Activated sludge settlers perform two functions simultaneously: clarification and sludge thickening. Depending on the values of the mixed liquor and return sludge concentrations and on the sludge settling characteristics either one of these functions may limit the solids loading rate that can be applied to the settler, which in turn determines the required settler surface area.

A method is developed to establish which of the two functions is determinant for the maximum solids loading rate in a particular design situation. Expressions are derived to calculate the required settler surface area for clarification and for thickening. These expressions are based on the experimentally verfied supposition that the zone settling velocity decreases exponentially with increasing suspended solids concentration. A graphical method to determine the optimal mixed liquor and return sludge concentrations for minimum construction and operational cost is presented.

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