An Electrochemical CO ₂ Reduction Reaction-Backed Nourished Biofilm System for In-Situ Precision Control of Carbon Supply in Wastewater

Biofilm-based wastewater treatment is constrained by limited internal mass transfer, restricting O2 and substrate penetration, promoting CO2 accumulation, inducing nutrient-starving inner-layer metabolic inactivity, and compromising biofilm integrity. Microbial electrochemical systems (MESs) partially enable CO2 conversion but remain incompatible with real wastewater and fail to reactivate inner-layer biofilms. Here, we present a CO2–Reduction–Backed Nourished Biofilm (CBNB) system with a polyamide membrane–electrode–biofilm sandwich architecture that couples selective membrane-guided gas transport with Sn-catalyzed in situ CO2 reduction. The system enriches CO2 at the electrode interface to ∼2.3× bulk wastewater, enhances CO2 selectivity over O2, and delivers controlled carbon supplementation that restores inner-layer metabolic activity without inducing outer-layer overgrowth. A mechanistic insight is derived from the carbon mass distribution and biofilm kinetics models. Simulations show that ∼40% of electrochemically generated HCOOH is retained within inner layers, achieving 99.5% theoretical carbon recovery and increasing the inner-layer metabolic activity by over 100%. Techno-economic analysis indicates a 2.4-fold increase in COD removal per kWh relative to conventional biofilms and MES. By integrating in situ CO2 utilization within biofilm interiors, this system overcomes mass-transfer limitations, sustains high metabolic activity, preserves structural stability, avoids unnecessary biomass accumulation, and provides a scalable framework for energy-efficient, carbon-recovering wastewater treatment.