Drinking Water Distribution: Pressure, Quality, and Cross-Connections
What You Must Understand
Distribution protects finished water from the plant to the tap. Core topics: system hydraulics (pressure, head loss, flow), storage (elevated and ground tanks for pressure and emergency supply), valves and hydrants, backflow prevention, and water quality maintenance (disinfectant residual, disinfection byproducts, corrosion control).
Pressure basics: static pressure depends on elevation head-roughly 0.433 psi per foot of water (2.31 ft per psi). Friction loss increases with flow and pipe roughness and decreases with larger diameter. Minimum service pressure is typically around 20 psi; systems must avoid negative pressure that could draw contamination in. Cross-connection control relies on air gaps (best) and reduced-pressure-zone or double-check assemblies; know which device suits which hazard level. Disinfectant residual must be maintained throughout the system; low residual invites biofilm and nitrification in chloraminated systems. Flushing manages stagnation, sediment, and biofilm; dead ends are problem areas.
Traps to Watch
- 0.433 psi/ft vs. 2.31 ft/psi: mixing the conversion direction is a classic error.
- Air gap is the only true backflow prevention; mechanical devices can fail.
- Chloramine nitrification: questions may ask for signs (falling residual, rising nitrite) and fixes (breakpoint chlorination, flushing).
- Pressure vs. flow: adequate pressure does not guarantee adequate flow; pipe diameter and friction govern flow.
Rule of Thumb
To find static pressure from a tank elevation, multiply height of water above the point (feet) by 0.433 psi/ft. A 100-ft column gives about 43.3 psi before friction losses. Then subtract friction loss for flowing conditions.
Study Focus
Practice pressure conversions, backflow device selection, and residual management scenarios.