Biological Treatment Practice Questions for Level 3 · Advanced

1. Question 1

   An aeration blower's air delivery falls at the same speed. Inlet filter pressure drop is normal, discharge pressure rises, and the header pressure after the distribution valve falls. Which area should be checked first?

   1. Distribution valve and pipe between taps
   2. Inlet filter and intake duct before the blower
   3. Blower motor controls and speed feedback
   4. Air header downstream of the second pressure tap
   Show the answer

   Answer: A. Distribution valve and pipe between taps

2. Question 2

   Nitrate removal in an anoxic zone falls after aeration is increased upstream. Anoxic DO rises from 0.2 to 1.8 mg/L. Influent carbon and internal recycle flow are steady, and the mixer is running. What is the most likely limit on denitrification?

   1. Oxygen carried into the anoxic zone
   2. Too little biodegradable carbon entering
   3. Too little nitrate returned by recycle
   4. Poor mixing in the anoxic basin
   Show the answer

   Answer: A. Oxygen carried into the anoxic zone

3. Question 3

   Effluent ammonia rises while aeration DO remains adequate. Mixed-liquor pH has fallen to 6.3, and alkalinity leaving the aeration basin is nearly depleted. Influent load and solids retention time are steady. What should be adjusted first?

   1. Alkalinity addition
   2. RAS flow
   3. WAS rate
   4. Internal recycle flow
   Show the answer

   Answer: A. Alkalinity addition

4. Question 4

   Phosphorus removal worsens after nitrate in the return sludge increases. The anaerobic-zone mixer works, DO remains near zero, and influent readily biodegradable carbon is steady. Which change most directly restores the anaerobic condition?

   1. Raise aerobic-zone DO
   2. Increase the final clarifier RAS rate
   3. Increase influent carbon to the anaerobic zone
   4. Reduce nitrate entering the zone
   Show the answer

   Answer: D. Reduce nitrate entering the zone

5. Question 5

   A plant lowers its aeration DO setpoint. Nitrite then rises while effluent ammonia remains low. Actual DO matches the new setpoint, and pH, temperature, solids retention time, and ammonia loading remain at their normal values. What best explains the change?

   1. Higher WAS rate
   2. Nitrite oxidation became limited by oxygen
   3. Falling pH inhibited the nitrifying organisms
   4. Higher ammonia loading exceeded treatment capacity
   Show the answer

   Answer: B. Nitrite oxidation became limited by oxygen

Operator Master has 7 Level 3 · Advanced biological treatment questions, free.

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