Pressurized Recycling Design Example

For the purposes of this example, the solids to be thickened are excess activated sludge with an initial solids concentration of 7000 mg/l. The operating temperature is 20°C and the operating pressure selected is 60 psig (5.1 atm). A 60% air saturation has been achieved at a pressurized detention time of one min. The A/S ratio value selected is 0.03. The volume of incoming excess activated sludge to be thickened is 124,800 gal/day with an overflow rate of 2500 gal/ft2/day. Applying Equation 7.46(4) yields the following:

FIG. 7.46.4 Pressure flotation design data plots. Data plot procedure: 1. Conduct rise rate tests at various initial solids concentrations (a through d in panel A) covering the range of expected unit influent solids concentrations; 2. Determine the initial rise rate Rr for each concentration as the slope of a line tangent to the initial lineal rise rate portion of each curve; 3. Plot the concentration of floated solids (% by weight) versus the A/S for each influent solids level; 4. Plot the subnatant SS concentration (unit effluent SS) versus the A/S for each influent solids level; and 5. Select the design A/S ratio to achieve the solids capture or concentration objective, or alternatively, optimize for maximum concentration and capture.

FIG. 7.46.4 Pressure flotation design data plots. Data plot procedure: 1. Conduct rise rate tests at various initial solids concentrations (a through d in panel A) covering the range of expected unit influent solids concentrations; 2. Determine the initial rise rate Rr for each concentration as the slope of a line tangent to the initial lineal rise rate portion of each curve; 3. Plot the concentration of floated solids (% by weight) versus the A/S for each influent solids level; 4. Plot the subnatant SS concentration (unit effluent SS) versus the A/S for each influent solids level; and 5. Select the design A/S ratio to achieve the solids capture or concentration objective, or alternatively, optimize for maximum concentration and capture.

The required unit surface area is as follows: 124,800 gal/day

2500 gal/ft2/day

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