Level 3 · Advanced sample questions
5 free sample questions on disinfection for the Level 3 · Advanced wastewater treatment operator exam. Try each one, then open the answer to check your work.
One UV bank shows lower intensity than the others. UV transmittance, channel level, lamp power, and lamp age are similar across all banks. A manual sleeve cleaning restores the affected bank, but its automatic cleaning cycle does not. What should be checked first?
Answer: B. Automatic sleeve cleaner
Manual cleaning restores intensity, showing that sleeve fouling is the immediate cause and that the lamps can still produce normal output. Failure of the automatic cycle to produce the same result points to the sleeve-cleaning mechanism.
Chlorine feed rate and contact-tank flow stay steady, but residual at the tank outlet falls. A new influent sample has higher ammonia, and the residual measured just after dosing also falls. Which explanation best fits the change?
Answer: A. Increased chlorine demand
Higher ammonia reacts with the applied chlorine, so more of it is used up and the residual is already lower right after dosing. Because feed rate and flow stay steady, increased chlorine demand explains the change before contact-tank hydraulics could affect it.
A sulfur dioxide ejector discharges into an effluent wet well. Gas feed falls as the wet-well level rises and returns when the level falls. Ejector inlet-water pressure stays steady, the gas supply is adequate, and the vacuum tubing passes its leak test. What should be evaluated first?
Answer: A. Ejector discharge backpressure
Gas feed changes with wet-well level while inlet-water pressure, gas supply, and vacuum tubing remain satisfactory. A higher wet-well level increases pressure against the ejector discharge, so discharge backpressure should be evaluated first.
A chlorine contact-basin dye test produces an early peak followed by a long, weak tail. Actual flow and operating volume match the detention-time calculation, and release and sampling times were checked. What best explains the result?
Answer: B. Part of the flow takes a short path through the basin
Verified flow, volume, and timing make calculation or recording errors unlikely. An early tracer peak indicates that some water reaches the outlet quickly, while the long tail indicates slower zones, a pattern consistent with short-circuiting and uneven basin hydraulics.
Effluent flow falls while the dechlorination pump stays at a fixed rate. Chlorine residual before dechlorination remains on target. Grab tests confirm rising sulfite throughout the outlet channel, with no detectable final chlorine. Which adjustment best fits these results?
Answer: A. Reduce sulfite feed to match the lower effluent flow
Chlorine entering dechlorination remains on target, but a fixed sulfite rate is being applied to less water. Confirmed excess sulfite and no final chlorine show overfeed, so the sulfite rate should be reduced to match the lower effluent flow.
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