Biogas slurry reflux enhances the organic loading rate of high-solid anaerobic digestion of kitchen waste by alleviating fatty acids accumulation

Abstract

The inhibition of fatty acids metabolism is a key limiting factor for organic loading rate (OLR) enhancement during the high-solid anaerobic digestion (HSAD) of kitchen waste (KW). Biogas slurry reflux (BSR) has been demonstrated to enhance system stability and biogas production. However, the mechanism through which BSR modulates fatty acids metabolism, thereby increasing OLR, remains unclear. Therefore, by conducting thermodynamic evaluation and multi-omics analyses, this study explored the feasibility of BSR for enhancing OLR in HSAD and its regulatory role in fatty acids metabolism throughout a semi-continuous long-term (363-day) pilot experiment. In the first OLR increase stage, the system operated stably when the OLR was below 6.0 g·VS/L·d, and the average daily methane yield exceeded 400 mL/g VS. Moreover, functional genes related to fatty acids degradation were enriched. However, fatty acids metabolism was inhibited at an OLR of 6.0 g·VS/L·d, resulting in a noticeable accumulation of fatty acids, leading the system to the brink of collapse. Notably, the implementation of BSR promoted an OLR increase, reaching levels of 7.5 g·VS/L·d. Thermodynamic analyses demonstrated that BSR promoted the thermodynamic degradation of propionate and butyrate under equivalent OLR conditions. Particularly, BSR enhanced the enrichment of Syntrophaceticus and Syntrophomonas, thereby promoting syntrophic acetate oxidizing-hydrogenotrophic methanogenesis. Metagenome and metabolome analyses suggested that BSR promoted medium and long-chain fatty acids degradation and methanogenesis under high OLR (OLR exceeding 6.0 g·VS/L·d). These findings provide systematic insights into the regulation of fatty acids biochemical metabolism through BSR, thus promoting an increase in OLR within plug-flow HSAD.