Build a Water Filter
Engineer your own water filtration system and learn how dirty water can be cleaned using natural materials.
Adult supervision recommended for younger children.
An adult should help with cutting the plastic bottle. This experiment can be messy — do it outside or on a tray.
Important Safety Note!
- • DO NOT drink the filtered water. Your filter removes visible dirt, but it does not remove bacteria or chemicals that could make you sick.
- • An adult should cut the plastic bottle with scissors or a knife.
- • Wash hands after handling dirty water.
You Will Need
Step-by-Step Instructions
Cut the Bottle
Have an adult cut the plastic bottle in half. You will use the top half (with the cap) turned upside down as a funnel, sitting inside the bottom half which acts as a collection cup. Remove the cap or poke small holes in it.
Layer 1: Cotton
Place a thick layer of cotton balls at the very bottom of the upside-down funnel (at the neck of the bottle). This is the finest filter and will catch the smallest particles.
Layer 2: Activated Charcoal (Optional)
If you have activated charcoal, add a layer about 3cm thick on top of the cotton. Charcoal absorbs chemicals, bad smells, and tiny particles. Rinse the charcoal with clean water first to remove dust.
Layer 3: Fine Sand
Add a layer of fine sand about 5cm thick. Fine sand catches small dirt particles that make it past the gravel.
Layer 4: Coarse Sand / Pebbles
Add a layer of coarse sand or small pebbles about 5cm thick. This layer catches medium-sized particles.
Layer 5: Gravel
Add gravel or small rocks as the top layer, about 5cm thick. This is the first thing the dirty water hits, catching the biggest bits like leaves and twigs.
Make Dirty Water
Mix water with soil, a few small leaves, and tiny twigs to make dirty water. Stir it up well so it looks brown and murky.
Filter!
Slowly pour the dirty water into the top of your filter. Watch it trickle down through each layer and collect in the bottom cup. Compare the colour of the filtered water to the original dirty water.
Extension: Improve Your Design
Run the filtered water through a second time. Is it even cleaner? Try different layer arrangements. What happens if you change the order? Can you design the most effective filter?
Filter Layer Order (bottom to top)
Water flows from top to bottom, getting cleaner at each layer.
What Happened?
The dirty brown water became much cleaner and clearer after passing through your filter! Each layer trapped different-sized particles:
- • Gravel stopped the biggest debris like leaves and twigs.
- • Coarse sand caught medium-sized dirt particles.
- • Fine sand trapped very small particles of silt.
- • Activated charcoal (if used) absorbed chemicals and colour through a process called adsorption.
- • Cotton caught the finest remaining particles.
However, the water is still not safe to drink because the filter cannot remove invisible bacteria, viruses, and dissolved chemicals. Real water treatment plants use additional steps like UV light, chlorine treatment, and advanced membranes to make water truly safe.
The Science Behind It
Filtration is the process of passing a liquid through a material that traps unwanted particles. The filter material acts as a physical barrier — particles larger than the gaps in the material get stuck, while water passes through.
Different materials have different-sized gaps between their particles. Gravel has big gaps (catches big stuff), sand has tiny gaps (catches small stuff), and cotton has extremely tiny gaps (catches very fine particles).
Activated charcoal works differently — it uses a process called adsorption (not absorption). Charcoal has an incredibly porous surface with millions of tiny holes. Chemicals and impurities stick to these surfaces as water passes through.
This is essentially the same process that happens in nature! When rain falls on the ground, it filters through layers of soil, sand, and rock, becoming cleaner as it reaches underground aquifers. This natural filtration is how many communities around the world get their water.
Global Connection: The Water Crisis
Around the world, approximately 2 billion people do not have access to safely managed drinking water. Many communities rely on rivers, ponds, or wells that may contain bacteria, parasites, and pollutants.
Simple filtration systems — similar to what you just built but more advanced — are used by organisations like WaterAid and UNICEF to provide clean water in developing communities. Ceramic filters, bio-sand filters, and solar disinfection are affordable technologies saving lives every day.
In Australia, we are lucky to have clean water from the tap, but droughts remind us that water is a precious resource that must be managed carefully.
Think About It
Why is the order of the layers important? What would happen if you put cotton on top and gravel on the bottom?
Even though the filtered water looks clean, why is it still not safe to drink? What invisible things might still be in it?
Imagine you are living in a remote village with no clean water supply. How could you use materials found in nature to build a filter?
What can we do in our daily lives to conserve water and keep waterways clean?
Knowledge Check
Test what you have learned! Select the correct answer for each question.
Question 1
What is filtration?
Question 2
Which layer in your filter catches the biggest particles?
Question 3
Why is the filtered water still not safe to drink?
Question 4
How does activated charcoal help clean water?
Question 5
Approximately how many people worldwide lack access to safely managed drinking water?
Key Concepts Summary
- ● Filtration separates particles from a liquid by passing it through a material that traps the particles.
- ● Different materials filter different-sized particles: gravel (large), sand (small), cotton (very fine).
- ● Activated charcoal removes chemicals through adsorption — impurities stick to its porous surface.
- ● Physical filtration cannot remove bacteria, viruses, or dissolved chemicals — additional treatment is needed.
- ● Clean water access is a global challenge — about 2 billion people lack safely managed drinking water.