Sewage Treatment Operator Class III Licensing Exam Q&A with Rationales update

Sewage Treatment Operator Class III v v v v




Licensing Exam Questions And Correct An v v v v v




swers (Verified Answers) Plus Rationales
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2026 Q&A | Instant Download Pdf
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1. What is the primary purpose of secondary treatment in a sewage t
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reatment plant? v




A. Remove grit v




B. Settle out large solids
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C. Biologically degrade organic matter v v v




D. Add disinfectant
v




Answer: C v




Rationale: Secondary treatment uses microorganisms to biologically oxidize
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v and remove dissolved and colloidal organic matter.
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2. Which microorganism group is primarily responsible for nitrification?
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,A. Protozoa
B. Fungi
C. Autotrophic bacteria v




D. Viruses
Answer: C v




Rationale: Autotrophic nitrifying bacteria convert ammonia to nitrite and nit
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rate.



3. The measure of oxygen consumed by microorganisms to degrade
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organic matter over 5 days is known as:
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A. COD
B. TOC
C. BOD₅
D. MPN
Answer: C v




Rationale: BOD₅ measures biochemical oxygen demand over a 5-day period.
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4. What is the ideal DO concentration for an activated sludge aeration t
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ank?

A. 0–1 mg/L
v




B. 2–4 mg/L
v

,C. 6–8 mg/L v




D. >10 mg/L v




Answer: B v




Rationale: Maintaining 2–4 mg/L DO supports aerobic microbial activity.
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5. Sludge with excessive filamentous bacteria typically produces:
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A. Good settling v




B. Bulking sludge v




C. Sand accumulation
v




D. Anaerobic odors v




Answer: B v




Rationale: Filamentous overgrowth causes fluffy, poorly settling sludge.
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6. The most common disinfectant used in wastewater treatment is:
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A. Ozone
B. UV light
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C. Chlorine
D. Bromine
Answer: C v




Rationale: Chlorine is widely used due to its effectiveness and low cost.
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7. In the activated sludge process, F/M ratio stands for:
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, A. Flow/Mass
B. Feed/Mixing
C. Food-to-Microorganism ratio v




D. Filter/Membrane
Answer: C v




Rationale: F/M describes the relationship between organic load and biomass
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concentration.



8. A primary clarifier mainly removes:
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A. Dissolved solids v




B. Nitrates
C. Settleable and floating solids
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D. Pathogens
Answer: C v




Rationale: Primary treatment relies on physical settling and skimming.
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9. The main purpose of anaerobic digestion is to:
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A. Oxygenate sludge v




B. Stabilize sludge and reduce volume
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C. Increase nitrogenv




D. Remove grit v