February 2007, Vol. 19, No.2

Certification Quiz

Test Your Knowledge of Biosolids

    True or False Questions:
  1. T F Settling primary sludge can increase total solids concentration to between 4% and 7%.
  2. T F In a two-stage anaerobic digester, almost all stabilization and gas production occur in the primary tank.
  3. T F In a two-stage anaerobic digester, the secondary tank serves four purposes: as a storage vessel for digested solids, as a standby primary tank, as a source for seed, and, in some cases, as a quiescent basin for supernatant withdrawal.
  4. T F Continuously feeding solids into an anaerobic digester is the worst possible feeding schedule.
  5. T F Solids should be withdrawn from the top of the digester.

  6. Multiple Choice Questions:
  7. What are the benefits of egg-shaped digesters?

  8. A. The cone-shaped top prevents foaming.
    B. The shape eliminates corners, thus preventing dead corners for mixing, and the steep slope at the bottom reduces grit accumulation.
    C. For the same capacity, egg-shaped digesters cost less to construct than large-diameter cylindrical tanks with slightly sloped bottoms.
    D. They are easily transported.

  9. The ratio of air to digester gas must be correct for combustion to occur. At what concentration is digester gas typically explosive?

  10. A. 1 volume of gas in 15 volumes of air.
    B. 1 volume of gas in 12 volumes of air
    C. 1 volume of gas in 5 volumes of air.
    D. 1 volume of gas in 2 volumes of air.

  11. Stabilization of liquid sludge by lime addition typically requires that pH be maintained at what level and for how long?

  12. A. pH 9 held for 1 hour.
    B. pH 10 held for 5 hours.
    C. pH 11 held for 3 hours.
    D. pH 12 held for 2 hours.


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.