An electrochemically integrated bioreactor for a sustainable production of grid-gas using wastewater streams

Microbial electrochemical technologies have often been investigated to perform the conversion of carbon dioxide (CO2) to methane (CH4), yet practical implementation remains constrained by low achievable current densities, inefficient hydrogen transfer, and limited long-term stability, particularly under real wastewater conditions. Here, we present an electrochemically integrated bioreactor system (EiBS) that couples a zero-gap proton exchange membrane (PEM) electrolyzer with a packed bed biological reactor operated at 34 °C, enabling in-situ hydrogen (H2) production for direct microbial reduction of CO2 to CH4 without external gas handling. Using a sulfide-based cathode catalyst and a non-compressible anode architecture, the system achieved its maximal methane production rates per electrode area of 1712 LCH4 m-2 d-1 at current densities up to 85 mA cm-2, which is a two-fold improvement compared to similar systems in the literature metrics reported for such directly coupled electrochemical-microbiological assemblies. Herein, the corresponding electron recovery and energy efficiency were 81% and 37%, respectively, while energy efficiencies of 45% were recorded for operation at 70 mA cm-2. The electrolyzer operated stably over 160 days using sewage-derived growth medium before demanding an electrode replacement, while the fully integrated EiBS ran continuously for more than 380 days demonstrating exceptional robustness under operation with a nutrient-rich real wastewater stream. Through the determination of the thermodynamic-dependent 13C fractionation resulting from the methanogenic CO2-reduction (αCO2-CH4), a very-high H2 availability for the Methanobacterium and Methanospirillum ssp. dominated microbiome was identified (extrapolated estimations of ΔG values as low as -150 kJ per mol CH4). Thus, by leveraging real wastewater as a nutrient source, we provide a proof-of-concept on how renewable grid-gas with net zero CO2 emissions can be produced efficiently and at high rates via in-situ H2 production.