June 2009, Vol. 21, No.6

Certification Quiz

Test Your Knowledge of Activated Sludge

    True or False Questions:
  1. T F Color can be indicative of activated sludge health.
  2. T F An overabundance of algae in an activated sludge plant may indicate low nutrient levels.
  3. T F Settled bacteria in an aeration basin provide much of the treatment.
  4. T F The activated sludge process is an anaerobic, suspended growth biological treatment method.
  5. T F In the activated sludge process, the wastewater oxygen demand may be separated into two categories: carbonaceous and nitrogenous.
  6. T F A well-designed and operated activated sludge process, treating typical municipal wastewater, should achieve a CBOD effluent quality of 5 to 15 mg/L.
  7. T F The loading on a nitrification system consists of ammonia plus organic nitrogen and is typically referred to as total nitrogen.

  8. Multiple Choice Questions:
  9. Mesophilic bacteria present in the activated sludge aeration basin thrive in what temperature range?
  10. A. 65°F –75°F.
    B. 70°F –80°F.
    C. 80°F –90°F.
    D. 115°F –125°F.

  11. Why are chemicals added to the activated sludge process?
  12. A. To control desirable microorganisms.
    B. To decrease the amount of required nutrients, such as nitrogen.
    C. To provide a nonbuffering solution.
    D. To improve clarification.

  13. What are the basic components of the conventional activated sludge process?
  14. A. Thickeners and digesters.
    B. Screens and classifiers.
    C. Sand filters and chlorine contact chambers.
    D. Biological reactors and clarifiers.

  15. To prevent excessive solids buildup in the activated sludge process, what must be done to some of the solids in the process?
  16. A. They must be chlorinated.
    B. They must be returned to the headworks.
    C. They must be wasted.
    D. They must be discharged with effluent.

  17. Which of the following is an example of an activated sludge process?
  18. A. Trickling filter.
    B. Sequencing batch reactor.
    C. Rotating biological contactor.
    D. Secondary aerobic biofilter.

  19. Poor sludge settling as a result of filamentous organisms has been associated with which of the following dissolved-oxygen concentrations in mixed liquor?
  20. A. More than 3 mg/L.
    B. More than 1 mg/L.
    C. Less than 3 mg/L.
    D. Less than 1 mg/L.

  21. What does a high mixed liquor respiration rate (or SOUR) describe?
  22. A. A young sludge that is underoxidized.
    B. An old sludge that is overoxidized.
    C. A possible slug load.
    D. a and c.

  23. In an activated sludge system, what is perhaps the most important parameter affecting biological activity?
  24. A. pH.
    B. Alkalinity.
    C. Dissolved oxygen.
    D. Temperature.

  25. Given the following data, determine the SVI.

    Mixed liquor suspended solids 4500 mg/L
    30-minute settleability 450 mL/L
  26. A. 80 mL/g.
    B. 100 mL/g.
    C. 120 mL/g.
    D. 130 mL/g.

  27. A conventional activated sludge plant has a flow of 1.5 mgd. If the desired return rate is to be 15% of the flow, how many gallons of return activated sludge should be pumped back to the aeration basin?
  28. A. 0.225 mgd.
    B. 0.150 mgd.
    C. 0.125 mgd.
    D. 0.100 mgd.

  29. Given the following information, calculate the MCRT.

    Influent TSS  315 mg/L
    Waste activated sludge total suspended solids  6300 mg/L
    Mixed liquor suspended solids  3300 mg/L
    Effluent total suspended solids  15 mg/L
    Influent flow 7.5 mgd
    Waste activated sludge flow 0.06 mgd
    Primary clarifier volume 0.4 million gal
    Aeration basin volume 1.2 million gal
    Secondary clarifier volume  0.33 million gal
  30. A. 6 days.
    B. 8 days.
    C. 12 days.
    D. 10 days.

Questions were developed by Ron Moeller, operations specialist at Kennedy/Jenks Consultants (San Francisco), and Ann Nuttall, utility systems operations administrator of water pollution control for the City of Tulsa, Okla. The Association of Boards of Certification (Ames, Iowa) Validation and Examination Committee reviewed the questions.  

Answer Key:
Click here to show or hide the answer key


Junkins, Randy, Kevin Deeny, and Thomas Eckhoff (1983). The Activated Sludge Process: Fundamentals of Operation. West Chester, Pa.: Ann Arbor Science.

Arthur, Robert M. and staff of Arthur Technology (1983). Procedures and Practices in Activated Sludge Process Control. Fond du Lac, Wis.: Ann Arbor Science.

Water Environment Federation (2001). Activated Sludge Process Control Training Manual. Alexandria, Va.: Water Environment Federation.

Water Environment Federation (1996). Operation of Municipal Wastewater Treatment Plants; Manual of Practice No. 11, Fifth Ed. Alexandria, Va.: Water Environment Federation.