Mechanistic study of the effect of leachate recirculation ratios on the carboxylic acid productions during a two-phase food waste anaerobic digestion

Abstract

Food waste (FW) bioconversion to valuable carboxylic acids in leach bed reactors (LBRs) has received increasing attention over the years. Wherein, leachate recirculation is a specific factor when operating LBR to facilitate solid waste degradation. However, its impact and mechanism on the regulation of soluble metabolic products (SMPs) production remain underexplored. Aiming to comprehend the effect of leach recirculation ratio selection on manipulating FW acidogenesis, this study investigated the changes in SMPs production and the corresponding mechanisms in LBRs under different leachate recirculation ratios (LRRs), i.e., 0%, 25%, 50% and 75%. The results showed that the total SMP production increased by approximately 2.3–4.2 folds concurrently with the increasing LRR when compared to that without leachate recirculation. Besides, the SMPs distribution was also ratio-dependent. Mechanistic investigation revealed that the application of leachate recirculation not only alleviated the microbial loss but also improved the abundance of functional gene encodings on fatty acids biosynthesis by 38.5–65.4%, enhancing the SMP production. Moreover, the leachate recirculation at different levels significantly changed the key bacteria responsible for SMPs production. By increasing the LRR, the main functioning acid producer mainly altered from Lactobacillus to Clostridium, Bifidobacterium, Megasphaera and Caproicoproducens. Consequently, the dominated SMP was changed from lactate to butyrate and/or medium-chain fatty acids. Together, this study offers a mechanistic insight into the role of the LRR on FW acidogenesis and emphasize the significance of leachate recirculation on enhancing and manipulating SMP production.