The sedimentation phase of the activated sludge process has a large influence on the effluent quality of secondary wastewater treatment plants. Increasingly stringent effluent guidelines emphasize the need to improve the performance of secondary settlers. Full scale studies of rectangular settlers, at a secondary treatment plant with an average flow of 4 m3/s, were made. The non-settleable fraction of the effluent suspended solids defines an upper limit to settler efficiency. Polymer can be used to enhance settling when dealing with peak flow situations. The mass of solids in the settler, needed to calculate a mass balance of the activated sludge process, can be estimated by a simple model based on the sludge blanket depth and the average concentration of the sludge blanket at a central location in a settler. On-line instruments are useful for monitoring rapid and periodic changes in the state of the activated sludge process.

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