Net-negative carbon valorization in wastewater treatment via sequential thermochemical-electrochemical coupling

Wastewater treatment plants simultaneously emit greenhouse gases and rely on external carbon sources, presenting both a challenge and an opportunity for carbon circularity. We develop a comprehensive life-cycle assessment of 32 Waste-to-Chemical pathways that integrate thermochemical and electrochemical conversions in sequential or parallel architectures to convert CH₄ and CO₂ into reusable carbon sources for in-plant utilization. Sequential thermochemical-electrochemical coupling consistently delivers the strongest climate benefit, with formate identified as the optimal product, reducing emissions up to ~35% reduction relative to direct-emission baselines. We further validate this pathway experimentally using commercially available Pd/Al₂O₃ catalysts for CH₄ thermochemical oxidation and Bi₂O₃ catalysts for CO₂ electroreduction in a porous solid electrolyte reactor. Residual heat generated during CH₄ oxidation enhances downstream CO₂ electroreduction, enabling 97.2 ± 1.2% Faradaic efficiency toward separation-free formate at 150 mA cm^-2 and 45 °C. These results validate that commercially relevant catalysts and thermally coupled operation can deliver experimentally validated performance aligned with system-level projections. This framework establishes a scalable strategy for embedding carbon circularity into wastewater infrastructure and transforming wastewater treatment plants into distributed platforms for low-carbon chemical production.