Source identification of N2O produced during simulated wastewater treatment under different oxygen conditions using stable isotopic analysis

Abstract

Nitrous oxide (N₂O), a potent greenhouse gas which is important in climate change, is predicted to be the most dominant ozone depleting substance. It is mainly produced by oxidation of hydroxylamine (NH₂OH) or reduction of nitrite (NO₂^-) during microbiological processes such as nitrification and denitrification. 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 denitrification in WWTP. We investigated the N₂O production mechanisms during batch experiments that simulate wastewater treatment with activated sludge under various dissolved oxygen (DO) concentrations by stable isotope analysis. About 125mL of water was sampled from 30L incubation chamber for several times during the incubation, and concentration and isotopomer ratios of N₂O and N-containing species were measured using gas chromatography/isotope ratio mass spectrometry (GC/IRMS). Ammonium (NH₄⁺) consumption was accompanied by increment of nitrite (NO₂^-), and at the same time dissolved N₂O concentration gradually increased to 4850 and 5650 nmol kg^-1, respectively, during the four-hour incubation when DO concentrations were 0.2 and 0.5 mg L^-1. Observed low SP values (0.2-8.9‰ at DO-0.2 mg L^-1, -5.3-6.3‰ at DO-0.5 mg L^-1, -1.0-8.3‰ at DO-0.8 mg L^-1) in N₂O and relationship of nitrogen isotope ratios between N₂O and its potential substrates (NH₄⁺, NO₃^-) suggested that N₂O produced under the aerobic condition derived mainly from NO₂^- reduction by ammonia-oxidizing bacteria (nitrifier–denitrification).

DOI: http://doi.dx.org/10.5564/mjc.v15i0.313

Mongolian Journal of Chemistry  15 (41), 2014, p4-10