Characteristics of rapid-biofiltering anammox reactor (RBAR) for low nitrogen wastewater treatment

•RBAR achieved 86.2% TN removal stably under NLR of 4.86 kgN/m3·d and HRT of 32 min.•△pH and NRR in RBAR followed the relationship of △pH = 0.066NRR + 0.255.•High upflow velocity selected and kept granular sludge of 21.61 gSS/L in RBAR.•Anammox activity of granular sludge in RBAR could reach 0.82 gN/gVSS·d.•Relative abundance of Candidatus Brocadia in biofilm of RBAR could reach 71.1%. This research provides an important approach for rapid treatment of low nitrogen wastewater through anaerobic ammonium oxidation (anammox), which was realized in a rapid-biofiltering anammox reactor (RBAR). The operation mode of continuous upward flow and gradually shortened hydraulic retention time (HRT) accumulated anammox bacteria effectively in RBAR, where carmine anammox granular sludge and thick biofilm were co-existed, leading the biomass concentration and the specific anammox activity to reach 21.61 gSS/L and 0.82 gN/gVSS·d in the main functional zone. Moreover, the relative abundance of anammox bacteria in the whole reactor was more than 50%, and the relative abundance of Candidatus Brocadia in the biofilm of 20–47 cm zone reached 71.10%. Results showed that the removal rate and effluent concentration of total nitrogen remained stable at 86.24% and 14.20 mg/L (below 15 mg/L) averagely, under HRT of 32 min when the the nitrogen loading rate was 4.86 kgN/m3·d.