Characterizing the variability of food waste quality: A need for efficient valorisation through anaerobic digestion

Optimizing food waste anaerobic digestion in Kuwait: Experimental insights and empirical modelling using artificial neural networks

This study investigates, for the first time, the anaerobic digestion of food waste in Kuwait to optimize methane production through a combination of artificial neural network (ANN) modelling and continuous reactor experiments. The ANN model, utilizing eight hidden neurons and a 70-20-10 split for training, validation and testing sets, yielded mean squared error values of 0.0056, 0.0048 and 0.0059 and coefficient of determination (R²) values of 0.9942, 0.9986 and 0.9892, respectively. Methane percentages in biogas were predicted using six parameters: biomass type, pH, organic loading rate (OLR), hydraulic retention time (HRT), temperature and reactor volume. To validate the ANN results, continuous reactor experiments were conducted under an OLR of 3 kg VS m⁻³ d⁻¹ and HRT of 20 days at varying temperatures (35°C, 40°C, 45°C, 50°C and 55°C). The experiments demonstrated optimal methane production in the mesophilic range, with ANN predictions closely aligning with experimental data up to 45°C. However, deviations were observed at higher temperatures, particularly under thermophilic conditions beyond 50°C. This study provides novel insights into waste-to-energy initiatives in Kuwait and highlights the potential of integrating computational models with empirical data to enhance biogas production processes.

Contaminants, biochemical methane potential, and biodegradability of different bio-waste categories: guidance for anaerobic digestion

This study evaluated the anaerobic digestion suitability of bio-waste from different sources by comparing their biochemical methane potential (BMP), biodegradability (BI), and content of contaminants (heavy metals and physical impurities) - an often-overlooked factor but one of particular concern in bio-waste. Predominant heavy metals included Cu and Zn, while recurring physical impurities comprised plastics and organic non-biodegradable matter. Food waste from food processing plants were most suitable, exhibiting low contamination and high biogas conversion (BMP > 549 NmLCH4/gVS and BI > 86 %). Conversely, organic fractions from mechanical biological treatment were highly contaminated, while green waste displayed low biogas conversion (BMP < 368 NmLCH4/gVS and BI < 72 %). Food waste from households and medium/large-sized producers also demonstrated high biogas conversion, but variable contamination levels could compromise their suitability. Assessing contaminants alongside BMP and BI provides a comprehensive approach for selecting suitable bio-waste feedstocks that can be introduced in biogas plants.

Enhanced anaerobic digestion of freezing and thawing pretreated cow manure with increasing solid content: kinetics and microbial community dynamics

High solid anaerobic digestion has proved the mainstream technology for the treatment of organic wastes. However, the high molecular weight and complex lignocellulosic structure of cow manure (CM) make it indigestible and inefficient, leading to limit the hydrolysis step of anaerobic digestion at high solid content. To mitigate this bottleneck, an improved cost-effective freezing and thawing pretreatment technique was proposed in this study. The freezing and thawing pretreatment of raw CM without any dilution was done for 20 days. The maximum cumulative methane yield (487 mL CH4 g- 1VS) was achieved at a total solid (TS) of 5% followed by TS of 10% and 15%, which was 13%, 20% and 21% higher than obtained from untreated CM, respectively. The kinetic results revealed that the biodegradable materials could be utilized at increasing TS with decreasing hydrolysis rate. The pretreatment significantly enhanced the methylotrophic methanogenic pathway during high solid anaerobic digestion, which was contrary to the general concept that the process is usually dominated by acetoclastic and hydrogenotrophic methanogens. This study is very important to understand the effect of solid content but also important to understand the effect of freezing and thawing pretreatment on process parameters and microbial community dynamics in high solid anaerobic digestion.

Co-digestion and co-treatment of sewage and organic waste in mainstream anaerobic reactors: operational insights and future perspectives

The global shift towards sustainable waste management has led to an intensified exploration of co-digestion and co-treatment of sewage and organic waste using anaerobic reactors. This review advocates for an integrated approach where organic waste is treated along with the sewage stream, as a promising solution to collect, treat, and dispose of organic waste, thereby reducing the environmental and economic burden on municipalities. Various efforts, ranging from laboratory to full-scale studies, have been undertaken to assess the feasibility and impacts of co-digestion or co-management of sewage and organic waste, using technologies such as up-flow anaerobic sludge blankets or anaerobic membrane bioreactors. However, there has been no consensus on a standardized definition of co-digestion, nor a comprehensive understanding of its impacts. In this paper, we present a comprehensive review of the state-of-the-art in liquid anaerobic co-digestion systems, which typically operate at 1.1% total solids. The research aims to investigate how the integration of organic waste into mainstream anaerobic-based sewage treatment plants has the potential to enhance the sustainability of both sewage and organic waste management. In addition, utilizing the surplus capacity of existing anaerobic reactors leads to significant increases in methane production ranging from 190 to 388% (v/v). However, it should be noted that certain challenges may arise, such as the necessity for the development of tailored strategies and regulatory frameworks to enhance co-digestion practices and address the inherent challenges.

Effect of biochar addition on the anaerobic digestion of food waste: microbial community structure and methanogenic pathways

This study assessed the effects of the addition of biochar prepared at 700 °C with different dosages on the anaerobic digestion of food waste. The biochar addition at a concentration of 10.0 g/L increased the cumulative methane yield by 128%, and daily methane production was also significantly promoted. The addition of biochar derived from poplar sawdust significantly increased the relative abundance of dominant bacteria for anaerobic digestion by 85.54-2530% and promoted the degradation of refractory organic matter and the transfer of materials between the hydrolysis and acid production stages. Further analysis has demonstrated that Bathyarchaeia and hydrogenotrophic methanogens were enriched by the biochar addition. Meanwhile, the relative abundances of functional genes, including C5-branched dibasic acid metabolism, and pyruvate metabolism, were increased by 11.38-26.27%. The relative abundances of genes related to major amino acid metabolism, including histidine metabolism, lysine biosynthesis, and phenylalanine, tyrosine, and tryptophan biosynthesis, were increased by 11.96-15.71%. Furthermore, the relative abundances of genes involved in major replication and repair were increased by 14.76-22.76%, and the major folding, sorting, degradation, and translation were increased by 14.47-19.95%, respectively. The relative abundances of genes related to major membrane transport and cell motility were increased by 10.02 and 83.09%, respectively.

Safe Circular Food Systems: A Transdisciplinary Approach to Identify Emergent Risks in Food Waste Nutrient Cycling

With growing awareness of the environmental, economic, and social costs associated with food waste, there is a concerted effort on multiple scales to recover the nutrient value of discarded food. These developments are positive, but the rapid movement toward alternatives and the complexity of solving problems located at the intersection of economic, social, and environmental systems also have the potential to produce unanticipated risks. This paper draws upon long-term stakeholder-engaged research throughout New England, with a focus on Maine, to develop a transdisciplinary, systems-based model of the potential social, economic, and environmental risks of food waste nutrient cycling. Our effort is intended to help inform the creation of safe, functional, and environmentally benign circular food systems.

Valorization of food waste: A comprehensive review of individual technologies for producing bio-based products

BACKGROUND

The escalating global concerns about food waste and the imperative need for sustainable practices have fuelled a burgeoning interest in the valorization of food waste. This comprehensive review delves into various technologies employed for converting food waste into valuable bio-based products. The article surveys individual technologies, ranging from traditional to cutting-edge methods, highlighting their respective mechanisms, advantages, and challenges.

SCOPE AND APPROACH

The exploration encompasses enzymatic processes, microbial fermentation, anaerobic digestion, and emerging technologies such as pyrolysis and hydrothermal processing. Each technology's efficacy in transforming food waste into bio-based products such as biofuels, enzymes, organic acids, prebiotics, and biopolymers is critically assessed. The review also considers the environmental and economic implications of these technologies, shedding light on their sustainability and scalability. The article discusses the role of technological integration and synergies in creating holistic approaches for maximizing the valorization potential of food waste. Key finding and conclusion: This review consolidates current knowledge on the valorization of food waste, offering a comprehensive understanding of individual technologies and their contributions to the sustainable production of bio-based products. The synthesis of information presented here aims to guide researchers, policymakers, and industry stakeholders in making informed decisions to address the global challenge of food waste while fostering a circular and eco-friendly economy.

The management and prevention of food losses and waste in low- and middle-income countries: A mini-review in the Africa region

This mini-review analyses food losses and waste (FLW) management in low- and middle-income countries (LMICs) and identifies potential strategies to improve FLW management efficiency on the African continent. To achieve this aim, a search of grey and published scientific literature-case studies, feasibility studies, theses, peer-reviewed journals, governments and technical reports was performed. Food waste (FW) per capita in sub-Saharan Africa (SSA) was determined to be between 6 and 11 kg capita-1 year-1. Factors militating against FLW management include a lack of infrastructure, waste reduction and mandatory waste management plans, financial support for food redistribution programmes, awareness and a lack of knowledge of FW management and effective approaches. Poor recovery systems, a lack of incentives in FW recycling programmes, a lack of a regulatory and policy framework and institutional weaknesses as well as a lack of sufficient and appropriate education programmes to improve FW source separation and collection rates are all significant challenges in the African region, with negative consequences for the environment and public health. Except for fuel conversion and food scraps for digestion to recover energy, there is a huge potential for composting and using FW as a digestate, which could eventually lead to a reduction in the amount of FW being landfilled or incinerated. The study explores potential interventions to reduce amount of FLW and form a basis for future research in this field and improving FW management efficiency in LMCs, especially on the continent of Africa. It also provides information that could assist researchers, policymakers and decision-makers reduce amount of FLW, aid in the utilization of FW for energy production, and reduce greenhouse gas emissions in the continent, as well as support the achievement of other sustainable development goals, such as 12.3, which is particularly important in the context of the African continent, which is dependent on food imports.

Impact of dewatering inorganic coagulants on anaerobic digestion treating food waste leachate

Anaerobic digestion of food waste leachate (FWL) provides a viable solution for waste treatment and energy production. Returning solids from digested sludge to the reactor maintains a high microbial concentration and enhances digestion efficiency. However, this requires coagulants because the digestate has low dewaterability. This study analyzed methane production and microbial communities using biochemical methane potential tests for inorganic coagulants (AlCl3, Al2(SO4)3, FeCl3, and Fe2(SO4)3) in FWL treatment. Cumulative methane production was the highest in the control and decreased in the order of Fe2(SO4)3, AlCl3, FeCl3, and Al2(SO4)3. Iron ions inhibited H2S production while aluminum ions increased it compared to the control group. Despite the absence of significant changes in microbial communities following coagulant injection, a substantial increase in damaged cells was observed. These findings highlight the intricate repercussions of coagulant introduction in anaerobic digestion, emphasizing notable alterations in methane production dynamics and the integrity of microbial cells.

Exploitation of microbial activities at low pH to enhance planetary health

Awareness is growing that human health cannot be considered in isolation but is inextricably woven with the health of the environment in which we live. It is, however, under-recognized that the sustainability of human activities strongly relies on preserving the equilibrium of the microbial communities living in/on/around us. Microbial metabolic activities are instrumental for production, functionalization, processing, and preservation of food. For circular economy, microbial metabolism would be exploited to produce building blocks for the chemical industry, to achieve effective crop protection, agri-food waste revalorization, or biofuel production, as well as in bioremediation and bioaugmentation of contaminated areas. Low pH is undoubtedly a key physical-chemical parameter that needs to be considered for exploiting the powerful microbial metabolic arsenal. Deviation from optimal pH conditions has profound effects on shaping the microbial communities responsible for carrying out essential processes. Furthermore, novel strategies to combat contaminations and infections by pathogens rely on microbial-derived acidic molecules that suppress/inhibit their growth. Herein, we present the state-of-the-art of the knowledge on the impact of acidic pH in many applied areas and how this knowledge can guide us to use the immense arsenal of microbial metabolic activities for their more impactful exploitation in a Planetary Health perspective.

Acidogenic fermentation of food waste to generate electron acceptors and donors towards medium-chain carboxylic acids production

This study investigated the optimum pH, temperature, and food-to-microorganisms (F/M) ratio for regulating the formation of electron acceptors and donors during acidogenic fermentation to facilitate medium-chain carboxylic acids (MCCAs) production from food waste. Mesophilic fermentation at pH 6 was optimal for producing mixed volatile fatty acids (719 ± 94 mg COD/g VS) as electron acceptors. Under mesophilic conditions, the F/M ratio (g VS/g VS) could be increased to 6 to generate 22 ± 2 g COD/L of electron acceptors alongside 2 ± 0 g COD/L of caproic acid. Thermophilic fermentation at pH 6 was the best condition for producing lactic acid as an electron donor. However, operating at F/M ratios above 3 g VS/g VS under thermophilic settings significantly reduced lactic acid yield. A preliminary techno-economic evaluation revealed that converting lactic acid and butyric acid generated during acidogenic fermentation to caproic acid was the most profitable food waste valorization scenario and could generate 442-468 €/t VS/y. The results presented in this study provide insights into how to tailor acidogenic fermentation reactions to desired intermediates and will help maximize MCCAs synthesis.

Recent advances in co-digestion conjugates for anaerobic digestion of food waste

Anaerobic digestion (AD) is a biological process that breaks down organic waste materials, such as food waste (FW) that produces biogas and digestate. The biogas can be utilized as biofuel, and digestate could be applied as fertilizer. However, AD of FW alone has limitations on optimal degradation, digester stability and biogas yield. Co-digestion of FW along with other organic wastes such as animal manure, agricultural residue, sewage sludge and industrial organic waste, has shown substantial improvement in degradation process with increased biogas yield. The inadequacies in FW for optimum AD, like low carbon-to-nitrogen ratio (C/N ratio), lack of trace elements and irregular particle sizes, can be nullified by adding appropriate co-digestion conjugates. This review aims to describe the characteristic inadequacies of FW and examines the effect on mesophilic co-digestion of FW with animal manure, waste sludge and agricultural wastes for biogas production optimization. A critical review on the impact of pretreatment and co-digestion to enrich the methane (CH4) content in biogas has been performed. The review also examines the microbial community shift due to co-digestion, which is critical for the stability of an anaerobic digester. Finally, it discusses the prospects and challenges for the widespread application of the co-digestion technique as an effective organic waste management practice.

Microbial community composition of food waste before anaerobic digestion

Anaerobic digestion is widely used to process and recover value from food waste. Commercial food waste anaerobic digestion facilities seek improvements in process efficiency to enable higher throughput. There is limited information on the composition of microbial communities in food waste prior to digestion, limiting rational exploitation of the catalytic potential of microorganisms in pretreatment processes. To address this knowledge gap, bacterial and fungal communities in food waste samples from a commercial anaerobic digestion facility were characterised over 3 months. The abundance of 16S rRNA bacterial genes was approximately five orders of magnitude higher than the abundance of the fungal intergenic spacer (ITS) sequence, suggesting the numerical dominance of bacteria over fungi in food waste before anaerobic digestion. Evidence for the mass proliferation of bacteria in food waste during storage prior to anaerobic digestion is presented. The composition of the bacterial community shows variation over time, but lineages within the Lactobacillaceae family are consistently dominant. Nitrogen content and pH are correlated to community variation. These findings form a foundation for understanding the microbial ecology of food waste and provide opportunities to further improve the throughput of anaerobic digestion.

Co-digestion of food waste and hydrothermal liquid digestate: Promotion effect of self-generated hydrochars

Hydrothermal treatment (HTT) can efficiently valorize the digestate after anaerobic digestion. However, the disposal of the HTT liquid is challenging. This paper proposes a method to recover energy through the anaerobic co-digestion of food waste and HTT liquid fraction. The effect of HTT liquid recirculation on anaerobic co-digestion performance was investigated. This study focused on the self-generated hydrochars that remained in the HTT supernatant after centrifugation. The effect of the self-generated hydrochars on the methane (CH₄) yield and microbial communities were discussed. After adding HTT liquids treated at 140 and 180 °C, the maximum CH₄ production increased to 309.36 and 331.61 mL per g COD, respectively. The HTT liquid exhibited a pH buffering effect and kept a favorable pH for the anaerobic co-digestion. In addition, the self-generated hydrochars with higher carbon content and large oxygen-containing functional groups remained in HTT liquid. They increased the electron transferring rate of the anaerobic co-digestion. The increased relative abundance of Methanosarcina, Syntrophomonadaceae, and Synergistota was observed with adding HTT liquid. The results of the principal component analysis indicate that the electron transferring rate constant had positive correlationships with the relative abundance of Methanosarcina, Syntrophomonadaceae, and Synergistota. This study can provide a good reference for the disposal of the HTT liquid and a novel insight regarding the mechanism for the anaerobic co-digestion.

Characteristics of Biogas Production and Synergistic Effect of Primary Sludge and Food Waste Co-Digestion

Co-digestion implementation in wastewater treatment plants enhances biogas yield, so this research investigated the optimal ratio of biodegradable waste and sewage sludge. The increase in biogas production was investigated through batch tests using basic BMP equipment, while synergistic effects were evaluated by chemical oxygen demand (COD) balance. Analyses were performed in four volume basis ratios (3/1, 1/1, 1/3, 1/0) of primary sludge and food waste with added low food waste: 3.375%, 4.675%, and 5.35%, respectively. The best proportion was found to be 1/3 with the maximum biogas production (618.7 mL/g VS added) and the organic removal of 52.8% COD elimination. The highest enhancement rate was observed among co-digs 3/1 and 1/1 (105.72 mL/g VS). A positive correlation between biogas yield and COD removal is noticed while microbial flux required an optimal pH, value of 8 significantly decreased daily production rate. COD reductions further supported the synergistic impact; specifically, an additional 7.1%, 12.8%, and 17% of COD were converted into biogas during the co-digestions 1, 2, and 3, respectively. Three mathematical models were applied to estimate the kinetic parameters and check the accuracy of the experiment. The first-order model with a hydrolysis rate of 0.23-0.27 indicated rapidly biodegradable co-/substrates, modified Gompertz confirmed immediate commencement of co-digs through zero lag phase, while the Cone model had the best fit of over 99% for all trials. Finally, the study points out that the COD method based on linear dependence can be used for developing relatively accurate model for biogas potential estimation in anaerobic digestors.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12155-023-10620-8.

Synergetic effects of anaerobic co-digestion of food waste and algae on biogas production

Anaerobic co-digestion of food waste and algae was assessed to offset the drawbacks of anaerobic mono-digestion of each substrate. Batch test results indicated that a food waste and algae mixture ratio of 8:2 facilitated the highest CH₄ yield (334 mL CH₄/g COD_Input). This ratio was applied to the anaerobic co-digestion reactor, resulting in a CH₄ yield that was twice that of the anaerobic mono-digestion reactors, thereby facilitating high operational stability. In contrast to the anaerobic mono-digestion, anaerobic co-digestion resulted in stable CH₄ production by overcoming volatile fatty acid accumulation and a decreased pH, even under a high organic loading rate (3 kg COD/m³∙d). Furthermore, a comparative metagenomic analysis revealed that the abundance of volatile fatty acid-oxidizing bacteria and hydrogenotrophic and methylotrophic methanogens was significantly increased in the anaerobic co-digestion reactor. These findings indicate that the anaerobic co-digestion of food waste and algae significantly improves CH₄ production and process stability.

Techno-economic assessment of biorefinery scenarios based on mollusc and fish residuals

Biorefineries aim to maximise resource recovery from organic sources that have been traditionally considered wastes. In this respect, leftovers from mollusc and seafood processing industries can be a source of multiple bioproducts such as protein hydrolysates (PH), calcium carbonate and co-composted biochar (COMBI). This study aims to evaluate different scenarios of biorefineries fed by mollusc (MW) and fish wastes (FW) to understand which is the most convenient to maximise their profitability. Results showed that the FW-based biorefinery obtained the highest revenues with respect to the amounts of waste treated, i.e., 955.1 €·t^-1 and payback period (2.9 years). However, including MW in the biorefinery showed to increase total income as a higher amount of feedstock could be supplied to the system. The profitability of the biorefineries was mainly dependent on the selling price of hydrolysates (considered as 2 €·kg^-1 in this study). However, it also entailed the highest operating costs (72.5-83.8% of total OPEX). This highlights the importance of producing high-quality PH in economic and sustainable way to increase the feasibility of the biorefinery.

Sustainable management of food waste; pre-treatment strategies, techno-economic assessment, bibliometric analysis, and potential utilizations: A systematic review

Food waste (FW) contains many nutritional components such as proteins, lipids, fats, polysaccharides, carbohydrates, and metal ions, which can be reused in some processes to produce value-added products. Furthermore, FW can be converted into biogas, biohydrogen, and biodiesel, and this type of green energy can be used as an alternative to nonrenewable fuel and reduce reliance on fossil fuel sources. It has been demonstrated in many reports that at the laboratory scale production of biochemicals using FW is as good as pure carbon sources. The goal of this paper is to review approaches used globally to promote turning FW into useable products and green energy. In this context, the present review article highlights deeply in a transdisciplinary manner the sources, types, impacts, characteristics, pre-treatment strategies, and potential management of FW into value-added products. We find that FW could be upcycled into different valuable products such as eco-friendly green fuels, organic acids, bioplastics, enzymes, fertilizers, char, and single-cell protein, after the suitable pre-treatment method. The results confirmed the technical feasibility of all the reviewed transformation processes of FW. Furthermore, life cycle and techno-economic assessment studies regarding the socio-economic, environmental, and engineering aspects of FW management are discussed. The reviewed articles showed that energy recovery from FW in various forms is economically feasible.

Co-digestion of cow manure and food waste for biogas enhancement and nutrients revival in bio-circular economy

Anaerobic co-digestion (AcoD) with suitable substrate ratios may have the potential to improve biogas process and could play a better role in nutrient management for biocircular economy. The goal of this study was to enhance biogas yield from AcoD of cow manure (CM) and canteen food waste (CFW), and pertinent co-digestion of suitable substrate ratios for nutrient management i. e NPK from linear to biocircular economy, using ruminant intestinal fluid as a source of inoculum. A mesophilic (37 ± 1 °C) laboratory-scale AcoD with varying CFW/CM ratios of (0:1, 1:4, 2:3, 1:1, 3:2, 4:1, and 1:0) based on wet weight was performed. The AcoD systems of different CFW/CM ratios were evaluated with a loading rate of 400 g/L in the presence of 100 g cow intestinal fluid (CIF) inoculation. All experimental AcoD systems yielded greater biogas (147-300 cm3/g VS) than the mono-digestion in which only CM (135 cm3/g VS) and CFW (146 cm3/g VS) were digested anaerobically. The AcoD system of CFW/CM with 4:1 showed the highest biogas yield (300 cm3/g VS), and VS and COD reduction rate (39.51% and 65.15%, respectively), and nutrient contents (6.53%). Moreover, the experiment results were verified by modified Gompertz model. This work provided a window of opportunity to examine the anaerobic co-digestion technology beyond biogas production and to put the current low-cost technology to use for nutrient management and as a better component of the biocircular economy for agriculture in Pakistan in order to achieve sustainable development goals.

Research trends and strategies for the improvement of anaerobic digestion of food waste in psychrophilic temperatures conditions

The organic fraction of municipal solid waste is mainly composed of food waste (FW), and traditional disposal practices for this fraction are generally considered to have negative environmental and economic impacts. However, the organic characteristics of this fraction could also be exploited through the anaerobic digestion of FW (FW-AD), which represents unique advantages, including the reduction of the area required for final disposal and environmental pollution and the same time the generation of renewable energy (mainly methane gas), and a by-product for agricultural use (digestate) due to its high nutrient content. Although approximately 88% of the world's population resides in areas with temperatures below 8 °C, psychrophilic conditions (temperatures below 20 °C) have hardly been studied, while mesophilic (66%) and thermophilic (27%) ranges were found to be more common than psychrophilic FW-AD (7%). The latter condition could decrease microbial activity and organic matter removal, which could affect biogas production and even make AD unfeasible. To improve the efficiency of the psychrophilic FW-AD process, there are strategies such as: measurement of physical properties as particle size, rheological characteristics (viscosity, consistency index and substrate behavior index), density and humidity, bioaugmentation and co-digestion with other substrates, use of inocula with psychrophilic methanogenic communities, reactor heating and modification of reactor configurations. However, these variables have hardly been studied in the context of psychrophilic conditions and future research should focus on evaluating the influence of these variables on FW-AD under psychrophilic conditions. Through a bibliometric analysis, this paper has described and analyzed the FW-AD process, with a focus on the psychrophilic conditions (<20 °C) so as to identify advances and future research trends, as well as determine strategies toward improving the anaerobic process under low temperature conditions.

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Temperature has a great influence on anaerobic digestion of food waste (FW-AD).

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Studies on the psychrophilic condition are limited, warranting further research.

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Physical properties of the substrate and inoculum influence psychrophilic FW-AD.

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The use of inocula adapted to low temperatures could increase biogas production.

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Changes in reactor configurations could improve biogas yield at low temperature.

Dehydrated Food Waste for Composting: An Overview

Food waste disposal has recently received much attention worldwide due to its major impact on environmental pollution and economic costs. Using high moisture content of food waste has the highest negative environmental impact due to increased greenhouse gas emissions, odor, and leachate. Drying technologies play an important role in reducing the moisture content of food waste, which is necessary for environmental sustainability and safety. The first part of this review highlights that sun-drying is the most cost-effective drying method. However, it has not been widely recommended for food waste management due to several limitations, including the inability to control sunray temperature and the inability to control end-product quality. Thermal drying eliminates moisture from food waste quickly, preventing hydrolysis and biodegradation. Thermal dryers, such as the GAIA GC-300 dryer, and cabinet dryer fitted with a standard tray, are the best alternative to sun drying. The second part of this review highlights that dehydrated food waste products are slightly acidic (4.7–5.1), have a high electrical conductivity (EC) value (4.83–7.64 mS cm-1), with high nutrient content, due to low pH levels, dehydrated food waste is not suitable for direct use as a fertilizer for the plants. So, the dried food waste should be composted before application to the plants because the composting process will dominate the limitation of phytotoxins, anoxia, salinity, and water repellence. Trench compost can be a good choice for decomposing dried organic waste because trench compost relies solely on soil decomposing microorganisms and insects.

Current Trends in Biological Valorization of Waste-Derived Biomass: The Critical Role of VFAs to Fuel A Biorefinery

The looming climate and energy crises, exacerbated by increased waste generation, are driving research and development of sustainable resource management systems. Research suggests that organic materials, such as food waste, grass, and manure, have potential for biotransformation into a range of products, including: high-value volatile fatty acids (VFAs); various carboxylic acids; bioenergy; and bioplastics. Valorizing these organic residues would additionally reduce the increasing burden on waste management systems. Here, we review the valorization potential of various sustainably sourced feedstocks, particularly food wastes and agricultural and animal residues. Such feedstocks are often micro-organism-rich and well-suited to mixed culture fermentations. Additionally, we touch on the technologies, mainly biological systems including anaerobic digestion, that are being developed for this purpose. In particular, we provide a synthesis of VFA recovery techniques, which remain a significant technological barrier. Furthermore, we highlight a range of challenges and opportunities which will continue to drive research and discovery within the field. Analysis of the literature reveals growing interest in the development of a circular bioeconomy, built upon a biorefinery framework, which utilizes biogenic VFAs for chemical, material, and energy applications.

Recent advances in conductive materials amended anaerobic co-digestion of food waste and municipal organic solid waste: Roles, mechanisms, and potential application

Recently, conductive materials (i.e., carbon-based and iron-based materials) as a feasible and attractive approach have been introduced to anaerobic co-digestion (ACoD) system for promoting its performance and stability through direct interspecies electron transfer. Owing to the key roles of conductive materials in ACoD process, it is imperative to gain a profound understanding of their specific functions and mechanisms. Here, this review critically examined the state of the art of conductive materials assisted ACoD of food waste and common municipal organic solid waste. Then, the fundamental roles of conductive materials on ACoD enhancement and the relevant mechanisms were discussed. Last, the perspectives for co-digestate treatment, reutilization, and disposal were summarized. Moreover, the main challenges to conductive materials amended ACoD in on-site application were proposed and the future remarks were put forward. Collectively, this review poses a scientific basis for the potential application of conductive materials in ACoD process in the future.

Enhancement of anaerobic digestion by co-digesting food waste and water hyacinth in improving treatment of organic waste and bio-methane recovery

In Kenya, 57% of the municipal solid waste generated is Food waste (FW) which has high organic content. However, the treatment and bioconversion of FW to biogas have always been challenging due to its rapid biodegradation, resulting from rapid hydrolysis and accumulation of volatile fatty acids and lowering pH in the bioreactor. In this study, the anaerobic digestibility of FW as a mono substrate was compared to co-digestion of FW with water hyacinth (WH) for improved biogas production and organic matter removal efficiency in a laboratory batch reactor. Different mix proportions of FW and WH were co-digested under mesophilic conditions (37 °C) at a dilution of 6% (w/v) Total Solids (TS) content. The TS of the substrates (Food waste and Water Hyacinth) were pre-processed to have a concentration of TS at 6% (60 g/L) to operate a wet AD which requires the substrate to be less than 15% TS. The proportions of WH: FW (v/v) were 100:0, 85:15, 70:30, 55:45, 30:70, 15:85, and 0:100. In the batch rectors the anaerobic co-digestion was conducted with Substrate to Inoculum (S/I) ratio of 1:1. FW is generally considered to have high volatile solids which hydrolyze rapidly lowering pH arising from excess production of Hydrogen which in presence of CO₂ and acetogenic bacteria leads to more production of acetate, formate and other long chain fatty acids which inhibits methanogenesis as a result of rapid acidification. The rapid acidification of the bioreactors that are used to treat FW results in the inhibition of the methanogenesis process. The co-digestion of the substrates could have improved the process parameters by reducing acidity caused by the high C/N ratio, reducing the inhibitory range, and increasing the buffer capacity which enhanced the bio-methane potential and the microbial activity. The batch experiments were set in triplicate for both cases of FW, WH, mixtures, and Inoculum. The results showed that the average gas yields after 81 days for the various mix proportions were 256.27and 357.69 ml/g-VS for mono-digestion of WH and FW respectively. For the mixtures of WH: FW the average reported biogas production were 305.01, 280.27, 548.91,616.01 and 270.87 ml/g-VS for mixtures of 15:85, 30:70, 55:45,70:30 and 85:15 respectively. The modified Gompertz model showed that the digesters with WH and FW alone had lag times of 2.599 and 1.052 days respectively. The mix substrates of WH: FW 85:15, 70:30, 55:45, 30:70 and 15:85 shown lag times of 2.456, 3.777, 2.574, 1.956 and 1.75 days respectively. A mix (WH: FW) of 70:30 had the highest maximum specific biogas production Rmax and the maximum biogas production potential of 18.19 mlCH₄/gVS per day and 607.7mlCH₄/gVS respectively. The R² and RSME values ranged from 0.9867 to 0.9963 and 2.663 to 9.359 respectively in all the digesters. The study shows that the co-digestion of WH and FW in the mix ratio of 70:30 improved the volume of biogas produced and organic matter removal efficiency reached 79%.

Anaerobic digestion of urban wastes: integration and benefits of a small-scale system

This work presents an integrated approach of anaerobic digestion (AD) at the urban scale, based on on-site collected wastes, experimentation in a pilot reactor, and model approach. To cope with urban waste limitations (season inflow, organic matter decrease, limited area), it was proposed a settlement of wastewater (WW), a drying of food waste (FW), and silage of green waste (GW). The results obtained highlight the performance of these pretreatments to concentrate and/or preserve the organic matter over time. Co-digestion in a 30-liter reactor was then successfully carried out, resulting in an 83% of volatile solids reduction and stable methane production of 321 ml CH₄/g VS_add, in 35 days. Finally, we developed a standard neighbourhood model with AD based on these experimental results. It appears that 66% of the primary sludge (PS) and 28% of the FWs produced daily could be treated by the proposal process. 14% of the annual production of GW could thus be processed daily. From an energetic point of view, the process generates a surplus of respectively, 2500 MJ/year and 38,000 MJ/year in terms of electricity and heat. The generation of 15 times more heat energy than electricity, supports an installation of anaerobic digestion, close to homes. Moreover, if these results are encouraging, we suggest different scenarios of co-digestion ratios and operational parameters for their optimization.

Dry Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste: Biogas Production Optimization by Reducing Ammonia Inhibition

The aim of this work is to optimize biogas production from thermophilic dry anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) by comparing various operational strategies to reduce ammonia inhibition. A pilot-scale plug flow reactor (PFR) operated semi-continuously for 170 days. Three scenarios with different feedstock, namely solely OFMSW, OFMSW supplemented with structural material, and OFMSW altered to have an optimal carbon-to-nitrogen (C/N) ratio, were tested. Specific biogas production (SGP), specific methane production (SMP), the biogas production rate (GPR), and bioenergy recovery were evaluated to assess the process performance. In addition, process stability was monitored to highlight process problems, and digestate was characterized for utilization as fertilizer. The OFMSW and the structural material revealed an unbalanced content of C and N. The ammonia concentration decreased when the optimal C/N ratio was tested and was reduced by 72% if compared with feeding solely OFMSW. In such conditions, optimal biogas production was obtained, operating with an organic loading rate (OLR) equal to 12.7 gVS/(L d). In particular, the SGP result was 361.27 ± 30.52 NLbiogas/kgVS, the GPR was 5.11 NLbiogas/(Lr d), and the potential energy recovery was 8.21 ± 0.9 MJ/kgVS. Nevertheless, the digestate showed an accumulation of heavy metals and low aerobic stability.

Improving the energetic utilization of household food waste: Impact of temperature and atmosphere during storage

Food waste (FW) from households represents a major fraction of municipal waste and it is often collected in separate biowaste bins. Until waste collection is carried out, storage conditions in the biowaste bin influence FW properties. To draw conclusions for an optimized waste utilization in anaerobic digestion (AD), the aim of this study was to evaluate the impact of storage duration (20 to 40 days) and temperature (5 °C and 20 °C) on inherent energy potentials of household FW during aerobic and anaerobic storage. Therefore, physico-chemical parameters of recipe-based FW samples with reproducible initial compositions were monitored. After 20 days of aerobic storage, water contents (WC) were reduced from 61.9% to 39.5% (20 °C) and from 63.9% to 50.3% (5 °C) while organic dry matter (oDM) concentrations were lowered by 4.3% (20 °C) and 1.1% (5 °C). Increased pH-values of 6.6 (initially 5.5) were only measured for FW stored aerobically at 20 °C. In total, the energy potential was decreased by 31% (20 °C) and by 16% (5 °C). Thus, storage temperature and duration are crucial parameters for optimized aerobic FW storage leading to higher energy yields in AD. Instead, anaerobic storage of FW decreased pH-values to <5 while increasing WC in all samples (up to 67% at 20 °C). As oDM concentrations were preserved almost completely, the energy potential losses were only marginal proving that energy contents of FW could be preserved at household level. Consequently, energy yields in AD of FW could be increased through anaerobic storage conditions.

Temporal Variation of Nitrogen and Sulfur Species of Food Waste and Sludge during Anaerobic Co-Digestion

Anaerobic co-digestion (AcoD) has been a widely accepted method to treat food waste (FW) and sewage sludge (SS). However, there is a knowledge gap regarding the key speciation transformation of nitrogen and sulfur in AcoD. Here, we explored the changes of nitrogen (N) and sulfur (S) compounds in liquid digestion and biogas, as well as the composition of microbial community structure and related metabolic functions. The results showed that H2S in the biogas was the main form of S in the early stage, and then, it was converted into SO42− and SO32−, while NH3 and NH4+ were the main forms of N during the AcoD. In addition, bacterial diversity was associated with N and S compounds; Syntrophomonas and Aminobacterium were positively correlated to H2S, NH3, NH4+ and SO32−, and Saccharibacteria_genera_incertae_sedis, Candidatus_Cloacamonas and Thermomonas were positively correlated to SO42− and NO2−. Additionally, the FAPROTAX prediction showed that the functional composition related to N and S metabolism was different from SS and inoculum after the AcoD. This study provides detailed information of conversion of N and S of the AcoD, which could lay a foundation for the subsequent regulation of the mechanism of nitrogen and sulfur compounds in the methanogenic process.

Multilinear Regression Model for Biogas Production Prediction from Dry Anaerobic Digestion of OFMSW

The aim of this study was to develop a multiple linear regression (MLR) model to predict the specific methane production (SMP) from dry anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW). A data set from an experimental test on a pilot-scale plug-flow reactor (PFR) including 332 observations was used to build the model. Pearson′s correlation matrix and principal component analysis (PCA) examined the relationships between variables. Six parameters, namely total volatile solid (TVSin), organic loading rate (OLR), hydraulic retention time (HRT), C/N ratio, lignin content and total volatile fatty acids (VFAs), had a significant correlation with SMP. Based on these outcomes, a simple and three multiple linear regression models (MLRs) were developed and validated. The simple linear regression model did not properly describe the data (R2 = 0.3). In turn, the MLR including all factors showed the optimal fitting ability (R2 = 0.91). Finally, the MLR including four uncorrelated explanatory variables of feedstock characteristics and operating parameters (e.g., TVSin, OLR, C/N ratio, and lignin content), resulted in the best compromise in terms of number of explanatory variables, model fitting and predictive ability (R2 = 0.87).

Mechanical pre-treatment of source-collected municipal biowaste prior to energy recovery by anaerobic digestion

Pre-treatments are usually necessary to prepare biowaste to anaerobic digestion. The major objectives are (i) to remove undesirable materials such as plastics and metals, which may contaminate the biowaste even if separated source-collection systems are implemented, and (ii) to extract the most readily biodegradable organic fractions from the waste stream. In this study, two wet mechanical pre-treatments, namely air-compressed press and worm screw press, were investigated on urban household biowaste. Two liquid to solid ratios were tested in each pre-treatment. Anaerobic digestion of pre-treated biowaste was studied by measuring their biomethane potentials and by controlled experiments in a continuously stirred-tank reactor with a feed load of 3.5 gVS.L^-1.d^-1. It was observed that increasing liquid to solid ratio in the pre-treatments allowed to increase the proportion of biodegradable organic matter extracted from the biowaste, up to 949 gCOD.kgTS^-1 from household biowaste. The biomethane potentials of pre-treated waste were very high (up 525 LCH₄.kgVS^-1) and COD (949 gCOD.kg^-1TS) from household biowaste. Anaerobic digestion in continuously stirred-tank reactor showed a very strong conversion of COD load (81%) and a high methane production up to 345 LCH₄.kgVS^-1.

"Food waste-wastewater-energy/resource" nexus: Integrating food waste management with wastewater treatment towards urban sustainability

Sustainable food waste management is a global issue with high priority for improving food security and conserving natural resources and ecosystems. Diverting food waste from the solid waste stream to the wastewater stream is a promising way for food waste source separation, collection, treatment, and disposal. Given the advances in wastewater treatment, this integrated system has great potential for the concurrent recovery of water, resource, and energy. To this end, many efforts from lab-scale to full-scale studies have been devoted to evaluating the feasibility and associated impacts on both solid waste and wastewater systems. This paper summarizes the current status of food waste diversion from the aspects of principle and application. The impacts of food waste diversion on solid waste treatment, sewer system, wastewater treatment, and environmental benefits have been comprehensively reviewed and analysed. In the context of the critical review, this paper further identified the challenges of food waste diversion in unified definitions of the field, sewer network assessment, emerging wastewater treatment technologies, scale-up studies, and policy drivers. Perspectives on the contribution of food waste diversion to a food waste management hierarchy were discussed for initiating the nexus of "food waste-wastewater-energy/resource". We conclude that food waste diversion could facilitate sustainable urban development, but the area-specific factors (e.g., household practices, water resource, sewerage system condition, and treatment techniques) require adequate evaluations to determine the implementation. The outcomes of this study could contribute to the practice and policy-making of food waste management towards urban sustainability.

Comparative Fuel Yield from Anaerobic Digestion of Emerging Waste in Food and Brewery Systems

Food waste (FW), a major part of the US waste stream, causes greenhouse gases within landfills, but there is an opportunity to divert FW to anaerobic digestion (AD) facilities that produce biogas and digestate fertilizer. The composition of FW inputs to AD determines the value of these products. This study provides insight into the effect of waste composition on the quality of AD products by first characterizing the biogas and digestate quality of anaerobically digested FW from four diets (paleolithic, ketogenic, vegetarian, and omnivorous), and then estimating the difference in biogas produced from codigested FW and brewery waste (BW). Waste feedstock mixtures were incubated in lab-scale bioreactors for 21 days with live inoculum. Biogas quality was monitored for 21–30 days in four trials. Samples were analyzed using a gas chromatograph for detection of methane (CH4) and carbon dioxide (CO2). The composition of the waste inputs had a significant impact on the quality of biogas but not on the quality of the digestate, which has implications for the value of post-AD fertilizer products. Wastes with higher proportions of proteins and fats enhanced biogas quality, unlike wastes that were rich in soluble carbohydrates. Codigestion of omnivorous food waste with carbon-rich agricultural wastes (AW) improved biogas quality, but biogas produced from BW does not necessarily improve with increasing amounts of AW in codigestion.

Residual municipal solid waste as co-substrate at wastewater treatment plants: An assessment of methane yield, dewatering potential and microbial diversity

Separately collected organic fraction of municipal solid waste, also known as biowaste, is typically used to fill the available capacity of digesters at wastewater treatment plants. However, this approach might impair the use of the ensuing digestate for fertilizer production due to the presence of sewage sludge, a contaminated substrate. Worldwide, unsorted municipal solid household waste, i.e. residual waste, is still typically disposed of in landfills or incinerated, despite its high content of biodegradables and recyclables. Once efficiently separated from residual waste by mechanical processes, the biodegradables might be appropriate to substitute biowaste at wastewater treatment plants. Thus, the biowaste would be available for fertilizer production and contribute to a reduction in the demand on non-renewable fertilizers. This study aimed at determining the technical feasibility of co-digesting the mechanically separated organic fraction of residual waste with sewage sludge. Further, key parameters for the implementation of co-digestion at wastewater treatment plants were determined, namely, degradation of the solids and organics, specific methane production, flocculant demand, and dewatered sludge production. The microbial community and diversity in both mono- and co-digestion was also investigated. Semi-continuous laboratory scale experiments showed that the co-substrate derived from the residual waste provided a stable anaerobic co-digestion process, producing 206 to 245 L of methane per kg of volatiles solids added to the digester. The dewaterability of the digestate increased by 4.8 percentage points when the co-substrate was added; however, there was also an increase in the flocculant demand. The specific dewatered sludge production was 955 kg per ton of total solids of co-substrate added to the digester. Amplicon sequencing analysis provided a detailed insight into the microbial communities, which were primarily affected by the addition of co-substrate. The microbiota was fully functional and no inhibition or problems in the anaerobic digestion process were observed after co-substrate addition.

Market waste composition analysis and resource recovery potential in Kumasi, Ghana

Municipal solid waste constitutes significant quantities of waste generated in markets. Markets produce substantial quantities of fruit and vegetable waste, a source of nuisance in landfills. In Ghana, market waste (MW) appears to be unexplored and has limited data available. The need for MW valorization in the face of a circular economy requires reliable knowledge of MW properties. The study determined the waste compositions of selected major markets from two different classes of settlement in Kumasi and the seasonal effect on the compositions. The chemical properties of organics were determined via proximate and ultimate analyses and the theoretical biomethane potential, with the Buswell equation. From the results, MW composition in the wet season is 59.6% organic, 11.4% plastics, 8.3% paper, 5.3% textiles, 4.7% inert, 4.1% miscellaneous, 2.1% metal, 1.8% glass and 2.8% leather. The dry season values are 45.8% organic, 14.6% plastics, 12.7% paper, 7.3% textiles, 6.4% inert, 4.3% miscellaneous, 2.3% metal, 2.6% glass and 3.9% leather. An ANOVA indicates significant differences between the two seasons and some waste components; organics, plastics, paper and cardboard, leather, and inert. The high calorific values recorded ranged from 14.8 MJ kg^-1 to 16.6 MJ kg^-1. The biogas potential and biomethane content ranged from 775.3 l/kgVS to 828.9 L/kgVS and 50% to 57% respectively.Implications: Market waste (MW) in Ghana appears to be an unchartered area and there is limited data on market generation and composition. The need for MW valorization requires reliable knowledge on MW properties. This study explores MW characteristics of six major market from two different classes of settlements in a developing country. Study findings suggest that the quantities of market organics are higher than household waste. Again, MW composition can be influenced by season and geographical location. Furthermore, the study establishes the potential of MW in considerable quantities of biogas and methane generation, in comparison with household waste.

Estimation of Sustainable Bioenergy Production from Olive Mill Solid Waste

The disposal of olive wastes and their wastewater is a major problem worldwide. An important recycling chain can be formed through biogas production and energy conversion from olive waste. This study developed an efficient and effective sustainable model for biogas production using anaerobic digestion conditions with the co-digestion of pretreated olive waste. The sample used was hard olive pomace, which was dried in an oven before being crushed to fine particles with a mortar and pestle. The sample was analyzed by a CE-440 Elemental Analyzer, and Fourier Transform Infrared Spectrophotometer (FTIR) analysis was performed using Shimadzu IRTracer-100. Through the analysis, a substantial amount of electrical energy of 769 kWh/t was found to be generated per ton of olive pomace due to the high volatile solid (VS) percentage of organic waste material incorporated during the calculation. Reduced land area for landfilling olive waste was calculated to be 108 m2 per year, whereas the potential to reduce landfill leachate production was evaluated to be 0.32 m3 per year.

Influence of the Preliminary Storage on Methane Yield of Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste

Anaerobic digestion (AD) is a suitable management option for the energy valorization of many wastes, including the organic fraction of municipal solid waste (OFMSW). However, in some cases, long storage after the separate collection of this waste is required for management reasons, especially when the amount of waste to be treated temporarily exceeds the capacity of available AD plants. This study evaluates the biochemical methane potential (BMP) of the OFMSW after preliminary storage of 2, 6, and 10 days, in order to assess whether they are still suitable for AD or not. Moreover, the accuracy of three kinetic models (first order, Gompertz, and logistic models) in estimating the methane yield of stored OFMSW is tested. The resulting methane yield was between about 500 and 650 NmL·gVS−1 and slightly increased with the increase of the storage time after collection. Overall, this study has demonstrated that storage of OFMSW, when the collected amount of solid waste exceeds the treatment capacity of AD plants, a storage time up to 10 days does not impact the methane yield of the process.

Thermally enhanced solubilization and anaerobic digestion of organic fraction of municipal solid waste

Organic fraction of municipal solid waste (OFMSW) is an ideal substrate for biogas production; however, complex chemical structure and being heterogeneous obstruct its biotransformation in anaerobic digestion (AD) process. Thermal pre-treatment of OFMSW has been suggested to enhance the solubilization and improve the anaerobic digestibility of OFMSW. This paper critically and comprehensively reviews the characterization of OFMSW (physical, chemical, bromatological) and enlightens the valuable properties of OFMSW for waste valorization. In following sections, the advantages and limitations of AD of OFMSW are discussed, followed by the application of temperature phased AD, and various thermal pre-treatments, i.e., conventional thermal, microwave, and thermo-chemical for high rate bioenergy transformation. Effects of pre-treatment on COD, proteins, sugars and VS solubilization, and biogas yield are discussed. Formation of recalcitrant during thermal pre-treatment and the effect on anaerobic digestibility are considered. Full scale application, and techno-economic and environmental feasibility of thermal pre-treatment methods are also revealed. This review concluded that thermophilic (55 °C) and temperature phased anaerobic digestion, temperature phased anaerobic digestion, TPAD (55 + 37 °C) processes shows effective and stable performance at low HRTs and high OLRs and achieved higher methane yield than mesophilic digestion. The thermal pre-treatment at a lower temperature (120 °C) improves the net energy yield. However, high-temperature pre-treatment (>150 °C) result in decreased biogas yield and even lower than the non-pre-treated OFMSW, although a high degree of COD solubilization. The OFMSW solubilization in terms of COD, proteins, and sugars cannot accurately reflect thermal/hybrid pre-treatments' potential. Thus, substrate pre-treatment followed by anaerobic digestibility of pretreated substrate together can evaluate the actual effectiveness of thermal pre-treatment of OFMSW.

Biogas and biofertilizer production from organic fraction municipal solid waste for sustainable circular economy and environmental protection in Malaysia

Waste management in Malaysia remains a persistent economic and environmental challenge. Up to date, more than 80% of Malaysian solid waste disposed at landfills and dumpsites. Therefore, Malaysia is facing an urgent need to move towards a sustainable solid waste management and thus resource recovery from organic solid waste. Hence, this study aims to investigate the feasibility of energy and bio fertilizer recovery from organic fraction municipal solid waste (OFMSW) via anaerobic digestion. The economic and environmental benefit analysis was investigated. Approximate and elementary analysis of OFMSW samples were carried out to estimate the potential production of biogas and bio fertilizer. It was found that organic waste contributes about 45% of the total MSW generated in Malaysia. Anaerobic digestion of 50% of organic waste is expected to produce 3941 MWh/day of electrical energy and 2500 t/day of bio fertilizer. In terms of environmental impacts, 2735 t/day of Carbon dioxide (CO₂) emission, 1128 m²/day of landfilling area and 481 m³/day of leachate can be avoided. A net revenue of 3300 million RM (1 US Dollar ≈ 4.15 RM) can be generated by the sales of electricity via Feed-in-Tariff (FiT), sales of biofertilizer to local agricultural industries and inclusive of the saving generated from the reduction of OFMSW landfilling operations and leachate treatment at landfills. Economic development can go hand-in-hand with environmental sound practices in the field of waste management.

Assessing agro-food system circularity using nutrient flows and budgets

Nutrient reuse and recycling is a key strategy towards more circular and sustainable food systems and depends on the specific conditions of the area under study, such as geography and the type of agricultural system. In this study we analysed nutrient flows and assessed the circularity of a livestock-dominated and export-oriented agro-food system at different system levels and spatial scales. We quantified the nitrogen (N), phosphorus (P) and potassium (K) flows and soil balances in the Belgian agro-food system at the sub-regional, regional and national scale, and assessed five P-based indicators that capture different aspects of circularity: total inputs, phosphorus use efficiency, share of reused to total input, recycling rate, and losses. We found that nutrient soil balances depend on the type of agricultural system: areas with intense livestock production accumulate up to 108 kgN/ha, 4.8 kgP/ha and 150 kgK/ha in their soil annually, whereas areas of mostly arable production have low N and K surpluses of <20 kg/ha and P deficits of < -10 kg/ha. We further found that Wallonia, the southern region of the country that is characterized by lower livestock densities and a partial reuse of sewage sludge, outperforms the Flemish region in the North in all five indicators. The food system in the whole of Belgium has a 34% phosphorus use efficiency rate and a 63% overall recycling rate, while 84% of the total inputs in agriculture are from secondary sources. Our results show that the type of production system is the most crucial determinant for circularity, and highlight the benefit of working at different levels and spatial scales to capture all aspects of circularity in agro-food systems.

Determination of the dewatered digestate amounts and methane yields from the co-digestion of biowaste as a basis for a cost-benefit analysis

Co-digestion is the simultaneous digestion of two or more substrates and a common practice at wastewater treatment plants (WWTPs). The amounts of methane and digested sludge produced are key parameters for evaluating the economic efficiency of co-digestion. However, the share of dewatered digestate produced from co-substrates is not known. Synergistic effects in co-digestion, i.e. a better biodegradability compared to the mono-digestion of each substrate, might reduce the amounts of digested sludge and increase methane yields. However, these effects might also influence the calculation of methane and digestate quantities from co-substrates. The main objective of this work was to provide a basis for the cost-benefit analysis of biowaste (BW) co-digestion at WWTPs for this data. Therefore, continuous and batch experiments with sewage sludge (SS) and BW co-digestion were conducted and evaluated for methane and digestate production, and possible synergistic effects. BW co-digestion led to an additional production of 0.35 t total solids (TS) of dewatered sludge per ton TS_added in continuous and 0.23 t TS of dewatered sludge per ton of TS_added in batch experiments. The methane yield from BW was 441 L/kg TS_added in continuous experiments and 482 L/kg TS_added batch test. No synergistic effects were observed in both batch and continuous co-digestion experiments. Batch tests were found to be suitable for a rough estimation of the co-digestion economic efficiency key parameters. Continuous experiments are recommended to obtain more robust data. A cost-benefit analysis found that electricity production from co-digestion can generate savings of 88-170 €/t TS_added compared to grid purchase.

Anaerobic co-digestion of fruit and vegetable waste: Synergy and process stability analysis

Anaerobic mono- and co-digestion of fruits and vegetable waste (FVW), slaughterhouse waste (SHW), and cattle manure (CM) under mesophilic conditions (35°C) were conducted through biochemical methane potential tests to investigate how the FVW in a co-substrate formulation improves the methane yield, the degradative synergy between substrates, and especially the stability of the process. The co-digestion of FVW:SHW and FVW:CM were evaluated with volatile solids (VS) ratios of 1:2, 1:1, and 2:1. The results indicated that the highest synergistic effect was found in the co-digestion FVW:CM at 1:1 VS ratio. However, the co-digestion FWV:SHW at 1:2 VS ratio increased the methane yield by 74.2% compared to the mono-digestion of FVW (776.3 mL CH₄ g VS_added^-1). As a critical condition in these processes, the stability was evaluated using the early warning indicator VFA/TA (volatile fatty acids/total alkalinity). The co-substrate SHW promotes greater stability in methane production as the soluble carbohydrate content in FVW increases. It was proposed that the high protein (49.04 ± 0.96% VS) and ammonia content (693 ± 3 mg L^-1) of SHW leads to the formation of a dampening system known as a carbonate-acetic buffer. It was concluded that balanced distribution between carbohydrates, proteins, and lipids is crucial to increase methane yields, and the low methane productions were associated with low N-NH₄⁺ concentrations (FVW:CM co-digestions). The results obtained in this study can serve as a basis to design full-scale digesters under similar operating conditions and with the same substrate:co-substrate relationships.Implications: The production of methane from the anaerobic digestion process of food, and lose waste presents a viable alternative of valorization and could help to mitigate environmental impacts. However, anaerobic digestion from these substrates carries high instabilities and low methane yields. The need to increase these yields and contribute to process stability must be considered in the selection of a co-substrate. In this context, this work aims to evaluate the best fruits and vegetable waste: co-substrateformulation, that promotes higher methane yield, a synergy between substrates, and to improve the AD process stability in the presence of perturbations in the substrate composition. We believe that our results could be helpful for the design processes for methane production from fruit and vegetable waste, to contribute to competitiveness with conventional energies and promote the sustainability of these processes.

Mechanical pretreatment of municipal biowaste to produce an aqueous slurry dedicated to anaerobic digestion

The present study investigated a wet mechanical pretreatment to improve methane production by anaerobic digestion from biowaste material by separating a biodegradable aqueous slurry fraction (ASF) from a more recalcitrant particulate fraction (PF). Four source-sorted municipal biowastes were studied, namely household (HBW), supermarket (SBW), restaurant (RBW), and green biowaste (GBW). The treatment consisted in soaking the waste in water and then pressing the slurry through a grid with 3-mm openings to separate the two fractions. Methane production of ASF and PF obtained from the four biowastes were measured using the BMP protocol and compared to the potential of the respective untreated biowaste. Results were very different for GBW as compared to the other three BWs. With GBW, which was the most lignocellulosic of the BW studied, only 17% of the initial methane potential was recovered in the ASF. The extraction was much better on the other biowastes and increased in the following order: HBW (58%) ≃ RBW (57%) < SBW (67%). The ASF from these biowastes exhibited low total solid contents and high BMPs (416, 408, and 423 NL_CH4.g^-1_vs for HBW, RBW, and SBW respectively). The experimental results obtained in this study therefore showed that wet pressing separation was an efficient pretreatment to improve and facilitate methane production by anaerobic digestion of biowaste such as HBW, RBW, and SBW.

A review on the valorisation of food waste as a nutrient source and soil amendment

Valorisation of food waste offers an economical and environmental opportunity, which can reduce the problems of its conventional disposal. Food waste is commonly disposed of in landfills or incinerated, causing many environmental, social, and economic issues. Large amounts of food waste are produced in the food supply chain of agriculture: production, post-harvest, distribution (transport), processing, and consumption. Food waste can be valorised into a range of products, including biofertilisers, bioplastics, biofuels, chemicals, and nutraceuticals. Conversion of food waste into these products can reduce the demand of fossil-derived products, which have historically contributed to large amounts of pollution. The variety of food chain suppliers offers a wide range of feedstocks that can be physically, chemically, or biologically altered to form an array of biofertilisers and soil amendments. Composting and anaerobic digestion are the main large-scale conversion methods used today to valorise food waste products to biofertilisers and soil amendments. However, emerging conversion methods such as dehydration, biochar production, and chemical hydrolysis have promising characteristics, which can be utilised in agriculture as well as for soil remediation. Valorising food waste into biofertilisers and soil amendments has great potential to combat land degradation in agricultural areas. Biofertilisers are rich in nutrients that can reduce the dependability of using conventional mineral fertilisers. Food waste products, unlike mineral fertilisers, can also be used as soil amendments to improve productivity. These characteristics of food wastes assist in the remediation of contaminated soils. This paper reviews the volume of food waste within the food chain and types of food waste feedstocks that can be valorised into various products, including the conversion methods. Unintended consequences of the utilisation of food waste as biofertilisers and soil-amendment products resulting from their relatively low concentrations of trace element nutrients and presence of potentially toxic elements are also evaluated.

A Conceptual Framework for Incorporation of Composting in Closed-Loop Urban Controlled Environment Agriculture

Controlled environment agriculture (CEA), specifically advanced greenhouses, plant factories, and vertical farms, has a significant role to play in the urban agri-food landscape through provision of fresh and nutritious food for urban populations. With the push towards improving sustainability of these systems, a circular or closed-loop approach for managing resources is desirable. These crop production systems generate biowaste in the form of crop and growing substrate residues, the disposal of which not only impacts the immediate environment, but also represents a loss of valuable resources. Closing the resource loop through composting of crop residues and urban biowaste is presented. Composting allows for the recovery of carbon dioxide and plant nutrients that can be reused as inputs for crop production, while also providing a mechanism for managing and valorizing biowastes. A conceptual framework for integrating carbon dioxide and nutrient recovery through composting in a CEA system is described along with potential environmental benefits over conventional inputs. Challenges involved in the recovery and reuse of each component, as well as possible solutions, are discussed. Supplementary technologies such as biofiltration, bioponics, ozonation, and electrochemical oxidation are presented as means to overcome some operational challenges. Gaps in research are identified and future research directions are proposed.

Dry Mesophilic Anaerobic Digestion of Separately Collected Organic Fraction of Municipal Solid Waste: Two-Year Experience in an Industrial-Scale Plant

In this paper, performance analysis over two years’ operation of an industrial anaerobic digestion (AD) plant of a separately collected organic fraction of municipal solid waste is presented. The continuous plug-flow AD plant is still regularly operating and it has been fully operational since September 2018. Since then, it has been supplied with 40,000 t/y of pretreated separately collected organic fraction of municipal solid waste from municipalities of the Calabria region in Southern Italy. The AD process is carried out in a mesophilic regime at 40 ± 0.5 °C, using a constant hydraulic retention time (HRT) of 22 days and a substrate with average total solids and average total volatile solids of 30.0% and 22.2%, respectively. In the last two years, the plant produced an average of 191 m3 and 860 m3 of biogas per tonne (t) of organic input material and of total volatile solids, respectively, with an average methane specific production of 508 m3/t (total volatile solids). The average CH4 percentage in the biogas was of 59.09%. The obtained results came out from the combination of high organic content of separately collected organic fraction of municipal solid waste, optimized pretreatment system and operating conditions adopted.

Blast-furnace slag cement and metakaolin based geopolymer as construction materials for liquid anaerobic digestion structures: Interactions and biodeterioration mechanisms

In order to promote the development of the biogas industry, solutions are needed to improve concrete structures durability in this environment. This multiphysics study aims to analyse the multiphases interactions between the liquid phase of an anaerobic digestion system and cementitious matrices, focusing on (i) the impacts of the binder nature on the anaerobic digestion process at local scale, and (ii) the deterioration mechanisms of the materials. Cementitious pastes made of slag cement (CEM III), innovative metakaolin-based alkali-activated material (MKAA), with compositions presumed to resist chemically aggressive media, and a reference binder, ordinary Portland cement (CEM I), were tested by immersion in inoculated cattle manure in bioreactors for a long period of five digestion cycles. For the first time it was shown that the digestion process was disturbed in the short term by the presence of the materials that increased the pH of the liquid phase and slowed the acids consumption, with much more impact of the MKAA. However, the final total production of biogas was similar in all bioreactors. Material analyses showed that, in this moderately aggressive medium, the biodeterioration of the CEM I and CEM III pastes mainly led to cement matrix leaching (decalcification) and carbonation. MKAA showed a good behaviour with very low degraded depths. In addition, the material was found to have interesting ammonium adsorption properties in the chemical conditions (notably the pH range) of anaerobic digestion.

Comparison of microbial communities during anaerobic digestion of kitchen waste: Effect of substrate sources and temperatures

In this study, batch experiments were conducted to compare the effect of temperature and substrate source on microbial communities in the anaerobic digestion of kitchen waste. The results showed that the microbial communities of anaerobic digestion were not sensitive to varied sources of waste, but shifted with the change in operating temperatures. In the reactors operated at mesophilic conditions, Levilinea, Syntrophomonas, Methanothrix, and Methanosphaerula, etc. were the dominant microbes during the process. While in thermophilic reactors, Levilinea, Ornatilinea, Methanosphaerula and Methanomassiliicoccus, etc. prevailed. Meanwhile, an enrichment in Coprothermobacter, Defluviitoga, Defluviitalea, Tepidimicrobium, Lutispora and Fonticella were observed as the temperature changed from mesophilic to thermophilic, suggesting these genera could be selectively enriched at thermophilic conditions. The results provided fundamental understanding of the microbiology that could support the scale up of food waste anaerobic digestion.