Biological Treatment Practice Questions for Level 4 · Expert

1. Question 1

   A step-feed aeration basin meets total airflow demand, but the first pass has low DO and rising ammonia while the final pass has 4.5 mg/L DO and low ammonia. Blower output is at its approved limit. What adjustment should be tested first?

   1. Shift air from the final pass to the first pass and trend ammonia
   2. Increase total blower output above its approved limit
   3. Reduce RAS until the first-pass DO begins to rise
   4. Add supplemental carbon to the final aeration pass
   Show the answer

   Answer: A. Shift air from the final pass to the first pass and trend ammonia

   Total air is limited, but the last pass has excess oxygen while the first pass shows the treatment need. Rebalancing air tests whether existing capacity can support ammonia removal where demand is highest.

2. Question 2

   After a temperature drop, Train 1 ammonia rises and its SRT is 8 days; Train 2 remains stable at 13 days. DO and settling are acceptable. Total plant wasting must remain unchanged for the next three days because solids storage is full. What is the best temporary plan?

   1. Increase RAS in both trains and hold wasting steady
   2. Raise DO in Train 1 and waste both trains equally
   3. Shift wasting toward Train 2 and track both SRTs
   4. Increase Train 2 wasting without reducing Train 1 wasting
   Show the answer

   Answer: C. Shift wasting toward Train 2 and track both SRTs

   The plant must hold total wasting constant while protecting the colder, short-SRT train. Redistributing wasting temporarily supports nitrifier inventory without increasing total solids production.

3. Question 3

   An internal recycle increase lowers final nitrate, which was already below target, but raises anoxic-zone DO from 0.2 to 0.9 mg/L. The plant's anoxic operating limit is 0.5 mg/L. Carbon feed is unchanged, and ammonia removal remains complete. What is the best next adjustment?

   1. Increase the recycle again
   2. Back the recycle down and compare nitrate removal
   3. Raise aerobic DO to increase nitrate supply to the anoxic zone
   4. Move supplemental carbon to the aerobic zone and compare nitrate
   Show the answer

   Answer: B. Back the recycle down and compare nitrate removal

   The higher recycle improved nitrate delivery but carried enough oxygen to interfere with anoxic conditions. A controlled reduction tests the balance.

4. Question 4

   Two centrifugal blowers share one air header. At low demand, both remain online near their minimum stable flow. One begins to surge whenever basin valves throttle. Which change best protects the blowers without wasting air?

   1. Raise the header-pressure target
   2. Open basin valves beyond demand
   3. Stage one blower off earlier
   4. Throttle both blower inlets
   Show the answer

   Answer: C. Stage one blower off earlier

5. Question 5

   A two-stage anoxic system has a fixed carbon limit. During a seven-day matched-load trial, moving 20% of the Stage 1 dose to Stage 2 raises nitrate leaving Stage 1 from 80 to 90 lb/day and lowers final nitrate by 45 lb/day. The control plan requires at least 20 lb/day of margin below the 120 lb/day Stage 1 limit, and prior step tests became nonlinear above 90 lb/day. The goal is to test whether more final nitrate removal is available before setting a permanent split. What is the best next step?

   1. Adopt the current split as the permanent setpoint
   2. Move 5% back to Stage 1 and repeat a seven-day trial
   3. Move another 20% to Stage 2 and repeat a seven-day trial
   4. Test a smaller shift within the required margin
   Show the answer

   Answer: D. Test a smaller shift within the required margin

Operator Master has 9 Level 4 · Expert biological treatment questions, free.

• Plant Operations35 questions
• Solids Handling11 questions
• Disinfection7 questions
• Laboratory7 questions
• Math and Calculations5 questions
• Pumps and Hydraulics5 questions
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• Grit Removal2 questions
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• Primary Treatment2 questions
• Regulations and Compliance2 questions