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

Highlights: • A stable PN/A process was achieved by hydroxylamine addition in the aerobic zone. • Hydroxylamine addition, with alternating anoxic/aerobic, suppressed NOB activity. • A superior effluent quality in a continuous reactor meets current discharge limits. • Both anammox (30.3%) and denitrification (28.1%) are the main N removal pathways. • Coupling PAOs (63.9%) and DPAOs (33.3%) were achieved in the reactor for P removal. 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⁻-N_produced/NH₄⁺-N_consumed ratio from 28.7% to 2OH 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 biofilm₁ and biofilm₂, 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.