Bio-electrochemical nitrogen removal in wastewater: Coupling anodic ammonium oxidation with hydrogenotrophic denitrification in a microbial electrolysis cell

The increasing need for efficient nitrogen removal in wastewater treatment has driven interest in innovative biological and electrochemical approaches that can simultaneously address ammonium and nitrate contamination. This study explored the performance of simultaneous anodic ammonium oxidation and hydrogenotrophic denitrification in a single-chamber microbial electrolysis cell (MEC). Different operational conditions, including varying ammonia‑nitrogen/nitrate‑nitrogen ratios, were selected to evaluate the MEC's ability to promote anaerobic ammonium oxidation along with organic matter removal. The results demonstrated that the single-stage MEC achieved 57.8 % total nitrogen removal efficiency under an optimal condition where the influent contained only ammonium nitrogen at 200 mg/L with no added nitrate. Adding nitrate in the influent of the microbial electrolysis system decreased ammonium removal in all the operational conditions. Additionally, chemical oxygen demand (COD) destruction and methane production occurred uninterrupted throughout all experimental stages. Therefore, simultaneous methane production and nitrogen removal was realized in this study. The microbial electrolysis system achieved 97.3 ± 0.5 % nitrate removal, outperforming the hydrogenotrophic anaerobic digestion system. While no nitrite accumulation was observed in the systems, nitrous oxide emission occurred in all the operational conditions. A microbial community analysis showed that the anode of the MEC reactor was dominated by Pseudothauera and clostridium, which explained the denitrification performance by the MEC. This highlights the potential of bioelectrochemical systems for simultaneous ammonium and nitrate removal in a single-stage wastewater treatment process.