Water Distribution
What this domain tests
The Distribution domain tests whether you can move finished water without degrading it: pressure, storage, mains, valves, hydrants, cross-connection control, disinfectant residual, flushing, and main-break repair. Maryland systems range from large metropolitan loops to small community systems with a single tank and a few miles of main.
Core rules and concepts
- Pressure: maintain positive pressure throughout the system. A common regulatory floor is 20 psi at all points under normal demand; confirm your system's permit, because many Maryland systems target higher pressures.
- Static head: pressure from elevation is 0.433 psi per foot of head. A tank water level 100 feet above a hydrant produces 43.3 psi static pressure before friction losses.
- Disinfectant residual: maintain a measurable residual in the distribution system. Low residuals at dead ends call for flushing, valve exercises, or storage turnover, not just more chlorine at the plant.
- Cross-connections: containment backflow assemblies protect the public system; isolation assemblies protect internal processes. Test reduced-pressure-zone assemblies annually and keep records.
- New mains and repairs: follow AWWA C651 practices. A common new-main method is chlorination to 25 mg/L for 24 hours, then flushing and bacteriological sampling before placing the main in service.
Worked example
A tank's high-water level is 180 feet above a hydrant. Static pressure is 180 × 0.433 = 77.9 psi. During a fire-flow test the level drops to 120 feet above the hydrant: 120 × 0.433 = 52.0 psi. If a nearby high-elevation customer falls below 20 psi during the test, the system has a pressure or storage problem even though the hydrant itself reads 52 psi.
Traps that cost marks
Confusing feet of head with psi; assuming a hydrant reading represents the whole system; ignoring dead ends and storage turnover; sampling before repair chlorination and flushing are complete; and treating a cross-connection survey as a one-time task.