August 2011, Vol. 23, No.8

Certification Quiz

Test your knowledge of thickening and dewatering

    True or False Questions:
  1. Generally, depending on the thickening method, thickening increases the solids content of sludge to between 4% and 10%.

  2. Multiple Choice Questions:
  3. Which of  the following is not true about the operation of a gravity belt thickener (GBT)?

  4. A. GBTs concentrate biosolids as water drains by gravity from the influent end.
    B. For optimum performance, chemical conditioning of the feed sludge typically is required.
    C. Water flows through the belt and collects as filtrate.
    D. For process control of the GBT, maintain an air-to-solids ratio of 0.005 to 0.006.

  5. A facility is dewatering a blend of primary and waste activated sludge to a total solids concentration of 16% with a vacuum filter using lime and ferric chloride for conditioning. The incinerator is using natural gas fuel at a rate of 1.055 kJ per ton of volatile sludge solids. What can be done to optimize the system and make the incinerator self-sustaining so that fuel does not need to be purchased? 

  6. A. Increase the feed rate and maintain a constant combustion airflow.
    B. Replace dewatering equipment with another type of unit to increase incinerator feed concentration to higher than 22% total solids.
    C. Add polymer to the incinerator for conditioning.
    D. Decrease the combustion airflow in the incinerator while maintaining a constant feed.

  7. Three organic polymers were evaluated for use in a centrifuge dewatering application. The polymer doses and costs are shown below. The performance for each polymer at these doses was equivalent. Which polymer has the lowest cost? 


    Dose (kg/Mg)

    Cost ($/kg)

    Polymer A



    Polymer B



    Polymer C



  8. A. Polymer A
    B. Polymer B
    C. Polymer C

  9. An operator is dewatering anaerobically digested biosolids on a 1-m belt filter press. The feed concentration is 2.5% solids, and the digested biosolids flow rate is 13.63 m3/h. What is the solids loading rate? 

  10. A. 5.45 kg/h
    B. 159 kg/h
    C. 341 kg/h
    D. 5450 kg/h

  11. Given the following data, determine the solids capture efficiency for a centrifuge.

    Feed sludge concentration


    Cake solids


    Centrate solids concentration


  12. A. 92.5%
    B. 93.0%
    C. 94.7%
    D. 95.5%

  13. A wastewater treatment plant uses a filter press to dewater anaerobically digested biosolids. Each filter press cycle will process 1 Mg of biosolids and conditioning chemicals. The typical ferric chloride and lime dosages are 50 and 250 kg/Mg, respectively. If the plant must dewater 3.5 Mg/d of biosolids, how many filter press runs are required per day?

  14. A. 3
    B. 4
    C. 5
    D. 6

  15. For 45 kg of biosolids based on dry weight, what is the total weight of solids that must be handled if the solids content of the material is 3% (after stabilization), 20% (after dewatering), and 75% (after drying)?


  16. A. 1500 kg, 225 kg, 60 kg
    B. 2100 kg, 225 kg, 85 kg
    C. 1100 kg, 158 kg, 55 kg

  17. A wastewater treatment plant dewaters 9.25 Mg/d of primary and secondary solids. The dewatering equipment produces cake with a total solids concentration of 20% and captures 98% of the solids feed. How much dewatered cake is produced per day?

  18. A. 33.8 Mg
    B. 45.3 Mg
    C. 51.6 Mg
    D. 52.3 Mg

Questions excerpted from the Operation of Municipal Wastewater Treatment Plants Study Guide, published in 2007 by the Water Environment Federation (Alexandria, Va.). 

Answer Key:
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