Classroom activity Grade 7 Careers & CTE

Wexholt's Barrier Test: A Membrane Technician Activity

A grade 7 careers activity in which teams test three barriers against a mixed sample, then build the flow diagram a membrane plant technician monitors.

Grade: Grade 7
Subject: Careers & CTE
Material: Classroom activity
Time: 55 min
Prep: Moderate prep
Materials: Pitcher, Clear plastic cups, Kitchen strainer or mesh sieve, Coffee filters, Clean sand and gravel, Ground pepper or craft glitter, Table salt, Food coloring, Spoons, Trays or baking pans, Paper towels, Chart paper and markers

Standards

RST.6-8.7 Common Core ELA

Integrate quantitative or technical information expressed in words in a text with a version of that information expressed visually (e.g., in a flowchart, diagram, model, graph, or table).

How this material aligns: Teams turn the written description of a membrane plant into a labeled flow diagram showing each barrier and the reading its technician watches.

All materials for RST.6-8.7 →

D2.Eco.2.6-8 C3 Framework

Evaluate alternative approaches or solutions to current economic issues in terms of benefits and costs for different groups and society as a whole.

How this material aligns: The closing debrief asks students to weigh running the membrane plant against buying treated water from a neighboring town, naming who gains and who pays under each option.

All materials for D2.Eco.2.6-8 →

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

Objectives

Students will test three barriers and record what each one removes from a mixed sample.
Students will build a labeled flow diagram of a membrane treatment plant in correct order.
Students will explain what a membrane plant technician monitors at each stage.

Vocabulary

Reverse osmosis: Reverse osmosis cleans water by pushing it through a membrane with holes so tiny that salt and most other dissolved things cannot fit through. It can even turn seawater into fresh water.

Also: membrane, barrier, concentrate, salt rejection

Teacher overview

This is a hands-on barrier test for teams of three or four, followed by a diagram build. Each team runs one mixed water sample through three barriers of decreasing pore size and records what each barrier catches, which follows the order of stages in a membrane treatment plant. Students commonly assume that a filter fine enough to catch dirt will also take out salt, so the sequence is built to let the salt survive every barrier a classroom can make. Students then map their results onto the plant a membrane technician runs in the fictional coastal town of Wexholt.

Activity Directions

Setup before class (moderate prep): Fill one pitcher with tap water and stir in salt until it dissolves. Add a few drops of food coloring and a spoonful of ground pepper or craft glitter, then stir in a small scoop of gravel and sand. This is the raw sample. Set out one tray per team with a strainer, two coffee filters, two clear cups, a spoon, and paper towels. Post the rule that no sample is tasted at any point.
Frame the job (7 min): Tell students that the town of Wexholt sits on the coast and treats brackish water with a membrane plant. Describe the technician who runs it. She watches pressure across each stage, checks how much salt the membranes are rejecting, and decides when a stage needs cleaning. Most technicians in this role hold a state water treatment operator license earned by exam after a high school diploma and on-the-job training, and some add a two-year degree in water technology or a four-year engineering degree to move into process work. Say that the team's three barriers stand for the first three things her plant does, and that the fourth stage cannot be built in a classroom.
Barrier one, the screen (8 min): Teams pour part of the raw sample through the strainer into a clear cup. They record what stayed in the strainer and what came through. Have them hold the cup up to the light before writing.
Barrier two, the filter (12 min): Teams pour the screened water through a coffee filter folded into a cup, with a spoonful of clean sand in the filter. This takes several minutes, and teams should record how the flow slows as material builds up. They record what the filter caught and what the water looks like now.
Barrier three, the second filter (8 min): Teams pour the filtered water through a fresh coffee filter with no sand. They record whether anything else is caught. Most teams find very little, which is the expected result.
The stage they cannot build (7 min): Ask teams whether the salt is gone. Let them notice they have no way to tell by looking. Explain reverse osmosis plainly. A pump pushes water hard against a membrane with openings so small that water molecules pass and most dissolved salt does not. The salt leaves in a separate stream called concentrate. Add that membrane treatment takes more electricity than conventional treatment, roughly 11 to 15 kWh per 1,000 gallons for seawater against about 1 to 2 kWh per 1,000 gallons for conventional treatment and delivery. Brackish water like Wexholt's is less salty, so it takes less than seawater.
Build the flow diagram (8 min): Each team draws the Wexholt plant on chart paper in order: screen, sand filter, cartridge filter, membrane, and the two streams that leave the membrane. Under each stage they write the reading the technician watches there, choosing from pressure across the stage, water clarity, and salt rejection. Teams post their diagrams.
Debrief and close (5 min): The activity ends when every team has posted a diagram and answered the handout questions. Ask three questions aloud. Why does the plant put the coarse barrier first rather than last? What would happen to the membranes if the sand filter were skipped? Third, if Wexholt could either run this plant or buy treated water from a neighboring town, who gains and who pays under each option, given the electricity the membranes need? Collect the handouts as the exit record.

Wexholt's Barrier Test: A Membrane Technician Activity — Grade 7 Careers & CTE

Name: ____________________________ Date: ______________

Student Handout

Your team is testing barriers for a membrane plant in the town of Wexholt. Record what you observe. Do not taste any sample. Answer each question in one or two sentences.

What did the strainer catch, and what passed through it?
What did the sand and coffee filter catch that the strainer did not?
Did the third barrier catch much? What does your answer tell you about the size of what was left?
After three barriers, is the salt still in the water? Explain how you know.
In your own words, describe what the membrane does that your three barriers could not do.
Name one reading the technician watches at a stage of the plant, and explain what a change in that reading would tell her.

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: Accept any accurate observation. Most teams report the gravel and larger sand staying in the strainer while colored water, pepper or glitter, and fine grit pass through.
Answer: Accept any accurate observation. Most teams report the filter catching the fine grit and most of the pepper or glitter, leaving water that is clearer but still colored.
Answer: Accept any accurate observation. Teams usually report that the third barrier catches little, which means the material still in the water is smaller than the filter's openings.
Answer: Yes, the salt is still there. Accept any reasoning noting that dissolved salt is not visible, that no barrier the team built stopped it, and that the water cannot be tasted to check.
Answer: Accept any answer describing separation of dissolved material. Sample response: the membrane lets water molecules through under pressure and holds back most of the dissolved salt, which leaves in the concentrate stream.
Answer: Accept any one reading with a matching explanation. Sample responses: rising pressure across a filter means it is clogging and needs cleaning, cloudier water means a stage is not doing its job, and falling salt rejection means a membrane is wearing out or leaking.

Differentiation

Support

Assign fixed team roles of pourer, recorder, and timekeeper, and post them where every team can see. Provide the flow diagram as a set of cut-out stage cards students place in order rather than draw from a blank page. Allow handout answers to be given as short phrases or labeled sketches.

Extension

Ask teams to design a fourth barrier from the materials on the tray that would catch something none of the first three caught, then explain why it would or would not work. Teams must predict the result before testing and account for any difference between the prediction and what happened.

ELL supports

Pre-teach "barrier," "membrane," and "concentrate" with the physical objects in hand before the teams begin. Provide a picture-supported recording sheet with a drawing of each barrier above its answer space. Pair students so that each team has one member who can restate the directions in the student's home language.

Extensions

Have students find out which of the stages in their diagram exists at the plant that serves their own community, using a utility's published water quality report or a plant tour if one is available. Students mark the stages their local plant has, note which ones it does not, and write one sentence explaining why a plant's source water determines the barriers it needs.