Nitrogen removal efficiency and mechanisms of an improved anaerobic-anoxic–oxic system for decentralized sewage treatment

[Display omitted] •Development of improved AAO system to treat variable wastewater flows in rural areas.•Successful achievement of removal rates of 91.5% for COD and 68.2% for TIN.•Improved removal efficiency, notably in aerobic zone, with enhanced SND performance.•Dominance of heterotrophic nitrifying and aerobic denitrifying bacteria in the system. The unstable operation and poor effluent quality often associated with decentralized sewage treatment systems due to fluctuating water flows have garnered significant attention. In this study, a novel integrated process combining anoxic denitrification and simultaneous nitrification and denitrification was developed to address these challenges. The improved anaerobic-anoxic-aerobic system achieved average effluent concentrations of 20.83 mg/L and 4.63 mg/L for chemical oxygen demand and NH4+-N, with average removal rates of 91 % and 68 %, respectively. Moreover, the aerobic zone demonstrated an impressive efficiency of 40.8 % for simultaneous nitrification and denitrification. The key bacteria groups driving the system’s performance were heterotrophic and aerobic nitrifying bacteria, which dominated the microbial populations. Overall, the system optimizes the traditional anaerobic-anoxic-aerobic process, providing an effective solution for fluctuating wastewater flows. It establishes a successful coexistence model for multiple microbial populations, highlighting its applicability for superior nitrogen removal performance, and reference for optimizing rural sewage treatment. The improved anaerobic-anoxic-aerobic system for fluctuating wastewater treatment has superior nitrogen removal performance depending on multiple microbial populations.