Synergistic Regulation of Homoacetogenesis and Electrochemical Electron Supply Enhances CO2-Assisted Caproate Production from Waste Activated Sludge

Abstract Municipal sewage treatment has become an energy-intensive sector associated with substantial CO2 emissions and waste activated sludge (WAS) generation. Chain elongation (CE), as a promising carbon resource recovery route, enables the conversion of sludge-derived short-chain fatty acids into medium-chain fatty acids (MCFAs). This study developed a microbial electrolysis system (MES) to couple CO2 utilization with caproate production from prefermented WAS, while chloroform and 2-bromoethanesulfonic acid were applied to regulate methanogenesis and homoacetogenesis. CO2 promoted caproate synthesis only under closed-circuit conditions, indicating that its positive role depended on electrochemical electron supply rather than simple substrate addition. The closed-circuit MES with chloroform and CO2 addition achieved the highest caproate concentration, yield, and selectivity, reaching 3092.9 ± 73.1 mg COD/L, 0.66 g COD/g COD, and 58.06%. Compared with 2-bromoethanesulfonic acid, chloroform more effectively inhibited homoacetogens, thereby reducing the diversion of CO2 and H2 toward acetate accumulation and redirecting carbon and reducing equivalents toward caproate formation. Electrochemical analyses confirmed enhanced cathodic biofilm activity, as indicated by the lowest charge transfer resistance of 2.401 Ω. Microbial and functional pathway analyses revealed the enrichment of electroactive and chain-elongating bacteria and the high abundance of key genes involved in the Wood-Ljungdahl pathway, acetyl-CoA generation, fatty acid biosynthesis, and reverse β-oxidation. These findings provide a low-carbon strategy for synergistic CO2 utilization and high-value WAS upgrading.