Lab / experiment Grade 9 Engineering & Design

Building and Testing a Multimedia Water Filter Column

A grade 9 engineering lab where students build a gravel, sand, and charcoal filter column, then test it by measuring water clarity and flow rate across two trials.

Grade: Grade 9
Subject: Engineering & Design
Material: Lab / experiment
Time: 55 min
Prep: Moderate prep
Materials: Clear 2-liter plastic bottle with the bottom cut off (1 per group), Coffee filter or two layers of paper towel (1 per group), Gravel or small pebbles (1.5 cups per group), Coarse sand (2.5 cups per group), Activated charcoal, aquarium type (2 cups per group), Cotton balls (2 per group), Muddy water sample, soil stirred into water (1 liter per group), Clear collection cups (2 per group), Graduated cylinder or measuring cup, Stopwatch or phone timer, Ruler, Cloudiness comparison card, 1 to 5 scale (1 per group), Safety goggles (1 per student)

Standards

HS-ETS1-1 NGSS

Analyze a major global challenge to specify qualitative and quantitative criteria and constraints for solutions that account for societal needs and wants.

How this material aligns: Students set measurable criteria and constraints for a working filter, a clarity target and a workable flow rate, before building and testing the column in this lab.

All materials for HS-ETS1-1 →

HS-ETS1-3 NGSS

Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics as well as possible social, cultural, and environmental impacts.

How this material aligns: In analysis question 6 students weigh the trade-off between faster flow and clearer water, evaluating their filter design against the prioritized criteria this standard requires.

All materials for HS-ETS1-3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-17.

Objectives

Students will build a layered filter column and test it on a muddy water sample.
Students will measure clarity and flow rate across two trials and evaluate the design trade-offs.

Vocabulary

Filtration: Filtration is the step where water passes through layers of sand, gravel, or other materials that trap the small particles still left in it. It works like a very fine strainer.
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.
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.

Also: media, clarity

Teacher overview

Real treatment plants clear cloudy water by passing it through layers of gravel, sand, and other media that trap particles. By rule, a conventional plant must keep finished water at or below 0.3 NTU in 95 percent of its samples. A classroom cannot measure NTU exactly, but this lab lets students build a layered filter column and test it the way engineers do. Students judge the design by two measures at once: how clear the water gets and how fast it flows through.

Building and Testing a Multimedia Water Filter Column — Grade 9 Engineering & Design

Name: ____________________________ Date: ______________

Procedure

Step 1 (safety, read first): Put on goggles. Use only clean tap water and garden soil to make the muddy sample. Do not drink or taste any water in this lab. Wipe up spills right away so no one slips.

Step 2 (set design targets): As a team, agree on two goals for your filter. One goal is a clarity target, such as reaching a 2 or lower on the cloudiness card. The other is a flow that is not too slow, such as passing 250 mL in under two minutes. Write both targets down.

Step 3 (build the column): Turn the bottle, with its bottom already cut off by the teacher, upside down to make a funnel. Place a cotton ball loosely in the neck. Add a 2-inch layer of gravel, then a 3-inch layer of sand, then a 2-inch layer of charcoal. Lay a coffee filter on top. Measure each layer with the ruler as you go.

Step 4 (baseline reading): Rate the muddy sample's clarity on the 1 to 5 cloudiness card, where 1 is clear and 5 is very cloudy. Record the reading. Measure out 250 mL of the muddy water.

Step 5 (run trial 1): Pour the 250 mL slowly through the filter into a clean cup. Start the stopwatch when you begin pouring. Stop it when the water stops dripping. Record the time. Rate the collected water's clarity on the same card.

Step 6 (find the flow rate): Divide the volume by the time to get a flow rate in milliliters per second. For example, 250 mL in 40 seconds is a flow rate of 6.25 mL/s.

Step 7 (run trial 2): Repeat steps 4 through 6 with a second 250 mL sample from the same source. Record the second set of readings so you can check whether your results are consistent.

Data & Analysis

Trial	Clarity before (1-5)	Clarity after (1-5)	Volume (mL)	Time (s)	Flow rate (mL/s)
1			250
2			250

What was the average "before" clarity rating across your two trials?
What was the average "after" clarity rating across your two trials?
Find the clarity improvement, the average before rating minus the average after rating. What does this number tell you about the filter?
First check the formula flow rate = volume divided by time on the Step 6 example of 250 mL in 40 seconds. Then calculate the flow rate for your own trial 1. Show your work.
A real plant must keep water at or below 0.3 NTU in nearly all samples. Could a column like yours alone meet that standard? Explain why or why not.
Suppose you made the sand layer twice as deep. Predict how that change would affect the clarity and the flow rate, and explain the trade-off between the two.

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: varies by group's data.
The average of the group's two recorded "before" ratings. Because both samples come from the same muddy source, the two before ratings should be close.
Answer: varies by group's data.
The average of the group's two recorded "after" ratings. This should be lower than the before average if the filter worked as expected.
Answer: varies by group's data.
Clarity improvement = average before minus average after. A larger positive number means the filter removed more visible cloudiness. Accept any value consistent with the group's own data and a clear explanation.
Answer: 6.25 mL/s for the Step 6 check; the trial 1 value varies by group's data.
For the check, 250 / 40 = 6.25 mL/s. For trial 1, flow rate = 250 mL divided by the group's own recorded time. Accept any value that equals the group's trial 1 volume divided by its recorded time.
Answer: No, not with this column alone.
A bottle column removes visible sediment but not the tiny particles that set a true 0.3 NTU reading. Real plants add coagulation, finer filtration, and continuous testing to reach that standard.
Answer: Clearer water but slower flow.
A deeper sand layer usually traps more particles, so clarity improves, but the water passes through more slowly, so the flow rate drops. The trade-off is that better clarity often costs speed, and a design must balance both.

Differentiation

Support

Build one demonstration column as a class before groups build their own, and provide a data table that asks for a single trial instead of two. Give a printed reminder that flow rate equals volume divided by time.

Extension

Have groups test one design change, such as a deeper sand layer or an added charcoal layer, then compare the new clarity and flow rate with their first design and write a short recommendation about which design they would build.

ELL supports

Provide picture cards for each media layer, gravel, sand, and charcoal, labeled with its name. Allow students to record clarity using the numbered card alone, and provide a sentence frame for question 6, such as "A deeper sand layer makes the water ___ but the flow ___."