Harnessing energy from food waste via anaerobic digestion: A comprehensive review

Managing food waste is key to tackling climate change

Anaerobic digestion (AD) of food waste (FW) is considered an environmentally sustainable process that can divert the disposal of FW to landfill and prevent greenhouse gas (GHG) emissions in managing the FW. Although several studies have attempted to demonstrate the AD of FW, low methane yields and a high incidence of process instability have been reported due to the rapid generation and accumulation of volatile fatty acids (VFAs). This paper reviews the recent research and development with high variation in FW composition, such as the carbon-to-nitrogen (C/N) ratio and, consequently, the effect of its physicochemical composition on process performance and methane yields. The paper highlights the significance of optimizing the anaerobic co-digestion (AcoD) of FW with carbon-rich substrates such as garden waste (GW) and/or the addition of trace elements as strategies that can improve the process performance and methane yields from FW. This review focuses on the factors effecting the feasibility of food organics and garden organics (FOGO) as a substrate for methane production. The review also critically analyses the prospects of enhancement of biomethane yield by optimizations of the impactful parameters. The progress in research related to these methods and identifying existing limitations to efficient AD of FOGO are the key findings of this review. This review also assesses the impact of nanotechnology on the process performance of the digester. The integration of FO and GO in AD processes has demonstrated enhanced biogas yields, improved process stability, and better waste management outcomes compared to the digestion of either substrate alone. Despite these advantages, challenges such as feedstock variability, process optimization, and the need for advanced pretreatment methods remain. Addressing these issues through continued research and technological innovations will be crucial for maximizing the efficiency and scalability of AD systems. Moreover, the economic feasibility and policy frameworks supporting AD need further development to promote broader adoption.

The C/N ratio and organic loading rates (OLRs) are the main constraint factors of anaerobic digestion (AD) which symbols in digestion mortifications due to the rapid accumulation of volatile fatty acids (VFAs) and total ammonia nitrogen (TAN). In this study, the addition of high C-content wastes; paper waste (PW), cardboard waste (CW), and tissue waste (TW) into food waste (FW) feedstock is individually taken into consideration. At first, batch anaerobic digestion is conducted at a balanced C/N ratio of waste materials and then CSTR anaerobic digestion is carried out at different OLRs to investigate the potential of biogas production and process stability, under mesophilic conditions. In batch anaerobic digestion, co-substrate (PW + FW) feedstock is outperformed the other co-substrate feedstocks in terms of biogas production and process stability due to slow formation of VFAs and stable microbial conversion of VFAs into biogas fuel. The highest specific biogas yield (SBY) of 651 mL/gVS and specific methane yield (SMY) of 350 mL/gVS with a 57% rate of biodegradability are obtained from (PW + FW) feedstock at OLR of 15 gVS/L d. Furthermore, (PW + FW) feedstock with superb performance is then subjected to investigate the optimum OLR in CSTR digester for 30 days. The SBY of 22 L/gVS and SMY of 13 L/gVS is obtained with the utmost rate of VS reduction of 96%. The pH, alkalinity, COD, TS reduction, VS reduction, VFAs, and TAN concentration are all in the optimal range at the C/N ratio of 25 and OLR of 2 gVS/L d. This study provides new information regarding the C/N ratio and OLR for AD own great potential in improving the methane yield and productivity of PW and FW.

The objective of this study was to determine methane yields (MY) of organic wastes in biogasification facilities according to the mixing ratio of food waste/food waste leachate and sewage sludge. One biogasification facility that treated sewage sludge only was compared with three biogasification facilities treating sewage sludge and food waste. The theoretical MY was derived based on analyses of carbohydrate, fat, and protein to examine the efficiency of the biogasification facility. The average actual MY was 0.424 Sm3CH4/kg volatile solids, which corresponded to 83.7% of theoretical MY. In the case of combined anaerobic digestion (CD) mixing with food waste/food waste leachate, inhibitory factors (volatile fatty acids [VFAs], total nitrogen [TN], and organic matter contents) showed the tendency to have relatively higher values in CD facilities than in the biogasification facility treating sewage sludge only. Mean concentrations of VFAs and TN in the anaerobic digester effluent, and the organic loading rate were 406 mg/L, 3,721 mg/L, and 1.62 kg volatile solids/m3 day, respectively. The influence of anaerobic digester effluent was in charge of 10% within the influent environmental loading rate from the sewage treatment plants associated with the biogasification facilities. Analyses of the microbial community showed that a remarkable change in the structure of methanogens was directly related to different MY in each plant. In particular, Methanoculleus and Methanosaeta increased with an increasing ratio of food waste/food waste leachate to sludge, while Methanococcus and Methanosarcina decreased. In conclusion, CD showed steady operational conditions and high efficiency of MY by injecting food waste/food waste leachate into the anaerobic digester. It met the current criteria for integrated treatment of organic waste in biogasification facilities in South Korea.

The increasing global energy demand, driven by urbanization and population growth, has intensified waste generation and dependence on fossil fuels, exacerbating climate change. This study investigates the performance of anaerobic digestion (AD) in converting organic waste into biogas and nutrient-rich digestate, with a focus on key operational parameters: pH, temperature, organic loading rate (OLR), particle size (PS), carbon-to-nitrogen ratio (C/N), hydraulic retention time (HRT), moisture content (MC), and the stages of microbial conversion. Laboratory-scale experiments demonstrated that maintaining a pH range of 6.8-7.2, mesophilic temperatures (35-37°C), and a C/N ratio between 20 and 30 significantly improved methane yield. Co-digestion strategies and electromagnetic field stimulation enhanced biogas production by up to 25%, addressing common issues of low methane content and process instability. The study concludes that optimizing substrate composition and reactor design can substantially increase AD efficiency, making it a viable technology for sustainable energy generation and waste management.

Energy from food waste

BACKGROUNDThe accumulative municipal solid waste (MSW) production calls for emerging efficient technology for its handling. Anaerobic digestion (AD) of MSW provides effective waste volatilization. The high C/N ratio of MSW and increasing organic loading rates (OLRs) are central AD restrictions that affect the AD process performance, inhibition due to aggregation of volatile fatty acids (VFAs), and rapid fall in pH.RESULTSThis study examines the consequence of OLRs on AD of MSW with a high C/N ratio of 406 in food waste (FW) to have a balanced C/N ratio of 30. Three batch scale digesters investigate under mesophilic conditions (35 °C) with OLRs of 10, 15, and 20 gVS L−1to assess the digester performance and stability in biogas yield, methane yield, and volatile solids (VS) reduction. The cumulative biogas and methane yield are observed to be 1336 and 776 mL gVS−1, respectively, with aVSreduction rate of 78%, at 10 gVS L−1. VFA/alkalinity ratio ranges from 0.02 to 0.01 at OLR of 10 and 15 gVS L−1, which designates a higher buffering capability of the digester. While VFA/alkalinity ratio of 0.48 observes at OLR of 20 gVS L−1. A rapid deprivation in digester performance and stability finds at ORL of 15 and 20 gVS L−1. The cumulative biogas yield and methane yield decrease with the increase in OLR from 10 to 20 gVS L−1.CONCLUSIONThis study provided sufficient information for better AD processes and operational circumstances that are an optimum and effective method to convert organic matter to biogas fuel. © 2021 Society of Chemical Industry (SCI).

Anaerobic co-digestion (AcoD) has emerged as an effective technology for treating poultry manure (PM) and food waste (FW), converting these organic materials into valuable biogas and bio-fertilizers. The management of PM and FW has become increasingly critical due to environmental challenges associated with their disposal, which often leads to pollution and greenhouse gas (GHG) emissions. By employing AcoD, both PM and FW can be processed simultaneously, optimizing resource utilization and enhancing waste management (WM) strategies. This strategy relies on the use of sustainable waste-to-energy technology (WET) that harnesses agricultural biomass as a clean, renewable, and carbon-neutral energy source. The advantages of AcoD over traditional anaerobic digestion (AD) include improved biogas yields, better nutrient balance, and enhanced microbial stability due to the diverse substrate mix. Co-digesting animal-based wastes, such as PM, with plant-based wastes like FW enables more efficient degradation of organic matter, leading to increased methane production. Various studies have demonstrated the successful co-digestion of PW with various types of FW, resulting in significant increases in methane yields, as a potent renewable energy source. This review critically highlights the characteristics and benefits of AcoD, focusing on the synergistic effects that various FW substrates have when paired with PM. Essential process parameters influencing microbial activity, such as pH, organic loading rate (OLR), temperature, carbon-to-nitrogen (C/N) ratio, and hydraulic retention time, are discussed for optimizing biogas production (BGP). The findings underscore the greenness of AcoD as an eco-friendly approach, reducing dependency on fossil fuels, and mitigating environmental pollution, thereby contributing to sustainable WM practices and BGP.

Parallel pilot-scale anaerobic digestion systems were conducted to evaluate the influence of system temperatures (30°C and 35°C) on digestion performance, greenhouse gas control and economic efficiency. Biogas productions (6.64- 12.96 m3/d) and methane yields (0.46-0.61 m3/kg VS) of 35°C digestion system were significantly higher than those of 30°C digestion system with the organic loading rate (OLR) of 2.0-4.5 kg VS/m3•d. Two regression equations of methane yields with increasing OLRs were fitted at 30°C and 35°C to predict the methane production of practical food waste (FW) digestion plants. By analyzing process stability, the optimal operating OLRs of 35°C digestion system (4.0 kg VS/m3•d) was found to be higher than that of 30°C digestion system (3.0 kg VS/m3•d), indicating that the 35°C digestion system had better processing capacity. The greenhouse gas emission under corresponding optimal operating OLR of 35°C digestion system was also calculated to be better than that of 30°C digestion system. Even the system temperature of 30°C was found to be more suitable for the digestion where OLR was less than 3.0 kg VS/m3•d, a higher operational temperature of 35°C was still a better choice for conventional high-solid digestion. Keywords: food waste, anaerobic digestion, pilot-scale, organic loading rate, greenhouse gas, economic efficiency DOI: 10.3965/j.ijabe.20160901.1748 Citation: Wang L, Zhu B N, Yuan H R, Liu Y P, Zou D X, Li X J. Comparative investigations on pilot-scale anaerobic digestion of food waste at 30°C and 35°C. Int J Agric & Biol Eng, 2016; 9(1): 109－1187.

This paper provides a comprehensive review of hybrid waste‐to‐energy (WTE) systems that integrate anaerobic digestion (AD) and biomass gasification, emphasizing their synergistic benefits in sustainable energy production and waste management. By combining biochemical and thermochemical processes, these hybrid systems maximize energy recovery, optimize resource utilization, and significantly mitigate environmental impacts. The study highlights the principles and operational dynamics of standalone AD and gasification technologies, showcasing how their integration addresses limitations such as incomplete biomass conversion and excessive digestate production. Hybrid systems demonstrate superior performance in converting diverse biomass feedstocks, including municipal solid waste (MSW), agricultural residues, and food waste, into renewable energy and valuable by‐products. Advancements in reactor designs, pretreatment techniques, and system configurations are discussed, with a focus on enhancing energy efficiency and reducing greenhouse gas (GHG) emissions. Pretreatment methods such as AD pretreatment and advanced sorting mechanisms are explored to address feedstock variability and improve process stability. Key synergies, such as utilizing waste heat from gasification to dry AD residues, further boost overall system efficiency. The paper identifies critical operational parameters such as feedstock composition and reactor conditions that influence system performance and explores emerging solutions. Economic and environmental benefits, such as improved energy yields and cost efficiency, demonstrate the potential of hybrid AD–gasification systems. Despite the advantages, challenges persist, particularly in scaling hybrid systems and managing feedstock variability. Infrastructural limitations and the complexity of balancing AD and gasification processes remain significant barriers to widespread adoption. By reviewing existing research and case studies, this paper underscores the critical role of hybrid systems in achieving global renewable energy goals and sustainable waste management practices. Ultimately, hybrid AD–gasification systems offer a promising pathway for transitioning to cleaner energy systems, maximizing waste valorization, and supporting the global shift toward a circular economy.

This study investigated the effect of organic loading rate (OLR) on anaerobic digestion (AD) of food waste under mesophilic and thermophilic conditions. Results showed that the performance of the AD system was distinctly influenced by temperature and OLR in terms of biogas production, intermediate metabolism, and degradation performance. The optimal OLR under thermophilic condition was 2.5 g of volatile solids (VS)/L/day with methane yield (MY) of 541 mL/g of VSadded. In addition, the optimal OLR under mesophilic condition was 1.5 g of VS/L/day with a MY of 371 mL/g of VSadded. At the same OLR, the MY under thermophilic condition was 33–49% higher than that under mesophilic condition. Under thermophilic condition, steady methane production and degradation efficiency were achieved with considerably high OLR of 7.5 g of VS/L/day. Under mesophilic condition, stability was obtained only when the OLR was controlled below 2.5 g of VS/L/day. Results also revealed that food waste is a highly desirable substrate w...

Anaerobic digestion (AD) is an essential technology for renewable energy production, yet process stability and methane yield remain sensitive to various factors, particularly under high organic loading rates (OLR). This study evaluates the effects of CO 2 enrichment on methane production, hydrogen concentration, oxidation-reduction potential (ORP), and pH in AD systems using food waste and maize silage as substrates. Under high OLR conditions, CO 2 enrichment increased the methane yield by 6.9 % for food waste and 7.2 % for maize silage. Elevated hydrogen levels in CO 2 -enriched reactors are consistent with the hypothesis of altered methanogenic routing (e.g., increased relevance of hydrogenotrophic methanogenesis), which should be verified by targeted microbial and/or isotopic analyses. ORP values were consistently less negative, especially in food waste trials, where they also served as early indicators of process disturbances. No long-term pH alterations were observed. The findings highlight the potential of CO 2 enrichment to enhance methane yield and improve process resilience. Monitoring CH 4 , H 2 , and ORP is essential to fully understand the underlying mechanisms and optimize AD performance under CO 2 -enriched conditions. • Successful CO 2 enrichment in the process of anaerobic digestion • Enhancement of methane yield through CO 2 enrichment of up to 7.2 % • A substantial variation in H 2 levels is observed, contingent on the substrate • The ORP in the CO 2 -enriched reactor was up to 50 mV higher (less negative) • Enhanced process stability by CO 2 enrichment, especially during high OLR

The introduction of food wastes into anaerobic digestion (AD) brings a promising scenario of increasing feedstock availability and overall energy production from AD. This study evaluated the biodegradability and methane potential from co-digestion of two typical food wastes, kitchen waste and chicken fat, with dairy manure. For single substrate, the bio-methane potential assays showed that kitchen waste had the highest methane yield of 352 L-CH4 kg–1-VS added, 92% more than dairy manure alone. Chicken fat at the same Volatile Solid (VS) level (2 g L–1) inhibited bio-methane production. Addition of kitchen waste and chicken fat to a VS percentage of up to 40% improved overall methane yield by 44% and 34%, respectively. Synergistic effect was observed when either combining two or three substrates as AD feedstock, possibly as a result of increased biodegradability of organic materials in chicken fat and kitchen waste compared with dairy manure. Addition of chicken fat improved methane yield more than kitchen waste. However, addition of chicken fat VS over 0.8 g L–1 should be cautiously done because it may cause reactor failure due to decrease in pH. The maximum methane yield was 425 L-CH4 kg–1-VS, achieved at a VS ratio of 2:2:1 for kitchen waste, chicken fat, and dairy manure. Results from batch AD experiment demonstrated that supplementing dairy manure to chicken fat and/or kitchen waste improved alkalinity of substrate due to the inclusion of more titratable bases in dairy manure, and therefore stabilized the methanogenesis and substantially improved biogas yield. A mixture of substrates of kitchen waste, chicken fat, and dairy manure at a ratio of 1:1:3 was fed to a continuously stirred tank reactor operated at organic loading rates of 3.28, 6.55, and 2.18 g-COD L–1-day (hydraulic retention time of 20, 10, and 30 days, respectively) under mesophilic condition, and methane production rate reached 0.65, 0.95, and 0.34 L-CH4 L–1-reactor-day.

Anaerobic digestion was conventionally applied for treating sewage sludge. However, the accumulation of solid waste particularly food waste has reach the critical levels worldwide. In practice, the food waste was dumped into the landfill for ultimate disposal. However, the greenhouse gases produced in the landfill makes this is no longer a preferable option. Anaerobic digestion was seen as an alternative for managing the food waste in a sustainable way. Methane, a renewable energy is potentially in replacing fossil fuel. However, the methane yield from the digestion of food waste inefficient. Therefore, a study of the co-digestion of sewage sludge and food waste was conducted to investigate the improvement of the methane yield. This study was conducted by using a mixture of domestic primary sewage sludge and food waste as a co-substrate for the anaerobic digester. The kinetics modified Gompertz modelling was applied to describe the anaerobic digestion process. A series of batch biochemical methane potential (BMP) assay was prepared using Automatic Methane Potential Test System (AMPTS II) to investigate the anaerobic digestibility of the mixture of domestic primary sewage sludge and food waste. The BMP assay showed that the co-digestion improved the ultimate methane yield by 32.6% higher than domestic primary sewage sludge alone, indicated that the co-substrate characteristics influencing the methane yield. Besides that, the greater VS/TS ratio of the substrate also resulted in the greater methane yield. The kinetics parameter from the modelling analysis were slightly lower as compared to the laboratory data.

Chapter 4 - Sustainable Waste-to-Energy Technologies: Anaerobic Digestion