Operator Class IV Exam Practice

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

• Which of the following is the primary purpose of the activated sludge

process?

a) Disinfection

b) Biological treatment of wastewater

c) Solids dewatering

d) Grit removal

Rationale: The activated sludge process is designed to biologically

remove organic matter from wastewater using microorganisms.

2. The term BOD stands for:

a) Biological Oxygen Demand 1 / 4

b) Biochemical Organic Degradation

c) Biochemical Oxygen Demand

d) Basic Oxygen Diffusion

Rationale: BOD measures the amount of dissolved oxygen needed by

microorganisms to decompose organic matter in water.

• Which of the following is a common method for disinfection in

wastewater treatment?

a) Sedimentation

b) Filtration

c) Chlorination

d) Aeration

Rationale: Chlorination is widely used to disinfect treated

wastewater before discharge to reduce pathogens.

• Which process is primarily responsible for nitrogen removal in

wastewater?

a) Sedimentation

b) Biological nitrification and denitrification

c) Chlorination

d) Grit removal

Rationale: Nitrogen is removed biologically through nitrification

(ammonia to nitrate) and denitrification (nitrate to nitrogen gas).

5. A common indicator of fecal contamination in wastewater is:

a) Nitrate 2 / 4

b) Ammonia

c) Coliform bacteria

d) Phosphate

Rationale: Coliform bacteria, such as E. coli, indicate potential

presence of pathogens.

• What is the typical purpose of primary clarification?

a) Biological treatment

b) Nutrient removal

c) Solids settling

d) Disinfection

Rationale: Primary clarification removes settleable solids and some

floatables from wastewater.

7. The chemical formula for chlorine gas is:

a) ClO₂

b) Cl₂

c) NaCl

d) HCl

Rationale: Chlorine gas consists of diatomic chlorine molecules, Cl₂.

• Which of the following is a characteristic of an ideal settling tank?

a) High turbulence

b) Rapid flow

c) Uniform velocity distribution

d) High oxygen content 3 / 4

Rationale: Uniform velocity allows solids to settle efficiently without

resuspension.

• What is the main purpose of sludge digestion?

a) Disinfection

b) Stabilization of organic solids

c) Nutrient addition

d) Grit removal

Rationale: Sludge digestion biologically stabilizes organic matter,

reducing odor and volume.

• What does TSS stand for in wastewater terminology?

a) Total soluble solids

b) Total suspended solids

c) Total sedimentary solids

d) Total sludge solids

Rationale: TSS measures the solids in water that are not dissolved

and can be removed by filtration.

• Which of the following units is used to express BOD?
• mg/L
• mg/L
• ppmv
• m³/day

Rationale: BOD is typically expressed in milligrams per liter (mg/L) to

quantify organic matter in water.

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