High NO and N2O accumulation during nitrite denitrification in lab-scale sequencing batch reactor: influencing factors and mechanism

Nitrous oxide (N2O) accumulation in biological nitrogen removal has drawn much attention in recent years; however, nitric oxide (NO) accumulation in denitrification was rarely studied. In this study, NO and N2O accumulation during nitrite denitrification in a lab-scale sequencing batch reactor (SBR) were investigated. Results showed that low pH (< 7) and high influent loading (> 360:90) (COD:NO2--N) caused serious NO and N2O accumulation. The maximal NO accumulation of 4.96 mg L-1 was observed at influent loading of 720:180 and the maximal N2O accumulation of 46.29 mg L-1 was found at pH of 6. The NO accumulation was far higher than the values reported in previous studies. In addition, the high NO accumulation could completely inhibit the activities of reductases involved in denitrification. High NO and N2O accumulation were mainly caused by significant free nitrous acid (FNA) and NO inhibition at low pH and high influent loading. There were significant differences on NO and N2O accumulation at different carbon to nitrogen (COD/N). Low COD/N (<= 4) could mitigate NO accumulation, but led to high N2O accumulation. It is speculated that NO accumulation is related to the rapid denitrification with accumulated electron in anaerobic stage at high COD/N. N2O accumulation is attributed to intense electron competition at low COD/N. High dissolved oxygen (DO) of 4.04 mg L-1 was detected during NO detoxification in this experiment, which is speculated to be partly caused by NO dismutation.