December 2007, Vol. 19, No.12
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:
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.
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.
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.
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.
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Questions were developed by Steve Spicer and reviewed by the Association of Boards of Certification (Ames, Iowa) Validation and Examination Committee.
Water Environment Federation (2002). Activated Sludge; Manual of Practice No. OM-9, Second Edition. Alexandria, Va.: Water Environment Federation.