Primary Treatment Practice Questions for Level 2 · Intermediate

1. Question 1

   Primary sludge in a hopper turns black, gas bubbles appear, and odors increase. What is the likely cause?

   1. Anaerobic breakdown of stored sludge.
   2. Hydraulic washout of settled solids.
   3. Excess surface skimming of floating scum.
   4. Dilution of sludge by high influent flow.
   Show the answer

   Answer: A. Anaerobic breakdown of stored sludge.

   Primary sludge can turn septic when it remains in the hopper too long. Anaerobic breakdown can darken the sludge and produce gas and odors.

2. Question 2

   What is the primary purpose of a scum baffle located ahead of a clarifier effluent weir?

   1. Direct settled sludge toward the center hopper.
   2. Increase the velocity beneath the inlet structure.
   3. Keep floating material from leaving with the effluent.
   4. Measure the depth of the sludge blanket.
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   Answer: C. Keep floating material from leaving with the effluent.

   The scum baffle blocks floating grease, foam, and debris from passing over the effluent weir.

3. Question 3

   Heavy scum repeatedly collects on a primary clarifier. What influent condition commonly contributes to it?

   1. A high load of dense mineral solids
   2. A high load of dissolved, nonfloatable material
   3. A low load of floatable organic material
   4. A high load of fats, oils, and grease
   Show the answer

   Answer: D. A high load of fats, oils, and grease

   Fats, oils, and grease are less dense than water and tend to float, so elevated influent FOG commonly increases primary scum.

4. Question 4

   A primary clarifier blanket rises between two large daily sludge withdrawals, then drops quickly after each withdrawal. Effluent quality and collector operation remain normal. Which change best addresses the pattern without changing daily sludge volume?

   1. Use smaller, frequent draws
   2. Use larger, less frequent draws
   3. Lower the sludge collector speed
   4. Raise the surface-skimmer speed
   Show the answer

   Answer: A. Use smaller, frequent draws

   Smaller, more frequent withdrawals limit blanket buildup while preserving the same total daily sludge volume. B increases the time between draws, C changes a collector that is working normally, and D changes scum removal rather than sludge withdrawal.

5. Question 5

   A two-hopper primary clarifier has a high blanket over Hopper 2. The common sludge pump delivers its normal 240 gpm. During a 10 min withdrawal, both branch valves receive a 100% open command, but local stem travel at Hopper 2 stops at 35% of the travel measured at Hopper 1. What should the operator inspect first?

   1. Inspect the skimmer-drive setting
   2. Inspect the common sludge-pump impeller
   3. Inspect Hopper 2 branch-line restriction
   4. Inspect Hopper 2 actuator linkage
   Show the answer

   Answer: D. Inspect Hopper 2 actuator linkage

   Both branches receive a 100% open command, but Hopper 2 reaches only 35% of the stem travel measured at Hopper 1, so its actuator linkage should be inspected. A addresses surface scum, B addresses common pump capacity, and C addresses a branch restriction; those are real nearby causes, but the local travel reading isolates the valve mechanism first.

Operator Master has 8 Level 2 · Intermediate primary treatment questions, free.

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