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

•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.