Methods of distribution of water

Water is delivered from the service reservoir to the consumers by one of three methods, depending on the position of the source and the ground levels.

1. Gravity system

The source or the service reservoir is at a level high enough above the town, so that water flows by gravity at the required pressure.

 Source/Reservoir
   \
     \  gravity flow
       \_______ town

• Merits: most economical and reliable, since there is no pumping and no power cost; simple to operate; very low maintenance; pressure is steady.
• Demerits: possible only where a high source exists; pressure at low ground may be too high so BPT or PRV are needed; large initial pipe cost for long transmission lines.

2. Pumping system (direct pumping)

Treated water is pumped directly into the mains without any storage; the pumps run according to the demand.

• Merits: high pressure can be achieved easily, useful for fire fighting; the reservoir cost is saved; suited for flat towns.
• Demerits: pumps must run all the time with varying load, so the cost is high and efficiency is low; any breakdown or power failure stops the supply; standby pumps are needed; pressure fluctuates with the demand.

3. Combined (dual) system - pumping with storage

Pumps lift water at a uniform rate into an elevated service reservoir, which supplies the town by gravity and also receives the excess flow in the low-demand hours.

 Source -> Pump -> [Elevated reservoir] -> distribution
                        |  (surplus stored in the off-peak hours)

• Merits: the pumps work at a constant, efficient rate; the supply continues for a time even if the pump fails; peak demands and fire are met from storage; the pump size and the running cost are less. This is the most common system.
• Demerits: needs the cost of the reservoir and also of the pumps and power; more complex operation and maintenance.

Choice

The gravity system is preferred where the topography allows; the combined system is the most practical in flat towns; direct pumping is used only for small schemes or as a temporary arrangement.