Toward Nitrous Oxide-Free Ammonium Treatment: Integrating Hydrogenotrophic Denitrification in Anoxic Bioanodes

Microbial electrolysis cells (MECs) offer a novel alternative to conventional biological ammonium (NH4+) removal by using electrodes rather than oxygen as electron acceptors. However, NH4+ oxidation at bioanodes can result in substantial nitrous oxide (N2O) emissions─a potent greenhouse gas with significant climate impact. We investigated the potential of hydrogenotrophic denitrification within the anodic compartment to mitigate N2O emissions during NH4+ removal. In situ hydrogen (H2) addition effectively reduced N2O concentrations to below 0.01 mg of N/L, without nitrate or nitrite accumulation. Batch experiments demonstrated that the suspended biomass exhibited a high capacity for N2O reduction, which was strongly dependent on H2 availability and linked to denitrification activities. Integrating hydrogenotrophic denitrification with NH4+ oxidation in MECs presents a promising approach for sustainable nitrogen management with near-zero N2O emission.