Grade III Practice Exam Questions And

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

1. The primary purpose of screening in wastewater treatment is to:

• Remove dissolved solids
• Remove grit
• Remove large solids and debris
• Reduce BOD

Rationale: Screening is the first step in wastewater treatment to remove

large solids such as sticks, rags, and plastics that could damage equipment or interfere with treatment processes.

2. The primary function of grit chambers is to:

• Remove grease 1 / 4

• Reduce BOD
• Remove sand, gravel, and other heavy inorganic particles
• Filter pathogens

Rationale: Grit chambers allow heavy inorganic particles like sand and

gravel to settle out before the wastewater moves to primary treatment, protecting pumps and downstream equipment.

• Which parameter measures the amount of oxygen required by

microorganisms to decompose organic matter in wastewater?

• Total Suspended Solids (TSS)
• Biochemical Oxygen Demand (BOD)
• Chemical Oxygen Demand (COD)
• pH

Rationale: BOD is a measure of the oxygen required by microorganisms to

break down organic material in wastewater over a specified time.

• What is the typical detention time for primary clarifiers in a wastewater

treatment plant?

• 5–10 minutes
• 1–2 hours
• 12–24 hours
• 3–4 days

Rationale: Primary clarifiers generally operate with a detention time of 1–

• hours, allowing solids to settle without significant decomposition. 2 / 4

5. The main purpose of aeration in activated sludge systems is to:

• Settle solids
• Provide oxygen to microorganisms
• Remove nutrients
• Disinfect effluent

Rationale: Aeration supplies dissolved oxygen to support aerobic

microorganisms that consume organic matter in the wastewater.

• Which of the following is a characteristic of anoxic conditions?
• High dissolved oxygen
• Presence of free chlorine
• Absence of dissolved oxygen but presence of nitrate
• High pH

Rationale: Anoxic conditions occur when dissolved oxygen is absent, but

nitrate is available to act as an electron acceptor during denitrification.

7. In wastewater treatment, flocculation is primarily used to:

• Aerate the water
• Aggregate small particles into larger flocs
• Kill pathogens
• Reduce ammonia

Rationale: Flocculation gently mixes water to encourage small particles to

form larger, settleable flocs. 3 / 4

• What is the main objective of secondary treatment?
• Remove heavy metals
• Remove grease and oils
• Remove dissolved and suspended organic matter
• Disinfect wastewater

Rationale: Secondary treatment focuses on reducing organic matter

through biological processes such as activated sludge or trickling filters.

9. Nitrification is a two-step biological process that converts:

• Ammonia to nitrogen gas
• Nitrite to ammonia
• Ammonia to nitrate via nitrite
• Nitrate to ammonia

Rationale: Nitrification involves the oxidation of ammonia to nitrite by

Nitrosomonas and then nitrite to nitrate by Nitrobacter.

10. Denitrification in wastewater treatment primarily converts:

• Ammonia to nitrate
• Nitrite to ammonia
• Nitrate to nitrogen gas
• Nitrogen gas to nitrate

Rationale: Denitrification is an anoxic process where bacteria reduce

nitrate to nitrogen gas, removing nitrogen from wastewater.

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