Level 4 Exam Practice Questions And

New Mexico Wastewater Operator Level 4 Exam Practice Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf

• The primary purpose of preliminary treatment in a wastewater plant is

to:

• Remove dissolved nutrients
• Remove heavy metals
• Remove large solids and debris
• Reduce BOD

Rationale: Preliminary treatment focuses on removing large solids and

debris through screens and grit chambers to prevent damage to downstream equipment.

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2. The term BOD stands for:

• Basic Oxygen Demand
• Biochemical Oxygen Demand
• Biological Oxidation Demand
• Bacterial Oxygen Deficiency

Rationale: BOD measures the amount of oxygen required by microorganisms

to decompose organic matter in water.

• Which unit process is primarily responsible for biological treatment of

wastewater?

• Grit chamber
• Primary clarifier
• Activated sludge system
• Disinfection

Rationale: The activated sludge system uses microorganisms to biologically

degrade organic matter in wastewater.

• Nitrification is a two-step process where ammonia is converted to

nitrate. Which bacteria oxidize ammonia to nitrite?

• Nitrosomonas
• Nitrobacter 2 / 4

• Pseudomonas
• Bacillus

Rationale: Nitrosomonas bacteria convert ammonia (NH₃) to nitrite (NO₂⁻),

while Nitrobacter converts nitrite to nitrate (NO₃⁻).

• Which of the following is a common disinfectant used in wastewater

treatment?

• Sodium hydroxide
• Calcium carbonate
• Chlorine
• Potassium permanganate

Rationale: Chlorine is widely used to disinfect treated wastewater before

discharge to kill pathogenic microorganisms.

6. Sludge digestion primarily reduces:

• pH
• Volatile solids
• Heavy metals
• Nitrogen

Rationale: Anaerobic or aerobic digestion reduces volatile solids in sludge,

decreasing its volume and odor potential.

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• Which of the following is a method of chemical phosphorus removal?
• Aeration
• Filtration
• Alum addition
• Sedimentation

Rationale: Aluminum sulfate (alum) precipitates phosphorus as insoluble

aluminum phosphate for removal.

8. The pH of raw sewage is typically in the range of:

A. 1–3

B. 6–8

C. 9–11

D. 12–14

Rationale: Raw wastewater usually has a near-neutral pH of 6–8, which is

suitable for microbial activity.

9. Tertiary treatment in wastewater plants is designed to:

• Remove grit and solids
• Stabilize sludge
• Remove nutrients and trace contaminants
• Aerate water
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