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Lab / experiment Grade 12 Science

Jar Test Coagulation on Brindle Creek Water: Finding the Dose

A grade 12 lab where students run a five-jar coagulation test on fictional muddy source water, rate settled clarity, and find the lowest effective alum dose.

Objectives

  1. Students will run a jar test, varying coagulant dose across five jars and rating settled-water clarity.
  2. Students will identify the lowest coagulant dose that produces good clarity.
  3. Students will explain why a classroom clarity rating is a proxy for, not a substitute for, a certified turbidity reading.

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.
Flocculation
Flocculation is the slow, gentle stirring step at a water plant that lets tiny sticky particles bump into each other and clump into bigger pieces called floc, heavy enough to settle out of the water.
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: optimal dose

Teacher overview

Before muddy river water can be filtered, treatment plants add a coagulant such as alum, which makes fine particles clump into floc heavy enough to settle. Operators find the right dose with a jar test, and this lab is a classroom version using fictional Brindle Creek source water. Students dose five jars with increasing amounts of alum, mix and settle them, and rate the clarity of the water above the settled layer. A classroom cannot measure turbidity in the exact units a plant reports, so the clarity rating here is a simple stand-in that still shows the same pattern.

Jar Test Coagulation on Brindle Creek Water: Finding the Dose — Grade 12 Science
Name: ____________________________ Date: ______________

Procedure

Step 1. Safety: put on goggles. Alum is a mild skin and eye irritant, so do not taste or drink any sample, wash your hands after handling it, and wipe up spills right away to avoid a slip.

Step 2. Prepare the turbid source water for the fictional Brindle Creek by stirring about 3 tablespoons of soil into 1.5 liters of water until it is evenly cloudy. Fill five identical jars each with the same 250 mL of this water and label them 1 through 5.

Step 3. Add the alum stock solution in increasing doses: jar 1 gets 0 drops and is your control, jar 2 gets 5 drops, jar 3 gets 10 drops, jar 4 gets 15 drops, and jar 5 gets 20 drops.

Step 4. Rapid mix: stir each jar quickly for 30 seconds right after dosing, which spreads the coagulant like a plant's flash mixer.

Step 5. Slow mix: stir each jar gently for 2 minutes to let small particles collide and build into larger floc.

Step 6. Settle: let all five jars stand completely undisturbed for 15 minutes.

Step 7. Rate the clarity of the water above the settled layer in each jar using the 1 to 5 card, where 1 is clear and 5 is very cloudy, and record it in the data table.

Step 8. If time allows, repeat steps 2 through 7 as a second trial so you can compare your two sets of ratings.

Data & Analysis

Jar Alum dose (drops) Clarity rating (1-5)
1 0
2 5
3 10
4 15
5 20

  1. Which jar, and which alum dose, produced the clearest settled water based on your ratings?

  2. What clarity rating did the control jar (0 drops) receive, and what does comparing it with the dosed jars show about what coagulation does?

  3. Describe the trend in clarity as the dose increased from 0 to 20 drops. Was the water always clearer at a higher dose?

  4. If any high-dose jar looked cloudier again, at what dose did clarity stop improving? Name that dose as the optimal dose from your data.

  5. Find the average clarity rating of the two lowest doses (jars 1 and 2) and the average of the two highest doses (jars 4 and 5). Which group settled clearer?

  6. A real plant must bring finished water to 0.3 NTU or below. Could a jar test like this reach that standard on its own? Explain the difference between your clarity rating and a certified turbidity reading.

  7. As an operator, why is the lowest dose that still gives good clarity usually the best choice? Consider both chemical cost and the amount of settled sludge produced.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

  1. Answer: Accept the jar with the lowest clarity rating in the student's data.
    In most runs a middle dose, often jar 3 or jar 4, gives the clearest water. Accept any jar the student's own ratings support, with a clear statement that a lower rating means clearer water.

  2. Answer: The control should be the cloudiest, showing coagulation is what clears the water.
    With no alum, the fine particles stay suspended, so the control usually keeps a high cloudiness rating. Comparing it with the dosed jars shows that adding coagulant is what lets particles clump and settle.

  3. Answer: Clarity improves as dose rises, then often levels off or worsens at the highest dose.
    Accept a description that matches the student's data. Many groups see clarity improve up to a point and then stop improving, or even get slightly worse, at the highest dose.

  4. Answer: Accept the dose where the student's ratings stop dropping.
    The optimal dose is the smallest dose that reaches the best clarity. If jar 4 and jar 5 have the same rating, the optimal dose is the jar 4 dose, since more alum past that point adds no benefit.

  5. Answer: Accept the two averages computed from the student's data; the high-dose group usually settles clearer.
    Each average is the mean of the two listed ratings. In most runs the higher-dose group has the lower (clearer) average, though the difference shrinks near the optimal dose.

  6. Answer: No, not with a jar test alone.
    A jar test shows coagulation and settling but does not include filtration, and a clarity card is a rough visual scale, not a calibrated turbidimeter reading in NTU. Reaching 0.3 NTU also requires filtration, and confirming it requires a certified instrument, so the classroom rating only shows the same trend, not the official number.

  7. Answer: The lowest effective dose saves chemical and produces less sludge.
    Beyond the optimal dose, extra alum adds cost and makes more settled sludge to handle without improving clarity. Choosing the smallest dose that still clears the water balances treatment quality against chemical use and waste, the trade-off an operator manages daily.

Differentiation

Support

Pre-mix the turbid water and pre-label the jars so groups begin at the dosing step, and provide a simplified data table for a single trial. Demonstrate one rapid-mix and slow-mix cycle for the class before groups start.

Extension

Have groups test a narrower range of doses around their optimal jar (for example 8, 10, and 12 drops) to pin the best dose more precisely, then write a short recommendation to the plant naming the dose and the reason.

ELL supports

Provide picture cards for each step and a labeled photo showing a clear jar next to a cloudy jar. Allow students to record results with the clarity numbers and a ranking rather than full sentences, and pair the clarity card with its numbers.

Extensions

Have students find out what coagulant their local treatment plant uses from its water quality report, then write one question for the utility asking how operators decide the daily dose as the river's cloudiness changes with the weather.