Practice Examination Questions And

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

• Which process removes settleable solids from raw wastewater before

biological treatment?

• Activated sludge
• Trickling filtration
• Primary clarification
• Disinfection

Primary clarification removes heavier solids via sedimentation before biological steps.

• What is the normal pH range for effective nitrification?

A. 4–5

B. 5–6 1 / 4

C. 6.5–7

D. 7.0–8.5

Nitrifying bacteria require near-neutral to slightly alkaline conditions for optimal activity.

3. The purpose of return activated sludge (RAS) is to:

• Remove pathogens
• Maintain biological mass in the aeration basin
• Thicken sludge
• Provide alkalinity

RAS supplies enough microorganisms to sustain treatment efficiency.

4. The primary organism responsible for nitrification is:

• Anaerobic heterotrophs
• Autotrophic nitrifiers
• Fungi
• Protozoa

Autotrophic nitrifiers oxidize ammonia to nitrate using inorganic carbon sources.

5. A high F/M ratio typically results in:

• Old sludge conditions
• Young, rapidly growing biomass
• Low oxygen uptake 2 / 4

• Settled sludge bulking

High food availability promotes young, high-growth microbial populations.

6. Grit removal protects downstream equipment by removing:

• Organics
• Fats and oils
• Inorganic heavy particles
• Pathogens

Grit chambers target sand, gravel, and similar abrasive materials.

7. A typical dissolved oxygen level for conventional aeration basins is:

• 0.1 mg/L
• 0.5 mg/L
• 1 mg/L
• 2 mg/L or greater

Maintaining ≥2 mg/L ensures adequate oxygen for aerobic biological processes.

8. Sludge bulking is often caused by excessive growth of:

• Amoebas
• Ciliates
• Filamentous bacteria
• Rotifers

Filamentous organisms produce poor settling sludge.

• Chlorination for disinfection works primarily by: 3 / 4

• Oxidizing organic carbon
• Destroying cell walls and metabolic functions of pathogens
• Adding alkalinity
• Reducing nitrate

Chlorine disrupts cell membranes and enzymatic processes.

10. A composite sample is best used to:

• Measure peak flows
• Assess storm load
• Represent average conditions over time
• Track specific spills

Composite sampling provides a time-weighted or flow-weighted average.

11. The primary hazard of hydrogen sulfide (H₂S) in sewers is:

• Corrosion only
• Infectious risk
• Toxicity and explosion risk
• Vitamin depletion

H₂S can be lethal and contributes to corrosion and explosive conditions.

12. Sludge age is defined as:

• Wasting frequency
• Digester residence time
• Average time solids remain in the system
• / 4