November 2007, Vol. 19, No.11

Certification Quiz

Test Your Knowledge of Thickening

    True or False Questions:
  1. Overflow from an efficiently operating gravity thickener typically contains less than 350 mg/L suspended solids.
  2. Gravity thickeners typically will produce underflow sludge ranging from 16% to 22% solids when thickening 100% primary sludge.
  3. Dissolved-air flotation thickening uses air bubbles to increase the density of water, thus enabling solid particles to float more readily.
  4. Mixed liquor sludge volume index for waste activated sludge is one of the principal physical–chemical characteristics measured to predict flotation performance.
  5. Gravity belt thickeners are operated using four main variables: polymer feed rate, solids feed rate, belt speed, and mechanical adjustments.

  6. Multiple Choice Questions:
  7. What percentage of suspended solids typically is removed by dissolved-air flotation?

  8. A. 57% to 63%.
    B. 65% to 70%.
    C. 75% to 80%.
    D. 85% to 99%.

  9. What is the typical range of loading rates for primary sludge into a gravity thickener?

  10. A. 10 to 15 lb/ft2•d (49 to 73 kg/m2•d).
    B. 20 to 30 lb/ft2•d (100 to 150 kg/m2•d).
    C. 35 to 40 lb/ft2•d (171 to 195 kg/m2•d).
    D. 45 to 50 lb/ft2•d (220 to 244 kg/m2•d).

  11. After initial installation of a centrifuge thickener, when should the first inspection of the rotating assembly be conducted?

  12. A. After 500 hours of operation.
    B. After 1000 hours of operation.
    C. After 1500 hours of operation.
    D. After 3000 to 4000 hours of operation.

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