Development Of Design Criteria

Because of the varying nature of sludge and the lack of sufficient design information, environmental engineers should initiate laboratory or pilot studies to develop the design information. Specifically, they should obtain the rate of VSS destruction, the maximum percent of expected VSS reduction, and the oxygen requirements for various degrees of VSS destruction.

To simplify laboratory procedures, environmental engineers can estimate the destruction rate coefficient and oxygen requirements from a batch study by using the following procedure:

1. Obtain three or four batch units with approximately 2 to 4l of excess activated sludge in each. Vary the concentration in the batch units to cover the spectrum of anticipated concentrations in the proposed digester.

2. Aerate the units and after they are completely mixed, perform the following analysis on each:

a. SS, mg/l b. VSS, mg/l c. Oxygen uptake, mg/l per day

3. Continue to aerate the systems for 25 to 30 days and perform the preceding analyses every 3 days on each unit (see Table 7.44.1).

4. Plot the VSS and SS concentration remaining versus the sludge age (aeration time). From this plot, approximate the oxidizable or degradable fraction of the solids, i.e., the residual VSS remaining after 25 or 30 days of digestion can be taken as the nondegradable portion of the volatile matter (1775 mg/l in Table 7.44.1). The volatile solids destroyed during this aeration are the maximum degradable portion of the volatile matter.

5. For each sampling period, recalculate the remaining degradable VSS. Plot the degradable VSS remaining as a function of detention time (see Figure 7.44.7), and calculate the reaction rate coefficient (k). Note the effect of initial VSS on the destruction rate, and indicate this relationship, if noticeable.

6. Record the oxygen uptake rate as a function of sludge age or aeration time (see Figure 7.44.8). When sizing aeration equipment, estimate the oxygen utilization value as the average value exerted during aeration for

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