Class IV Practice Examination Questions

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

Q1. What is the primary purpose of secondary treatment in a wastewater facility?

• Removal of grit
• Removal of heavy metals
• Removal of biodegradable organic matter
• Sludge thickening

Rationale: Secondary treatment focuses on biological oxidation of organic

matter measured as BOD.Q2. What does BOD primarily measure?

• Nitrogen concentration
• Chemical oxidation strength 1 / 4

• Oxygen required for microbial degradation of organics
• Suspended solids content

Rationale: BOD quantifies dissolved oxygen needed for microorganisms to

oxidize organic materials.

Q3. Nitrification is the biological conversion of:

• Nitrate to nitrogen gas
• Ammonia to nitrate
• Nitrite to ammonia
• Nitrogen gas to ammonia

Rationale: Nitrification occurs in two steps: ammonia → nitrite → nitrate.

Q4. The primary microorganisms responsible for nitrification are:

• Protozoa
• Nitrifying bacteria
• Fungi
• Anaerobic archaea

Rationale: Nitrifiers (Nitrosomonas, Nitrobacter) oxidize ammonia and

nitrite.

Q5. The F/M ratio is defined as:

• Food mass over facility maintenance
• Pounds of BOD applied per pound of MLVSS
• Flow rate divided by MLSS
• Flow rate multiplied by BOD

Rationale: F/M compares loading to the mass of microorganisms. 2 / 4

Q6. Sludge bulking in activated sludge is commonly caused by:

• High DO
• Toxic influent
• Excessive filamentous bacteria
• Low solids retention time

Rationale: Filamentous organisms cause poor settling and bulking.

Q7. The purpose of anaerobic digestion is to:

• Increase sludge volume
• Convert nitrate to ammonia
• Remove grit
• Stabilize sludge by converting organics to methane and CO₂

Rationale: Anaerobic digestion reduces volatile solids and produces biogas.

Q8. The optimum DO level in an activated sludge aeration basin is:

• 0.2 mg/L
• 1–2 mg/L
• 2–4 mg/L
• 8 mg/L

Rationale: Most systems target 2–4 mg/L to support aerobic

microorganisms.

Q9. Chlorine disinfection effectiveness increases with:

• Higher pH
• Lower pH
• Higher alkalinity 3 / 4

• Lower temperature

Rationale: Hypochlorous acid (more effective) predominates at lower pH.

Q10. The primary function of a grit chamber is to remove:

• Oils and greases
• Heavy, inorganic particles
• Dissolved solids
• Settleable biomass

Rationale: Grit chambers remove sand, gravel, and similar materials.

Q11. Sludge age is also known as:

A. MLSS

B. F/M

• Solids retention time
• Sludge blanket depth

Rationale: Sludge age describes average time solids remain in the system.

Q12. The term MLVSS refers to:

• Mineral volatile suspended solids
• Volatile fraction of MLSS representing active biomass
• Maximum liquid volume of sludge
• Mineral load volume

Rationale: MLVSS indicates the active microbial mass.

Q13. A common cause of foaming in aerobic digesters is:

• Low pH
• Excessive grit
• / 4