Distribution networks

The distribution network carries treated water from service reservoirs to every consumer, at enough pressure and in enough quantity, around the clock where possible. It is the largest part of a water scheme's capital cost and the source of most losses and complaints.

Layouts include dead-end (tree) systems, simple but prone to stagnant water and supply cuts when a pipe breaks; grid (looped) systems, where water reaches each point by more than one route, giving better pressures and reliability; and ring and radial systems for planned areas. Networks are divided into district metered areas so that flows can be measured and leaks found. Service reservoirs, elevated or ground-level, balance hourly demand against steady supply, store water for fire and emergencies, and set pressure.

Pipes are designed for peak hourly demand, which can be two to three times average, with minimum residual pressures specified by building height and velocities kept in a reasonable range to limit head loss and water hammer. Pipe materials include ductile iron, mild steel, HDPE, PVC and DI or steel with linings. Valves isolate sections, air valves release trapped air at high points, scour valves drain low points and fire hydrants are spaced along streets. Continuous pressurised supply, rather than intermittent supply, is the goal, because intermittent supply draws contaminated water into pipes when they empty.