Biological anoxic-oxic activated sludge process, which treated domestic wastewater, was investigated to examine the effects of SRT and DO on nitrogen removal and emission of N20, which is causing global warming. The results obtained were as follows: The greater part of N20 emission was observed through nitrification, that amounted to about 4.5 times as that through denitrification. Alternation of SRT or MLSS, to some extent, had no influence on N20 emission through denitrification in anoxic tanks, as far as the nitrogen removal was made effectively under appropriate nitrogen loading. The amount of N20 emitted through nitrification at the run of 10-days SRT reaced about twice as that at the run of 20-days SRT, which led to that enough SRT could suppress N20 emission through nitrification. Besides, keeping high nitrification efficiency was important for suppression of N20 emission. It was clarified that control of carrying DO into the anoxic tank by nitrified water could reduce N20 emission through denitrification in anoxic tanks.
