Ammonium and organic carbon co-removal under Feammox heterotrophic conditions at low Fe³⁺ concentrations

Feammox, the anaerobic oxidation of ammonium coupled to Fe³⁺ reduction, offers a promising alternative for nitrogen removal under low-oxygen and high-organic load conditions. In this study, Feammox was successfully enriched from Anammox sludge by adjusting environmental conditions. Ammonium and organic carbon were co-removed at low Fe³⁺ concentrations (200 mg/L), indicating that Feammox can operate under heterotrophic conditions. Ferrous iron generation, XRD analysis, and PCR amplification of functional genes (nirK, narG, nxrA) support the occurrence of Feammox and iron cycling. Key microorganisms such as Geobacter spp., Shewanella spp., Ferrovum myxofaciens, Albidiferax ferrireducens, and Acidimicrobium were identified. Microbial community analysis confirmed the coexistence of taxa related to Anammox and Feammox pathways. These findings demonstrate that Feammox is feasible at lower Fe³⁺ concentrations than previously reported and under conditions compatible with real wastewater, highlighting its potential for application in systems with high C/N ratios and variable composition.