Anodic ammonium oxidation coupling to anammox without external nitrite addition enhanced by carbon granule and...

Microbial electrolytic cell (MEC) can achieve autotrophic nitrogen removal, which is a promising means for treating low C/N ammonium-rich wastewater. However, it still suffers from insufficient ammonium oxidation rate and low total nitrogen (TN) removal. This study optimized the autotrophic nitrogen removal performance of a single-chamber MEC reactor using carbon granule (CG) materials as the bioanode, combined with supercapacitor (SC) as power supply. In the anoxic anodic system with only ammonium as the electron donor, the CG-anode/SC-powered system achieved removal loads of 35.31 ± 4.89 and 27.44 ± 4.80 mg L^-1 d^-1 for NH₄⁺-N and TN, respectively. This TN removal load was 7 and 5.8 times higher than those using carbon felt and carbon brush anodes, respectively, and 2.7 times higher than those using a direct current power supply. Microbial community composition and functional gene abundance analyses showed that the CG anode was occupied by microbes associated with the anaerobic ammonia oxidation (Candidatus_Kuenenia, Ignavibacterium, norank__c__SJA-28, and norank__f__A4b), combined with the key gene abundance increase of hzs and hdh in the anammox pathway. With the lower current and coulomb efficiency of the CG-anode/SC-powered system, it demonstrated that the mechanism on the enhanced nitrogen removal was the electrically driven anodic partial nitrification with electrode as electron acceptor for nitrite production in situ, and then combined with anammox for nitrogen removal. This study constructs a novel autotrophic nitrogen removal system combined with SC-MEC and anammox, providing a viable strategy for the treatment of low C/N ammonium-rich wastewater.