Lab / experiment Grade 9 Engineering & Design
Brackwell Creek Jar Test: Finding the Best Coagulant Dose
A grade 9 lab where students run a coagulation jar test on turbid water, compare alum doses, and find the dose that clears the water best before it settles.
Objectives
- Students will run a coagulation jar test using a range of alum doses on turbid water.
- Students will identify the dose that clears the water best and explain the trade-offs of overdosing.
Vocabulary
- Coagulation
- Coagulation is the first cleaning step at a drinking water plant. Operators quickly mix in a chemical that makes tiny bits of dirt stop pushing each other away so they can start sticking together.
- Alum
- Alum is the most common chemical that water plants add to make tiny dirt particles clump together. A pinch of alum can help a whole jar of muddy water turn clear.
- 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: settling
Teacher overview
Brackwell Creek is a fictional muddy stream a small plant wants to treat. Real plants add a coagulant such as alum, which makes fine mud clump together into larger bits that settle out. Adding too little alum leaves the water cloudy, and adding too much can push the mud back apart. This lab lets students find the best dose using a jar test, the same tool plant operators use. A classroom cannot measure turbidity in NTU, so students rate clarity on a simple 1-to-5 scale instead.
Procedure
Put on goggles. Wear safety goggles for the whole lab. The teacher prepares the alum stock solution; students do not mix the chemical themselves.
Make the turbid water. Stir 2 teaspoons of soil into every 3 liters of water until it looks evenly cloudy. Make about 4 liters for each group, enough for two trials of six 300 mL jars.
Fill the jars. Measure 300 mL of turbid water into each jar of the first set of 6. Number the jars 1 through 6.
Rate the starting clarity. Hold jar 1 against white paper with a flashlight behind it. Rate its clarity from 1 (clear) to 5 (very cloudy) and record it as the "before" value. All jars start the same, so this value is shared.
Add the doses. Using the dropper, add alum to the jars in this order: jar 1 gets 0 drops (the control), jar 2 gets 2 drops, jar 3 gets 4 drops, jar 4 gets 6 drops, jar 5 gets 8 drops, and jar 6 gets 10 drops.
Mix, then wait. Using one rod per jar, stir all six jars at the same time, quickly for 1 minute, then slowly for 3 minutes, then stop. Let all jars sit still for 15 minutes so the clumps can settle.
Rate the settled clarity. Rate each jar's clarity again from 1 to 5 using the card and flashlight. Record each value. Do not taste or drink any of the water, and wash your hands when finished.
Run a second trial. While the first set of jars settles, fill the second set of six jars with fresh turbid water and repeat the dosing, mixing, settling, and rating steps to check that the result holds.
Data & Analysis
Record the settled clarity rating (1 to 5) for each jar in both trials.
| Jar | Alum drops | Clarity trial 1 | Clarity trial 2 |
|---|---|---|---|
| 1 | 0 | ||
| 2 | 2 | ||
| 3 | 4 | ||
| 4 | 6 | ||
| 5 | 8 | ||
| 6 | 10 |
Which jar gave the clearest settled water? State how many drops of alum it received.
What was the settled clarity rating of the control jar (0 drops)? Use the average of your two trials.
By how many clarity levels did your best jar improve over the control? (Control rating − best rating.)
In one class's data the control rated 5 and the best jar rated 1 on the 1-to-5 scale. What was the percent improvement in the clarity rating? (Difference ÷ control × 100.)
Real plants must keep finished water at or below 0.3 NTU in nearly all samples. Could this jar test alone prove a plant meets that limit? Explain why or why not.
Adding more alum than the best dose can make the water cloudy again. Explain the trade-off between adding enough alum and adding too much, including chemical cost and the extra sludge that must be handled.
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: The jar with the lowest clarity number. Most groups find that a middle dose, often 4 or 6 drops, gives the clearest water. Accept the dose that matches the student's own data.
Answer: A value from 1 to 5, usually near 5 for the muddy control. It should equal the average of the student's two control readings.
Answer: A whole number of clarity levels equal to the control rating minus the best rating. Accept any value that matches the student's own data and shows the subtraction.
Answer: 80 percent.
Improvement = (5 − 1) ÷ 5 × 100 = 4 ÷ 5 × 100 = 80%.Answer: No. A jar test with a 1-to-5 eye rating cannot measure NTU, and 0.3 NTU is far lower than the eye can judge. The test shows which dose works best, but a plant needs a real turbidity meter and repeated samples to prove it meets the limit.
Answer: Accept any answer that names both sides. Enough alum clears the water, but too much wastes chemical, costs more money, and leaves more sludge to remove and dispose of, and it can even make the water cloudy again.
Differentiation
Support
Run one jar as a class demonstration first, so students see the mixing and settling steps before their own trial. Provide a data table for a single trial instead of two.
Extension
Have students test one more variable, such as a longer settling time or a warmer water sample, and write a short comparison of how it changed the best dose or the final clarity.
ELL supports
Provide picture cards for "coagulation," "settle," and "clarity," and a labeled 1-to-5 clarity scale with a shaded jar next to each number. Allow students to record ratings using the numbered card alone.