Many plant owners, operators and designers consider the nitrifying activated sludge process as being a high hydraulic residence time (HRT) process. However, in recent years a number of techniques have been developed to enhance the performance of activated sludge plants for nitrification. The result, when the proper combination of these techniques is used, can be a significant reduction in the biological process hydraulic residence time and a corresponding reduction in facility costs and space requirements. Several of these factors are discussed in this paper, including a discussion of the mechanistic basis for each technique and its impact on nitrification process sizing. Some full-scale North American examples are also discussed.

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