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 (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.

Die Strukturchemie der 2-Chalkogeno-1,3,4,5-tetraisopropylimidazoline / The Structural Chemistry of the 2-Chalcogeno-1,3,4,5-tetraisopropylimidazolines

1,3,4,5-Tetraisopropyl-2-thioimidazoline (6b) was prepared by condensation of N,N′-diisopropylthiourea and isobutyroin. 1,3,4,5-Tetraisopropyl-2-oxoimidazoline (6a), 1,3,4,5-tetraisopropyl-2- selenoimidazoline (6c) and 1,3,4,5-tetraisopropyl-2-telluroimidazoline (6d) were obtained from 2,3- dihydro-1,3,4,5-tetraisopropylimidazol-2-ylidene and dinitrogen oxide or selenium and tellurium, respectively. The crystal structure analyses revealed the presence of the A-type rotamer for 6a while for 6c a paddlewheel-like orientation of the isopropyl substituents was found (I type). In crystals of 6b, both A- and E-type molecules are present while in solution at room temperature the I-type rotamer is detected. A dynamic 1H and 13C{1H} NMR study of 6b in the range of 185.5 to 423.3K revealed a temperature-dependent rotation of the isopropyl substituents (ΔH‡ = 15:7(9) kcal mol-1, ΔS‡ = -5(3) calK-1 mol-1, ΔG‡ = 17:2(9) kcal mol-1) with I as the rotamer of lowest energy. This result is confirmed by MO calculations which indicate A and C being the next stable rotamers. The barrier of rotation of the isopropyl substituents about the C-N bond from I to C (B3LYP ΔH‡ =13:7 kcal mol-1,ΔG‡ =15:4 kcal mol-1) is slightly lower than that about the C-C bond from I to G.