December 2011, Vol. 23, No.12

Certification Quiz

Test your knowledge of clarifiers

    True or False Questions:
  1. A clarifier is a confined space.
  2. Wastewater enters peripheral feed clarifiers at the center and flows out to the edge.
  3. The sludge blanket depth in an optimally designed secondary clarifier should be limited to 0.3 to 0.6 m (1 to 2 ft).

  4. Multiple Choice Questions:
  5. The weekly operating report for secondary clarification should include which of the following?
  6. A. Clarifier loading
    B. Return sludge flow
    C. Temperature
    D. pH

  7. A significant portion of odor emissions from a primary clarifier come from which of the following conditions?
  8. A. Low solids capture
    B. Hydraulic overload
    C. Excess oxygen release in the primary sludge
    D. Turbulence at the effluent weir

  9. Optimizing primary clarifier performance could result in what?
  10. A. Reduced dewatered solids concentration
    B. Starving the activated sludge system
    C. Increased aerobic digester-gas production
    D. Improved nitrification in the activated sludge system

  11. The process control goal is to find a blanket or level that provides a thick sludge without adversely affecting tank-removal efficiency, overloading collector equipment, or what?
  12. A. Creating a thick scum
    B. Decreasing the sludge pH
    C. Allowing decomposition in the bottom of the clarifier
    D. Shearing the sludge

  13. Using the following data, calculate the sludge volume index in mL/g.

    Settled sludge volume (mL/L)


    Mixed liquor suspended solids (mg/L)



  14. A. 1.4 mL/g
    B. 140 mL/g
    C. 7.3 mL/g
    D. 280 mL/g

  15. Using the following data, calculate how many kilograms of primary sludge should be removed per day.

    Influent flow (ML/d)


    Influent total suspended solids (mg/L)


    Primary effluent total suspended solids (mg/L)



  16. A. 2280 kg/d
    B. 5320 kg/d
    C. 7610 kg/d
    D. 19,100 kg/d

  17. Using the following data, calculate the final clarifier solids loading rate. 

    Wastewater flow (m3/d)


    Return activated sludge flow (m3/d)


    Mixed liquor suspended solids (mg/L)


    Clarifier surface area (m2)



  18. A. 149.5 kg/d•m²
    B. 345.2 kg/d•m²
    C. 498.7 kg/d•m²
    D. 996.1 kg/d•m²

Questions written by Jorj Long, deputy director of Operations Technology at Black & Veatch (Overland Park, Kan.) and reviewed by the Operations forum Editorial Advisory Board and the Association of Boards of Certification (Ankeny, Iowa) Wastewater Treatment Validation and Examination Committee. 

©2011 Water Environment Federation. All rights reserved.

Answer Key:
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Water Environment Federation (2008). Operation of Municipal Wastewater Treatment Plants; Manual of Practice No. 11, Sixth Edition. Alexandria, Va.: Water Environment Federation.