How to build a water filter

Purify your water at home with a filter made from rocks, sand and coal

Image credit: Future PLC/ © Illustrations by Ed Crooks

1. Create the body

First, cut right around the outside of a large plastic bottle, so that you have two separate pieces – the top and the bottom. Take a big piece of cotton wool and press it into the hole at the lid of the bottle to plug the opening. The fine fibres of the cotton wool will catch tiny particles of dirt floating in your water. Place the top of the bottle upside down into the bottom of the bottle.

Image credit: Future PLC/ © Illustrations by Ed Crooks

2. Start filling

The aim here is to remove as much dirt and debris from your dirty water as possible. Start by putting a one-centimetre layer of charcoal in the bottom of the bottle to cover the cotton wool, then pouring in some sand on top of that to cover it. The layer should be around two centimetres thick. Then, take some small pieces of gravel and pour them on top of the sand to create another layer.

Image credit: Future PLC/ © Illustrations by Ed Crooks

3. Layer on layer

Next, add some larger gravel to your filter. Try to keep the layers even and make sure the previous layer is completely covered by the next one. Finally, top the filter off with some larger stones, making sure that the gravel is totally covered underneath. You should now be able to see that the gaps between particles in each layer decrease from top to bottom.

Image credit: Future PLC/ © Illustrations by Ed Crooks

4. Make your mixture

To test your filter, you will need some dirty water. Take a jug of water, pour in some soil and mix it around with a spoon. You’ll notice that some of the soil will dissolve in the water, while other bits will remain floating. Next, throw in some grass and leaves. These are the kinds of things that you might find in river water so they will make your experiment more authentic.

Image credit: Future PLC/ © Illustrations by Ed Crooks

5. Pour it in

Now you can pour your dirty water into the filter. The water contains particles of lots of different sizes, and these will be stopped at different points in the filtration process. The stones should stop the leaves and grass, while some soil will make it down further to the gravel or sand. The water left at the end will look much cleaner, but it’s still not safe to drink!



The gaps within the filter get smaller as the water trickles through it, and fewer particles can get through these gaps. By the time it reaches the bottom, the filter has stopped almost all the dirt particles, leaving much cleaner water filtering out the bottom of the bottle.

Disclaimer: Neither Future Publishing nor its employees can accept any liability for any adverse effects experienced during the course of carrying out these projects or at any time after. Always take care when handling potentially hazardous equipment or when working with electronics and follow the manufacturer’s instructions.

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