Disinfection Practice Questions for Level 4 · Expert

1. Question 1

   Two hypochlorite pumps alternate weekly. Pump 2 displays 12 gal/hr, but tank drawdown and an independent line-flow test both show 8 gal/hr. Its stroke feedback reads 100%, and contact-basin residual falls when Pump 2 is online. What should be done first?

   1. Transfer to Pump 1, inspect Pump 2, and verify its output
   2. Increase both pumps' residual setpoint through the next peak
   3. Shorten the pump-alternation schedule to one day
   4. Recalibrate the contact-basin residual analyzer before changing pumps
   Show the answer

   Answer: A. Transfer to Pump 1, inspect Pump 2, and verify its output

   Pump 2 cannot deliver the required flow at full stroke. The available pump maintains treatment while Pump 2's suction, check valves, and drive are inspected and its output is confirmed before return.

2. Question 2

   During wet weather, flow rises 25% and UV transmittance drops from 70% to 55%. All duty banks are at maximum output. Starting the standby bank raises predicted dose from 31 to 39 mJ/cm2, while the plant operating minimum is 40 mJ/cm2. The wet-weather plan allows excess secondary effluent to be held in an offline basin. What is the best response?

   1. Start standby and divert enough flow to hold dose
   2. Start standby and accept 39 mJ/cm2 for the event
   3. Lower the operating minimum until transmittance recovers
   4. Increase downstream chlorine without changing UV operation
   Show the answer

   Answer: A. Start standby and divert enough flow to hold dose

   Standby capacity alone does not meet the stated minimum, so the approved offline basin must hold enough flow to restore the required UV dose.

3. Question 3

   A reuse system must maintain a free-chlorine operating target. Its chemical-feed loop currently controls to total chlorine residual. During intermittent ammonia peaks, total residual stays on target, but verified testing shows free chlorine falls and combined chlorine rises. Pump delivery and analyzer calibration are acceptable. What control improvement should be evaluated?

   1. Raise the fixed total-residual setpoint high enough to cover every historical ammonia peak, and keep the existing control loop unchanged.
   2. Validate a control strategy that responds to free chlorine and ammonia conditions, with operating limits and independent checks.
   3. Move the total-residual analyzer upstream of chlorine addition so the loop responds sooner.
   4. Hold chlorine feed constant during ammonia peaks and use final total residual as the only acceptance check.
   Show the answer

   Answer: B. Validate a control strategy that responds to free chlorine and ammonia conditions, with operating limits and independent checks.

   Total residual can remain on target while ammonia converts free chlorine to combined chlorine. A validated strategy using free-chlorine and ammonia conditions controls the parameter that matters.

4. Question 4

   A UV system's dose control assumes 55% transmittance. Verified online and laboratory results have remained near 70%, and delivered dose is consistently above the operating minimum. The operating plan permits a revised control curve after a documented trial. Which trial is best?

   1. Test a lower fixed-output schedule across representative flows, with UVT checks and manual fallback.
   2. Validate a UVT-based dose curve across representative flows, with operating limits and automatic fallback.
   3. Validate a flow-only dose curve across representative flows, with lamp-age checks and automatic fallback.
   4. Test one-bank operation at average flow, with the existing dose display and operator-controlled fallback.
   Show the answer

   Answer: B. Validate a UVT-based dose curve across representative flows, with operating limits and automatic fallback.

   A validated UVT-responsive curve tests the energy opportunity across representative flows while preserving documented limits and automatic fallback protection.

5. Question 5

   One of two ozone contact basins loses transfer efficiency. Its upstream pressure is high, downstream pressure is low, and all diffuser zones are weak. Valve feedback shows open, but branch flow is not independently measured. Which test plan best confirms the fault location before equipment is opened?

   1. Map dissolved-ozone residual through both basins under matched loading
   2. Compare generator output and header setpoint with an independent analyzer
   3. Calibrate only the upstream pressure transmitter
   4. Verify both pressures, valve travel, and strainer differential
   Show the answer

   Answer: D. Verify both pressures, valve travel, and strainer differential

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

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