Developing a Novel Electrocatalytic Anammox Process to Drive High-Efficiency Nitrogen Removal under Ultralow Nitrite Conditions in an Upflow Electrochemical Anaerobic Membrane Bioreactor

Anaerobic ammonium oxidation (anammox) is a promising enormous energy- and resource-saving technology for nitrogen removal. Conventional anammox relies on nitrite (NO₂^––N) for the oxidation of ammonium (NH₄⁺–N) to dinitrogen gas (N₂). However, the current partial nitrification process for NO₂^––N production exhibits inherent instability and substantial energy demands, restricting its practical application. To address such limitations, an upflow dual-chamber electrocatalysis-driven anammox membrane bioreactor (UEA-MBR), by employing ruthenium–iridium-coated titanium alloy electrodes, was constructed to minimize NO₂^––N dependence while achieving high nitrogen elimination. The newly developed UEA-MBR showed excellent performance in nitrogen removal at a NO₂^––N concentration (NH₄⁺–N/NO₂^––N = 1:0.4) even much lower than the theoretical. This UEA-MBR achieved a remarkable total nitrogen removal efficiency of 89.7% and maintained 60.8% even without NO₂^––N supply. Besides the conventional anammox, the electrocatalytic anammox process by directly oxidizing NH₄⁺–N to N₂ while delivering electrons was confirmed as well in the bioanode. Moreover, the generated electrons, after being transferred into the cathode, were utilized to further drive the anammox-generated NO₃^––N to NO₂^––N via partial denitrification, which provided the self-produced NO₂^––N for the anodic conventional anammox process via recirculation of effluent and ensured the high-nitrogen removal. This electrocatalytic system enriched the key functional genus electroactive Candidatus_Brocadia (23.5%) and norank_o_Fimbriimonadales (13.8%) and upregulated the genes of hzs, nirS, and nirB, establishing a robust nitrogen removal bioreactor. This integrated UEA-MBR shows great potential in breaking the barrier of conventional NO₂^––N-dependent anammox, offering a novel strategy toward sustainable and low-carbon wastewater treatment.