Effect of Fe2+ addition on anammox consortia, nitrogen removal performance and functional genes analysis during start-up of anammox process

Abstract Fe2+ is widely used in wastewater treatment and plays an important role in the anammox system. In this study, the effects of Fe2+ on the anammox system was investigated with 0, 1, and 5 mM Fe2+ in reactors. During the start-up, the nitrogen removal performance was changed significantly, the nitrogen removal efficiency and nitrogen removal rate reached 94% and 81.5 g N m−3 d−1 in the reactor, respectively, at the 1 mM Fe2+ after 63 days of operation. Moreover, the extracellular polymeric substances were improved, and showed linear correlation with Fe2+ concentration. The 16S metagenomic analysis proved that Fe2+ addition increased the relative abundances of anammox bacteria-Candidatus Brocadia than that of control. The results of RT-qPCR also indicated that Fe2+ oxidation activated the central metabolism by up-regulating the key functional genes (hdh and hao) in anammox bacteria. The difference between functional genes, and Candidatus Brocadia abundances in the sludge led to differences in ammonium removal efficiency and nitrite removal efficiency between all reactors, because Ca. Brocadia has great influence on anaerobic ammonium oxidation. ARE and NRE will be improved when the suitable amount of Fe2+ was added into the sludge. Overall, the anammox system can be quickly improved even at higher concentrations when the anammox consortium encounter with an appropriate amount of Fe2+.