Fe(OH)3 induced the Anammox system to perform extracellular electron transfer for enhancement of NH4+ removal

It is widely found that the ratio of nitrite consumption to ammonia consumption (ΔNO2--N/ΔNH4+-N) in Anammox systems is lower than its stoichiometry of 1.32, but the reason is unclear. In this study, ΔNO2--N/ΔNH4+-N of an Anammox system decreased to 0.80 with the addition of Fe(OH)3, which was significantly lower than a control system without the addition of Fe(OH)3. The lower ratio or more NH4+ removal was likely related to the reduction of Fe(OH)3 that served as extracellular electron acceptor to anaerobically oxidize NH4+, namely Feammox. Chemical or biological oxidation of the produced Fe2+ formed the Fe(III)/Fe(II) cycle to likely cause the continuous Feammox in the Anammox system. Isotopic analysis verified that the Anammox sludge collected from the Fe(OH)3-added reactor produced more 30N2 by Feammox. Raman analysis showed that the outer membrane cytochrome c of Fe(OH)3-added Anammox sludge could participate in the extracellular electron transfer during Feammox. Metagenomic analysis further confirmed that the gene encoding redox enzymes of cytochrome c of Candidatus Brocadia were enriched with adding Fe(OH)3. The results suggested that the Fe(OH)3 induced Anammox bacteria to perform extracellular electron transfer, allowing the Anammox system to effectively remove NH4+ via the Feammox pathway.