Licensing Exam Practice Test 100

Sewage Treatment Operator Class III Licensing Exam Practice Test 100 Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary purpose of secondary treatment in a sewage

treatment plant?

• Remove grit
• Settle out large solids
• Biologically degrade organic matter
• Add disinfectant

Answer: C

Rationale: Secondary treatment uses microorganisms to biologically

oxidize and remove dissolved and colloidal organic matter.

• Which microorganism group is primarily responsible for nitrification? 1 / 4

• Protozoa
• Fungi
• Autotrophic bacteria
• Viruses

Answer: C

Rationale: Autotrophic nitrifying bacteria convert ammonia to nitrite and

nitrate.

• 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.

• What is the ideal DO concentration for an activated sludge aeration

tank?

• 0–1 mg/L
• 2–4 mg/L 2 / 4

• 6–8 mg/L
• >10 mg/L

Answer: B

Rationale: Maintaining 2–4 mg/L DO supports aerobic microbial activity.

5. Sludge with excessive filamentous bacteria typically produces:

• Good settling
• Bulking sludge
• Sand accumulation
• Anaerobic odors

Answer: B

Rationale: Filamentous overgrowth causes fluffy, poorly settling sludge.

6. The most common disinfectant used in wastewater treatment is:

• Ozone
• UV light
• Chlorine
• Bromine

Answer: C

Rationale: Chlorine is widely used due to its effectiveness and low cost.

• In the activated sludge process, F/M ratio stands for: 3 / 4

• Flow/Mass
• Feed/Mixing
• Food-to-Microorganism ratio
• Filter/Membrane

Answer: C

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

biomass concentration.

8. A primary clarifier mainly removes:

• Dissolved solids
• Nitrates
• Settleable and floating solids
• Pathogens

Answer: C

Rationale: Primary treatment relies on physical settling and skimming.

9. The main purpose of anaerobic digestion is to:

• Oxygenate sludge
• Stabilize sludge and reduce volume
• Increase nitrogen
• Remove grit
• / 4