Class 3 Water Distribution Exam Prep (50 Questions)
1. What is the minimum required chlorine residual before the first customer in Ontario?
0.2 mg/L free chlorine.
2. What is the purpose of looping a water distribution system?
To maintain pressure, chlorine residual, and reliability by providing multiple flow paths.
3. What does “water age” refer to in distribution systems?
The time water spends in the system, from treatment to customer use.
4. What problems are associated with high water age?
Loss of chlorine residual, DBP formation, and increased bacterial growth.
5. What is a dead-end main?
A section of water main with no outlet, prone to stagnation.
6. What is the purpose of unidirectional flushing (UDF)?
To improve cleaning efficiency, restore chlorine residuals, and reduce water waste.
7. What is the minimum pressure requirement during normal operating conditions?
35 psi.
8. What is the minimum pressure requirement during fire flow events?
20 psi.
9. What is the typical design fire flow for commercial areas?
2,500–3,500 gpm for at least 2 hours (varies by local codes).
10. What is the main cause of nitrification in distribution systems?
Ammonia from chloramine breakdown promoting bacterial activity.
11. What is the most effective method for controlling nitrification?
Maintaining a strong chlorine residual and periodic flushing.
12. What is the typical velocity limit for water mains?
Less than 5 ft/s during normal operation.
13. What does a water hammer result from?
Sudden changes in velocity from pump or valve operation.
14. What device is used to reduce water hammer?
Surge tanks, slow-closing valves, or air chambers.
15. What is the Hazen-Williams C-factor?
A measure of a pipe’s smoothness and resistance to flow.
16. What does a low C-factor indicate?
A rough pipe interior, higher friction loss, and reduced capacity.
17. What is the primary advantage of ductile iron pipe over cast iron?
Higher strength, flexibility, and corrosion resistance.
18. What is cathodic protection used for?
To prevent corrosion of metallic pipes and tanks.
19. What is the difference between static and residual pressure?
Static pressure = pressure with no flow.
Residual pressure = pressure while water is flowing.
20. What is the minimum distance between a water main and a sanitary sewer (Ontario guideline)?
3 m horizontally, with the water main above the sewer.
21. What is the typical lifespan of a PVC water main?
50–75 years.
22. What is the primary disadvantage of asbestos cement pipe?
Brittle and health concerns during cutting or removal.
23. What is the function of a blowoff hydrant?
To flush dead-end mains and remove stagnant water.
24. What is the function of an altitude valve?
To automatically control filling of a storage tank and prevent overflow.
25. What is the function of a PRV (Pressure Reducing Valve)?
To reduce downstream pressure to safe operating levels.
26. What is the function of a PSV (Pressure Sustaining Valve)?
To maintain upstream pressure while allowing downstream flow.
27. What is the recommended chlorine residual at the farthest point in a system?
At least 0.05 mg/L free chlorine.
28. What is a tracer wire used for?
To locate buried non-metallic pipes such as PVC or HDPE.
29. What is the purpose of air release valves?
To remove trapped air from high points in the distribution system.
30. What is the primary cause of taste and odor problems in distribution systems?
Algae, bacteria, or chemical reactions such as chlorine decay.
31. What is the most important safety step before entering a valve chamber or meter pit?
Test the atmosphere for oxygen, H₂S, and explosive gases.
32. What is the maximum day demand (MDD)?
The highest average daily demand occurring in a year.
33. What is the peak hour demand (PHD)?
The highest water demand in a single hour.
34. What is non-revenue water?
Water produced but not billed due to leaks, theft, or meter errors.
35. What is the purpose of a cross-connection control program?
To prevent contamination from backflow or back-siphonage.
36. What is the function of a double check valve assembly (DCVA)?
To prevent backflow of pollutants in moderate hazard situations.
37. What is the function of a reduced pressure zone (RPZ) device?
To prevent backflow of contaminants in high hazard conditions.
38. What is the recommended minimum separation between water and gas mains?
At least 0.3 m (1 ft).
39. What is the purpose of an interconnection between two distribution systems?
To provide emergency backup supply and redundancy.
40. What is a common cause of pipe corrosion in water mains?
Low pH, dissolved oxygen, and stray electrical currents.
41. What is the primary benefit of using GIS in water distribution systems?
Accurate mapping of mains, valves, and hydrants for operations and planning.
42. What is the typical spacing for hydrants in residential areas?
150 m (500 ft).
43. What is the most important parameter to check when responding to a customer complaint of low pressure?
Check pressure at the customer location and compare with system pressure.
44. What is the purpose of swabbing a water main?
To clean the pipe interior and remove biofilm or deposits.
45. What is the most important step before returning a repaired main to service?
Disinfect, flush, and collect bacteriological samples.
46. What is the safe exposure limit for chlorine gas?
0.5 ppm (TLV-TWA, ACGIH).
47. What is the purpose of secondary disinfection?
To maintain a disinfectant residual throughout the distribution system.
48. What is the difference between chlorine demand and chlorine residual?
Demand = amount consumed by reactions.
Residual = remaining disinfectant available.
49. What is the most effective way to reduce disinfection by-products (DBPs)?
Reduce natural organic matter (NOM) before chlorination and maintain proper chlorine dose.
50. What is the first step an operator should take when notified of a main break?
Isolate the section by closing valves and notify affected customers.
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