Operator Level II Practice Exam

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

• The primary purpose of primary clarification in wastewater treatment

is to:

• Remove nutrients
• Remove settleable solids
• Disinfect effluent
• Reduce biological oxygen demand

Rationale: Primary clarification allows settleable solids to settle out,

reducing the load on secondary treatment.

• Which of the following is a common method for measuring

biochemical oxygen demand (BOD)? 1 / 4

• Titration with iodine
• Incubation at 20°C for 5 days
• Spectrophotometry at 420 nm
• Membrane filtration

Rationale: BOD5 is measured by incubating a wastewater sample at 20°C

for 5 days and measuring oxygen depletion.

3. Activated sludge process is primarily designed to:

• Chemically remove phosphorus
• Biologically remove organic matter
• Remove heavy metals
• Increase pH

Rationale: Activated sludge uses microorganisms to metabolize organic

matter, reducing BOD and suspended solids.

• Which microorganism group is most responsible for nitrification in

wastewater treatment?

• Fungi
• Nitrosomonas and Nitrobacter
• Algae
• Protozoa

Rationale: Nitrosomonas oxidize ammonia to nitrite, and Nitrobacter

oxidize nitrite to nitrate in nitrification. 2 / 4

5. In a trickling filter, the primary mechanism of treatment is:

• Sedimentation
• Biological oxidation by attached microorganisms
• Chemical precipitation
• Filtration

Rationale: Trickling filters rely on microbial biofilms on media surfaces to

degrade organic matter.

• What is the typical mixed liquor suspended solids (MLSS) range in a

Level II activated sludge plant?

• 50–100 mg/L
• 100–200 mg/L
• 2000–4000 mg/L
• 5000–7000 mg/L

Rationale: MLSS is the concentration of suspended solids in the aeration

tank; Level II plants commonly maintain 2000–4000 mg/L.

7. Denitrification in wastewater treatment requires:

• Aerobic conditions
• High temperatures
• Anoxic conditions and a carbon source
• Ultraviolet light

Rationale: Denitrification occurs under anoxic conditions where nitrate is

reduced to nitrogen gas using an organic carbon source. 3 / 4

• Which sludge treatment process reduces pathogens and odors most

effectively?

• Gravity thickening
• Anaerobic digestion
• Chemical coagulation
• Filtration

Rationale: Anaerobic digestion stabilizes sludge, reduces pathogens, and

decreases odor.

9. In wastewater treatment, F/M ratio refers to:

• Flow rate to media
• Food to microorganism ratio
• Filtration to mixing ratio
• Flocculation to mixing ratio

Rationale: F/M ratio indicates the amount of organic matter (food)

available per unit of microorganisms, critical for process control.

10. Chlorination of treated wastewater is primarily used to:

• Remove BOD
• Remove phosphorus
• Disinfect effluent
• Enhance sedimentation

Rationale: Chlorination kills or inactivates pathogenic microorganisms

before discharge or reuse.

• / 4