Simultaneous removal of NOx− and NH4+ driven by Fe(II)/Fe(III) cycle under the continuously-feeding mode

Fe(III) (hydr)oxides have been reported to serve as the extracellular electron acceptor of iron-reducing bacteria to oxidize NH4+, and the resulted Fe(II) may be oxidized by NOx− to Fe(III) that continuously participates to the next round of NH4+ removal, but whether such the Fe(II)/Fe(III) cycle can simultaneously remove NOx− and NH4+ under continuously feeding modes remains unknown. In this study, a NH4+-containing wastewater with and without NO2− or NO3− was fed into anaerobic reactors continuously to investigate the nitrogen removal with the Fe(II)/Fe(III) cycle. The results showed that, under a hydraulic retention time (HRT) of 48 h, the NH4+-N removal efficiencies in the NO3−-fed and NO2−-fed reactors reached about 50.3 % and 42.7 % respectively, accompanied with the complete removal of NO3− and NO2−. The experiment of intermittent addition of 14NOx− into the serum bottle containing 15NH4+ showed that the Fe(II) content decreased rapidly after NOx− addition, and then gradually increased, indicating the occurrence of iron cycle. The isotope experiment also demonstrated that 29N2 and 30N2 were generated after adding NOx−, and the Anammox microorganisms were undetected in all groups. The above results indicated that NOx− and NH4+ could be simultaneously removed under the continuously-feeding mode, in which Fe(II)/Fe(III) cycle was the essential reason for the nitrogen removal.