High-efficient nitrogen removal with low demand of Fe source and mechanism analysis driven by Fe(II)/Fe(III) cycle

[Display omitted] •Stable and efficient nitrogen removal of 92% was achieved by Fe(II)/Fe(III) cycle.•Operating of Feammox coupled with NDFO process saves aeration and carbon source addition.•N2O was not detected, achieving reduction of greenhouse gas emissions.•Fe-reducing and Fe-oxidizing bacteria played an important role in the Fe cycle.•Metagenomic analysis revealed functional potential in the Fe(II)/Fe(III) cycle. Feammox (Fe(III) reduction coupled with anaerobic ammonium oxidation) is less studied for nitrogen removal in wastewater treatment, and the limited studies show its low removal efficiency and need for continuous resupply of Fe(III) sources. In this study, sustainable and efficient nitrogen removal was obtained by the Fe redox cycle with only 1 g/L Fe2O3 dosage. The removal efficiencies of NH4+-N and TN were 100% and 92.45%, respectively, significantly higher than previous studies (∼50%). Fe(III) was reduced to Fe(II) by Feammox and subsequently regenerated by NDFO (NOx--N-dependent Fe(II) oxidation) and chemical oxidization. NOx--N produced by Feammox was also reduced to N2 by NDFO. The iron-reducing bacteria (Ferruginibacter and Dechloromonas) and the iron-oxidizing bacteria (Rhodanobacter and Thermomonas) were enriched by the addition of Fe2O3 and played important roles in Feammox and NDFO, respectively. Results of X-ray diffraction showed that Fe(OH)2 was produced from Feammox and then transformed to lepidocrocite by Fe(II) oxidation. Metagenomic analysis showed D/ANRA (dissimilatory/ assimilatory nitrate reduction to ammonia) were the potential nitrogen conversion pathways. N2O as an intermediate product of Feammox or NDFO was undetected. The genes associated with Fe transport were upregulated, accelerating the transfer of electrons between microorganisms and Fe(Hydr)oxide. The process for nitrogen removal by the Fe redox cycle requires no aeration and no organic carbon source, and can reduce the need for Fe(III) sources. It offers a promising method for nitrogen removal from wastewater.