Production of nitrous oxide gas during nitrification of wastewater

Abstract

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.