The science idea
Rivers, ponds and rain run-off carry mud, leaves, tiny plants and many other things. Water that looks dirty is clearer after it has passed through layers of grains, and this is the idea behind the sand filters used in water-treatment plants. In this project you build a small version and test how well it works. It is a good project for the environment theme, because it shows both the technology of cleaning water and its limits.
The first idea is filtration, which is separation by size. A mixture of water and solid particles is passed through a porous material. The water and anything dissolved in it, such as salt, pass through, and particles that are larger than the gaps are trapped. The layers of your filter have gaps of different sizes: the gravel traps leaves and lumps, the coarse sand traps grains of silt, and the fine sand traps finer mud. Because the finest sand has the smallest gaps, it also makes the water flow more slowly. That is the trade-off of every filter: finer filtering gives cleaner water but a slower flow.
The second idea is adsorption. The surface of charcoal, especially of activated charcoal, is full of tiny pores, and some molecules stick to it. That is how a charcoal layer can take out some colour and smell. Adsorption is not absorption: the molecules stay on the surface and are not soaked up inside. Kitchen charcoal is much less effective than the activated charcoal used in water purifiers and aquarium filters, so expect only a modest effect.
The most important idea is the limit of the filter. A sand-and-charcoal filter does not remove bacteria, viruses or most dissolved chemicals. A real treatment plant uses four steps: coagulation (a chemical such as alum makes tiny particles stick together), sedimentation (the clumps settle), filtration, and disinfection (chlorine or ultraviolet light kills germs). Your bottle does only the filtration step, so the water that comes out must never be drunk. Water for drinking should be boiled for at least a minute after the water starts to boil vigorously, or treated with a proper purifier.
You can make the project into a real investigation by measuring clarity with a see-through test: look down through a tall glass at a black cross on white paper, and measure how deep the water can be before the cross is no longer visible. You can compare filters with different layers, and measure the flow rate.
Materials and tools
Cost: Free
A plastic bottle, sand, gravel, cotton and a little cooking charcoal are household or garden items.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Plastic bottle, 1.5 to 2 litres, clean and with its cap | 1 | A used cold-drink bottle that never held chemicals. |
| Gravel or small pebbles | About 1 cup | From the garden or the road side. Wash them well. |
| Coarse sand | About 1 cup | From a building site or a garden. Wash it until the water runs clear. |
| Fine sand | About 1 cup | River or builder’s fine sand, or sieve ordinary sand through a cloth. |
| Charcoal from cooking fire (not briquettes with chemicals) or activated charcoal tablets | A handful | Wood charcoal from a stove. Crush it in a cloth with a hammer (adult), and wash well. |
| Cotton wool or a clean cotton cloth | A small wad | From the first-aid kit or an old cotton cloth. |
| Glass jars or clear cups | 4 | To hold the muddy water and the filtered water. |
| Garden soil, a few dry leaves, water | 1 spoon of soil, 500 mL water | To make the muddy sample. |
| Black marker, white paper, ruler and a stopwatch | 1 each | For the clarity test and timing. |
Tools
- Scissors and craft knife (adult)
- A tall, clear glass or cylinder for the clarity test
- A spoon
- A measuring jug or a cup marked in mL
- A hammer and a cloth for crushing the charcoal (adult)
Safety first
Read this before you start
- Do not drink or taste any water from this project, before or after filtering. It is not safe. Label the jars “NOT FOR DRINKING”.
- An adult must cut the bottle and crush the charcoal. Wear a mask or cover your nose when crushing and washing charcoal, because the dust is black and irritating.
- Use only plain wood charcoal. Do not use charcoal briquettes that have chemicals, nor the ash from burnt plastic or rubbish.
- Wash your hands with soap after handling soil, sand, charcoal and dirty water.
- Pour the used muddy water on the garden, not down the drain, and do not leave water standing, because mosquitoes breed in it.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Prepare the bottle
An adult cuts off the bottom of the bottle about 5 cm from the base. Keep the cap, make a few small holes in it with a hot nail so that water can drip out, or leave the neck stuffed with cotton and the cap off.
Tip: File or tape the sharp cut edge so that it does not hurt your hands.
Wash the filter materials
Rinse the gravel, the two kinds of sand and the crushed charcoal in a bucket of water until the water poured off looks clear. Spread them out to dry a little.
Common mistake: Skipping this step. Dusty sand and charcoal will cloud the water that you are trying to clean.
Stand the bottle
Turn the bottle upside down with its neck down. Place the neck in a clean glass or a cup, or stand the bottle in a tall can with a hole cut in it so that it stays upright.
Tip: Test that it does not tip over when you fill it. Use a heavier holder if it does.
Add the cotton
Push a wad of cotton wool into the neck so that it fills the neck and the first few centimetres above it. Press it down gently.
Common mistake: Packing the cotton too tight. Then the water will hardly drip. A loose wad is better.
Add the charcoal layer
Pour in about 3 cm of crushed, washed charcoal on top of the cotton, and level it with the back of a spoon.
Tip: Crush the pieces to the size of rice grains, not powder. Powder clogs the filter.
Add the fine sand
Add about 3 cm of fine sand on top of the charcoal and level it.
Common mistake: Mixing the layers. Pour each layer slowly and keep them separate.
Add the coarse sand and gravel
Add 3 cm of coarse sand and then 3 cm of gravel. The top layer of gravel protects the sand from being dug up when you pour water.
Tip: Place a spoon or a saucer on the gravel and pour the water onto it to stop the layers from being dug up.
Rinse the filter
Pour clean tap water through the filter until the water that drips out looks clear. Discard this water. Wait for the dripping to slow down.
Common mistake: Testing the filter before rinsing. The first water that comes out is dirty with charcoal dust.
Make the muddy sample
Mix one spoon of garden soil and a few crushed dry leaves in 500 mL of water in a jar. Stir well. Pour a little into the tall glass and do the clarity test: put a black cross on paper under the glass, look down and see how deep the water can be before the cross disappears.
Tip: Do the clarity test in the same place and the same light every time.
Filter the sample
Stir the muddy water again and pour it slowly onto the gravel. Time how long it takes for the first 100 mL to drip into the glass. Collect the water in a clean jar.
Common mistake: Pouring too fast. The water spills over the sides or digs holes in the layers.
Test the filtered water
Do the clarity test on the filtered water. Write down how much clearer it is and how long it took. Look at the water in the jar: is it colourless? Does it have any smell?
Tip: If it is still cloudy, pour the filtered water through again. Many filters need two passes.
Try a variation
Change one thing, such as leaving out the charcoal, using only fine sand, or adding a pinch of alum (phitkari) to the muddy water, stirring it, and leaving it for 20 minutes before filtering. Measure the clarity and time again.
Common mistake: Changing several things at once. Change one thing at a time so you know what made the difference.
How it should look when it is finished
An upside-down plastic bottle held over a clear glass. Through the clear plastic you can see bands of grey gravel, light and darker sand and a black charcoal layer above a white cotton wad. Muddy brown water is poured in at the top, and clearer water drips slowly into the glass, with a label “NOT FOR DRINKING”.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
When you pour muddy water on the gravel, it trickles down between the grains. Each layer has gaps of a different size. The big gaps of the gravel stop leaves and lumps. The coarse sand stops silt, and the fine sand, which has the smallest gaps, stops the finest mud. Water, with anything dissolved in it, is small enough to pass.
The charcoal grains have a huge surface area, because they are full of tiny pores. Some of the dissolved molecules that cause colour and smell stick to this surface. This is called adsorption. The cotton at the bottom stops any grains from falling out with the water.
The filter does not remove germs, which are smaller than the gaps, nor dissolved salts and chemicals, so the water can still make you ill. A real plant adds coagulation and disinfection to deal with these.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The water drips very slowly or not at all | The cotton is packed too tight, the sand is too fine, or the charcoal is powder. | Loosen the cotton, use a coarser sand and crush the charcoal into grains rather than powder. |
| The filtered water is black or grey | Charcoal dust. | Rinse the filter with a lot of clean water first, and keep the charcoal in grains. |
| The filtered water is still cloudy | Very fine mud passes through the layers. | Pass it through a second time, add a finer layer or let the water settle first. |
| Water goes round the sides | The layers are not tight against the bottle. | Press the layers down gently and make the top layers level. |
| The layers mix when pouring | Water is poured from too high or too fast. | Pour onto a spoon or saucer placed on the gravel. |
| There is a smell in the filtered water | Dirty sand or the charcoal is not fresh. | Wash the materials well and use fresh charcoal. Remember that you should not drink this water. |
Test and record
Use the same amount (500 mL) and the same muddy sample each time. The clarity depth is the greatest depth of water at which the black cross can still be seen (in cm); a larger number means clearer water.
| Filter tested | Clarity depth before (cm) | Clarity depth after (cm) | Time for first 100 mL (s) | Volume collected (mL) | Notes (colour, smell) |
|---|---|---|---|---|---|
| Full filter (all layers) | |||||
| Without charcoal | |||||
| Sand only | |||||
| With alum first |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- What is filtration?
The separation of solid particles from a liquid by passing it through a porous material that traps the particles and lets the liquid through.
- Why are the layers in this order, with the coarse material on top?
The big gaps at the top trap the large particles first. If the fine sand were on top, it would clog at once, and the water would barely flow.
- What is the job of the charcoal?
Its surface adsorbs some of the dissolved substances that cause colour and smell. This is adsorption, and kitchen charcoal is only weakly effective.
- Is the filtered water safe to drink? Why?
No. Germs and dissolved chemicals pass through. Water for drinking must be boiled or treated by a proper purifier.
- What is the difference between adsorption and absorption?
In adsorption molecules stick to the surface of a solid, as on charcoal. In absorption a substance is taken into the inside of another, like a sponge.
- What does alum do?
It makes tiny suspended particles clump together (coagulation) so that they settle faster or are trapped more easily.
- Name the four steps of a water-treatment plant.
Coagulation, sedimentation, filtration and disinfection.
- Why does a finer filter give a slower flow?
Finer material has narrower gaps, and it is harder for water to pass through them, so the water flows more slowly.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a water filter from a plastic bottle and to test how well it clears muddy water.
- Apparatus
- A plastic bottle, gravel, coarse sand, fine sand, charcoal, cotton wool, glass jars, soil, a stopwatch, a ruler and a black cross on paper.
- Procedure
- Describe how you built the filter layer by layer, rinsed it, made the muddy sample, measured the clarity before and after, and timed the flow. Include the diagrams and state any variation you tried.
- Observation
- The table of clarity depth, time and volume for each filter.
- Result
- The filtered water was clearer: the clarity depth rose from ... cm to ... cm. The full filter took ... s for the first 100 mL.
- Conclusion
- Layers of gravel, sand and charcoal remove suspended particles and some colour, but not germs or dissolved chemicals, so the water is not safe to drink and must be boiled or treated.
Take it further
- Measure how the flow rate changes with the thickness of the fine sand layer, and plot a graph.
- Compare coarse sand, fine sand and a mixture, and find the best balance between clarity and speed.
- Test a coloured solution (food colour) through the charcoal layer to see how much colour it removes.
- Compare the filtered water with tap water using the clarity test.
- Find out how your town or village purifies its drinking water and draw a flow chart.
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