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

WW secondary: activated sludge: practice questions

Our Water/Wastewater Operator bank has 19 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 · Wastewater Treatment Class I

What is the food-to-microorganism (F/M) ratio?

  1. The ratio of return sludge flow to waste sludge flow in gallons per day
  2. The ratio of chlorine to ammonia
  3. The ratio of air to water
  4. The ratio of incoming BOD load to the mass of microorganisms in the aeration basin
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D. The ratio of incoming BOD load to the mass of microorganisms in the aeration basin — F/M is controlled by wasting.

Question 2 · Wastewater Treatment Class I

What is mean cell residence time (sludge age)?

  1. The time water stays in the secondary clarifier before it flows over the weirs
  2. The time sludge stays in the digester
  3. The average time microorganisms stay in the system
  4. The time water stays in the clarifier
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C. The average time microorganisms stay in the system — Longer sludge age supports nitrification.

Question 3 · Wastewater Treatment Class I

What does a high SVI (e.g., over 150) usually indicate?

  1. Excellent settling with a compact sludge blanket and clear effluent
  2. Poorly settling, bulking sludge, often from filamentous organisms
  3. Too much grit
  4. Low influent flow
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B. Poorly settling, bulking sludge, often from filamentous organisms — Causes include low DO and low F/M.

Question 4 · Wastewater Treatment Class I

What is denitrification?

  1. Converting ammonia to nitrate in an aerated zone
  2. Removing phosphorus with alum
  3. Bacterial conversion of ammonia to nitrite and nitrate in a well-aerated basin
  4. Bacteria converting nitrate to nitrogen gas in an anoxic zone
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D. Bacteria converting nitrate to nitrogen gas in an anoxic zone — Needs a carbon source and little or no DO.

Question 5 · Wastewater Treatment Class I

Which process variation uses long aeration times and low F/M, producing less sludge?

  1. Trickling filter
  2. High-rate activated sludge
  3. Primary clarification
  4. Extended aeration
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D. Extended aeration — Common in small package plants and oxidation ditches.

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