N2O emissions from secondary clarifiers and their contribution to the total emissions of the WWTP.

Recent studies indicate that the emissions of nitrous oxide, N2O, constitute a major part of the carbon footprint of wastewater treatment plants. Generally, it has been assumed that the N2O emissions from secondary clarifiers are insignificant. Nevertheless, several researchers have reported denitrification occurring in the secondary clarifier basins. The objective of this study was to measure the N2O emissions from secondary clarifiers and weigh the portion they could represent of the overall emissions at wastewater treatment plants. On-line measurements during several days were carried out at four different municipal WWTPs in Finland in cold weather conditions (March) and in warm weather conditions (June-July). An attempt was made to define the conditions, in which N2O emissions from secondary clarifiers may occur. It was evidenced that large amounts of N2O can be emitted from the secondary clarifiers, and that the emissions were related to the season. In addition to the seasonal variability, prominent diurnal fluctuation in the emissions was detected as well; however, differences in emissions between the plants were observed. It was assumed that part of the N2O released in secondary clarification was originally formed in the activated sludge basin. The emissions from secondary clarification thus seem to be depending on the nitrification and denitrification accomplished in the DN-process and on the amount of sludge stored in the secondary clarifiers