Production of Nitrous Oxide Gas during Denitrification of Wastewater

Laboratory-scale experiments were conducted to examine nitrous oxide (N 2 O) production during denitrification. Substrate containing acetate (909 mgl -1 as COD), yeast extract (91 mgl -1 as COD) and KNO 3 with COD/NO 3 -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). N 2 O in produced nitrogenous gas far exceeded 10% in some conditions, and up to 8% of influent NO 3 -N was transformed to N 2 O. Low COD/NO 3 -N, short SRT and low pH were favored conditions for N 2 O production. COD/NO 3 -N of 1.5 and 2.5 at an SRT shorter than 10 days, and COD/NO 3 -N of 3.5 or 4.5 at an SRT shorter than 1 day resulted in N 2 O production together with incomplete nitrate and nitrite removal. Lower COD/NO 3 -N caused higher production of N 2 O. N 2 O 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 N 2 O readily produced N 2 O even when substrate with high COD/NO 3 -N was dosed in batch experiments, which suggests that N 2 O producing species may be accumulated during continuous operation in favorable conditions.