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Industrial Wastewater Treatment Operator Class III Practice Exam Questions And Correct Answers (Verified Answers) Plus Rationales 2026 Q&A | Instant Download Pdf
1. The primary purpose of preliminary treatment in wastewater plants is to:
- Remove dissolved organics
- Remove large solids and grit
- Reduce BOD
- Disinfect the water
Rationale: Preliminary treatment protects downstream equipment by
removing debris and grit that could cause damage or clogging.
2. The term BOD refers to:
- Biochemical Organic Determinant
- Biochemical Oxygen Demand
- Biological Oxidation Determination
- Basic Oxygen Dissolution 1 / 3
Rationale: BOD measures the oxygen used by microorganisms to
decompose organic matter in wastewater.
- Which process primarily removes suspended solids?
- Chlorination
- Sedimentation
- Filtration
- Aeration
Rationale: Sedimentation allows heavier solids to settle by gravity before
biological treatment.
- What is the main purpose of secondary treatment?
- Disinfection
- Sludge thickening
- Biological oxidation of organic matter
- Removal of grit
Rationale: Secondary treatment uses microorganisms to oxidize and
stabilize organic pollutants.
5. In activated sludge, the term “MLSS” stands for:
- Mixed Liquor Solid Substrate
- Main Liquid Settling Solids
- Mixed Liquor Suspended Solids
- Microbial Load Solids System
Rationale: MLSS measures suspended solids in the aeration basin,
representing microorganisms and organic matter. 2 / 3
- Which condition causes sludge bulking?
- High dissolved oxygen
- Filamentous bacteria growth
- Low sludge age
- Excess chlorine
Rationale: Filamentous bacteria prevent proper settling, leading to bulking
sludge.
- What is the typical pH range for effective biological treatment?
A. 3–5
B. 5–6
C. 6.5–8.5
D. 9–10
Rationale: Most microorganisms perform optimally within a near-neutral
pH range.
- What is a common method to control odor in wastewater plants?
- Adding sodium chloride
- Maintaining aerobic conditions
- Increasing influent flow
- Adding sulfur compounds
Rationale: Aerobic environments prevent formation of hydrogen sulfide
and other odor-causing gases.
9. A clarifier is designed to:
- Aerate the wastewater
- / 3
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