Nitric oxide and Nitrous oxide accumulation, oxygen production during nitrite denitrification in an anaerobic/anoxic sequencing batch reactor: exploring characteristics and mechanism.

Abstract

Nitrite denitrification has received increasing attention due to its high efficiency, low energy consumption, and sludge yield. However, the nitric oxide (NO) and nitrous oxide (N2O) which are harmful to the environment, microorganisms, and humans are produced in this process. In order to mitigate NO and N2O production, the biological mechanisms of NO and N2O accumulation, as well as NO detoxification during nitrite denitrification in a sequencing batch reactor were studied. Results showed that the peak of NO accumulation increased from 0.29 [Formula: see text] 0.01 to 3.12 [Formula: see text] 0.34mg L-1 with the increase of carbon to nitrogen ratio (COD/N), which is caused by the sufficient electron donor supply for NO2--N reduction process at high COD/N. Furthermore, the result suggested that NO accumulation with no pH adjustment was 12 times higher than that with pH adjustment. It is related to the inhibition on NO reductase caused by the high free nitrous acid (FNA) and NO concentration with no pH adjustment. The pathways of NO detoxification included NO emission, reduction, and dismutation, and the more NO produced, the high proportion of NO dismutation pathway. Result showed that the maximum of oxygen production during NO dismutation reached to 1.39mg L-1. N2O accumulation was mainly associated with FNA and NO inhibition, COD/N. The peak of N2O accumulation presented a completely opposite trend at pH adjustment and no pH adjustment, it is because that the higher FNA and NO concentration at high COD/N without pH adjustment will inhibit the N2O reductase activity, resulting in the N2O reduction was hindered during nitrite denitrification.