How to Make a Water Filter from Sand and Gravel

Learn to build a simple water filter using sand and gravel for an engaging science experiment that teaches water purification basics.

1. Gather Your Materials. You'll need a clear plastic bottle (2-liter works best), coarse gravel or small pebbles, fine sand, cotton balls or coffee filter, scissors, rubber band, and dirty water for testing (mix soil or mud with clean water). Having multiple layers of different-sized materials is key to effective filtration.
2. Prepare the Filter Container. Cut the bottom off your plastic bottle, leaving the neck and cap intact. Turn the bottle upside down so the neck points downward. Remove the cap to create your water outlet. This inverted bottle becomes your filter housing, with water flowing from wide opening to narrow spout.
3. Layer Your Filter Materials. Start by placing cotton balls or a coffee filter at the neck of the bottle to prevent materials from falling out. Add a layer of fine sand (about 2 inches), then a layer of coarse sand or small gravel (2 inches), followed by larger gravel or pebbles (2-3 inches). Each layer should be distinct and level for best results.
4. Test Your Water Filter. Hold your filter over a clean container and slowly pour your dirty test water into the top. Watch as the water moves through each layer, becoming cleaner as it progresses. The filtered water should come out much clearer than what went in, though it may take several minutes for all water to filter through.
5. Observe and Discuss Results. Compare the filtered water to your original dirty water and discuss what happened. Talk about how each layer trapped different sized particles, with gravel catching big debris and sand filtering smaller particles. This is similar to how groundwater gets cleaned naturally as it moves through soil and rock layers.