Advanced nitrogen removal in a single return anaerobic/aerobic/anoxic/aerobic (AnOAO) bioreactor treating municipal wastewater through hydroxylamine addition: Performance and microbial community

•Achieving NAR (98%) of AnOAO reactor by continuously adding NH2OH in only 2 days.•Stable NAR (95%) maintained by decreasing NH2OH dosing with 1 day every 4 days.•TINinf and TINeff were 42.9 and 4.8 mg N/L at aerobic HRT (4.6 h) and DO (<1mg/L)•Advanced N-removal by real sewage with low C/N (2.8) without external carbon source.•AOB activity increased to 12.78 mg N/(g VSS∙h) while NOB decreased by 81%