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.

Effects of calcium magnesium phosphate fertilizer, biochar and spent mushroom substrate on compost maturity and gaseous emissions during pig manure composting.

This study used a laboratory-scale system to investigate the effects of calcium magnesium phosphate fertilizer (CaMgP), biochar, and spent mushroom substrate (SMS) on compost maturity and gasous emissions during pig manure composting. The results showed that the addition of CaMgP, Biochar or SMS had no negative effect on the quality and maturity of compost, and all three additives could reduce the emissions of ammonia (NH3), hydrogen sulfide (H2S), dimethyl sulfide (Me2S) and dimethyl disulfide (Me2SS). Among them, the effect of adding CaMgP on NH3 emission reduction was the most obvious, reduced 42.90%. The emission reduction of CaMgP to H2S was similar to that of SMS, which decreased by 34.91% and 32.88% respectively. The emission reduction effects of the three additives on Me2S and Me2SS were obvious, all of which were over 50%. However, only adding SMS reduced the N2O emission by 37.08%.

Techno-economic assessment of industrial food waste composting facility: Evaluating bulking agents, processing strategies, and market dynamics.

Techno-economic assessment (TEA) of a valorization of bulking agent (BA) ratios on the food waste compost value chain is made to assess economic feasibility. TEA was performed with two plans (Plan A: existing composting facilities; Plan B: new composting facilities) and each plan was under four scenarios. The BA (i.e. corn stalks, garden waste, and watermelon seedlings) ratio of 5 % (S1), 10 % (S2), 20 % (S3), and garden waste with a ratio of 20 % (S4). Results indicate that S2, with a net present value (NPV) of 128.9 million, represents Plan A's most economically viable scenario. Although the total operating costs of S4 were 18.9 %-23.5 % higher, 25.6 %-42.2 % higher total revenue made S4 have an NPV of 92.9 million, making it the most viable scenario in Plan B. All scenarios show positive NPV within a ± 20 % fluctuation range. Organic fertilizer price, government subsidies, and processing capacity were the key factors influencing NPV.

Effects of digestion duration on energy efficiency, compost quality, and carbon flow during solid state anaerobic digestion and composting hybrid process.

This study investigated the effects of anaerobic digestion duration on methane yield, net energy production, and humification of compost during solid state anaerobic digestion (SSAD) and composting hybrid process for food waste treatment. Carbon flow and balance were used to evaluate organic methanation and humification inclination of carbon in the whole SSAD and aerobic composting system. Results showed that SSAD for 15 (AD-15) and 21 days (AD-21) could increase net energy production and degraded organic matter contained in the mixtures to achieve high biological stability. The cumulative net energy production between the AD-15 and AD-21 treatments was not significantly different, which was 8.3% higher than that in SSAD for 30 days (AD-30). Furthermore, digestate (AD-15 and AD-21) composting for 3 days reached maturity and absence of phytotoxic substances. Carbon fixed into humus of the AD-21 treatment (11.6%) was not significantly different from that of AD-15 (12.0%). However, the total amount of carbon fixed into compost in AD-15 was 6.6% higher than that in AD-21. Moreover, the CO2 -C loss of the AD-15 treatment (22.9%) was slightly higher than that of AD-21 (20.6%). Thus, AD-21 treatment achieved the most effective use of carbon during SSAD and composting hybrid process for food waste treatment. These results could provide valuable insights for the effective management of food waste in practice.

Bacterial dynamics for gaseous emission and humification during bio-augmented composting of kitchen waste with lime addition for acidity regulation.

This study investigated the impacts of lime addition and further microbial inoculum on gaseous emission and humification during kitchen waste composting. High-throughput sequencing was integrated with Linear Discriminant Analysis Effect Size (LEfSe) and Functional Annotation of Prokaryotic Taxa (FAPROTAX) to decipher bacterial dynamics in response to different additives. Results showed that lime addition enriched bacteria, such as Taibaiella and Sphingobacterium as biomarkers, to strengthen organic biodegradation toward humification. Furthermore, lime addition facilitated the proliferation of thermophilic bacteria (e.g. Bacillus and Symbiobacterium) for aerobic chemoheterotrophy, leading to enhanced organic decomposition to trigger notable gaseous emission. Such emission profile was further exacerbated by microbial inoculum to lime-regulated condition given the rapid enrichment of bacteria (e.g. Caldicoprobacter and Pusillimonas as biomarkers) for fermentation and denitrification. In addition, microbial inoculum slightly hindered humus formation by narrowing the relative abundance of bacteria for humification. Results from this study show that microbial inoculum to feedstock should be carefully regulated to accelerate composting and avoid excessive gaseous emission.

Co-composting of kitchen waste with agriculture and forestry residues and characteristics of compost with different particle size: An industrial scale case study.

Since the implementation of domestic waste classification in China, the kitchen waste production has increased rapidly. The unique physical and chemical properties of kitchen waste make it impossible for direct composting for composting alone. This study investigated the co-composting of kitchen waste with agriculture and forest residues at an industrial scale at the Nangong Composting Plant (Located in Beijing). Cornstalks, garden waste, and watermelon seedlings were composted with kitchen waste, with the added agriculture and forestry residues comprising 5%, 10% and 20% of the weight. Industrial composting was performed 30 days at a scale of 165-180 tone. The mixed compost products were screened to different particle sizes, and the maturity, humification, and calorific value were analyzed. The kitchen waste mixed with 20% agricultural complementary materials reached hyperthermophilic temperature (82 °C), had reduced moisture content (45%), and resulted in better composting performance at an industrial scale. By adding 20% complementary materials to kitchen waste produced mature compost with a higher germination index (GI) (91%) by adjusting the pH, electrical conductivity (EC), carbon to nitrogen ratio (C/N), and moisture content. The compost in the 5% and 10% complementary materials treatments did not fully mature and had a GI of<10%, influenced by the higher EC and NH4+-N content. The property of final compost with different particle size vary greatly. The small particle size compost (≤45 mm) had higher uniformity, maturity, and humification degree, and it was suitable to use as a fertilizer; the larger particle size (>45 mm) had more material with lower calorific value (8000-10,000 kJ·kg-1), and could be used as refuse-derived fuel. To make better use of kitchen waste compost, 45-mm particle size screening is suggested at an industrial-scale composting plant. These results support industrial-scale kitchen waste composting in China.

Anaerobic digestion of agricultural wastes from liquid to solid state: Performance and environ-economic comparison.

Performance and environ-economic impacts were compared for anaerobic digestion (AD) of dairy manure and cucumber residues from liquid to solid state. Environ-economic evaluation of the overall AD process at different total solids (TS) was performed with the estimated treatment capacity of 9600 tons/year and service life of 20 years. Results showed that TS increase from 6% to 22% enhanced both cumulative and volumetric methane (CH4) production. Further TS increase to 25%, however, reduced CH4 yield. Environ-economic assessment indicated that TS increase enhanced volumetric waste treatment capacity and thus AD environmental footprints. Environmental credits from digestate and biogas utilization could compensate the adverse environmental impacts of other processes in AD plants. Furthermore, biogas and nutrients in digestate determined AD net-present value. As a result, solid state AD was more profitable with higher CH4 yield and more nutrients in both biosolids and digested effluent of digestate than its liquid and hemi-solid counterparts.

Bacterial dynamics and functions for gaseous emissions and humification in response to aeration intensities during kitchen waste composting.

This study revealed bacteria dynamics and functions for gaseous emissions and humification during kitchen waste composting under different aeration intensities (i.e. 0.24, 0.36, and 0.48 L kg-1 DM min-1) using high-throughput sequencing with Functional Annotation of Prokaryotic Taxa. Results show that aeration increase restrained bacteria (e.g. Lactobacillus and Acinetobacter) for fermentation, nitrate reduction, and sulphur/sulphate respiration, but enriched thermophilic bacteria (e.g. Thermomonospora and Thermobifida) for aerobic chemohetertrophy, xylanolysis, cellulolysis, and methylotrophy. Thus, high aeration intensity (i.e. above 0.36 L kg-1 DM min-1) effectively alleviated the emission of greenhouse gases and hydrogen sulphide, and meanwhile facilitated the production of humus precursors and ammonia. Nevertheless, humification was limited by the conclusion of composting under high aeration conditions due to the consumption of humus precursors for bacterial activity. Thus, aeration intensity should be regulated at different stages indicated by temperature to balance gaseous emissions and humification during kitchen waste composting.

Regulating bacterial dynamics by lime addition to enhance kitchen waste composting.

This study examined bacterial dynamics in response to lime addition to enhance kitchen waste composting using modular network analysis. Bacterial communities could be separated into three meta-modules corresponding to the mesophilic, thermophilic, and mature stage of composting. Lime addition at 1% (wet weight) suppressed acidogens and denitrifiers (e.g. Lactobacillus and Acinetobacter) at the mesophilic stage to reduce greenhouse gas emissions. The matrix pH and temperature were also increased by lime addition via hydrogen reaction to favor bacterial growth and activity. Thus, thermophilic bacteria (e.g. Thermoactinomycetaceae and Planifilum) were enriched with lime addition to facilitate lignocellulose biodegradation for humus formation at the thermophilic stage. Further lime addition to 1.5% reduced ammonia emission at the thermophilic stage via chemical fixation. Moreover, lime inhibited denitrifiers but proliferated nitrifiers at the mature stage to decrease nitrous oxide emission and enhance nitrate content, respectively. As such, lime addition improved both biotic and abiotic composting performance.

Biochar amendment to advance contaminant removal in anaerobic digestion of organic solid wastes: A review.

Anaerobic digestion (AD) has been widely applied to convert organic solid wastes into biogas, a renewable energy, and digestate, a bio-fertilizer, to sustain waste management. Nevertheless, several vexing contaminants in OSWs restrict digestate application in agriculture. Biochar has been evidenced to effectively improve AD by promoting organic biodegradation and alleviating the accumulation of inhibitory substances (e.g. ammonia and volatile fatty acids). Furthermore, biochar could advance contaminant removal in AD given its highly porous, conductive and alkaline features. Thus, this review aims to highlight the role of biochar amendment to advance contaminant removal in AD of OSWs. Key contaminants, such as antibiotics, heavy metals, microplastics, polycyclic aromatic hydrocarbons, furfural and 5-hydroxy methyl furfural (5-HMF) that ubiquitously present in OSWs were demonstrated. The underlying mechanisms of biochar to amend the removal of these contaminants by AD were discussed. Furthermore, future perspectives to the development of biochar-assisted AD for OSWs treatment were provided.