Sewage Treatment Operator Class III
Licensing Exam Practice Test 100
Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary purpose of secondary treatment in a sewage
treatment plant?
A. Remove grit
B. Settle out large solids
C. Biologically degrade organic matter
D. Add disinfectant
Answer: C
Rationale: Secondary treatment uses microorganisms to biologically
oxidize and remove dissolved and colloidal organic matter.
2. Which microorganism group is primarily responsible for nitrification?
,A. Protozoa
B. Fungi
C. Autotrophic bacteria
D. Viruses
Answer: C
Rationale: Autotrophic nitrifying bacteria convert ammonia to nitrite and
nitrate.
3. The measure of oxygen consumed by microorganisms to degrade
organic matter over 5 days is known as:
A. COD
B. TOC
C. BOD₅
D. MPN
Answer: C
Rationale: BOD₅ measures biochemical oxygen demand over a 5-day
period.
4. What is the ideal DO concentration for an activated sludge aeration
tank?
A. 0–1 mg/L
B. 2–4 mg/L
,C. 6–8 mg/L
D. >10 mg/L
Answer: B
Rationale: Maintaining 2–4 mg/L DO supports aerobic microbial activity.
5. Sludge with excessive filamentous bacteria typically produces:
A. Good settling
B. Bulking sludge
C. Sand accumulation
D. Anaerobic odors
Answer: B
Rationale: Filamentous overgrowth causes fluffy, poorly settling sludge.
6. The most common disinfectant used in wastewater treatment is:
A. Ozone
B. UV light
C. Chlorine
D. Bromine
Answer: C
Rationale: Chlorine is widely used due to its effectiveness and low cost.
7. In the activated sludge process, F/M ratio stands for:
, A. Flow/Mass
B. Feed/Mixing
C. Food-to-Microorganism ratio
D. Filter/Membrane
Answer: C
Rationale: F/M describes the relationship between organic load and
biomass concentration.
8. A primary clarifier mainly removes:
A. Dissolved solids
B. Nitrates
C. Settleable and floating solids
D. Pathogens
Answer: C
Rationale: Primary treatment relies on physical settling and skimming.
9. The main purpose of anaerobic digestion is to:
A. Oxygenate sludge
B. Stabilize sludge and reduce volume
C. Increase nitrogen
D. Remove grit
Licensing Exam Practice Test 100
Questions And Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary purpose of secondary treatment in a sewage
treatment plant?
A. Remove grit
B. Settle out large solids
C. Biologically degrade organic matter
D. Add disinfectant
Answer: C
Rationale: Secondary treatment uses microorganisms to biologically
oxidize and remove dissolved and colloidal organic matter.
2. Which microorganism group is primarily responsible for nitrification?
,A. Protozoa
B. Fungi
C. Autotrophic bacteria
D. Viruses
Answer: C
Rationale: Autotrophic nitrifying bacteria convert ammonia to nitrite and
nitrate.
3. The measure of oxygen consumed by microorganisms to degrade
organic matter over 5 days is known as:
A. COD
B. TOC
C. BOD₅
D. MPN
Answer: C
Rationale: BOD₅ measures biochemical oxygen demand over a 5-day
period.
4. What is the ideal DO concentration for an activated sludge aeration
tank?
A. 0–1 mg/L
B. 2–4 mg/L
,C. 6–8 mg/L
D. >10 mg/L
Answer: B
Rationale: Maintaining 2–4 mg/L DO supports aerobic microbial activity.
5. Sludge with excessive filamentous bacteria typically produces:
A. Good settling
B. Bulking sludge
C. Sand accumulation
D. Anaerobic odors
Answer: B
Rationale: Filamentous overgrowth causes fluffy, poorly settling sludge.
6. The most common disinfectant used in wastewater treatment is:
A. Ozone
B. UV light
C. Chlorine
D. Bromine
Answer: C
Rationale: Chlorine is widely used due to its effectiveness and low cost.
7. In the activated sludge process, F/M ratio stands for:
, A. Flow/Mass
B. Feed/Mixing
C. Food-to-Microorganism ratio
D. Filter/Membrane
Answer: C
Rationale: F/M describes the relationship between organic load and
biomass concentration.
8. A primary clarifier mainly removes:
A. Dissolved solids
B. Nitrates
C. Settleable and floating solids
D. Pathogens
Answer: C
Rationale: Primary treatment relies on physical settling and skimming.
9. The main purpose of anaerobic digestion is to:
A. Oxygenate sludge
B. Stabilize sludge and reduce volume
C. Increase nitrogen
D. Remove grit
