Grade by Grade
C. Matter and Energy · Pure Substances and Mixtures

Design Challenge: Build a Water Filter

A1.3 A1.4 A1.5 C2.7 C2.3About 60 minutes
Learning goals
  • I can use an engineering design process to design, build, test and improve a water filter.
  • I can set criteria and constraints for a design.
  • I can explain which parts of a mixture my filter can and cannot remove.
  • I can follow safety rules and explain why filtered water is still not safe to drink.

Words to know

engineering design process
A series of steps engineers use to solve a problem: define, research, plan, build, test, evaluate, improve and share.
criteria
What a design must do to be successful. For example: the water must come out clearer.
constraints
Limits on a design, such as time, cost, size or materials.
prototype
A first working model of a design, used for testing.
turbidity
How cloudy a liquid is because of particles in it. Clear water has low turbidity.
contaminant
Any unwanted substance in water, such as dirt, germs or dissolved chemicals.

The challenge

You have a jar of muddy water: a heterogeneous mixture (a suspension) of water, soil, bits of leaves and maybe some tiny gravel. Your job: design and build a filter that makes the water as clear as possible.

Important: even very clear filtered water is not safe to drink. A home-made filter can't remove germs or dissolved substances. Treating drinking water is a job for trained water treatment operators.

The engineering design process

  1. Define the problem: what must the design do? Set the criteria and constraints.
  2. Research and brainstorm: what materials might trap particles of different sizes?
  3. Plan: sketch the design with labels. List materials.
  4. Build a prototype.
  5. Test it fairly and record results.
  6. Evaluate and improve: what worked? What would you change? Build version 2.
  7. Communicate: share your design, results and improvements.
1. Define2. Research3. Plan4. Build5. Test6. Evaluate7. Communicateimprove andbuild againshare the design, results and changes
The design process is a loop, not a straight line. After testing and evaluating, engineers usually improve and build again, then communicate what they learned.
Criteria (what it must do)Constraints (limits)
Water coming out is clearer than water going inOnly use materials from the list (no store-bought filters)
Filters 250 mL of muddy waterMust fit in one 2 L plastic bottle
Finishes filtering in under 5 minutesBuild time: 30 minutes
Can be emptied and reusedGrown-up does all cutting
Sample criteria and constraints. You can change them, but write them down before you build.

How layered filters work

Real water treatment plants often pass water through beds of sand and gravel. In your home-made filter, you can use layers that each trap particles of different sizes:

  • Coarse layer (gravel or small pebbles): catches big pieces like leaves and twigs.
  • Medium layer (sand): catches smaller grains of soil.
  • Fine layer (coffee filter, cotton balls, cloth): catches tiny particles.

Why not use only the finest layer? It would clog quickly with big particles. Putting the coarse layer on top protects the fine layers.

What a filter can't do: it can't remove dissolved substances (like salt or sugar) because they pass through with the water. It can't reliably remove germs either. Water treatment plants add other steps, like disinfecting with chlorine, to make water safe.

A homemade filter made from an upside-down plastic bottle with layers of gravel, sand and cotton, dripping clear water into a jar beside a jar of muddy water.
The coarse gravel on top catches big pieces, the sand catches smaller grains, and the cotton catches tiny particles. Filtered water looks clear but is not yet safe to drink. AI-generated image
gravel or pebbles: catches big piecessand: catches smaller grainscotton or coffee filter: tiny particlesmuddy water inclearer water out (NOT safe to drink)coarsefine
A layered bottle filter, coarse on top to fine at the bottom. The gravel protects the finer layers from clogging. Even clear water from a home-made filter is not safe to drink.
Worked example

Let's evaluate a prototype

Noor's first filter, from top to bottom: coffee filter, sand, gravel. She poured 250 mL of muddy water in. It took 12 minutes, and the water came out slightly cloudy. Evaluate it against the criteria and suggest an improvement.

  1. Compare with the criteria: clearer than before? Yes, a bit. Under 5 minutes? No (12 minutes). So it partly meets the criteria.
  2. Find the cause: the coffee filter is on top, so all the mud hits the finest layer first and clogs it. That's why it's so slow.
  3. Improve: flip the order: gravel on top, then sand, then the coffee filter (or cotton) at the bottom near the bottle neck.
  4. Test again with the same amount of the same muddy water (stir the jar first so it's mixed the same way), and compare the time and clarity.

The prototype was too slow because the fine layer clogged. Version 2 should go from coarse on top to fine on the bottom, then be re-tested the same way.

Just for fun

Engineers have a saying: if your first prototype works perfectly, you probably didn't test it hard enough. Noor's 12-minute filter wasn't a failure; it was version 1.

Hands-on activity

Build, test, improve

A grown-up does all the cutting. Cut edges of plastic bottles can be sharp.

You need: a 2 L clear plastic bottle, a jar or glass, gravel or small pebbles (rinsed), clean sand (play sand), cotton balls, a coffee filter, an elastic band, a measuring cup, a timer, garden soil and water. Optional: a piece of clean cloth.

  1. Make the muddy water: stir 2 big spoonfuls of soil into 500 mL of water. Keep a little in a glass as your "before" sample.
  2. Prepare: a grown-up cuts the bottle in half. Turn the top half upside down (like a funnel) and set it into the bottom half. Cover the bottle's mouth with a coffee filter held by an elastic band.
  3. Plan and sketch your layers with labels. Write your criteria and constraints.
  4. Build your prototype by adding layers.
  5. Test: stir the muddy water, then pour 250 mL in. Time how long until it stops dripping. Compare the clarity: hold each glass in front of a page of printed text. Can you read the words through it?
  6. Rate the clarity on a scale (1 = can't see the text, 5 = very clear).
  7. Improve one thing (layer order, thickness or material) and test again the same way.
  8. Share it: make a labelled diagram, a results table for version 1 and version 2, and a sentence on why you made your change.

What to look for: a filter with gravel on top, sand in the middle and cotton or a coffee filter at the bottom usually gives much clearer water than a single layer. The water may still be a bit cloudy or brownish, because some particles are extremely tiny and some substances from the soil are dissolved.

Safety: do not drink the filtered water, even if it looks clear. Wash your hands after handling soil. Pour used water and soil into the garden, not the sink (sand and gravel can clog drains).

Your turn

1

What is the difference between a criterion and a constraint?

2

Why should the coarse layer go on top?

3

Name two things a home-made filter cannot remove.

4

Why is it important to test version 1 and version 2 in exactly the same way?

5

What is a prototype?

6

Is muddy water homogeneous or heterogeneous? What type of mixture is it?

More practice: warm-up

1

What does turbidity measure?

2

Is it safe to drink water from your home-made filter?

3

Who does the cutting of the plastic bottle?

4

What is the first step of the engineering design process?

5

Which layer catches leaves and twigs?

More practice: on your own

1

A filter made only of cotton balls packed tightly works very slowly. Explain why and suggest a fix.

2

Before filtering, a teaspoon of salt is stirred into the muddy water. After filtering, will the clear water still contain salt? Explain. (Don't taste it to find out!)

3

Why do we hold the glass in front of printed text?

4

Give one constraint that real water treatment engineers might have.

5

Why is it a good idea to only change one thing between version 1 and version 2?

Challenge

1

Version 1 took 4 minutes and scored 2/5 for clarity. Version 2 (thicker sand layer) took 7 minutes and scored 4/5. Which is better? Use the criteria (under 5 minutes, clearer water) to explain, and suggest a version 3.

2

A town's water treatment plant lets water sit in big tanks before filtering it. How does this help the filters?

3

Explain, using the words suspension and solution, why filtering works on muddy water but not on salt water.

Check your understanding

Quiz

1.

In a design challenge, "must finish filtering in under 5 minutes" is a...

2.

The best order for a layered filter, from top to bottom, is...

3.

A home-made filter cannot remove...

4.

Why should you not drink water from a home-made filter?

5.

A first working model built for testing is called a...

6.

After testing, the next step in the engineering design process is to...

For parents and teachers

Big idea: This design challenge uses the engineering design process (A1.3) to apply separation by filtration (C2.7) to a heterogeneous mixture (C2.3). It also practises safety (A1.4) and communicating results (A1.5).

How to help: Do the cutting yourself. Let your child plan, build and decide on improvements, even if the first version doesn't work well. Failed tests are part of engineering. Ask: "What criterion did it meet? What would you change, and why?"

Common difficulties:

  • Thinking clear water is clean water. Germs and dissolved substances can't be seen.
  • Changing several things between tests.
  • Putting the finest layer on top.

Safety: you cut the bottle (sharp edges). Nobody drinks the filtered water. Dispose of muddy water and sand outdoors, not down the sink.