Secondary Clarification Practice Questions for Level 2 · Intermediate

1. Question 1

   An operator transfers a hazardous chemical into a spray bottle for another shift to use later. What is required?

   1. Leave it unmarked if the original container remains nearby.
   2. Add a workplace label with its identity and hazards.
   3. Write the transfer date and the operator's initials only.
   4. Store it in a locked cabinet until the next shift arrives.
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   Answer: B. Add a workplace label with its identity and hazards.

   The portable-container exception applies only to immediate use by the employee who made the transfer. A bottle left for another shift needs a workplace label.

2. Question 2

   Which observation most clearly distinguishes denitrification-related rising sludge from filamentous bulking in a secondary clarifier?

   1. Fine solids remain suspended after the larger floc settles.
   2. Gas bubbles lift previously settled sludge toward the surface.
   3. Light floc settles slowly throughout the settling test.
   4. Dense floc settles rapidly and forms a compact blanket.
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   Answer: B. Gas bubbles lift previously settled sludge toward the surface.

   Denitrification can form nitrogen gas within settled sludge, causing bubbles to lift the sludge after it initially settles.

3. Question 3

   RAS withdrawal from a secondary clarifier increases for a short period while influent flow, solids loading, and settling remain stable. Which blanket response is most likely before a new equilibrium develops?

   1. The blanket moves lower because solids leave the clarifier faster.
   2. The blanket moves higher because return flow adds solids to the clarifier.
   3. The blanket moves higher because faster withdrawal improves thickening.
   4. The blanket stays at the same depth because underflow cannot affect inventory.
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   Answer: A. The blanket moves lower because solids leave the clarifier faster.

   Increasing underflow withdrawal generally lowers the blanket in the short term when other conditions remain stable.

4. Question 4

   Secondary-clarifier underflow withdrawal increases while feed, blanket depth, and settling remain comparable. Which usual RAS response is expected?

   1. Concentration decreases as less thickening occurs before withdrawal.
   2. Concentration increases as less thickening occurs before withdrawal.
   3. Concentration decreases because solids remain in the blanket longer.
   4. Concentration increases because the withdrawal rate removes more water only.
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   Answer: A. Concentration decreases as less thickening occurs before withdrawal.

   Faster underflow withdrawal generally reduces thickening time and can dilute RAS concentration.

5. Question 5

   RAS from a secondary clarifier becomes black and develops an offensive odor. Aeration-basin mixed liquor remains brown, basin DO is within its normal range, the clarifier blanket has climbed over two shifts, and verified RAS flow is below normal. Which condition best explains the observations?

   1. A toxic influent is suppressing biological activity throughout the aeration basin.
   2. Excessive RAS withdrawal is returning sludge before stable settling can occur.
   3. Extended clarifier detention is allowing the return sludge to become septic.
   4. Denitrification is lifting settled sludge after nitrate forms in the blanket.
   Show the answer

   Answer: C. Extended clarifier detention is allowing the return sludge to become septic.

   Brown basin mixed liquor and normal basin DO make a basin-wide biological failure less likely. The rising blanket and low verified RAS flow increase solids detention in the clarifier, which can allow the return sludge to become septic, black, and offensive.

Operator Master has 8 Level 2 · Intermediate secondary clarification questions, free.

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