Formation of nitrous oxide in a gradient of oxygenation and nitrogen loading rate during denitrification of nitrite and nitrate.

Nitrous oxide (N(2)O) emission has been observed during denitrification of nitrate via nitrite as intermediate. With a laboratory-scale reactor (2.4 L), the N(2)O emission was characterized under a gradient of DO concentration from 0 to 0.7 mg/L, different ratio of nitrite versus nitrate and different nitrite feed mode. The N(2)O emission was influenced by the level of dissolved oxygen (DO) and nitrite accumulation. The higher DO level and the higher ratio of nitrite versus nitrate resulted in the higher N(2)O emission. Using nitrite as sole electron acceptor at the same loading rate, the sequence of N(2)O emission with three different feed modes was: pulse>step-wise>continuous feed. The N(2)O emitted in pulse feed reactors was 3.1-4.2 and 8.2-11.7 folds of that in the step-wise feed and continuous feed reactors, respectively. With continuous feed mode, the impact of DO concentration on the mass of N(2)O emitted was limited while the higher N(2)O emission occurred at the higher nitrite loading rate.