Level 3 · Advanced sample questions
5 free sample questions on biological treatment for the Level 3 · Advanced wastewater treatment operator exam. Try each one, then open the answer to check your work.
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?
Answer: A. Distribution valve and pipe between taps
Pressure rises before the distribution valve but falls after it, locating the added pressure loss between those two measurements. The valve and connecting pipe should be checked for a restriction or incorrect position.
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?
Answer: A. Oxygen carried into the anoxic zone
The loss of nitrate removal begins with increased upstream aeration and a large rise in anoxic-zone DO. Because carbon, nitrate recycle, and mixing are steady, oxygen carryover is the condition most directly limiting denitrification.
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?
Answer: A. Alkalinity addition
Adequate DO and steady load and SRT make oxygen supply and biomass retention less likely limits. Nearly depleted alkalinity and low pH can inhibit nitrification, so alkalinity addition is the first adjustment to evaluate.
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?
Answer: D. Reduce nitrate entering the zone
An anaerobic zone must be free of both dissolved oxygen and nitrate as usable electron acceptors. Because mixing, DO, and influent carbon are steady, reducing the increased nitrate entering with RAS most directly restores the required condition.
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?
Answer: B. Nitrite oxidation became limited by oxygen
The change follows the lower DO setpoint while pH, temperature, SRT, and loading stay normal. Continued low ammonia shows ammonia oxidation is occurring, but rising nitrite indicates the nitrite-oxidation step has become oxygen-limited.
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