Graphic organizer Grade 6 Engineering & Design

Choosing a Disinfection Method for the Tinbury Wells

A grade 6 scoring matrix that rates chlorine tablets, a liquid feed pump, and an ultraviolet reactor against four rules for a fictional small water system.

Grade: Grade 6
Subject: Engineering & Design
Material: Graphic organizer
Time: 25 min
Prep: No prep
Materials: None

Standards

MS-ETS1-2 NGSS

Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.

How this material aligns: Students run three competing disinfection methods through one scoring matrix of four written rules, then defend the method that survives the comparison.

All materials for MS-ETS1-2 →

MS-ETS1-1 NGSS

Define the criteria and constraints of a design problem with sufficient precision to ensure a successful solution, taking into account relevant scientific principles and potential impacts on people and the natural environment that may limit possible solutions.

How this material aligns: The matrix separates three criteria a method can meet partly from one constraint it must meet outright, which is the precision this standard asks of a problem statement.

All materials for MS-ETS1-1 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-09-23.

Objectives

Students will score three disinfection methods against four written rules in a matrix.
Students will explain why a failed constraint rules an option out while a failed criterion does not.
Students will recommend one method and name what they would monitor after it is installed.

Vocabulary

Disinfection: Disinfection is the step that kills or disables germs in water so it is safe to drink. Water plants usually disinfect with chlorine, ultraviolet light, or ozone.
Chlorine residual: Chlorine residual is the small amount of chlorine still left in the water after it leaves the treatment plant. It keeps protecting the water from germs all the way to your faucet.

Also: ultraviolet, criterion, constraint

Teacher overview

This organizer is a scoring matrix: three competing methods run across the columns, four written rules run down the rows, and students mark each cell as a pass or a fail before recommending one method. The setting is Tinbury, a fictional village of about sixty homes served by two wells. The misconception to plan for surfaces as soon as students begin scoring: they treat every unmet rule as equally fatal and simply count the passes. One of the four rules is a constraint, which no design can trade away, while the other three are criteria a design can meet partly and still be worth building. Two of the three methods here tie on raw count, so the recommendation turns entirely on telling those two kinds of rule apart.

Choosing a Disinfection Method for the Tinbury Wells — Grade 6 Engineering & Design

Name: ____________________________ Date: ______________

Organizer

Tinbury is a fictional village. Its two wells draw clear groundwater that already tests free of grit, and the district now has to add disinfection before the water reaches homes. Read the method file and the rules, then fill in the matrix.

Method file

Method 1, chlorine tablets in a flow-through feeder. Water passes over a stack of tablets and dissolves a little of each one. No electric power is needed. Chlorine stays in the water as it travels to homes. The operator refills the stack in about 20 minutes a week. The dose drifts as the tablets wear down and the stack gets shorter.
Method 2, a liquid chlorine feed pump. A small pump adds a measured amount of liquid chlorine and speeds up or slows down with the well pump, so the dose stays steady. Chlorine stays in the water as it travels to homes. The pump runs on electric power. The operator mixes solution and checks the pump for about 45 minutes a week.
Method 3, an ultraviolet reactor. Water passes a bright lamp, and the light damages germs so they cannot multiply. The lamp runs on electric power. Nothing is left in the water after it leaves the lamp. The operator changes the lamp once a year and spends about 15 minutes a week on checks.

Tinbury's rules

Rule 1 (criterion): the water reaching the farthest home must still carry a chlorine residual of at least 0.2 mg/L, so an operator can tell whether a pipe break has let dirty water in.
Rule 2 (criterion): the operator can spend no more than 60 minutes a week running this system.
Rule 3 (criterion): the dose must stay steady even though the well pump starts and stops all day.
Rule 4 (constraint): the well house has no electric power, and running a line to it would cost more than the whole disinfection project.

Scoring matrix

Rule	Method 1, tablets	Method 2, feed pump	Method 3, ultraviolet
Rule 1: residual at the far end
Rule 2: under 60 minutes a week
Rule 3: steady dose
Rule 4: no electric power needed
Rules passed, out of 4
Criterion or constraint failed?

Recommendation box

Question	Your answer
Which method do you recommend?
Which rule does your choice fail, and what would you do about it?
What would change if power reached the well house next year?

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

Completed Exemplar

Rule	Method 1, tablets	Method 2, feed pump	Method 3, ultraviolet
Rule 1: residual at the far end	Pass, chlorine stays in the water	Pass, chlorine stays in the water	Fail, nothing is left after the lamp
Rule 2: under 60 minutes a week	Pass, about 20 minutes	Pass, about 45 minutes	Pass, about 15 minutes
Rule 3: steady dose	Fail, the dose drifts as tablets wear down	Pass, the pump matches the well pump	Pass, the lamp treats all water alike
Rule 4: no electric power needed	Pass, no power needed	Fail, the pump needs power	Fail, the lamp needs power
Rules passed, out of 4	3	3	2
Criterion or constraint failed?	A criterion, Rule 3	A constraint, Rule 4	A criterion and a constraint

Question	Exemplar answer
Which method do you recommend?	Method 1, the tablet feeder. It ties Method 2 at three rules passed, but the rule it fails is a criterion, while the rule Method 2 fails is the constraint about power, and a constraint cannot be traded away.
Which rule does your choice fail, and what would you do about it?	Rule 3, the steady dose. The operator would test the chlorine residual at the far end twice a week instead of once, and refill the tablet stack before it runs low, so the drift is caught early.
What would change if power reached the well house next year?	Rule 4 would stop ruling out Method 2, and the feed pump would then pass all four rules. The district would compare the cost of the pump against the extra testing time the tablets need.

Differentiation

Support

Give these students a matrix with the Method 3 column already scored, since that column has the clearest fails, and read the method file aloud before they begin. Provide a two-line card defining criterion and constraint and leave it on the desk during the work.

Extension

Have students add a fifth rule of their own about cost, price each method with numbers they invent and state as invented, then rescore all three columns and say whether the recommendation holds.

ELL supports

Provide a labeled drawing of each method installed in a well house, with arrows showing where water enters and leaves. Post the words "residual," "dose," "drift," and "reactor" with a short meaning beside each. Allow the matrix cells to be filled with "P" or "F" and a two-word reason.

Extensions

Have students read the public water quality report their own utility publishes each year, find the section on disinfection, and write a short paragraph naming which method their system uses, whether one of these three or another, and one reason it may have been chosen.