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December 2007, Vol. 19, No.12
Certification Quiz
Test Your Knowledge of Activated Sludge
True or False Questions:
- High-rate activated sludge processes typically have a mean cell residence time of 5 to 15 days.
- Typical F:M values for nitrification are 0.25 to 0.45 d-1.
- If an activated sludge is not settling or compacting well, and the SVI is greater than 150 mL/g, there may be excessive growth of filamentous organisms.
- In a diffused-air system, violent turbulence on the surface of the water in the biological reactor tank indicates high dissolved-oxygen levels.
- The presence of a very dark or black foam in the biological reactor tank indicates either insufficient aeration, which results in anaerobic conditions, or that industrial wastes, such as dyes and inks, are present.
- Typically, in a well-operated bioreactor, 10% to 25% of the surface will be covered with a 2- to 3-in. layer of light tan foam.
Multiple Choice Questions:
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Which of the answers below are probable causes for filamentous sludge bulking?
A. Insufficient nutrients.
B. Uniform organic loading.
C. Low influent sulfide concentrations.
D. High dissolved-oxygen concentrations in biological reactors.
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What is the average SRT range for a single-stage nitrification system when temperatures range from 60°F to 70°F?
A. 1 to 3 days.
B. 5 to 7 days.
C. 8 to 10 days.
D. 12 or more days.
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What is the peak clarifier surface overflow rate for the facility with two clarifiers that is described in the table? Clarifier’s surface area each = 7850 ft2. |
Average plant flow = 7.5 mgd. |
Peak plant flow = 15.0 mgd. |
A. 753 gal/d•ft2.
B. 827 gal/d•ft2.
C. 955 gal/d•ft2.
D. 1029 gal/d•ft2.
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What is the solids loading rate for the facility described in the table?
Total clarifier surface area = 15,700 ft2. |
Plant flow = 7.5 mgd. |
Return flow rate = 3.8 mgd. |
MLSS = 2500 mg/L. |
A. 0.25 lb MLSS/h/ft2.
B. 0.63 lb MLSS/h/ft2.
C. 0.79 lb MLSS/h/ft2.
D. 1.19 lb MLSS/h/ft2.
Questions were developed by Steve Spicer and reviewed by the Association of Boards of Certification (Ames, Iowa) Validation and Examination Committee.
Answer Key:
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1. False. High-rate activated sludge processes typically have a mean cell residence time of 1 to 3 days. A MCRT range of 5 to 15 days is indicative of a conventional activated sludge system.
2. False. Typical F:M values for nitrification are 0.10 d-1 or less. F:M values of 0.25 to 0.45 are typical for systems removing BOD only.
3. True.
4. False. In a diffused-air system, violent turbulence on the surface of the water in the biological reactor tank indicates a broken pipe or diffuser.
5. True.
6. True.
7. a.
8. c.
9. c. Peak surface overflow rate = (plant flow ÷ clarifier surface area). So, peak surface overflow rate = (15,000,000 gal/d ÷ [7850 ft² × 2 clarifiers]) = 955 gal/d•ft².
10. b. Solids loading rate = [(plant flow + return flow)(conversion factor)] ÷ [(24 h/d)(clarifier surface area)]. So, [(7.5 mgd + 3.8 mgd)(8.34 [lb/mil. gal ÷ mg/L]) ÷ [(24 h/d)(15,700 ft²)] = 0.63 MLSS/h/ft².
References:
Water Environment Federation (2002). Activated Sludge; Manual of Practice No. OM-9, Second Edition. Alexandria, Va.: Water Environment Federation.