Study Questions

1. Prepare a table that summarizes the typical SRT values, bioreactor suspended solids concentrations, operating temperatures, and pH values for the three types of aerobic digestion. Describe how the differences in operating conditions result in differences in performance.

2. List the benefits and drawbacks of aerobic digestion and describe where it is typically applied.

3. Discuss how ASM No. 1 can be used to evaluate A/AD. Describe the steps required to apply the model, including procedures to calibrate it.

4. What factors determine the biodegradable fraction of waste solids leaving an activated sludge system? Express the results in terms of the simplified model of Chapter 5.

5. Describe the batch technique used to determine the decay coefficient and nonbiodegradable fraction of waste solids, including the data analysis. Why must constant temperature and pH values be maintained during the test?

6. A batch aerobic digestion test was performed on waste activated sludge from a pilot plant treating a soluble wastewater. The temperature was 20°C and the pH was maintained at a value above 6.5. The results are presented in Table SQ12.1. (a) Determine the concentrations of the volatile and fixed nonbiodegradable suspended solids, (b) Determine the decay coefficients for both volatile and fixed suspended solids.

7. A completely mixed aerobic digester is to be designed to treat the waste solids characterized in Study Question 6. The solids will be thickened to 10,000 mg/L prior to digestion but no additional thickening will be practiced in the digester. The winter operating temperature will be 10°C, and the temperature correction factor, 8, is 1.04. The flow rate of the thickened solids will be 500 m'/day. (a) What SRT is required to provide 38% VSS destruction during winter operating conditions? (b) The temperature of the bioreactor can be elevated to 25°C by insulation and selection of the oxygen transfer device. What SRT is required to achieve a 38% VSS destruction at this temperature? (c) What is the SOUR of the digested solids at 25°C? (d) What is the oxygen requirement for the aerobic digester? (e) What volume must the digester have at 25°C? (f) Assuming that FSS are lost during digestion in a first order manner, what is the percent reduction in TSS at 25°C? (g) How much power is required to mix the bioreactor at 25°C?

8. Use the simple model of Chapter 5 to define the characteristics of the waste solids produced by an activated sludge system operating at 20°C and an SRT of 5 days. For these calculations use the wastewater characteristics presented in Table E8.4 and the stoichiometric and kinetic parameters in Table E8.5. Assume that all slowly biodegradable substrate is

Table SQ12.1

Batch Aerobic Digestion Data

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