Lab / experiment Grade 11 Engineering & Design

Aldwyn Reservoir Jar Test: Finding the Optimal Alum Dose

A grade 11 lab where students run a five-jar coagulation test on cloudy water, plot clarity against alum dose, and scale the best dose to a plant feed rate.

Grade: Grade 11
Subject: Engineering & Design
Material: Lab / experiment
Time: 2 sessions, ~90 min total
Prep: Moderate prep
Materials: Five clear 9-ounce plastic cups or small beakers per group, 250 mL graduated cylinder or measuring cup, Food-grade pickling alum, about 2 grams per group, 1-liter bottle for mixing the alum stock solution, Pitcher or jar holding at least 1.5 liters, for the cloudy water, Scale that reads to 0.1 gram, shared, Marker for labeling cups, Two 10 mL syringes or droppers marked in mL, Dry clay or sifted garden soil, about 6 grams per group, Clarity tube: a clear straight-sided tube at least 30 cm tall and no more than about 3 cm across inside, marked in centimeters, with a black cross drawn on white paper taped under the base, Stirring rods or plastic spoons, Stopwatch or phone timer, Graph paper, Safety goggles, Paper towels

Standards

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: Analysis question 4 reads the point where added alum stops buying clarity, and question 8 asks students to recommend a dose to the fictional plant with one reason from their own clarity data and one from the cost of overdosing, the criteria-and-trade-off evaluation this standard describes.

All materials for HS-ETS1-3 →

HSS-ID.B.6 Common Core Math

Represent data on two quantitative variables on a scatter plot, and describe how the variables are related, including fitting a function to the data, using functions fitted to data to solve problems, and informally assessing the fit of a function by plotting and analyzing residuals.

How this material aligns: In Step 10 students plot clarity depth against alum dose as paired measurements on two quantitative axes, then in analysis questions 3 and 4 they describe how the two variables are related and read the curve's flattening at the high-dose end, the represent-and-describe portion of this standard.

All materials for HSS-ID.B.6 →

HSN-Q.A.1 Common Core Math

Use units as a way to understand problems and to guide the solution of multi-step problems; choose and interpret units consistently in formulas; choose and interpret the scale and the origin in graphs and data displays.

How this material aligns: Analysis questions 5 and 6 carry milligrams per liter, million gallons per day, and pounds per gallon through the feed-rate formula, the unit-guided calculation this standard targets.

All materials for HSN-Q.A.1 →

Download Word file

This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-07-23.

Objectives

Students will run a five-jar coagulation test across a range of alum doses and record clarity data.
Students will plot clarity against dose and identify the dose beyond which clarity stops improving.
Students will convert a chosen dose into a plant feed rate in pounds per day.

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.
Jar test: A jar test is a small-scale experiment operators run in the lab before treating a whole tank of water, testing different amounts of chemical in separate jars to see which dose clears the water best. It helps operators avoid wasting chemicals or under-treating the water.

Also: optimal dose

Teacher overview

Before a treatment plant settles the fine clay that makes raw water cloudy, an operator has to decide how much coagulant to feed. Too little leaves the particles suspended, while too much wastes chemical and creates extra sludge to haul away, and because the right dose depends on the raw water that day, plants find it with a bench test rather than a formula. This lab runs that bench test. Groups dose five jars of the same cloudy water with a rising series of alum doses, mix and settle them under matched conditions, measure how clear each one becomes, and then convert the best dose into the pounds per day the fictional Aldwyn plant would feed. Teachers should watch the mixing steps most closely, because the comparison between doses is only valid if every jar receives the same fast stir and the same slow stir.

Aldwyn Reservoir Jar Test: Finding the Optimal Alum Dose — Grade 11 Engineering & Design

Name: ____________________________ Date: ______________

Procedure

Step 1, safety (3 minutes). Put on goggles and keep them on for the whole lab. Use only food-grade pickling alum. Do not drink or taste any water in this lab, and wipe spills right away so the floor stays dry.

Step 2, mix the raw water (7 minutes). Stir about 6 grams of dry clay or sifted soil into 1.5 liters of tap water. Let the container sit for 5 minutes so the heaviest grit falls out, then stir it once more. This cloudy water stands in for raw water drawn from the fictional Aldwyn Reservoir.

Step 3, mix the alum stock (5 minutes). Dissolve 1.0 gram of alum in 1.0 liter of tap water and stir until no crystals remain. This stock holds 1,000 mg/L of alum. Adding 1 mL of it to a 250 mL jar raises that jar by 4 mg/L, because 1 milligram of alum spread through a quarter of a liter is a dose of 4 mg/L.

Step 4, set up the five jars (5 minutes). Label the cups 1 through 5. Pour 250 mL of the cloudy water into each one. Work quickly so all five jars start with water of the same cloudiness.

Step 5, dose the jars (3 minutes). Add stock solution with the syringe as follows: jar 1 gets 0 mL, jar 2 gets 1 mL, jar 3 gets 2 mL, jar 4 gets 3 mL, and jar 5 gets 4 mL. Record the matching doses of 0, 4, 8, 12, and 16 mg/L in the data table. Jar 1 is the control.

Step 6, rapid mix (2 minutes). Stir every jar hard for 30 seconds, counting the same number of strokes in each. The fast stir spreads the alum through the water before it starts to react.

Step 7, slow mix (5 minutes). Stir every jar slowly and steadily for 5 minutes. Watch for floc, the small clumps that form as fine clay sticks together. Record whether the floc you see is absent, small, or large in each jar.

Step 8, settle (20 minutes). Stop stirring and leave the jars completely still for 20 minutes. Do not bump the table. Use this time to draw the axes of your graph, with dose in mg/L on the horizontal axis and clarity depth in centimeters on the vertical axis.

Step 9, measure clarity (8 minutes). Pour the clear water from the top of jar 1 into the clarity tube slowly, without stirring up the settled layer. Stop as soon as the black cross under the base disappears, and record that water depth in centimeters. If the jar runs out of water before the cross disappears, record "more than" the final depth. Rinse the tube and repeat for jars 2 through 5.

Step 10, plot (7 minutes). Plot your five points of clarity depth against dose and connect them with a smooth line. Steps 1 through 10 fill the first session; answer the analysis questions in a short second session.

Data & Analysis

Jar	Stock added (mL)	Alum dose (mg/L)
1	0	0
2	1	4
3	2	8
4	3	12
5	4	16

Which jar reached the greatest clarity depth, and what alum dose did that jar receive?
Subtract the clarity depth of jar 1, which got no alum, from the clarity depth of your best jar. How many centimeters of clarity did the alum add?
Divide your answer to question 2 by the clarity depth of jar 1, then multiply by 100. What percent improvement did the best dose produce over the untreated control?
Look at the right-hand side of your graph, from 12 mg/L to 16 mg/L. Did the line keep climbing, flatten out, or turn down? Explain what that shape tells you about feeding more alum than the water needs.
The fictional Aldwyn plant treats 3.0 MGD. Using pounds per day = dose in mg/L times flow in MGD times 8.34, find the alum feed rate in pounds per day for a dose of 8 mg/L.
Now repeat the calculation from question 5 using the dose from your own best jar. How many pounds per day would the fictional plant feed at that dose?
A conventional filtration plant must keep its finished water at or below 0.3 NTU in 95 percent of monthly samples. Explain why a clarity-tube reading from this lab cannot show whether the fictional Aldwyn plant would meet that limit.
Overdosing costs money and leaves more sludge to haul away. State the dose you would recommend to the fictional Aldwyn plant, and give one reason based on your clarity data and one based on cost.

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 highest clarity depth. Most groups find the greatest clarity in jar 3 or jar 4, at 8 mg/L or 12 mg/L; accept whichever jar the group's own recorded depths identify.
Answer: the difference in centimeters. The value equals the best jar's clarity depth minus jar 1's clarity depth, and it should be positive if the alum worked as expected.
Answer: a percent improvement. The value equals the question 2 difference divided by jar 1's clarity depth, times 100; accept any figure consistent with the group's own two measurements and a correct setup.
Answer: the line flattens or turns down. Once the clay particles are neutralized, extra alum adds little clarity and can reverse the particles' charge so they stay suspended again, so the curve levels off or dips rather than climbing. If a group's line kept climbing through 16 mg/L, accept that answer when the group explains that its water's best dose may lie above the range tested.
Answer: 200.2 lb/day. The feed rate is 8 mg/L times 3.0 MGD times 8.34, which equals 200.16, or about 200.2 pounds per day.
Answer: a feed rate matching the group's own dose. Multiply the group's best dose by 3.0 and by 8.34; a 12 mg/L dose gives 300.2 pounds per day, and a 4 mg/L dose gives 100.1 pounds per day.
Answer: the clarity tube is not a turbidimeter. It measures how deep water must be before a printed cross disappears, which is a rough visual comparison, so it cannot resolve the tenths of a unit that the 0.3 NTU standard is written in.
Answer: a dose with two reasons. Accept any dose the group's data supports, provided the clarity reason cites their measured depths and the cost reason notes that a higher dose buys little added clarity while raising chemical spending and sludge volume.

Differentiation

Support

Prepare the cloudy water and the alum stock before class so these groups begin at Step 4, and give them a data table with the doses already filled in. Provide the graph axes drawn and labeled, and work question 5 as a modeled example on the board with the units written under each factor.

Extension

Have groups run a sixth jar at a dose halfway between their best jar and the next one, predict its clarity depth from the graph before measuring, and then compare the prediction with the measurement. Ask them to explain in a paragraph why a plant repeats this bench test whenever the raw water changes, rather than locking in one dose for the year.

ELL supports

Pre-teach "coagulation," "floc," "dose," and "clarity" with a labeled sequence of drawings showing loose clay particles, clumped floc, and settled floc with clear water above. Post the sentence frame "Jar ___ was clearest, so the best dose was ___ milligrams per liter" for question 1, and allow floc observations to be recorded as sketches instead of words.