Water Distribution
Water Distribution
This domain tests the buried and stored system: pressure, storage, mains, valves, hydrants, cross-connection control, flushing, disinfectant residual, sampling, and repair practices. In North Dakota, cold winters, frost, and long dead-end mains make storage turnover, main disinfection, and freeze protection practical exam topics.
Core rules
- Keep positive pressure throughout the system; pressure below zero invites contamination.
- Storage must turn over; older or oversized tanks can lose residual and form disinfection byproducts.
- Backflow prevention is required at cross-connections; an air gap is the strongest protection.
- Flushing velocity should move sediment; calculate volume before you flush.
- Main repairs need disinfection and bacteriological sampling before return to service.
Worked example
An 8-inch main is 1,200 ft long. Area = 0.7854 × (8/12)^2 = 0.349 ft². Volume = 0.349 ft² × 1,200 ft = 419 ft³. Convert: 419 ft³ × 7.48 gal/ft³ = 3,135 gal. At 100 gpm flushing, time = 3,135 ÷ 100 = 31 min. The reasoning: convert diameter to feet, find cross-sectional area, multiply by length, then convert cubic feet to gallons.
Traps
Using 8 inches directly in the formula; forgetting 7.48 gal/ft³; confusing psi with feet of head (1 psi ≈ 2.31 ft); flushing without checking residual and valve direction; taking bacteriological samples from a dirty or improperly sterilized tap.