N2O emission was studied in a continuous process via nitrite for real urban wastewater treatment. The relationship between the gaseous forms and the liquid kinetic rates of nitritation and denitritation was investigated. N2O mass load and global nitrogen mass balance were quantified. The emission factor of the N2O (gN2Oemitted/kg mixed liquor volatile suspended solids [MLVSS]/d) was calculated. Incrementing the nitritation rate permits the reduction of N2O emission by 78%. Instead, an N2O decrease of 93% was observed by increasing the denitritation velocity. The determinant role of the anoxic phase in the production of dinitrogen oxide was identified. The contribution of N2O emission from the anoxic phase (4.8 gN2O-N/kgMLVSS/d) was enhanced under limiting denitritation conditions (kd lower than 0.08 kgNOx-N/kgMLVSS/d). N2O production increased by five times with the accumulation of nitrites in the solution up to 200 mg/L. Strict correlation between free ammonia concentration and nitritation rate was found as a possible signal of further ammonia oxidizing bacteria selection.

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