Build a Multi-Layer Water Filter
The same layered-filtration idea behind real water treatment systems, built with household materials.
Related field: Environmental Engineering
Mission Briefing
Objective
Build a layered filter that visibly improves the clarity of dirty water.
Constraints
- Only sand, gravel, charcoal, and cotton or a coffee filter are allowed as filter materials
- The starting water must be visibly dirty but not hazardous — mix in dirt or food coloring, not anything toxic
- The filtered water is not safe to drink, even if it looks clear — this is a clarity demonstration, not a purification system
Concept You're Testing
Filtration — physically removing particles from water by forcing it through layers of different materials, a foundational step in real water treatment before any chemical or biological treatment happens.
What You'll Need
- A plastic bottle with the bottom cut off (adult help recommended), or a funnel
- Sand, small gravel or pebbles, and crushed charcoal (activated charcoal if available)
- Cotton balls or a coffee filter
- A container to catch the filtered water
- Dirty water to test with (mix soil or food coloring into tap water)
Ask an adult to help cut the plastic bottle — the cut edges can be sharp.
Instructions
- 1Turn the bottle upside down to use as a funnel, or use a funnel directly.
- 2Layer the materials from bottom to top: cotton or coffee filter first, then charcoal, then sand, then gravel on top.
- 3Slowly pour your dirty water sample through the top of the filter.
- 4Collect what comes out the bottom in a clean container.
- 5Compare the filtered water's clarity to the original dirty water, holding both up to light.
- 6Try changing the layer order or thickness and test again to see what changes the result.
How to Measure Results
Compare the clarity of the water before and after filtering — hold both samples up to a light source and describe the visible difference, or time how long it takes to read text through each sample.
Skills You'll Practice
- Why layer order matters in a filtration system, not just which materials are used
- The difference between making water look clean and making it safe to drink
- Comparing a before-and-after result methodically instead of just eyeballing it once
- How a real-world environmental engineering process scales up from this same basic idea
If It Doesn't Work the First Time
That's normal — this challenge follows the same design process real engineers use. A failed test just tells you where to go back and improve.
Figure out what problem you're actually solving, and what would even count as success. Skip this step and jump straight to building, and a lot of bad designs happen right here.