December 2007, Vol. 19, No.12

Certification Quiz

Test Your Knowledge of Activated Sludge

    True or False Questions:
  1. High-rate activated sludge processes typically have a mean cell residence time of 5 to 15 days.
  2. Typical F:M values for nitrification are 0.25 to 0.45 d-1.
  3. 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.
  4. In a diffused-air system, violent turbulence on the surface of the water in the biological reactor tank indicates high dissolved-oxygen levels.
  5. 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.
  6. 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.

  7. Multiple Choice Questions:
  8. Which of the answers below are probable causes for filamentous sludge bulking? 
  9. A. Insufficient nutrients.
    B. Uniform organic loading.
    C. Low influent sulfide concentrations.
    D. High dissolved-oxygen concentrations in biological reactors.

  10. What is the average SRT range for a single-stage nitrification system when temperatures range from 60°F to 70°F? 
  11. A. 1 to 3 days.
    B. 5 to 7 days.
    C. 8 to 10 days.
    D. 12 or more days.

  12. 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.



  13. A. 753 gal/d•ft2.
    B. 827 gal/d•ft2.
    C. 955 gal/d•ft2.
    D. 1029 gal/d•ft2.

  14. 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.




  15. 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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Water Environment Federation (2002). Activated Sludge; Manual of Practice No. OM-9, Second Edition. Alexandria, Va.: Water Environment Federation.