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

Comprehensive review of technologies for separate digestate treatment and agricultural valorisation within circular and green economy

Anaerobic digestion (AD) has the potential to catalyse the shift from a linear to a circular economy. However, effective treatment and management of both solid (DSF) and liquid (DLF) digestate fraction treatment and management require adopting sustainable technologies to recover valuable by-products like energy, biofuels, biochar, and nutrients. This study reviews state-of-the-art advanced technologies for DSF and DLF treatment and valorisation, using life cycle assessment (LCA) and techno-economic analysis (TEA) in integrated digestate management (IDM). Key findings highlight these technologies' potential in mitigating environmental impacts from digestate management, but there's a need to improve process efficiency, especially at larger scales. Future research should prioritize cost-effective and eco-friendly IDM technologies. This review emphasizes how LCA and TEA can guide decision-making and promote sustainable agricultural practices. Ultimately, sustainable IDM technologies can boost resource recovery and advance circular economy principles, enhancing the environmental and economic sustainability of AD processes.

A new paradigm for whole-chain low-carbon utilization of food waste secondary waste based on multivariate evaluations

The rapid growth of China's food and beverage industry has led to a significant increase in food waste, presenting major challenges for its disposal. Anaerobic digestion is the primary treatment method, but its by-products-biogas slurry (BS) and biogas residue (BR)-pose additional treatment challenges. This study proposes and evaluates three management scenarios for these by-products: (1) BS sewage treatment with BR incineration (S1), (2) BS land application with BR composting (S2), and (3) BS sewage treatment with BR composting (S3). The scenarios were comprehensively assessed using material flow analysis, life cycle assessment, and economic benefit analysis. The findings show that S2 achieves the highest carbon utilization efficiency (80.16%) and delivers superior environmental and economic benefits. Scaling up S2 by 2030 could reduce national carbon emissions by 1817.8 kilotons and generate $11.14 billion in economic profit. This study offers a novel model for the sustainable, low-carbon utilization of food waste by-products, providing valuable insights for future organic waste management.

A study on value addition of cow dung-based anaerobic sludge for biomethane and bio-oil production via co-liquefaction with rice straw and clam shells as a catalyst

The waste management sector is moving towards sustainable approaches for facilitating resource-recovery possibilities. Agriculture residue (rice straw), cow dung (cattle waste), and clam shells from the ocean are the primary waste materials possessing a huge value addition opportunity. In this study, the effective usage of rice straw and anaerobic sludge from cow dung for bio-energy production was studied. Cow dung was initially anaerobically processed for the generation of biomethane and sludge in a digester for a retention time of 40 days. The anaerobic sludge with rice straw was hydrothermally processed in varying proportions of 1 : 0, 0 : 1, 1 : 1,1 : 2, 2 : 1, 3 : 1, 1 : 3 and temperatures of 240-360 °C for 1 hour with varying biomass loads of 50, 75, 100, 125, and 150 g. Additionally, clam shells, one of the best bioresources, were used as a catalyst in the hydrothermal process at concentrations of 0.2-1 wt%. The maximum bio-oil produced was 36.23 wt% at a temperature of 320 °C, with a biomass load of 100 g, mixed proportion of 2 : 1 and catalyst loading of 0.6 wt%. The produced bio-oil comprised hydrocarbons, aldehydes, and carboxylic acids, as confirmed through GC-MS. In the anaerobic study, ≈0.018 m3 cumulative gas was produced at a retention time of 40 days. The biochar had a higher carbon content and its feasibility for further usage shows promise towards sustainability.

Improving bio-conditioning dewatering performance of food waste anaerobic digestate at low ambient temperatures by heating treatment

Food waste anaerobic digestate (FWAD) containing high concentrations of contaminants must be purified or recycled. Bio-conditioning dewatering followed by activated sludge process (BDAS) has emerged as a promising technology for treating FWAD. However, the bio-conditioning dewatering as a pivotal step of BDAS is often negatively affected by low ambient temperatures often occurred in winter. This study investigated the role of heating FWAD in improving the bio-conditioning dewatering performance of FWAD. Batch experiments demonstrated that the bio-conditioning dewatering efficiency increased with temperature rise. Notably, due to the low energy consumption, 50°C was considered to be the most appropriate heating treatment temperature, realizing a drastic reduction of specific resistance to filtration (SRF) of bio-conditioned FWAD from initial 1.24 × 1012 m/kg in the control at a ambient temperature of 10°C to 5.42 × 1011 m/kg and a saving of 25% in bio-conditioning reagents cost. The results of the pilot-scale and large-scale experiments revealed that heating treatment made the bio-conditioning dewatering more stable regardless of the fluctuation of ambient temperature in practical engineering. The decrease in the viscosity of bio-conditioned FWAD and the enhancement in microbial fermentation liquor flocculation capacity through heating treatment played pivotal roles in improving the bio-conditioning dewatering performance of FWAD. This work provides a cost-effective strategy to achieve efficient bio-conditioning dewatering at a relatively low ambient temperature, which was helpful in the engineering application of the novel BDAS process in wastewater treatment.

Biogas Digestate and Sewage Sludge as Suitable Feeds for Black Soldier Fly (Hermetia illucens) Larvae

Hermetia illucens larvae can use organic wastes as a substrate, which makes them an interesting potential feed. However, waste may contain heavy metals, which are limited in feed. Here, we investigated the ability of H. illucens to grow on organic wastes and measured their heavy metal bioaccumulation. The larvae were fed with food waste, biogas digestates, and sewage sludge. When the first adult fly was visible, the tests were stopped and the larvae immediately processed. The samples (wastes before use, larvae after feeding) were analysed for mineral nutrient and heavy metal content using AAS and ICP-OES, respectively. The results show that the weight of the larvae fed with food waste increased sevenfold, which was broadly in line with expectations. Those fed with sewage sludge and digestate from biogas station increased threefold. While the larvae fed with sewage sludge exceeded the limits for heavy metals, particularly Cd and Pb, in feedstock, those fed with biogas digestate and food waste did not. These findings add to the literature showing the suitability of different wastes as H. illucens feed, and the importance of excluding waste contaminated with heavy metals from larvae intended for use as animal feed, or else diverting these larvae to non-feed uses.

A critical review of biochar as an environmental functional material in soil ecosystems for migration and transformation mechanisms and ecological risk assessment

At present, biochar has a large application potential in soil amelioration, pollution remediation, carbon sequestration and emission reduction, and research on the effect of biochar on soil ecology and environment has made positive progress. However, under natural and anthropogenic perturbations, biochar may undergo a series of environmental behaviors such as migratory transformation, mineralization and decomposition, and synergistic transport, thus posing certain potential risks. This paper outlines the multi-interfacial migration pathway of biochar in "air-soil-plant-animal-water", and analyzes the migration process and mechanism at different interfaces during the preparation, transportation and application of biochar. The two stages of the biochar mineralization process (mineralization of easily degradable aliphatic carbon components in the early stage and mineralization of relatively stable aromatic carbon components in the later stage) were described, the self-influencing factors and external environmental factors of biochar mineralization were analyzed, and the mineral stabilization mechanism and positive/negative excitation effects of biochar into the soil were elucidated. The proximity between field natural and artificially simulated aging of biochar were analyzed, and the change of its properties showed a trend of biological aging > chemical aging > physical aging > natural aging, and in order to improve the simulation and prediction, the artificially simulated aging party needs to be changed from a qualitative method to a quantitative method. The technical advantages, application scope and potential drawbacks of different biochar modification methods were compared, and biological modification can create new materials with enhanced environmental application. The stability performance of modified biochar was compared, indicating that raw materials, pyrolysis temperature and modification method were the key factors affecting the stability of biochar. The potential risks to the soil environment from different pollutants carried by biochar were summarized, the levels of pollutants released from biochar in the soil environment were highlighted, and a comprehensive selection of ecological risk assessment methods was suggested in terms of evaluation requirements, data acquisition and operation difficulty. Dynamic tracing of migration decomposition behavior, long-term assessment of pollution remediation effects, and directional design of modified composite biochar materials were proposed as scientific issues worthy of focused attention. The results can provide a certain reference basis for the theoretical research and technological development of biochar.

Empirical and simulated investigation of the solid waste reverse supply chain: A complex adaptive system perspective

Solid waste is increasing rapidly worldwide. In this study, the solid waste (household waste, construction and demolition waste and industrial waste) management systems are treated as reverse supply chain to analyze the critical operational issues based on complex adaptive system theory. At the single-layer, the complexity of the various nodes at a layer arises from rational decision-making and behavioral heterogeneity. The solid waste generation layer is employed as an example to investigate the complexity of node behavioral decisions. Regression analysis results reveal that both endogenous (Attitude, Subjective norm, and Perceived behavioral control) and exogenous factors (Economic incentive, Government supervision, Technical support) positively influence sorting behavior. The effect of Economic incentive (β=0.327P<0.001) and Attitude (β=0.249P<0.001) on sorting behavior are the largest. In the multi-layer system, different layers communicate with each other through the material and financial flows and have cross-layer impacts. An agent-based model is developed to investigate the multi-layer feedforward influence mechanism of changes in key layers (e.g., sorting rate, disposal rate) and the material and financial flows adaptive adjustment direction of the solid waste reverse supply chain. High rate of participation and accuracy of source sorting can shorten material flow paths and reduce storage and transportation costs. The increase in disposal rate encourages the transition of solid waste from backfill to resource utilization. This study provides a practice reference for solid waste reverse supply chain and related enterprises managers.

Wasted Food, Wasted Resources? A Critical Review of Environmental Impact Analysis of Food Loss and Waste Generation and Treatment

Food loss and waste (FLW) comes with significant environmental impacts and thus prevents a sustainable food system transition. Here we conducted a systematic review of 174 screened studies that assessed the environmental impacts of FLW generation and treatment. We found that the embodied impacts of FLW along the supply chain and impacts from FLW treatment received equal attention, but few studies have included both. The reviewed studies show narrow geographical (mostly conducted in industrialized countries) and food supply chain (mostly focused on the consumption stage) coverage. Life cycle analysis (LCA), material flow analysis (MFA), or their combination are the most commonly used to quantify FLW related environmental impacts. More method standardization, integration, and innovation and better FLW data with regional and stage resolution from a first-hand source are badly needed. Among the various proposed mitigation strategies covering technology, economy, behavior, and policy aspects, process optimization and waste management options are the most discussed. Our review calls for a more holistic environmental impact assessment of FLW generation and treatment and analysis of the trade-offs among different environmental impact categories and between supply chain stages, which would better inform relevant policy on effective environmental impact mitigation strategies toward sustainable food systems.

Enhancing sustainable crop cultivation: The impact of renewable soil amendments and digestate fertilizer on crop growth and nutrient composition

Utilizing digestate as a fertilizer enhances soil nutrient content, improves fertility, and minimizes nutrient runoff, mitigating water pollution risks. This alternative approach replaces commercial fertilizers, thereby reducing their environmental impact and lowering greenhouse gas emissions associated with fertilizer production and landfilling. Herein, this study aimed to evaluate the impact of various soil amendments, including carbon fractions from waste materials (biochar, compost, and cocopeat), and food waste anaerobic digestate application methods on tomato plant growth (Solanum lycopersicum) and soil fertility. The results suggested that incorporating soil amendments (biochar, compost, and cocopeat) into the potting mix alongside digestate application significantly enhances crop yields, with increases ranging from 12.8 to 17.3% compared to treatments without digestate. Moreover, the combination of soil-biochar amendment and digestate application suggested notable improvements in nitrogen levels by 20.3% and phosphorus levels by 14%, surpassing the performance of the those without digestate. Microbial analysis revealed that the soil-biochar amendment significantly enhanced biological nitrification processes, leading to higher nitrogen levels compared to soil-compost and soil-cocopeat amendments, suggesting potential nitrogen availability enhancement within the rhizosphere's ecological system. Chlorophyll content analysis suggested a significant 6.91% increase with biochar and digestate inclusion in the soil, compared to the treatments without digestate. These findings underscore the substantial potential of crop cultivation using soil-biochar amendments in conjunction with organic fertilization through food waste anaerobic digestate, establishing a waste-to-food recycling system.

Using fresh vegetable waste from Chinese traditional wet markets as animal feed: Material feasibility and utilization potential

To develop new animal feed sources and establish a sustainable food upcycling system, the material feasibility and feeding potential of fresh vegetable waste (FVW) were clarified in this study. First, the FVW output of wet markets in Hangzhou, China was tracked and predicted. The results showed that the retail waste ratio of FVW in wet markets reached 9.3 %, predicting that China's FVW will reach 9034 kt in 2030. Second, the study revealed that the nutritive value of FVW was comparable to that of traditional alfalfa feed, suitable for use as animal feed. However, we found a high probability of microbial contamination. Therefore, FVW should have stricter classification and collection methods. Under this premise, the feeding utilization potential of FVW in wet markets is large. In 2030, the crude protein content may replace 2737 kt of alfalfa, saving 7.7 E + 08 m3 of water and 75,018 ha of land.

Carbon/nitrogen flows and associated microbial communities in full-scale foodwaste treatment plants

Microorganisms play key roles in the conversion of organic matter in foodwaste. However, both the microbially-mediated element (carbon/C and nitrogen/N) flows and associated microbial communities in foodwaste treatment plants (FWTPs) remain unclear. This study collected samples of different foodwaste treatment units from five full-scale FWTPs to analyze the C/N flows and microbial communities in foodwaste treatment processes. Results showed that 39.8-45.0% of organic carbon in foodwaste was converted into biogas. Hydrolytic acidogenic bacteria (e.g., Lactobacillus and Limosilactobacillus) and eukaryota (e.g., Cafeteriaceae, Saccharomycetales, and Agaricomycetes) were more abundant in feedstock and pretreatment units. Redundancy analyses showed that acidogens were major players in the transformation of foodwaste organic matter. Populations of W27 and Tepidanaerobacter were major contributors to the difference in conversion of C/N in these FWTPs. This study could support foodwaste treatment efficiencies improvement by providing insights into C/N flows and associated microbiota in FWTPs.

A Critical Review of Data Science Applications in Resource Recovery and Carbon Capture from Organic Waste

Municipal and agricultural organic waste can be treated to recover energy, nutrients, and carbon through resource recovery and carbon capture (RRCC) technologies such as anaerobic digestion, struvite precipitation, and pyrolysis. Data science could benefit such technologies by improving their efficiency through data-driven process modeling along with reducing environmental and economic burdens via life cycle assessment (LCA) and techno-economic analysis (TEA), respectively. We critically reviewed 616 peer-reviewed articles on the use of data science in RRCC published during 2002-2022. Although applications of machine learning (ML) methods have drastically increased over time for modeling RRCC technologies, the reviewed studies exhibited significant knowledge gaps at various model development stages. In terms of sustainability, an increasing number of studies included LCA with TEA to quantify both environmental and economic impacts of RRCC. Integration of ML methods with LCA and TEA has the potential to cost-effectively investigate the trade-off between efficiency and sustainability of RRCC, although the literature lacked such integration of techniques. Therefore, we propose an integrated data science framework to inform efficient and sustainable RRCC from organic waste based on the review. Overall, the findings from this review can inform practitioners about the effective utilization of various data science methods for real-world implementation of RRCC technologies.

Impacts of digestate-based compost on soil property and nutrient availability

The treatment of digestate from food waste (DFW) has emerged as the bottleneck for food waste anaerobic digestion. DFW generally contains abundant nutrients that can be recycled by composting. However, the effect of DFW-based compost on soil improvement has not been extensively explored. In this study, soil properties were improved by adding various amounts of DFW-based compost, and the growth conditions of Pak choi were monitored. The results indicated that the DFW-based compost could provide nitrogen, calcium, magnesium, and organic matter, thereby enhancing the growth of Pak choi, accumulating chlorophyll, and improving photosynthesis efficiency. As the amount of added DFW-based compost increased from 0% to 20%, the fresh biomass, leaf weight, and root weight of Pak choi increased by 242%, 262%, and 99%, respectively. The total chlorophyll content was 2.62 mg g-1 in control and increased to 12.45 mg g-1 in the group with 20% DFW-based compost, benefiting the photochemical efficiency of Pak choi. However, the growth was inhibited when the addition amount exceeded 20%, potentially due to excessive nutrient supplementation. Overall, the addition of 20% of DFW-based compost was suggested to promote the growth of Pak choi by providing proper nutrients.

Environmental impact assessment of multi-source solid waste based on a life cycle assessment, principal component analysis, and random forest algorithm

As a national pilot city for solid waste disposal and resource reuse, Dongguan in Guangdong Province aims to vigorously promote the high-value utilization of solid waste and contribute to the sustainable development of the Greater Bay Area. In this study, life cycle assessment (LCA) coupled with principal component analysis (PCA) and the random forest (RF) algorithm was applied to assess the environmental impact of multi-source solid waste disposal technologies to guide the environmental protection direction. In order to improve the technical efficiency and reduce pollution emissions, some advanced technologies including carbothermal reduction‒oxygen-enriched side blowing, directional depolymerization‒flocculation demulsification, anaerobic digestion and incineration power generation, were applied for treating inorganic waste, organic waste, kitchen waste and household waste in the park. Based on the improved techniques, we proposed a cyclic model for multi-source solid waste disposal. Results of the combined LCA-PCA-RF calculation indicated that the key environmental load type was human toxicity potential (HTP), came from the technical units of carbothermal reduction and oxygen-enriched side blowing. Compared to the improved one, the cyclic model was proved to reduce material and energy inputs by 66%-85% and the pollution emissions by 15%-88%. To sum up, the environmental impact assessment and systematic comparison suggest a cyclic mode for multi-source solid waste treatments in the park, which could be promoted and contributed to the green and low-carbon development of the city.

A Review on the Challenges and Choices for Food Waste Valorization: Environmental and Economic Impacts

Valorization of food waste (FW) is instrumental for reducing the environmental and economic burden of FW and transitioning to a circular economy. The FW valorization process has widely been studied to produce various end-use products and summarize them; however, their economic, environmental, and social aspects are limited. This study synthesizes some of the valorization methods used for FW management and produces value-added products for various applications, and also discusses the technological advances and their environmental, economic, and social aspects. Globally, 1.3 billion tonnes of edible food is lost or wasted each year, during which about 3.3 billion tonnes of greenhouse gas is emitted. The environmental (-347 to 2969 kg CO₂ equiv/tonne FW) and economic (-100 to $138/tonne FW) impacts of FW depend on the multiple parameters of food chains and waste management systems. Although enormous efforts are underway to reduce FW as well as valorize unavoidable FW to reduce environmental and economic loss, it seems the transdisciplinary approach/initiative would be essential to minimize FW as well as abate the environmental impacts of FW. A joint effort from stakeholders is the key to reducing FW and the efficient and effective valorization of FW to improve its sustainability. However, any initiative in reducing food waste should consider a broader sustainability check to avoid risks to investment and the environment.

Insights into the influence of digestate-derived biochar upon the microbial community succession during the composting of digestate from food waste

The by-product from the anaerobic digestion of food waste (FW) called the digestate (DFW) needs proper disposal because of its high environmental burden. Composting can transform DFW into a nutrient-containing soil improver via a series of microbial metabolic activities. However, the long composting time and high amount of ammonia emission are the key concerns of DFW composting. In the present study, the effect of DFW-derived biochar (BC-DFW) on microbial succession and its involvement in nitrogen transformation and humification during DFW composting were investigated. The results indicated that the BC-DFW accelerated bacterial and fungal evolution, and the bacterial diversity was augmented by increasing the amount of BC-DFW. In particular, Cryomorpha, Castellaniella, Aequorivita, and Moheibacter were enriched by the addition of BC-DFW, thereby enhancing the degradation of organic matter and nitrogen transformation and increasing the germination index. The group with 25% BC-DFW contained a higher relative abundance of Cryomorpha (2.08%, 2.47%) than the control (0.39%, 1.72%) on days 19 and 35 which benefited the degradation of organic matter. The group with 25% BC-DFW quickly enhanced the growth of Nitrosomonas, thereby accelerating the conversion of ammonium-nitrogen to nitrate-nitrogen and reducing the phytotoxicity of the composting product.

A review on biological methodologies in municipal solid waste management and landfilling: Resource and energy recovery

Rapid industrialization and urbanization growth combined with increased population has aggravated the issue of municipal solid waste generation. MSW has been accounted for contributing tremendously to the improvement of sustainable sources and safe environment. Biological processing of MSW followed by biogas and biomethane generation is one of the innumerable sustainable energy source choices. In the treatment of MSW, biological treatment has some attractive benefits such as reduced volume in the waste material, adjustment of the waste, economic aspects, obliteration of microorganisms in the waste material, and creation of biogas for energy use. In the anaerobic process the utilizable product is energy recovery. The current review discusses about the system for approaching conversion of MSW to energy and waste derived circular bioeconomy to address the zero waste society and sustainable development goals. Biological treatment process adopted with aerobic and anaerobic processes. In the aerobic process the utilizable product is compost. These techniques are used to convert MSW into a reasonable hotspot for resource and energy recovery that produces biogas, biofuel and bioelectricity and different results in without risk and harmless to the ecosystem. This review examines the suitability of biological treatment technologies for energy production, giving modern data about it. It likewise covers difficulties and points of view in this field of exploration.

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.

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.

Mechanism of digestate-derived biochar on odorous gas emissions and humification in composting of digestate from food waste

Emissions of odorous gases and prolonged composting duration are the key concerns in the composting of digestate from food waste (DFW). In this study, different amounts of biochar derived from DFW (BC-DFW) were introduced in the composting process of DFW to decrease the emissions of ammonia (NH₃) and volatile sulfur compounds (VSCs) and composting duration. The addition of BC-DFW increased the temperature and germination index during DFW composting. The group with 25% BC-DFW exhibited a 30% smaller composting duration. Significant amounts of NH₃ and VSCs emissions were observed in the initial phase of DFW composting. Dimethyl disulfide (DMDS) was a prominent contributor to the odor associated with VSCs. The addition of BC-DFW facilitated the adsorption of NH₃ and VSCs, and the corresponding contents decreased by 5-21% and 15-20%, respectively. Moreover,the BC-DFW accelerated the transformation of ammonium-nitrogen (NH₄⁺-N) to nitrate-nitrogen (NO₃^--N), thereby alleviating the NH₃ volatilization. The addition of 25% BC-DFW minimized the NH₃ emission and enhanced the generation of humic-acid-like matter, thereby promoting humification. Therefore, the addition of 25% BC-DFW was optimal for promoting the degradation of organic matter and humification and odor emission reduction (e.g., NH₃, DMDS).

Producing insect protein from food waste digestate via black soldier fly larvae cultivation: A promising choice for digestate disposal

The treatment of food waste digestate with high salinity is a big challenge. This paper evaluated the possibility of using black soldier fly larvae for food waste digestate disposal and insect protein production. Results showed that both digestates from hydrogen and methane fermentations were rich in protein and lipid contents, which benefited the BSFL cultivation. The BSFL reared on digestates from hydrogen and methane fermentations of food waste performed better in pre-pupal weight (19.12% and 41.13% higher, respectively), body length (3.62% and 18.21% higher, respectively) and crude protein contents (7.85% and 39.05% higher, respectively) than that reared on raw food waste. In addition, the maximum body weight growth rate (R_m) of BSFL cultivated on both digestates were 28.28% and 47.10% higher than that of BSFL cultivated on raw food waste, respectively. During BSFL cultivation, organic matter reduction between 40.97% and 46.07% were achieved. Digestates from hydrogen and methane fermentations represent favorable feeding substrates for BSFL cultivation. Using BSFL to treat AD digestate not only provides a digestate disposal approach, but also produces insect biomass and organic fertilizer as value-added byproducts, which shows tremendous potential in digestate disposal.

Pyrolysis of food waste and food waste solid digestate: A comparative investigation

The effects of anaerobic digestion (AD) on pyrolysis were elaborated by comparing the pyrolysis performance of food waste (FW) and food waste solid digestate (FWSD). The pyrolysis mechanisms of FW and FWSD were revealed by experimental and kinetic analysis. The properties and potential applications of pyrolytic products from FW and FWSD were discussed. The results showed that part of organic matters of FW were consumed during AD, which altered the pyrolysis performance of FWSD. The pyrolytic gas from FW had better quality due to its higher lower heating value (LHV) (20.52 kJ/Nm³). The pyrolytic oil and biochar derived from FWSD showed better qualities as oil fuel and carbon-based absorbent. Pyrolysis of FWSD produced less nitrogen-containing pollutants (NCPs) indicated that AD coupled with pyrolysis is more environmental-friendly to treat FW. This study provides potential approach and theoretical guidance for the treatment and resource utilization of FW and FWSD.

Improvement in the physicochemical characteristics of biochar derived from solid digestate of food waste with different moisture contents

The management of digestate from food waste (DFW) has become a worldwide challenge. Pyrolysis is a promising technology to generate biochar from the DFW. However, unlike other biomass, DFW usually has high salt and moisture content, which affects the properties of biochar generated from pyrolysis. The characteristics of biochar derived from DFW with different MCs (5%, 20%, 40%, and 60%) were investigated in the present study. It was found that more micropore and mesopore structures were generated in the biochar with the increase of MC from 5% to 60%, resulting in the Brunauer-Emmett-Teller surface area of the biochar increased from 89.23 m² g^-1 to 117.75 m² g^-1. The MC could also promote the variation of oxygen-containing functional groups and the generation of amorphous carbon structures, which are beneficial for the adsorption property of the biochar. Pyrolysis could stabilize the metals in the biochar, while MC has little effect on the metal speciations. These results provide fundamental information on the impact of MC on the properties of biochar derived from DFW and are important for the optimization of the pre-drying process.

Analysis of Main Factors on Evaluation and Selection of Wet Waste Disposal Modes: A Case Study of Universities in Shanghai, China

This paper explores greenhouse gas emission intensity and economy of centralized and on-site wet waste disposal mode, while comprehensively evaluating the two modes for decision-making. Based on the fieldwork in Shanghai’s 20 campuses of 15 universities, multiple scenarios that can reflect the different levels of technology and management in reality, were set for the following studies. The greenhouse gas emissions generated from centralized and on-site disposal modes of wet waste were calculated in two emission scenarios using Life Cycle Assessment, Life Cycle Inventory, and the IPCC 2006 method. Additionally, the continuous cost input from the universities for the two disposal modes was analyzed in three cost-input scenarios using the Net Present Value method. Furthermore, a comprehensive evaluation of the two modes was also conducted by using Analytic Hierarchy Process and Fuzzy Comprehensive Evaluation under the five main factors of greenhouse gas emission—control, economy, stability, education and innovation, and bargaining power for municipal sanitation departments. The results revealed that the centralized disposal mode is superior to the on-site disposal mode in terms of greenhouse gas emission control and economy. The centralized disposal mode is a more rational choice due to the better comprehensive evaluation performance. It was also emphasized that the construction of the wet waste disposal system is so complicated that the academic community and the policymakers may have to pay more attention to the integration of system design, industrial development, and other aspects of wet waste disposal.

Anaerobic Digestate from Biogas Plants—Nuisance Waste or Valuable Product?

Biogas production in waste-to-energy plants will support the decarbonization of the energy sector and enhance the EU’s energy transformation efforts. Digestates (DG) formed during the anaerobic digestion of organic wastes contain large amounts of nutrients. Their use for plant fertilization allows for diversifying and increasing the economic efficiency of farming activities. However, to avoid regional production surpluses, processing technologies allowing the acquisition of products that can be transported over long distances are required. This study therefore aimed at determining the effect of applied methods of DG treatment on the chemical composition of the resulting products and their effect on the yields and chemical composition of plants. The following digestate-based products (DGBPs) were tested: two different digestates (DGs), their liquid (LF) and solid fractions (SF) and pellets from DGs (PDG), and pellets form SFs (PSF). Results from the experiment show that during SF/LF separation of DGs, >80% of nitrogen and 87% of potassium flows to LFs, whereas >60% of phosphorus and 70% of magnesium flows to SFs. The highest yields were obtained using untreated DGs and LFs. The application of DGs and LFs was not associated with a leaching of nutrients to the environment (apparent nutrients recovery from these products exceeded 100%). Pelletized DG and SF forms can be used as slow-release fertilizer, although their production leads to significant nitrogen losses (>95%) by ammonia volatilization.

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.

Life cycle assessment and society willingness to pay indexes of food waste-to-energy strategies

Food waste (FW) has received increasing attention because of its immense production quantities and significance to resource and environmental impacts related to disposal approaches. We combined life cycle assessment (LCA) with society's willingness to pay (WTP) index to evaluate energy, water, and environmental impacts on three food waste-to-energy (FWTE) options in China. For anaerobic digestion (AD) mode, the results showed that 1140 MJ of energy consumption could be saved by power generation from methane, power transmission, and biodiesel production from per ton of FW; the cost of climate change for treating FW was 137.8 kg CO₂e t^-1 FW, failing to be climate-sound due to the end life of digestate in practice. The total impact to AD mode in the form of monetized value for WTP was 13.3 CNY t^-1 FW, of which the collection and transportation, pretreatment, AD reaction, wastewater treatment, biodiesel production, and residue landfilling stages contributed by 10.5%, 6.5%, 19.3%, 27.6%, 4.7%, and 75.7%, respectively, while biogas utilization offset it by 43.9%. Notably, a considerable amount of water used in AD prevented it from showing an advantage compared to incineration (-5.1 CNY t^-1 FW), which performed best overall attributing to the generated electricity compensated for primary energy demand, water, and terrestrial acidification to a great extent. Landfilling turned out to be an unappealing FW disposal method due to the low landfill gas capture ratio. Given that AD is touted for its environmental benefits, potential approaches-such as developing a reliable and supportive technology to facilitate digestate recycling into agriculture-were discussed to improve its competitiveness and attractiveness. Our study employed a way to accumulate and compare impact indicators to better interpret FW management impacts and advantages, considering energy recovery, resource recycling, and the environment.

Use of activated carbon to reduce ammonia emissions and accelerate humification in composting digestate from food waste

Management of digestate from food waste (DFW) is becoming the bottleneck of the food waste anaerobic digestion. Composting is a feasible method to dispose the DFW and convert it to organic fertilizer; however, high ammonia (NH₃) emissions and long composting time are key concerns in this process. In this study, the mechanism of activated carbon (AC) on the loss of NH₃ and humification during DFW composting was investigated. The use of AC could promote humification, shorten 50% of the DFW composting period, and decrease the NH₃ emissions by 34%. Results of the microbial analysis indicated that the AC could promote the growth of key microbes (i.e., Wallemia genus for fungi; and Fastidiosipila genus for bacteria). The Cladosporium and Fastidiosipila genera developed in the fractions closely and loosely attached to the AC, respectively, leading to faster degradation of lignocellulose matter. In addition, AC could enrich the Ammoniibacillus genus, reducing nitrogen loss.

Resource potential and global warming potential of fruit and vegetable waste in China based on different treatment strategies

Fruit and vegetable waste (FVW) contains rich resources that can be recovered by methods such as incineration, anaerobic digestion to generate heat energy, biogas, and preservation by ensiling. However, a horizontal comparison of the resource potential and environmental impact of different recycling methods employed for FVW has not been conducted. This study quantifies and computes the recycling potential and global warming potential (GWP) of anaerobic digestion, ensiling, and incineration of the FVW generated during primary production in China. First, a gray model was employed to estimate the FVW output in 2030, based on the FVW produced between 2002 and 2017. Next, the resource potential and GWP of anaerobic digestion, incineration, and ensiling were evaluated. Finally, an optimization method was utilized to analyze possible strategies of FVW recycling in 2030. Results indicate that FVW output in China is expected to increase to 170 Mt by 2030, highlighting the need for efficient treatment options. Further, the resource potential and GWP of different waste treatment strategies were notably different. The recycling potential of ensiling was the highest at 1950 MJ/t; while the GWP of anaerobic digestion was the lowest at -31 kg CO2eq. An optimization analysis suggested that the optimal target of 100% would be attained if all FVW is ensiled in 2030. The study provides a basis for informed technical decision-making related to FVW recycling options in the future.

A multi-perspective review on microbial electrochemical technologies for food waste valorization

The worldwide generation of food waste (FW) has been increasing enormously due to the growing food industry and population. However, FW contains a large amount of biodegradable organics that can be converted to clean energy, which can potentially minimize the utilization of fossil fuels. Conventional biowaste valorization technologies, such as anaerobic digestion and composting, have been adopted for FW management for recovering useful biogas and compost. However, they are often limited by high capital and operation costs, low recovery efficiency, slow process kinetics, and system instability. On the other hand, microbial electrochemical technologies (METs) have been highly promising for efficiently harvesting bioenergy and high value-added products from FW. Hence, this article critically reviews up-to-date studies on applying various METs regarding their value-added products recovery efficiencies from FW. Moreover, this review lists existing challenges, ways to optimize the system performance and provides perspectives on future research needs.

Long-term characterization and resource potential evaluation of the digestate from food waste anaerobic digestion plants

The management of digestate from food waste (DFW) has become a big challenge for anaerobic digestion (AD) plants. It is crucial to understand the characteristics of DFW for its beneficial utilization. This study investigated the long-term characteristics of DFW from an industrial-scale AD plant in China for 16 months. The result showed that the characteristics of the DFW were relatively stable. The DFW contained considerable amounts of organic matter (23-40% of lignin and 12-26% of protein) and abundant nutrients (N, P, and K), with high concentrations of metals (e.g., 55.17 mg g^-1 and 15.55 mg g^-1 of Ca and Fe) and sulfur (1.40%) on a dry basis. Based on the results, pyrolysis and composting were evaluated as optional conversion ways of DFW. The pyrolysis temperature range of 500 °C to 600 °C was recommended for producing biochar. In this temperature range, the Brunauer-Emmett-Teller surface area of the produced biochar is over 120 m² g^-1. The composting offered the best potential for recovering the nutrients from DFW, but the high ammonia gas content (6970 ppm) should be paid attention to during composting.

Digested Sludge Quality in Mesophilic, Thermophilic and Temperature-Phased Anaerobic Digestion Systems

Anaerobic digestion (AD) technology is commonly used to treat sewage sludge from activated sludge systems, meanwhile alleviating the energy demand (and costs) for wastewater treatment. Most often, anaerobic digestion is run in single-stage systems under mesophilic conditions, as this temperature regime is considered to be more stable than the thermophilic one. However, it is known that thermophilic conditions are advantageous over mesophilic ones in terms of methane production and digestate hygienisation, while it is unclear which one is better concerning the digestate dewaterability. Temperature-phased anaerobic digestion (TPAD) is a double-stage AD process that combines the above-mentioned temperature regimes, by operating a thermophilic digester followed by a mesophilic one. The aim of this study is to compare the digestate quality of single-stage mesophilic and thermophilic AD and TPAD systems, in terms of the dewaterability, pathogenic safety and lower calorific value (LCV) and, based on the comparison, consider digested sludge final disposal alternatives. The research is conducted in lab-scale reactors treating waste-activated sludge. The dewaterability is tested by two methods, namely, centrifugation and mechanical pressing. The experimental results show that the TPAD system is the most beneficial in terms of organic matter degradation efficiency (32.4% against 27.2 for TAD and 26.0 for MAD), producing a digestate with a high dewaterability (8.1–9.8% worse than for TAD and 6.2–12.0% better than for MAD) and pathogenic safety (coliforms and Escherichia coli were not detected, and Clostridium perfringens were counted up to 4.8–4.9 × 103, when for TAD it was only 1.4–2.5 × 103, and for MAD it was 1.3–1.8 × 104), with the lowest LCV (19.2% against 15.4% and 15.8% under thermophilic and mesophilic conditions, respectively). Regarding the final disposal, the digested sludge after TAD can be applied directly in agriculture; after TPAD, it can be used as a fertilizer only in the case where the fermenter HRT assures the pathogenic safety. The MAD digestate is the best for being used as a fuel preserving a higher portion of organic matter, not transforming into biogas during AD.

Effect of the compound bacterial agent on microbial community of the aerobic compost of food waste

In our study, we used 16SrRNA and ITS to investigate the microbial community composition and the effect of compound bacterial agent on the microbial community composition in the aerobic composting process of food waste (FW). At the bacterial level, the main phyla of Group A (compost naturally) were Proteobacteria and Firmicutes, and the main species were Pseudomonas_sp._GR7, Bacillus licheniformis and Pediococcus acidilactici. The main phyla of Group B (compost with compound bacterial agent) were Proteobacteria, Firmicutes and Streptophyta, and the main species were Klebsiella pneumoniae, Cronobacter sakazakii, Macrococcus caseolyticus, Enterococcus faecalis, Citrobacter freundii and Bacillus velezensis. It is worth noting that M. caseolyticus may be able to improve the effect of odour which is an important sensory index during aerobic composting. At the fungal level, the main phylum of both Groups A and B was Ascomycota, and the main species of Group A were Paecilomyces variotii, Byssochlamys spectabilis and Aspergillus fumigatus. The main species of Group B were Ogataea polymorpha and Millerozyma farinosa. Finally, the degradation rate of Group B was 81% that was about 15% higher than that of Group A, indicating that the compound bacterial agent could effectively improve the degradation rate and the composting process, while the low abundance of the compound bacterial agent in the composting process might be due to the small initial addition or the inhibition of other bacteria or fungi in the composting process.

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.

The implications of facility design and enabling policies on the economics of dry anaerobic digestion

Diverting organic waste from landfills provides significant emissions benefits in addition to preserving landfill capacity and creating value-added energy and compost products. Dry anaerobic digestion (AD) is particularly attractive for managing the organic fraction of municipal solid waste because of its high-solids composition and minimal water requirements. This study utilizes empirical data from operational facilities in California in order to explore the key drivers of dry AD facility profitability, impacts of market forces, and the efficacy of policy incentives. The study finds that dry AD facilities can achieve meaningful economies of scale with organic waste intake amounts larger than 75,000 tonnes per year. Materials handling costs, including the disposal of inorganic residuals from contaminated waste streams and post-digester mass (digestate) management, are both the largest and the most uncertain facility costs. Facilities that utilize the biogas for vehicle fueling and earn associated fuel credits collect revenues that are 4-6x greater than those of facilities generating and selling electricity and 10-12x greater than facilities selling natural gas at market prices. The results suggest important facility design elements and enabling policies to support an increased scale of organic waste handling infrastructure.