Class 4 Exam Practice Questions And

Québec Wastewater Treatment Operator Class 4 Exam Practice Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary purpose of preliminary treatment in a wastewater

treatment plant?

a) Remove nutrients

b) Remove large solids and debris

c) Disinfect wastewater

d) Reduce BOD

Rationale: Preliminary treatment removes large solids, grit, and debris to

protect downstream equipment and processes.

• Which parameter is most commonly used to measure the strength of

organic pollution in wastewater? 1 / 4

• pH

b) Total Suspended Solids

c) Biochemical Oxygen Demand (BOD)

d) Conductivity

Rationale: BOD measures the amount of oxygen required by microorganisms

to decompose organic matter in wastewater.

• What is the function of a grit chamber?

a) Remove grease and oil

b) Reduce BOD

c) Remove sand, gravel, and other heavy particles

d) Adjust pH

Rationale: Grit chambers remove heavy inorganic particles that can cause

wear on pumps and other equipment.

• Which of the following processes is classified as secondary treatment?

a) Screening

b) Sedimentation

c) Activated sludge process

d) Chlorination

Rationale: Secondary treatment removes dissolved and colloidal organic

matter using biological processes.

• What is the main goal of tertiary treatment?

a) Remove grit 2 / 4

b) Polish effluent to remove nutrients and pathogens

c) Aerate wastewater

d) Sludge digestion

Rationale: Tertiary treatment further improves effluent quality by removing

residual nutrients, pathogens, and suspended solids.

• What is the typical purpose of a primary clarifier?

a) Aerate wastewater

b) Settle suspended solids

c) Remove nutrients

d) Disinfect effluent

Rationale: Primary clarifiers allow heavy solids to settle, reducing solids load on secondary treatment.

• In the activated sludge process, what is the function of the return

activated sludge (RAS)?

a) Disinfect effluent

b) Remove nutrients

c) Recycle microorganisms to maintain biological activity

d) Remove grit

Rationale: RAS returns a portion of settled sludge to the aeration tank to

maintain sufficient microbial population.

• Which of the following is a common method for sludge stabilization?

a) Filtration 3 / 4

b) Anaerobic digestion

c) Sedimentation

d) Screening

Rationale: Anaerobic digestion reduces pathogens, odor, and volatile solids

in sludge.

• Which parameter is used to measure the concentration of suspended

solids in water?

a) BOD

b) COD

c) TSS (Total Suspended Solids)

d) DO

Rationale: TSS quantifies the amount of particles suspended in wastewater.

• What is the ideal dissolved oxygen (DO) range for the aeration

tank in an activated sludge system?

• 0–1 mg/L
• 1–2 mg/L
• 2–4 mg/L
• 5–8 mg/L

Rationale: DO between 2–4 mg/L provides sufficient oxygen for

microorganisms without excessive energy consumption.

• Which of the following chemicals is commonly used for

disinfection in wastewater treatment?

• / 4