Nitrous oxide emission during nitrification and denitrification in a full-scale night soil treatment plant

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.