Disinfection Practice Questions for Level 2 · Intermediate

1. Question 1

   A plant normally operates past breakpoint. Flow and chlorine feed stay steady, but free chlorine residual falls as ammonia entering the contact basin rises. What is the likely cause?

   1. The contact time became longer.
   2. The chlorine solution gained strength.
   3. The wastewater chlorine demand decreased.
   4. The wastewater chlorine demand increased.
   Show the answer

   Answer: D. The wastewater chlorine demand increased.

   Ammonia reacts with chlorine and increases chlorine demand. With steady flow and feed, a higher demand can leave a lower residual.

2. Question 2

   Validated samples from two chlorine-contact trains are collected at matched outlet locations and analyzed immediately. The trains receive the same applied dose concentration and have comparable influent chlorine demand, temperature, and pH. Which difference can explain a lower outlet residual in one train?

   1. Greater effective detention permits more chlorine consumption before the outlet.
   2. Shorter effective detention permits more chlorine consumption before the outlet.
   3. Lower influent chlorine demand consumes more of the applied chlorine.
   4. Less contact with the wastewater causes greater residual decay.
   Show the answer

   Answer: A. Greater effective detention permits more chlorine consumption before the outlet.

   With comparable dose, demand, water conditions, and sampling timing, greater effective detention allows more chlorine consumption before the outlet, producing a lower residual.

3. Question 3

   During a tracer study, tracer appears at a contact basin's outlet much earlier than the basin's theoretical detention time. What does this result indicate?

   1. Solids are settling throughout the contact basin.
   2. Flow is short-circuiting through the contact basin.
   3. Disinfectant demand is increasing within the basin.
   4. Tracer is reacting completely within the basin.
   Show the answer

   Answer: B. Flow is short-circuiting through the contact basin.

   Early tracer breakthrough indicates that part of the flow is taking a faster path through the basin instead of using the full available volume.

4. Question 4

   As pH increases in water containing free chlorine, which change generally reduces disinfection effectiveness under otherwise similar conditions?

   1. The fraction present as hypochlorous acid decreases while hypochlorite ion increases.
   2. The fraction present as hypochlorous acid increases while hypochlorite ion decreases.
   3. Both free-chlorine species convert to combined chlorine at the same rate.
   4. The relative fractions remain constant while only total chlorine concentration changes.
   Show the answer

   Answer: A. The fraction present as hypochlorous acid decreases while hypochlorite ion increases.

   Higher pH shifts free chlorine toward the less effective hypochlorite ion.

5. Question 5

   With flow, lamp output, and channel geometry unchanged, what is the expected effect of lower wastewater UV transmittance?

   1. UV penetrates farther through the water and delivered dose increases.
   2. UV penetration decreases and delivered dose to the water is reduced.
   3. UV penetration is unchanged because electrical lamp power is constant.
   4. UV penetration increases while exposure time decreases by the same amount.
   Show the answer

   Answer: B. UV penetration decreases and delivered dose to the water is reduced.

   Lower UVT means more UV is absorbed or scattered before penetrating the water.

Operator Master has 10 Level 2 · Intermediate disinfection questions, free.

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