Laboratory-scale experiments were conducted to examine nitrous oxide (N2O) production during the nitrification process. Substrate containing 400 mgN l−1 of NH4HCO3 as a nitrogen source was continuously fed into a 10 or 3 liter completely mixed reactor with varied dissolved oxygen (DO) and SRT. The DO of the continuous systems were fixed at 0.1, 0.2, 0.5, 1.7 and 6.8 mg l−1. The SRT were controlled at 3, 5, 10 and 20 days. Total amount of N2O production reached higher than 36 mgN l−1 in some conditions, and up to 16% of nitrified nitrogen was converted into N2O. N2O production was observed in all tested runs. Lower DO and shorter SRT resulted in higher N2O production. Incomplete nitrification was found at DO lower than 0.5 mg l−1 and an SRT shorter than 5 days. The highest concentration and conversion rate of N2O production was obtained at DO of 0.2 mg l−1 or when SRT was 3 days. The results suggest that N2O production is significant during nitrification process. The accumulation of NO2 may contribute to the high amount of N2O production.

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