Enhanced nitrogen removal and minimization of N₂O emission in a constant-flow multiple anoxic and aerobic process

Nitrogen discharged from wastewater treatment plants contributes greatly to the eutrophication of receiving water bodies. The multiple anoxic and aerobic (AO) process can enhance nitrogen removal, while the concomitant emission of nitrous oxide (N₂O) attracts growing attention. Nitrogen removal and N₂O emission in a constant-flow multiple AO process were evaluated. The effects of dissolved oxygen (DO), carbon source, as well as feeding mode on N₂O emission were investigated. The removal percentages of ammonia and total nitrogen were 99.8% and 74.1%, respectively. Limited DO, the presence of organic carbon, and high nitrite concentrations enhanced N₂O emission during nitrification. Endogenous denitrification of nitrite showed a high N₂O emission potential, especially for one-time dosing of nitrite. nosZ, encoding N₂O reductase, was widely distributed in Proteobacteria (39.6%), β-proteobacteria (31.6%) and the genera of Ralstonia (20.0%). The study suggested that high DO concentration and the absence of organic carbon during nitrification, avoidance of endogenous denitrifying nitrite during denitrification, and the constant-feeding operation mode were all beneficial to minimize N₂O emission.