Operator Class IV Practice Exam

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

• What is the primary purpose of a secondary clarifier in a wastewater

treatment plant?

• To remove large solids from raw wastewater
• To disinfect effluent
• To separate biological solids from treated water
• To aerate wastewater

Rationale: Secondary clarifiers are used to settle out biological solids after

the secondary treatment process, clarifying the water before discharge or further treatment. 1 / 4

• Which microorganism is primarily responsible for nitrification in

wastewater treatment?

• Escherichia coli
• Clostridium
• Nitrosomonas and Nitrobacter
• Pseudomonas

Rationale: Nitrification is a two-step aerobic process where Nitrosomonas

convert ammonia to nitrite, and Nitrobacter convert nitrite to nitrate.

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

activated sludge system?

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

Rationale: MLSS concentrations in activated sludge systems typically range

from 2000 to 4000 mg/L to maintain effective treatment.

• Which of the following is a common method of sludge stabilization?
• Sedimentation
• Filtration
• Anaerobic digestion
• Chlorination

Rationale: Anaerobic digestion stabilizes sludge by biologically decomposing

organic matter, reducing odor and pathogen content. 2 / 4

• What is the function of a bar screen in a wastewater treatment plant?
• To disinfect water
• To remove fine particles
• To remove large debris and rags
• To aerate wastewater

Rationale: Bar screens remove large solids and debris from raw wastewater

to protect downstream equipment.

• Which chemical is commonly used for phosphorus removal in

wastewater treatment?

• Chlorine
• Alum (Aluminum sulfate)
• Sodium hydroxide
• Lime

Rationale: Alum reacts with soluble phosphorus to form insoluble aluminum

phosphate, which can be removed by sedimentation.

• In a trickling filter, what provides the surface area for microbial growth?
• Sand
• Rock, plastic media, or other inert media
• Activated carbon
• Gravel filter

Rationale: Trickling filters use media with high surface area where

microorganisms form a biofilm to degrade organic matter. 3 / 4

• What is the main purpose of chlorination in wastewater treatment?
• Remove nutrients
• Reduce BOD
• Disinfect effluent
• Increase dissolved oxygen

Rationale: Chlorination is primarily used to kill or inactivate pathogens in

treated wastewater.

• Which of the following is an indicator organism for fecal contamination?
• Staphylococcus aureus
• Salmonella
• Escherichia coli
• Clostridium perfringens

Rationale: E. coli is commonly used as an indicator of fecal contamination in

water and wastewater.

• What is the purpose of a dissolved oxygen (DO) probe in an aeration

basin?

• Measure pH
• Monitor oxygen levels for aerobic microbial activity
• Determine sludge volume
• Measure temperature

Rationale: DO probes help operators maintain adequate oxygen for

microorganisms to degrade organic matter efficiently.

• / 4