| Sumario: | Abstract In this study, a novel step-feed three-stage integrated anoxic/oxic biological aerated filter (STIAOBAF) process was developed to enhance nitrogen removal from the synthetic domestic wastewater through optimizing influent flow distribution ratio (IFDR) for three stage reactors (R1, R2, R3). Long-term operation demonstrated that the maximum nitrogen removal efficiency was achieved at the IFDR of 30%:50%:20%. The corresponding effluent total nitrogen (TN) was less than 10 mg/L, superior to the first A grade discharge standard of China (Effluent TN < 15 mg/L). The IFDR was further optimized to 32%:49%:19% by response surface methodology (RSM) model, thus obtaining the highest TN removal efficiency of 81.4%. Nitrogen profiles suggested the 2nd stage reactor was the greatest significant contributor for nitrogen removal of the whole system. Microbial community analysis revealed that Chloroflexi , Bacteroidetes , Firmicutes , and Acidobacteria were abundant in anoxic zones, while Planctomycetes , Bacteroidetes and Verrucomicrobia were rich in oxic zones. Nitrogen removal-associated functional bacterial groups (Nitrospira , Thauera , Azospira and Candidatus Kuenenia) were also identified, supporting high-rate nitrogen removal through the combination of anoxic denitrification with aerobic simultaneous nitrification and denitrification (SND). The STIAOBAF will offer a compact and robust alternative for advanced nitrogen removal from the sewage. Graphical abstract Image Highlights • The STIAOBAF process was successfully developed to enhance TN removal. • The optimal IFDR of the system was 32%:49%:19%. • Nitrogen removal was attributed to anoxic denitrification with aerobic SND. • Variations in microbial community in aerobic and anoxic zones were observed. • Nitrogen removal-associated functional bacterial groups were identified.
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