Practice Examination Questions And

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

• What is the primary purpose of a wastewater treatment plant?
• Generate drinking water
• Remove pollutants to protect public health and the environment
• Produce electricity
• Store stormwater

Answer: b

Rationale: Treatment plants are designed to remove contaminants before

discharge to protect ecosystems and human health.

• Which process is most associated with removing settleable solids?
• Chlorination
• Primary clarification 1 / 4

• Aeration
• Filtration

Answer: b

Rationale: Primary clarifiers allow heavier solids to settle by gravity.

• What does BOD measure?
• Chemical concentration
• Odor strength
• Oxygen needed by microorganisms to decompose organic matter
• Turbidity

Answer: c

Rationale: Biological Oxygen Demand quantifies biodegradable organic

load.

4. A lift station is used to:

• Reduce odors
• Pump wastewater from lower to higher elevation
• Aerate sludge
• Remove grit

Answer: b

Rationale: Lift stations overcome elevation differences in collection

systems.

• What is a typical detention time for a primary clarifier? 2 / 4

• 30 seconds
• 1–2 minutes
• 1–2 hours
• 24 hours

Answer: c

Rationale: Primary clarifiers require hours, not minutes, for adequate

settling.

6. The main purpose of aeration in secondary treatment is:

• Disinfection
• Sedimentation
• Provide oxygen for biological activity
• Chemical precipitation

Answer: c

Rationale: Aeration supports microorganisms that break down organic

pollutants.

7. Activated sludge is:

• Chemical coagulant
• Biomass of microorganisms used for biological treatment
• Sand filtration material
• Primary sludge

Answer: b

Rationale: Activated sludge contains living microbes that degrade

organics. 3 / 4

• What does TSS stand for?
• Total Solids Saturation
• Total Suspended Solids
• Thickened Sludge Solids
• Total System Solids

Answer: b

Rationale: TSS measures solids suspended in water that can be removed by

settling.

9. A common method of sludge thickening is:

• Chlorination
• Gravity thickening
• UV disinfection
• Filtration only

Answer: b

Rationale: Gravity thickeners concentrate sludge by settling.

10. Chlorination is mainly used for:

• Nutrient removal
• Solids removal
• Disinfection
• pH adjustment

Answer: c

Rationale: Chlorine kills pathogenic organisms prior to discharge.

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