Planning of distribution; intermittent supply

General considerations in planning a distribution system

1. Demand: population and demand for the design period, with peak factors and fire demand.
2. Source and topography: position of the source and reservoir, and ground levels, to decide between gravity, pumping or a combination.
3. Layout: choose dead end, grid iron, ring or radial layout to suit the roads and the town plan.
4. Pressure: keep the residual head at least 10–15 m at the tap (7–10 m for rural stand posts) and the maximum pressure about 60–70 m, using zoning or BPT if more.
5. Velocity: 0.6–3 m/s in the pipes.
6. Reservoir: provide balancing, emergency and fire storage at a suitable high point close to the demand.
7. Appurtenances: sectional valves, air valves, scour valves at low points and dead ends, and hydrants.
8. Quality protection: avoid dead ends, cross connections and low spots where water stagnates.
9. Materials and cost: pipe material, class and cost; ease of operation and maintenance; future expansion.

When intermittent supply is recommended

1. When the source yield is limited and cannot provide for the continuous 24 h supply (for example in dry-season springs).
2. When the pumping is costly or the power supply is available only for a few hours.
3. When the head available in the system is small and only part of the area can be given water at a time (zone-by-zone rotation).
4. When the treatment capacity is small, or in the early years when the network is incomplete.
5. In small or rural towns where consumers can store water without difficulty.

Drawbacks of the intermittent system

1. Pipes are empty during the off period, so polluted water can be sucked in through leaks, and quality is poor.
2. Larger pipes, valves and meters are required, because the whole day's demand is delivered in a few hours.
3. Consumers must store water, which wastes space and can become contaminated.
4. No water for fire fighting during the off period.
5. Frequent pressure change and wet–dry cycles cause corrosion and pipe damage; air in the pipes distorts meter readings.
6. Supply is unfair to people at the far ends or on high ground.