of Qualification Class III Exam Practice

Ontario Wastewater Operator Certificate of Qualification Class III Exam Practice Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary purpose of a grit chamber in a wastewater

treatment plant?

• Remove organic solids
• Remove sand, gravel, and heavy inorganic solids
• Increase dissolved oxygen
• Adjust pH

Rationale: Grit chambers are designed to remove heavy inorganic

materials such as sand, gravel, and small stones to prevent wear on pumps and downstream equipment.

• Which microorganism is most responsible for nitrification in

wastewater treatment? 1 / 4

• Escherichia coli
• Nitrosomonas and Nitrobacter
• Bacillus subtilis
• Pseudomonas aeruginosa

Rationale: Nitrosomonas oxidize ammonia to nitrite, and Nitrobacter

oxidize nitrite to nitrate, completing nitrification.

3. The purpose of secondary clarifiers in activated sludge systems is to:

• Settle out biomass from treated water
• Remove grit and sand
• Increase aeration
• Adjust pH

Rationale: Secondary clarifiers separate the biological sludge from the

treated effluent to allow recycling or disposal.

4. Chlorination is primarily used in wastewater treatment to:

• Remove solids
• Disinfect effluent
• Remove phosphorus
• Reduce BOD

Rationale: Chlorine destroys pathogenic microorganisms to reduce the

risk of disease in the receiving environment.

• What is the typical range of dissolved oxygen (DO) required in an

aeration tank for an activated sludge process?

• 0–1 mg/L 2 / 4

• 1–2 mg/L
• 2–4 mg/L
• 5–10 mg/L

Rationale: Maintaining DO at 2–4 mg/L ensures sufficient oxygen for

aerobic bacteria without wasting energy.

• What is the purpose of a bar screen at a wastewater treatment plant?
• Settle solids
• Remove large debris such as rags and sticks
• Reduce BOD
• Control odor

Rationale: Bar screens protect downstream equipment by removing

large solids that could clog pumps or damage equipment.

• What does BOD5 measure?
• Total solids in wastewater
• The amount of oxygen required by microorganisms to decompose

organic matter in five days

• Nitrogen content
• pH level

Rationale: BOD5 is a standard measure of the biodegradable organic

content in wastewater over five days at 20°C.

• What is the main advantage of using extended aeration in activated

sludge systems?

• Reduces pH 3 / 4

• Simplifies sludge handling and improves effluent quality
• Removes heavy metals
• Reduces temperature

Rationale: Extended aeration treats wastewater for longer periods,

reducing sludge production and improving effluent quality.

• Which sludge treatment method is most commonly used to reduce

volume and pathogens?

• Aeration
• Anaerobic digestion
• Coagulation
• Filtration

Rationale: Anaerobic digestion breaks down organic matter, reduces

pathogen levels, and stabilizes sludge for disposal.

• What is the primary purpose of a flow equalization basin?
• Remove BOD
• Buffer hydraulic and organic load variations
• Disinfect effluent
• Remove grit

Rationale: Flow equalization basins regulate flow and pollutant

concentrations to prevent overloading downstream processes.

• Which chemical is commonly used for phosphorus removal in

wastewater?

• Sodium hypochlorite
• / 4