Using Fe(II)/Fe(III) as catalyst to drive a novel anammox process with no need of anammox bacteria

A novel ‘anammox’ in the absence of anammox bacteria was confirmed to occur in an anaerobic sludge slurry system, in which Fe(II)/Fe(III) cycle driven by NO2−-induced Fe(II) oxidation and subsequent NH4+-induced Fe(III) reduction (Feammox) pushed the nitrogen removal. Results showed that Fe(II) contents significantly (p<0.05) decreased and Fe(III) accordingly increased with NO2− addition, indicating that Fe(II) was anaerobically oxidized to Fe(III). With depletion of NO2−, the Fe(II) content began to increase which was a result of Feammox. Consequently, 96.0% NH4+-N of the NO2−-added reactor was removed during 18 days operation, while NH4+-N content remained essentially unchanged in the control in which NO2− was not added. X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) analysis indicated that FeOOH was produced from chemical Fe(II) oxidation with NO2−. During the treatment, anammox bacteria was not detected, but the relative abundance of Geobacter of the NO2−-added group increased by 13 folds. Isotope experiment in 15NH4+-containing reactors found that much more 30N2 and 29N2 in the 14NO2−-added group were produced than those in the control (without 14NO2−), confirming that 14NO2− induced Fe(II) oxidation to participate in Feammox for 15NH4+ removal. Also, NO2− could be produced from partial denitrification of NO3−, meaning that NO3− as a more common species might substitute NO2− to trigger this new anammox process.