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

Microbial electrolysis cells (MECs) offer a novel alternative to conventional biological ammonium (NH 4 + ) removal by using electrodes rather than oxygen as electron acceptors. However, NH 4 + oxidation at bioanodes can result in substantial nitrous oxide (N 2 O) emissions─a potent greenhouse gas with significant climate impact. We investigated the potential of hydrogenotrophic denitrification within the anodic compartment to mitigate N 2 O emissions during NH 4 + removal. In situ hydrogen (H 2 ) addition effectively reduced N 2 O 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 N 2 O reduction, which was strongly dependent on H 2 availability and linked to denitrification activities. Integrating hydrogenotrophic denitrification with NH 4 + oxidation in MECs presents a promising approach for sustainable nitrogen management with near-zero N 2 O emission.