How a simple water filter works — Engineering, 7–10 years
Different layers can catch different kinds of dirt, but clearer water is not necessarily safe to drink.
Idea
In a simple filter, water passes through materials with spaces between their grains. Gravel catches larger pieces, sand catches smaller particles and cloth can hold back some bits, so the water may look cleaner when it comes out.
Why it matters
People need ways to remove visible dirt from water, especially when water must be used for washing or an experiment. The problem is that dirt can have many sizes, so one layer cannot catch everything; several layers work on different particles.
Worked example
Cut the bottom from a clean plastic bottle and turn it upside down. Cover the neck with cloth, then add a layer of fine sand, a layer of gravel and another cloth. Pour muddy water slowly through it and compare the first and last water in a clear glass; the last may have fewer visible particles.
A common trap
Clear water can look clean enough to drink, so it is reasonable to trust your eyes. But germs and dissolved chemicals may pass through a simple filter without being visible; filtered experiment water must not be tasted or called safe without proper treatment.
Where it is used
Water-treatment systems use several stages to remove particles, germs and some chemicals, with each stage designed for a particular problem. Camping filters can improve water from a stream, but their instructions matter, and some water still needs boiling or another approved treatment.
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