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Lesson plan Grade 8 Engineering & Design

Choosing PFAS Treatment for Tannery Springs: A Design Trade-Off Study

A grade 8 engineering lesson where students weigh three PFAS treatment options with a decision matrix, judging removal, cost, energy, and waste trade-offs.

Objectives

  1. Students will list design criteria for a PFAS treatment choice and assign each a weight.
  2. Students will use a weighted decision matrix to compare three treatments and defend a recommendation.

Vocabulary

PFAS
A large family of human-made chemicals used in things like nonstick pans, waterproof fabric, and firefighting foam. They are called forever chemicals because they take a very long time to break down.
Granular activated carbon
Small black grains of specially treated carbon that grab contaminants as water flows through them. Utilities use big tanks of it to remove PFAS, bad tastes, and odors.
Ion exchange
A water treatment method that swaps unwanted charged particles in the water for harmless ones held on tiny resin beads. It is used to remove PFAS and to soften hard water.
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: trade-off, decision matrix

Teacher overview

PFAS are a group of long-lasting chemicals that show up in some drinking-water sources and do not break down easily. The federal limit for two of them, PFOA and PFOS, is 4 parts per trillion each, and systems must comply by April 2029, although in May 2026 EPA proposed letting eligible systems apply for a two-year exemption to April 2031. In this lesson students act as engineers for the fictional town of Tannery Springs, which found PFAS above the limit in its wells and must pick one of three treatments. Because no single option wins on every measure, students use a weighted decision matrix so their recommendation rests on stated criteria rather than a first guess. The lesson is about the reasoning process, so there is no single correct treatment; a defensible, well-justified choice is the goal.

Procedure

Hook (5 min): Introduce PFAS as chemicals that resist breaking down. Tell students that Tannery Springs tested its wells and found PFOA and PFOS above the federal limit of 4 parts per trillion each, so the town must act.

Background (12 min): Present the three treatment options with simple descriptions. Granular activated carbon uses beds of specially treated carbon that trap PFAS as water passes through, and the carbon must be replaced or reactivated over time. Ion exchange uses resin beads that swap PFAS out of the water, and the spent resin must be handled as waste. Reverse osmosis pushes water through membranes that block PFAS, but it uses the most energy and produces a concentrated reject stream that must be managed. Note that the rule is in motion: the limit is set and the deadline is April 2029, but EPA has proposed a two-year exemption that eligible systems could apply for, so timing is part of the decision.

Model the matrix (10 min): Show the decision matrix. List four criteria down the side: removal effectiveness, cost, energy use, and waste produced. Model assigning each criterion a weight from 1 to 5 based on how much the town cares about it, then scoring each option from 1 to 5 and multiplying score by weight.

Group work (15 min): In groups of three, students assign their own weights, score all three options, total the weighted points, and pick a recommendation. Each group writes one sentence naming the biggest trade-off of their choice.

Share-out and exit ticket (8 min): Two or three groups share their recommendation and reasoning. As an exit ticket, each student writes one trade-off of the option their group chose. Collect and check that students name a real downside, not just a benefit; if many list only benefits, revisit the meaning of trade-off next class.

Choosing PFAS Treatment for Tannery Springs: A Design Trade-Off Study — Grade 8 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

You are engineers for Tannery Springs. The town's wells tested above the federal limit of 4 parts per trillion for PFOA and PFOS, and the town must choose one treatment. Use the decision matrix to compare the three options fairly.

Criteria to score (weight each from 1 to 5, then score each option from 1 to 5, where 5 means the option does best on that criterion):

  • Removal effectiveness
  • Cost to build and run
  • Energy use
  • Waste produced

The three options: granular activated carbon, ion exchange, and reverse osmosis.

  1. Which criterion did you give the highest weight, and why does it matter most to a town?
  2. After totaling weighted points, which treatment did your group recommend?
  3. Name one trade-off, a real downside, of the treatment your group chose.
  4. Why does EPA's proposal to let eligible systems apply for a two-year exemption, from April 2029 to April 2031, matter to how quickly the town must decide?

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: Answers vary; a strong response names a criterion such as removal effectiveness and explains that a treatment that does not reliably get PFOA and PFOS below the 4 parts per trillion limit fails the town's core goal, so it deserves a high weight.
  2. Answer: Answers vary by the weights each group chose. Accept any of the three treatments when the recommendation follows from the group's own weighted totals rather than a guess.
  3. Answer: Accept a real downside that matches the choice: carbon must be replaced and disposed of, ion exchange creates spent resin waste, and reverse osmosis uses the most energy and makes a concentrated reject stream.
  4. Answer: An exemption, if the town qualifies and the proposal becomes final, would give more time to fund and build, but the PFAS is already above the limit, so waiting still exposes residents; a strong answer weighs public health against the extra time.

Differentiation

Support

Give these students a decision matrix with the weights already filled in so they can focus on scoring and totaling. Provide the three option cards with the key facts underlined.

Extension

Ask students to add a fifth criterion of their own, such as how long each option takes to install, justify its weight, and re-run their totals to see whether the recommendation changes.

ELL supports

Provide the option cards with a simple icon for each treatment and a short glossary for PFAS, carbon, resin, and membrane. Offer sentence frames such as "I recommend ___ because it scores high on ___, but its trade-off is ___."