Co-existing Anammox, Ammonium-Oxidizing, and Nitrite-Oxidizing Bacteria in Biocathode-Biofilms Enable Energy-Efficient Nitrogen Removal in a Bioelectrochemical Desalination Process

In this study, a nitritation-anammox biocathode microbial desalination cell (NiA_moxMDC) was studied for its potential in nitrogen and salinity removal. A maximum power density of 1.007 W/m³ and an average TDS removal of 53.9 ± 4.1% were accomplished. The average COD (chemical oxygen demand) removal in each cycle was around 72.1 ± 1.6%, and the corresponding average CE (Coulombic efficiency) of the NiA_moxMDC was 25.6 ± 6.1%. The net energy produced by the NiA_moxMDC at an aeration rate of 3 mL/min was 0.031 kWh/m³. The energy consumption by the NiA_moxMDC was between 0.12 and 0.58 kWh/kg-N, which is significantly lower than other conventional and bioelectrochemical nitrogen removal processes. At an aeration rate of 3 mL/min or a dissolved oxygen concentration of 0.8 mg-O₂/L, 97.7% of the NH₄⁺-N removal and 74.7% of the total nitrogen removal were achieved. Microbial community analysis of the nitritation-anammox cathode showed a relative abundance species of 29.3% belonging to the order of Ignavibacteriales. The second most abundant phylum was affiliated to the Planctomycetes phylum (20.5%) belonging to the order of Candidatus Brocadiales. Aerobic ammonium-oxidizing bacteria (AOBs), Nitrospira multiformis, Nitrosomonas europaea genome sequence, and Nitrosomonas sp. with relative abundances of 0.1%, 0.05%, and 1.2%, respectively, were found in the biocathode chamber. The co-existence of AOBs and anammox bacteria in the NiA_moxMDC cathode was confirmed. Thus, bioelectrochemical nitritation-anammox approach has been shown to be a synergistic solution for resource-efficient wastewater treatment.