title: Isotopic analysis of N2O produced in a conventional wastewater treatment system operated under different aeration conditions.
abstract: BACKGROUND
  Dissolved oxygen (DO) concentration is a key parameter of nitrous oxide (N2O), a greenhouse gas, emitted from wastewater treatment systems. No study of stable isotopes has described N2O production during conventional activated sludge (CAS) treatment under different DO concentrations.
  
  
  METHODS
  Concentrations and isotope ratios, including intramolecular site preference of (15)N in NNO (SP), of N2O 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)N values of NH4(+) and the δ(15)N and δ(18)O values of NO3(-) were measured using IRMS.
  
  
  RESULTS
  Aeration tank water was supersaturated with N2O. 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 NH4(+) was emitted as gaseous N2O 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, NH2OH oxidation (nitrification) was the main contributor to N2O production at about 90% and 57%, respectively, at the aeration tank middle and end. At other sampling points, 55-63% of the N2O was produced by bacterial NO2(-) reduction (nitrifier-denitrification) with a lower nitrification contribution. For all sampling points in the lower aeration experiments, NO2(-) reduction was a major N2O production pathway.
authors.affiliation: Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan.
authors.forename: Azzaya
authors.initials: A
authors.lastname: Tumendelger
authors.forename: Sakae
authors.initials: S
authors.lastname: Toyoda
authors.forename: Naohiro
authors.initials: N
authors.lastname: Yoshida