Effect of anoxic to aerobic duration ratios on nitrogen removal and nitrous oxide emission in the multiple anoxic/aerobic process

Characteristics of nitrogen removal and nitrous oxide (N₂O) emission in the multiple anoxic/aerobic (AO) process were examined in three sequencing batch reactors (SBRs) with different anoxic durations (50 min, SBR_H; 40 min, SBR_M; 30 min, SBR_L) and a fixed aerobic duration of 30 min. The highest total inorganic nitrogen removal percentage of 85.8% was obtained in SBR_H, while a minimum N₂O emission factor of 1.9% was obtained in SBR_L. During nitrification batch experiments, the N₂O emission factor and emission rate were both lower in SBR_H than SBR_L. More N₂O production was obtained during denitrification in SBR_H when denitrifiers utilized intracellular organic carbon. Nitrite reduction by heterotrophs was the main N₂O production pathway during simultaneous nitrification and denitrification in SBR_H and SBR_L, with the N₂O emission factor of 31.3% and 36.3%, respectively. Adequate anoxic duration and lowering aerobic nitrite concentrations could be adopted to mitigate N₂O emission in the multiple AO process. The dominant microorganisms at the phylum level in all reactors were Proteobacteria and Bacteroidetes, while the abundance of Nitrospira was the highest in SBR_H with relatively lowest dissolved oxygen concentrations.