Electro-driven valorization of waste activated sludge into caprylate (C₈) via syngas-ethanol synergy

Microbial chain elongation (CE) is a promising route for recovering value-added medium-chain carboxylic acids (MCCAs) from waste streams. Yet, compared with caproate (C₆), caprylate (C₈) biosynthesis remains challenging due to weak electron donor availability and insufficient understanding of C₈-forming microbial assembly. This study examined ethanol-syngas synergy for caprylate upgrading of sludge fermentation broth in microbial electrosynthesis (MES). The two-stage syngas-ethanol reactor achieved the highest caprylate production of 4388 mg COD/L with 69.20 % selectivity, 1.40-51.65 times higher than other reactors. Ethanol supplementation enhanced syngas utilization, while syngas availability in turn promoted ethanol-driven CE, synergistically strengthening carbon flux toward caprylate. Microbial community analysis revealed a functional interaction network composed of homoacetogens, electroactive bacteria, hydrolytic/fermentative bacteria, and chain-elongating bacteria, which drove caprylate synthesis by channeling carbon and electrons derived from syngas and complex organics into the CE process. The two-stage syngas-ethanol reactor exhibited enhanced electron storage capacity and reduced charge transfer resistance, which accounted for its superior electron efficiency (53.69 %). Life cycle assessment and techno-economic analysis indicated that this strategy effectively reduced carbon emissions by 33 %, while also lowering operating costs by approximately 50 % due to reduced reliance on ethanol. This study offers a novel strategy integrating wastewater treatment with high-value chemical production, contributing to the advancement of carbon neutrality goals.