Investigation of dissolved N₂O production processes during wastewater treatment system in Ulaanbaatar

Abstract

Nitrous oxide (N₂O) is an increasing greenhouse gas in the troposphere and a potential destroyer of stratospheric ozone layer. Wastewater treatment plant (WWTP) is one of the anthropogenic N₂O sources because inorganic and organic nitrogen compounds are converted to nitrate (NO₃-, in the case of standard system) or N₂ (in the case of advanced system) by bacterial nitrification and denitrifcation processes in WWTP. These major processes can be distinguished by isotopocule analysis. In order to reveal production mechanisms of N₂O in a standard wastewater treatment, we made water sampling at the central WWTP in Ulaanbaatar. The water samples collected from seven stations including biological reaction tanks were measured for concentration and isotopocule ratios of dissolved N₂O and other inorganic nitrogen. Dissolved N₂O concentration was extremely higher than that expected under atmospheric equilibrium (about 9 nmol/l) at all stations, indicating that this system is a potential source of N₂O. It showed a gradual increase with the progress of biological reaction and the highest concentration (335.7 nmol/l) was observed at station N5-4 of the aeration tank when the DO was 5.7 mg/l. Nitrification by nitrifying bacteria could actively occur by the concentration of NH₄+ decreased whereas NO₂- and NO₃- showed a temporal and monotonic increase, respectively, under high DO concentration. Although the reported values of site preference (SP) of N₂O, the difference in ¹⁵N/¹⁴N ratio between central (α) and terminal (β) nitrogen, produced via NO₂- reduction (SP(ND)), including both nitrifier and denitrifier denitrification, and NH₂OH oxidation (SP(HO)) ranged from -10.7‰ to 0‰ and 31.4‰ to 36.3‰, respectively, the observed SP at aeration tank was close to SP(ND) rather than SP(HO). It was ranged from 0.4‰ to 13.3‰ when N₂O concentration was high, implying that the NO₂- reduction made a greater contribution to N₂O production. Slightly elevated SP (13.3‰) only at station N5-1 was derived from the mixing of N₂O produced via NH₂OH oxidation and the maximal contribution of this pathway was estimated to be about 40%. In other words, the contribution of NO₂- reduction was more than 60%.