Make dirty water clean - How to filter drinking water

Most young children have not given too much thought to clean water. They turn on the tap and clean, ready to drink water comes out. Some are used to drinking water out of a water filter system.

This nature education experiment shows children how dirty muddy water can be cleaned by filtering it through gravel and sand and shows them what goes on inside a water filtering system.

You will need:

• sand
• blotting paper or 3 or 4 coffee filters
• clean flower pot
• transparent jug
• large measuring cup or pouring jug
• Scissors
• Tablecloth or dish towel
• Gravel
• Cotton wool
• Dirty water (you can make tap water dirty by adding food colouring, cooking oil, small bits of plastic or soil)
• Glass

Have small groups of children take turns making a water filter (or if necessary the class can sit in a circle and watch as you do it)

• Put blotting paper or 3 or 4 coffee filters, then sand, and finally gravel in a clean flower pot
• Place the flower pot in a larger transparent container so the filtered water can drip through and the children can observe it
• In a large measuring cup, place dirt, bits of plant matter and water
• Mix it up to make muddy water
• Pour the muddy water into the flower pot as shown in the image on the right.
• Children observe the color of the water dripping out the bottom of the pot into the transparent container
• Ask questions - Where did the dirt go? Why did cleaner water come out?
• Take the filter apart and try and find the dirt and bits of plant matter

What's happening?

Your filter gets rid of solid large particles that are not dissolved in the dirty water. The size of the particles that can be removed by filtration depends on the size of the filter you use.

A filter with small holes will stop more particles getting through than a filter with large holes.

Your filter won't get rid of food colouring, because food colouring is dissolved in the water. A technique called distillation is used in laboratories to get rid of substances dissolved in water.

The water you drink from taps is cleaned by lots of processes. Filtration is just the start. The water is also disinfected to get rid of microbes.

More: Try arranging the layers in a different order and compare the color of the filtered water.