Isotopic analysis of N_2O produced in a conventional wastewater treatment system operated under different aeration conditions

摘要

Dissolved oxygen (DO) concentration is a key parameter of nitrous oxide (N_2O), a greenhouse gas, emitted from wastewater treatment systems. No study of stable isotopes has described N_2O production during conventional activated sludge (CAS) treatment under different DO concentrations. METHODS: Concentrations and isotope ratios, including intramolecular site preference of ~(15)δ in NNO (SP), of N_2O were measured using gas chromatography/isotope ratio mass spectrometry (GC/IRMS) for samples from seven points in a wastewater treatment plant (WWTP) operated with three aeration conditions. The δ~(15)δ values of NH_4~+ and the δ~(15)δ and δ~(18)O values of NO_3 ~- were measured using IRMS. RESULTS: Aeration tank water was supersaturated with N_2O. The highest value, 3700 nmol kg~(-1), was observed at the aeration tank end and in settled sludge under the lowest aeration condition. About 0.03% of the influent NH_4~+ was emitted as gaseous N_2O at the lowest aeration condition. The conversion rate was 0.14% under the highest aeration condition. The SP values were significantly higher at the middle and end of the aeration tanks under the highest aeration condition, but were nearly zero or slightly negative under lower aeration conditions. CONCLUSIONS: Under the highest aeration condition, NH_2OH oxidation (nitrification) was the main contributor to N_2O production at about 90% and 57%, respectively, at the aeration tank middle and end. At other sampling points, 55-63% of the N_2O was produced by bacterial NO_2 ~- reduction (nitrifier-denitrification) with a lower nitrification contribution. For all sampling points in the lower aeration experiments, NO_2 ~- reduction was a major N_2O production pathway.