Science activity
Water Filter Engineering
Layer pebbles, sand, and a coffee filter in a cup. Pour muddy water through and watch it come out cleaner!
Materials and setup
- plastic bottle (cut in half)
- sand
- gravel
- activated charcoal (pet store)
- cotton balls
- muddy water
- cups for collecting filtrate
How this changes by age
Pre-K (ages 3–4)
Layer pebbles, sand, and a coffee filter in a cup. Pour muddy water through and watch it come out cleaner!
Steps
- Place a coffee filter in the bottom of a clear plastic cup with small holes poked in the bottom.
- Add a layer of sand on top of the filter, then a layer of small pebbles.
- Make muddy water in another cup by stirring dirt into water.
- Slowly pour the muddy water through your filter into a third cup underneath.
- Look at the water that came out. Is it cleaner than what you poured in? Talk about what the filter caught.
- SAFETY — DO NOT DRINK the filtered water. Home charcoal, sand, and pebble filters DO NOT remove germs or dissolved chemicals. This is a demonstration of how filters work, NOT a way to make safe drinking water.
- Tell a grown-up one thing that surprised you.
Learning objectives
- Observe that materials can clean dirty water
- Build a simple layered structure
- Compare 'before' and 'after' visually
Kindergarten (ages 5–6)
Build TWO filters with different layers. Test which one cleans water better. Hold a paper behind each cup — through which can you read the words?
Steps
- Build Filter A: pebbles + sand + coffee filter.
- Build Filter B: pebbles + sand + cotton balls + coffee filter.
- Make a single batch of muddy water and pour the same amount through each filter.
- Hold a piece of paper with big writing behind each cup of filtered water. Through which can you read the words more clearly?
- Decide which filter worked better and tell a grown-up why you think so.
- SAFETY: Even though the water LOOKS cleaner, DO NOT DRINK IT. Home filters made of sand, pebbles, and cotton do NOT remove germs that can make you sick. Real drinking water is treated with chemicals that kill germs.
- Tell a grown-up one thing that surprised you.
Learning objectives
- Compare two designs side by side
- Use a simple visual test (readability) as a measurement
- Explain a result using evidence
Early elementary (ages 6–8)
Design a 3-stage filter (settling layer, sand, charcoal). Measure clarity by reading words through the filtered water. Test 3 designs and pick the best.
Steps
- Cut a plastic bottle in half. Use the top half upside down (cap end down, with cap removed) as your filter funnel. Place a cotton ball at the neck.
- Build Design 1: cotton + sand + gravel (pour water in top, gravel on top so it filters down through sand and out the cotton).
- Build Design 2: cotton + activated charcoal (from a pet store) + sand + gravel.
- Build Design 3: settling cup first (let muddy water sit for 5 min so big particles fall to the bottom), then pour the top water through cotton + charcoal + sand + gravel.
- Test each design with the SAME amount of muddy water from the SAME batch. Score each by counting how many words of standard-size text you can read through a clear cup of the filtrate.
- Pick a winner. Write a short explanation: which design scored highest, and why do you think it worked?
- SAFETY: DO NOT DRINK any of the filtered water, even if it looks clear. Home filters remove dirt particles but NOT bacteria, viruses, or chemicals that can make you sick.
- In one sentence, tell a parent or sibling what surprised you today.
Learning objectives
- Design and compare multi-stage filters
- Hold variables constant when testing designs
- Use a measurable proxy (readable words) for clarity
Upper elementary (ages 8–10)
Build a multi-stage water treatment system. Calculate gallons-per-hour throughput. Research how municipal water treatment plants work and identify which stages your filter mimics.
Steps
- Build a multi-stage filter using a cut plastic bottle: cotton at the neck, then activated charcoal, then fine sand, then coarse sand, then gravel on top.
- Test with muddy water. Time how long it takes for a measured volume (e.g., 500 mL) to fully filter through. Calculate the throughput: liters per hour, and convert to gallons per hour.
- Research municipal water treatment. Most plants use: (1) screening, (2) coagulation/flocculation, (3) sedimentation, (4) filtration, (5) disinfection. Find a diagram from a credible source.
- Map each stage of YOUR filter to a stage of the municipal process. Which stages does your filter mimic? Which stages does it skip?
- Write 1 short paragraph explaining clearly: filtered water from your design is NOT safe to drink. Why not? (Microbes — bacteria, viruses, parasites — and dissolved chemicals are not removed by sand and charcoal alone. Disinfection with chlorine or UV is required for safe drinking water.)
- Submit a 1-page report: design diagram, throughput calculation, mapping to municipal stages, and the safety paragraph.
- In one sentence, tell a parent or sibling what surprised you today.
Learning objectives
- Build and characterize a multi-stage filter with a real flow-rate calculation
- Connect a hands-on build to real municipal infrastructure
- Communicate a critical safety limitation precisely
Safety and evidence note
Read the full activity before beginning. An adult should supervise tools, heat, food, outdoor work, movement, and experiments as appropriate. Completion records that the activity was done; the child’s explanation, work sample, photo, or demonstration is stronger evidence of learning than a completion check alone.
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