Transition from Anammox to Feammox metabolic modes: Regulation strategies for nitrite in Anammox enrichment cultures

This study achieved the metabolic transition from anaerobic ammonium oxidation (Anammox) to Fe(III)-mediated ammonium oxidation (Feammox) using iron-carbon micro-electrolytic spheres as a slow-release iron source through a stepwise reduction in influent NO₂⁻-N concentration. The results demonstrated that sustained Feammox activity was governed by nitrate-dependent ferrous oxidation (NDFO) metabolism combined with oxygen-regulated Fe(III) regeneration, resulting in a peak total nitrogen removal efficiency of 91.6 % at 40 mg/L NO₂⁻-N. While exclusive NH₄⁺-N feeding inhibited Feammox activity, this suppression was reversible upon NO₂⁻-N supplementation. Intriguingly, Anammox activity remained robust despite decreasing NO₂⁻-N levels and showed significant positive correlation with Feammox activity, suggesting shared metabolic modules. Metagenomic profiling further identified Ca. Brocadia as the core functional genus driving NH₄⁺-N oxidation, highlighting its niche adaptation in iron-mediated systems. These mechanistic insights establish a framework for designing energy-efficient nitrogen removal processes leveraging iron-redox cycling.