Page 1 of 99
ABC Water Treatment Operator Class III exam
Actual exam Questions with Correct Answers
and explanations LATEST THIS YEAR -JUST
RELEASED
Summarized Exam Coverage – ABC Water Treatment Class III
Source water protection and watershed management, coagulation/flocculation (jar testing, chemical
dose optimization), sedimentation (tube/plate settlers, sludge removal), filtration (membrane, direct,
conventional, pressure), disinfection (chlorine, chloramines, ozone, UV, CT calculations), fluoridation
and corrosion control, softening (lime-soda ash, ion exchange), iron/manganese removal, taste and odor
control (oxidation, activated carbon), plant hydraulics (pumping, flow measurement), process control
and optimization (turbidity, particle counts), instrumentation (SCADA, analyzers), safety (confined space,
chlorine handling, HAZMAT), water quality sampling and laboratory (coliform, DBP precursors),
regulations (SWTR, LT2, DBP Rule, Lead and Copper), troubleshooting, math (CT, detention time, loading
rates, chemical feed, percent strength), and management.
1. A Class III operator at a surface water plant sees a sudden increase in raw water turbidity from 5 NTU
to 45 NTU after a storm. The first process adjustment should be:
A) Increase the filtration rate
B) Increase the coagulant dose and monitor settled water turbidity
C) Decrease the coagulant dose
D) Backwash all filters immediately
Answer: B – Higher turbidity requires more coagulant to neutralize particles; jar test can guide dose
adjustment.
2. A plant uses chlorine gas. The operator hears a hissing sound near the cylinder valve. The immediate
action is:
A) Tighten the valve with a wrench
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B) Evacuate the area and don SCBA before closing the cylinder valve
C) Spray water on the leak
D) Call the supervisor from inside the room
Answer: B – Chlorine gas is toxic; SCBA is required; stop the leak by closing the cylinder valve if safe.
3. A water system receives a Tier 2 public notice violation for exceeding the maximum contaminant level
(MCL) for total trihalomethanes (TTHMs). The most likely cause is:
A) High turbidity in filtered water
B) Long detention time in clearwell with free chlorine and high organics
C) Low pH in distribution
D) Excessive fluoride
Answer: B – TTHMs form when chlorine reacts with natural organic matter; long contact time increases
formation.
4. A conventional plant’s settled water turbidity is 4 NTU after sedimentation. Filtered water turbidity is
0.15 NTU. What action should the operator take?
A) Increase filtration rate
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B) Optimize coagulation to lower settled water turbidity to ≤2 NTU
C) Backwash filters more frequently
D) No action, filtered water is within limits
Answer: B – SWTR requires settled water turbidity ≤2 NTU for conventional treatment; 4 NTU is too high
and risks filter overload.
5. A membrane filtration plant experiences a rapid increase in transmembrane pressure (TMP). The first
troubleshooting step is:
A) Acid clean the membranes
B) Perform a integrity test
C) Increase the backwash frequency
D) Reduce the flux rate
Answer: C – Increasing TMP often indicates fouling; increasing backwash frequency or chemically
enhanced backwash (CEB) is first step.
6. A water treatment plant uses chloramines for disinfection. The operator measures free ammonia in
the finished water at 0.5 mg/L. This indicates:
, Page 4 of 99
A) Proper chloramine formation
B) Excess ammonia beyond the chlorine-to-ammonia ratio
C) Breakpoint chlorination has been reached
D) Free chlorine residual is too high
Answer: B – Chloramines require a Cl₂:NH₃-N weight ratio of about 5:1; excess ammonia remains as free
ammonia.
7. A Class III operator calculates the CT value for inactivation of Giardia. The required CT is 120 mg·min/L
at 15°C. The actual CT is 80 mg·min/L. What action is required?
A) Reduce flow rate to increase contact time
B) Increase chlorine residual or contact time
C) Increase temperature
D) No action, 80 is acceptable
Answer: B – CT must meet or exceed the required value; increase residual or detention time.
8. A plant adds 2 mg/L of chlorine to raw water. The chlorine demand is 1.5 mg/L. The free chlorine
residual will be approximately:
ABC Water Treatment Operator Class III exam
Actual exam Questions with Correct Answers
and explanations LATEST THIS YEAR -JUST
RELEASED
Summarized Exam Coverage – ABC Water Treatment Class III
Source water protection and watershed management, coagulation/flocculation (jar testing, chemical
dose optimization), sedimentation (tube/plate settlers, sludge removal), filtration (membrane, direct,
conventional, pressure), disinfection (chlorine, chloramines, ozone, UV, CT calculations), fluoridation
and corrosion control, softening (lime-soda ash, ion exchange), iron/manganese removal, taste and odor
control (oxidation, activated carbon), plant hydraulics (pumping, flow measurement), process control
and optimization (turbidity, particle counts), instrumentation (SCADA, analyzers), safety (confined space,
chlorine handling, HAZMAT), water quality sampling and laboratory (coliform, DBP precursors),
regulations (SWTR, LT2, DBP Rule, Lead and Copper), troubleshooting, math (CT, detention time, loading
rates, chemical feed, percent strength), and management.
1. A Class III operator at a surface water plant sees a sudden increase in raw water turbidity from 5 NTU
to 45 NTU after a storm. The first process adjustment should be:
A) Increase the filtration rate
B) Increase the coagulant dose and monitor settled water turbidity
C) Decrease the coagulant dose
D) Backwash all filters immediately
Answer: B – Higher turbidity requires more coagulant to neutralize particles; jar test can guide dose
adjustment.
2. A plant uses chlorine gas. The operator hears a hissing sound near the cylinder valve. The immediate
action is:
A) Tighten the valve with a wrench
, Page 2 of 99
B) Evacuate the area and don SCBA before closing the cylinder valve
C) Spray water on the leak
D) Call the supervisor from inside the room
Answer: B – Chlorine gas is toxic; SCBA is required; stop the leak by closing the cylinder valve if safe.
3. A water system receives a Tier 2 public notice violation for exceeding the maximum contaminant level
(MCL) for total trihalomethanes (TTHMs). The most likely cause is:
A) High turbidity in filtered water
B) Long detention time in clearwell with free chlorine and high organics
C) Low pH in distribution
D) Excessive fluoride
Answer: B – TTHMs form when chlorine reacts with natural organic matter; long contact time increases
formation.
4. A conventional plant’s settled water turbidity is 4 NTU after sedimentation. Filtered water turbidity is
0.15 NTU. What action should the operator take?
A) Increase filtration rate
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B) Optimize coagulation to lower settled water turbidity to ≤2 NTU
C) Backwash filters more frequently
D) No action, filtered water is within limits
Answer: B – SWTR requires settled water turbidity ≤2 NTU for conventional treatment; 4 NTU is too high
and risks filter overload.
5. A membrane filtration plant experiences a rapid increase in transmembrane pressure (TMP). The first
troubleshooting step is:
A) Acid clean the membranes
B) Perform a integrity test
C) Increase the backwash frequency
D) Reduce the flux rate
Answer: C – Increasing TMP often indicates fouling; increasing backwash frequency or chemically
enhanced backwash (CEB) is first step.
6. A water treatment plant uses chloramines for disinfection. The operator measures free ammonia in
the finished water at 0.5 mg/L. This indicates:
, Page 4 of 99
A) Proper chloramine formation
B) Excess ammonia beyond the chlorine-to-ammonia ratio
C) Breakpoint chlorination has been reached
D) Free chlorine residual is too high
Answer: B – Chloramines require a Cl₂:NH₃-N weight ratio of about 5:1; excess ammonia remains as free
ammonia.
7. A Class III operator calculates the CT value for inactivation of Giardia. The required CT is 120 mg·min/L
at 15°C. The actual CT is 80 mg·min/L. What action is required?
A) Reduce flow rate to increase contact time
B) Increase chlorine residual or contact time
C) Increase temperature
D) No action, 80 is acceptable
Answer: B – CT must meet or exceed the required value; increase residual or detention time.
8. A plant adds 2 mg/L of chlorine to raw water. The chlorine demand is 1.5 mg/L. The free chlorine
residual will be approximately:
