1. Prepare a table comparing the following factors for the eight major types of activated sludge: SRT, HRT, MLSS concentration, recycle ratio, bioreactor configuration, and design approach.
2. List the main advantages and disadvantages of an activated sludge system in comparison to anaerobic wastewater treatment processes. When would an activated sludge process be used in comparison to an anaerobic process?
3. List the benefits and drawbacks of each of the eight major activated sludge variations. When is each typically applied?
4. Explain why the SRT is generally maintained between 3 and 15 days to obtain an activated sludge that settles well.
5. Three benefits have been attributed to HPOAS: (1) an increased rate of treatment; (2) increased density of biomass with an associated increased settling rate; and (3) reduced rate of excess biomass production Discuss and evaluate these claims.
6. Discuss the role of both floe-forming and filamentous bacteria in the formation of a good settling activated sludge, explaining why an optimum balance exists.
7. Explain the kinetic selection mechanism as applied to SAS. Describe how the selector should be configured to take advantage of this mechanism, and explain why.
8. Discuss the types of filamentous microorganisms that often occur in activated sludge systems and relate the filament types to the environmental conditions favoring them.
9. Discuss the impacts of DO concentration on the performance of activated sludge systems. Identify all potential effects and their typical importance.
10. Discuss the relationship between filament growth and activated sludge floe structure.
11. Describe and contrast the various indices used to characterize activated sludge settleability.
12. Discuss the benefits of plug-flow conditions within an activated sludge system. Describe how the process loading factor for the initial contact zone should be selected to optimize sludge settleability.
13. Why should the MLSS concentration in an activated sludge system normally lie between 500 and 5,000 mg/L as TSS? What factors affect the choice of the value?
14. Why must both an upper and a lower limit be placed on the mixing energy supplied per unit volume to an activated sludge bioreactor? List appropriate limit values for diffused air and mechanical surface aeration systems.
15. Why are heat losses from diffused air activated sludge systems less than those from systems using mechanical surface aerators? Does the HRT affect heat loss, and if so how?
16. Using the wastewater characteristics in Table E8.4, the stoichiometric and kinetic parameters in Table E8.5, and the temperature correction factors in Table El 0.1, design a CM AS system to treat an average wastewater flow rate of 30,000 mVday using an SRT of 7 days. Assume a constant loading. Use mechanical surface aeration and justify all assumptions and decisions. The lowest sustained winter temperature is 13°C and the highest sustained summer temperature is 24°C.
17. The diurnal peak loading for the CMAS system considered in Study Question 16 is twice the average loading. What is the peak oxygen requirement during both low and high temperature operating conditions? Does consideration of the peak oxygen requirement affect the selection of the bioreactor volume? What bioreactor volume and MLSS concentration would you recommend under this condition? Why?
18. Design a CAS system for the situation considered in Study Question 16. Use a diffused air oxygen transfer system and assume that the in-process oxygen transfer efficiency is 12%. Also assume that the hydraulic characteristics of the CAS bioreactor are equivalent to three tanks in scries.
19. Use a computer code implementing ASM No. 1 or a similar model to evaluate the steady-state distribution of oxygen requirements in the CAS system sized in Study Question 18.
20. Assume that the influent flow and pollutant concentrations for the problem considered in Study Question 18 vary as indicated in Figure 6.2. Use a computer code implementing ASM No. 1 or a similar model to determine the effect of these diurnal variations on the system effluent quality and the oxygen requirement in each equivalent tank of the CAS system designed in Study Question 18. Do this for the summer conditions only. How does the flow-weighted average effluent quality compare with the steady-state value obtained in Study Question 18? Would it be possible to deliver oxygen rapidly enough to meet the peak oxygen requirement in the first tank of the system sized in Study Question 18? Why? If the system is incapable of meeting the peak oxygen requirement, what modifications to the design would be required? What other impacts would such changes have?
21. Repeat Study Question 16 for an SAS system. Assume that the main bioreactor is a single completely mixed vessel, and determine the required sizes for all tanks in the system. Assume that diffused aeration is to be used with a 12% oxygen transfer efficiency.
22. An EAAS system is to be designed to treat the wastewater described in Study Question 16. Select and justify the SRT that will be used for the design. Determine the mass of MLSS in the system, the waste solids quantity, and the oxygen requirement under both summer and winter conditions. Assume that the bioreactor will be configured as an oxidation ditch with vertical mechanical surface aerators. The in-process oxygen transfer capacity of the aerators is 1.2 kg OV(kW-hr), and the minimum volumetric power input required to maintain solids in suspension is 7 kW/ 1000 m\ Determine the allowable range in bioreactor volumes.
23. A four pass SFAS system with an SRT of 7 days is to be used to treat the wastewater considered in Study Question 16. The total bioreactor volume is 5,625 m' and the RAS flow rate is 15,000 m'/day. Influent wastewater is to be distributed uniformly to each of the four passes. Determine the MLSS concentration in each of the four passes, as well as the oxygen requirement. Use a diffused air oxygen transfer system and assume that the in-process oxygen transfer efficiency is 12 percent. Is the bioreactor volume acceptable as far as the constraints on mixing energy and oxygen transfer are concerned? Why? How does the winter effluent soluble substrate concentration compare to that from the CMAS system in Study Question 16?
24. For the problem considered in Study Question 16, design a CSAS system to produce an effluent with a concentration of readily biodegradable substrate of less than 5 mg/L as COD, while maintaining an SRT of 7 days. For this design assume that the contact tank MLSS concentration will be 2,500 mg/L as TSS and that the stabilization basin MLSS concentration will be 8,000 mg/L as TSS. Use a diffused air oxygen transfer system and assume that the in-process oxygen transfer efficiency is 12%. Size the contact and stabilization basins and check the oxygen transfer, mixing, and floe shear requirements.
25. Use a computer code implementing ASM No. 1 or a similar model to determine the distribution of the steady-state oxygen requirement between the contact and stabilization basins for Study Question 24 and compare the requirements to the estimates made in that study question.
26. An SBRAS process is to be designed to treat the wastewater defined in Study Question 16. An effective SRT of 15 days is to be used, and the SVI is expected to be less than or equal to 120 mL/g. Develop a plot that demonstrates the effect on the total SBR volume of the number of operating cycles per day and the fraction of the total bioreactor cycle devoted to fill plus react.
27. Discuss the factors that must be considered when selecting a wasting frequency for an activated sludge system.
28. Define each of the following terms and tell what each indicates about the operation of an activated sludge system: clumping, ashing, pin floe, straggler floe, and dispersed growth.
29. Discuss the factors that must be considered during selection of the dosing rate and dose point for application of chlorine to control filamentous sludge bulking.
30. Why is SRT control considered to be a long-term operational strategy?
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