Operator Class III Practice Exam

North Dakota Wastewater Treatment Operator Class III Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• The primary purpose of preliminary treatment in a wastewater plant is

to:

• Remove dissolved nutrients
• Remove large solids and grit
• Remove microorganisms
• Neutralize pH

Rationale: Preliminary treatment targets large debris and grit to prevent

damage to pumps and downstream equipment.

• Which of the following processes primarily removes suspended solids

from wastewater? 1 / 4

• Disinfection
• Aeration
• Primary sedimentation
• Nitrification

Rationale: Primary sedimentation allows heavier suspended solids to settle

by gravity.

3. The primary goal of biological treatment is to:

• Remove grit
• Reduce BOD (Biochemical Oxygen Demand)
• Remove large solids
• Filter wastewater

Rationale: Biological treatment uses microorganisms to consume organic

matter, reducing BOD.

4. Activated sludge is a process used to:

• Disinfect wastewater
• Treat wastewater biologically by aeration
• Remove grit
• Chlorinate effluent

Rationale: Activated sludge relies on microorganisms in aerated tanks to

degrade organic material.

• Which microorganism group is primarily responsible for nitrification?
• Protozoa 2 / 4

• Chemolithotrophic bacteria (Nitrosomonas and Nitrobacter)
• Fungi
• Algae

Rationale: Nitrifying bacteria oxidize ammonia to nitrite and nitrate.

6. Denitrification converts:

• Ammonia to nitrate
• Nitrate to nitrogen gas
• BOD to solids
• Phosphate to organic phosphorus

Rationale: Denitrification is an anaerobic process converting nitrate into

nitrogen gas, removing nitrogen from wastewater.

7. The function of a clarifier is to:

• Aerate wastewater
• Allow solids to settle out by gravity
• Disinfect water
• Neutralize pH

Rationale: Clarifiers allow suspended solids to settle, separating solids

from treated water.

8. A typical BOD test measures oxygen consumption over:

• 1 day
• 3 days 3 / 4

• 5 days
• 7 days

Rationale: The standard BOD5 test measures oxygen demand over five

days at 20°C.

• If BOD removal efficiency is 85%, and influent BOD is 200 mg/L,

effluent BOD is approximately:

• 20 mg/L
• 30 mg/L
• 30 mg/L
• 50 mg/L

Rationale: Effluent BOD = Influent BOD × (1 – removal efficiency) = 200 × (1 – 0.85) = 30 mg/L.

10. The purpose of chlorination in wastewater treatment is to:

• Remove BOD
• Remove solids
• Disinfect and kill pathogens
• Neutralize pH

Rationale: Chlorination destroys harmful bacteria and pathogens before

discharge.

11. In a trickling filter system, wastewater flows over media to:

• Sediment solids
• Provide surface for microbial growth
• / 4