of Qualification Class V Practice Exam

Ontario Wastewater Operator Certificate of Qualification Class V Practice Exam 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?

• Remove dissolved nutrients
• Remove heavy metals
• Remove large solids and grit
• Reduce BOD

Rationale: Preliminary treatment is designed to remove large solids,

grit, and debris to protect downstream equipment.

• Which microorganism is most commonly responsible for nitrification

in wastewater treatment?

• Escherichia coli 1 / 4

• Nitrosomonas and Nitrobacter
• Clostridium
• Pseudomonas

Rationale: Nitrosomonas oxidizes ammonia to nitrite, and

Nitrobacter oxidizes nitrite to nitrate, which are key steps in nitrification.

3. The term “CBOD” refers to:

• Chemical Biochemical Oxygen Demand
• Carbonaceous Biochemical Oxygen Demand
• Complete Biochemical Oxygen Demand
• Combined Biochemical Oxidation Demand

Rationale: CBOD measures the oxygen demand of organic matter in

wastewater excluding nitrogenous demand.

• What is the main function of an aeration tank in a conventional

activated sludge system?

• Remove grit
• Provide oxygen for microbial degradation of organic matter
• Clarify effluent
• Disinfect wastewater

Rationale: Aeration tanks supply oxygen to microorganisms so they

can break down organic matter efficiently.

• Which parameter is most commonly used to measure the organic

strength of wastewater? 2 / 4

• pH
• Total Solids

C) BOD

• Turbidity

Rationale: Biochemical Oxygen Demand (BOD) indicates the amount

of biodegradable organic material present.

6. In the activated sludge process, sludge age is also known as:

• Hydraulic retention time
• Mean cell residence time (MCRT)
• Detention time
• Sludge settling time

Rationale: Sludge age or MCRT refers to the average time

microorganisms remain in the system.

• What is the purpose of chlorination in wastewater treatment?
• Remove solids
• Disinfect the effluent
• Reduce BOD
• Promote sedimentation

Rationale: Chlorination is used to inactivate pathogens in the treated

effluent.

• Which of the following can cause bulking sludge in an activated sludge

system?

• High F/M ratio 3 / 4

• Filamentous bacteria overgrowth
• Low temperature
• Excessive aeration

Rationale: Filamentous bacteria prevent proper settling, causing

bulking sludge.

9. The term “hydraulic retention time” refers to:

• Sludge age
• Effluent quality
• The average time wastewater stays in a treatment unit
• Flow rate

Rationale: HRT is calculated by dividing the volume of the tank by the

influent flow rate.

• Which of the following is considered a primary pollutant in raw

sewage?

• Nitrogen
• Phosphorus
• Pathogenic microorganisms
• Dissolved oxygen

Rationale: Pathogens are present in raw sewage and are considered

primary pollutants until removed or disinfected.

11. The main goal of secondary treatment is to:

• Remove grit and sand
• Remove biodegradable organic matter
• / 4