Digestate management for high-solid anaerobic digestion of organic wastes: A review

Integration of Digestate-Derived Biochar into the Anaerobic Digestion Process through Circular Economic and Environmental Approaches-A Review

The growing energy consumption and the need for a circular economy have driven considerable interest in the anaerobic digestion (AD) of organic waste, offering potential solutions through biogas and digestate production. AD processes not only have the capability to reduce greenhouse gas emissions but also contribute to the production of renewable methane. This comprehensive review aims to consolidate prior research on AD involving different feedstocks. The principles of AD are explored and discussed, including both chemical and biological pathways and the microorganisms involved at each stage. Additionally, key variables influencing system performance, such as temperature, pH, and C/N ratio are also discussed. Various pretreatment strategies applied to enhance biogas generation from organic waste in AD are also reviewed. Furthermore, this review examines the conversion of generated digestate into biochar through pyrolysis and its utilization to improve AD performance. The addition of biochar has demonstrated its efficacy in enhancing metabolic processes, microorganisms (activity and community), and buffering capacity, facilitating Direct Interspecies Electron Transfer (DIET), and boosting CH4 production. Biochar also exhibits the ability to capture undesirable components, including CO2, H2S, NH3, and siloxanes. The integration of digestate-derived biochar into the circular economy framework emerges as a vital role in closing the material flow loop. Additionally, the review discusses the environmental benefits derived from coupling AD with pyrolysis processes, drawing on life cycle assessment investigations. Techno-economic assessment (TEA) studies of the integrated processes are also discussed, with an acknowledgment of the need for further TEA to validate the viability of integrating the biochar industry. Furthermore, this survey examines the techno-economic and environmental impacts of biochar production itself and its potential application in AD for biogas generation, aiming to establish a more cost-effective and sustainable integrated system.

Enhanced thermophilic high-solids anaerobic digestion of organic fraction of municipal solid waste with spatial separation from conductive materials in a single reactor volume

Despite benefits such as lower water and working volume requirements, thermophilic high solids anaerobic digestion (THSAD) often fails due to the rapid build-up of volatile fatty acids (VFAs) and the associated drop in pH. Use of conductive materials (CM) can promote THSAD through stimulation of direct interspecies electron transfer (DIET), while the need for their constant dosing due to poor separation from effluent impairs economic feasibility. This study used an approach of spatially separating magnetite and granular activated carbon (GAC) from the organic fraction of municipal solid waste (OFMSW) in a single reactor for THSAD. GAC and magnetite addition could both mitigate the severe inhibition of methanogenesis after VFAs build-up to ∼28-30 g/L, while negligible methane production was observed in the control group. The highest methane yield (286 mL CH4/g volatile solids (VS)) was achieved in magnetite-added reactors, while the highest maximum CH4 production rates (26.38 mL CH4/g VS/d) and lowest lag-phase (2.83 days) were obtained in GAC-added reactors. The enrichment of GAC and magnetite biofilms with various syntrophic and potentially electroactive microbial groups (Ruminiclostridium 1, Clostridia MBA03, Defluviitoga, Lentimicrobiaceae) in different relative abundances indicates the existence of specific preferences of these groups for the nature of CM. According to predicted basic metabolic functions, CM can enhance cellular processes and signals, lipid transport and metabolism, and methane metabolism, resulting in improved methane production. Rearrangement of metabolic pathways, formation of pili-like structures, enrichment of biofilms with electroactive groups and a significant improvement in THSAD performance was attributed to the enhancement of the DIET pathway. Promising results obtained in this work due to the spatial separation of the bulk OFMSW and CM can be useful for modeling larger-scale THSAD systems with better recovery of CM and cost-effectiveness.

A comprehensive review on the preparation of biochar from digestate sources and its application in environmental pollution remediation

The preparation of biochar from digestate is one of the effective ways to achieve the safe disposal and resource utilization of digestate. Nevertheless, up to now, a comprehensive review encompassing the factors influencing anaerobic digestate-derived biochar production and its applications is scarce in the literature. Therefore, to fill this gap, the present work first outlined the research hotspots of digestate in the last decade using bibliometric statistical analysis with the help of VOSviewer. Then, the characteristics of the different sources of digestate were summarized. Furthermore, the influencing factors of biochar preparation from digestate and the modification methods of digestate-derived biochar and associated mechanisms were analyzed. Notably, a comprehensive synthesis of anaerobic digestate-derived biochar applications is provided, encompassing enhanced anaerobic digestion, heavy metal remediation, aerobic composting, antibiotic/antibiotic resistance gene removal, and phosphorus recovery from digestate liquor. The economic and environmental impacts of digestate-derived biochar were also analyzed. Finally, the development prospect and challenges of using biochar from digestate to combat environmental pollution are foreseen. The aim is to not only address digestate management challenges at the source but also offer a novel path for the resourceful utilization of digestate.

A critical review on phycoremediation of pollutants from wastewater-a novel algae-based secondary treatment with the opportunities of production of value-added products

Though the biological treatment employing bacterial strains has wide application in effluent treatment plant, it has got several limitations. Researches hence while looking for alternative biological organisms that can be used for secondary treatment came up with the idea of using microalgae. Since then, a large number of microalgal/cyanobacterial strains have been identified that can efficiently remove pollutants from wastewater. Some researchers also found out that the algal biomass not only acts as a carbon sink by taking up carbon dioxide from the atmosphere and giving oxygen but also is a renewable source of several value-added products that can be extracted from it for the commercial use. In this work, the cleaning effect of different species of microalgae/cyanobacteria on wastewater from varied sources along with the value-added products obtained from the algal biomass as observed by researchers during the past few years are reviewed. While a number of review works in the field of phycoremediation technology was reported in literature, a comprehensive study on phycoremediation of wastewater from different industries and household individually is limited. In the present review work, the efficiency of diverse microalgal/cyanobacterial strains in treatment of wide range of industrial effluents along with municipal wastewater having multi-pollutants has been critically reviewed.

Feasibility of housefly larvae-mediated vermicomposting for recycling food waste added digestate as additive

The development of methods for the efficient treatment and application of food waste digestate is an important research goal. Vermicomposting via housefly larvae is an efficient way to reduce food waste and achieve its valorization, however, studies on the application and performance of digestate in vermicomposting are rarely. The present study aimed to investigate the feasibility of the co-treatment of food waste and digestate as an additive via larvae. Restaurant food waste (RFW) and household food waste (HFW) were selected to assess the effects of waste type on vermicomposting performance and larval quality. Waste reduction rates of 50.9%-57.8% were observed in the vermicomposting of food waste mixed with digestate at a ratio of 25%, which were slightly lower than those for treatments without the addition of digestate (62.8%-65.9%). The addition of digestate increased the germination index, with a maximum value of 82% in the RFW treatments with 25% digestate, and decreased the respiration activity, with a minimum value of 30 mg-O₂/g-TS. The larval productivity of 13.9% in the RFW treatment system with a digestate rate of 25% was lower that without digestate (19.5%). Materials balance shows that larval biomass and metabolic equivalent had decreasing trends as the amount of digestate increased and HFW vermicomposting exhibited lower bioconversion efficiency than that of RFW treatment system regardless of the addition of digestate. These results suggest that mixing digestate at a low ratio (25%) during vermicomposting of food waste especially RFW could lead to considerable larval biomass and generate relatively stable residues.

The Antibiotics Degradation and Its Mechanisms during the Livestock Manure Anaerobic Digestion

Antibiotics are administered to livestock at subtherapeutic levels to promote growth, and their degradation in manure is slow. High antibiotic concentrations can inhibit bacterial activity. Livestock excretes antibiotics via feces and urine, leading to their accumulation in manure. This can result in the propagation of antibiotic-resistant bacteria and antibiotic resistance genes (ARGs). Anaerobic digestion (AD) manure treatment technologies are gaining popularity due to their ability to mitigate organic matter pollution and pathogens, and produce methane-rich biogas as renewable energy. AD is influenced by multiple factors, including temperature, pH, total solids (TS), substrate type, organic loading rate (OLR), hydraulic retention time (HRT), intermediate substrates, and pre-treatments. Temperature plays a critical role, and thermophilic AD has been found to be more effective in reducing ARGs in manure compared to mesophilic AD, as evidenced by numerous studies. This review paper investigates the fundamental principles of process parameters affecting the degradation of ARGs in anaerobic digestion. The management of waste to mitigate antibiotic resistance in microorganisms presents a significant challenge, highlighting the need for effective waste management technologies. As the prevalence of antibiotic resistance continues to rise, urgent implementation of effective treatment strategies is necessary.

Sustainable management and recycling of anaerobic digestate solid fraction by composting: A review

The aim of the study was to review and discuss the management and recycling of anaerobic digestate solid fraction by composting process in the context of circular bioeconomy and sustainable development. The conversion of the solid fraction into compost can be recognized as novel process-enhancing supplements for land reclamation. Moreover, the solid fraction of the digestate is a valuable substrate for compost production, both as a monosubstrate and as a valuable additive for other raw materials to enrich in organic matter. These results should serve as reference point to target adjusting screws for anaerobic digestate solid fraction by composting process improvement, its implementation in modern bioeconomy perspective as well as provide a guideline for effective waste management.

Biochar-enhanced agricultural application of liquid digestate from food waste anaerobic digestion for celery cultivation

In this research, the performance of biochar-enhanced agricultural application of food waste liquid digestate for celery cultivation was investigated to reveal its utilization potential and environmental impacts. Liquid digestate demonstrated a good agronomic effect, with a significant fertilization efficiency of 42.3 % during celery growth. With liquid digestate addition (270 t/ha), the same level of harvested celery yield of 15,345 kg/ha was achieved compared with chemical fertilizer utilization of 15,495 kg/ha. Based on the same nitrogen input, the liquid digestate application increased the sugar content of the harvested celery (7 %-15 %) while decreasing the nitrate content (29 %-45 %). The harvested celery with liquid digestate application indicated higher contents of total nitrogen, total phosphorus and total potassium levels than those in the chemical fertilizer group. Liquid digestate as a fertilizer supplemented the soil with nutrients, including phosphorus, potassium and organic matter, but did not cause excessive accumulation. The inorganic nitrogen content of the leachate increased as applied liquid digestate increased. However, it remained 20 %-60 % lower than that of chemical fertilizer at the same fertilization efficiency. After applying liquid digestate, there was no significant increase was observed in soil salinity. The coupled addition of biochar helps to improve the overall effects of liquid digestate for agricultural application and reduce negative environmental impacts. This study demonstrates that returning liquid digestate to agricultural fields as fertilizer is an environmentally and economically beneficial practice.

Increasing the value of Phragmites australis straw in a sustainable integrated agriculture model (SIAM) comprising edible mushroom cultivation and spent mushroom substrate compost

Because of the regular annual harvest, Phragmites australis used in wetland protection produces an abundance of straw, resulting in a large amount of straw waste. As a result, the extra straw should be used in a convenient and efficient manner. A comprehensive analysis of P. australis straw use in Pleurotus cultivation and spent mushroom substrate compost was performed in this study to increase its value. The lignocellulose content in the straw was shown to meet the nutritional requirements of the Pleurotus mushroom. Immediately thereafter, the replacement of bagasse with P. australis proved to be reasonable for Pleurotus eryngii substrate and could generate a profit of ¥17,400 per 10,000 cultivation bags. Substituting P. australis for approximately 20 % to 40 % of bagasse is recommended for Pleurotus ostreatus cultivation and can yield a profit of approximately ¥16,000. Spent mushroom substrate compost was confirmed to increase the organic matter content, and post-compost use of this substrate as a fertilizer could increase economic income by approximately ¥1000 for every 10,000 bags. Overall, this recycling pathway for P. australis resources presents positive ecological and social benefits, and the model is a sustainable and eco-friendly solution for agricultural waste worthy of promotion and further application.

Assessment of economic viability and production costs for the innovative microfiltered digestate

This paper assesses the economic viability of the production process of an innovative form of digestate known as "microfiltered digestate", suitable for use as a fertilizer in driplines for permanent crops. A Break-Even Analysis was performed to determine the Break-Even Point which identifies the minimum quantity of products necessary to cover production costs at a certain selling price. Our results show that the production of microfiltered digestate provides positive economic viability for anaerobic digestion managers, providing them with an attractive market outlet and, at the same time, a new form of income. The experimental study was stated in Sicily, but it is replicable in any territorial context with anaerobic digestion plants. The results are in line with the principles of circular economy and are very current with reference to the use of organic fertilizers instead of chemical ones.

The effect of fertilization with microfiltered liquid digestate on the quality parameters of Citrus fruits

Nowadays, the adoption of sustainable agricultural practices, including the reduction of synthetic fertilizers, has become a challenge for the agriculture sector. In this experimental work, the effect of the liquid fraction of digestate (by-product of the anaerobic digestion process) as a fertilizer was evaluated. The aim of the research was to verify to which extent digestate can affect growth and quality parameters of orange fruits, comparing the results to those obtained for fruits grown on soil treated with conventional mineral fertilizers. To assess the effectiveness of the treatments, different qualitative and quantitative parameters of Citrus fruits were measured. In particular, the results showed slight differences between the two treatments, suggesting that digestate may be used for the production of high-quality fruits. Moreover, in some orchards, the Citrus fruits of the plants treated with digestate showed a higher concentration of health-promoting compounds, such as vitamin C, flavonoids, phenolic content, when compared to the control group. Thus, digestate can be considered an optimal source of plant nutrients and can be used as a crop growth promoter, since it represents an effective strategy for reducing the mineral fertilizers input.

Valorization of Anaerobic-Fermentation Liquid Digestates-Membrane-Based Process Development

Complete valorization of various wastes and effluents, with significant organic content, remains a great challenge in the pursuit of a circular economy. The approach based on anaerobic fermentation, leading to valuable biogas production, has been broadly accepted and employed as an attractive processing scheme. However, despite notable research efforts, complete valorization of the digestates (involving recovery of nutrients/by-products and full recycling/reuse of treated water) requires additional work for sustainable process development. This study aims to make a contribution in this direction by demonstrating a systematic methodology for valorizing the liquid digestate. The proposed membrane-based processing scheme involves UF-membrane pretreatment of the liquid digestate (for sludge separation) and subsequent NF/RO membrane treatment for reuse/recycling of the permeate; the concentrate, enriched in "nutrients" (phosphate and ammonium compounds), can be utilized for soil fertilization, with further conditioning/processing. By performing targeted laboratory experiments and advanced simulations, the membrane-based process was developed to a relatively high technology-readiness level, including a pilot unit design/construction and preliminary testing with satisfactory results. Through pilot testing in industrial environment, further process development and optimization will be pursued, towards practical applications. The demonstrated methodology is also considered appropriate for systematic development of membrane-based processes to valorize/treat a variety of similar effluents.

Calcium hypochlorite-coupled aged refuse promotes hydrogen production from sludge anaerobic fermentation

This work investigated the effect of calcium hypochlorite (CH) coupled aged refuse (AR) treatment on the enhanced hydrogen generation from sludge anaerobic dark fermentation (SADF). The enhanced mechanism was systematically revealed through sludge disintegration, organic matter biotransformation, and microbial community characteristics, etc. The experimental data showed that CH coupled AR increased the hydrogen yield to 18.1 mL/g, significantly higher than that in the AR or CH group alone. Mechanistic analysis showed that CH-coupled AR significantly promoted sludge disintegration and hydrolysis processes, providing sufficient material for hydrogen-producing bacteria. Microbiological analysis showed that CH-coupled AR increased the relative abundance of responsible hydrogen-producing microorganisms. In addition, CH-coupled AR was very effective in reducing phosphate content in the fermentation liquid and fecal coliforms in the digestate, thus facilitating the subsequent treatment of fermentation broth and digestate. CH coupled AR is an alternative strategy to increase hydrogen production from sludge.

Effect of magnetite particle size on propionate degradation in the propionate-based anaerobic system

The size effect of magnetite (Fe₃O₄) on the degradation of propionate (PA) in the PA-based anaerobic system was investigated. The sequential bench-scale experiments were conducted. Results showed that the effects of different sized magnetite particles on PA degradation varied, and reaction cycles also played a role in substrate removal/degradation. With the increase of reaction cycle, nano-magnetite promoted PA degradation and CH₄ production, which caused faster PA degradation rate (0.997 g/L·d) than the control group (CK) without magnetite (0.834 g/L·d), whereas the groups with micron- and millimeter-sized magnetite had slower PA degradation rates (0.746 and 0.636 g/L·d) than CK group. The particle size or surface characteristics of the magnetite may become the main factor determining the PA degradation rate. Furthermore, the analysis of PA conversion and volatile fatty acids (VFAs) distribution showed the C6-dismutation pathway, which converses PA to butyrate, enhanced by the introduction of magnetite. Microbial community analysis showed that PA was degraded mainly by methyl-malonyl-CoA (MMC) pathway. The relative abundance of Syntrophobacter that catalyze MMC pathway in the group with nano-magnetite were much higher after three reaction cycles at 39 %, as compared to micro-magnetite at 28 %, and millimeter-sized magnetite at 27 %, which contributed to faster degradation of PA. Functional enzyme-encoding genes for the four methanogenesis pathways were identified with reference to KEGG database entries. The methanogenesis pathway using acetate was the most abundant pathway in all groups. The observations have important implications for enhancing the PA removal in PA-inhibited anaerobic digester.

A new cutting-edge review on the bioremediation of anaerobic digestate for environmental applications and cleaner bioenergy

Circular agriculture and economy systems have recently emerged around the world. It is a long-term environmental strategy that promotes economic growth and food security while reducing negative environmental consequences. Anaerobic digestion (AD) process has a high contribution and effective biodegradation route for bio-wastes valorization and reducing greenhouse gases (GHGs) emissions. However, the remaining massive digestate by-product contains non-fermented organic fractions, macro and/or micro-nutrients, heavy metals, and metalloids. Direct application of digestate in agriculture negatively affected the properties of the soil due to the high load of nutrients as well as the residuals of GHGs are emitted to the environment. Recycling and valorizing of anaerobic digestate is the main challenge for the sustainable biogas industry and nutrients recovery. To date, there is no global standard process for the safe digestate handling. This review described the biochemical composition and separation processes of anaerobic digestate. Further, advanced physical, chemical, and biological remediation's of the diverse digestate are comprehensively discussed. Moreover, recycling technologies such as phyco-remediation, bio-floc, and entomoremediation were reviewed as promising solutions to enhance energy and nutrient recovery, making the AD technology more sustainable with additional profits. Finally, this review gives an in-depth discussion of current biorefinery technologies, key roles of process parameters, and identifies challenges of nutrient recovery from digestate and prospects for future studies at large scale.

Bioreactor-scale production of rhamnolipids from food waste digestate and its recirculation into anaerobic digestion for enhanced process performance: Creating closed-loop integrated biorefinery framework

Reaching industrially relevant productivities in bioprocesses and their efficient integration in the existing industrial infrastructure remain as important challenges in the circular economy to create closed loop sustainability framework. Using anaerobic digestion (AD) biorefinery as a model, the present work addressed these problems via integration of next-generation rhamnolipids production with AD. A high rhamnolipids concentration of 10.25 ± 1.34 g/L was obtained by fed-batch fermentation using food waste digestate as medium. Digestate-derived rhamnolipids contained Rha-C₁₀-C₁₀ and Rha-Rha-C₁₀-C₁₀ as the predominant congeners. These were used back in single-phase AD to demonstrate their effect on sludge solubilization and digestion efficiency. A dosage of 0.02 g rhamnolipids/g total suspended solids was found to be optimal which enhanced the hydrolysis-acidogenesis reactions to up to 27% over control. It however retarded methane production which could be overcome by the prolongation of digestion time. Finally, the value chain appreciation by the proposed process was demonstrated by a feasibility analysis.

Impact of solid digestate processing on carbon emission of an industrial-scale food waste co-digestion plant

Anaerobic digestion (AD) has been widely applied for treating organic waste and is known as a carbon-offsetting process. However, most studies relied on laboratory-scale experiments or literature to calculate carbon emissions from AD process, and the impact of digestate processing was overlooked. This study assessed the carbon footprint for an industrial food waste co-digestion plant with operational data. The results indicated that carbon emission before digestate treatment is -88.5 ± 4.4 kg CO₂-eq/t. The major source of carbon emission is electricity provision, followed by fuel combustion, unburned biogas, and fugitive gas emissions, while waste oil recovery and biogas utilization offset the carbon emissions. Considering digestate treatment and disposal options, the plant's net carbon emissions are as follows: -86.1 ± 6.2 kg CO₂-eq/t (incineration) < -80.7 ± 6.5 kg CO₂-eq/t (land application) < 6.7 ± 12.2 kg CO₂-eq/t (landfilling). This work aims at providing a roadmap for making site-specific calculations of the carbon footprint for AD process.

A comparison of digestate management options at a large anaerobic digestion plant

Increased biogas production from increasing numbers of anaerobic digestion (AD) facilities has increased the mass of digestate applied to agricultural land close to AD plants and has led to an oversupply in some regions. This necessitates long distance digestate transportation accompanied by economic, environmental, and social drawbacks. This work assesses the performance of three different digestate management options (MOs); land application of whole digestate (MO1), digestate separation (MO2), and digestate separation and evaporation (MO3), combined with centralised or decentralised digestate storage. All MOs required the same landbank area, whilst MO2 and MO3 reduced digestate management costs by 9% and 37% (if recovered heat is used) respectively. GHG emissions from MO2 were 41% lower than MO1 if renewable electricity was used. MO3 reduced GHG emissions by 63% compared to MO1, if renewable electricity and recovered heat were used. MO2 required the same centralised digestate storage volume as MO1 while MO3 required 44% of the centralised storage volume. Centralised digestate storage required a maximum of 79 days for digestate transportation (33 trucks/day, 20 m³ capacity) to land for MO1 and MO2, and 35 days for MO3. Decentralised digestate storage required 63 storage tanks and 15 trucks/day for MO1, 69 tanks and 15 trucks/day for MO2, and 68 tanks and 7 trucks/day for MO3. Tank size ranged from 500 m³ to 20,000 m³. MO3 combined with decentralised storage could reduce the cost and GHG emissions (if recovered energy is used), vehicle movements, and the number of storage tanks required for digestate management.

Unraveling the metabolic shift in anaerobic digestion pathways associated with the alteration of onion skin waste concentration

Onion skin waste (OSW) is common waste in developing countries, which can cause severe environmental pollution when not properly treated. Value-added products can be chemically extracted from OSW; however, that process is not economically feasible. Alternatively, dry anaerobic digestion (DAD) of OSW is a promising approach for both energy recovery and environment protection. The main hurdles during DAD of OSW can be the hydrolysis and acidification. In batch tests, sludge digestate (SD) rich with methanogens was co-digested with different fractions of OSW for enhancing hydrolysis and raising biogas productivity. The cumulative biogas production (CBP) was 36.6 ± 0.3 mL for sole DAD of SD (100% SD) and increased up to 281.9 ± 14.1 mL for (50% SD: 50% OSW) batch. Self-delignification of OSW took place by SD addition, where the lignin removal reached 75.3 ± 10.5% for (85% SD: 15% OSW) batch. Increasing the fraction of OSW (45% SD: 55% OSW) reduced the delignification by a value of 68.8%, where initial lignin concentration was 9.48 ± 1.6% in dry weight. Lignin breaking down resulted a high fraction of phenolic compounds (345.6 ± 58.8 mg gallic acid equivalent/g dry weight) in the fermentation medium, causing CBP drop (219.0 ± 28.5 mL). The presence of elements (K, Ca, Mg, Fe, Zn, Mn, S and P) in OSW improved the enzymatic activity, facilitated phenolic compounds degradation, shifted the metabolism towards acetate fermentation pathway, and raised biogas productivity. Acidogenesis was less affected by phenolic compounds than methanogenesis, causing higher H₂ contents and lower CH₄ contents, at batches with high share of OSW.

Fed-batch anaerobic digestion of raw and pretreated hazelnut skin over long-term operation

This study provided important insights on the anaerobic digestion (AD) of hazelnut skin (HS) by operating a fed-batch AD reactor over 240 days and focusing on several factors impacting the process in the long term. An efficient reactor configuration was proposed to increase the substrate load while reducing the solid retention time during the fed-batch AD of HS. Raw HS produced maximally 19.29 mL CH₄/g VS_add/d. Polyphenols accumulated in the reactor and the use of NaOH to adjust the pH likely inhibited AD. Maceration and methanol-organosolv pretreatments were, thus, used to remove polyphenols from HS (i.e. 82 and 97%, respectively) and improve HS biodegradation. Additionally, organosolv pretreatment removed 9% of the lignin. The organosolv-pretreated HS showed an increment in methane potential of 21%, while macerated HS produced less methane than the raw substrate, probably due to the loss of non-structural sugars during maceration.

Assessment of a full-scale solid-state anaerobic co-digestion: A multi-component substrate analysis by using ORWARE

Long-term sustainable biogas production requires different raw material alternatives, especially when reducing the most desirable organic substrate, food waste, which has been set as a goal in the 2030 Agenda. In Sweden, horse manure (HM) is generated in large quantities, and due to its physical and chemical characteristics, it has the potential to be used as a raw material to produce biogas through anaerobic digestion (AD). In order to investigate the challenges that HM digestion can impose in terms of methane yield and/or digestate quality, the modified ORganic WAste REsearch (ORWARE) AD model was applied. The aim was to study the effects of different substrates and combinations of these on the AD process during a full-scale solid-state (SS)-AD. In this sense, the model allows for the analysis of the digestion process of multicomponent substrates at the element level. The simulation results suggested that the replacement of green waste (GW) by HM with wood chips as bedding material gave the best improvement in terms of energy turnover; the liquid fraction of the digestate of this mixture of substrates presented the highest concentration in all the nutrients analyzed, specifically in total carbon-biological and phosphorus. The nutrient concentrations in the digestate from the aforementioned scenario are in line with the SPCR120 certification.

Energy recovery from food waste and garden and park waste: Anaerobic co-digestion versus hydrothermal treatment and anaerobic co-digestion

The feasibilities of the anaerobic co-digestion of two of the most relevant biowastes, food waste and garden and park waste, were evaluated and compared with the hydrothermal treatment of each waste and the anaerobic co-digestion of raw biowastes with the process water generated. The effects on the process stability and energy recovery were also analyzed. Anaerobic digestion was the best option for food waste treatment from an energetic point of view, with 81% recovery of the energy stored in the feedstock, while the highest energy recovery from garden and park waste was obtained for the solid fraction generated from hydrothermal treatment (85.5%). In addition, the anaerobic co-digestion of food waste with 5% of the process water generated from garden and park waste showed a similar energy recovery to that of food waste only (∼80%), thus improving the biological stability of the process.

A critical review of process parameters influencing the fate of antibiotic resistance genes in the anaerobic digestion of organic waste

The overuse and inappropriate disposal of antibiotics raised severe public health risks worldwide. Specifically, the incomplete antibiotics metabolism in human and animal bodies contributes to the significant release of antibiotics into the natural ecosystems and the proliferation of antibiotic-resistant bacteria carrying antibiotic-resistant genes. Moreover, the organic feedstocks used for anaerobic digestion are often highly-rich in residual antibiotics and antibiotic-resistant genes. Hence, understanding their fate during anaerobic digestion has become a significant research focus recently. Previous studies demonstrated that various process parameters could considerably influence the propagation of the antibiotic-resistant genes during anaerobic digestion and their transmission via land application of digestate. This review article scrutinizes the influences of process parameters on antibiotic-resistant genes propagation in anaerobic digestion and the inherent fundamentals behind their effects. Based on the literature review, critical research gaps and challenges are summarized to guide the prospects for future studies.

The challenges and perspectives for anaerobic digestion of animal waste and fertilizer application of the digestate

The increase in livestock production creates a serious problem of managing animal waste and by-products. Among the wide range of waste valorization methods available, anaerobic digestion is very promising. It is a form of material recycling that also produces renewable energy in the form of biogas, which is reminiscent of energy recycling. The effluent and digestate from the anaerobic digestion process need to be processed further. These materials are widely used in agriculture due to their composition. Both the liquid and solid fractions of digestate are high in nitrogen, making them a valuable source for plants. Before soil or foliar application, conditioning (e.g., with inorganic acids) and neutralization (e.g., with potassium hydroxide) is required to eliminate odorous compounds and microorganisms. Various methods of conducting the process by anaerobic digestion (use of additives increasing activity of microorganisms, co-digestion, multiple techniques of substrate preparation) and the possibility of controlling process parameters such as optimal C/N ratio (15-30), optimal temperature (psychrophilic (<20 °C), mesophilic (35-37 °C) and thermophilic (55 °C) for microorganism activity ensure high efficiency of the process. Literature data describing tests of various digestates on different plants prove high efficiency, determined by yield increase (even by 28%), nitrogen uptake (by 20%) or phosphorus recovery rate (by 43%) or increase of biometric parameters (e.g., leaf area).

Low-temperature anaerobic digestion of chicken manure at high organic and nitrogen loads - strategies for controlling short chain fatty acids

Objective of this work was to investigate the technical feasibility of low-temperature, closed-loop two-stage (liquid-solid) anaerobic digesters to treat chicken-manure (TS:68%; NH₃:8 g/L) as a sole-feedstock. Effect of pH, temperature, treatment-duration, organic loading rate (OLR) and inoculum-recirculation ratio on short chain fatty acids (SCFA) production was studied. Digesters were operated at 20 ± 1 °C for 282-d over 4 batch-runs (∼70-d/batch) at an OLR of 8.78-4.3 gVS/L/d. Results showed that specific methane yield above 0.6 LCH₄/gVS was feasible with a methane concentration > 60%. SCFA speciation of the entire system was monitored through the liquid-digester. Among SCFA indicators, the ratios of propionic-to-acetic acids, (butyric + valeric)-to-acetic acids, and total SCFA-to-alkalinity were observed within the limit, i.e., below 1.4, 0.3 and 0.8, respectively, indicating high-digester stability. This strategy allowed early detection, diagnosis of process failures in high-solids digester in fed-batch mode, and re-evaluation of operating protocol to enrich performance with economic-benefits.

Selecting the optimal nutrients recovery application for a biogas slurry based on its characteristics and the local environmental conditions: A critical review

Digestate (effluent of biogas plants) became the main bottleneck for biogas industry expansion because it often exceeds the capacity of surrounding croplands as fertilizer. Nutrients recovery from digestate is a promising solution for closing nutrients cycles and generating high value-added byproducts. In fact, numerous nutrients recovery technologies were reported and utilized for that purpose. However, each technology has optimum working conditions, while digestates have different characteristics due to the different substrates, digestion conditions, and handling methods. On the other hand, no protocol has been reported yet for selecting the optimal nutrients recovery technology or sequenced technologies for different digestates regarding their characteristics and the surrounding environmental conditions. In this study, an interactive flowchart was suggested and discussed for selecting the most appropriate technology or sequential techniques among the different alternatives. The whole digestate utilization technologies, solid-liquid separation technologies, liquid and solid processing technologies were included.

Molecular diversity of liquid digestate from anaerobic digestion plants for biogenic waste

The treatment and valorization of liquid digestate (ADLD) after anaerobic digestion of biogenic waste are challenging. This study used ultra-high resolution mass spectrometry to determine the molecular characteristics of ADLD collected from different full-scale plants for food waste treatment. The results indicated that there were regular differences in the dissolved organic matter (DOM) indicators among the samples from dry and wet anaerobic processes. ADLD DOM had higher H/C and O/C, and contained more easily degradable proteins. In addition, sCOD and pH were the drivers of the molecular distribution of ADLD common compounds. The same common compounds were present in the ADLD from different anaerobic digestion plants. They had a significant correlation with physicochemical characteristics. The compounds relating to plant hormones and nutrients as well as xenobiotics were both identified, suggesting that comprehensive considerations should be taken into account for the land application of ADLD.

Comprehensive evaluation of gradient controlled anaerobic digestion and pyrolysis integration processes: A case study of Sargassum treatment

In this study, an integrated process of gradient controlled anaerobic digestion (AD) and pyrolysis (PY) was proposed for energy recovery of Sargassum (SA). The methanogenic performance of SA and the PY performance of subsequent digestate were investigated. And the energy flow of the integrated process was comprehensively evaluated. The results showed that two methanogenic peaks occurred, which were appeared on the 7th day (9.57 mL/g VS) and the 17th day (15.74 mL/g VS), respectively. The structure and toughness of SA were destroyed by AD, which altered the subsequent PY performance. For the energy flow, the AD (lasted for 14 days) integrated PY process showed a superior performance compared with direct PY, as the total energy recovery increased from 5.88 to 6.42 MJ/kg TS. This study laid a foundation for the successful integration of AD and PY, which provided a guidance for the clean treatment and energy recovery of biomass.

Oriented conversion of agricultural bio-waste to value-added products - A schematic review towards key nutrient circulation

Agriculture bio-waste is one of the largest sectors for nutrient circulation and resource recovery. This review intends to summarize the possible scheme through coupling chemical conversion of crop straws to biochar and biological conversion of livestock waste to value-added products thus reaching key nutrient circulation. Chemical conversion of crop straws to biochar was reviewed through summarizing the preparation methods and functional modification of biochar. Then, high-solid two-phase anaerobic conversion of agriculture bio-waste to value-added products and improved performance of bio-conversion through byproduct gases reuse and biochar supplementation were reviewed. Finally, high quality compost production through amendment of biochar and residual digestate was proposed with analysis of reduced nitrogen emission and carbon balance. The biological mechanism of synergistic regulation of carbon and nitrogen loss during bio-conversion with biochar was also reviewed. This will provide a model for synergistic conversion of agricultural wastes to value added products pursuing key nutrient circulation.

Tunable active sites on biogas digestate derived biochar for sulfanilamide degradation by peroxymonosulfate activation

Conversion of digestate into biochar-based catalysts is an effective strategy for disposal and resource utilization. The active sites on biochar correlated with reactive species formation in peroxymonosulfate (PMS) system directly. Clarifying the structure-performance relationship of digestate derived biochar in PMS system was essential for decomposition of contaminants. Herein, dairy manure digestate derived biochar (DMDB) was prepared for PMS activation and sulfamethoxazole (SMX) degradation. The higher pyrolysis temperature could promote effective sites generation. Especially, the DMDB-800 catalyst exhibited excellent performance for PMS activation, achieving 90.2% degradation of SMX within 60 min. Based on the correlation analysis between log (k) values and active sites, defects, graphite N and CO were identified as dominant sites for PMS activation. The ¹O₂ oxidation and surface electron transfer were critical routes for SMX degradation. Besides, the degradation pathways of SMX were proposed according to DFT calculations and intermediates determination. The cleavage of the sulfonamide bond, hydroxylation of the benzene ring and oxidation of the amino group mainly occurred during SMX degradation. Overall, this study provides deep insights into the enhanced mechanism of tunable active sites on DMDBs for PMS activation, boosting the application of digestate biochar for water treatment in advanced oxidation systems.

Sewage-water treatment with bio-energy production and carbon capture and storage

Typical large-scale sewage-water treatments consume energy, occupy space and are unprofitable. This work evaluates a conceivable two-staged sewage-water treatment at 40,000 m³/d of sewage-water with sewage-sludge (totaling 10kg^COD/m³) that becomes a profitable bioenergy producer exporting reusable water and electricity, while promoting carbon capture. The first stage comprises microbial anaerobic digesters reducing the chemical oxygen demand (COD) by 95% and producing 60%mol methane biogas. The effluent waters enter the subsequent aerobic stage comprising microbial air-fed digesters that extend COD reduction to 99.7%. To simulate the process, up-to-date anaerobic/aerobic digester models were implemented. A biogas-combined-cycle power plant with/without post-combustion carbon capture is designed to match the biogas production, supplying electricity to the process and to the grid. Results comprehend electricity exportation of 13.21 MW (7.92 kWh/t^{Reusable-Water}) with -9.957t^CO2/h of negative carbon emission (-0.6 kg^CO2-Emitted/kg^COD-Removed). The biogas-combined-cycle without carbon capture achieves 21.08 MW of power exportation, while a 37.3% energy penalty arises if carbon capture is implemented. Configurations with/without carbon capture reach feasibility at 125 USD/MWh of electricity price, with respective net present values of 6.86 and 85.07 MMUSD and respective payback-times of 39 and 12 years. These results demonstrate that large-scale sewage-water treatment coupled to biogas-fired combined-cycles and carbon capture can achieve economically feasible bioenergy production with negative carbon emissions.

Sustainable management and recycling of food waste anaerobic digestate: A review

Anaerobic digestion (AD) is a widely used technology to valorise food waste for biogas production yet a considerable amount of digestate remains under-utilised. Sustainable management and recycling of the nutrient-rich food waste anaerobic digestate (FWD) is highly desirable for closing resource loop and actualising circular economy. This work reviews the distinct properties of FWD and the existing treatment technologies. FWD shows great prospects as a nutrient source for microalgal cultivation and biofuel production. Emerging technologies such as thermal conversion (e.g., pyrolysis and hydrothermal treatment) of FWD into value-added products such as functionalised biochar/hydrochar with diverse applications would be attractive and warrant further research investigation. Integrated AD with subsequent valorisation facilities is highly encouraged to achieve complete utilisation of resources and reduce carbon emissions.

The role of waste management in reducing bioplastics' leakage into the environment: A review

Bioplastics are becoming more and more widespread as substitutes for petroleum-derived plastics due to their biodegradability. Bioplastics degradation under different environments has been described and reported to depend mainly on bioplastics' compositions and the environmental conditions. Incomplete degradation during waste management processes and leakage of bioplastics into the environment are becoming major concerns that need to be further investigated. In this context, the present paper aimed to review recent literature dealing with biodegradation of bioplastics under industrial (e.g. anaerobic digestion and composting) and natural (e.g. soil and water) environments, and to link it to the potential bioplastics' leakage into the environment. Reviewed data were used to estimate the potential role of waste management processes in decreasing the potential leakage of bioplastics. Depending on bioplastics' type and processing conditions, waste management can effectively reduce bioplastics' potential leakage, decreasing the concentration of these materials that can reach the natural environments.

Improving anaerobic digestion mass balance calculations through stoichiometry and usual substrate characterization

In recent years, anaerobic digestion (AD) of organic waste has raised as a winning strategy to produce energy and organic fertilizer. To optimize such a technology, mass balance is needed to model or simply monitor the process. This paper describes a theoretical framework allowing process indicators to be derived from estimates of organic waste elemental composition. Using a database of 128 typical feedstocks, semi-empirical equations are provided to estimate this elemental composition in case of a missing analysis. Then, a revised stoichiometric reaction of anaerobic digestion is proposed considering biomass yield and nitrogen. Biomass yield induces a more accurate estimation of the ammonia production and the volatile solids (VS) loss. The stoichiometric reaction allows the prediction of biogaz quality and mass loss and a correction factor for VS removal is proposed. The use of real case studies highlights the need to consider this correction (correction factor range: 1.01-1.2), especially for dry AD.

Emerging bioelectrochemical technologies for biogas production and upgrading in cascading circular bioenergy systems

Biomethane is suggested as an advanced biofuel for the hard-to-abate sectors such as heavy transport. However, future systems that optimize the resource and production of biomethane have yet to be definitively defined. This paper assesses the opportunity of integrating anaerobic digestion (AD) with three emerging bioelectrochemical technologies in a circular cascading bioeconomy, including for power-to-gas AD (P2G-AD), microbial electrolysis cell AD (MEC-AD), and AD microbial electrosynthesis (AD-MES). The mass and energy flow of the three bioelectrochemical systems are compared with the conventional AD amine scrubber system depending on the availability of renewable electricity. An energy balance assessment indicates that P2G-AD, MEC-AD, and AD-MES circular cascading bioelectrochemical systems gain positive energy outputs by using electricity that would have been curtailed or constrained (equivalent to a primary energy factor of zero). This analysis of technological innovation, aids in the design of future cascading circular biosystems to produce sustainable advanced biofuels.

Mixing effects on decentralized high-solid digester for horticultural waste: Startup, operation and sensitive microorganisms

This work studied the use of a horizontal high-solid digester for the decentralized anaerobic treatment of horticultural waste (fallen leaves), where the effect of intermittent mixing by a modified double helical ribbon impeller was investigated. Before experimental verification, the flow pattern and theoretical mixing time were first characterized by CFD simulation. Subsequently, three mixing time intervals (i.e., 3 min/3 hr; 18 min/3 hr; 108 min/3 hr) and one control setup (i.e., without mixing) were compared for their performance during start-up and semi-continuous operation. It was found that minimal mixing was necessary for an efficient digester's start-up but increased mixing intensity for semi-continuous operation. The results were further interpreted by correlating the digester performance and microbial communities. Those microorganisms sensitive to increased mixing intensity were highlighted and analysed.

Anaerobic digestion of organic waste: Recovery of value-added and inhibitory compounds from liquid fraction of digestate

Anaerobic digestion, as an eco-friendly waste treatment technology, is facing the problem of low stability and low product value. Harvesting value-added products beyond methane and removing the inhibitory compounds will unleash new vitality of anaerobic digestion, which need to be achieved by selective separation of certain compounds. Various methods are reviewed in this study for separating valuable products (volatile fatty acids, medium-chain carboxylic acids, lactic acid) and inhibitory substance (ammonia) from the liquid fraction of digestate, including their performance, applicability, corresponding limitations and roadmaps for improvement. In-situ extraction that allows simultaneous production and extraction is seen as promising approach which carries good potential to overcome the barriers for continuous production. The prospects and challenges of the future development are further analyzed based on in-situ extraction and economics.

Food waste treating by biochar-assisted high-solid anaerobic digestion coupled with steam gasification: Enhanced bioenergy generation and porous biochar production

A food waste treating system was proposed in this study by combining biochar-assisted high-solid anaerobic digestion and subsequent steam gasification of the digestate. The effect of solid level, biochar dosage in anaerobic digestion on the properties of biogas, syngas, and final biochar products were investigated. Results showed that at a high total solid level and biochar dosage of 25 g/L and 50 g/L, the accumulative methane yield reached 110.3 mL CH₄/g VS and 126.7 mL CH₄/g VS, respectively. From steam gasification of different digestates under 850 °C for 15 min, a maximum of 34.92 mmol/g for the hydrogen yield and 11.44 MJ/m³ for the higher heating value could be obtained for the syngas. Furthermore, the by-product produced from steam gasification was a nutrient-enriched porous biochar, which was suitable to be used as compost. This study demonstrated a pathway for food waste treating to produce methane-enriched biogas, hydrogen-enriched syngas, and nutrient-enriched biochar.

Pretreatment by composting increased the utilization proportion of pig manure biogas digestate and improved the seedling substrate quality

Anaerobic digestion of organic residues has the potential to significantly contribute to a shift from fossil to renewable energy, while the remaining biogas digestate need to be treated or used for a second time. In this study, the pig manure biogas digestate (PMBD) was evaluated as a potential part of seedling substrates and composting was considered a pretreating method to improve its characters. Composting was carried out firstly in a forced aeration composting system (100 L), in which perlite and sawdust were used as additives in different proportions separately or together. Based on the comparison of the physicochemical characters of different seedling substrates formulas mixed with PMBD or pig manure biogas digestate pretreated by composting (CPMBD), selected seedling substrates were analyzed by bioassay experiment. The results showed that pretreatment by composting and the additives (perlite and sawdust) used in composting decreased the pH value of PMBD and make it suitable for seedling substrates, especially composted with perlite and sawdust. Both PMBD and CPMBD with low proportion in the substrates improved plant growth of lettuce and tomato, while CPMBD was better than PMBD. However, when the proportions of PMBD was increased more than 20% and CPMBD was increased more than 40%, plant growth inhibition was observed. Tomato was more sensitive than lettuce to the physicochemical characters of the substrate. In summary, PMBD pretreated by composting not only increases the uses of digestate, but also enhances plant growth and hence yield.

High-solid digestion from cellulosic ethanol stillage with activated sludge of simultaneous propionate degradation and methanogenesis

High solid anaerobic digestion (HSAD) was an emerging bioconversion technology which had the advantages of small digester, less digestate and low heating energy. A one-stage anaerobic system in CSTR by inoculating activated sludge of simultaneous propionate degradation and methanogenesis was proposed to improve the high-solid digestion performance and to stabilize the reaction process. Semi-continuous mode was successfully used to perform HSAD from cellulosic ethanol whole stillage at an initial substrate loading of 15.4% (w/w) dry matter content with different OLRs from 1.5 to 5.0 gVS·L^-1 d^-1 at an HRT of 30 days. The average methane yield during whole digestion reached 349.9 mL⋅gVS^-1 with a total VS removal rate of 61.3%. The acclimation mechanism of multifunctional activated sludge was also explored by analyzing the functional property, physiological activity and microbial community structure. The results indicated the feasibility and efficiency of multifunctional activated sludge in a semi-continuous high-solid stirred tank reactor system.

Solid digestate disposal strategies to reduce the environmental impact and energy consumption of food waste-based biogas systems

This study examined the environmental impacts and energy consumption of three solid digestate treatment scenarios to quantify their impacts on the entire food waste (FW)-based biogas system: (1) incineration; (2) composting, and; (3) landfill. The results showed that composting had the largest net energy consumption, but least total environmental impact of 57.3 kWh and 8.75 E-03, respectively, whereas landfill showed the opposite pattern. Moreover, there were significant differences (p < 0.05) and relatively high contributions between the digestate treatment subunits among the three scenarios. The most significant contributions of digestate subunits in methods 1-3 to the 100-year global warming potential (GWP₁₀₀) were 70.5%, 52.5%, and 103.4%, respectively. The results indicated that solid digestate treatment had a significant impact, and reasonable disposal of solid digestate could significantly reduce the environmental impacts and energy consumption of the entire FW-based biogas system.

Valorization of anaerobic digestion digestate: A prospect review

Anaerobic digestion is recognized as promising technology for bioenergy production from biowaste, with huge quantity of digestate being produced as the residual waste. The digestate contains substantial amounts of organic and inorganic matters that be considered highly risky contaminants to the receiving environments if not properly treated, but also potential renewable resources if are adequately recovered. This prospect review summarized the current research efforts on digestate valorization, including aspects of resource recovery and the proposed applications, particularly on the conversion techniques and economic feasibility. The prospects for digestate valorization were highlighted at the end of this review.

The valorization of the anaerobic digestate from the organic fractions of municipal solid waste: Challenges and perspectives

The anaerobic digestion is a well-established process for the treatment of organic solid waste, pursuing its conversion into a methane rich gas destined to energy generation. Research has largely dealt with the enhancement of the overall bioconversion yields, providing several strategies to maximize the production of bio-methane from the anaerobic processing of a wide variety of substrates. Nevertheless, the valorization of the process effluents should be pursued as well, especially if the anaerobic digestion is regarded in the light of the circular economy principles. Aim of this work is in identifying the state of the art of the strategies to manage the digestate from the anaerobic processing of the organic fractions of municipal solid waste. Conventional approaches are described and novel solutions are figured out in order to highlight their potential scale up as well as to address future research perspectives.

Investigation of hydrodynamics in high solid anaerobic digestion by particle image velocimetry and computational fluid dynamics: Role of mixing on flow field and dead zone reduction

High solid anaerobic digestion (HSAD) was a potential organic waste treatment. Compared with low solid anaerobic digestion, it had the advantages of small footprint, less digestate, and low heating energy. However, HSAD's methane production is poor, mainly due to the complex hydrodynamics. In this study, computational fluid dynamics were utilized for HSAD's hydrodynamics investigation at 14.3% solid content and compared to the particle image velocimetry measurement. Then, effects of mixing on hydrodynamics were investigated. The results indicated that the diameter of impeller was critical for the radial mixing, and the distance between the impellers dictated the axial mixing. Besides, rotating speed affected flow velocities significantly, but displayed less effect on expanding the mixing range. Furthermore, HSAD's treating capacity could be increased at large extent by optimizing mixing. The visualization of the hydrodynamics in this study could potentially offer conceptual basis for HSAD's design in practical engineering.

Effect of different aerobic hydrolysis time on the anaerobic digestion characteristics and energy consumption analysis

Aerobic hydrolysis of stover before anaerobic digestion is beneficial to improve the biodegradability of corn stover. Aerobic hydrolysis of corn stover at 43 °C was conducted to investigate the effects of hydrolysis time (0 h, 8 h, 16 h, and 24 h) on the degradation of lignocellulose from corn stover and material conversion. Further anaerobic digestion and energy consumption analysis with the digestion temperature of 36 °C were carried out. The accumulation rate of volatile fatty acids began to slow down after 16 h of hydrolysis, and the concentration of acetic acid reached 221.85 mmol/L at 24 h of hydrolysis. The degradation rate of lignocellulose was obviously increased after hydrolysis. When the hydrolysis time was 16 h, it reached the maximum cumulative methane production with 268.75 ml/g VS. In terms of biogas production and energy conversion efficiency, it is more appropriate to choose 16 h as hydrolysis time in biogas engineering.

Composition-oriented estimation of biogas production from major culinary wastes in an anaerobic bioreactor and its associated CO2 reduction potential

Despite the wide applications of dry anaerobic digestion (AD), a number of fundamental issues, such as composition-oriented estimation of biogas production and CO₂ reduction potential, were not well understood yet. The objective of this study was to establish composition-oriented models for prediction of biogas production and the associated shift of microbial communities. Three important factors regarding feedstock, including loading, carbon-to-nitrogen ratio, and solid-to-liquid ratio, were found to significantly affect the biogas production. The biogas production and digestion kinetics were evaluated with the response surface methodology. The major contribution to biogas production was found to be hydrogenotrophic methanogens (82.6 ± 0.4%). The net CO₂ reduction potential was assessed from the life-cycle approach, and a substantial amount of CO₂ generation (i.e., 2.8-6.7 tonne/tonne-VS) could be reduced by AD, compared to incineration, revealing that dry AD for food waste treatment should be one of the essential practices in the portfolio of global CO₂ mitigation.