Amino acids as in-situ electron donors drive medium chain fatty acids production from sludge acidogenic fermentation liquid by electro-fermentation enhancement

The production of medium chain fatty acids (MCFAs) from waste biomass by chain elongation often requires the supplementation of exogenous chemical electron donor (ED), to support the chain elongator. This study explored the feasibility of producing MCFAs using amino acids in acidogenic fermentation liquid from protein-rich sludge hydrolysate, and proposed a two-stage electro-fermentation process to realize MCFAs production without exogenous chemical ED. Stage I aimed to convert organic substrates into short chain fatty acids (SCFAs) through anodic alkaline acidogenic fermentation. Stage II utilized residual amino acids as in-situ ED in acidogenic fermentation liquid to generate MCFAs in cathodic chamber under open circuit (OC) and 0.9 V conditions. Results indicated that acetate (6.9 g/L) and butyrate (4.5 g/L) were accumulated as the main products in stage I, and 2.7 g/L of amino acids remained in acidogenic fermentation liquid. In stage II, the remained amino acids were consumed as in-situ ED to drive the conversion of acetate and butyrate into caproate, and 1.1 g/L and 3.01 g/L caproate were respectively determined at OC and 0.9 V conditions. Microbial community and function prediction mechanisms revealed the presence of abundant chain elongator (Clostridium_sensu_stricto_12) and electroactive bacteria (Desulfovibrio) in cathodic biofilm, as well as the organic degrader (Pseudomonas) in plankton, these microbes synergistically converted SCFAs and amino acids into MCFAs. This study for the first time explored the feasibility of using amino acids as alternative chemical ED to drive chain elongation, and provides a novel technical solution for alleviating dependence on exogenous chemical ED in the production of MCFAs from waste biomass.