Wastewater Treatment Operator Exam
Practice Questions and Correct Answers
(Verified Answers) Plus Rationale 2026
Q&A| Instant Download Pdf
Domain 1: Wastewater Fundamentals and Preliminary Treatment
Question 1 — Wastewater Characteristics
Which substance contributes most directly to the organic load in domestic
wastewater?
A. Sand
B. Chloride
C. Biodegradable organic matter
D. Dissolved oxygen
Answer: C. Biodegradable organic matter
Rationale: Biodegradable organic matter provides food for microorganisms and
contributes significantly to the biochemical oxygen demand of wastewater.
Question 2 — BOD
BOD is primarily a measure of:
A. Total dissolved salts
B. Oxygen required by microorganisms to biologically stabilize organic matter
C. Amount of suspended grit
D. Chlorine concentration
Answer: B. Oxygen required by microorganisms to biologically stabilize organic
matter
,Rationale: Biochemical oxygen demand, or BOD, indicates the amount of dissolved
oxygen microorganisms require to break down biodegradable organic material
under specified test conditions.
Question 3 — TSS
Total suspended solids are:
A. Solids completely dissolved in wastewater
B. Particles that can be retained from wastewater by a specified filtration
procedure
C. Only biological organisms
D. Only inorganic chemicals
Answer: B. Particles that can be retained from wastewater by a specified
filtration procedure
Rationale: TSS represents suspended particulate material in wastewater and is
commonly used to evaluate treatment performance.
Question 4 — Screening
The primary purpose of a bar screen is to remove:
A. Dissolved ammonia
B. Large debris and solids
C. Dissolved oxygen
D. Chlorine residual
Answer: B. Large debris and solids
Rationale: Screens remove larger materials such as rags, plastics, sticks, and other
debris that could damage pumps or interfere with downstream treatment.
Question 5 — Grit Removal
A grit chamber is designed primarily to remove:
,A. Dissolved organic matter
B. Heavy inorganic particles such as sand and gravel
C. Dissolved ammonia
D. Bacteria
Answer: B. Heavy inorganic particles such as sand and gravel
Rationale: Grit removal protects downstream equipment and reduces
accumulation of abrasive inorganic solids. Typical grit includes sand, gravel, and
similar dense particles.
Question 6 — Grit Settling
Grit settles primarily because of its:
A. High biological activity
B. Relatively high specific gravity
C. High chlorine demand
D. Low temperature
Answer: B. Relatively high specific gravity
Rationale: Grit particles are denser than most organic wastewater solids, allowing
them to settle when the grit chamber is properly designed and operated.
Question 7 — Flow Measurement
A Parshall flume is commonly used to:
A. Remove grease
B. Measure wastewater flow
C. Digest sludge
D. Disinfect effluent
Answer: B. Measure wastewater flow
Rationale: A Parshall flume is a hydraulic flow-measuring device commonly used in
wastewater treatment facilities.
, Domain 2: Primary Treatment
Question 8 — Primary Clarifier
The main purpose of a primary clarifier is to:
A. Add oxygen to wastewater
B. Remove settleable solids and floating materials
C. Destroy all pathogens
D. Remove dissolved salts
Answer: B. Remove settleable solids and floating materials
Rationale: Primary clarification uses gravity to remove settleable solids as primary
sludge and floating materials such as grease and scum.
Question 9 — Detention Time
Clarifier detention time refers to:
A. Sludge age
B. The average time wastewater remains in the clarifier
C. Pump operating time
D. Chlorine contact time only
Answer: B. The average time wastewater remains in the clarifier
Rationale: Detention time is commonly calculated by dividing basin volume by
flow rate. It provides an estimate of how long wastewater remains in the
treatment unit.
Question 10 — Scum Removal
Scum in a primary clarifier consists primarily of:
A. Settled grit
B. Floating materials such as fats, oils, and grease
Practice Questions and Correct Answers
(Verified Answers) Plus Rationale 2026
Q&A| Instant Download Pdf
Domain 1: Wastewater Fundamentals and Preliminary Treatment
Question 1 — Wastewater Characteristics
Which substance contributes most directly to the organic load in domestic
wastewater?
A. Sand
B. Chloride
C. Biodegradable organic matter
D. Dissolved oxygen
Answer: C. Biodegradable organic matter
Rationale: Biodegradable organic matter provides food for microorganisms and
contributes significantly to the biochemical oxygen demand of wastewater.
Question 2 — BOD
BOD is primarily a measure of:
A. Total dissolved salts
B. Oxygen required by microorganisms to biologically stabilize organic matter
C. Amount of suspended grit
D. Chlorine concentration
Answer: B. Oxygen required by microorganisms to biologically stabilize organic
matter
,Rationale: Biochemical oxygen demand, or BOD, indicates the amount of dissolved
oxygen microorganisms require to break down biodegradable organic material
under specified test conditions.
Question 3 — TSS
Total suspended solids are:
A. Solids completely dissolved in wastewater
B. Particles that can be retained from wastewater by a specified filtration
procedure
C. Only biological organisms
D. Only inorganic chemicals
Answer: B. Particles that can be retained from wastewater by a specified
filtration procedure
Rationale: TSS represents suspended particulate material in wastewater and is
commonly used to evaluate treatment performance.
Question 4 — Screening
The primary purpose of a bar screen is to remove:
A. Dissolved ammonia
B. Large debris and solids
C. Dissolved oxygen
D. Chlorine residual
Answer: B. Large debris and solids
Rationale: Screens remove larger materials such as rags, plastics, sticks, and other
debris that could damage pumps or interfere with downstream treatment.
Question 5 — Grit Removal
A grit chamber is designed primarily to remove:
,A. Dissolved organic matter
B. Heavy inorganic particles such as sand and gravel
C. Dissolved ammonia
D. Bacteria
Answer: B. Heavy inorganic particles such as sand and gravel
Rationale: Grit removal protects downstream equipment and reduces
accumulation of abrasive inorganic solids. Typical grit includes sand, gravel, and
similar dense particles.
Question 6 — Grit Settling
Grit settles primarily because of its:
A. High biological activity
B. Relatively high specific gravity
C. High chlorine demand
D. Low temperature
Answer: B. Relatively high specific gravity
Rationale: Grit particles are denser than most organic wastewater solids, allowing
them to settle when the grit chamber is properly designed and operated.
Question 7 — Flow Measurement
A Parshall flume is commonly used to:
A. Remove grease
B. Measure wastewater flow
C. Digest sludge
D. Disinfect effluent
Answer: B. Measure wastewater flow
Rationale: A Parshall flume is a hydraulic flow-measuring device commonly used in
wastewater treatment facilities.
, Domain 2: Primary Treatment
Question 8 — Primary Clarifier
The main purpose of a primary clarifier is to:
A. Add oxygen to wastewater
B. Remove settleable solids and floating materials
C. Destroy all pathogens
D. Remove dissolved salts
Answer: B. Remove settleable solids and floating materials
Rationale: Primary clarification uses gravity to remove settleable solids as primary
sludge and floating materials such as grease and scum.
Question 9 — Detention Time
Clarifier detention time refers to:
A. Sludge age
B. The average time wastewater remains in the clarifier
C. Pump operating time
D. Chlorine contact time only
Answer: B. The average time wastewater remains in the clarifier
Rationale: Detention time is commonly calculated by dividing basin volume by
flow rate. It provides an estimate of how long wastewater remains in the
treatment unit.
Question 10 — Scum Removal
Scum in a primary clarifier consists primarily of:
A. Settled grit
B. Floating materials such as fats, oils, and grease
