Practice Examination Questions And

Indiana Wastewater Operator Class II Practice Examination Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

(Covering: wastewater treatment processes, math, safety, collection

systems, regulations, lab, microbiology)

• What is the primary purpose of secondary treatment?
• Removal of grit
• Removal of biodegradable organic matter
• Disinfection
• Sludge thickening

Rationale: Biological secondary treatment reduces BOD by removing

dissolved and suspended organics.

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• The typical range of dissolved oxygen (DO) in an activated sludge

aeration basin is:

• 0–0.5 mg/L
• 0.5–1.0 mg/L
• 1.5–3.0 mg/L
• 5–7 mg/L

Rationale: Adequate DO for mixed liquor is usually between 1.5–3.0 mg/L

to maintain aerobic conditions.

• What does BOD measure?
• Nitrogen concentration
• Microbial density
• Amount of oxygen required to degrade organic matter
• Chlorine demand

Rationale: BOD quantifies oxygen needed by microorganisms to consume

biodegradable organics.

• Which microorganism is primarily responsible for nitrification?
• Protozoa
• Fungi
• Nitrifying bacteria
• Anaerobic archaea 2 / 4

Rationale: Nitrification is carried out by autotrophic nitrifying bacteria

such as Nitrosomonas and Nitrobacter.

5. A clarifier’s main function in activated sludge treatment is:

• Aeration
• Solids-liquid separation
• Disinfection
• Odor control

Rationale: Secondary clarifiers settle biomass and return settled solids to

the aeration tank.

6. Chlorine residual after disinfection ensures:

• Sludge stabilization
• Odor removal
• Continued pathogen control in the distribution system
• pH correction

Rationale: A residual confirms adequate disinfectant remains to inactivate

pathogens.

7. F/M ratio compares: 3 / 4

• Flow to mechanical equipment
• Food to microorganisms
• BOD load to mixed liquor mass
• MLSS to DO

Rationale: F/M describes organic load relative to biomass available to

treat it.

8. Sludge bulking is often caused by:

• Excess DO
• Filamentous organisms
• Too much chlorine
• Low flow

Rationale: Filamentous bacteria hinder settling and create bulking sludge.

• What is a typical MLVSS/MLSS ratio?

A. 10%

B. 65–75%

C. 90%

D. 30%

Rationale: The volatile fraction represents living biomass and commonly

ranges 65–75%.

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