Optimizing Resource Utilization Of Food Waste And Digestate Through Multi-substrate Co-disposal And Its Microbial Mechanisms

Anaerobic digestion（AD）,aerobic composting,and land use are the dominant technologies for the resource utilization of food waste（FW）and its digestate（FWD）.The easy spoilage and acidification issues of FW make it hard to maintain the stability of AD system.Meanwhile,the disadvantages such as low carbon-to-nitrogen ratio（C/N）,high moisture content,and odor in FWD lead to low resource efficiency in aerobic composting and land use.Multi-substrate co-disposal is a promising approach for enhancing the biotransformation and land use efficiency of FW and its digestate,with microorganisms playing a critical role in this process.However,the synergistic resource utilization strategy for FW and its digestate with other organic solid waste is still under investigation,and the microbial mechanisms underlying this process remain unclear.In this study,multi-substrate co-disposal was adopted to enhance the AD performance of FW,to increase the humification efficiency and product quality of FWD composting,and to improve the application effectiveness of FWD compost on agricultural soil quality and crop yield.By using amplicon and metagenomic sequencing analysis approaches,this study elucidated the driving mechanisms of microbial communities and related metabolic traits that promote the resource utilization of FW and its digestate.The main results of this study are as follows:1.A multi-substrate Aco D strategy for FW was proposed,and the microbial community characteristics were analyzed.Results showed that the low organic loading Aco D of restaurant food waste（RFW）,household food waste（HFW）and rice straw（RS）had a synergistic methanogenic effect,achieving a maximum synergistic index of 1.28 at a volatile solids（VS）ratio of 0.45:0.45:0.1.The maximum methane yield of 452.9±1.0 m L/g-VS was obtained when the VS ratio was 0.81:0.09:0.10.Physicochemical analysis indicated that the Aco D of RFW,HFW and RS slowed down the hydrolysis rate and promoted the release of soluble organic matter in the later stage of AD process,thus providing richer substrates for methane production than mono-digestion.Community structure analysis indicated that the increase in diversity of bacteria associated with organic matter hydrolysis,acidogenesis,and acetogenesis,as well as the increase in the relative abundance of Methanosaeta,were key factors promoting methane production in Aco D of multi-substrate.2.The functional traits of the core microbiota in multi-substrate Aco D of FW under high organic loading conditions were revealed.Results showed that the maximum synergistic index of 1.28 was obtained at the initial stage of high organic loading Aco D when the VS ratio of RFW,HFW and RS was 0.45:0.45:0.1.Physicochemical analysis showed that Aco D diminished the accumulation of volatile fatty acids（VFAs）,mainly acetate and butyrate,in the early stage.Ecological niche analyses showed that Aco D altered the succession of key populations and reduced the relative abundance of bacteria associated with hydrolysis and acidogenesis,while enriching acetogenic bacteria and acetate-utilizing methanogens.Genome-reassembly and functional traits analyses revealed that the core bacterial species were T78 sp.and UBA4923 sp.,which had the greatest potential to metabolize starch,polysaccharides,acetate,propionate and butyrate.In this line,Aco D alleviated the acidification by reducing the metabolic potential of core microbial populations involved in carbohydrate hydrolysis and VFAs production.Besides,the synergistic relationship between syntrophic bacteria and Methanothrix sp.,and the enhanced metabolic potential（10.5%～59.1%）associated with the acetotrophic and CO₂reduction methanogenic pathways dominated by Methanothrix sp.,provided a further elucidation of the microbial mechanisms underlying the promotion of methane production.3.The strategy of co-composting of FWD with agricultural solid waste and microbial driving mechanism was explored.Results showed that,when FWD and RS were co-composted at a C/N of 15,the maximum amount of FWD treatment was achieved and the corresponding seed germination index（96.9±2.5%）,organic matter（42.3±0.0%）and total nutrient（8.3±0.1%）were better than the national and international composting standards.The addition of spent mushroom substrate（SMS）at the optimized C/N accelerated temperature rise and humification,improved the levels of seed germination index,organic matter and nutrient,whilst reduced the salinity and heavy metal content in the compost,resulting in optimal compost quality.Microbial community analysis showed that SMS promoted the enrichment of various carbohydrate-and amino acid-degrading bacteria（e.g.Thermobifida fusca,Thermobacillus composti,Novibacillus thermophilus）,cellulose-degrading and organic nitrogen-mineralising fungi（e.g.Thermothelomyces thermophilus,Thermothielavioides terrestris）in initial and thermophilic stages,which accelerated the succession of microbial population towards humification direction.Redundancy analysis showed that environmental factors including temperature,humic acid,degree of polymerization,EC and ammonia nitrogen had significant influences on the evolution of both bacterial and fungal populations.In addition,correlation analysis between functional genes and environmental factors revealed that SMS facilitated compost humification via promoting the functional capabilities of carbohydrates,amino acids,lipid metabolism,terpenoids,polyketides and xenobiotic degradation and metabolism during composting.4.The potential for synergistic land use of FWD compost and steel slag-based silicon fertilizer was studied,and the response of soil microbial community and functions was elucidated.Results showed that,when FWD compost was applied alone,a 5%（w/w）application rate was the most effective in promoting the growth of Brassica chinensis L.in facility soils,with fresh weight,plant height,and leaf number improving by 120.1%,53.4%,and 71.4%,respectively.Co-application of FWD compost and steel slag-based silicon fertilizer further increased the nutrients in plants and soil and reduced the risk of soil salinization.The highest increase in fresh weight（348.0%）was observed with co-application of 5%FWD compost and 0.8%silicon fertilizer.Correlation analysis indicated that the fertilization promoted the growth and nutrient uptake of Brassica chinensis L.by increasing the effective nutrient and organic matter content of the soil.Microbial community structure analysis showed that application of FWD compost enriched soil bacteria and fungi related to organic matter hydrolysis,disease suppression,and nitrogen cycling,such as Cellvibrio,Lysobacter,Sphingomonas,Thermothielavioides terrestris,Thermothelomyces thermophilus and Fusarium oxysporum,while reducing populations of various animal,plant,and human pathogens,such as Xanthomona,Stenotrophomonas,Colletotrichum higginsianum and Eremothecium gossypii.Co-application of FWD compost and steel slag-based silicon fertilizer further optimized microbial community composition and diversity.Redundancy analysis showed that soil available potassium and organic matter exerted the greatest impact on bacterial and fungal community structure,respectively.Metabolic functional analysis showed that the application of FWD compost facilitated soil carbohydrate hydrolysis,central carbon metabolism,organic nitrogen conversion and ammonium generation pathways.In addition,compared to the application of FWD compost alone,co-application of FWD compost and steel slag-based silicon fertilizer,promoted small molecular organic acids,nitrogen fixation,and assimilatory nitrate reduction pathways,further facilitating soil nutrient conversion.In summary,this study explored the technical strategies of synergistic disposal of multiple-substrate to improve methane production efficiency in AD of FW,to optimize the humification and product quality of aerobic composting of FWD,and to promote the high-value agricultural application of FWD compost.Using amplicon and metagenomic methods,the microbial driving mechanisms involved in the resource utilization process were revealed.These findings provide the technical and theoretical basis for promoting the AD efficiency of FW,as well as the resource recovery ability of FWD in China.