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

A critical review on characterization, human health risk assessment and mitigation of malodorous gaseous emission during the composting process

Composting has emerged as a suitable method to convert or transform organic waste including manure, green waste, and food waste into valuable products with several advantages, such as high efficiency, cost feasibility, and being environmentally friendly. However, volatile organic compounds (VOCs), mainly malodorous gases, are the major concern and challenges to overcome in facilitating composting. Ammonia (NH3) and volatile sulfur compounds (VSCs), including hydrogen sulfide (H2S), and methyl mercaptan (CH4S), primarily contributed to the malodorous gases emission during the entire composting process due to their low olfactory threshold. These compounds are mainly emitted at the thermophilic phase, accounting for over 70% of total gas emissions during the whole process, whereas methane (CH4) and nitrous oxide (N2O) are commonly detected during the mesophilic and cooling phases. Therefore, the human health risk assessment of malodorous gases using various indexes such as ECi (maximum exposure concentration for an individual volatile compound EC), HR (non-carcinogenic risk), and CR (carcinogenic risk) has been evaluated and discussed. Also, several strategies such as maintaining optimal operating conditions, and adding bulking agents and additives (e.g., biochar and zeolite) to reduce malodorous emissions have been pointed out and highlighted. Biochar has specific adsorption properties such as high surface area and high porosity and contains various functional groups that can adsorb up to 60%-70% of malodorous gases emitted from composting. Notably, biofiltration emerged as a resilient and cost-effective technique, achieving up to 90% reduction in malodorous gases at the end-of-pipe. This study offers a comprehensive insight into the characterization of malodorous emissions during composting. Additionally, it emphasizes the need to address these issues on a larger scale and provides a promising outlook for future research.

Exogenous additive ferric sulfate regulates sulfur-oxidizing bacteria in cow manure composting to promote carbon fixation

Enhancing carbon fixation in the composting process was of great significance in the era of massive generation of organic solid waste. In this study, the experimental results showed that the contents of dissolved organic matter (DOM) in the experimental group (CT) were 37.58% higher than those in the control group (CK). The CO2 emission peaked on day 5, and the value of CK was 1.34 times that of CT. Significant differences were observed between the contents of sulfur fractions in CT and CK. This phenomenon may be due to the suppression of sulfur-reducing gene expression in CT. On day 51 of composting, the abundance of sulfur-oxidizing bacteria (SOB) Rhodobacter (5.33%), Rhodovulum (14.76%), and Thioclava (23.83%) in CT was higher than that in CK. In summary, the composting fermentation regulated by Fe2(SO4)3 increased the sulfate content, enhanced the expression of sulfur-oxidizing genes and SOB, and ultimately promoted carbon sequestration during composting.

Predicting maturity and identifying key factors in organic waste composting using machine learning models

The measurement of germination index (GI) in composting is a time-consuming and laborious process. This study employed four machine learning (ML) models, namely Random Forest (RF), Artificial Neural Network (ANN), Support Vector Regression (SVR), and Decision Tree (DT), to predict GI based on key composting parameters. The prediction results showed that the coefficient of determination (R2) for RF (>0.9) and ANN (>0.9) was higher than SVR (<0.6) and DT (<0.8), suggesting that RF and ANN displayed superior predictive performance for GI. The SHapley additive exPlanations value result indicated that composting time, temperature, and pH were the important features contributing to GI. Composting time was found to have the most significant impact on GI. Overall, RF and ANN were suggested as effective tools for predicting GI in composting. This study offers the reliable approach of accurately predicting GI in composting processes, thereby enabling intelligent composting practices.

Multi-stage aeration regime to regulate organic conversion toward gas alleviation and humification in food waste digestate composting

Aerobic composting has been considered as a pragmatic technique to convert food waste digestate into high-quality biofertiliser. Nevertheless, massive gaseous emission and immature product remain the primary challenges in food waste digestate composting. Thus, the performance of multi-stage aeration regimes to improve gaseous emissions and organic humification during food waste digestate composting was investigated in this study. In addition to continuous aeration with a constant intensity of 0.3 L kg·dry mass (DM)-1·min-1, two multi-stage decreased aeration regimes were designed as "0.3-0.2-0.1" and "0.3-0.1-0.1" L·kg·DM-1·min-1 from the thermophilic to cooling and then mature stages, respectively. Results showed that the decreased aeration regimes could alleviate nitrous oxide (N2O) and ammonia (NH3) emission and slightly enhance humification during composting. The alleviated N2O and NH3 emission were mainly contributed by abiotically reducing gaseous release potential as well as biotically inactivating denitrifers (Pusillimonas and Pseudidiomarina) and proliferating Atopobium to reduce nitrate availability under lower aeration supply. The "0.3-0.2-0.1 L kg·DM-1·min-1" regime exhibited a more excellent performance to alleviate N2O and NH3 emission by 27.5% and 16.3%, respectively. Moreover, the decreased aeration regimes also favored the enrichment of functional bacteria (Caldicoprobacter and Syntrophomonas) to accelerate lignocellulosic biodegradation and thus humic acid synthesis by 6.5%-11.2%. Given its better performance to improve gaseous emissions and humification, the aeration regime of "0.3-0.2-0.1 L kg·DM-1·min-1" are recommended in food waste digestate composting in practice.

Contribution of sulfur-containing precursors to release of hydrogen sulfide in sludge composting

Hydrogen sulfide (H2S) production during composting can impact the environment and human health. Especially during the thermophilic phase, H2S is discharged in large quantities. However, in sludge composting, the contributions of different sulfur-containing precursors to H2S fluxes, key functional microorganisms, and key environmental parameters for reducing H2S flux remain unclear. Analysis of cysteine (Cys), methionine (Met), and sulfate (SO42-) concentrations, multiple stepwise regression analysis, and Kyoto Encyclopedia of Genes and Genomes (KEGG) annotation analysis of metagenomes showed that Cys was the main contributor to the production of H2S and that Met was among the main sources during the first three days of composting, while the SO42- contribution to H2S was negligible. Fifteen functional genera involved in the conversion of precursors to H2S were identified by co-occurrence network analysis. Only Bacillus showed high temperature resistance (>50 °C) and the ability to reduce H2S. Redundancy analysis showed that total carbon (64.0 %) and pH (23.3 %) had significant effects on functional bacteria. H2S had a quadratic relationship with sulfur-containing precursors. All microbial network sulfur-containing precursors metabolism modules showed a highly significant relationship with Cys.

Functional redundancy is the key mechanism used by microorganisms for nitrogen and sulfur metabolism during manure composting

The microbial ecological functions associated with the nitrogen and sulfur cycles during composting have not been thoroughly elucidated. Using metagenomic sequencing, the microbial mechanisms underlying the nitrogen and sulfur metabolism during livestock and poultry manure composting were investigated in this study. The findings demonstrate that functional redundancy among microorganisms is a crucial factor for the nitrogen and sulfur cycling during livestock and poultry manure composting. Processes such as organic sulfur synthesis, assimilatory sulfate reduction, ammonia assimilation, and denitrification were found to be prevalent. Additionally, there was a certain degree of conservation in nitrogen and sulfur conversion functions among microorganisms at the phylum level. All high-quality metagenomic assembly genomes (MAGs) possessed carbon fixation potential, with 86.3 % of MAGs containing both nitrogen and sulfur conversion genes. Except for bin30, other MAGs encoding sulfur oxidation enzymes were found to be associated with at least one denitrification gene. This suggests a potential interplay between nitrogen and sulfur metabolism among microorganisms. 45, 19, 1, 31, 1, and 2 MAGs could completely regulate organic sulfur synthesis, assimilatory sulfate reduction, thiosulfate oxidation to sulfate, glutamine synthase-glutamate synthase pathway (GS-GOGAT), denitrification, and dissimilatory nitrate reduction, respectively by encoding the required enzymes. TN and pH were the key factors driving the functional redundancy in nitrogen and sulfur microbial community.

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 change in substance and microbial community structure during the co-composting of kitchen waste anaerobic digestion effluent, sewage sludge and Chinese medicine residue

Anaerobic digestion is a resource recovery method for organic waste, gaining attention due to carbon reduction. Disposing of anaerobic digestion effluent (ADE) is crucial for developing anaerobic digestion, but conventional wastewater treatment fails to effectively recover nutrients contained in the ADE. In the present study, the ADE without solid-liquid separation was mixed with sewage sludge and Chinese medicine residue for the composting, where the ADE could be recovered at high temperature through humification. Besides, the nitrogen balance, humification process, and microbial dynamics during the composting process were studied. The results showed that the group supplemented with ADE could increase the nitrogen retention efficiency by 2.21 % compared to the control group. High ammonia nitrogen content and salinity did not negatively affect the maturity and phytotoxicity of compost products and even increase the humification degree of compost products. Moreover, additional ADE may not alter microbial community structure, which could contribute to microbial succession. This is the first time to investigate the substance transformation and shift in microbial community structure while applying composting process for ADE treatment, in which the anaerobic-aerobic collaborative disposal process provides an alternative solution for the recovery of ADE.

Regulation of fungal communities during pig manure composting

The role of protein shell (PS) amendment in altering the fungal community during pig manure (PM) composting was investigated. Six different dosages of PS based on the dry weight of PM (0 %, 2.5 %, 5 %, 7.5 %, 10 %, and 12 %; T1-T6, respectively) were mixed with wheat straw to make the initial feedstock and composted for 42 days. The results showed that Ascomycota, Basidiomycota, and Giomeromycota were the most abundant phyla in all treatments. However, the relative abundance of Giomeromycota was the highest in the control treatment, although a substantially greater population was observed in all treatments. Genus abundance declined steadily from T1 to T6; however, T4 and T6 had smaller populations. Correlation analysis also suggested that T6 amendment increased the overall fungal dynamics and organic matter degradation. Thus, T6 was more efficient to enhance the overall fungal population and dynamics with considerable network connections among all the analyzed parameters.

A comprehensive review on the decentralized composting systems for household biodegradable waste management

Municipal solid waste primarily consists of household biodegradable waste (HBW). HBW treatment is a crucial step in many countries due to rapid urbanization. Composting is an effective technique to treat HBW. However, conventional composting systems are unable to produce matured compost (MC), as well as releasing huge amounts of greenhouse and odorous gases. Therefore, this review attempts to suggest suitable composting system to manage HBW, role of additives and bulking agents in composting process, identify knowledge gaps and recommend future research directions. Centralized composting systems are unable to produce MC due to improper sorting and inadequate aeration for composting substrate. Recently, decentralized compost systems (DCS) are becoming more popular due to effective solid waste reduction at the household and/or community level itself, thereby reducing the burden on municipalities. Solid waste sorting and aeration for the composting substrate is easy at DCS, thereby producing MC. However, Mono-composting of HBW in DCS leads to production of immature compost and release greenhouse and odorous gases due to lower free air space and carbon-to-nitrogen ratios, and higher moisture content. Mixing HBW with additives and bulking agents in DCS resulted in a proper initial substrate for composting, allowing rapid degradation of substrate due to longer duration of thermophilic phase and produce MC within a shorter duration. However, people have lack of awareness about solid waste management is the biggest challenge. More studies are needed to eliminate greenhouse and odorous gases emissions by mixing different combinations of bulking agents and additives (mainly microbial additives) to HBW in DCS.

Microbes from mature compost to promote bacterial chemotactic motility via tricarboxylic acid cycle-regulated biochemical metabolisms for enhanced composting performance

This study aims to reveal the underlying mechanisms of mature compost addition for improving organic waste composting. Composting experiments and metagenomic analysis were conducted to elucidate the role of mature compost addition to regulate microbial metabolisms and physiological behaviors for composting amelioration. Mature compost with or without inactivation pretreatment was added to the composting of kitchen and garden wastes at 0%, 5%, 10%, 15%, and 20% (by wet weight) for comparison. Results show that mature compost promoted pyruvate metabolism, tricarboxylic acid (TCA) cycle, and oxidative phosphorylation to produce heat and energy to accelerate temperature increase for composting initiation and biological contaminant removal (>78%) for pasteurization. Energy requirement drives bacterial chemotactic motility towards nutrient-rich regions to sustain organic biodegradation. Nevertheless, when NADH formation exceeded NAD+ regeneration in oxidative phosphorylation, TCA cycle was restrained to limit continuous temperature increase and recover high intracellular NAD+/NADH ratio to secure stable oxidation reactions.

Inhibition insights of hydrothermal liquid digestate in anaerobic digestion: Impact on organics conversion and inhibitor degradation

Hydrothermal liquid digestate has been widely accepted as a substrate in anaerobic digestion (AD) for energy recovery. However, the potential negative impacts of hydrothermal liquid digestate on AD remain unclear. In this study, the organic biodegradability of hydrothermal liquid digestate produced from hydrothermal treatment (HTT) at different temperatures was analyzed, and the formation and degradation process of potential inhibitory substances were discussed. Results demonstrated that the AD lag phase of hydrothermal liquid digestate increased from 3 days at raw liquid digestate to 5-21 days. When the HTT temperature reached 220 °C, the methane yield decreased by 48%, and more than 71% of the organics in the hydrothermal liquid digestate were not utilized by AD. Biorefractory substances, such as fulvic and humic acids, accumulate in the hydrothermal liquid digestate. Potential inhibitory substances from Maillard reactions mainly affect the methanogenesis of AD. Most inhibitory substances were degraded within 7-22 days, with the degradation rate following the order of pyrroles > pyrazines > ketones > imidazoles > indoles. The AD community structure and methane conversion were partially re-established after most inhibitory substances were degraded. This study provides valuable information on eliminating the potential negative effects of hydrothermal liquid digestate on AD.

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.

Effects of digestate-encapsulated biochar on plant growth, soil microbiome and nitrogen leaching

The increasing amount of food waste and the excessive use of mineral fertilizers have caused detrimental impacts on soil, water, and air quality. Though digestate derived from food waste has been reported to partially replace fertilizer, its efficiency requires further improvement. In this study, the effects of digestate-encapsulated biochar were comprehensively investigated based on growth of an ornamental plant, soil characteristics, nutrient leaching and soil microbiome. Results showed that except for biochar, the tested fertilizers and soil additives, i.e., digestate, compost, commercial fertilizer, digestate-encapsulated biochar had positive effects on plants. Especially, the digestate-encapsulated biochar had the best effectiveness as evidenced by 9-25% increase in chlorophyll content index, fresh weight, leaf area and blossom frequency. For the effects of fertilizers or soil additives on soil characteristics and nutrient retention, the digestate-encapsulated biochar leached least N-nutrients (<8%), while the compost, digestate and mineral fertilizer leached up to 25% N-nutrients. All the treatments had minimal effects on the soil properties of pH and electrical conductivity. According to the microbial analysis, the digestate-encapsulated biochar has the comparable role with compost in improving the soil immune system against pathogen infection. The metagenomics coupling with qPCR analysis suggested that digestate-encapsulated biochar boosted the nitrification process and inhibited the denitrification process. This study provides an extensive understanding into the impacts of the digestate-encapsulated biochar on an ornamental plant and offers practical implications for the choice of sustainable fertilizers or soil additives and food-waste digestate management.

Effects of biochar supported nano zero-valent iron with different carbon/iron ratios on two-phase anaerobic digestion of food waste

The promotion effects of biochar supported nano zero-valent iron (BC/nZVI) with different carbon/iron ratios on two-phase anaerobic digestion (AD) of food waste (FW) were studied. Results suggested that when the carbon/iron ratio was 3:1 AD system showed the best performance, with the concentration of volatile fatty acids (VFAs) in acidogenic phase (AP) and the cumulative methane production in methanogenic phase (MP) increased by 31.4% and 24.8%, respectively. Metagenomic analysis demonstrated that BC/nZVI increased the relative abundance of Defluviitoga in AP, and promoted the growth of Methanothrix in MP. Metabolic pathway analysis in AP indicated that BC/nZVI mainly promoted the abundances of acetate kinase and butyrate kinase to enhance acid production. Methane metabolism pathway analysis in MP revealed that BC/nZVI increased methane production by promoting the module of M00357 and activating related enzymes. The results of this sutdy showed that BC/nZVI promoted AD of FW mainly through acetoclastic methanogenic pathway.

Digestate-derived carbonized char and activated carbon: Application perspective

The flourishment of anaerobic digestion (AD) on waste treatment emphasizes the importance of digestate valorization, which plays an essential role in determining the benefits provided by the AD process. The perception of digestate gradually shifts from waste to products to realize the concept of circular economy and maximize the benefits of digestate valorization. This review first outlined the current status of digestate valorization, focusing on thermal-chemical methods. The novel valorization methods were then summarized from the recent research, illustrating prospects for digestate valorization. Limits and perspectives are finally addressed. Methods for preparing digestate-derived activated carbon and impurity effects were elucidated. Inherent mineral content/inorganic impurity could be a niche for downstream use. High surface area and well-developed pore structure are essential for satisfying downstream use performance, but they are not the only factors. Digestate char applications other than use as an energy fuel are suggested.

Investigation of biochar amendments on odor reduction and their characteristics during food waste co-composting

The odor emission such as ammonia (NH₃) and hydrogen sulfide (H₂S) during the composting process is a severe problem that adversely affects the environment and human health. Therefore, this study aimed to (1) evaluate the variation of physicochemical characteristics during the co-composting of food waste, and sawdust mixed biochar; (2) assess the efficiency of biochar-composting combined amendment materials for reducing odor emissions and their maturity. The raw materials including food waste (FW), straw dust (SD), and biochar (BC) were prepared and homogeneously mixed with the weight ranging from 120.0 kg to 135.8 kg with five treatments, BC0 (Control), BC1 (5 % biochar), BC2 (5 % distilled water washed biochar), BC3 (10 % biochar), BC4 (20 % biochar). Adding biochar could change physicochemical properties such as temperature, moisture, and pH during composting. The results indicated applying biochar-composting covering to minimalized NH₃ and H₂S aided by higher porous structure and surface functional groups. Among trials, biochar 20 % obtained the lowest NH₃ (2 ppm) and H₂S (3 ppm) emission on day 16 and stopping their emission on day 17. The NH₃/NH₄⁺ adsorption on large specific surface areas and highly porous micro-structure of biochar lead to reduced nitrogen losses, while nitrification (NH₄⁺ ➔ NO₂^- ➔ NO₃^-) may also contribute to nitrogen retention. The H₂S concentration decreased with increasing the biochar proportion, suggesting that biochar could reduce the H₂S emission. Correlation analysis illustrated that temperature, moisture, and oxygen are the most critical factors affecting H₂S and NH₃ emissions (p <0.05). The physicochemical properties and seed germination index indicated that the compost was mature without phytotoxicity. These novelty findings illustrated that the biochar amendment is an effective solution to reduce odor emission and enhances the maturity of compost mixture, which is promising to approach in real-scale conditions and could apply in agricultural fields.

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.

Intermittent aeration to reduce gaseous emission and advance humification in food waste digestate composting: Performance and mechanisms

This study investigated the performance and mechanisms of intermittent aeration to regulate gaseous emission and humification during food waste digestate composting. In addition to continuous aeration, three intermittent aeration regimes were conducted with the on-off interval ratio at 3:1, 2:1, and 1:1 within each 30 min, respectively. Results showed that intermittent aeration regimes reduced gaseous emission and enhanced humification during composting. In particular, intermittent aeration with the on/off ratio of 1:1 was more effective to reduce organic mineralization than other regimes, which alleviated the emission of nitrous oxide and ammonia by 63.1% and 75.7% in comparison with continuous aeration, respectively. In addition, this aeration regime also enhanced the content of humic acid by 24.1%. Further analysis demonstrated that prolonging aeration-off intervals could enrich facultative bacteria (e.g. Atopobium and Clostridium) from digestate and inhibit the proliferation of several aerobic bacteria (e.g. Caldicoprobacter and Marinimicrobium) to retard organic mineralization for humification.

Effect of moisture content, aeration rate, and C/N on maturity and gaseous emissions during kitchen waste rapid composting

To determine factors affecting compost maturity and gaseous emissions during the rapid composting of kitchen waste, an orthogonal test was conducted with three factors: moisture content (MC) (55%, 60%, 65%), aeration rate (AR) (0.3,0.6 and 0.9 L·kg^-1DM·min^-1) and C/N ratio (21, 24, 27). The results showed that the importance of factors affecting compost maturity was: C/N > AR > MC, optimal conditions were: C/N of 24, AR of 0.3 L·kg^-1DM·min^-1and MC of 65%. For gaseous emissions, the sequence of essential factors affecting NH₃ emissions was: C/N > MC > AR, and the optimal parameters for NH₃ reduction were: C/N of 27, MC of 65%, and AR of L·kg^-1DM·min^-1. The important factors affecting N₂O and H₂S emissions are both: MC > C/N > AR, while their best parameters were different. The optimal parameters for N₂O emission reduction were MC of 60%, AR of 0.3 L·kg^-1DM·min^-1 and C/N of 24, while these for H₂S were MC of 55%, AR of 0.3 L·kg^-1DM·min^-1 and C/N of 21. The C/N mainly affected the compost maturity and AR further affected the maturity and pollutant gas emissions by influencing the temperature and O₂ content. Considering comprehensively the maturity and gaseous reduction, the optimal control parameters were: MC of 60%-65%, AR of L·kg^-1DM·min^-1, and C/N of 24-27.