Boosting medium-chain fatty acids production from waste activated sludge via cation exchange resin: Role of endogenous redox mediators and metabolic shift

The production of medium-chain fatty acids (MCFAs) from waste activated sludge (WAS) is hindered by the recalcitrant extracellular polymeric substances (EPS) matrix and inefficient carbon chain elongation (CE). Here, we propose a cation exchange resin (CER)-based strategy to enhance MCFAs production. CER pretreatment shortened the fermentation time by 4 days and increased the MCFAs yield to 12.2 g COD/L, with caprylate (C8) dominating (70.4 % of total MCFAs). Mechanistically, CER destabilized EPS and lysed cells by removing multivalent metal ions, releasing biodegradable organics and endogenous redox mediators (e.g., humic acids). These mediators enhanced hydrolysis, acidogenesis, and CE efficiency. Microbial community analysis revealed enrichment of chain-elongating bacteria (e.g., Clostridium), while metabolic pathway analysis showed CER suppressed competing pathways (e.g., excessive ethanol oxidation) and optimized CE process and site. This study provides a sustainable CER-based approach for WAS valorization, bridging the gap between sludge management and circular bioeconomy.