How It Works

This post was transferred from my old blog. See the original post (with comments) at:


I spent my first three months here in Malawi working with gravity fed water systems.  Taking advantage of my new cartoon drawing kick, I thought I’d take a few minutes to explain how one works.

The pictures I used are from a real gravity fed system, but obviously the cartoon schematic below is not accurate or to scale. I just drew it to try to show all the fundamental elements of a system, and how they fit together.

How it Works

1. The Intake

There are two major types of intakes: river intakes and spring intakes. A river intake involves damming a small river. Water pools behind the concrete dam, and then flows into a pipe. Excess water overflows the dam and continues down the river.


A river intake on the Sankhulani scheme, shown with Mr. Alizeo (repair team member) and Mr. Utumbe (government water supervisor).

A spring intake involves finding a spring right where it bubbles up on the ground, and capping it to protect it from external contamination. All of the water then goes into a pipe and down into the system.

image A spring intake on the Sankhulani scheme.

2. Screening Tank

Sorry, I don’t actually have a picture of one of these. The basic purpose of a screening tank is to get sediment and other debris out of the water before it goes to people’s taps. It’s been a while since I’ve seen one, but this is a (very) rough approximation of how it works.

Screening Tank

A “schematic” of a screening tank. Water flows in one side, debris settle to the bottom, and water flows out the other. A dividing wall in the middle eliminates water flow at the bottom of the tank, ensuring that debris settle.

3. Sedimentation Tank

On most of the scheme’s I’ve worked with, I don’t think there’s really any design difference between a sedimentation tank and a storage tank. A sedimentation tank is basically just a big tank where any sediment that makes it past the screening tank can settle to the bottom.

image Mr. Friday (Sankhulani scheme repair team member), cracking open the hatch on a sedimentation tank.

4. Break Pressure Tank

Water flowing downhill can build up a lot of pressure. Excess pressure can damage taps, break pipe connections, and cause all kinds other problems. That’s why break pressure tanks are used to reduce water pressure. They’re very simple. Water flows into a concrete tank, hits the far wall, sloshes around a lot, and then flows out the other side. Goodbye pressure.


Mr. Alizeo and Mr. Utumbe opening up a break pressure tank. The concrete slab in front of the tank has a valve under it, in case you want to shut off water flowing into the tank. The top pipe on the right is an overflow pipe, and the bottom one is an outlet pipe (for draining the tank before cleaning).


The inside of the tank. The pipe on the right is the inflow, the pipe on the left is the outflow. Notice that they’re not directly aligned, ensuring that the water will have to slosh around in the tank before flowing out. The pipe at the top of the picture is the outlet pipe for cleaning.

5. River Crossing

Most gravity fed water systems have many river crossings, which are like Achilles’ Heels for the systems. During heavy rains, trees frequently wash down the river, knocking out the pipes. When the pipes get knocked out, whole sections of the system lose water, and repairs usually can’t be done for months (until the rains slow down). Without water, people stop paying monthly contributions, meaning that there’s less money available to do the repair when the time comes. Dealing with river crossings is definitely a tough gig for scheme management.


A river crossing. (Note: crossings are supposed to be done with galvanized steel pipe to reduce the risk of washout, but once that pipe gets washed out, it often gets replaced with PVC, increasing the risk of a washout next year.)

6. Air Valve

Although these systems are powered by gravity, water still sometimes has to flow uphill. (See the first schematic). When it does, there is a risk of air getting caught in the pipe. If air does get caught in the pipe, it essentially gets trapped (since it can’t flow through the water), builds up pressure, and blocks flow. It can sometimes also burst the pipe.

To prevent this, air valves are used to let the air out of the pipe. I’m not sure how they work exactly (I’d love to take one apart some day, but the chance hasn’t come up) but somehow they allow air to exit the pipe but not water.


Mr. Friday inspecting an air valve that’s obviously broken. This picture gives a good idea of why it’s broken: too much water pressure. The valve pretty much just flew off, but we found it close by. See the next picture for Mr. Friday’s clutch repair strategy.


All better, assuming the water pressure never gets strong enough to knock these rocks off. The pipe at the bottom is the main line for the system. The riser coming off of it is leads to the air valve.

7. Storage Tank

Think you can take a guess at this one? Well, you’re right. A storage tank is pretty much what you’d think it is: a tank for storing water. This helps regulate water pressure in the system, and also ensures that taps towards the end of the line have water even when flow from the intake is low.


There it is. A storage tank. The pipes coming out of the top allow air to get out when the water flows in (and vice versa) preventing air locking.


Inside the storage tank. One of the echoier places I’ve ever been.

8. Water Tap

Finally, a water tap. The point to these systems. Where the magic happens.

The big gravity fed schemes in Thyolo, where I was working, have between 80 and 100 taps each. This gives communities way denser water coverage than drilling boreholes (one borehole with a pump costs around $10,000, so you have to be sparing with them) meaning women and children have to spend less time walking each day to fetch water. The water is also usually safer than what’s available form traditional sources.

imageA water tap on the Sankhulani scheme. To give an idea of scale, the intake for the system is on the far side of that forested mountain in the background. To further that idea of scale, this tap is only about halfway down to the end of the system. These things are big.



A water tap on a smaller system in Thyolo, this time with community members posed for a photo. Nice.

Well, hope that was interesting. Gravity fed systems are an example of a pretty simple technology (just simple pipes and concrete) that can make a pretty big difference in people’s lives. Keep in mind though, these things need to be maintained. Imagine cleaning all those tanks, fixing all the breakages, searching along kilometers of underground mainline every time there’s a leak. And it’s all up to community volunteers. It says something about the strength and work ethic of Malawians that these things ever keep running.

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14 responses to “How It Works

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