How to Make a Water Filter

by Deb on June 10, 2011

Filter

This one is interesting – I started it at home several weeks ago with my girls and we’ve been fiddling around with it, but then it came up in a year 7 class as well. So I know it works with a wide range of ages! It’s simple to do but there is one bit that can be tricky to get at home, which is why the school ones worked better.

It’s surprisingly easy to make your own water filter, using a bottle and sand from your garden plus a bit of cottonwool, some different sized sieves and lots of containers. It may look a bit strange, but it does work to make the water safe to drink.

  1. Drill a hole in the bottle cap. Put in a cotton wool ball to stop the sand coming through and put the lid back on. Cut the bottom off the bottle.
  2. Get a bucket of sand from the garden, try to have it fairly dry so it will sift easily – so not the potting mix! Where we are, the sand is very mixed with lots of clay and small rocks from sand up to gravel size. This needs to be sifted to work properly. If you have very uniform sand like beach sand you can use it straight, but kids think sifting is a lot of fun.
  3. If you are sifting start with your largest sieve, either one from the kitchen or use some flywire. Sift a little at a time, putting the rocks left in the sieve aside in a container and catching the rest.
  4. Now sift the remainder with a finer sieve – this is the bit that’s hard to find! Have a look around, some types of flyscreen are finer than others. If you have a mesh toy bag or laundry bag it might work. Fish or butterfly nets are generally too fine and delicate but they might work if you’re desparate. And this step is only necessary if you have very mixed sand as we do.
  5. Keep the larger material out of the sieve as well as catching what comes through – you should end up with three containers with fine, medium and large grains.

    small filter

    You can see the sifted layers in one we made at school.

  6. Now pour them into the bottle with the finest at the bottom, then the medium then the largest rocks.
  7. Stand your filter over a container. Depending on the bottle you’ve used, you might be able to put it directly in the container or have a frame to support it, we put one inside a concrete brick.pouring
  8. Pour some dirty water in and watch it drip through!

What’s happening:

Filters are basically microscopic sieves, the size of the holes depends on how closely the grains are packed together. If you have nicely sorted grains that are around the same size, they will pack together with only small gaps:

Sorted

Sorted grains only have small gaps between them.

Whereas unsorted grains will have large gaps:

Unsorted

Unsorted grains pack randomly they they end up with large gaps.

The sand works just like a sieve and blocks the large particles getting through, the sorted grains will stop more. These types of filters actually get better over time, as you use it the bits it is trapping help to block up holes and make the gaps smaller.

On the one we made at home we included activated charcoal at the bottom, which you can get from pet shops for fish tanks. This works on the atomic level and will strip out things like chlorine.

Make your own and enjoy some freshly filtered water.

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{ 1 comment… read it below or add one }

Teacher Laura Oreamuno. June 20, 2011 at 4:32 am

I am teaching (enjoying activities) with kids from 4 to 6 years on any kind of experience connected with science and around the world. Last friday did the water filter as you show us, but the cap was one of those for drinkinkg water…so no need to drill holes, you pull and the drop of water comes out!! They were fascinated and ran for a dish to collect the water for birds!! Thanks.

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