Washington Wastewater Treatment Plant Operator Group III Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

Washington Wastewater Treatment Plant Operator Group III Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary purpose of preliminary treatment in a wastewater

treatment plant?

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

Answer: B

Rationale: Preliminary treatment (screening, grit removal) is designed to

remove large solids, debris, and grit before further treatment to protect downstream equipment and processes. 1 / 4

• In primary clarifiers, the removal of suspended solids is based mainly

on:

• Biological oxidation
• Chemical precipitation
• Sedimentation by gravity
• Filtration

Answer: C

Rationale: Primary clarifiers rely on gravity settling; suspended solids settle

out under quiescent conditions.

3. A trickling filter is a type of:

• Activated sludge process
• Attached-growth biological treatment
• Tertiary filtration
• Clarification process

Answer: B

Rationale: Trickling filters are fixed-film (attached-growth) systems where

biofilm grows on a media and degrades the organic matter.

• What is F/M ratio in activated sludge theory?
• Flow per meter of tank
• Food (BOD) to microorganism (MLSS) ratio
• Fractional mass per flux rate
• Filtration to maintenance ratio 2 / 4

Answer: B

Rationale: F/M ratio is the mass of food (measured as BOD) per mass of

microorganisms (measured as MLSS), a key parameter in biological process control.

5. Mixed Liquor Suspended Solids (MLSS) refers to:

• Solids in the raw influent
• Solids in the clarifier effluent
• Concentration of suspended solids in the aeration tank
• Settleable solids in primary sludge

Answer: C

Rationale: MLSS is the concentration of mixed (activated) biomass plus non-

biological solids in the aeration basin.

• What is SRT (Solids Retention Time) in an activated sludge system?
• The time wastewater spends in the clarifier
• The hydraulic retention time of the aeration tank
• The average time that biomass is kept in the system
• The time sludge spends in the digester

Answer: C

Rationale: SRT (or MCRT) represents the average age of the microorganisms

in the system, which affects biomass stability and performance.

• A drop in dissolved oxygen (DO) concentration in the aeration tank

could be caused by: 3 / 4

• Reduced influent BOD
• Mechanical failure of the blower
• Excessive recirculation rate
• Lower temperature

Answer: B

Rationale: If the blower fails or malfunctions, air supply drops and DO will

fall, impairing biological processes.

• For nitrification to occur in activated sludge, which condition is

essential?

• High F/M ratio
• Low dissolved oxygen (e.g., <0>
• Adequate alkalinity and moderate temperature
• High ammonia concentration only

Answer: C

Rationale: Nitrifying bacteria require sufficient alkalinity, adequate DO

(often >1.5 mg/L), and moderate temperatures to convert ammonia to nitrate.

9. Chlorination is most commonly used for:

• Primary treatment
• Biological oxidation
• Disinfection of effluent
• Sludge thickening
• / 4