Water distribution
Water distribution tests how well you can move treated water without losing pressure, residual, or public health protection. Maine questions emphasize pressure zones, storage, cross-connection control, main repair, flushing, and coliform response.
Core rules
- Pressure is created by elevation: static pressure (psi) = height difference (ft) × 0.433 psi/ft. Water surface elevation, not tank bottom, sets the head.
- Maine rules require at least 20 psi at all points in the distribution system under normal conditions and during fire flow; below that, contamination can enter through leaks.
- An air gap is the best backflow protection. Reduced-pressure zone assemblies and double-check valves must be tested by certified testers at required intervals.
- Main breaks: isolate, relieve pressure, repair, disinfect the repaired section, flush, and collect bacteriological samples before returning to service.
- Keep storage turnover adequate. Long detention time lowers residual, raises disinfection byproduct precursors, and can cause nitrification in chloraminated systems.
- Flush dead ends and hydrants on a schedule; record residual and turbidity.
Worked example: static pressure
A tank water surface is 160 ft above a hydrant at elevation 60 ft. Difference = 100 ft. Static pressure = 100 × 0.433 = 43.3 psi. If friction loss at fire flow is 10 psi, residual pressure at the hydrant is 43.3 − 10 = 33.3 psi. That is above 20 psi, so the test passes. If the tank surface were only 120 ft, static would be 26.0 psi and residual 16.0 psi, which fails.
Traps
Using tank bottom elevation; forgetting friction loss; mixing gpm with psi; assuming a backflow device protects a connection without testing; failing to sample after a main repair; and ignoring low-pressure areas at high elevations.