, Question 1
A surface water treatment plant uses alum coagulation at pH 6.5. Jar tests show
that increasing the alum dose from 20 mg/L to 30 mg/L improves turbidity
removal but drops filtered water pH to 6.0. Which adjustment best maintains
optimal coagulation while preventing lead and copper corrosion in the
distribution system?
A. Increase alum dose to 35 mg/L and add chlorine for oxidation.
B. Reduce alum dose to 15 mg/L and add a polymer as a coagulant aid.
C. Maintain 30 mg/L alum and add caustic soda to raise pH to 7.5 after
filtration.
D. Switch to ferric chloride at 25 mg/L without pH adjustment.
Correct Answer: C - Maintain 30 mg/L alum and add caustic
soda to raise pH to 7.5 after filtration.
RATIONALE
Alum lowers pH; adding caustic after filtration restores pH to 7.5,
minimizing lead/copper solubility while preserving coagulation.
Reducing alum may worsen turbidity, and ferric chloride also lowers
pH, increasing corrosion risk.
Question 2
A conventional rapid sand filter operates at 4 gpm/ft² with a media depth of 24
inches and effective size of 0.5 mm. After a backwash, the filter runs for 36
hours before terminal head loss of 8 ft. If the operator increases the loading rate
to 6 gpm/ft², which change in filter performance is most likely?
A. Longer filter runs due to increased scour of particles.
B. Shorter filter runs and increased risk of media loss.
C. Improved particle removal efficiency due to deeper penetration.
D. No change in run time but higher backwash water use.
Correct Answer: B - Shorter filter runs and increased risk of
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,media loss.
RATIONALE
Higher loading rates increase head loss development and shorten filter
runs, while also raising shear forces that can cause media loss. Particle
removal may deteriorate, not improve, due to reduced contact time.
Question 3
A groundwater system chlorinates with free chlorine at pH 8.2, 0.5 mg/L
residual, and 10°C. The system must achieve 4-log virus inactivation.
According to the CT table, the required CT is 6 mg-min/L. What is the
minimum contact time (minutes) in a baffled clearwell with a baffling factor of
0.7?
A. 17.1 minutes
B. 12.0 minutes
C. 8.6 minutes
D. 24.0 minutes
Correct Answer: A - 17.1 minutes
RATIONALE
CT = residual × time × baffling factor. Rearranging: time = CT /
(residual × baffling factor) = 6 / (0.5 × 0.7) = 17.1 minutes. Other
options miscalculate by omitting the baffling factor or misplacing the
decimal.
Question 4
A distribution system experiences a pressure drop to 15 psi during peak
demand. The system has a 12-inch main with a C-factor of 100. Which
operational action is most appropriate to restore pressure while maintaining
chlorine residual?
A. Throttle a downstream valve to increase pressure upstream.
Page 3
, B. Increase pump speed and add booster chlorination at a strategic
location.
C. Flush hydrants to reduce stagnation.
D. Reduce storage tank overflow to maintain level.
Correct Answer: B - Increase pump speed and add booster
chlorination at a strategic location.
RATIONALE
Increasing pump speed raises flow and pressure; booster chlorination
maintains residual in the affected area. Throttling valves would
worsen pressure, flushing may lower pressure further, and reducing
overflow does not address demand.
Question 5
A treatment plant uses ozone for primary disinfection. Bromate formation is
detected at 12 µg/L, exceeding the MCL of 10 µg/L. Which operational change
is most effective at reducing bromate while maintaining disinfection?
A. Increase ozone dose and contact time.
B. Lower ozone dose, add hydrogen peroxide, and use chloramines for
secondary disinfection.
C. Switch to free chlorine with a higher CT.
D. Add ammonia before ozonation to form bromamines.
Correct Answer: B - Lower ozone dose, add hydrogen peroxide,
and use chloramines for secondary disinfection.
RATIONALE
Lowering ozone dose and adding HO (advanced oxidation) can reduce
bromate, while chloramines provide a stable secondary residual.
Increasing ozone would worsen bromate; free chlorine may form
brominated DBPs; adding ammonia before ozone can increase
bromate.
Page 4
A surface water treatment plant uses alum coagulation at pH 6.5. Jar tests show
that increasing the alum dose from 20 mg/L to 30 mg/L improves turbidity
removal but drops filtered water pH to 6.0. Which adjustment best maintains
optimal coagulation while preventing lead and copper corrosion in the
distribution system?
A. Increase alum dose to 35 mg/L and add chlorine for oxidation.
B. Reduce alum dose to 15 mg/L and add a polymer as a coagulant aid.
C. Maintain 30 mg/L alum and add caustic soda to raise pH to 7.5 after
filtration.
D. Switch to ferric chloride at 25 mg/L without pH adjustment.
Correct Answer: C - Maintain 30 mg/L alum and add caustic
soda to raise pH to 7.5 after filtration.
RATIONALE
Alum lowers pH; adding caustic after filtration restores pH to 7.5,
minimizing lead/copper solubility while preserving coagulation.
Reducing alum may worsen turbidity, and ferric chloride also lowers
pH, increasing corrosion risk.
Question 2
A conventional rapid sand filter operates at 4 gpm/ft² with a media depth of 24
inches and effective size of 0.5 mm. After a backwash, the filter runs for 36
hours before terminal head loss of 8 ft. If the operator increases the loading rate
to 6 gpm/ft², which change in filter performance is most likely?
A. Longer filter runs due to increased scour of particles.
B. Shorter filter runs and increased risk of media loss.
C. Improved particle removal efficiency due to deeper penetration.
D. No change in run time but higher backwash water use.
Correct Answer: B - Shorter filter runs and increased risk of
Page 2
,media loss.
RATIONALE
Higher loading rates increase head loss development and shorten filter
runs, while also raising shear forces that can cause media loss. Particle
removal may deteriorate, not improve, due to reduced contact time.
Question 3
A groundwater system chlorinates with free chlorine at pH 8.2, 0.5 mg/L
residual, and 10°C. The system must achieve 4-log virus inactivation.
According to the CT table, the required CT is 6 mg-min/L. What is the
minimum contact time (minutes) in a baffled clearwell with a baffling factor of
0.7?
A. 17.1 minutes
B. 12.0 minutes
C. 8.6 minutes
D. 24.0 minutes
Correct Answer: A - 17.1 minutes
RATIONALE
CT = residual × time × baffling factor. Rearranging: time = CT /
(residual × baffling factor) = 6 / (0.5 × 0.7) = 17.1 minutes. Other
options miscalculate by omitting the baffling factor or misplacing the
decimal.
Question 4
A distribution system experiences a pressure drop to 15 psi during peak
demand. The system has a 12-inch main with a C-factor of 100. Which
operational action is most appropriate to restore pressure while maintaining
chlorine residual?
A. Throttle a downstream valve to increase pressure upstream.
Page 3
, B. Increase pump speed and add booster chlorination at a strategic
location.
C. Flush hydrants to reduce stagnation.
D. Reduce storage tank overflow to maintain level.
Correct Answer: B - Increase pump speed and add booster
chlorination at a strategic location.
RATIONALE
Increasing pump speed raises flow and pressure; booster chlorination
maintains residual in the affected area. Throttling valves would
worsen pressure, flushing may lower pressure further, and reducing
overflow does not address demand.
Question 5
A treatment plant uses ozone for primary disinfection. Bromate formation is
detected at 12 µg/L, exceeding the MCL of 10 µg/L. Which operational change
is most effective at reducing bromate while maintaining disinfection?
A. Increase ozone dose and contact time.
B. Lower ozone dose, add hydrogen peroxide, and use chloramines for
secondary disinfection.
C. Switch to free chlorine with a higher CT.
D. Add ammonia before ozonation to form bromamines.
Correct Answer: B - Lower ozone dose, add hydrogen peroxide,
and use chloramines for secondary disinfection.
RATIONALE
Lowering ozone dose and adding HO (advanced oxidation) can reduce
bromate, while chloramines provide a stable secondary residual.
Increasing ozone would worsen bromate; free chlorine may form
brominated DBPs; adding ammonia before ozone can increase
bromate.
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