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Water & Wastewater Operator (WPI/ABC Class I) — Practice Exam

WT treatment: filtration: practice questions

Our Water/Wastewater Operator bank has 10 questions on this topic. Here are 5 of them with answers; the full mock exam draws from every topic, like the real one.

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Question 1 · Water Treatment Class I

What is a typical backwash rate for a rapid sand filter?

  1. About 100 gpm per square foot
  2. About 15 to 20 gpm per square foot
  3. About 0.1 gpm per square foot
  4. About 1 gpm per square foot, the same as the filtration rate
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B. About 15 to 20 gpm per square foot — Enough to expand the bed roughly 20–50%.

Question 2 · Water Treatment Class I

What are mudballs in a filter?

  1. Anthracite chips worn smooth
  2. Algae colonies on the surface
  3. Clumps of floc and media that form when backwashing is inadequate
  4. Activated carbon granules that are added to the media to remove taste and odor
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C. Clumps of floc and media that form when backwashing is inadequate — Surface wash or air scour helps prevent them.

Question 3 · Water Treatment Class I

What is the maximum combined filter effluent turbidity allowed at any time for conventional filtration?

  1. 0.3 NTU
  2. 1 NTU
  3. 5 NTU
  4. 10 NTU
Show the answer

B. 1 NTU — 40 CFR 141.173 and 141.551.

Question 4 · Water Treatment Class I

What causes air binding in a filter?

  1. Dissolved air coming out of solution when pressure in the bed drops below atmospheric
  2. Too much backwash water pushed through the underdrain at the start of a run
  3. Too much backwash water entering through the underdrain and lifting the gravel support layers
  4. High water temperature only
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A. Dissolved air coming out of solution when pressure in the bed drops below atmospheric — Negative head lets gas form in the media.

Question 5 · Water Treatment Class I

Which conditions usually signal that a filter needs backwashing?

  1. Head loss reaches its limit, effluent turbidity rises, or a set run time passes
  2. The chlorine residual rises, or the plant flow drops at night
  3. The raw water pH drops below 7
  4. The chlorine residual leaving the clearwell rises above 2 mg/L or the plant flow drops at night
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A. Head loss reaches its limit, effluent turbidity rises, or a set run time passes — Terminal head loss or turbidity breakthrough ends a run.

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