Water distribution practice test D1 & D2 COMPLETE 250
QUESTIONS AND VERIFIED SOLUTIONS
SECTION 1 — DISINFECTION
Questions 1–50
1. What is the primary purpose of disinfecting a potable water
distribution system?
A. Increase water hardness
B. Destroy or inactivate disease-causing microorganisms
C. Increase pipe pressure
D. Remove all minerals
Answer: B
Rationale: Disinfection protects public health by inactivating pathogenic
organisms. It is one of the principal barriers against waterborne disease.
2. Which disinfectant is most commonly used in drinking-water
distribution systems?
A. Chlorine
B. Calcium
C. Nitrogen
D. Carbon dioxide
Answer: A
Rationale: Chlorine is widely used because it is effective, relatively
economical, and can provide a measurable residual throughout the
distribution system.
3. The amount of chlorine remaining after chlorine demand has been
satisfied is called:
,A. Chlorine dose
B. Chlorine residual
C. Chlorine demand
D. Chlorine concentration factor
Answer: B
Rationale: Residual is the disinfectant remaining in the water after
reactions and demand have consumed part of the applied chlorine.
4. Chlorine demand is calculated as:
A. Residual − dose
B. Dose + residual
C. Dose − residual
D. Residual × dose
Answer: C
Rationale: Chlorine demand is the amount consumed by reactions with
contaminants and other constituents: demand = dose − residual.
5. If chlorine is applied at 4.0 mg/L and the measured residual is 1.2
mg/L, what is the chlorine demand?
A. 2.8 mg/L
B. 3.2 mg/L
C. 4.8 mg/L
D. 5.2 mg/L
Answer: A
Rationale: Demand = dose − residual = 4.0 − 1.2 = 2.8 mg/L.
6. In water, 1 mg/L is approximately equivalent to:
A. 0.1 ppm
B. 1 ppm
C. 10 ppm
,D. 100 ppm
Answer: B
Rationale: For dilute water solutions, 1 mg/L is approximately equal to
1 part per million.
7. CT represents:
A. Chlorine temperature
B. Chlorine concentration multiplied by contact time
C. Chlorine tank capacity
D. Chlorine treatment cost
Answer: B
Rationale: CT is commonly expressed as disinfectant concentration ×
contact time and is used to evaluate disinfection effectiveness.
8. A water plant has a chlorine concentration of 2 mg/L and a contact
time of 30 minutes. What is the CT value?
A. 15 mg-min/L
B. 30 mg-min/L
C. 60 mg-min/L
D. 90 mg-min/L
Answer: C
Rationale: CT = C × T = 2 × 30 = 60 mg-min/L.
9. Chlorine demand tends to increase when water contains:
A. More organic and reducing substances
B. Less contact with contaminants
C. Pure distilled water only
D. No reactive substances
Answer: A
, Rationale: Organic matter, ammonia, iron, manganese, sulfide, and
other reactive substances can consume chlorine.
10. Free chlorine residual refers primarily to:
A. Chlorine available as disinfectant after demand reactions
B. Chlorine permanently bound to sediment
C. Chloride ions only
D. Chlorine gas in the cylinder
Answer: A
Rationale: Free chlorine is the residual chlorine species available to
provide ongoing disinfecting action.
11. Chloramines are formed when chlorine reacts with:
A. Ammonia or nitrogen-containing compounds
B. Calcium carbonate only
C. Sand
D. Oxygen only
Answer: A
Rationale: Chlorine reacts with ammonia to form chloramine species
such as monochloramine.
12. Which statement best describes chloramination?
A. It uses chlorine and ammonia to form chloramines
B. It eliminates all disinfectant residual
C. It uses only ozone
D. It removes all minerals from water
Answer: A
Rationale: Chloramination intentionally forms chloramine residuals for
disinfection and distribution-system residual maintenance.
QUESTIONS AND VERIFIED SOLUTIONS
SECTION 1 — DISINFECTION
Questions 1–50
1. What is the primary purpose of disinfecting a potable water
distribution system?
A. Increase water hardness
B. Destroy or inactivate disease-causing microorganisms
C. Increase pipe pressure
D. Remove all minerals
Answer: B
Rationale: Disinfection protects public health by inactivating pathogenic
organisms. It is one of the principal barriers against waterborne disease.
2. Which disinfectant is most commonly used in drinking-water
distribution systems?
A. Chlorine
B. Calcium
C. Nitrogen
D. Carbon dioxide
Answer: A
Rationale: Chlorine is widely used because it is effective, relatively
economical, and can provide a measurable residual throughout the
distribution system.
3. The amount of chlorine remaining after chlorine demand has been
satisfied is called:
,A. Chlorine dose
B. Chlorine residual
C. Chlorine demand
D. Chlorine concentration factor
Answer: B
Rationale: Residual is the disinfectant remaining in the water after
reactions and demand have consumed part of the applied chlorine.
4. Chlorine demand is calculated as:
A. Residual − dose
B. Dose + residual
C. Dose − residual
D. Residual × dose
Answer: C
Rationale: Chlorine demand is the amount consumed by reactions with
contaminants and other constituents: demand = dose − residual.
5. If chlorine is applied at 4.0 mg/L and the measured residual is 1.2
mg/L, what is the chlorine demand?
A. 2.8 mg/L
B. 3.2 mg/L
C. 4.8 mg/L
D. 5.2 mg/L
Answer: A
Rationale: Demand = dose − residual = 4.0 − 1.2 = 2.8 mg/L.
6. In water, 1 mg/L is approximately equivalent to:
A. 0.1 ppm
B. 1 ppm
C. 10 ppm
,D. 100 ppm
Answer: B
Rationale: For dilute water solutions, 1 mg/L is approximately equal to
1 part per million.
7. CT represents:
A. Chlorine temperature
B. Chlorine concentration multiplied by contact time
C. Chlorine tank capacity
D. Chlorine treatment cost
Answer: B
Rationale: CT is commonly expressed as disinfectant concentration ×
contact time and is used to evaluate disinfection effectiveness.
8. A water plant has a chlorine concentration of 2 mg/L and a contact
time of 30 minutes. What is the CT value?
A. 15 mg-min/L
B. 30 mg-min/L
C. 60 mg-min/L
D. 90 mg-min/L
Answer: C
Rationale: CT = C × T = 2 × 30 = 60 mg-min/L.
9. Chlorine demand tends to increase when water contains:
A. More organic and reducing substances
B. Less contact with contaminants
C. Pure distilled water only
D. No reactive substances
Answer: A
, Rationale: Organic matter, ammonia, iron, manganese, sulfide, and
other reactive substances can consume chlorine.
10. Free chlorine residual refers primarily to:
A. Chlorine available as disinfectant after demand reactions
B. Chlorine permanently bound to sediment
C. Chloride ions only
D. Chlorine gas in the cylinder
Answer: A
Rationale: Free chlorine is the residual chlorine species available to
provide ongoing disinfecting action.
11. Chloramines are formed when chlorine reacts with:
A. Ammonia or nitrogen-containing compounds
B. Calcium carbonate only
C. Sand
D. Oxygen only
Answer: A
Rationale: Chlorine reacts with ammonia to form chloramine species
such as monochloramine.
12. Which statement best describes chloramination?
A. It uses chlorine and ammonia to form chloramines
B. It eliminates all disinfectant residual
C. It uses only ozone
D. It removes all minerals from water
Answer: A
Rationale: Chloramination intentionally forms chloramine residuals for
disinfection and distribution-system residual maintenance.
