July 2012, Vol. 24, No.7

Certification Quiz

Test your knowledge of thickening

    True or False Questions:
  1. The purpose of sludge thickening is to decrease the volume of sludge and increase the efficiencies of downstream processes.
  2. Both gravity belt thickening and rotary-drum thickening concentrate solids through filtration.
  3. In a gravity thickener, the depth of the sludge blanket varies with temperature, and higher temperatures require more-shallow blankets to promote compaction thickening.
  4. As a rule, positive-displacement pumps create more shear on solids. This shear is why positive-displacement pumps are not preferred for thickening and dewatering.

  5. Multiple Choice Questions:
  6. What is a possible reason for a gravity thickener not performing well?
  7. A. Overflow rate is too high
    B. Thickened sludge pumping rate is too high
    C. Short-circuiting of flow through tank
    D. All of the above

  8. What is the normal retention time range for primary sludge in a gravity thickener, depending on the temperature?
  9. A. 0.5 to 1 days
    B. 1 to 2 days
    C. 2 to 3 days
    D. 3 to 5 days

  10. If septicity (floating solids, foul odors, and reduced underflow currents) occurs after solids have been stored too long in a gravity thickener, which of the following can be done to help address the problem?
  11. A. Elutriation of the thickener feed
    B. Aeration of the thickener feed
    C. Addition of chlorine or hydrogen peroxide
    D. All of the above

  12. What should be the solids feed temperature range in a gravity thickener operation if the secondary-to-primary sludge ratio ranges between 4:1 and 6:1?
  13. A. 0°C to 5°C (32°F to 41°F)
    B. 5°C to 10ºC (4°F to 50°F)
    C. 15°C to 20°C (59°F to 69°F)
    D. 25°C to 30°C (77°C to 86°C)

  14. What is the minimum underflow pumping velocity to prevent solids deposition during thickened sludge pumping?
  15. A. 0.3 m/s (1 ft/s)
    B. 0.46 m/s (1.5 ft/s)
    C. 0.6 m/s (2 ft/s)
    D. 0.76 m/s (2.5 ft/s)

Questions developed by LaShell Stratton-Childers and reviewed by the Operations forum editorial advisory board and the Association of Boards of Certification (Ankeny, Iowa) Wastewater Treatment Validation and Examination Committee. 

Answer Key:
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