Schematic diagram of rural gravity scheme

A gravity water supply system uses the natural fall of the ground from a high-level source to carry water to the village without pumping. It is the most common, cheapest and most reliable scheme in the hills of Nepal.

Schematic diagram

  Spring (high level)
       |
  [Spring box / Intake]
       |  pipe (GI/HDPE)
  [Sedimentation tank]
       |
  [Valve + Reservoir tank] <- chlorination
       |
  [Break pressure tank] (if head > 60 m)
       |
  Distribution pipes with air/wash valves
       |
  [Tap stands / Public standposts]

Function of components

1. Source (spring/stream): gives the required yield throughout the year and is usually at a higher level than the village.
2. Intake / spring box: collects the spring water, protects it from pollution, has an overflow and a washout, and controls the flow into the pipe.
3. Transmission pipeline: carries water by gravity to the tank. Pipes are laid below ground where possible.
4. Sedimentation tank: settles silt and sand, especially for stream sources, before the water enters the reservoir tank.
5. Reservoir (storage) tank: stores water to balance the continuous inflow against the variable demand, commonly 33 to 50 percent of the daily demand. Disinfection (chlorination) is usually done here.
6. Break pressure tank (BPT): reduces the static head to atmospheric pressure so that pipes and fittings are not overstressed.
7. Valves (air, wash-out, sluice, reflux): release trapped air at high points, drain sediment at low points, control the flow and prevent back flow.
8. Distribution pipes: carry water from the tank to the settlements.
9. Tap stands / public standposts: the points where users collect water, with a soak pit for waste water.