Secondary Clarification Practice Questions for Level 4 · Expert

1. Question 1

   During warm afternoons, one of two secondary clarifiers develops a narrow surface current from its feedwell to one section of the effluent weir. TSS rises only at that section. Blanket depth, settleability, total flow, and RAS remain normal. Temperature profiling shows incoming mixed liquor is 5°F warmer than the clarifier contents. The parallel clarifier has an energy-dissipating inlet and no surface current. Which trial best tests the suspected cause?

   1. Increase RAS from both clarifiers and compare their blanket depths throughout the afternoon.
   2. Lower plant wasting and compare mixed-liquor solids the next day.
   3. Add polymer to both trains and compare settling rates and localized effluent TSS.
   4. Test an approved inlet-energy-dissipation setup and map temperature and TSS.
   Show the answer

   Answer: D. Test an approved inlet-energy-dissipation setup and map temperature and TSS.

   The localized warm inflow and surface current point to a density-current problem. Testing inlet energy dissipation while mapping temperature and TSS directly evaluates that mechanism.

2. Question 2

   A secondary clarifier's RAS valve closes briefly whenever the sludge collector reverses. The blanket rises during each event and falls after RAS flow resumes. The collector must reverse to protect its drive, and the RAS pump remains stable. Which controlled test best isolates the cause of the blanket cycling?

   1. Increase RAS flow for one full cycle while retaining the valve-close command.
   2. Reduce collector-reversal speed for one full cycle while retaining the valve-close command.
   3. Temporarily inhibit the valve-close command for one monitored reversal while retaining the approved collector protection.
   4. Advance the valve-close command so it begins before the collector reversal starts and ends after the reversal is complete.
   Show the answer

   Answer: C. Temporarily inhibit the valve-close command for one monitored reversal while retaining the approved collector protection.

   The repeatable RAS interruption is the change aligned with the blanket cycling. Temporarily inhibiting only that command isolates the suspected cause while retaining collector protection.

3. Question 3

   Peak testing shows secondary clarifier surface overflow rate remains within its approved limit, but solids loading exceeds its limit and blankets rise. RAS capacity, settling, and flow distribution are acceptable, and the process cannot reduce MLSS without losing required solids retention time. Which capital option best addresses the demonstrated constraint?

   1. Install larger RAS pumps while keeping the same clarifier area.
   2. Deepen the existing clarifiers without adding surface area.
   3. Add effluent-weir length while keeping the same solids-separation area.
   4. Add clarifier surface area sized for the peak solids load. ---
   Show the answer

   Answer: D. Add clarifier surface area sized for the peak solids load. ---

   Solids loading is the demonstrated limit. Adding clarifier surface area directly reduces the pounds of solids applied per square foot while the other stated constraints remain acceptable.

4. Question 4

   Filamentous bulking increases after internal recycle is moved ahead of the selector. Selector DO rises, readily biodegradable carbon is unchanged, and total aeration capacity is adequate. Which trial best tests the suspected cause?

   1. Raise final-zone DO for one week
   2. Increase RAS concentration by settling
   3. Move the recycle past the selector
   4. Reduce wasting until the blanket falls
   Show the answer

   Answer: C. Move the recycle past the selector

5. Question 5

   A cold-weather ammonia increase begins after one train loses biomass during maintenance. The other train has complete nitrification and spare aeration, but its clarifier is near the plant's blanket limit. What is the best recovery plan?

   1. Send all influent to the stable train
   2. Stop wasting from both trains
   3. Raise airflow in the damaged train only
   4. Seed the train within loading limits
   Show the answer

   Answer: D. Seed the train within loading limits

Operator Master has 6 Level 4 · Expert secondary clarification questions, free.

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