20 Disinfection & Treatment Chemistry Practice Questions & Answers

Every Disinfection & Treatment Chemistry practice question from the Water Treatment Operator Practice Test, with the correct answer and a short explanation.

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1. A plant applies ozone ahead of filtration and then feeds chloramine into the water leaving the clearwell. What is the purpose of the chloramine feed?
- A.To strip taste and odor compounds that passed through the filter beds
- B.To hold a lasting disinfectant residual out in the distribution system✓ Answer
- C.To lower the pH of the finished water before it enters the pipe network
- D.To achieve the pathogen inactivation credit required before filtration
Primary disinfection is the inactivation achieved inside the plant; secondary disinfection is the residual carried out into the pipes. Ozone is a powerful primary disinfectant but leaves no residual at all, so a second chemical must be fed. Chloramine is a common choice because it persists far longer than free chlorine in long mains and forms much less trihalomethane along the way.
Source: 40 CFR Part 141 Subpart H, Surface Water Treatment Rule (residual disinfectant entering and within the distribution system); AWWA Water System Operations: Water Treatment, disinfection chapterReport a problem with this question
2. An operator raises the finished-water pH from 7.0 to 8.5 for corrosion control while holding the free chlorine residual at the same measured value. What happens to the disinfecting strength of that residual?
- A.It is unchanged, because the measured free chlorine residual is the same value
- B.It decreases, because more of the free chlorine is present as hypochlorite ion✓ Answer
- C.It increases, because more of the free chlorine is present as hypochlorous acid
- D.It decreases, because part of the free chlorine has become combined residual
Free available chlorine is the sum of hypochlorous acid and hypochlorite ion, and the split between them is set by pH. Hypochlorous acid dissociates as pH rises, so above roughly neutral-to-slightly-alkaline water the weaker hypochlorite ion dominates. Because hypochlorous acid is the far more effective disinfecting species, the same measured residual does less inactivation at pH 8.5 and a larger CT is required.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — chlorine chemistry (hypochlorous acid / hypochlorite ion equilibrium)Report a problem with this question
3. Chlorine is fed at steadily increasing doses to a water that contains ammonia, and total chlorine residual is plotted against dose. The residual rises to a hump, falls to a low point, then rises again. What is happening between the hump and the low point?
- A.Applied chlorine is escaping from the water because the pH has dropped
- B.Applied chlorine is combining with the ammonia to build more chloramines
- C.Applied chlorine is oxidizing and destroying the chloramines already formed✓ Answer
- D.Applied chlorine is being consumed by iron, manganese and hydrogen sulfide
The breakpoint curve has a fixed shape. Chlorine first satisfies fast reducing agents with no residual, then builds combined residual with ammonia up to the hump; past the hump the extra chlorine oxidizes those chloramines, so total residual falls until the breakpoint low point. Beyond the breakpoint the ammonia is gone and free available residual rises roughly in step with further dose.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — breakpoint chlorinationReport a problem with this question
4. A plant applies a chlorine dose of 3.6 mg/L. After the contact basin, the total chlorine residual is 1.1 mg/L and the free chlorine residual is 0.8 mg/L. What are the chlorine demand and the combined residual?
- A.Demand 2.5 mg/L and combined residual 1.9 mg/L
- B.Demand 3.3 mg/L and combined residual 0.8 mg/L
- C.Demand 2.5 mg/L and combined residual 0.3 mg/L✓ Answer
- D.Demand 2.8 mg/L and combined residual 0.3 mg/L
Chlorine demand is the dose minus the residual that survives, so 3.6 − 1.1 = 2.5 mg/L. Total residual is free plus combined, so the combined portion is 1.1 − 0.8 = 0.3 mg/L. Subtracting the free residual instead of the total residual from the dose is the usual slip and gives 2.8 mg/L.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — chlorine demand, free and combined residual calculationsReport a problem with this question
5. A small plant switches from chlorine gas to calcium hypochlorite while delivering the same amount of available chlorine. What change in the treated water should the operator expect?
- A.The pH will hold steady and the total hardness will drop slightly
- B.The pH will fall and the alkalinity will be consumed noticeably
- C.The pH will rise and the calcium hardness will increase slightly✓ Answer
- D.The pH will rise and the alkalinity will be destroyed very rapidly
Chlorine gas hydrolyzes in water to hypochlorous acid and hydrochloric acid, so it pushes pH down and eats alkalinity. Calcium hypochlorite is a basic salt: it releases hypochlorite and calcium, so it nudges pH upward and adds a little calcium hardness. Sodium hypochlorite also raises pH but contributes sodium rather than hardness.
Source: AWWA Water System Operations: Water Treatment — chlorine gas and hypochlorite chemistryReport a problem with this question
6. A system that chloraminates feeds chlorine and then ammonia, and holds the chlorine to ammonia-nitrogen weight ratio inside a narrow band. Why is that band held so tightly?
- A.Too little chlorine raises trihalomethanes, and too much raises the hardness of the finished water
- B.Too little chlorine turns monochloramine into free chlorine, and too much yields a weaker oxidant
- C.Too little chlorine leaves free ammonia that feeds nitrifiers, and too much forms odorous dichloramine✓ Answer
- D.Too little chlorine raises the pH sharply, and too much strips alkalinity from the finished water
Monochloramine is the species the system wants, and it is favored only inside a fairly narrow chlorine to ammonia-nitrogen weight ratio. Below the band, unreacted free ammonia remains and nitrifying bacteria oxidize it, which destroys the residual and drives up nitrite and bacterial counts. Above the band, dichloramine and nitrogen trichloride form and generate taste and odor complaints. Note the ratio is by weight and is expressed against ammonia-nitrogen, not against ammonia.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — chloramination and nitrification control; AWWA Water System Operations: Water TreatmentReport a problem with this question
7. A plant treating 0.85 MGD must apply a chlorine dose of 2.4 mg/L using calcium hypochlorite that is 65% available chlorine. Water weighs 8.34 lb/gal. How many pounds of the calcium hypochlorite product are required per day?
- A.17.0 lb/day
- B.11.1 lb/day
- C.26.2 lb/day✓ Answer
- D.3.1 lb/day
First find the pounds of chlorine needed: (2.4 mg/L)(0.85 MGD)(8.34 lb/gal) = 17.0 lb/day. That is chlorine, not product, so divide by the available-chlorine decimal: 17.0 / 0.65 = 26.2 lb/day of calcium hypochlorite. Stopping at 17.0 is the classic purity error, and multiplying by 0.65 instead of dividing gives 11.1.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — chemical feed rate and percent purity calculationsReport a problem with this question
8. A system must feed 42 lb of chlorine per day using a sodium hypochlorite solution that is 12.5% available chlorine by weight and weighs 10.0 lb/gal. How many gallons of solution are needed per day?
- A.33.6 gal/day✓ Answer
- B.40.3 gal/day
- C.5.3 gal/day
- D.336 gal/day
Percent available chlorine is a weight percent, so each gallon carries (10.0 lb/gal)(0.125) = 1.25 lb of chlorine. Dividing the requirement by that gives 42 / 1.25 = 33.6 gal/day. Using 8.34 lb/gal — the weight of water rather than the weight of the solution — gives 40.3 and understates the feed.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — hypochlorite solution strength and feed rate calculationsReport a problem with this question
9. A clearwell holds 250,000 gallons and is operating at a flow of 1,500 gpm. The regulatory agency has approved a baffling factor of 0.5 for the basin, and the free chlorine residual at the outlet is 1.2 mg/L. What CT value has the basin provided?
- A.167 mg/L·min
- B.100 mg/L·min✓ Answer
- C.83 mg/L·min
- D.200 mg/L·min
Theoretical detention time is 250,000 gal / 1,500 gpm = 166.7 minutes. CT credit uses the effective contact time, which is that detention time multiplied by the approved baffling factor: 166.7 × 0.5 = 83.3 minutes. CT is then (1.2 mg/L)(83.3 min) = 100 mg/L·min. Skipping the baffling factor gives 200 and would claim roughly twice the disinfection actually delivered, because short-circuiting sends part of the flow through much faster than the average.
Source: EPA Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources — effective contact time (T10) and baffling factorsReport a problem with this question
10. Two operating days are compared at the same target log inactivation with the same disinfectant. On the second day the water is colder and the pH is higher. How does the required CT compare with the first day?
- A.Lower, because higher pH shifts the chlorine toward the stronger species
- B.Higher, but only from the pH, since temperature does not affect the CT
- C.Lower, because colder water slows the decay of the chlorine in the basin
- D.Higher, because colder water and higher pH both raise the CT that is needed✓ Answer
Inactivation is a reaction, and reactions slow as temperature falls, so colder water needs more concentration-time product for the same log kill. Higher pH shifts free chlorine toward the weaker hypochlorite ion, which also raises the CT needed. Both effects push in the same direction, which is why compliance is documented using the day's lowest temperature, highest pH and lowest residual — the least favorable conditions. A more resistant organism or a weaker disinfectant raises the requirement in the same way.
Source: EPA Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources — effect of temperature and pH on required CTReport a problem with this question
11. A surface water source has a documented history of Cryptosporidium oocysts. Which added barrier gives the most dependable inactivation of that particular organism?
- A.Ultraviolet light applied to the filtered water ahead of the clearwell✓ Answer
- B.Chloramine fed at the head of the plant to extend the contact time
- C.Sodium hypochlorite fed at the filter effluent to raise the residual
- D.Free chlorine fed at a higher dose into the clearwell for longer contact
Cryptosporidium oocysts have a thick wall that makes them extremely resistant to chlorine and even more so to chloramine, so raising a chlorine dose or extending chlorine contact buys very little inactivation of this organism. Ultraviolet light damages the organism's nucleic acid at doses that are easy to deliver, and ozone also works; those are the practical inactivation barriers. UV leaves no residual, so a chemical residual is still needed for the distribution system.
Source: EPA Long Term 2 Enhanced Surface Water Treatment Rule (40 CFR Part 141 Subpart W) — Cryptosporidium treatment technique toolbox; AWWA Water System Operations: Water Treatment, alternative disinfectantsReport a problem with this question
12. An ultraviolet reactor is delivering less dose than usual, although lamp output and flow have not changed. Which explanation fits best?
- A.Alkalinity has risen and is absorbing the light before it reaches the water
- B.Water temperature has risen and increased the ultraviolet demand of the water
- C.Free chlorine residual entering the reactor has fallen below its set point
- D.Ultraviolet transmittance has fallen and the lamp sleeves have fouled✓ Answer
UV works only where the light actually reaches the organism, so delivered dose depends on how far the light penetrates. Color, turbidity and dissolved organics lower ultraviolet transmittance, and scale or biofilm on the quartz sleeves blocks light at the source; either one cuts delivered dose with lamp output unchanged. UV adds no residual and forms no byproducts, so it never substitutes for the chemical residual carried into distribution.
Source: AWWA Water System Operations: Water Treatment — ultraviolet disinfection, UV transmittance and sleeve foulingReport a problem with this question
13. Which statement correctly pairs a disinfectant with the byproduct most closely associated with it?
- A.Chlorine dioxide forms bromate when the source water carries bromide
- B.Free chlorine forms chlorite when the source water carries bromide
- C.Chloramine forms chlorate when the source water carries bromide
- D.Ozone forms bromate when the source water carries bromide✓ Answer
Ozone is a strong enough oxidant to convert bromide in the source water into bromate, which is why bromide-bearing sources are the main chemistry concern when ozone is considered. Chlorite and chlorate come from chlorine dioxide and its generation chemistry, while free chlorine reacting with natural organic matter is what produces trihalomethanes and haloacetic acids.
Source: EPA Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules (40 CFR Part 141 Subparts L and V) — regulated byproducts by disinfectantReport a problem with this question
14. Trihalomethane results are climbing at the far ends of the distribution system in warm weather, and the plant must keep the inactivation it is credited for. Which change attacks the cause without weakening disinfection?
- A.Move the chlorine application point up to the raw water intake
- B.Raise the plant chlorine dose so residual survives out to the far ends
- C.Raise storage tank levels so more finished water is held in reserve
- D.Take out more organic precursor material before the chlorine is applied✓ Answer
Trihalomethanes form when free chlorine reacts with natural organic matter, and formation grows with precursor concentration, chlorine dose, temperature and contact time. Removing precursors first — enhanced coagulation, activated carbon, or moving the chlorine application point downstream of clarification — cuts formation while the same inactivation is still delivered. Raising the dose, chlorinating the raw water, and increasing storage all add dose or water age and make the numbers worse.
Source: EPA Stage 2 Disinfectants and Disinfection Byproducts Rule (40 CFR Part 141 Subpart V) and EPA enhanced coagulation requirements; CSUS Water Treatment Plant Operation — DBP precursor controlReport a problem with this question
15. In the context of disinfection byproducts, what is a precursor?
- A.A mineral that speeds the breakdown of chlorine residual in storage
- B.The portion of applied chlorine that remains after the demand is met
- C.Organic material that reacts with the disinfectant to form byproducts✓ Answer
- D.The byproduct formed in the pipe network after water leaves the plant
A precursor is the raw material for the byproduct, not the byproduct itself: the natural organic matter washed in from the watershed, commonly tracked as total organic carbon. Because the byproduct cannot form without it, the preferred control is to remove precursors before disinfecting rather than to cut back on disinfection — inactivating pathogens remains the first obligation, and the byproduct risk is managed around it.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — disinfection byproduct formation and TOC precursorsReport a problem with this question
16. Raising the aluminum sulfate dose has pulled the coagulated water's pH and alkalinity below the range the plant wants to hold. Which chemical addition corrects both?
- A.Soda ash, because it adds carbonate alkalinity and lifts the pH✓ Answer
- B.Sulfuric acid, because it neutralizes the alkalinity that is left
- C.Carbon dioxide, because it adds carbonate alkalinity and lifts the pH
- D.Citric acid, because it buffers the water against a further pH drop
Aluminum sulfate hydrolyzes in water and releases acid, so it consumes alkalinity and drags pH down; the more coagulant fed, the more alkalinity is destroyed. Soda ash (sodium carbonate) supplies carbonate alkalinity and raises pH, as do lime and caustic soda. Carbon dioxide dissolves to form carbonic acid and pushes pH down, which is exactly why it is used for recarbonation after lime softening, and the two acids listed lower pH as well.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — coagulation alkalinity demand and pH adjustment chemicalsReport a problem with this question
17. Finished water leaving the plant has a negative Langelier Saturation Index. What does that indicate about the water in the distribution piping?
- A.It is undersaturated with calcium carbonate and tends to be corrosive✓ Answer
- B.It is in balance with calcium carbonate and will neither scale nor etch
- C.It is supersaturated with calcium carbonate and tends to deposit scale
- D.It is undersaturated with dissolved oxygen and cannot corrode the pipe
The index is the measured pH minus the pH at which the water would be saturated with calcium carbonate. A negative value means the water is below saturation, so it can dissolve calcium carbonate rather than deposit it, and it tends to be aggressive toward pipe walls and toward lead-tin solder joints. A positive value means the opposite tendency, scale formation. The index gives a direction only; it does not tell the operator how fast corrosion will proceed.
Source: 40 CFR Part 141 Subpart I, Lead and Copper Rule — corrosion control treatment and water quality parameters; AWWA Water System Operations: Water Treatment, Langelier Saturation IndexReport a problem with this question
18. A utility with lead service lines and lead-soldered joints begins feeding orthophosphate to the finished water. What is the mechanism it is relying on?
- A.It lowers the water's pH so lead compounds stay in a solid form
- B.It binds dissolved lead so the metal passes on through the tap
- C.It builds a low-solubility film on the pipe wall that holds lead in✓ Answer
- D.It raises alkalinity enough to settle lead out in the water main
Orthophosphate reacts with lead at the pipe surface to build a passivating layer of low-solubility lead phosphate, and it is that film, not the chemical in the bulk water, that limits how much lead dissolves into the water sitting in the service line. Because the film depends on stable chemistry, pH and alkalinity must be held steady while it forms and afterward; an abrupt change in water quality or disinfectant can strip the film and release lead.
Source: 40 CFR Part 141 Subpart I, Lead and Copper Rule — optimal corrosion control treatment (phosphate inhibitors and passivation)Report a problem with this question
19. An operator feeding potassium permanganate ahead of the filters for manganese removal sees pink water in the filter effluent. What does this indicate, and what should be done?
- A.The permanganate dose is too low, so the feed rate should be raised
- B.The chlorine dose is too high, so the chlorine feed should be cut back
- C.The filter media is exhausted, so the media should be replaced now
- D.The permanganate dose is too high, so the feed rate should be lowered✓ Answer
Permanganate is fed to oxidize soluble manganese to an insoluble form that the filter can capture, and it should be fully consumed by the time the water leaves the filter. Pink or purple color in the filter effluent is unreacted permanganate carrying through, which is the visible signature of an overfeed. The correct response is to trim the feed to the point where the manganese is oxidized and the color is gone, checking manganese in the filtered water as the dose is reduced.
Source: CSUS Office of Water Programs, Water Treatment Plant Operation — iron and manganese oxidation with potassium permanganateReport a problem with this question
20. An operator must bring the free chlorine residual in a batch of treated water down to zero. Which pair of chemicals will do that?
- A.Calcium hypochlorite and sodium chlorite
- B.Sodium thiosulfate and ascorbic acid✓ Answer
- C.Potassium permanganate and ozone
- D.Sodium hydroxide and soda ash
Dechlorination requires a reducing agent, because the job is to reduce free chlorine to chloride so it no longer acts as an oxidant. Sodium thiosulfate and ascorbic acid both do this, as do sulfur dioxide and sodium bisulfite. Sodium hydroxide and soda ash only raise pH and alkalinity, while permanganate, ozone, hypochlorite and chlorite are themselves oxidants or oxidant sources and would add to the load rather than remove it.
Source: AWWA Water System Operations: Water Treatment — dechlorination agents (sulfite compounds, sodium thiosulfate, ascorbic acid)Report a problem with this question

Practice questions written against the standardized Water Treatment Operator Need-to-Know Criteria published by Water Professionals International (formerly the Association of Boards of Certification) and standard references from the CSUS Office of Water Programs and AWWA. This site is not affiliated with or endorsed by WPI/ABC, AWWA, or the US EPA. Operator certification is issued by your state's certifying authority, which sets plant classification tiers, operator grades, eligibility, and the passing standard — confirm those with your state before testing. Contaminant limits and monitoring requirements are set federally and are revised over time, so no answer here should be relied on as a current regulatory value; consult the regulations in force for your system. This bank covers the drinking-water treatment exam only — wastewater treatment, wastewater collection, and water distribution are separate certifications. About the Need-to-Know Criteria →

1. A plant applies ozone ahead of filtration and then feeds chloramine into the water leaving the clearwell. What is the purpose of the chloramine feed?

2. An operator raises the finished-water pH from 7.0 to 8.5 for corrosion control while holding the free chlorine residual at the same measured value. What happens to the disinfecting strength of that residual?

3. Chlorine is fed at steadily increasing doses to a water that contains ammonia, and total chlorine residual is plotted against dose. The residual rises to a hump, falls to a low point, then rises again. What is happening between the hump and the low point?

4. A plant applies a chlorine dose of 3.6 mg/L. After the contact basin, the total chlorine residual is 1.1 mg/L and the free chlorine residual is 0.8 mg/L. What are the chlorine demand and the combined residual?

5. A small plant switches from chlorine gas to calcium hypochlorite while delivering the same amount of available chlorine. What change in the treated water should the operator expect?

6. A system that chloraminates feeds chlorine and then ammonia, and holds the chlorine to ammonia-nitrogen weight ratio inside a narrow band. Why is that band held so tightly?

7. A plant treating 0.85 MGD must apply a chlorine dose of 2.4 mg/L using calcium hypochlorite that is 65% available chlorine. Water weighs 8.34 lb/gal. How many pounds of the calcium hypochlorite product are required per day?

8. A system must feed 42 lb of chlorine per day using a sodium hypochlorite solution that is 12.5% available chlorine by weight and weighs 10.0 lb/gal. How many gallons of solution are needed per day?

9. A clearwell holds 250,000 gallons and is operating at a flow of 1,500 gpm. The regulatory agency has approved a baffling factor of 0.5 for the basin, and the free chlorine residual at the outlet is 1.2 mg/L. What CT value has the basin provided?

10. Two operating days are compared at the same target log inactivation with the same disinfectant. On the second day the water is colder and the pH is higher. How does the required CT compare with the first day?

11. A surface water source has a documented history of Cryptosporidium oocysts. Which added barrier gives the most dependable inactivation of that particular organism?

12. An ultraviolet reactor is delivering less dose than usual, although lamp output and flow have not changed. Which explanation fits best?

13. Which statement correctly pairs a disinfectant with the byproduct most closely associated with it?

14. Trihalomethane results are climbing at the far ends of the distribution system in warm weather, and the plant must keep the inactivation it is credited for. Which change attacks the cause without weakening disinfection?

15. In the context of disinfection byproducts, what is a precursor?

16. Raising the aluminum sulfate dose has pulled the coagulated water's pH and alkalinity below the range the plant wants to hold. Which chemical addition corrects both?

17. Finished water leaving the plant has a negative Langelier Saturation Index. What does that indicate about the water in the distribution piping?

18. A utility with lead service lines and lead-soldered joints begins feeding orthophosphate to the finished water. What is the mechanism it is relying on?

19. An operator feeding potassium permanganate ahead of the filters for manganese removal sees pink water in the filter effluent. What does this indicate, and what should be done?

20. An operator must bring the free chlorine residual in a batch of treated water down to zero. Which pair of chemicals will do that?