Nitrous oxide (N2O) emissions from nitrification-denitrification processes in a full-scale night soil treatment plant were measured, and patterns and control of the N2O production were investigated. Estimated N2O emissions ranged from 4.4 to 1,190 gN/(m3 of influent), corresponding to a conversion ratio of influent nitrogen to N2O-N of 0.24-55%. N2O was produced in the intermittent aeration tank (IAT) where nitrification and denitrification were carried out alternately. The produced N2O was either stripped out to the off-gas or remained in the effluent in dissolved form. The former accounted for more than 99.5% of the total emissions. The latter flowed into the following anoxic tank, where 60-98% of N2O was reduced. A significant difference in the extent of N2O supersaturation in mixed liquor of IAT was observed between the cases of high and low N2O emissions. In IAT, N2O tended to be produced discretely either in aerobic or in anoxic phases. It seemed that the completeness of nitrification and denitrification in IAT, indicated from a mass balance between NH4-N and NO3-N and from NO2-N accumulation in mixed liquor of IAT, was one of the important factors affecting the N2O production. This completeness was decided by the time ratio of aerobic and anoxic phases. External addition of methanol to IAT seemed to reduce N2O emissions.

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