Dinitrogen oxide (N₂O) emission in the treatment of urban wastewater via nitrite: influence of liquid kinetic rates

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 (gN₂O_emitted/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.