Class D Water Operator License Exam
Practice Questions And Correct Answers
(Verified Answers) Plus Rationales
2025|2026 Q&A | Instant Download Pdf
1. Which agency regulates public drinking water systems in the United
States?
A. EPA
B. CDC
C. EPA
D. OSHA
The U.S. Environmental Protection Agency (EPA) enforces drinking
water regulations under the Safe Drinking Water Act.
2. The primary purpose of chlorination in drinking water treatment is to:
A. Disinfect and kill pathogens
B. Remove hardness
C. Reduce turbidity
, D. Control taste and odor
Chlorination targets harmful microorganisms to make water safe for
human consumption.
3. Which chemical is most commonly used to adjust pH upward in water
treatment?
A. Sulfuric acid
B. Alum
C. CO₂
D. Sodium hydroxide
Sodium hydroxide (caustic soda) is often added to raise pH levels in
treated water.
4. Which of the following is a primary standard under the Safe Drinking
Water Act?
A. Odor
B. Nitrate
C. Temperature
D. Color
Primary standards address contaminants with health impacts, such
as nitrates, lead, and microbial agents.
5. Which process removes suspended particles from water by adding
chemicals that cause them to clump together?
A. Sedimentation
B. Filtration
, C. Coagulation/Flocculation
D. Disinfection
Coagulation and flocculation use chemicals like alum to bind
particles into larger flocs for removal.
6. The pH scale ranges from:
A. 0 to 14
B. 1 to 12
C. 0 to 10
D. 1 to 14
The pH scale measures acidity/alkalinity from 0 (acidic) to 14
(alkaline), with 7 as neutral.
7. Which water quality test measures cloudiness caused by suspended
solids?
A. Color test
B. Hardness test
C. pH test
D. Turbidity test
Turbidity is measured in NTUs (nephelometric turbidity units) and
indicates suspended matter.
8. The maximum contaminant level (MCL) for nitrate in drinking water is:
A. 1 mg/L
B. 10 mg/L
C. 50 mg/L
, D. 0.5 mg/L
EPA sets the nitrate MCL at 10 mg/L to prevent health issues like
methemoglobinemia.
9. Which device measures water flow rate?
A. Barometer
B. Thermometer
C. Flow meter
D. Turbidimeter
Flow meters measure water volume passing through a system,
critical for treatment control.
10. Backwashing a filter is done to:
A. Remove trapped particles from the filter media
B. Add chlorine to the water
C. Adjust pH
D. Increase hardness
Backwashing reverses water flow through a filter to clean
accumulated debris.
11. Which form of chlorine has the highest available chlorine
content?
A. Chlorine gas
B. Sodium hypochlorite
C. Calcium hypochlorite
Practice Questions And Correct Answers
(Verified Answers) Plus Rationales
2025|2026 Q&A | Instant Download Pdf
1. Which agency regulates public drinking water systems in the United
States?
A. EPA
B. CDC
C. EPA
D. OSHA
The U.S. Environmental Protection Agency (EPA) enforces drinking
water regulations under the Safe Drinking Water Act.
2. The primary purpose of chlorination in drinking water treatment is to:
A. Disinfect and kill pathogens
B. Remove hardness
C. Reduce turbidity
, D. Control taste and odor
Chlorination targets harmful microorganisms to make water safe for
human consumption.
3. Which chemical is most commonly used to adjust pH upward in water
treatment?
A. Sulfuric acid
B. Alum
C. CO₂
D. Sodium hydroxide
Sodium hydroxide (caustic soda) is often added to raise pH levels in
treated water.
4. Which of the following is a primary standard under the Safe Drinking
Water Act?
A. Odor
B. Nitrate
C. Temperature
D. Color
Primary standards address contaminants with health impacts, such
as nitrates, lead, and microbial agents.
5. Which process removes suspended particles from water by adding
chemicals that cause them to clump together?
A. Sedimentation
B. Filtration
, C. Coagulation/Flocculation
D. Disinfection
Coagulation and flocculation use chemicals like alum to bind
particles into larger flocs for removal.
6. The pH scale ranges from:
A. 0 to 14
B. 1 to 12
C. 0 to 10
D. 1 to 14
The pH scale measures acidity/alkalinity from 0 (acidic) to 14
(alkaline), with 7 as neutral.
7. Which water quality test measures cloudiness caused by suspended
solids?
A. Color test
B. Hardness test
C. pH test
D. Turbidity test
Turbidity is measured in NTUs (nephelometric turbidity units) and
indicates suspended matter.
8. The maximum contaminant level (MCL) for nitrate in drinking water is:
A. 1 mg/L
B. 10 mg/L
C. 50 mg/L
, D. 0.5 mg/L
EPA sets the nitrate MCL at 10 mg/L to prevent health issues like
methemoglobinemia.
9. Which device measures water flow rate?
A. Barometer
B. Thermometer
C. Flow meter
D. Turbidimeter
Flow meters measure water volume passing through a system,
critical for treatment control.
10. Backwashing a filter is done to:
A. Remove trapped particles from the filter media
B. Add chlorine to the water
C. Adjust pH
D. Increase hardness
Backwashing reverses water flow through a filter to clean
accumulated debris.
11. Which form of chlorine has the highest available chlorine
content?
A. Chlorine gas
B. Sodium hypochlorite
C. Calcium hypochlorite
