Operator Class 3 Practice Exam

Québec Wastewater Treatment Operator Class 3 Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

1. The primary purpose of a grit chamber in wastewater treatment is to:

a) Remove dissolved organic matter

b) Settle heavy inorganic particles

c) Reduce BOD

d) Neutralize pH

b) Settle heavy inorganic particles

Rationale: Grit chambers are designed to remove sand, gravel, and

other dense particles that can damage equipment and reduce treatment efficiency. 1 / 4

• Which of the following is the most common method for disinfecting

treated wastewater?

a) Filtration

b) Chlorination

c) Sedimentation

d) Aeration

b) Chlorination

Rationale: Chlorination is widely used to kill pathogenic

microorganisms before discharge or reuse.

• What is the typical function of a primary clarifier?

a) Reduce nutrient concentration

b) Remove settleable solids

c) Aerate wastewater

d) Add chemicals

b) Remove settleable solids

Rationale: Primary clarifiers allow solids to settle by gravity,

reducing suspended solids load for secondary treatment.

4. In activated sludge systems, the main purpose of aeration is to:

a) Reduce pH

b) Provide oxygen for microbial metabolism

c) Remove grit

d) Increase BOD

b) Provide oxygen for microbial metabolism 2 / 4

Rationale: Oxygen is required for aerobic bacteria to metabolize

organic matter in the wastewater.

• Which parameter measures the amount of oxygen consumed by

microorganisms in 5 days?

a) COD

b) BOD5

c) TSS

d) DO

b) BOD5

Rationale: BOD5 (Biochemical Oxygen Demand over 5 days) is a

standard measure of organic pollution in wastewater.

6. Nitrification in wastewater treatment converts:

a) Ammonia to nitrate

b) Nitrate to ammonia

c) Nitrogen gas to ammonia

d) Nitrite to organic nitrogen

a) Ammonia to nitrate

Rationale: Nitrification is a two-step aerobic process performed by

autotrophic bacteria, converting ammonia to nitrite and then nitrate.

• Denitrification requires which condition to occur?

a) Aerobic

b) Anaerobic

c) High pH 3 / 4

d) Low temperature

b) Anaerobic

Rationale: Denitrifying bacteria reduce nitrate to nitrogen gas in the

absence of oxygen.

• What is the main purpose of a secondary clarifier?

a) Remove heavy metals

b) Settle biological flocs

c) Neutralize pH

d) Filter solids

b) Settle biological flocs

Rationale: Secondary clarifiers separate the biomass (activated

sludge) from treated water for recycling or disposal.

9. Total Suspended Solids (TSS) in wastewater primarily measure:

a) Dissolved nutrients

b) Particulate matter

c) Dissolved oxygen

d) Pathogens

b) Particulate matter

Rationale: TSS represents the solids that are suspended in water and

can affect treatment processes and water quality.

• Which of the following is an advantage of extended aeration

systems?

a) Requires less energy than trickling filters

• / 4