Certification Exam Practice Test

Drinking Water Operator Class C 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 drinking water

treatment?

• Remove dissolved minerals
• Improve taste and odor
• Destabilize small particles for removal
• Add chlorine residual

Coagulation destabilizes colloidal particles so they can clump and be removed in later treatment steps.

• Which chemical is most commonly used as a coagulant? 1 / 4

• Chlorine
• Lime
• Aluminum sulfate (alum)
• Fluoride

Alum is the most widely used coagulant for surface water treatment systems.

• What is the ideal pH range for coagulation with alum?

A. 3–4

B. 5–6

C. 6–7

D. 8–10

Alum works best in slightly acidic to neutral pH, typically around 6–7.

• What is the main purpose of flocculation?
• Remove pathogens
• Form larger flocs from coagulated particles
• Add corrosion control chemicals
• Remove hardness

Flocculation gently mixes water so microflocs can grow into larger, settleable flocs.

• / 4

• Sedimentation typically removes what percentage of suspended solids?

A. 10–20%

B. 20–30%

C. 30–50%

D. 60–80%

Sedimentation can remove up to 80% of solids when coagulation and flocculation are effective.

• Which filtration method is most common in drinking water treatment?
• Reverse osmosis
• Cartridge filters
• Rapid sand filtration
• Slow sand filtration

Rapid sand filters are widely used in municipal water treatment plants.

• What is the purpose of backwashing a filter?
• Add sand
• Remove trapped solids
• Reduce pH
• Add coagulant

Backwashing lifts and scours the filter media to remove accumulated particles. 3 / 4

• The minimum free chlorine residual in the distribution system is

typically:

• 0.1 mg/L
• 0.5 mg/L
• 0.2 mg/L
• 1.0 mg/L

Most standards require at least 0.2 mg/L free chlorine residual in the distribution system.

• Chlorine gas is classified as which type of hazard?
• Flammable
• Corrosive only
• Toxic and corrosive
• Odor nuisance

Chlorine gas is both highly toxic and corrosive, requiring strong safety controls.

• What is the best method to detect chlorine gas leaks?
• Smelling
• Ammonia vapor
• pH strips
• / 4