Certification Exam Practice Test

Drinking Water Operator Class A Certification Exam Practice Test Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary purpose of coagulation in water treatment?
• Remove dissolved gases
• Reduce hardness
• Destabilize suspended particles
• Disinfect water

Rationale: Coagulation neutralizes particle charges, allowing them to form

floc for removal.

• Which chemical is most commonly used as a coagulant in drinking water

treatment? 1 / 4

• Chlorine gas
• Aluminum sulfate (alum)
• Potassium permanganate
• Sodium hypochlorite

Rationale: Alum is the most widely used coagulant for turbidity and

particle removal.

3. The optimal pH range for alum coagulation is generally:

A. 3–4

B. 5.5–7.5

C. 8.5–10

D. 10–12

Rationale: Alum performs best in a slightly acidic to neutral pH range.

4. The primary goal of filtration is to remove:

• Dissolved minerals
• Organic gases
• Remaining suspended solids and pathogens
• VOCs

Rationale: Filtration captures fine particulates and microorganisms not

removed during sedimentation.

5. The CT concept in disinfection refers to:

• Concentration of turbidity
• Concentration × contact time 2 / 4

• Chemical type
• Chlorine tonnage

Rationale: CT measures disinfectant concentration multiplied by the

required contact time for inactivation.

• Which disinfectant provides the longest-lasting residual in the

distribution system?

• UV light
• Ozone
• Free chlorine
• Chloramines

Rationale: Free chlorine maintains a strong and long-lasting residual

compared to other disinfectants.

7. Chlorine gas is:

• Lighter than air
• Heavier than air
• Same weight as air
• Insoluble in water

Rationale: Chlorine gas is heavier and settles near the ground, increasing

exposure risk.

• The maximum contaminant level (MCL) for turbidity in treated water

(filtration required) is typically: 3 / 4

A. 5.0 NTU

B. 2.0 NTU

C. 0.3 NTU

D. 0.1 NTU

Rationale: Most regulations require 0.3 NTU or less at the combined filter

effluent 95% of the time.

9. Hardness in drinking water is primarily caused by:

• Sodium and potassium
• Calcium and magnesium
• Chloride and sulfate
• Iron and manganese

Rationale: Hardness results from multivalent cations, mostly calcium and

magnesium.

10. The most common method for measuring chlorine residual is:

• Turbidimeter
• Nephelometer
• DPD colorimetric method
• pH probe

Rationale: The DPD method is standard for measuring free and total

chlorine.

• What is breakpoint chlorination?
• / 4