Exam Practice Questions And Correct

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

• Which of the following is the primary purpose of preliminary treatment

in a wastewater treatment plant?

• Reduce nutrient levels
• Remove large solids and grit
• Disinfect wastewater
• Reduce biochemical oxygen demand

Answer: B

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

protect downstream equipment and improve the efficiency of subsequent treatment processes. 1 / 4

• What is the main function of a primary clarifier?
• Aerate wastewater
• Remove dissolved oxygen
• Settle suspended solids
• Neutralize pH

Answer: C

Rationale: Primary clarifiers allow heavier solids to settle by gravity, forming primary sludge, while grease and scum are removed from the surface.

• Which chemical is commonly used for phosphorus removal in

wastewater treatment?

• Sodium hypochlorite
• Alum (Aluminum sulfate)
• Calcium carbonate
• Sodium hydroxide

Answer: B

Rationale: Alum reacts with phosphate ions to form insoluble aluminum

phosphate, which can then be removed by sedimentation.

• Which of the following parameters is most critical in controlling an

activated sludge system?

• Temperature
• Dissolved oxygen (DO)
• pH
• Turbidity 2 / 4

Answer: B

Rationale: Adequate DO is essential for aerobic microorganisms to

metabolize organic matter efficiently in activated sludge processes.

5. The primary purpose of disinfection in wastewater treatment is to:

• Remove suspended solids
• Kill or inactivate pathogenic microorganisms
• Reduce nutrients
• Neutralize odors

Answer: B

Rationale: Disinfection reduces or eliminates pathogens to protect public

health before wastewater is discharged or reused.

6. In nitrification, ammonia is first converted to:

• Nitrogen gas
• Nitrite (NO2-)
• Nitrate (NO3-)
• Ammonium (NH4+)

Answer: B

Rationale: Nitrification is a two-step biological process: ammonia is oxidized to nitrite by Nitrosomonas and then to nitrate by Nitrobacter.

• Which of the following best describes the sludge age (mean cell

residence time) in an activated sludge process?

• Time wastewater remains in the primary clarifier 3 / 4

• Time solids settle in the secondary clarifier
• Average time microorganisms stay in the system
• Time required for disinfection

Answer: C

Rationale: Sludge age affects microbial population stability, system

performance, and nutrient removal efficiency.

• What is the primary purpose of an aeration basin in secondary

treatment?

• Provide oxygen to microorganisms
• Remove grit and debris
• Neutralize pH
• Settle solids

Answer: A

Rationale: Aeration basins supply oxygen to aerobic microorganisms to

break down organic matter in wastewater.

• Which of the following is the primary mechanism for removing

suspended solids in a secondary clarifier?

• Filtration
• Coagulation
• Sedimentation
• Adsorption
• / 4