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 (CO 2 ) to methane (CH 4 ), 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 (H 2 ) production for direct microbial reduction of CO 2 to CH 4 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 L CH4 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 13 C fractionation resulting from the methanogenic CO 2 -reduction (α CO2-CH4 ), a very-high H 2 availability for the Methanobacterium and Methanospirillum ssp. dominated microbiome was identified (extrapolated estimations of ΔG values as low as −150 kJ per mol CH 4 ). Thus, by leveraging real wastewater as a nutrient source, we provide a proof-of-concept on how renewable grid-gas with net zero CO 2 emissions can be produced efficiently and at high rates via in-situ H 2 production.