Effect of anoxic/aerobic phase fraction on N2O emission in a sequencing batch reactor under low temperature

Laboratory scale anoxic/aerobic sequencing batch reactor (A/O SBR) was operated around 15 °C to evaluate the effect of anoxic/aerobic phase fraction (PF) on N2O emission. The ammonia removal exhibited a decrease trend with the increase of PF, while the highest total nitrogen removal was achieved at PF = 0.5. Almost all the N2O was emitted during the aerobic phase, despite of the PF value. However, the net emission of N2O was affected by PF. Under the premise of completely aerobic nitrification, the lowest N2O emission was achieved at PF = 0.5, with a N2O–N conversion rate of 9.8%. At lower PF (PF = 0.2), N2O emission was stimulated by residual nitrite caused by uncompleted denitrification during the anoxic phase. On the other hand, the exhaustion of the easily degradable carbon was the major cause for the high N2O emission at higher PF (PF = 0.5). The N2O emission increased with the decreasing temperature. The time-weighted N2O emission quantity at 15 °C was 2.9 times higher than that at 25 °C.