Water Distribution
Water Distribution
This domain tests the distribution system as a hydraulic and public-health system: pressure, storage, main disinfection, cross-connection control and bacteriological monitoring. Questions are usually simple physics wrapped in field situations - converting head to pressure, tracing a chlorine residual complaint, or deciding whether a repaired main can be returned to service.
Core rules to hold onto
- Pressure: 1 psi = 2.31 ft of water. Static pressure is elevation difference between the water surface (tank or grade) and the point of concern; friction loss subtracts from it. Systems are typically designed to hold at least 20 psi under all conditions, including fire flow, because lower pressures invite contamination.
- Head loss: Hazen-Williams with a C factor - older unlined iron (C ≈ 100) loses far more head than cement-lined ductile iron or PVC (C ≈ 130–140) at the same flow.
- Storage: equalization volume plus fire flow plus emergency reserve; stored water is also where chlorine residual is lost and disinfection by-products form.
- Cross connections: the hierarchy is air gap first, then a reduced-pressure-zone assembly for high hazard, then a double check for low hazard. Test annually.
- New and repaired mains: chlorinate per AWWA C651 practice (commonly 25 mg/L free chlorine held 24 hours), flush to a normal residual, then collect bacteriological samples before placing the main in service.
Worked example
A tank water surface sits at elevation 340 ft; a tap is at 265 ft. Static head is 75 ft = 32.5 psi. At 500 gpm the 1,200-ft main loses 6.1 ft of head. Residual head = 75 − 6.1 = 68.9 ft, so pressure at the tap = 68.9 ÷ 2.31 = 29.8 psi. Comfortable now - but a fire flow of 1,500 gpm on the same main could push that below the 20 psi floor.
Traps that cost marks
Subtracting psi from ft without converting; ignoring that a positive total coliform result triggers repeat samples and, for E. coli, public notice; treating a chlorine residual complaint as a treatment problem when it is usually long detention time or a dead-end; and returning a repaired main to service on a flush alone, without bacteriological results.