Comparison of inhibitory roles on nitrite-oxidizing bacteria by hydroxylamine and hydrazine during the establishment of partial nitrification

[Display omitted] •NH2OH has a stronger inhibitory role on NOB than N2H4.•Inhibition of NOB by NH2OH or N2H4 combined with aeration time control is reversible.•The addition of NH2OH or N2H4 leads to NO production during nitrification.•Inhibition of Nitrospira could be related to the toxicity of NO. The inhibitory roles of hydroxylamine (NH2OH) and hydrazine (N2H4) on nitrite-oxidizing bacteria were investigated in a comparative study. The results showed that nitrite accumulation was achieved by adding 5 mg-N/L NH2OH or N2H4 to two parallel sequencing batch reactors, with nitrite accumulation rate reaching 95.83% and 86.58% within 15 days after adopting aeration time control, respectively. Correspondingly, the maximum level of NO in typical cycles caused by NH2OH addition was 0.18 mg-N/L, which was higher than obtained for N2H4. NH2OH or N2H4 showed strong inhibition on Nitrospira and promoted the enrichment of Nitrosomonas, with the effects of NH2OH being more significant. However, nitritation began to deteriorate after the cessation of inhibitors addition. In conclusion, NH2OH was a better inhibitor than N2H4 for Nitrospira. The inhibitory role of NH2OH was primarily related to NO toxicity, while for N2H4 it was attributed to its own toxicity, with NO playing a smaller role.