Operator Level IV Practice Exam

Oregon Wastewater Treatment Operator Level IV Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• The primary purpose of preliminary treatment in a wastewater

treatment plant is to:

• Remove dissolved organics
• Remove nutrients
• Remove large solids and grit
• Disinfect effluent

Rationale: Preliminary treatment focuses on removing large solids, grit,

and debris to prevent damage to downstream equipment.

2. The most common method of primary sludge thickening is:

• Aeration 1 / 4

• Gravity thickening
• Chemical precipitation
• Filtration

Rationale: Gravity thickening allows sludge to settle and consolidate,

reducing volume before further treatment.

3. The nitrification process in biological treatment converts:

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

Rationale: Nitrification is a two-step aerobic process converting ammonia

to nitrite and then nitrate.

• Which of the following is a common indicator of effective coagulation?
• Dissolved oxygen
• Turbidity reduction
• pH decrease
• Chlorine demand

Rationale: Coagulation floc formation and subsequent sedimentation

reduces turbidity, indicating process efficiency.

5. The primary function of a clarifier in wastewater treatment is to:

• Aerate the wastewater
• Remove nutrients 2 / 4

• Settle suspended solids
• Reduce BOD chemically

Rationale: Clarifiers allow suspended solids to settle out by gravity,

producing clarified effluent.

6. A sudden drop in DO in an aeration basin is often caused by:

• Excessive chlorine
• Low influent BOD
• Shock loading of organic matter
• Nitrification inhibition

Rationale: High organic loads consume oxygen rapidly, lowering dissolved

oxygen levels.

7. The chemical used most commonly for phosphorus removal is:

• Sodium hypochlorite
• Aluminum sulfate (alum)
• Calcium carbonate
• Magnesium hydroxide

Rationale: Alum reacts with phosphate to form an insoluble precipitate

that can be removed in sedimentation.

8. The term “sludge age” or “mean cell residence time” refers to:

• The age of the plant
• Average time microorganisms remain in the system 3 / 4

• Time for primary treatment
• Hydraulic retention time in clarifier

Rationale: Sludge age indicates the average time biomass stays in the

system, affecting nitrification and BOD removal.

9. Chlorine residual in a distribution system is necessary to:

• Remove TSS
• Maintain disinfection and prevent regrowth
• Improve pH
• Remove ammonia

Rationale: Maintaining a chlorine residual ensures pathogens do not

regrow in the system.

• The pH range optimal for nitrification in activated sludge

systems is:

A. 4.5–5.5

B. 6.0–6.5

C. 6.5–8.0

D. 8.5–9.5

Rationale: Nitrifying bacteria perform best in slightly alkaline conditions

between 6.5 and 8.0.

11. The primary purpose of an anaerobic digester is to:

• Aerate sludge
• Stabilize organic matter and reduce volume
• / 4