Water Purification Science Project

A bottle filter with gravel, sand and charcoal turns muddy water clear in a few minutes. It’s a good project. It also makes water look safe when it isn’t, and that turns out to be the best thing to explain in your write-up.

This is a small-scale version of what a city filtration plant does. It shows three of the real steps: settling, filtering and adsorption. It skips the step that makes water drinkable, and a good project says so.

What you need

• Two clear plastic bottles, 1 or 2 liters
• Scissors
• A coffee filter, cotton balls or a piece of cloth
• A rubber band
• Coarse gravel, rinsed
• Fine sand, rinsed
• Activated charcoal, the aquarium kind, rinsed until the water runs clear
• A cup of garden soil
• A measuring cup and a timer

NASA JPL’s version uses gauze and cotton balls. Science Buddies’ version uses a column of gravel, fine sand, activated carbon and paper. Both work, so use what you have.

Build it

1. Cut one bottle about a third of the way up from the bottom. The top part is your filter. The bottom part catches the water.
2. Stretch the coffee filter or cloth over the mouth of the top part and hold it with the rubber band.
3. Turn the top part upside down and set it in the bottom part, like a funnel.
4. Layer from the neck up: charcoal, then fine sand, then gravel. About an inch or two of each.
5. Mix the soil into two cups of water in the second bottle. That’s your dirty water.

The order matters. The gravel catches the big stuff first so it doesn’t clog the sand. The sand catches finer grit. The charcoal at the bottom does something different, covered below.

Run it

1. Shake the dirty water and set aside a small cup of it for comparison.
2. Start the timer and pour the rest slowly into the filter.
3. Record how long it takes to drip through and how much water you get back.
4. Pour the filtered water through again. Repeat three times.

Science Buddies scores clarity on a 1 to 5 scale, where 5 looks like plain tap water and 1 looks unchanged. That’s easy to do by eye against a white sheet of paper. JPL adds two more measurements: time, and how much water you recover.

Turn it into an experiment

A build is a demonstration. An experiment changes one thing and measures what happens. Pick one variable:

• Layer order. Does sand on top clog faster than gravel on top?
• Layer depth. Does twice as much sand give clearer water, and how much slower is it?
• Materials. Charcoal vs. no charcoal. Sand vs. cotton balls.
• Settling first. Let the dirty water sit for an hour before filtering. Is the result clearer? Faster?

That last one is sedimentation, the third step at a real treatment plant. Plants speed it up with alum, and the CDC’s travel guidance uses about 1/4 teaspoon per quart of cloudy water. Pickling alum from the grocery store works for a demonstration. Have an adult handle it.

What’s happening, in science terms

The gravel and sand work by size. Particles bigger than the gaps between grains get stuck. That’s filtration, the same idea as the sand beds at a filtration plant.

The charcoal works by chemistry. Activated charcoal is full of tiny pores, which gives it a huge surface area, and some dissolved chemicals stick to that surface. That’s called adsorption, with a “d.” It’s why charcoal can pull out some color and smell that sand can’t.

Settling works by gravity. Heavy particles sink. Tiny ones take a very long time, which is why plants add a chemical to clump them first.

The part to put in your conclusion

Clear water isn’t clean water.

Both source projects say so directly. Science Buddies: “Do not drink the filtered water.” NASA JPL: “No matter how ‘clean’ your filtered water looks, you should never drink it because it still may contain pollutants you can’t see.”

Here’s why, with numbers. The CDC lists bacteria at about 0.5 by 2 to 8 microns and viruses at about 0.03 microns. The gaps between sand grains are far bigger than that, so germs go straight through. So do dissolved things like salt and lead, which is why even the best bottle filter can’t make seawater drinkable.

Real systems close that gap two ways. They use much finer filters, rated in fractions of a micron. And they disinfect afterward with chlorine, UV or boiling. The purifying water guide covers the home versions of both, and DIY sand and charcoal filters looks at what these filters can and can’t do outside a classroom.

A strong project ends by naming the step it left out.

Going further

Test the disinfection step. You can’t safely grow bacteria at home, but you can test for chlorine. Pool test strips show whether bleach-treated water still has chlorine left after 30 minutes.

Try distillation. Salt water in a pot, a cup hanging from the upside-down lid. How to distill water walks through it. Once the cup water cools, taste it: the salt stays behind in the pot. This one needs an adult and a stove.

Compare to the space station. JPL notes that the International Space Station’s system recycles about 93 percent of the water on board. Look up what it does that your bottle filter doesn’t.