D Practice Exam Questions And Correct

Minnesota Wastewater Operator Class D Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• What is the primary purpose of a wastewater treatment plant?
• To remove trash from water
• To protect public health and the environment by treating

wastewater

• To produce drinking water
• To generate electricity

Rationale: Wastewater treatment plants are designed to remove

pollutants and pathogens to protect public health and the environment.

• Which of the following is a typical parameter measured in influent

wastewater? 1 / 4

• pH of drinking water
• BOD (Biochemical Oxygen Demand)
• Chlorine residual
• Fluoride

Rationale: BOD measures the amount of oxygen needed by microorganisms

to decompose organic matter in wastewater, a key indicator of wastewater strength.

3. The first step in most wastewater treatment plants is:

• Filtration
• Screening
• Chlorination
• Aeration

Rationale: Screening removes large solids and debris that could damage

equipment or interfere with downstream processes.

• What is the function of primary settling tanks?
• To disinfect wastewater
• To remove settleable solids from wastewater
• To add nutrients to wastewater
• To aerate the wastewater

Rationale: Primary settling tanks allow solids to settle out by gravity,

reducing the suspended solids load before secondary treatment. 2 / 4

5. Secondary treatment primarily removes:

• Large debris
• Dissolved and suspended organic matter
• Heavy metals
• Oil and grease

Rationale: Secondary treatment uses biological processes to break down

dissolved and suspended organic matter that remains after primary treatment.

• Activated sludge is an example of which type of treatment?
• Primary treatment
• Secondary treatment
• Tertiary treatment
• Chemical treatment

Rationale: Activated sludge is a biological process where microorganisms

consume organic matter during secondary treatment.

7. The purpose of chlorination in wastewater treatment is to:

• Increase pH
• Disinfect the effluent
• Reduce BOD
• Remove nitrogen

Rationale: Chlorination kills pathogenic organisms in treated wastewater

before discharge to the environment. 3 / 4

• Which of the following can indicate a malfunctioning aeration system?
• Clear effluent
• Odor of rotten eggs (hydrogen sulfide)
• Proper sludge settling
• Normal DO levels

Rationale: A strong hydrogen sulfide odor can indicate low oxygen levels,

meaning the aeration system is not functioning properly.

• What is the ideal pH range for biological treatment in activated sludge

systems?

A. 2–4

B. 5–6

C. 6.5–8.5

D. 9–11

Rationale: Most microorganisms in activated sludge function optimally in

the neutral to slightly alkaline range (6.5–8.5).

• Which of the following best describes BOD?
• Chemical Oxygen Demand
• Total Dissolved Solids
• The oxygen required by microorganisms to decompose organic

matter

• Dissolved oxygen in effluent
• / 4