Lab / experiment Grade 7 Engineering & Design
Ashford Mills Filter Trade-Off: Flow Rate Versus Clarity
Students build three filter designs, time how fast each runs, rate how clear the water is, and find the trade-off between speed and clarity.
Objectives
- Students will build three filter designs and measure the flow time and water clarity of each.
- Students will use their data to describe the trade-off between fast flow and clear water and propose a better design.
Vocabulary
- Flow rate
- Flow rate measures how much water moves past a point in a certain amount of time, like gallons per minute through a garden hose. Bigger pipes and stronger pushes give bigger flow rates.
- Turbidity
- Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.
Also: filter, clarity, trade-off
Teacher overview
The fictional Ashford Mills water works needs a filter that does two jobs at once. It must run fast enough to treat enough water, and it must make the water clear enough to drink. These two goals can pull against each other. A filter with more layers usually makes clearer water, but it also runs slower. In this lab students build three filter designs, time each one, and rate how clear the water comes out. Real plants must keep water at or below 0.3 NTU, a very low cloudiness reading, so students also judge whether a simple filter could ever reach that goal.
Procedure
- Your teacher will cut the bottom off each bottle ahead of time. Turn each bottle upside down. Each one is now a funnel.
- Put a coffee filter in the neck of each bottle. Now build three filters. Filter 1 gets gravel only. Filter 2 gets gravel, then a layer of sand on top. Filter 3 gets gravel, then sand, then a layer of cotton balls on top.
- Stir the muddy water so it is fully mixed. Rate its cloudiness with the 1-to-5 card. Write that number down. This is your starting water.
- Pour 250 mL of muddy water into Filter 1. Start the timer as you pour. Stop the timer when the water stops dripping. Write down the flow time in seconds.
- Rate the clarity of the water that came out. Use the 1-to-5 card. A 1 means clear. A 5 means very cloudy. Write the number down.
- Do the same steps for Filter 2 and Filter 3. Use a fresh 250 mL of muddy water each time. Stir the muddy water again before each pour.
- Safety note: do not drink or taste any water in this lab. The muddy water and the filtered water are for testing only. Wash your hands when you finish.
Data & Analysis
Record your results in the table. Lower clarity numbers mean clearer water.
| Design | Flow time (seconds) | Clarity after (1-5) |
|---|---|---|
| Filter 1: gravel | ||
| Filter 2: gravel + sand | ||
| Filter 3: gravel + sand + cotton |
Which design had the shortest flow time? That design was the fastest.
Which design gave the clearest water? That is the design with the lowest clarity number.
Suppose one design filtered its water in 40 seconds and another design took 55 seconds. How many seconds faster was the first design?
Look at your fastest design and your clearest design. Are they the same design? Describe the trade-off between speed and clarity that your data shows.
A real plant must keep its water at or below 0.3 NTU in nearly all of its samples. Based on your results, could a bottle filter like yours meet that goal on its own? Explain why or why not.
Use your data to describe a new filter that mixes the best parts of two of your designs. Your goal is a filter that is both fast and clear.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: Accept the design with the lowest recorded flow time in the student's own data. In most groups the gravel-only filter runs fastest because water passes through large gaps quickly.
Answer: Accept the design with the lowest clarity number in the student's own data. In most groups the gravel-sand-cotton filter gives the clearest water because it has the most layers to trap sediment.
Answer: 15 seconds.
55 − 40 = 15 seconds faster for the first design.Answer: Accept any answer that matches the student's data and names the trade-off. Usually the clearest filter is not the fastest, so making water clearer tends to make it flow more slowly.
Answer: No, not on its own. A bottle filter removes larger visible sediment, but 0.3 NTU is far lower than a simple filter can reach. Real plants add finer filtering, chemical treatment, and constant testing to get there.
Answer: Accept any design that combines features supported by the data, such as a gravel base for fast flow topped with a thin cotton layer for clarity, leaving out the slower sand layer. The reasoning should point to which tested design contributed each part.
Differentiation
Support
Build one demonstration filter with the class before groups start. Give these groups a data table with only two designs to test instead of three, and pre-mark the clarity card with a picture at each number.
Extension
Have groups test a fourth design with a charcoal layer added, then compare its flow time and clarity to their first three. Ask them to decide whether the charcoal was worth the change in flow speed.
ELL supports
Provide picture cards for each filter layer (gravel, sand, cotton, coffee filter) labeled with its name. Allow students to record clarity using the numbered card alone. Give a sentence frame for question 4: "My fastest filter was ___, and my clearest filter was ___, so the trade-off is ___."