Operator Grade 3 Practice Exam Questions

Rhode Island Wastewater Treatment Operator Grade 3 Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• Which microorganism is primarily responsible for nitrification in

activated sludge systems?

• Escherichia coli
• Pseudomonas
• Nitrosomonas
• Bacillus

Rationale: Nitrosomonas bacteria oxidize ammonia to nitrite, a key

step in nitrification.

• What is the typical pH range for optimal biological treatment in an

activated sludge process?

A. 4.5 – 5.5 1 / 4

B. 6.0 – 6.5

C. 6.5 – 8.0

D. 8.5 – 9.5

Rationale: Biological treatment is most effective in a neutral to

slightly alkaline pH range.

• Which chemical is commonly used for phosphorus removal in

wastewater?

• Sodium hypochlorite
• Alum (Aluminum sulfate)
• Ferric cyanide
• Calcium carbonate

Rationale: Alum precipitates phosphorus, allowing it to be removed

in the sludge.

• What is the main purpose of a grit chamber in a wastewater treatment

plant?

• Remove organic matter
• Remove sand, gravel, and heavy inorganic solids
• Remove dissolved nutrients
• Aerate the wastewater

Rationale: Grit chambers prevent abrasive materials from damaging

downstream equipment.

• Which factor most influences sludge settling in a secondary clarifier?
• Water temperature 2 / 4

• Mixed liquor suspended solids (MLSS) concentration
• pH
• Color of influent

Rationale: Proper MLSS concentration ensures sludge flocculation

and settling efficiency.

• In a trickling filter, what is the primary function of the biofilm?
• To filter out large debris
• To biologically degrade organic matter
• To chemically disinfect wastewater
• To remove grit

Rationale: Microorganisms in the biofilm metabolize organic

pollutants in the wastewater.

• What is the primary nitrogen compound removed during

denitrification?

• Ammonia
• Nitrite
• Nitrate
• Urea

Rationale: Denitrifying bacteria convert nitrate into nitrogen gas,

reducing nitrogen levels in effluent.

• Which of the following is considered a secondary treatment process?
• Screening
• Grit removal 3 / 4

• Activated sludge process
• Chlorination

Rationale: Secondary treatment biologically removes dissolved and

suspended organic matter.

• Which unit operation is primarily used for the physical separation of

solids from liquids?

• Aeration
• Coagulation
• Sedimentation
• Disinfection

Rationale: Sedimentation allows heavier solids to settle out of

wastewater by gravity.

• What is the primary purpose of chlorination in wastewater

treatment?

• Remove solids
• Remove phosphorus
• Disinfect effluent to kill pathogens
• Neutralize pH

Rationale: Chlorination is a chemical disinfection process aimed at

reducing harmful microorganisms.

• Which of the following is a typical indicator of organic load in

wastewater?

• pH
• / 4