Laboratory-scale experiments were conducted to examine nitrous oxide (N2O) production during denitrification. Substrate containing acetate (909 mgl-1 as COD), yeast extract (91 mgl-1 as COD) and KNO3 with COD/NO3-N of 1.5, 2.5, 3.5 and 4.5 was continuously fed to 3 l mixed flow reactors with varied solid retention time (SRT). N2O in produced nitrogenous gas far exceeded 10% in some conditions, and up to 8% of influent NO3-N was transformed to N2O. Low COD/NO3-N, short SRT and low pH were favored conditions for N2O production. COD/NO3-N of 1.5 and 2.5 at an SRT shorter than 10 days, and COD/NO3-N of 3.5 or 4.5 at an SRT shorter than 1 day resulted in N2O production together with incomplete nitrate and nitrite removal. Lower COD/NO3-N caused higher production of N2O. N2O production at pH of 6.5 was significantly higher than that at pH of 7.5 although pH of 7.5 and 8.5 showed less difference. The mixed liquor from the continuous reactor producing N2O readily produced N2O even when substrate with high COD/NO3-N was dosed in batch experiments, which suggests that N2O producing species may be accumulated during continuous operation in favorable conditions.

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