## Gravity Thickener Design Example

For the purposes of this example, the solids to be thickened are excess activated sludge with an initial solids concentration of 7 gm/l (0.434 lb/ft3). This sludge continuously enters the thickener, and the thickened sludge is continuously withdrawn at a concentration of 21 gm/l (1.302 lb/ft3). The G L is determined from data plots (see Figure 7.46.2) 0.52 lb/ft2/hr. The volume of sludge to be concentrated is 5200 gph (693.3 ft3/hr). The required

FIG. 7.46.2 Gravity thickener design data plots. Data plot procedure: 1. Conduct settling tests at various initial solids concentrations (a through d in panel A), covering the range of expected thickener influent solids concentrations; 2. Determine the settling velocity Vs for each concentration as the slope of a line tangent to the initial lineal settling rate portion of each curve; 3. Plot Vs values versus solids concentration, and construct a line of best fit; 4. Compute values of the flux rate G for a variety of solids concentrations (G = solids concentration, lb/ft3 X settling velocity, ft/hr), and plot as in panel C; and 5. Determine the value of the limiting flux rate GL as the intercept on the flux rate axis of a line tangent to the flux curve drawn from the desired underflow solids concentration.

FIG. 7.46.2 Gravity thickener design data plots. Data plot procedure: 1. Conduct settling tests at various initial solids concentrations (a through d in panel A), covering the range of expected thickener influent solids concentrations; 2. Determine the settling velocity Vs for each concentration as the slope of a line tangent to the initial lineal settling rate portion of each curve; 3. Plot Vs values versus solids concentration, and construct a line of best fit; 4. Compute values of the flux rate G for a variety of solids concentrations (G = solids concentration, lb/ft3 X settling velocity, ft/hr), and plot as in panel C; and 5. Determine the value of the limiting flux rate GL as the intercept on the flux rate axis of a line tangent to the flux curve drawn from the desired underflow solids concentration.

thickener area can be determined by using Equation 7.46(3) as follows:

27.14 ft

A good selection for this application is to provide two 27-to 30-ft diameter units equipped with vertical pickets. With one unit in service, the unit overflow rate (rate of effluent discharge) is as follows:

124,800 gal/day

578.6 ft2

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