Practice Exam Questions And Correct

Ohio Wastewater Operator Class III 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?

• To remove dissolved nutrients
• To remove large solids and grit
• To disinfect the effluent
• To remove bacteria

Rationale: Preliminary treatment removes large solids and grit to prevent

damage and clogging in downstream equipment.

• Which of the following is a typical BOD5 concentration in domestic

wastewater? 1 / 4

• 50 mg/L
• 100 mg/L
• 200 mg/L
• 250 mg/L

Rationale: Domestic wastewater usually has a BOD5 around 200–300

mg/L, reflecting organic matter that can be biologically oxidized in 5 days.

3. The main purpose of aeration in an activated sludge process is to:

• Remove phosphorus
• Provide oxygen for microorganisms to degrade organic matter
• Clarify sludge
• Reduce heavy metals

Rationale: Aeration supplies dissolved oxygen to support microbial

metabolism in organic matter removal.

• Which of the following is a symptom of nitrification inhibition in a

treatment plant?

• Increase in ammonia concentration in effluent
• Decrease in sludge volume
• Higher pH in effluent
• Reduced TSS in effluent

Rationale: Nitrification inhibition prevents conversion of ammonia to

nitrate, causing ammonia accumulation. 2 / 4

• What is the typical detention time in a primary clarifier?
• 5–10 minutes
• 30–45 minutes
• 1.5–2.5 hours
• 6–8 hours

Rationale: Primary clarifiers are designed to allow 1.5–2.5 hours for

settling of solids.

• Which microorganism is primarily responsible for nitrification?
• Escherichia coli
• Nitrosomonas and Nitrobacter
• Bacillus subtilis
• Pseudomonas aeruginosa

Rationale: Nitrosomonas oxidizes ammonia to nitrite, and Nitrobacter

converts nitrite to nitrate.

• What is the function of a sludge digester?
• Remove heavy metals
• Stabilize organic solids by microbial digestion
• Aerate the wastewater
• Remove grit

Rationale: Sludge digesters reduce volatile solids and pathogens through

anaerobic or aerobic microbial processes. 3 / 4

• Which chemical is commonly used for phosphorus removal in

wastewater treatment?

• Sodium hypochlorite
• Aluminum sulfate (alum)
• Calcium hydroxide
• Ferric cyanide

Rationale: Alum precipitates phosphorus as aluminum phosphate, reducing

phosphorus in effluent.

• What is the typical MLSS (Mixed Liquor Suspended Solids)

concentration in an activated sludge system?

• 50–100 mg/L
• 200–500 mg/L
• 2000–4000 mg/L
• 6000–8000 mg/L

Rationale: MLSS in conventional activated sludge is usually 2000–4000

mg/L to maintain sufficient biomass for treatment.

• What is the typical sludge age (SRT) for a nitrifying activated

sludge system?

• 1–2 days
• 3–5 days
• 10–15 days
• 20–30 days
• / 4