Oxygen-to-ammonium-nitrogen ratio as an indicator for oxygen supply management in microoxic bioanodic ammonium oxidation

Abstract

Microbial electrolysis cells (MECs) have been proven effective for oxidizing ammonium (NH4+), where the anode acts as an electron acceptor, reducing the energy input by substituting oxygen (O2). However, O2 has been proved to be essential for achieving high removal rates MECs. Thus, precise control of oxygen supply is crucial for optimizing treatment performance and minimizing energy consumption. Unlike previous studies focusing on dissolved oxygen (DO) levels, this study introduces the O2/NH4+-N ratio as a novel control parameter for balancing oxidation rates and the selectivity of NH4+ oxidation towards dinitrogen gas (N2) under limited oxygen condition. Our results demonstrated that the O2/NH4+-N ratio is a more relevant oxygen supply indicator compared to DO level. Oxygen served as a more favorable electron acceptor than the electrode, increasing NH4+ oxidation rates but also resulting in more oxidized products such as nitrate (NO3−). Additionally, nitrous oxide (N2O) and N2 production were higher with the electrode as the electron acceptor compared to oxygen alone. An O2/NH4+-N ratio of 0.5 was found to be optimal, achieving a balance between product selectivity for N2 (51.4 % ± 4.5 %) and oxidation rates (344.6 ± 14.7 mg-N/L*d), with the columbic efficiency of 30.7 % ± 2.0 %. Microbial community analysis revealed that nitrifiers and denitrifiers were the primary bacteria involved, with oxygen promoting the growth of nitrite-oxidizing bacteria, thus facilitating complete NH4+ oxidation to NO3−. Our study provides new insights and guidelines on the appropriate oxygen dosage, offering strategies into optimizing operational conditions for NH4+ removal using MECs.