July 2009, Vol. 21, No.7
Test Your Knowledge of Preliminary Treatment
True or False Questions:
- T F If bar spacing in the screening process is too narrow, organics that should be treated by subsequent processes will be captured and removed.
- T F Trash racks, also called bar racks, are constructed of heavy, parallel rectangular or round steel bars with 0.06- to 0.25-in. (2- to 6-mm) spacing set in a channel.
- T F Bar racks are sloped at an angle between 50° and 75° from vertical.
- T F Water in aerated grit tanks is more buoyant than nonaerated water.
Multiple Choice Questions:
Which choice below lists the parts of a screenings press?
A. Decanter, ram, wedge.
B. Ram, dryer, hopper.
C. Ram, hopper, wedge.
D. Wedge, ram, chain.
Why is adequate ventilation required in a grit-removal chamber?
A. To prevent creating a vacuum.
B. To prevent forming a corrosive atmosphere.
C. To promote aeration for biological activity.
D. To promote heavier grits to settle faster.
What is the likely cause of excessive grit in the
A. Flow velocity is too low.
B. Flow velocity is too high.
C. Mechanical rake is moving too slowly.
D. Mechanical rake is moving too fast.
Calculate the approximate monthly screenings-disposal cost for a 10-mgd wastewater treatment plant. Assume that on a typical day, 1.2 yd3, 2.3 yd3, and 0.7 yd3 of compacted screenings were removed during the first, second, and third 8-hour shifts, respectively. The plant pays $45 to dump each 12-ft-long, 4-ft-wide, 5-ft-high container at the landfill.
A. $106 per month.
B. $147 per month.
C. $384 per month.
D. $662 per month.
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Questions were developed by Steve Spicer and reviewed by the Association of Boards of Certification (Ames, Iowa) Validation and Examination Committee.
Water Environment Federation (2007). Operation of Municipal Wastewater Treatment Plants; Manual of Practice No. 11, Fifth Edition. Alexandria, Va.: Water Environment Federation.