The strategy of nitrate selective inhibition of NOB: transition from complete nitrification to partial nitrification.

Stable and efficient partial nitrification (PN) can provide the necessary substrate for anaerobic ammonium oxidation (Anammox). However, rapidly establishing and maintaining stable PN has always been challenging. This study innovatively proposes using nitrate (NO3--N) as a selective inhibitor of nitrite-oxidizing bacteria (NOB), thereby achieving the stable maintenance of PN. Based on batch tests that identified 100 mg/L NO3--N as the optimal concentration for NOB inhibition, two sequencing batch reactors (SBRs) were operated: the control R1 (without NO3--N addition in the influent) and the experimental R2 (with 100 mg/L of NO3--N added to the influent). In contrast to R1, which remained under complete nitrification, R2 achieved long-term stable PN with a maximum nitrite accumulation ratio (NAR) of almost 100%. The microbial community analysis showed that AOB (Nitrosomonas, Nitrosospira) in R2 is obviously enriched, NOB (Nitrospira) is continuously inhibited, and the relative abundance of Comamonadaceae reaches 47.58%, and this change may synergistically lead to the accumulation of nitrite (NO2--N). Preliminary mechanistic analysis suggests that high NO3--N concentrations may promote the shift from complete nitrification to PN by inhibiting the activity of nitrite oxidoreductase (NXR) and altering the niche balance between AOB and NOB. This study indicates the feasibility of using NO3--N as an environmentally benign regulating agent, thereby providing both a novel technical approach and theoretical support for stable operation of combined PN/Anaerobic Ammonium Oxidation (Anammox) processes.