N₂O Emission and Hydroxylamine Oxidase (HAO) Activity in a Nitrogen Removal Process Based on Activated Sludge with Three COD/NH₄⁺ Ratios

This study dealt with nitrous oxide (N₂O) emission and hydroxylamine oxidase (HAO) activity of waste sludge in a nitrification and denitrification process employing three carbon nitrogen (C/N) ratios in a sequencing batch reactor (SBR). The experimental results indicated that N₂O emission increased dramatically after the C/N ratio in the sludge increased from 6.5 to 9.3, which was greater than the N₂O emission at two other C/N ratios (3.5, 6.5). The HAO activity in the anoxic period was higher with all three C/N ratios than in the aerobic period. The results suggest that N₂O was produced primarily in the aerobic period and the main source of the N₂O emission resulted from denitrification by nitrifying bacteria and aerobic hydroxylamine oxidation. When a relatively deficient carbon source existed, the N₂O emission under anoxic conditions was affected by the HAO activity and vice versa. When the HAO activity was relatively high, it was found that more N₂O was released.