Biological treatment in wastewater treatment plants (WWTPs) is a source of nitrogen oxides (N2O, NO and NO2) emitted to the atmosphere. Aerobic ammonia-oxidising bacteria (AOB) have been suggested to be the main source of these emissions. In a full-scale sludge liquor treatment plant at Sjölunda WWTP, it was shown that significant emissions of N2O, NO and NO2 do occur. The plant is operated with nitritation alone, which gives an environment enriched in aerobic AOB. During normal operation, emissions of N2O, NO and NO2 were found to be 3.8%, 0.06% and 0.01% of the ammonium nitrogen load. The N2O emissions were larger than the recommended estimated figure of the Intergovernmental Panel on Climate Change (IPCC) for a complete wastewater treatment plant. The N2O emissions correlated positively with the length of the previous anoxic period, i.e., settling and decantation, and with the ammonium oxidation rate. The NO and NO2 emission profiles were similar and dependent on ammonium oxidation and DO level, but the NO2 concentrations were always lower.

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