Nitrifiers activity and community characteristics under stress conditions in partial nitrification systems treating ammonium-rich wastewater

Long-term exposure of nitrifiers to high concentrations of free ammonia (FA) and free nitrous acid (FNA) may affect nitrifiers activity and nitrous oxide (N₂O) emission. Two sequencing batch reactors (SBRs) were operated at influent ammonium nitrogen (NH₄-N) concentrations of 800 mg/L (SBR_H) and 335 mg/L (SBR_L), respectively. The NH₄-N removal rates in SBR_H and SBR_L were around 2.4 and 1.0 g/L/day with the nitritation efficiencies of 99.3% and 95.7%, respectively. In the simulated SBR cycle, the N₂O emission factors were 1.61% in SBR_H and 2.30% in SBR_L. N₂O emission was affected slightly by FA with the emission factor of 0.22%–0.65%, while N₂O emission increased with increasing FNA concentrations with the emission factor of 0.22%–0.96%. The dominant ammonia oxidizing bacteria (AOB) were Nitrosomonas spp. in both reactors, and their relative proportions were 38.89% in SBR_H and 13.36% in SBR_L. Within the AOB genus, a species (i.e., operational taxonomic unit [OTU] 76) that was phylogenetically identical to Nitrosomonas europaea accounted for 99.07% and 82.04% in SBR_H and SBR_L, respectively. Additionally, OTU 215, which was related to Nitrosomonas stercoris, accounted for 16.77% of the AOB in SBR_L.