## Design Example

In this example, the task is to design an aerobic digester for excess activated sludge from a secondary clarifier. The influent SS to the digester contain 12,000 mg/l SS, of which 83% are volatile. The design flow to the digester is 50,000 gpd, and the goal is 85% reduction of the degradable VSS content. The procedure is as follows:

1. Prepare batch reactors and correlate the data as shown in Table 7.44.1 and Figure 7.44.7. From the latter, the rate coefficient (k) is 0.141 (base 10), corrected to 0.324 for the logarithmic base e.

2. Calculate the nondegradable residue and the degradable concentration in the effluent.

Influent VSS (X1): thus:

1070 + 2900 3970 mg/l

3. Calculate the required detention time:

Required volume _ 0.05 mgd (18) _ 0.9 mg _ 120,000 ft3

4. Calculate the oxygen requirements. Compute the average oxygen requirement by calculating the area under the curve in Figure 7.44.8 and dividing it by the design detention time.

Area under the oxygen curve Average daily use

7310 mg/l O2 7310/18 406 mg/l day

Similarly, estimate the average VSS during a test period to determine the specific oxygen requirements.

Area under curve Average VSS concentration Specific oxygen utilization

43,040 day-mg/l 2390 mg/l 406/2390 0.17 mg/mg/day

The total oxygen requirements based on an average VSS concentration X2 in the digester effluent of 3970 mg/l are then:

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