Operator Level III Practice Exam

Oregon Wastewater Treatment Operator Level III Practice Exam Questions And ( 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 large solids and grit
• Disinfect the effluent
• Reduce BOD

Rationale: Preliminary treatment, including screening and grit removal, is

designed to remove large solids and grit to protect downstream equipment and processes. 1 / 4

• Which of the following best describes BOD5?
• Total solids in wastewater
• The amount of oxygen consumed by microorganisms over 5 days
• The amount of dissolved oxygen in effluent
• The pH level over 5 days

Rationale: BOD5 measures the oxygen demand of wastewater over a 5-day

period, indicating the amount of biodegradable organic material.

• In activated sludge systems, what is the function of the aeration tank?
• Remove grit
• Settle solids
• Provide oxygen for microbial degradation of organics
• Disinfect wastewater

Rationale: The aeration tank introduces oxygen to support microorganisms

that consume organic pollutants.

• What is the typical retention time for secondary clarifiers?
• 5–10 minutes
• 15–30 minutes
• 2–4 hours
• 6–12 hours

Rationale: Secondary clarifiers are designed to allow enough time for

biomass to settle, usually 2–4 hours. 2 / 4

• Which pathogen is most commonly used as an indicator of fecal

contamination?

• Salmonella
• E. coli
• Hepatitis A
• Giardia

Rationale: E. coli is a reliable indicator of fecal contamination and

potential pathogenic presence in wastewater.

6. Nitrification in wastewater treatment converts:

• Ammonia to nitrate
• Nitrite to ammonia
• Nitrate to ammonia
• Nitrogen gas to ammonia

Rationale: Nitrification is a biological oxidation process where ammonia is

converted to nitrate by nitrifying bacteria.

• Denitrification is carried out under which conditions?
• Aerobic
• Anoxic
• Anaerobic
• Microaerophilic

Rationale: Denitrification occurs in the absence of oxygen (anoxic

conditions) to convert nitrate to nitrogen gas. 3 / 4

• Which chemical is commonly used for disinfection in wastewater

treatment?

• Ammonia
• Chlorine
• Phosphate
• Lime

Rationale: Chlorine is widely used as a disinfectant to kill pathogens in

treated wastewater.

9. The purpose of sludge digestion is to:

• Increase solids
• Stabilize organic material
• Remove phosphorus
• Increase BOD

Rationale: Sludge digestion biologically stabilizes organic matter, reducing

odors and pathogen content.

• Which parameter is most affected by the addition of coagulants

in a treatment plant?

• pH
• Turbidity

C. BOD

• Dissolved oxygen
• / 4