Mechanisms of N₂O production in biological wastewater treatment under nitrifying and denitrifying conditions

• Published in: Water research (Oxford) 2012-03, Vol.46 (4), p.1027-1037
• Main Authors: Wunderlin, Pascal, Mohn, Joachim, Joss, Adriano, Emmenegger, Lukas, Siegrist, Hansruedi
• Format: Article
• Language: English
• Subjects: activated sludge; aerobic conditions; Aerobiosis; ammonia; Ammonia - chemistry; anaerobic conditions; Batch Cell Culture Techniques; Biodegradation, Environmental; biological production; Bioreactors - microbiology; carbon; Denitrification; denitrifying bacteria; dissolved oxygen; greenhouse gases; Hydroxylamine - chemistry; municipal wastewater; nitrates; nitric oxide; Nitrification; Nitrites - analysis; nitrous oxide; Nitrous Oxide - analysis; oxidation; Oxidation-Reduction; oxygen; ozone; Sewage - microbiology; stratosphere; Waste Disposal, Fluid; wastewater treatment; Water Pollutants, Chemical - isolation & purification; Water Purification - methods
• ISSN: 0043-1354; 1879-2448
• DOI: 10.1016/j.watres.2011.11.080

Nitrous oxide (N₂O) is an important greenhouse gas and a major sink for stratospheric ozone. In biological wastewater treatment, microbial processes such as autotrophic nitrification and heterotrophic denitrification have been identified as major sources; however, the underlying pathways remain unclear. In this study, the mechanisms of N₂O production were investigated in a laboratory batch-scale system with activated sludge for treating municipal wastewater. This relatively complex mixed population system is well representative for full-scale activated sludge treatment under nitrifying and denitrifying conditions. Under aerobic conditions, the addition of nitrite resulted in strongly nitrite-dependent N₂O production, mainly by nitrifier denitrification of ammonia-oxidizing bacteria (AOB). Furthermore, N₂O is produced via hydroxylamine oxidation, as has been shown by the addition of hydroxylamine. In both sets of experiments, N₂O production was highest at the beginning of the experiment, then decreased continuously and ceased when the substrate (nitrite, hydroxylamine) had been completely consumed. In ammonia oxidation experiments, N₂O peaked at the beginning of the experiment when the nitrite concentration was lowest. This indicates that N₂O production via hydroxylamine oxidation is favored at high ammonia and low nitrite concentrations, and in combination with a high metabolic activity of ammonia-oxidizing bacteria (at 2 to 3 mgO₂/l); the contribution of nitrifier denitrification by AOB increased at higher nitrite and lower ammonia concentrations towards the end of the experiment. Under anoxic conditions, nitrate reducing experiments confirmed that N₂O emission is low under optimal growth conditions for heterotrophic denitrifiers (e.g. no oxygen input and no limitation of readily biodegradable organic carbon). However, N₂O and nitric oxide (NO) production rates increased significantly in the presence of nitrite or low dissolved oxygen concentrations.