Achieving stable mainstream nitrogen and phosphorus removal assisted by hydroxylamine addition in a continuous partial nitritation/anammox process from real sewage.

Hydroxylamine (NH2OH) as the putative intermediate for anammox ensures the robustness of partial nitritation/anammox (PN/A) process; however, the feasible for NH2OH addition to improve the stability of PN/A process under low-strength ammonia (NH4+-N) condition need to be further investigated. In this study, the restoration and steady operation of mainstream PN/A process were investigated to treat real sewage with in situ NH2OH added in a continuous alternating anoxic/aerobic with integrated fixed-film activated sludge (A3-IFAS) reactor. Results showed that the deteriorated PN/A process caused by nitrate (NO3--N) built-up was rapidly restored with a distinct decrease of the NO3--Nproduced/NH4+-Nconsumed ratio from 28.7% to <10.0% within 20 days, after 5 mg N/L of NH2OH was added daily into the aerobic zone of A3-IFAS reactor. After 230 days of operation, the average total nitrogen (TN) and phosphate (PO43--P) removal efficiencies of 80.8% and 91.5%, respectively were stably achieved, with average effluent sCOD, NH4+-N, TN and PO43--P concentrations reaching 23.1, 2.3, 7.7 and 0.4 mg/L, respectively. Microbial community characterization revealed Candidatus Brocadia (3.60% and 2.92%) and Ignavibacteriae (1.56% and 2.66%) as the dominant anammox bacteria and denitrifying bacteria, respectively, jointly attached in the biofilm_1 and biofilm_2, while Candidatus Microthrix (5.17%) dominant in floc sludge was main responsible for phosphorus removal. This study confirmed that NH2OH addition is an effective strategy for nitrite-oxidizing bacteria suppression, contributing to the in situ restoration of PN/A process and high stable mainstream nitrogen and phosphorus removal in a continuous PN/A process from real sewage.