Coupling microbial electrosynthesis and chain elongation for simultaneous biomethane upgrading and carboxylate recovery: A tunable carbon valorization strategy.

Anaerobic digestion of sewage sludge is a cornerstone technology for energy recovery, yet the CO2-rich biogas stream constrains deep carbon recovery. Here, we developed a coupled microbial electrosynthesis (MES)-chain elongation platform as a tunable post-treatment strategy to simultaneously upgrade biomethane (bio-CH4) and valorize CO2 into medium-chain carboxylates (MCCAs). Using defined synthetic matrices to systematically isolate metabolic drivers, operation revealed a kinetic transition from electron-supply limitation to mass-transfer limitation (kLa) beyond 3.0 V. An operating window at 3.0 V with 260 mmol L-1 ethanol balanced these constraints, delivering 95.0 % CH4 and 36.3 mmol L-1 MCCAs. Thermodynamic analysis and 16S rRNA profiling support a push and pull metabolic framework, where cathodic H2 pushes homoacetogenesis while ethanol-driven chain elongation pulls carbon toward MCCAs. Techno-economic analysis suggested a positive operating margin (benefit-to-cost ratio = 1.85), with sensitivity analysis confirming that the high value of MCCAs provides an economic buffer against downstream separation costs. Life cycle assessment further indicated that this operating window minimized cradle-to-gate impacts when bio-CH4 upgrading and MCCAs valorization were assessed jointly. These findings highlight the potential of this electro-biological strategy to advance water resource recovery facility carbon management by offering a tunable approach to balance energy recovery and chemical production.