A gravity water system stores its water in a covered tank or cistern high enough to feed the taps and big enough for at least 3 days of supply, which the parent page puts at 400-600 L for a family of four and 5,000-10,000 L for a community of 30. Fit every tank with a lid, an insect-screened vent, an overflow, a drain and an outlet above the bottom, and treat it as a confined space that nobody enters without a rope and a second person outside.
A storage tank or cistern holds water between the source and the taps. It smooths out a source that fluctuates and, if placed high, creates the pressure that pushes water through the pipes.
Why A Gravity System Needs A Tank
A spring varies with the season; use comes in bursts in the morning and evening. A tank fills in quiet hours and covers the peaks. If the source fails, the tank is the reserve while you repair it. And a tank placed above the taps makes pressure of about 0.1 bar per metre of height, even when the spring itself is at the same level as the house. The wider principles are in Storage Tanks.
Three Kinds To Build Or Find
| Kind | Notes | Page |
|---|---|---|
| Ferrocement | Thin shell of cement mortar over mesh; the same shell works as ground-level cistern or elevated tank | Ferrocement Tanks |
| Stone or masonry | Thick walls; best below ground, where the earth supports the wall | Stone & Masonry Cisterns |
| Salvaged tank | Plastic or metal tanks from buildings and farms; check what it held before using it for drinking water | Salvaged Modern Pipe for the same history checks |
Do not use a tank that held fuel, chemicals or manure for drinking water. Scrubbing does not remove what plastic absorbs.
Size It
The parent page’s minimum is 3 days of supply, which for a family of four is 400-600 L and for a community of 30 is 5,000-10,000 L. The arithmetic is worked in Tank Sizing & Placement; Tank Sizing shows how a buffer for peaks and a reserve for outages add up.
Fit It
- A lid that seals against animals, dirt and mosquitoes.
- A vent with insect mesh, so air can move as the level changes.
- An overflow at the inlet level, as large as the inlet (Overflow Pipe), ending in an air gap above the ground or a drain so dirty water cannot rise back into the tank.
- A drain at the bottom for cleaning.
- An outlet a little above the bottom so sediment stays behind (Gravity Tap).
- A float valve on the inlet to stop filling when full, where the supply is pressurised (Float Valves).
- A firm base. 1,000 L of water weighs 1,000 kg.
Keep It Clean
Clean and disinfect the tank before first use and at intervals (Water Purification). Stored water can pick up bacterial regrowth, insects and dust; see Water Storage.
Common Mistakes
- No overflow, so the tank floods at the top and erodes its base.
- An overflow joined to a drain pipe that can carry dirty water back.
- A second, non-drinking supply connected to the tank. That is a cross-connection (Plumbing Separation).
- A tank placed below the highest tap.
- Too small a tank to ride out a dry week.
Safety
A lidded tank is a confined space, and a full tank is a crushing load
A closed tank can be short of oxygen and can hold carbon dioxide or hydrogen sulphide that pools at the bottom. Nobody enters without the lid open at two points, a harness tied to a line held by an attendant, and a second person outside who pulls the line and never climbs in. Do not test the air with a flame. Build the base and any tower for the full weight of the water.
What’s Next
- Learn next: Tank Sizing & Placement — size and height
- Combine with: Ferrocement Tanks — the build most readers will use first