermore, CNCs obtained using the APS oxidation method showed a new peak at 1742 cm−1, indicating the oxidation of hydroxyl groups into carboxylic acid groups.

3.1.2.3.  Thermal properties The thermal characteristics of CNCs can be determined using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA).



Nanocellulose from Food Industry Waste  375

In TGA, the thermal stability of a material is evaluated by recording the weight change with respect to temperature under constant heating rate or time under isothermal conditions in a controlled environment (Perera et al., 2021). DSC measures the temperature and heat flow associated with transitions in materials as a function of temperature. These thermal parameters are useful in obtaining quantitative and qualitative information on physical changes, including changes in heat capacity or endothermic/ exothermic processes (Thulasisingh et al., 2021). Coelho et al. (2020) used TGA to investigate the thermal properties of CNCs obtained from grape pomace using the sulphuric acid hydrolysis method. The initial thermal degradation and highest degradation temperature of CNCs were lower than that of fresh pomace and cellulose. TGA results also showed the lower thermal stability of CNCs (obtained using acid hydrolysis) when compared to cellulose obtained from sugarcane bagasse (Evans et al., 2019), wheat bran (Xiao et al., 2019), apple pomace (Melikoğlu et al., 2019), and amla pomace (Gupta et al., 2021). The significant decrease in molecular weight and disintegration of the heavily sulphated amorphous portions of CNCs during acid hydrolysis might be the reason for the lower thermal stability of CNCs (Dai et al., 2018; Melikoğlu et al., 2019).

3.1.2.4. Crystallinity Changes in the crystalline structure of the CNCs during the entire production process can be investigated using the X-ray diffraction (XRD) technique. Due to the removal of pectin, hemicellulose, lignin, or any other non-cellulosic components during production, the crystallinity of CNCs is generally higher than that of the source material (Rajinipriya et al., 2018). The high crystallinity of CNCs is a critical factor that could play a major role in defining their mechanical and thermal characteristics as well as their potential for reinforcing in nanocomposite applications (Gupta et al., 2021). Coelho et al. (2018) reported that the crystallinity of CNCs obtained from grape pomace using sulphuric acid hydrolysis increased with an increase in the hydrolysis time. The crystallinity index of CNCs obtained using 30 minutes of hydrolysis with 10 minutes of ultrasonication was 70.62%; however, the crystallinity index increased to 74.89% using a 60-minute hydrolysis, followed by 10 minutes ultrasonication. These results indicated that the crystallinity of CNCs depends on the source material, production techniques, and the process factors involved in their production.

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3.1.2.5. Particle size and zeta potential The particle size of CNCs can be determined in spherical approximation by using Dynamic Light Scattering (DLS), which mathematically considers the CNCs as spheres moving with Brownian motion irrespective of their actual physical morphology (Zhang et al., 2020). In addition, the size of CNCs can be measured using TEM and AFM images. As per the study of Gupta et al. (2021), the average diameter of CNC obtained from APCF using an AFM image was 23.45 nm; however, the average diameter calculated from the TEM image was 44.35 nm. Melikoğlu et al. (2019) utilised DLS and AFM to measure the average particle size of CNCs produced using sulphuric acid hydrolysis of apple pomace. The DLS data showed an average particle size of 20.65 ± 3.66 nm, while the AFM images showed a 7.9 ± 1.25 nm diameter and an average length of 28 ± 2.03 nm. These results indicated that the particle size of CNCs is highly dependent on measurement techniques, source material, and production techniques. The zeta potential (ZP) value measured using a DLS instrument is a good indicator of the stability of CNCs’ suspension in the aqueous medium (Sai Prasanna and Mitra, 2020). In general, a colloidal suspension having ZP of more than 30 mV or less than −30 mV is deemed to be stable, where the nanomaterials have enough surface charges to repel one another while preventing agglomeration (Gupta et al., 2021). However, a suspension is more stable when the absolute ZP value is high (Zhang et al., 2020). In a study, the ZP values of CNCs obtained from lemon seeds using sulphuric acid hydrolysis, APS oxidation, and TEMPO oxidation method were −40.27 mV, −31.27 mV, and −55.67 mV, respectively (Zhang et al., 2020). The ZP of less than −30 mV might be because of the formation of negatively charged SO₃ˉ groups during acid hydrolysis. The negative charges found on the CNC’s surface are due to electrostatic repulsive forces, which enhance the potentiality of CNCs to disperse more freely in suspensions (Sai Prasanna and Mitra, 2020).

3.2. Cellulose nanofibres (CNFs) from food waste 3.2.1. Preparation techniques of CNFs CNFs are fibrous, slender 3−100 nm nanofibres that can be isolated from the cell wall of the food waste materials using vigorous mechanical disintegration, biological pretreatments, and chemical pretreatments or combination with each other, as depicted in detail in Fig. 4 (Ma et al., 2020).



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Fig. 4.   Overview of preparation techniques of cellulose nanofibres (CNF).

During the mechanical disintegration of CNF, dry cellulose pumps can be utilised for the production of CNF; however, the product has poor mechanical properties. The most commonly used methods for CNF extraction are refining, homogenisation, and grinding (Nechyporchuk et al., 2016). For the homogenisation process, two types of equipment are mainly used, which are homogenisers and microfluidisers. The cellulose fibres are exposed to shear and impact forces, leading to the nanofibrillation of the cellulose fibres (Rol et al., 2019). During the refining technique, the cellulose fibre walls are swelled and peeled in an aqueous

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medium. Due to the increase in the surface and the volume of the extracted CNF, they are more suitable for chemical and biological pretreatments. Disk refiners, PFI mills, and valley beaters are the most commonly used devices for the refining technique (Nechyporchuk et al., 2016). During the grinding technique of the CNF extraction, the fibres pass through static and rotating grinding disks of the super mass collider grinders to produce CNF of a diameter of 20–90 nm (Nechyporchuk et al., 2016). The chemical and enzymatic pretreatment methods can produce CNF with functional groups and require less energy than mechanical methods. In addition, during enzymatic and biological treatments, the cellulose fibres are weakened, leading to better nanofibrillation (Rol et al., 2019). During enzyme hydrolysis, the enzymes are used as catalytic agents to enhance the hydrolysis of cellulose to improve the fibrillation process (Nechyporchuk et al., 2016). In a study, Perzon et al. (2020) prepared CNF from sugar beet using the enzymatic pretreatment method. For this treatment, the following enzymes were added to the sugar beet pulp: beta-glucanase (endo-1,3(4)β-glucanase), polygalacturonase (endo-1,4-α-galacturonidase), endo-xylanase (endo-1,4-β-xylanase), cellulase (endo-1,4-β-glucanase), and alpha-amylase (endo-1,4-α-glucanase). During chemical pretreatment, the structure of cellulose is modified based on the different chemical modification methods. Carboxylation via TEMPO-mediation, carboxylation via periodate–chlorite oxidation, sulphonation, carboxymethylation, phosphorylation, sulphoethylation, cationisation, periodate oxidation, quaternisation, and ozonation are some of the chemical treatment methods used to produce CNFs (Nechyporchuk et al., 2016; Rol et al., 2019). Leite et al. (2017) prepared CNF from cassava root bagasse and peelings using the chemical pretreatment technique. First, the bagasse and peeling samples were alkali-treated for 14 hours at 25°C with KOH solution. To remove the metallic ions, a Q-chelating treatment with EDTA was done at 70°C for 1 hour. At 90°C for 3 hours, hydrogen peroxide, NaOH, diethylenetriaminepentaacetic acid, and MgSO4 were used in a bleaching stage. To improve fibre delignification, the residual solid was given a second alkaline treatment with KOH solution. Furthermore, combined treatments of the different methods have been carried out to prepare CNF from food waste. Tibolla et al. (2018) isolated CNF from banana peels by chemical and mechanical treatments. The first step was to carry out the chemical treatment by alkaline treatment for 14 hours; this was followed by bleaching at 70°C for 1 hour and



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acid hydrolysis at 80°C for 1 hour. The obtained nanofibre suspensions were mechanically treated with high-pressure homogenising.

3.2.2. Properties of CNFs 3.2.2.1. Morphology Analysing the morphology of the extracted CNF is a crucial process since the dimensions and the characteristics of the CNF depend upon the extracted source and the production method (Kumari et al., 2019). The morphology of CNF is generally assessed using TEM, SEM, and AFM (Tibolla et al., 2018). When using different microscopy techniques and different sample preparation methods for analysis, slightly different observations can be produced (Nechyporchuk et al., 2016). Tibolla et al. (2018) compared the CNF images of SEM, TEM, and AFM; these images revealed morphological changes during the chemical treatment steps of banana peel extracted CNF. Furthermore, these CNFs also showed a web-like network with entangled cellulosic filaments. Widiarto et al. (2019) prepared CNF extracted from cassava peel using two different procedures. Procedure 1 used mechanical disintegration by homogenisation and ultrasonication, while procedure 2 used acid hydrolysis. Morphological studies of the CNF were conducted using SEM and TEM. According to the SEM images, the raw cassava peal had more than 60% starch constituent. CNF extracted from procedure 1 had a regular arrangement of cellulose fibres where a high degree of crystallinity was observed. In the samples extracted from procedure 2, a less visible structure was observed. The TEM images were like a web-like structure (with procedure 1 isolated CNF) while it showed a needle-like configuration (for procedure 2 extracted CNF) as shown in Fig. 5.

3.2.2.2. Chemical properties The chemical properties of the CNF are determined by using FTIR (Kumari et al., 2019). FTIR allows for the analysis of the changes in the chemical structure of the CNF during and after treatments. Costa et al. (2018) extracted CNF from banana peels using ultrasound and a high-pressure homogeniser. The freeze-dried aqueous suspension of CNF was used for the assessment of the changes in the chemical structure caused by vibrations or stretch during mechanical treatment from FTIR.

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(a)

(c)

(b)

(d)

(e)

Fig. 5.   SEM images of (a) raw cassava peel, (b) CNF procedure 1, (c) CNF procedure 2, TEM images of (d) CNF procedure 1 and (e) CNF procedure 2 (Widiarto et al., 2019).

Changes in intramolecular hydrogen bonding of cellulose II are represented by the band at 3337 cm–1. The peak at 1102 cm–1 confirmed the transition from cellulose I to II. The presence of aromatic rings and conjugated carbonyl groups in the polyphenolic groups of the lignin structure was confirmed by the absorption band of about 1609 cm–1 that was unaffected by mechanical treatments.

3.2.2.3. Thermal properties The thermal stability of the extracted CNF can be observed using TGA and DSC (Kumari et al., 2019). Costa et al. (2018) extracted CNF from the banana peel and bract by the ball milling-assisted ultrasonication method. The thermal stability of the samples was evaluated using both TGA and DSC. During TGA analysis, a major weight loss of 66.42% was observed between 201 and 214°C (for banana brat) and 284 and 368°C (for banana peel), which may attribute to the disintegration of cellulose molecules through pyrolysis. Furthermore, increased thermal stability was observed in CNF when compared to the native microfibres. The final



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residual weight was 8.96% (w/w) for banana brat and 3.26% (w/w) for banana peel. When considering the DSC results, a major endothermic peak with energy absorption of 152.5 J/g (339.5°C) and 107.5 J/g (342°C) for banana peel and banana brat, respectively, was observed. These endothermic peaks were attributed to the degradation of cellulose.

3.2.2.4. Crystallinity The crystallinity of the CNF can be determined by using XRD. The crystallinity index (%) is determined using the Segal method (Kumari et al., 2019). The crystallinity index is proportional to the number of secondary molecular bonds and the degree of compaction in crystalline areas (Tibolla et al., 2018). Czaikoski et al. (2020) isolated CNF from the cassava peel using a combination of chemical (acid hydrolysis or TEMPO-mediated oxidation) and physical method. The crystallinity index for acid hydrolysis (53.47%) samples was higher than the TEMPO-mediated oxidation (46.67%) treated CNF samples. Acid hydrolysis works on the amorphous fibrils components, lignin, and hemicellulose, aiding their extraction and, as a result, concentrating the crystalline sections of the material. Hongrattanavichit and Aht-Ong (2020) prepared CNF from sugarcane bagasse using a water-based steam explosion and high-pressure homogenisation method. They accessed the crystallinity of their samples at different stages (raw fibrer, microcrystalline fibres, and CNF). The lowest crystallinity index was 49.86% which attributes to the raw fibres with a broad peak at 21–23°. This was a result of the presence of non-cellulose components, such as hemicellulose and lignin. The highest crystallinity index was observed as 81.42% in the microcrystalline fibres, resulting in increased toughness and mechanical properties of the cellulose structure. The crystalline index of CNF is lower than that of the microcrystalline cellulose, which is 72.76%. This was a result of the high pressure of the microfluidiser’s interaction chamber that caused the disordering and dislocating of cellulose packing at the crystal region.

3.2.2.5. Particle size and zeta potential The DLS technique can be used to determine the particle size and the ZP of the CNFs. DLS can determine the hydrodynamic diameter of the CNF suspension. ZP is utilised to determine the dispersion stability of the CNF

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colloidal suspension (Kumari et al., 2019). The nanoparticle suspensions with the high ZP are electrically stable compared to those with low ZP, where the suspension is inclined to aggregate. In a study, Czaikoski et al. (2020) produced CNF from cassava peel by combining acid hydrolysis or TEMPO-mediated oxidation and ultrasonic disintegration. The ZP of the samples was determined using Nano ZS at a detection angle of 173°. The ZP of the samples that underwent acid hydrolysis was −49 mV, whereas the catalytic oxidation pretreated CNF had a ZP of −42 mV, which indicated that both the extracted CNF had electrostatic stability. The nanofibre diameter usually ranges from 2 to 20 nm, and it is a few micrometers long. The diameter of the CNF is usually determined from TEM images. The particle size of CNF depends on the extraction method and the extraction source. The particle size of CNF extracted from banana peel by chemical treatment and mechanical treatment was 2.89–4.65 nm (Tibolla et al., 2018), lignocellulosic biomass of lemongrass from enzymatic hydrolysis was 105.7 nm (Kumari et al., 2019), cassava root bagasse and peelings from chemical treatment were 2.4–5.4 nm (peelings) and 2.3–3.2 nm (bagasse) (Leite et al., 2017), cassava peel from chemical and physical treatment was 5−16 nm (Czaikoski et al., 2020), banana peel and bract from thr ball milling assisted ultrasonication method were 73 nm (banana peel) and 89 nm (banana bract) (Harini et al., 2018), and banana peels from enzymatic hydrolysis and mechanical treatment were 3.3–3.5 nm (Costa et al., 2018) as presented in Table 2.

4.  Potential Applications of Nanocellulose Nanocellulose has proven effective in a wide range of applications due to its desirable and exceptional characteristics, such as high aspect ratio, availability, enhanced mechanical properties, biocompatibility, and availability. Some emerging potential applications of food waste-derived nanocellulose in nanocomposite development, textile industry, environmental solutions, food industry, and biomedical and healthcare industries are discussed in this chapter (Table 3).

4.1. Nanocomposite development Nanocomposites are a blend of two or more materials or phases in which at least one of the materials has a dimension in the nanometre range



Nanocellulose from Food Industry Waste  383 Table 3.   Potential application of food waste-derived nanocellulose.

Food waste

Nanocellulose type

Potential application

Findings of the study

Refs.

Olive oil solid waste

CNC

Wastewater treatment

· CNC serves as selective magnetic absorbents for methylene blue. · CNCs showed an excellent affinity for methylene blue removal from wastewater.

Jodeh et al. (2018)

Lime residues

CNF

Packaging film

· The diameter of the CNF obtained is between 5 and 28 nm. · CNF application as edible packaging films.

Jongaroontaprangsee et al. (2018)

Sugar bagasse

CNC

Drug delivery

· Addition of CNW to starch Mauricio et al. (2015) microparticles caused variations in the release mechanism of Vitamin B12. · CNW and starch functioned as a drug release regulating factor.

Pineapple leaf fibres

CNF

Tissue implants, · Effectively strengthened the Cherian et al. (2011) such as vascular polyurethane. grafts and heart · At 5% concentration, the valves stiffness and strength of the composite were enhanced by 2600% and 300%, respectively.

Palm oil waste

CNC

Coating membranes, food and agricultural packaging, and automotive applications

· Composite's tensile strength Haafiz et al. (2016) increased by 61%. · PLA composition changed significantly. · Young's modulus increased. · Elongation at break decreases.

Sugar palm

CNC

Food packaging

· The incorporation of 22.8–41.84% CNCs increased the crystallinity of the films.

Turmeric waste

CNF

Bionanocomposite · Increase in tensile strength Gopi et al. (2019) with and Young's modulus. antibacterial activity

Ilyas et al. (2018)

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(1–100 nm) (Kargarzadeh et al., 2017). Nanocellulose has been applied in the development of nanocomposites with polymers, such as poly(3hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) (Yu et al., 2012), polyvinyl alcohol (PVA) (George et al., 2010), polylactide acid (PLA) (Fortunati et al., 2012), and many others. They achieve functions such as improved biodegradability, excellent oxygen barrier, enhanced mechanical properties, and several other beneficial characteristics which are not present in their pure states. Leal et al. (2019) developed a CNC composite film by combining starch, poly(butylene adipate-co-terephthalate) (PBAT), glycerol, annatto, and citric acid in different concentrations. The composite film (made using all substances) exhibited better mechanical and barrier characteristics, where the maximum stress (MPa) was approximately two times greater and water vapour permeability was 2.5 times smaller than that of the film prepared with only starch, PBAT, and glycerol. Due to good mechanical and barrier properties, the CNC-based composite film was found to be effective in packaging minimally processed mango. In another study, Sánchez-Gutiérrez et al. (2021) extracted CNF from olive tree pruning (a waste product of olive oil production) and incorporated them into PVA to develop a biodegradable packaging film. It significantly improved the optical qualities, tensile strength, thermal stability, barrier characteristics, and vapour permeability of the developed nanocomposite film.

4.2. Textile industry The unique properties of nanocellulose offer diverse possibilities for the textile industry and have led to the development of “smart” clothing with distinct features and benefits (Sawhney, Condon, and Singh, 2007). The applications of nanocellulose in modern fabrics include medical textiles with antimicrobial properties, fire-retardant commercial textiles, military textiles for minimising injuries, accidents, and infections, automotive textiles with self-cleaning, antiallergy, and smart textiles. Nanomaterials can also be used in creating connected clothing with abilities to detect and respond to external stimuli using electrical, colour, and physiological signals (Mishra et al., 2014; Ngô and Van De Voorde, 2014). The ability of a material to withstand heat impact and preserve its qualities, such as strength, toughness, or elasticity, at a particular



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temperature is known as thermal stability. This is an important criterion in textile, which could influence its ability to withstand heat and retard flame. There has been significant research interest in the use of fireretardant nanofillers that can effectively halt the burning process of natural fibre composite (Kovačević et al., 2021). In a study, Mandal and Chakrabarty (2011) extracted nanocellulose from sugarcane bagasse and investigated their thermal properties. The authors reported high thermal stability of the nanocellulose materials, which can be beneficial in the production of flame-retarding textile materials. In addition, flame retardants, such as boric acid, halogens, phosphorus, minerals, and nanometric substances, can be incorporated into nanocellulose composites to enhance their antifire characteristics (Mngomezulu and John, 2017).

4.3. Environmental remediation The rising concern for environmental protection has resulted in the development of innovative forms of bio-based and biodegradable materials for various engineering applications (Balaji et al., 2018; Bassas-Galia et al., 2017). The use of nanocellulose-based composites as flocculants, adsorbents, photocatalysts, and membranes is a recent development in environmental remediation. For example, nanocellulose has been proposed as an adsorbent for residual antibiotics, which are commonly found in wastewater containing medical residue, aquaculture effluent, and industrial discharge (Shak, Pang, and Mah, 2018). A significant amount of dye wastewater is released into the water regularly, posing a serious hazard to the environment and human health. Methylene blue (MB) is a cationic dye that forms a stable solution in water at room temperature (Deng et al., 2012). Due to its high toxicity, it is detrimental to human health above a specific quantity. As a result, it is critical to develop effective and low-cost solutions to remove MB and other dyes from wastewater and refresh the environment. In a study conducted by Jodeh et al. (2018), CNC isolated from olive oil industry’s solid waste was used to extract methylene blue from wastewater. The author reported that the efficiency of MB removal from wastewater was dependent on pH, and the highest efficiency was obtained at a pH of 12. A thermodynamic study also showed spontaneous adsorption of MB from wastewater at various temperatures.

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4.4. Food industry Nanocellulose is highly suitable for modifying the physicochemical, sensory, and nutritional properties of foods. They can be used to stabilise foams or emulsions by adsorbing to air–water or oil–water interfaces, self-assemble in aqueous solutions to create gel networks, and act as fillers or fat replacers (Mu et al., 2019). In addition, nanocellulose has also been explored in the development of reinforced biodegradable materials for food packaging, coatings, and films with advanced functionality, such as antioxidant and antibacterial properties, carbon dioxide emitter, moisture absorbers, ethylene, oxygen scavengers, coatings, and films (Yu et al., 2021). Haafiz et al. (2016) developed a reinforced PLA composite of cellulose nanowhiskers obtained from palm oil waste and suggested the potential application of the nanocomposites in coating membranes, food, and agricultural packaging. Moreover, Alzate-Arbeláez et al. (2019) obtained nanocellulose from the banana rachis (a waste product from banana processing) and combined it with the extract of Andean berries, which is very rich in polyphenol, and investigated the antioxidant properties and thermal stability of the polyphenolic banana rachis nanocomplex. They reported that the nanocomplex suppressed lipid peroxidation in an emulsified oil system, suggesting the ability of the nanocomposite to function as a food antioxidant.

4.5. Biomedical and healthcare industry Nanocellulose has huge potential for a wide range of applications in biomedicine and healthcare industries due to its unique properties, such as superior biocompatibility, high surface area, mechanical strength, nonimmunogenic nature, and ease of surface modification (Ge et al., 2019; Luo et al., 2019; S. Xiao et al., 2015; S. Yu et al., 2021). Chemical functionalisation of nanocellulose can change its characteristics, making it more suitable for various biomedical and health care applications, such as cartilage bone regeneration, wound healing, dental application, nanocellulose-based nanohybrids for tissue engineering, and specific delivery of drugs, protein, plasmids (Leal et al., 2019). Nanocellulose-based particles, nanopaper, and nanofoam have been reported to have unique colloidal properties, a large specific surface area, good rheological properties, non-toxicity, and biodegradability and can be



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utilised as a universal excipient for the delivery of poorly soluble medicines (Löbmann et al., 2017). Drug release at specific locations and a prolonged medication release profile can be produced using a suitable nanocellulose as a matrix material, implying a prospective application for tailored drug development and therapy (Löbmann et al., 2017). In a study by Mauricio et al. (2015), the authors isolated cellulose nanowhiskers (CNWs) from sugar bagasse and developed a microhydrogel composite from the CNWs and starch. Vinyl bonds were introduced to both CNW and starch to facilitate the development of the microhydrogel composite in which CNW served as a covalent cross-linker. They reported that CNW could function as an emulsifying agent, improving both homogeneity and sphericity of the microparticles. The addition of CNW to the starch microparticles caused changes in the release mechanism of the model drug (Vitamin B12), resulting in the reduction of the release pace by 2.9, and the combination of CNW and starch functioned as a drug release regulating factor.

5. Conclusion and Future Prospects Food waste can be converted into high-value products like nanocellulose (CNF and CNC) using various methodologies. Irrespective of the production techniques and processing conditions, nanocellulose has promising characteristics, such as good thermal stability, excellent morphology, good chemical properties, and high crystallinity with desirable zeta potential. Food waste-derived nanocelluloses have been utilised successfully in developing nanocomposites, textiles, environmental remediation, food, biomedical, and healthcare industries. Even if food wastes appear to be a promising source of nanocellulose, future work is required to develop different techniques for the preparation of nanocellulose from different types of food industry wastes using and improving their characteristics for specific applications. In addition, waste generated from most food processing industries is generally a mixture of different food products. Therefore, future work is also required to produce nanocellulose from a mixture of different food wastes and analyse their characteristics. The proportion of different food waste in a mixture could be optimised to obtain nanocellulose with desirable characteristics. For preparing CNC, methods such as APS oxidation and TEMPO oxidation could be explored using a wider range of food industry wastes, and their properties could be analysed. Furthermore, the feasibility of food waste-derived nanocellulose could be investigated for additional applications, such as construction, electronics,

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water purification, and many others. Regardless, the food industry wastes have a great potential to be a promising source of nanocellulose alongside wood and agricultural residues due to their low cost and availability.

Acknowledgement The authors would like to acknowledge the funding from Technological University Dublin under the Researcher Award 2020 and 2021.

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© 2023 World Scientific Publishing Europe Ltd. https://doi.org/10.1142/9781800612891_0015

Chapter 15

The Potential Use of Food Waste in Biocementation Process for Eco-Efficient Construction Materials Wilson Mwandira*,‡, Maria Mavroulidou*,§, Michael Gunn*,¶, Hemda Garelick†,‖, and Diane Purchase†,** *

London South Bank University, London, UK

Department of Natural Science, Faculty of Science and Technology, Middlesex University, London, UK

†

[email protected] [email protected] ¶[email protected] ‖[email protected] **[email protected] ‡

§

Abstract Biocement is emerging as a novel and sustainable alternative to current conventional construction materials because the microorganisms used are renewable, environmentally friendly, and safe. However, its implementation is hampered by the prohibitive cost associated with the raw materials required for possible industrial applications. Thus,

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in this chapter, we examine recent claims that introducing food waste usage into the biocementation process would lead to reduced costs and increased efficiency. The huge quantity of worldwide food waste generation is both an important resource and an environmental burden if not properly managed. Therefore, the motivation to use food waste is both environmental and economical as it makes the biocementation process more cost-effective, sustainable, and easier to implement. This chapter reviews recent literature on the use of food waste in biocement technology and examines other food waste yet to be investigated as raw material for culture media, cementing agent, biopolymer, and enzyme to generate biocement economically and sustainably. Keywords: food waste; biocementation processes; growth media; construction industry

1. Introduction A large proportion of the world’s population is living in urban areas and this trend is forecast to increase to 68% by the year 2050 (UNDESA, 2018). Both the increase in population and the shift to the urban area present a challenge to infrastructure development. Concrete, cement mortar, and burnt clay brick are used to build housing units and support infrastructure, but these building materials emit significant carbon dioxide (CO2) in their manufacturing process and are a substantial contributor to the global carbon footprint (Miller et al., 2018). Cement is the source of about 8% of the world’s CO2 emissions. In 2019, cement production was the only CO2 emission source that had an increase of 5.1% and this trend is projected to continue (Olivier and Peters, 2020). It is in this light that many researchers have embarked on developing civil engineering technology that has a low carbon footprint to preserve and protect the environment from further degradation. One of the techniques drawing international attention has been biocementation, using microorganisms to produce biominerals acting as cementing agents (biocement). For instance, the most widely explored biocementation process, Microbially Induced Calcite Precipitation (MICP), uses microorganisms to precipitate calcite. Biocement is one of the most researched alternatives to conventional construction materials, such as ordinary portland cement, and can also be used to improve soils encountered in construction (i.e., as a foundation for infrastructure or construction materials) and repair concrete or heritage



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structures (Charpe et al., 2019; Ivanov, 2020). Biocement used for construction purposes is produced at near room temperature hence avoiding high temperature and energy consumption (Tang et al., 2020); this prevents emissions of greenhouse gases. This technology has been proposed in the construction industry for the production of biobricks (Alonso et al., 2018), healing concrete (Pungrasmi et al., 2019), and reducing the porosity of concrete by pore clogging (Badiee et al., 2021) thus improving its durability. In geotechnical applications, biocementation has been proposed for the prevention of liquefaction (Riveros and Sadrekarimi, 2020), surface soil erosion, and to stabilise soils by increasing their strength and stiffness (Moravej et al., 2018). The adoption of biocementation has thus increased in a number of civil engineering applications and requires further investigation, as such innovative environmentally superior alternatives to OPC globally are critical to the global long-term reduction and mitigation of CO2 emissions from civil infrastructure construction towards sustainable engineering solutions. The challenge currently is to improve biocementation technology to make it applicable in the field (Naveed et al., 2020). This entails replacing the laboratory analytical grade reagents with readily available and low-cost raw materials to make the technology more sustainable. When applied in practice, the cost of these media ranges from 10% to 60% of the total operating costs of the biocementation process making it uneconomic to be implemented as a green technology without further developments (Rajasekar, et al., 2017; Iqbal et al., 2021). As a result of this, despite numerous demonstrations of this technology at a laboratory scale, biocementation is still considered expensive for commercial implementation due to the cost of raw materials (Mujah et al., 2017). Thus, the use of food waste provides a good source of alternative raw materials that could drastically reduce the cost of biocementation implementation and treatment. According to Food and Agriculture Organization (FAO), approximately 17% of food is discarded in the waste bins of households, retailers, restaurants, and other food services and ends up in landfills (FAO, 2021). In this chapter, we critically evaluate recent claims that introducing food waste usage into biocementation technology would lead to reduced costs and increased efficiency. Biocementation raw materials used for various biocementation processes are considered individually to evaluate the possibility of using food waste as a sustainable source of these raw materials.

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1.1. Types of biocementation processes Various types of construction-related biocementation processes have been identified. Their classification is based on the specific pathway and the related mechanisms to achieve biocementation. The major types of construction biocementation processes are shown in Table 1. The four most recognised biocementation processes for use in the civil engineering/construction sector involve urea hydrolysis, denitrification, dissimilatory sulphate reduction, and photosynthesis (Castro-Alonso et al., 2019). After a brief description of each biocementation process and the chemical reactions involved (listed in Table 1), possible food waste that can be used as raw materials according to the process is identified in subsequent subsections. Urea hydrolysis: Urea hydrolysis is a biochemical reaction process comprising a complex chain of reactions involving urea and urease enzyme. Urea is hydrolysed into ammonia and carbonic acid (Equations (1) and (2)) in Table 1. The produced carbonic acid is consequently converted into bicarbonate (Equation (3)). With the assistance of carbonic anhydrase, ammonium and hydroxide are formed due to ammonia hydrolysis (Equation (4)). To be useful for civil engineering material and Table 1.   Types of biocementation processes. Type of biocementation process

Overall pathway

Refs.

Urea hydrolysis

CO(NH2)2 + H2 → NH2COOH + NH3 (1) NH2COOH + H2O → NH3 + H2CO3 (2) H2CO3↔HCO3– + H+ (3) NH3 + 2H2O → 2NH4+ + 2OH– (4) Cell−Ca2+ + CO32− → Cell−CaCO3 (5)

Fujita et al. (2017); Naveed et al. (2020); Ran and Kawasaki (2016)

Denitrification

Ca(CH3COOH)2 + NO3− → CaCO3 + 0.8N2 + 3CO2 +3H2O + OH– (6)

O’Donnell et al. (2017)

Dissimilatory sulphate reduction

6CaSO4 + 4H2O + 6CO2 → CaCO3 + 4H2S + 2S + 11O2 (7)

Castro-Alonso et al. (2019); Peng et al. (2018)

Photosynthesis

Ca2+ + 2HCO3− → CaCO3 + CO2 + H2O (8) Irfan et al. (2019); Ca2+ + HCO3− + OH− → CaCO3 + 2H2O (9) Rajasekar et al. 2HCO3− ↔ CO2 + CO3− + H2O (10) (2017)



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construction, the microbial process requires a cation (Ca2+ or Mg2+). Thus, the presence of a cation and increased pH around the microenvironment of microbe will induce biomineral precipitation (Equations (5)). Denitrification: Biocementation via the denitrification process occurs when the organic matter is oxidised and then this is followed by denitrification. In this process, NO3− is used as an electron acceptor which produces CO2, NO2, OH−, and N2 (Equation (6)). Due to the consumption of H+, an alkaline microenvironment is created which increases the pH, and consequently, biomineral precipitation occurs in the presence of soluble cation. Dissimilatory sulphate reduction: Biocementation via dissimilatory sulphate reduction occurs via sulphate-reducing bacteria (SRB). SRB thrive in anaerobic environments that use sulphate as a terminal electron acceptor and have the ability to promote dissolution, diffusion, and precipitation (Equation (7)). Using these abilities, they are capable of forming biominerals by dissolution, diffusion, and biomineral precipitation. This process occurs when cation ions react with carbon dioxide (CO2) under an alkaline condition due to sulphide removal. Photosynthesis: Biocementation occurs via the photosynthesis process where the produced HCO3− in the photosynthetic process is dissociated into CO2 and OH− by carbonic anhydrase. Due to the production of OH−, the pH increases thus inducing biomineralisation in the presence of a cation (Equation (10)).

2. Food Waste as a Growth Media 2.1. Microbial growth and use of alternative culture media Microorganisms grow in batch cultures in which nutrients are purposefully added to support their growth. Therefore, for each biocementation mechanism described above, biocementation will occur at a different rate, due to the different growth rates and availability of nutrients for the microorganisms involved in the process. To grow and control microorganism activity, a growth medium is first provided. This may be a solid, liquid, or semisolid food containing the nutrients needed to sustain a microorganism. Specifically,

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microorganisms require a source of nutrients (carbon and nitrogen source) and other minor elements for their growth and survival (Lapierre et al., 2020). To replace the expensive laboratory-grade nutrient media, composites with food waste used as a source of one or more of these elements can be considered; in the first instance, food waste can be a source of carbon and nitrogen (Kumar et al., 2020). The different types of food waste that can be used as nutrient media for microbes used in biocementation are discussed briefly in this section.

2.2. Fruit and vegetable waste One-third of fruits and vegetables produced globally go to waste (FAO, 2019). The fact that such a substantial amount of food is produced but not eaten by humans has negative socio-economic and environmental impacts (FAO, 2021). Fruit and vegetable waste that can be investigated for growth media includes edible and inedible parts, such as vegetables, banana peels, shells, as well as scraped portions of vegetables or slurries (Anbu et al., 2017; Kumar et al., 2020). This waste can be collected from homes, supermarkets, canteens, and restaurants. The feasibility of using vegetable waste (e.g., cabbage, long bean, cucumber, and spinach) as an inexpensive nutrient source for biocementation was investigated by Omar et al. (2018) who used the end product of vegetable waste fermentation for the biocementation process. The results showed that the substrate from vegetable waste could replace the role of nutrient media and broth for bacterial growth by using the fermentation process. Despite the positive results in this study, more studies need to be carried out to include other fruit and vegetable waste. Nevertheless, although not widely used in the cultivation of microbes for biocementation, vegetable waste as nutrient media seems feasible and should be investigated further.

2.3. Dairy industry waste With the ever-increasing demand and supply of dairy products, the amount of dairy wastewater is projected to also increase (Shi et al., 2021). This food waste presents a good potential resource as a source of culture solution for the biocementation process. Thus, a number of researchers focused on food waste from the dairy industry that included lactose



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mother liquor (LML) (Achal et al., 2009) and whey (Kahani et al., 2020; Chaparro et al., 2021). Both LML and whey are the watery part of milk that results from industrial cheese-making processes and have significant carbohydrate, protein, and mineral content (Roufou et al., 2021). A study on LML as a growth medium showed that it can be used as a good source of nutrients that can support the growth and urease activity of ureolytic bacteria Sporosarcina pasteurii (NCIM 2477). The study revealed that urease production was 353 U mL−1 in LML medium. When compared to standard media, no significant difference was found in terms of growth and urease production. Thus, LML was suggested as an alternative source for standard media (Achal et al., 2009). Similarly, the studies on whey and powdered whey as a growth medium demonstrated that this waste could be used as a growth medium. Additionally, the study on powdered whey demonstrated the feasibility of cultivation of the bacteria in a non-sterile but sanitised medium to reduce the cost further by eliminating the need for sterilisation (Kahani et al., 2020).

2.4. Brewery industry waste Another source of food waste that can be used for biocementation technology is the brewery industry, whose main waste streams are brewer’s spent grain, hot trub, and brewer’s yeast. The usage of the food waste from the brewery industry is important as the three residues contain proteins and carbohydrates in their cellular structure. These residues have a high carbon content of at least 45.6% which makes them suitable for use as growth media (Mathias et al., 2015). The use of brewer’s yeast as one of the potential growth media has shown good results and amounted to a 96% reduction in the material cost for the biocementation process (Gowthaman et al., 2019). Brewer’s spent grain, a by-product of the brewery, is another alternative that has not been fully investigated. Promising results were obtained using spent grain extract as an alternative growth medium for Streptomyces malaysiensis AMT-3 (Nascimento et al, 2011). Spent grain, hot trub, and brewer’s yeast have not been comprehensively investigated as alternative media to support biocementation despite being known sources of carbon and nitrogen and a source of nutrients. Furthermore, it would be useful to investigate these three materials in unsterilised conditions when culturing microbes for biocementation. This appears to be feasible and would drastically reduce the cost of culturing.

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2.5. Other food wastes Table 2 presents a summary of food waste mentioned earlier that has already been used in biocementation studies as well as suggestions of other food waste yet to be investigated. Based on a number of scientific publications in other research areas, these suggested food wastes showed potential as culturing media formulation for biocementation. In Table 2, the alternative culture media are divided according to the source of raw material of the food waste. First, naturally discarded fruit waste materials, such as pineapple, apple, mango, jack fruit, green and yellow banana, sweet lime, and pomegranate, contain simple and complex sugars that are metabolisable by microorganisms (Anbu et al., 2017; Sagar et al., 2018; Nikseresht et al., 2020). For example, mango pulp waste was evaluated as an alternative culture medium for the production of the bacteria Komagataeibacter xylinus (García-Sánchez et al., 2020). Table 2.   Potential food waste that can be used as growth media for the biocementation process. Food waste

Waste type

Refs.

Vegetable waste

· Vegetables (cabbage, long bean, cucumber, and spinach)

Omar et al. (2018)

Dairy waste

· Lactose mother liquor

Achal et al. (2009)

· Whey

Chaparro et al. (2021); Kahani et al. (2020)

· Corn steep liquor

Fahmi et al. (2018)

· Brewer’s yeasts

Gowthaman et al. (2019)

· Chicken manure

Yoosathaporn et al. (2016)

· Tofu wastewater

Fang et al. (2019)

Brewery waste Other food waste

Suggested food Kumar et al. (2020) · Edible and inedible parts of waste not yet waste fruits and vegetables utilised for Dianursanti et al. (2014) · Food wash water (rice, biocementation cowpeas, peas, chickpeas, soy protein, and mung beans) · Irish, sweet potato, and yam pulp residue

Hayek et al. (2013)

· Residues from sugar cane and Nikseresht et al. (2020) vinasse



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Second, recent studies have suggested that wastewater from various grains, such as legumes (peas, chickpeas, soybeans, cowpeas, lentils, and mung beans) (Chaparro-Acuña et al., 2020) and cereals (corn, barley, and rice) (Nunkaew et al., 2012), could also be used to formulate alternative media. For example, a study by Fang et al. (2019) demonstrated a 27.8% increase in compressive strength using wastewater from the preparation of tofu as a nutrient source. These authors report that tofu wastewater has been proven to be effective in growing microorganisms, which included bacteria, fungi, and yeast. Finally, sugarcane molasses and vinasse, Irish and sweet potatoes, and yam pulp residue can be used as alternative media for culturing. These sources are similar as they are all rich in carbohydrates and are produced in large quantities. The literature shows that the composition of molasses varies depending on the source of the raw material and technology used in processing it. Thus, high total solids of 30–36% sucrose are found and some smaller quantities of carbohydrates, organic acids, proteins, and nitrogen compounds (Eggleston et al., 2017). These components are a major source of carbon and nitrogen; however, they have not been investigated for use as a nutrient source (Nikseresht et al., 2020). In addition, Irish and sweet potato peels were found to be both a rich source of carbohydrates and also certain amino acids, vitamins, minerals, and dietary fibre (Padmaja, 2009; Arapoglou et al., 2010). A study by Hayek et al. (2013) who profiled the nutritional value of sweet potatoes discovered that the tuber had a great potential to replace the expensive media ingredients for Lactobacillus thus lowering costs. Therefore, given the satisfactory results in the above-mentioned studies, these wastes could easily replace conventional culture media also for biocementation technology.

3. Food Waste as a Source of Cementation Media 3.1. Calcium sources from food waste As calcium is the main source of cations used for the formation of biocement, many researchers focused on specific food waste with high calcium content as alternatives for cementation media. These include eggshells (Dayakar et al., 2019; Sugata et al., 2020), oyster and scallop shells (Liang et al., 2019; Gowthaman et al., 2021), and bovine bones (Gowthaman et al., 2021). These wastes have been studied because they contain over 90% of calcium carbonate. To extract calcium ions from the

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shells, vinegar (Sugata et al., 2020) or nitric acid (Liang et al., 2019) has been used. The results from a study by Liang et al. (2019) suggest that the use of oyster shells, scallop shells, and eggshells as a source of soluble calcium extracted in dilute nitric acid indicated that the strength of the biocemented sands when applying different recycled calcium sources ranged from 845.1 to 1454.6 kPa, which is reasonable for construction works. Similar results were found in another study that revealed that using eggshells as a calcium source led to a 74.32% increase in the UCS for expansive soil (Sugata et al., 2020). Recently, a study by Gowthaman et al. (2021) used scallop powder as a calcium source to biocement amorphous peat to enhance its mechanical properties. The results revealed that the UCS of over 100 kPa after 28 days of curing time was around 6.5 times higher than that of the untreated peat. With a large number of restaurants and homes discarding these wastes, their use for biocementation is much more sustainable and cost-effective. Other sources of calcium that could be investigated are bones from fish, cows, chickens, or pigs from food waste. The huge quantity of bone waste generated worldwide is an environmental burden if not properly managed (Adeyemi and Adeyemo, 2007; Lapierre et al., 2020; Abylkhani et al., 2021). As cited, as much as 85% of all salts contained in bone are calcium phosphate and 10% are in the form of calcium carbonate (Supriadi et al., 2021). A recent study by Gowthaman et al. (2021b) investigated the use of bone meal powder as a low-cost source of calcium and phosphate. The results from this study showed that the material cost of the biocementation treatment was reduced by around 14 times compared to the conventional methods. Therefore, waste bones can be used as a calcium source in the biocementation process, however, further investigation is required for possible pilot and field implementations of the concept.

3.2. Incorporation of biopolymers from food waste in biocementation process Biopolymers as a standalone biocementing agent are biodegradable; therefore, many researchers have sought to make composite materials by incorporating MICP and biopolymers to increase their strength and durability (Ashraf et al., 2017). The biopolymers that have been extensively investigated include xanthan gum (Sujatha et al., 2021), gellan gum (Dikshit et al., 2021), and beta-glucan (Soldo et al., 2020). Biopolymers



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are generally derived from food waste and agro-waste. Various types of food waste discussed earlier (diary waste, brewery waste, discarded vegetables and fruits, and bones) can be used as substrates for the production of biopolymers (Ranganathan et al., 2020). Therefore, food waste would be a good source of biopolymers being incorporated into the biocementation process. This would improve the efficiency of the biocementation process and would make the process more sustainable. Sustainability lies in the fact that biopolymers are eco-friendly and biocompatible (Rahman et al., 2021). The incorporation of biopolymers in the biocementation process increases the nucleation sites for bioprecipitation due to the highly charged specific surface of biopolymers that enhances their interaction with fine soil particles (Chang et al., 2015). Studies have used xanthan gum and found that the biopolymer achieved higher strength comparable to cement (Devrani et al., 2021). Another study suggested that the incorporation of biopolymers in biocementation increases biocementation beyond those using the conventional biocementation process (Nawarathna, Nakashima, and Kawasaki, 2018). Others used chitosan which is an amino polysaccharide biopolymer that enhances biocementation (Nawarathna et al., 2019). The results indicated that although even without the inclusion of chitosan CaCO3 can nucleate and grow efficiently, the process could be accelerated by 38% by adding chitosan. A study by Spencer et al. (2020) used jute fibre which is cellulose and biopolymer and this was also reported to enhance the biocementation process. Cellulose can be processed from vegetable and fruit waste (SzymanskaChargot et al., 2017). Other examples of biopolymers might be processed from food waste and used in the biocementation process and these include the following: · Dairy waste from the dairy processing industry can be processed to produce polyhydroxyalkanoates (PHA) and xanthan gum. · Molasses, spent grains, and waste from brewery processes can be processed to produce PHA (Nielsen et al., 2017). · Materials from vegetable washing water, fruit peels, molasses, beet pulp, and distillate; fruit and vegetable processing can be processed to produce cellulose (fibre), PHA, pectin, lignin, and xanthan gum that can be incorporated into the biocementation process. · Food waste from dairy and bovine bones, blood and internal organs, eggshells, and shells of seafood can produce chitin/chitosan and PHA.

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However, the health risks associated with blood and internal organs require further assessment and evaluation.

3.3. Food waste as a source of urease and asparaginase enzymes used for biocementation So far, this chapter has discussed studies that have utilised microorganisms as the source of urease in the biocementation process. However, due to the cost involved in the cultivation of bacteria, enzymes could alternatively be produced from fruit and vegetable food waste, for instance, using various seeds as a source of urease enzyme used in the biocementation. For instance, Dilrukshi et al. (2018) and Imran et al. (2021) used urease extracted from watermelon seeds, a urease-rich food waste, for enzymatically induced calcium carbonate precipitation (EICP). However, not many researchers have taken this route to investigate other food waste but used commercial enzymes for EICP. Therefore, urease-rich agricultural wastes appear to be a more promising source for low-cost urease for biocementation. Additionally, asparaginase enzyme has been found to aid the EICP process for sand biogrout development (Li et al., 2015). Utilisation of food waste, such as squid pen and cooked chicken bone for asparaginase production using E.coli culture, has been reported (Batool et al., 2015). Therefore, the conversion of food waste into valuable biomolecules like enzymes reduces not only the biocementation process expenses but also the risk of environmental pollution from the discarded waste.

4. Advantages and Disadvantages of Using Food Waste in a Biocementation Process 4.1. Advantages · The use of food waste in the biocementation process is economical. Food waste is an untapped resource as well as readily available. Its use would drastically reduce the cost of implementation of the biocementation technology and its adoption thereof. · Second, the use of food waste usage in biocementation technology is sustainable and environmentally friendly by reducing the need for landfilling. It is a known fact that landfilling of food waste contributes to the emission of greenhouse gases as it rots and produces methane



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(Bian, Xin, and Chai, 2019; Mønster, Kjeldsen, and Scheutz, 2019). Methane is a greenhouse gas more potent than carbon dioxide (Saunois et al., 2020). Landfilling is also a problem due to the scarcity of landfill space. · Additionally, food waste can be readily used in the denitrification process for biocementation. This could make denitrification cheaper and more efficient, thus encouraging its use with the advantage of avoiding harmful by-products, such as ammonia, produced using the ureahydrolysis route (Pham et al., 2018). · The diversity of food waste is an opportunity if local materials can be sourced as opposed to transporting in remote areas. · In conclusion, food waste is a diverse resource that can be used for any biocementation process. Different microorganisms use various mechanisms, such as fermentation, urea hydrolysis, denitrification, dissimilatory sulphate reduction, and photosynthesis.

4.2. Disadvantages Potential problems with the future use of food waste in biocementation include the following: · Competing usage of food waste by other technologies. Food waste is used by various industries, such as biodiesel production (Karmee and Lin, 2014), food waste composting for organic fertilisers (Keng et al., 2020), biogas (Caruso et al., 2019), and energy production (Negri et al., 2020). These competing uses may affect the availability and quality of the resource and should be considered. · Microbial processes depend on factors such as type of microbe, temperature, pH, concentrations of electron donors and acceptors, and the concentration of nutrients and metabolites (Haouzi and Courcelles, 2018; Sadasivuni et al., 2020; Mendonça et al., 2021). Due to these factors, the manipulation of microbial growth factors is complex and for one to achieve the desired product from food waste might be cumbersome. · Limited quality control and processing of food waste. Many food wastes are of poor quality and can contain residual herbicides, pesticides, and weedkillers. This would limit their use for ground improvement applications.

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· Food waste unavailability in localised regions and insufficient bulk quantity. This may disadvantage unpopulated areas and it may require a substantial cost to transport materials as food waste is more in urban and peri-urban areas. · The cost of conversion of food waste might be high making it costinefficient (Ma and Liu, 2019). For example, pretreatment is required to convert food waste into biopolymers that are used as cementing agents. The cost of conversion of food waste into the required form for biocementation needs to be thoroughly investigated to pilot scale level as opposed to relying on laboratory results and proof of theory.

5. Future Prospects Biocementation technology is a process that has emerged as an attractive alternative in the construction industry, using microorganisms for biocement, biobricks, soil strengthening, and stabilisation. However, a barrier to its implementation at an industrial scale is the high cost of some essential ingredients required for the biocementation process. The use of food waste in the biocementation process as an alternative source of raw materials has been extensively proposed for various construction materials, but there has been insufficient investigation about selective, efficient, and cost-effective products that can be procured from this waste source. However, utilising food waste for construction via biocementation applications potentially could revolutionise the construction industry, also making it more sustainable and cost-effective. Despite the advantages and prospects of biocementation using food waste, more investigations are needed to progress and refine this technology, into reducing the cost of the biocementation process and minimise drawbacks affecting its commercial applications. At present, scaling up from proof-of-concept research to field-relevant industrial-scale applications is urgently needed and critical. Food waste as a source of raw materials for biocementation offers a sustainable and promising approach towards green building technology.

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© 2023 World Scientific Publishing Europe Ltd. https://doi.org/10.1142/9781800612891_0016

Chapter 16

Production of Biodegradable Fibres from Food Waste through Electrospinning and Their Prospective Medical Applications: An Emerging Method for Combating the COVID-19 Pandemic Md Ariful Haque*,§, Sik Chun Johnny Lo*,¶, Jin-Hua Mou*, Anshu Priya*, Zi-Hao Qin*, Zubeen Jyotiwadan Hathi*, Chrysanthi Pateraki†, Dimitris Ladakis†, Apostolis Koutinas†, Chenyu Du‡, and Carol Sze Ki Lin*,‖ School of Energy and Environment, City University of Hong Kong, Hong Kong, China *

Department of Food Science and Human Nutrition, Agricultural University of Athens, Athens, Greece

†

School of Applied Sciences, University of Huddersfield, Huddersfield, UK

‡

§

[email protected] [email protected] ‖[email protected]

¶

Abstract Food waste valorisation through the manufacture of value-added products has been studied for nearly a decade in many parts of the world. Despite 419

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the prevalence of recent studies, increasing food waste remains a problem. The transformation of food waste into useful products could be harnessed to tackle the coronavirus disease 2019 (COVID-19) pandemic. The current method of extracting nutrients from wasted food and facilitating their utilisation by microbes generates various platform chemicals and fuels, such as succinic acid, lactic acid, ethanol and hydrogen, and polymers, such as homopolymer polyhydroxybutyrate (PHB), copolymer poly(3hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and polylactic acid (PLA). A thorough review of scientific articles and reports on food wastebased biopolymer production and its possible applications in combating the COVID-19 pandemic is presented in this chapter. The key microbes used to produce either biopolymers or their building blocks are Haloferax mediterranei, Cupravidus necator, and Lactobacillus casei Shirota. The biopolymers and components derived from these microbes can be used to produce green, biodegradable, non-woven fabrics. The flexibility and biodegradability of these biopolymers also make them suitable for applications in the medical sector. Through the process of electrospinning, such fabrics can be used to produce biodegradable personal protective equipment (PPE) and, thereby, combat COVID-19 sustainably. The implementation of food waste valorisation helps not only in managing waste and reducing environmental pollution but also in generating resources, such as medical textiles, that can meet long-term sustainable development goals on a large scale. Keywords: biopolymer; electrospinning; microbes; personal protective equipment; valorisation

1. Introduction The production of materials such as biodegradable polymers and fibres has recently garnered a lot of interest. The continuous depletion of finite petroleum resources, coupled with price hikes and detrimental environmental effects, has increased the adoption of practices such as the recycling of waste resources, which are then used as feedstock for the production of environmentally friendly biodegradable polymers. The recycling of approximately 1.3 billion tonnes food waste generated globally (FAO, IFAD, UNICEF et al., 2018) offers both a cheaper and more sustainable source for biodegradable polymer production and a means of tackling the global food waste burden. Such processes facilitate the



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extraction of nutrients and functional molecules from food waste, which can then be used for the production of value-added products. The consequent minimisation of disposal to landfills also reduces the carbon footprint. Biodegradable polymers can be degraded and catabolised to carbon dioxide and water by naturally occurring microorganisms, such as bacteria, fungi, or algae (Shi et al., 2011). Their degradation does not generate any substances that are harmful to the environment (Mecking, 2004). There are two types of biodegradable polymers: those obtained from natural sources and those obtained from synthetic processes. Artificially derived biodegradable polymers have an advantage over naturally occurring polymers, as the former have a wide spectrum of applications, and their mechanical properties, such as flexibility, can be tailor-made for different degradation rate requirements. Despite being biocompatible, naturally occurring polymers have been relatively less explored due to their antigenicity and batch-to-batch variations in productivity (Doppalapudi et al., 2013). However, some naturally occurring biodegradable polymers, such as collagen, have been used in medical applications for over 1000 years, whereas synthetically produced biodegradable polymers have only been used for about 50 years (i.e., since the 1960s) (Schmitt et al., 1963, 1967). Advances in synthetic biology and bioengineering have facilitated the production of promising biopolymer candidates for various applications. For example, hyaluronate is used as a biomaterial in medical applications, as an additive in cosmetic products and foods (e.g., xanthan and dextran) and as a biopolyester in packaging materials (Schmid et al., 2015; Choi et al., 2019; Lv et al., 2015). In this chapter, we focus on biodegradable polymers that are derived from food waste and the potential medical applications of such polymers in combating the coronavirus disease 2019 (COVID-19) pandemic. This chapter covers a wide range of existing food waste treatment methods that are used to recover nutrients, methods for the utilisation of nutrient-rich hydrolysates in the fermentative production of biopolymers, and the potential applications of food waste-derived fibres in combating the COVID-19 pandemic. A simplified overview of food waste-derived fibres and their relevance to the pandemic is depicted in Fig. 1.

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Fig. 1.  Overview of food waste valorisation methods for COVID-19 applications. PHBV: poly(3-hydroxybutyrate-co-3-hydroxyvalerate); PLA: polylactic acid; PPE: personal protective equipment.

2. Food Waste Biorefining: An Approach for the Extraction of Nutrients for the Manufacture of Value-Added Products Food waste biorefining involves a cascade of biological processes through which various categories of food waste are transformed into value-added products, such as biofuels, platform chemicals, and other bio-based materials (Tsegaye et al., 2021). Food waste generally has a high organic content in the form of soluble sugars, starches, lipids, proteins, cellulose, and other compounds, making it a valuable feedstock for fermentation (Loizdou et al., 2017). Various biological processes that use food waste as a substrate for the production of methane (Cho et al., 1995), hydrogen (Han et al., 2004; Cappai et al., 2018), lactic acid (Sakai et al., 2004), succinic acid (Praneetrattananon et al., 2005), ethanol (Volynets et al., 2017; Konti et al., 2020; Moon et al., 2019; Prasoulas et al., 2020; Moon et al., 2021), and single-cell proteins and enzymes have been explored (Kadim et al., 2015; Aggelopoulos et al., 2014; Haque et al., 2016). Aside from the fermentative production of these chemicals, complete biorefining methods have also been developed for the production of succinic acid (Sakai et al., 2004), lactic acid (Moon et al., 2005), and polyhydroxybutyrate (PHB)



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(Bhattacharyya et al., 2015; Garcia et al., 2011). However, hydrolysis of the solid content in food waste is a key rate-limiting step in bioprocess development. In this section, we present various techniques of food waste biorefining.

2.1. Food waste biorefineries: Development and applications Broadly, three different techniques are used for food waste biorefinery development: (i) bioconversion of food waste into value-added products via enzymatic processes or through the involvement of microorganisms; (ii) thermochemical conversion of food waste through processes such as pyrolysis, liquefaction, and gasification, in which the waste is subjected to high temperatures and chemicals are used as solvents; and (iii) valorisation of food waste through chemical processes that use chemicals as both solvents and catalysts. The integration of one or two of the above processes has recently gained much attention owing to research on increasing the efficiency of the conversion yield via biorefining. Nutrient-rich hydrolysates or resource-rich streams derived from food waste through processes, such as anaerobic fermentation, enzymatic hydrolysis, and acid or alkaline hydrolysis, have been used to produce or purify chemicals, materials, and fuels using either biological or downstream processing techniques. The biological production of polymeric compounds of industrial interest has been extensively studied over the past few decades. The conversion of low-value feedstock or substrates into high-value products not only ensures sustainability but also facilitates the transition towards a circular economy (Zabanioutou and Kamaterou, 2019; Mehta et al., 2021). Recently, biopolymers, such as polysaccharides, polyhydroxyalkanoates (PHAs), aliphatic polyesters, and polylactides, have gained much attention for their potential applications in the transition from fossil fuel-based plastics to bioplastics. The high yields of biopolymers from food waste hydrolysis and fermentation techniques have been explored using different bacteria, such as Haloferax mediterranei and Cupravidus necator (Aramvash et al., 2016; Koller, 2015; Poomipuk et al., 2014).

2.2.  Recycling of food waste and its medical applications Food waste from the supply chain contains considerable amounts of valuable nutrients, proteins, dietary fibres, polysaccharides, fatty acids,

424  M. A. Haque et al.

flavouring compounds, phytochemicals, such as polyphenols, and other compounds of various interest. These resources can be extracted, purified, and concentrated for further use as functional ingredients in the food, pharmaceutical, cosmetic, and healthcare industries (Ravindran et al., 2016; Joana Gil-Chávez et al., 2013). The utilisation of nutrients embedded in food waste by microorganisms produces high-value compounds, such as PHAs. PHAs have various medical applications, such as in the manufacture of subcutaneous implants, intravenous microparticles, and compressed oral tablets that modulate the kinetic release of hormones, antitumor agents, and antibiotics (Williams et al., 2005; McLeod et al., 1988; Gangrade et al., 1991; Juni and Nakano, 1987).

2.3. Food waste valorisation for manufacturing of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) biopolymers Among all the PHA copolymers, PHBV (3-hydroxybutyrate) (3HB) and 3-hydroxyvalerate (3HV) have lower crystallinity, higher flexibility, and lower melting temperatures than PHB homopolymer (Kunioka et al., 1989; Scandola et al., 1992). The production of these biopolymers by bacteria requires the supplementation of inducers to the propionyl Co-A metabolic pathway. Nutrient-rich food waste hydrolysates derived from food waste hydrolysis can be used for the fermentative production of various chemicals and value-added products, including PHBV, succinic acid, lactic acid, biosurfactants, and single-cell proteins. The process of fermentative PHBV production using food waste as feedstock encompasses the hydrolysis of food waste to sugar-rich streams, which are used as carbon sources and other growth nutrients. Subsequently, the carbon- and nutrient-rich feedstocks are fed into a bioreactor for microbial fermentation in either batch or fed-batch cultivation systems, wherein various microbes, such as Cupravidus necator and Haloferax mediterranei, utilise the nutrients to metabolically produce PHBV. The supplementation of inducers alongside other nutrients in the fermentation system is a major concern associated with the fermentative production of PHBV. Short-chain volatile fatty acids, such as butyric acid, propanoic acid, acetic acid, levulinic acid, and valeric acid, play significant roles in the metabolic pathways of PHBV-producing bacteria. For instance, the inducer levulinic acid can improve the PHBV production



Production of Biodegradable Fibres  425

yield by 4.4 g/L when glucose is used as a carbon source (Wang et al., 2012). Aramvash et al. (2016) reported a PHBV yield of 0.55 g/L, using fructose as a carbon source and propionic acid as an inducer. Various downstream processing techniques, such as centrifugation, purification through a tangential filtration system, drying, and crystallisation, are important parameters that determine the quality of the commercial production of PHBV, which is a component of several economic and environmental sustainability initiatives. The production of PHBV biopolymers through food waste valorisation can circumvent current techno-economic and environmental issues, as it requires only low-cost (or zero-cost) feedstocks for biorefining.

2.4.  Lactic acid production from food waste hydrolysates Lactic acid is generally produced by the microbial fermentation of carbohydrate-rich feedstocks, such as starch and refined sugars. As the cost of the substrate is currently the main barrier to economically viable lactic acid production, we strongly recommend the use of cost-effective raw materials that have low toxicity and few pretreatment/processing requirements. Food waste, in addition to being a free resource, has more potential than other substrates due to its high carbohydrate content, amenability to pretreatment and effective waste management and low environmental impact (Rawoof et al., 2021). Kwan et al. (2016) developed an efficient method for the bioconversion of food waste to lactic acid via fungal hydrolysis and fermentation using Lactobacillus casei Shirota. They achieved a productivity of 2.61 g/L/h and a yield of 0.94 g/g from mixed food waste hydrolysates and attained an overall lactic acid conversion yield of 0.27 g/g from mixed food waste (Kwan et al., 2016). Although various microbes, such as Bacilli and fungi, have been studied for their abilities to bioconvert food waste to lactic acid, most are inefficient in terms of lactic acid production and/or substrate recovery (Eş et al., 2018). Conventional lactic acid production methods separate the substrate hydrolysis and fermentation processes, with each requiring its own set of optimal processing conditions. In contrast, simultaneous saccharification and fermentation methods can be carried out under the same conditions and in a single reactor, resulting in higher productivity, lower processing times, and lower levels of substrate inhibition (Cheng et al., 2019). The fermentation mode also significantly affects lactic acid production, with

426  M. A. Haque et al.

batch, fed-batch, and continuous modes differing in performance due to factors such as productivity, end-product inhibition, substrate utilisation, and risk of contamination. Immobilised cell bioreactors allow for continuous fermentation at higher cell densities and have higher productivity (Mou et al., 2021). This strategy was recently tested for lactic acid production using polyvinyl alcohol-immobilised Lactobacillus strains and found to improve the yield considerably (Chen et al., 2020; Nagarajan et al., 2020). A similar culture mode, the in situ fibrous bed bioreactor, is more attractive for such purposes as the immobilisation materials used (e.g., agricultural residues) are low-cost and effective (Li et al., 2018). This method thus shows a lot of promise for lactic acid production in a techno-economically feasible manner.

3. Food Waste-Derived Biopolymers for Non-Woven Fibre Synthesis: A Review Personal protective equipment (PPE), such as face masks and medical gowns, have been instrumental in reducing the risk of viral transmission and infection during the COVID-19 pandemic. Currently, most PPE is made from petroleum-based non-woven fabrics that are non-sustainable and non-biodegradable. To reduce wastage and dependency on fossil fuels, it is necessary to conduct research on the development of sustainable and eco-friendly non-woven materials. Biopolymers, which are derived from natural and/or fermentation products of plant-based materials and their by-products, are biodegradable and derived from renewable materials. Bio-friendly polymers will soon compete with traditional commodity plastics, as evidenced by sales growth rates of 20–30% per year (Mohanty et al., 2018). The COVID-19 pandemic has also accelerated research on bio-friendly polymers for non-woven fabrics, with materials such as PLA, PHA, and PHB showing promise.

3.1.  PLA non-woven fabrics Feng (2017) studied non-woven disordered mats made by melt-blowing at different die-to-collector distances. The resulting non-woven mats are porous, with good mechanical properties and wide applications in tissue engineering.



Production of Biodegradable Fibres  427

Rodchanasuripron et al. (2020) prepared non-woven PLA fibres using the rotational jet spinning method, in which polymers are heat-melted and centrifuged to obtain fibres. As the temperature and/or rotational speed is increased, the fibre diameter is reduced, facilitating the formation of beads. Higher rotational speeds lead to the formation of more hydrophobic fibres. Arrieta et al. (2020) produced electrospun mats of PLA blended with PHB and plasticised with oligomeric lactic acid (OLA). Increasing the amount of OLA plasticiser decreases the viscosity of the polymer solution and reduces the fibre diameter. The material has adequate biodegradability in compost, implying that it has potential applications in agricultural mulch films. Leonés et al. (2019) studied the shape memory behaviour of electrospun PLA nanofibres that were plasticised with OLA. Shape memory polymers are smart materials that change shape upon experiencing external stimuli but later return to their original shapes. The PLA–OLA formulation shows excellent shape memory behaviour at temperatures similar to those of the human body, indicating its potential utility in biomedical applications.

3.2.  PHA/PHB non-woven fabrics Vilchez et al. (2021) studied the use of PHB electrospun fibres, which are biocompatible and biodegradable, for encapsulation of the antioxidants curcumin (Cur) and quercetin (Que). The crystalline nature of the fibres decreases as the Cur or Que concentration increases. The antioxidant activities of both compounds are maintained after encapsulation in PHB electrospun fibres. Jang et al. (2021) explored the use of electrospun PHB as support for the immobilisation of phasin-fused lipases. The nanofibre non-woven mat can support an enzyme quantity 120 times greater than that supported by PHB polymer grains. The enzymes immobilised on electrospun PHB supports have higher stability and activity compared with those immobilised on conventional supports, indicating that electrospun PHB mats can be widely adopted for the development of various enzymatic processes. PHA non-woven fibres have also been used for the formation of fibrous scaffolds for biomedical applications. PHA electrospun fibres synthesised from waste glycerol have been shown to support stem cell

428  M. A. Haque et al.

growth at acceptable proliferation levels. Scaffold topology is an important criterion for cell viability; PHA electrospun fibres have been shown to significantly support cell adhesion and proliferation, whereas nonelectrospun PHA films have failed to do so (Canadas et al., 2014). PHA fibres blended with other polymers and plasticisers have better mechanical and biological properties and can increase PHA miscibility. Natural polymers, such as gelatine, zein, and cellulose acetate, have been blended with PHA to produce electrospun fibrous scaffolds, with gelatine used as a shell and PHA used as the core. Therefore, PHA can be augmented with other materials for tissue engineering applications (Sanhueza et al., 2019).

3.3. Methods and techniques for biopolymer non-woven fibre production: A review Various methods are available for the fabrication of polymer non-woven fibres, including melt-blowing, electrospinning, and solution-blowing spinning (SBS).

3.3.1. Melt-blowing Melt-blowing is one of the most widely adopted commercial non-woven fabric production techniques. It is particularly suitable for the production of polymers with high melt flow rates, such as polyethylene, polypropylene, polyamide, polyurethane, polyester, and thermoplastic elastomers (Kakoria and Sinha-Ray, 2018). Melt-blowing heats polymers to a molten state, which is rapidly drawn into fibres by hot gas streamed at a high velocity and co-axial with the polymer melt. The fibres are then collected on a screen to form a non-woven web (Zupančič et al., 2016; Yoo et al., 2009). Typically, melt-blowing results in the formation of microfibre nonwoven mats. The applications of melt-blowing for biopolymers are limited, as they are not stable and tend to denature at high temperatures.

3.3.2. Electrospinning Electrospinning is a well-developed non-woven fibre production method with good dispersion and mechanical properties. It typically uses electric force to draw charged threads of polymer into fibres with diameters of 1  µm–10 nm (Li and Xia, 2003). During electrospinning, a polymer



Production of Biodegradable Fibres  429

Fig. 2.   Working principle of electrospinning.

solution is charged using a high-voltage power supply, and the volume feed rate is maintained at a constant value using a pump controller. In the electric field, the repulsive electric force overcomes the surface tension of the polymer solution, which is then pushed from the needle tip into a collector. On its way to the collector, the solution jet is solidified as the solvent evaporates under the electric field, and the solid fibres are deposited onto the collector (Park, 2011). The working principle of electrospinning is illustrated in Fig. 2. The basic components are polymer solution, highvoltage supply, capillary pump with pipette or needle of small diameter (i.e., nozzle), and grounded collection platform. Electrospinning is different from conventional wet/dry fibre spinning and melt-blowing in terms of its driving force. The latter two techniques depend on aerodynamic drag, whereas electrospinning depends on electrostatic attraction. In conventional fibre spinning, the fibres are usually several microns in diameter, whereas in electrospinning, the fibre diameter is on the scale of 100 nm (Kakoria and Sinha-Ray, 2018).

3.3.3.  Solution blow spinning Solution blow spinning (SBS) is a novel process developed for the production of microfibres or nanofibres from a spinning solution. The process requires two parallel fluid streams: a polymer in a volatile solvent and a pressurised gas. The fibres are created along the direction of gas

430  M. A. Haque et al.

flow and deposited onto a collector (Vural et al., 2015; Medeiros et al., 2009). SBS requires a compressed air source, generally maintained at 150–200 m/s at 3–5 bar of pressure (Khansari et al., 2013). The polymer solution is fed to the nozzle via a syringe pump, in which the accelerated gas stretches the polymer jet to reduce the diameter (Khansari et al., 2013). The solvents then evaporate, and the polymeric fibres are collected either on a solid grid, a drum, or a continuous conveyor. SBS is similar to melt-blowing, with the key difference being that SBS uses solvent evaporation while melt-blowing uses cooling jet solidification. Unlike meltblowing, which produces microfibres, SBS results in nanofibres (Yoo et al., 2009).

3.4. Non-woven textiles for PPE produced from food waste-derived biopolymers using electrospinning Non-woven textiles for PPE, such as medical masks and medical gowns, can be fabricated from food waste-derived biopolymers. The new material aims to achieve superhydrophobicity and high conductivity and offers higher protection than the Filtering Face Pieces 3 (FFP3) standard, i.e., >99% protection not only for sub-microscopic particles or viruses (UNI EN 149:2009 standard) but also for nanoscopic particles up to 5–10 nm. This would enable such PPE to be used even in heavily virus-infected environments. The polymer blend would use a mixture of PLA and PHBV, which are biopolymers that can be derived from food waste and are biodegradable. PHBV has superior sensitisation properties and is less likely to cause delayed hyper-sensitivity reactions, as its degradation product is a natural constituent of human fluids and has an acceptably low degradation rate (Chang et al., 2016). PLA is often used as a base material due to its biocompatibility, biodegradability, and processability. The blending of PHBV with PLA improves the wettability of the material and improves the strength and ductility of PHBV (Chang et al., 2016). Non-woven PPE is usually made of polypropylene, which is nonbiodegradable. Non-woven fabrics are typically produced using meltblowing. However, this process generally requires the use of polymers with high melt flow rates (>1000 g/10 min) (Gahan and Zguris, 2000), which most of the biopolymers do not have. For instance, PLA has a melt flow rate of 10–70 g/10 min, which is far lower than the requirement for



Production of Biodegradable Fibres  431

melt-blowing (Barczewski and Mysiukiewicz, 2018). It is also difficult to produce biodegradable materials, such as PLA, with a high melt flow rate, as such materials are unstable and degrade easily. Therefore, PLA/PHBVbased non-woven textiles are produced by electrospinning, which allows for the rapid and efficient fabrication of spider web-like fibres without high melt flow rate requirements.

3.4.1. Effects of parameters during electrospinning Several factors affect the morphology and diameter of nanofibres produced through electrospinning. These factors belong to two categories: solution parameters (solvents and polymer concentration) and electrospinning parameters (voltage, solution flow rate, and needle to collector distance) (Theron et al., 2004).

3.4.2. Solvent effects The solvents used for the electrospinning of PLA and PHBV must be completely soluble. The boiling point of a solvent also determines its volatility. Volatile solvents with high evaporation rates facilitate easy evaporation from nanofibres during travel; however, if the evaporation rates are too high, they promote drying at the needle tip, which interferes with the electrospinning process. In contrast, solvents with low evaporation rates cause the deposition of solvent-containing nanofibres on the collector plate and increase the formation of beaded nanofibres (Sill and von Recum, 2008). Some alternative approaches use binary solvent systems to overcome these limitations. Casasola et al. (2014) studied the effects of dimethylformamide/acetone on fibre morphology and diameter. Increasing the amount of acetone yielded bead-free nanofibres but increased the fibre diameter. Figure 3 shows the effects of the solvent ratio on PLA nanofibre morphology.

3.4.3. Polymer concentration Polymer concentration is an important determinant of nanofibre diameter. Fibres with more uniform diameters are obtained at high polymer concentrations (Gu and Ren, 2005), which also increase viscosity and chain entanglement, leading to the formation of beadless electrospun nanofibres

432  M. A. Haque et al. (a)

(b)

(d)

(e)

(c)

Fig. 3.  Effect of the binary solvent ratio (dimethylformamide/acetone) on nanofibre morphology: scanning electron micrographs (1000×) of polylactic acid (PLA) nanofibres of 10% (w/v) PLA in dimethylformamide/acetone at ratios of (a) 8:2, (b) 6:4, (c) 5:5, (d) 4:6, and (e) 2:8.

(Haider et al., 2018). Further increases in concentration can impede solution flow and cause drying at the needle tip. In contrast, at low polymer concentrations, the electric field causes polymer fibres to fragment before reaching the collector plate (Haider et al., 2013).

3.4.4. Voltage Exposure of the polymer solution to a high-voltage electric field induces Taylor cone formation and produces nanofibres only at a critical voltage. Further increases in voltage may decrease the fibre diameter due to increased stretching of the polymer solution to certain threshold values (Sill and von Recum, 2008). Further increases in voltage may increase the fibre diameter and lead to bead formation due to increases in jet velocity and jet length (Deitzel et al., 2001).

3.4.5. Solution flow rate A minimum flow rate should be maintained for the production of uniform beadless electrospun nanofibres. Increasing the flow rate beyond a critical value will increase the pore size and fibre diameter and lead to the formation of beads and unspun droplets (Megelski et al., 2002). This is usually



Production of Biodegradable Fibres  433

attributed to incomplete drying of the fibre and low stretching of the polymer solution during its flight to the collector (Li and Wang, 2013).

3.4.6. Needle to collector distance A critical distance between the needle and the collector needs to be maintained for the formation of uniform, beadless, and defect-free nanofibres. Any changes to this distance will affect the morphology of the fibres owing to changes in the deposition time, evaporation rate, and stretching duration (Matabola and Moutloali, 2013).

3.5. Validation of PPE from food waste-derived biopolymers The American Society for Testing Materials (ASTM) F2100 is a wellrecognised international standard that is used to classify medical masks. It is based on key performance metrics, such as filtration efficiency and differential pressure (Table 1).

3.5.1. Bacterial filtration efficiency (BFE) The BFE determines the efficiency of a filtering layer in reducing bacteria-containing aerosols (ASTM F2101-19, 2019). The standard technique Table 1.   Key classification of medical masks based on the American Society for Testing Materials (ASTM) F2100. Property

Method

Level 1 Level 2 Level 3

BFE (%)

[ASTM F2101]

Bacterial filtration efficiency % (3 µm)

≥95

≥98

≥98

PFE (%)

[ASTM F2299]

Sub-micron particulate filtration efficiency % (0.1 µm)

≥95

≥98

≥98

Fluid resistance (mmHg)

[ASTM F1862]

Resistance to penetration of synthetic blood at certain pressure

80

120

160