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Abstract

An activated sludge process for reducing the amount of N.sub.2 O released to the atmosphere during biological oxidation of NH.sub.3, includes the following steps: mixing wastewater influent containing NH.sub.3 and BOD with activated sludge to provide a mixed liquor in a first enclosed aerobic treatment zone wherein an oxygen containing gas is introduced, the first enclosed aerobic treatment zone having two or more stages in which gas and liquor phases are staged and oxygen is transferred to the liquid phase and BOD is metabolized and NH.sub.3 is oxidized to NOx and N.sub.2 O, wherein the pN.sub.2 O is maintained at a level greater than 2 mm Hg; transferring the mixed liquor and gas to a second enclosed treatment zone having two or more stages, in which gas and liquid phases are staged, and wherein NOx and N.sub.2 O are reduced to N.sub.2 and an effluent gas is released to the atmosphere; transferring the mixed liquor to a settling zone wherein the activated sludge is separated from the mixed liquor to form a dense sludge and a clear supernatant; and returning at least a portion of the dense sludge to the enclosed aerobic treatment zone.

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First Claim