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Nitrite (NO2 (-)-N) accumulation in denitrification can provide the substrate for anammox, an efficient and cost-saving process for nitrogen removal from wastewater. This batch-mode study aimed at achieving high NO2 (-)-N accumulation over long-term operation with the acetate as sole organic carbon source and elucidating the mechanisms of NO2 (-)-N accumulation. The results showed that the specific nitrate (NO3 (-)-N) reduction rate (59.61 mg N VSS(-1) h(-1) at NO3 (-)-N of 20 mg/L) was much higher than specific NO2 (-)-N reduction rate (7.30 mg N VSS(-1) h(-1) at NO3 (-)-N of 20 mg/L), and the NO2 (-)-N accumulation proceeded well at the NO3 (-)-N to NO2 (-)-N transformation ratio (NTR) as high as 90 %. NO2 (-)-N accumulation was barely affected by the ratio of chemical oxygen demand (COD) to NO3 (-)-N concentration (C/N). With the addition of NO3 (-)-N, NO2 (-)-N accumulation occurred and the specific NO2 (-)-N reduction rate declined to a much lower level compared with the value in the absence of NO3 (-)-N. This indicated that the denitrifying bacteria in the system preferred to use NO3 (-)-N as electron acceptor rather than use NO2 (-)-N. In addition, the Illumina high-throughput sequencing analysis revealed that the genus of Thauera bacteria was dominant in the denitrifying community with high NO2 (-)-N accumulation and account for 67.25 % of total microorganism. This bacterium might be functional for high NO2 (-)-N accumulation in the presence of NO3 (-)-N.

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Anaerobic ammonium oxidation (anammox) has attracted extensive attention as a potentially sustainable and economical municipal wastewater treatment process. However, its large-scale application is limited by unstable nitrite (NO2 –-N) production and associated excessive nitrate (NO3 –-N) residue. Thus, our study sought to evaluate an efficient alternative to the current nitritation-based anammox process substituting NO2 –-N supply by partial-denitrification (PD; NO3 –-N → NO2 –-N) under mainstream conditions. Ammonia (NH4 +-N) was partly oxidized to NO3 –-N and removed via a PD coupled anammox (PD/A) process by mixing the nitrifying effluents with raw wastewater (NH4 +-N of 57.87 mg L–1, COD of 176.02 mg L–1). Excellent effluent quality was obtained with< 5 mg L–1 of total nitrogen (TN) despite frequent temperature fluctuations (25.7–16.3 °C). The genus Thauera (responsible for PD) was the dominant denitrifiers (36.4%–37.4%) and coexisted with Candidatus Brocadia (anammox bacteria; 0.33%–0.46%). The efficient PD/A allowed up to 50% reduction in aeration energy consumption, 80% decrease in organic resource demand, and lower nitrous oxide (N2O) production compared to conventional nitrification/denitrification process. Our study demonstrates that coupling anammox with flexible NO2 –-N supply has great potential as a stable and efficient mainstream wastewater treatment.