ABC Wastewater Treatment Operator Class III (Level
3) Certification Exam – Practice Questions & Answers
| Comprehensive Exam Prep Study Guide 2026/2027
Question 1
Turbidity in wastewater is caused by:
A. Color
B. Dissolved calcium
C. Hardness
D. Finely divided suspended material
Answer: D
Rationale: Turbidity in wastewater is caused by finely divided suspended material
that scatters light and gives the water a cloudy appearance.
Question 2
The gas most commonly associated with septic wastewater is:
A. Carbon dioxide
B. Carbon monoxide
C. Hydrogen sulfide
D. Methane
Answer: C
Rationale: Hydrogen sulfide is the gas most commonly associated with septic
wastewater, produced by bacterial reduction of sulfate compounds under anaerobic
conditions.
Question 3
How many cubic meters per second in 10 MGD?
A. 0.44
B. 1.44
C. 4.73
D. 24.62
Answer: A
pg. 1
,Rationale: 10 MGD = 10,000,000 gallons/day. Convert to m³/day (1 gallon =
0.003785 m³) = 37,850 m³/day. Divide by 86,400 seconds/day = 0.438 m³/s ≈ 0.44.
Question 4
Mechanical ventilation of a lift station is required to:
A. Lower temperatures to reduce proportion of hydrogen sulfide
B. Reduce chlorine demand
C. Reduce corrosion
D. Increase DO in raw wastewater
Answer: C
Rationale: Mechanical ventilation is required to reduce corrosion by removing
hydrogen sulfide gas and other corrosive gases from the lift station atmosphere.
Question 5
A wet well probe is usually used for determination(s) of level:
A. Approximate
B. Dual point
C. Continuous
D. Single point
Answer: D
Rationale: A wet well probe is usually used for single point determination of
level, providing on/off control for pumps.
Question 6
A pump is delivering at less than the expected rate of discharge. Which of the
causes listed below is incorrect?
A. Speed of motor too low
B. Pump not primed
C. Impeller clogged
D. Discharge head too low
Answer: D
Rationale: Discharge head too low would increase flow rate, not decrease it. The
other options all cause reduced discharge rates.
pg. 2
,Question 7
Metal pipes often fail because of:
A. Back siphonage
B. Browning movement
C. Galvanic action
D. Reverse osmosis
Answer: C
Rationale: Metal pipes often fail due to galvanic action, where dissimilar metals in
contact with an electrolyte create an electrochemical reaction causing corrosion.
Question 8
When a fire hydrant is operated, it should be:
A. Just opened enough to have a flow
B. Opened fully
C. Opened halfway
D. Regulated to flow required
Answer: B
Rationale: Fire hydrants should be opened fully when operated to prevent damage
to the valve and ensure proper flow.
Question 9
If the pump bearings on horizontal centrifugal pumps are over lubricated, the most
important effect is that:
A. Will result in smoother and more efficient operation of the pump
B. Will not make any difference in the operation of the pump
C. Will be wasted
D. Shaft or shaft sleeve will be damaged
Answer: D
Rationale: Over-lubrication can cause overheating and damage to the shaft or
shaft sleeve due to excessive pressure and friction.
Question 10
If packing is not maintained properly:
A. Cavitation damage will result
B. Impeller will corrode
pg. 3
, C. Loss of suction due to air being allowed to enter pump
D. Shaft or shaft sleeve will be damaged
Answer: C
Rationale: Improperly maintained packing allows air to enter the pump, causing
loss of suction and reduced pumping efficiency.
Question 11
Air flow through a trickling filter when the influent is warmer than the air
temperature is:
A. Upward through the filter
B. Downward through the filter
C. Inadequate and may require forced air ventilation
D. Improved by decreasing the recirculation
Answer: A
Rationale: When influent is warmer than the air, warmer air rises upward through
the filter, creating natural draft.
Question 12
An active sludge in the aeration tank with an abundant and available food supply
will have a relatively:
A. High oxygen uptake and fast settling rate
B. High oxygen uptake and slow settling rate
C. Low oxygen uptake and fast settling rate
D. Low oxygen uptake and slow settling rate
Answer: B
Rationale: Active sludge with abundant food has high oxygen uptake and a slow
settling rate due to the rapid growth of dispersed microorganisms.
Question 13
The reaction of chlorine and ammonia produces a compound which is called:
A. Ammonium chloride
B. Chloral hydrate
C. Chloramine
D. Hydrazine
Answer: C
pg. 4
3) Certification Exam – Practice Questions & Answers
| Comprehensive Exam Prep Study Guide 2026/2027
Question 1
Turbidity in wastewater is caused by:
A. Color
B. Dissolved calcium
C. Hardness
D. Finely divided suspended material
Answer: D
Rationale: Turbidity in wastewater is caused by finely divided suspended material
that scatters light and gives the water a cloudy appearance.
Question 2
The gas most commonly associated with septic wastewater is:
A. Carbon dioxide
B. Carbon monoxide
C. Hydrogen sulfide
D. Methane
Answer: C
Rationale: Hydrogen sulfide is the gas most commonly associated with septic
wastewater, produced by bacterial reduction of sulfate compounds under anaerobic
conditions.
Question 3
How many cubic meters per second in 10 MGD?
A. 0.44
B. 1.44
C. 4.73
D. 24.62
Answer: A
pg. 1
,Rationale: 10 MGD = 10,000,000 gallons/day. Convert to m³/day (1 gallon =
0.003785 m³) = 37,850 m³/day. Divide by 86,400 seconds/day = 0.438 m³/s ≈ 0.44.
Question 4
Mechanical ventilation of a lift station is required to:
A. Lower temperatures to reduce proportion of hydrogen sulfide
B. Reduce chlorine demand
C. Reduce corrosion
D. Increase DO in raw wastewater
Answer: C
Rationale: Mechanical ventilation is required to reduce corrosion by removing
hydrogen sulfide gas and other corrosive gases from the lift station atmosphere.
Question 5
A wet well probe is usually used for determination(s) of level:
A. Approximate
B. Dual point
C. Continuous
D. Single point
Answer: D
Rationale: A wet well probe is usually used for single point determination of
level, providing on/off control for pumps.
Question 6
A pump is delivering at less than the expected rate of discharge. Which of the
causes listed below is incorrect?
A. Speed of motor too low
B. Pump not primed
C. Impeller clogged
D. Discharge head too low
Answer: D
Rationale: Discharge head too low would increase flow rate, not decrease it. The
other options all cause reduced discharge rates.
pg. 2
,Question 7
Metal pipes often fail because of:
A. Back siphonage
B. Browning movement
C. Galvanic action
D. Reverse osmosis
Answer: C
Rationale: Metal pipes often fail due to galvanic action, where dissimilar metals in
contact with an electrolyte create an electrochemical reaction causing corrosion.
Question 8
When a fire hydrant is operated, it should be:
A. Just opened enough to have a flow
B. Opened fully
C. Opened halfway
D. Regulated to flow required
Answer: B
Rationale: Fire hydrants should be opened fully when operated to prevent damage
to the valve and ensure proper flow.
Question 9
If the pump bearings on horizontal centrifugal pumps are over lubricated, the most
important effect is that:
A. Will result in smoother and more efficient operation of the pump
B. Will not make any difference in the operation of the pump
C. Will be wasted
D. Shaft or shaft sleeve will be damaged
Answer: D
Rationale: Over-lubrication can cause overheating and damage to the shaft or
shaft sleeve due to excessive pressure and friction.
Question 10
If packing is not maintained properly:
A. Cavitation damage will result
B. Impeller will corrode
pg. 3
, C. Loss of suction due to air being allowed to enter pump
D. Shaft or shaft sleeve will be damaged
Answer: C
Rationale: Improperly maintained packing allows air to enter the pump, causing
loss of suction and reduced pumping efficiency.
Question 11
Air flow through a trickling filter when the influent is warmer than the air
temperature is:
A. Upward through the filter
B. Downward through the filter
C. Inadequate and may require forced air ventilation
D. Improved by decreasing the recirculation
Answer: A
Rationale: When influent is warmer than the air, warmer air rises upward through
the filter, creating natural draft.
Question 12
An active sludge in the aeration tank with an abundant and available food supply
will have a relatively:
A. High oxygen uptake and fast settling rate
B. High oxygen uptake and slow settling rate
C. Low oxygen uptake and fast settling rate
D. Low oxygen uptake and slow settling rate
Answer: B
Rationale: Active sludge with abundant food has high oxygen uptake and a slow
settling rate due to the rapid growth of dispersed microorganisms.
Question 13
The reaction of chlorine and ammonia produces a compound which is called:
A. Ammonium chloride
B. Chloral hydrate
C. Chloramine
D. Hydrazine
Answer: C
pg. 4
