of Qualification Class IV Exam Practice

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

• What is the primary purpose of a wastewater treatment plant?
• To produce drinking water
• To remove contaminants from wastewater before discharge
• To increase water hardness
• To filter stormwater only

Rationale: Wastewater treatment plants are designed to remove physical,

chemical, and biological contaminants from sewage to protect public health and the environment.

2. The BOD test measures:

• Dissolved oxygen in the water 1 / 4

• The amount of oxygen required by microorganisms to decompose

organic matter

• Total suspended solids
• Nutrient concentration

Rationale: BOD (Biochemical Oxygen Demand) indicates the organic

pollution load in wastewater and the oxygen required for microbial decomposition.

3. Nitrification in wastewater treatment involves:

• Conversion of nitrate to ammonia
• Conversion of ammonia to nitrate
• Removal of phosphorus
• Reduction of dissolved oxygen

Rationale: Nitrification is a two-step biological process where ammonia is

first oxidized to nitrite and then to nitrate by nitrifying bacteria.

4. Denitrification occurs under:

• Aerobic conditions
• Anoxic conditions
• High pH only
• High temperature

Rationale: Denitrification is the biological reduction of nitrate to nitrogen

gas under oxygen-limited (anoxic) conditions. 2 / 4

5. The primary purpose of sedimentation in a treatment plant is to:

• Kill bacteria
• Remove settleable solids from wastewater
• Oxidize ammonia
• Add chlorine

Rationale: Sedimentation allows heavy particles to settle out of wastewater

before further treatment.

• Which of the following is a common coagulant used in wastewater

treatment?

• Sodium chloride
• Aluminum sulfate
• Calcium carbonate
• Sodium bicarbonate

Rationale: Aluminum sulfate (alum) is widely used to destabilize and

aggregate fine particles for easier removal.

• Which process removes phosphorus from wastewater?
• Aeration
• Filtration
• Chemical precipitation
• Sedimentation only

Rationale: Chemical precipitation using salts like alum or ferric chloride binds phosphorus into solids that can be removed. 3 / 4

• What is the main objective of an aeration tank?
• Remove heavy metals
• Provide oxygen for biological treatment
• Clarify water
• Neutralize pH

Rationale: Aeration tanks supply oxygen to support the growth of

microorganisms that break down organic pollutants.

9. Activated sludge is:

• Raw sewage
• Settled sludge
• A mixture of microorganisms and wastewater
• Chemical sludge

Rationale: Activated sludge contains microorganisms that consume organic

matter in aeration systems.

10. Sludge digestion primarily aims to:

• Increase solids content
• Stabilize organic matter and reduce pathogens
• Remove nitrogen
• Remove phosphorus

Rationale: Anaerobic or aerobic digestion stabilizes sludge, reducing odor,

volume, and pathogen content.

• / 4