North Carolina Water Treatment
Operator Examination — Class III
Practice Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary purpose of coagulation in water treatment?
A. Disinfect water
B. Remove suspended particles
C. Adjust pH
D. Add fluoride
Rationale: Coagulation destabilizes suspended particles, allowing
them to aggregate and be removed during sedimentation and
filtration.
2. Which chemical is commonly used as a coagulant in water treatment?
A. Sodium hypochlorite
B. Aluminum sulfate
, C. Calcium carbonate
D. Hydrofluorosilicic acid
Rationale: Aluminum sulfate (alum) is widely used to coagulate
particles by forming flocs.
3. What is the typical pH range for effective coagulation with alum?
A. 4.0–5.0
B. 5.5–6.5
C. 6.0–7.5
D. 7.8–9.5
Rationale: Alum is most effective in slightly acidic to neutral pH
ranges, around 6.0–7.5.
4. Which process removes dissolved gases such as carbon dioxide from
water?
A. Filtration
B. Aeration
C. Coagulation
D. Sedimentation
Rationale: Aeration introduces air to water, allowing gases like CO₂
to escape.
5. The primary purpose of sedimentation in water treatment is to:
A. Kill bacteria
B. Remove settleable solids
C. Adjust pH
, D. Remove dissolved minerals
Rationale: Sedimentation allows heavier particles to settle out before
filtration.
6. Which type of filtration is most commonly used in Class III water
treatment plants?
A. Slow sand
B. Rapid sand
C. Reverse osmosis
D. Activated carbon only
Rationale: Rapid sand filters are widely used due to their efficiency
and ability to handle large volumes.
7. What is the main purpose of disinfection in water treatment?
A. Remove turbidity
B. Destroy pathogenic microorganisms
C. Remove minerals
D. Increase pH
Rationale: Disinfection ensures water is safe by eliminating harmful
bacteria, viruses, and protozoa.
8. Chlorine is added to water primarily for:
A. Taste improvement
B. Disinfection
C. Coagulation
D. pH adjustment
, Rationale: Chlorine is a strong oxidant used to inactivate pathogenic
microorganisms.
9. Free chlorine in water refers to:
A. Chlorine combined with ammonia
B. Chlorine bound to organics
C. Available chlorine not yet reacted
D. Chlorine gas in the atmosphere
Rationale: Free chlorine is the portion available to disinfect water
before forming chloramines or other compounds.
10. Combined chlorine in water is formed when:
A. Chlorine reacts with iron
B. Chlorine reacts with ammonia or nitrogen compounds
C. Chlorine reacts with calcium
D. Chlorine evaporates
Rationale: Combined chlorine, often called chloramines, results from
chlorine reacting with ammonia or organic nitrogen.
11. Which method is used to control taste and odor caused by algae
in raw water?
A. Sedimentation
B. Powdered activated carbon (PAC)
C. Fluoridation
D. Lime softening
Rationale: PAC adsorbs organic compounds causing taste and odor.
Operator Examination — Class III
Practice Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary purpose of coagulation in water treatment?
A. Disinfect water
B. Remove suspended particles
C. Adjust pH
D. Add fluoride
Rationale: Coagulation destabilizes suspended particles, allowing
them to aggregate and be removed during sedimentation and
filtration.
2. Which chemical is commonly used as a coagulant in water treatment?
A. Sodium hypochlorite
B. Aluminum sulfate
, C. Calcium carbonate
D. Hydrofluorosilicic acid
Rationale: Aluminum sulfate (alum) is widely used to coagulate
particles by forming flocs.
3. What is the typical pH range for effective coagulation with alum?
A. 4.0–5.0
B. 5.5–6.5
C. 6.0–7.5
D. 7.8–9.5
Rationale: Alum is most effective in slightly acidic to neutral pH
ranges, around 6.0–7.5.
4. Which process removes dissolved gases such as carbon dioxide from
water?
A. Filtration
B. Aeration
C. Coagulation
D. Sedimentation
Rationale: Aeration introduces air to water, allowing gases like CO₂
to escape.
5. The primary purpose of sedimentation in water treatment is to:
A. Kill bacteria
B. Remove settleable solids
C. Adjust pH
, D. Remove dissolved minerals
Rationale: Sedimentation allows heavier particles to settle out before
filtration.
6. Which type of filtration is most commonly used in Class III water
treatment plants?
A. Slow sand
B. Rapid sand
C. Reverse osmosis
D. Activated carbon only
Rationale: Rapid sand filters are widely used due to their efficiency
and ability to handle large volumes.
7. What is the main purpose of disinfection in water treatment?
A. Remove turbidity
B. Destroy pathogenic microorganisms
C. Remove minerals
D. Increase pH
Rationale: Disinfection ensures water is safe by eliminating harmful
bacteria, viruses, and protozoa.
8. Chlorine is added to water primarily for:
A. Taste improvement
B. Disinfection
C. Coagulation
D. pH adjustment
, Rationale: Chlorine is a strong oxidant used to inactivate pathogenic
microorganisms.
9. Free chlorine in water refers to:
A. Chlorine combined with ammonia
B. Chlorine bound to organics
C. Available chlorine not yet reacted
D. Chlorine gas in the atmosphere
Rationale: Free chlorine is the portion available to disinfect water
before forming chloramines or other compounds.
10. Combined chlorine in water is formed when:
A. Chlorine reacts with iron
B. Chlorine reacts with ammonia or nitrogen compounds
C. Chlorine reacts with calcium
D. Chlorine evaporates
Rationale: Combined chlorine, often called chloramines, results from
chlorine reacting with ammonia or organic nitrogen.
11. Which method is used to control taste and odor caused by algae
in raw water?
A. Sedimentation
B. Powdered activated carbon (PAC)
C. Fluoridation
D. Lime softening
Rationale: PAC adsorbs organic compounds causing taste and odor.
