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 to raise first-pass DO
   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

   During low night flow, a trickling-filter distributor slows, parts of the media dry, and morning soluble BOD rises. The nozzles and underdrains are clear, and the recirculation pump has capacity. What control trial is best?

   1. Raise recirculation to the pump's normal daytime rate, then trend motor amperage and end the trial when the distributor reaches daytime speed.
   2. Lower recirculation to increase media contact time, then trend morning soluble BOD before changing the setting again.
   3. Keep recirculation at the current setting, clean the already-clear nozzles on each shift, and review effluent quality after one week.
   4. Raise recirculation enough to restore even wetting, then trend distributor movement and effluent quality.
   Show the answer

   Answer: D. Raise recirculation enough to restore even wetting, then trend distributor movement and effluent quality.

   Recirculation can add the hydraulic load needed for steady distributor movement and even media wetting, so a controlled increase with trend review addresses the observed cause. It is like adding enough flow to keep a rotating sprinkler moving across the full bed. A targets daytime speed instead of even wetting and effluent response, B reduces the hydraulic load, and C repeats a nozzle check after the nozzles were found clear.

5. Question 5

   Ponding expands across a trickling filter at its normal hydraulic load. Distributor coverage is even, underdrains discharge freely, and matted growth and solids cover the media surface. Which diagnosis best fits the evidence?

   1. An underdrain blockage is backing water into laterals that still discharge freely.
   2. Uneven distributor speed is leaving dry sectors beneath several spray nozzles.
   3. Surface plugging is limiting drainage through the media.
   4. A failed recirculation pump is doubling the applied hydraulic load above normal.
   Show the answer

   Answer: C. Surface plugging is limiting drainage through the media.

   Matted surface material with free underdrain flow points to plugging near the top of the media. An underdrain restriction is a tempting cause of ponding, but the free discharge argues against it, like a clogged coffee filter above an open drain. B conflicts with even coverage, and D conflicts with the normal hydraulic load.

Operator Master has 15 Level 4 · Expert biological treatment questions, free.

• Plant Operations60 questions
• Math and Calculations55 questions
• Laboratory22 questions
• Regulations and Compliance15 questions
• Safety12 questions
• Solids Handling11 questions
• Disinfection9 questions
• Pumps and Hydraulics6 questions
• Secondary Clarification6 questions
• Headworks and Screening5 questions
• Primary Treatment3 questions
• Grit Removal2 questions