How to Build a Simple Water Filter Experiment

The bottle filter is the most-taught water activity in the world, and most of the time it is done wrong — one filter, one pour, everyone says “wow”, nobody learns anything. Done properly it is a genuine engineering investigation. Here is the properly version.

What you need

• A clear plastic bottle, cut in half. The top half becomes the funnel, upside down.
• Coffee filter paper or a piece of cloth
• Cotton wool
• Fine sand and coarse sand
• Small gravel or aquarium stones
• Optional: activated charcoal
• A jug of “dirty” water, made the same way every time
• A tray, because this is a messy activity

Step 1: make a standard dirty sample

This step gets skipped and it is the one that turns a demo into an experiment. Mix a fixed recipe — for example one litre of water, two spoons of soil, a spoon of shredded leaves, a small squeeze of cooking oil. Write the recipe down. Every test from now on uses exactly this.

Without a standard sample, you cannot compare filter A with filter B, because the water was different.

Step 2: build the first filter

Invert the top half of the bottle into the bottom half. Layer from the bottom of the funnel upward: filter paper or cloth, cotton wool, fine sand, coarse sand, gravel. Coarse material on top, fine material at the bottom.

Pour slowly. Fast pouring pushes dirt straight through the sides of the layers.

Step 3: measure the result, do not just look at it

“Cleaner” is an opinion. Turn it into data:

• Clarity: hold the sample over printed text and record how many lines you can read through it. Or compare against a set of reference cards.
• Time: how long the water took to pass through.
• Volume: how much went in, how much came out. Some is always retained by the layers.
• Colour and smell: recorded in words, consistently.

Step 4: change one thing

Now it becomes an investigation. Rebuild the filter with exactly one change and run the same sample through:

• Reverse the layer order — what happens?
• Double the sand depth
• Remove the cotton wool
• Add charcoal
• Pour faster

Record both results side by side. The conclusion should reference the numbers, not the impression.

The safety limit — say this out loud

A bottle filter removes particles. It does not remove bacteria, viruses, dissolved chemicals, heavy metals or anything else that would make water unsafe. Water that comes out of it looking perfectly clear can still be dangerous.

Nobody drinks anything from this activity. Say it before you start, not after someone has picked up the cup. It is also a genuinely interesting teaching point: clear is not the same as clean, and that distinction is the whole basis of water treatment.

Questions that push the thinking further

• Why does the order of the layers matter?
• What is each layer actually doing?
• What is still in the water that you cannot see?
• Real treatment plants use settling, filtration and disinfection. Which of those did we just do?
• How would you test whether your improvement was real, and not luck?

Take it further

• Build-a-Water-Filter STEM Kit — graded media, repeatable dirty-water sachets and a turbidity comparison card
• Clean Water Science Journal — the recording pages for this investigation
• Clean Water STEM Challenge Pack — the classroom version, as a group design challenge

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Written by the Watershed Learning curriculum team. We write and test every resource we sell. Factual content is checked against current, reputable sources before publication and reviewed when we update the article. Spotted something wrong? Tell us — we would rather know.
Last updated: 19 August 2026.