## Gravity Thickening

Gravity-type thickeners (see Figure 7.46.1), in which waste sludge is continuously fed and thickened sludge is continuously withdrawn, are commonly used in waste-water treatment systems. The required thickener area de

pends on the rate of solids arrival at the thickener bottom, which depends on the settling velocity of the sludge solids and the concentration of the thickened sludge removed from the unit. Solids move downward through the thickener at the combined flux rate given in the following equation:

where:

G = Solids flux rate, lb/ft2/hr

Cta = Solids concentration in thickener influent, lb/ft3 Vs = Solids settling velocity due to gravity, ft/hr Vu = Solids downward transport velocity due to underflow solids removal, ft/hr

The following equation expresses the downward transport velocity (Vu) due to concentrated solids withdrawal:

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