Water filter STEM challenge
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Learning Objectives
- 1Build a layered water filter that turns dirty water into clearer water
- 2Explain the role of each layer (cloth, charcoal, sand, gravel) in trapping particles
- 3Identify the variables that must stay the same to make the test fair
- 4Evaluate filter performance using clarity and filtration time as evidence
Mini Lesson
Around the world, billions of people use simple layered filters to clean cloudy water before drinking it. In this STEM challenge you will design, build, and test your own mini water filter using a plastic bottle and four materials: cloth (or a coffee filter), activated charcoal, sand, and gravel. Your goal is to turn a sample of dirty water into water that is clearly cleaner — and to do it as a fair test so you can compare your filter to a friend’s.
How a Layered Filter Works
Filtration is a physical separation method. Larger particles cannot fit through small spaces, so they get trapped while the water passes through. Each layer of a water filter is built from a different material with a different particle size. Together, they remove debris of every size from large leaves down to fine silt.
- Cloth or coffee filter (TOP) — traps the largest debris like leaves and dirt clumps before they clog the lower layers.
- Activated charcoal — adsorbs odors, some chemicals, and very fine particles. The black grains have a huge surface area.
- Sand — packs tightly to trap small grit and silt that slip past the charcoal.
- Gravel (BOTTOM) — supports the layers above and lets the cleaned water drain through into the collection cup.
Designing a Fair Test
A STEM challenge is also a science experiment. To compare two filters honestly, you must change only ONE thing at a time. Everything else — the dirty water, the bottle size, the pour speed — has to stay the same across all trials.
- Independent variable — the one thing you change (e.g. order of layers, or amount of charcoal).
- Dependent variable — what you measure (clarity of the output, time to filter 200 ml).
- Controlled variables — same dirty water sample, same container size, same total volume.
- Repeat each trial at least twice. One run could be lucky; two runs are evidence.
How Engineers Decide if it Worked
- Clarity — hold the filtered water against a white card. Can you see through it?
- Time — how many seconds did 200 ml take to drip through? Slow can be a sign of fine filtration.
- Smell — does the water still smell like the muddy original, or is it neutral? (Charcoal helps here.)
- Solids left in the cup — any visible particles? They tell you which layer let them through.
Important: a homemade filter makes water LOOK cleaner, but it does NOT make it safe to drink. Germs and dissolved chemicals can still pass through. To make filtered water drinkable, it must be boiled or treated with chlorine or another approved method.
Tip: pour the dirty water SLOWLY in a thin stream. If you dump it in fast you can blast a hole through the layers and the water will rush past untouched. Patience produces the cleanest result.
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