Exogenous inoculants enhance anaerobic digestion of food and kitchen waste: Metabolic and microbial mechanisms

Anaerobic digestion (AD) of food and kitchen wastes requires breaking the restriction of hydrolysis and improving efficiency. Hydrolytic microbiomes are often used to enhance the AD process, while the interaction of exogenous inoculants with indigenous communities is uncertain. This study investigated Lactobacillus lactis and Bacillus velezensis inoculants introduced in the strategy of biopretreatment and bioaugmentation to reveal the interaction of microbiomes and metabolic mechanisms. Results showed that both bioaugmentation (+47.8-58.7 %) and biopretreatment (+36.9-57.1 %) significantly enhanced the removal rate of crude cellulose. Whereas, introducing strategies elevated AD in different domains. Based on representative sequences and taxonomy, functional analyses indicated that bioaugmentation of Lactobacillus lactis could multiply the methanogenic pathway and promote the methane yield by 22.7-33.6 %, owing to the enhancement of syntrophic metabolism and hydrogenotrophic methanogenesis. Bioaugmentation of Bacillus velezensis promoted the last key step of the methanogenesis process that related to heterodisulfide reductase and used COB-S-S-COM as the terminal electron acceptor. While biopretreatment groups were dominated by strict acetotrophic methanogenesis, Methanosaeta was above 76 %. Biopretreatment, which added inoculant for hydrolysis, was tactically superior in reducing the digestion retention time by enhancing the pathway of beta-oxidation butyrate fermentation, shortening the lag time for 43.6-60.1 %. However, the methane yield decreased by 29.0-30.0 % due to the chemoheterotroph consumption. The combination of physicochemical pretreatment and bioaugmentation can achieve the highest methane production (+33.6 %). These results are expected to contribute to the rational application of exogenous inoculants and improve the AD efficiency of food and kitchen wastes.