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

   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

4. Question 4

   Two reuse trains share one chlorine-to-ammonia feed ratio. Train 1 has low pH, low monochloramine, and rising dichloramine; Train 2 has normal pH and chloramine species. Total residual is on target in both trains, and Train 1 has separate pH control. What is the best adjustment plan?

   1. Correct Train 1 pH, compare both trains, then adjust the shared ratio if needed
   2. Raise the shared chlorine feed and leave both pH controls unchanged
   3. Raise the shared ammonia feed until Train 1 monochloramine rises
   4. Lower the shared residual target for both trains
   Show the answer

   Answer: A. Correct Train 1 pH, compare both trains, then adjust the shared ratio if needed

5. Question 5

   Two vacuum chlorinators provide duty and standby feed, but both depend on one injector-water booster and one electrical panel. A peak-flow review shows either chlorinator can meet dose, while failure of either shared component stops both units. Which upgrade best provides true standby capacity?

   1. Add independent injector-water and power paths
   2. Add a third chlorinator to the same shared systems
   3. Increase gas-header pressure for both chlorinators
   4. Install a larger outlet analyzer
   Show the answer

   Answer: A. Add independent injector-water and power paths

Operator Master has 7 Level 4 · Expert disinfection questions, free.

• Plant Operations35 questions
• Solids Handling11 questions
• Biological Treatment9 questions
• Laboratory7 questions
• Math and Calculations5 questions
• Pumps and Hydraulics5 questions
• Safety4 questions
• Secondary Clarification3 questions
• Grit Removal2 questions
• Headworks and Screening2 questions
• Primary Treatment2 questions
• Regulations and Compliance2 questions