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Lesson plan Grade 12 Science

Granular Activated Carbon for PFAS at Fenwick Wells

A grade 12 science lesson where students size granular activated carbon treatment for a fictional PFAS-affected wellfield and evaluate ways to extend the carbon's life.

Objectives

  1. Students will calculate the percent removal a treatment step must achieve to bring a contaminant below its limit.
  2. Students will estimate how long a granular activated carbon bed lasts before breakthrough at a given flow.
  3. Students will evaluate design changes that extend the time between carbon change-outs.

Vocabulary

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.
Maximum contaminant level (MCL)
A maximum contaminant level, or MCL, is the highest amount of a certain substance that federal rules allow in drinking water. Water systems test regularly to prove their water stays under each limit.
Parts per trillion
A unit for measuring extremely small amounts of a chemical in water. PFAS limits are set in parts per trillion because these chemicals matter even in tiny doses.

Also: adsorption, breakthrough

Teacher overview

Granular activated carbon (GAC) is one of the main technologies utilities use to pull PFAS out of drinking water. The carbon adsorbs the contaminant onto its huge internal surface until the bed is exhausted and PFAS begins to "break through," at which point the carbon must be replaced. This lesson uses the fictional Fenwick Water District, whose Bellhaven wellfield exceeds the federal PFOA maximum contaminant level of 4.0 parts per trillion, to have students size the treatment and reason about its running cost.

The math is trace-level, and a common misconception is that a limit measured in parts per trillion must be unimportant because the number is small. The lesson is built to correct that: PFAS is regulated at parts per trillion precisely because it is harmful at very low concentrations and does not break down. Because the handout numbers are self-contained, a student who has only the handout can reproduce every answer.

Procedure

Hook (5 min): Ask students how small "one part per trillion" is, then offer a concrete comparison to hold in mind (about one drop of water in 20 Olympic-size swimming pools). Record two or three student guesses about why a contaminant would be limited at such a tiny level, without confirming any yet.

Direct instruction (12 min): Introduce PFAS as a family of long-lasting compounds and GAC as an adsorption treatment. State the federal maximum contaminant level of 4.0 parts per trillion for PFOA and PFOS, and note that systems must comply by April 2029. In May 2026 EPA proposed letting eligible systems apply for a two-year exemption, to April 2031; it would not be automatic for every system, the 4.0 ppt limits would not change, and the proposal was not final as of September 2026, so this is a rule still in motion. Define breakthrough and explain why bed life is measured in gallons treated per change-out.

Worked example (10 min): Model the removal math with numbers different from the handout so students see the method, not the answers. Suppose a different well in the fictional district reads 12 ppt and the plant must reach the 4.0 ppt limit. Show the removal formula, removal % = (influent - effluent) / influent x 100, giving (12 - 4.0) / 12 x 100 = about 67 percent required, and show that fresh GAC at 90 percent removal would leave 12 x 0.10 = 1.2 ppt.

Guided practice (10 min): Students complete handout items 1 through 3 with a partner while you circulate. Confirm that each pair sets up the removal percentage as a ratio of the change to the influent, not to the limit.

Independent practice (13 min): Students finish handout items 4 through 10 on their own, including the bed-life comparison and the short evaluation items.

Closure / exit ticket (5 min): On an index card, each student answers: "A well reads 10 ppt and the limit is 4.0 ppt. What minimum percent removal meets the limit?" Collect and check for the method (change over influent: (10 - 4) / 10 = 60 percent). If fewer than four of five cards show the correct setup, reteach the ratio at the next session.

Granular Activated Carbon for PFAS at Fenwick Wells — Grade 12 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose the Bellhaven wellfield that supplies the fictional Fenwick Water District measures PFOA at 20 ppt. The federal maximum contaminant level (MCL) for PFOA is 4.0 ppt. The plant treats 1.0 MGD through two granular activated carbon (GAC) vessels. Use the removal formula: removal % = (influent - effluent) / influent x 100.

  1. By how many ppt does the raw water (20 ppt) exceed the MCL of 4.0 ppt?

  2. What minimum percent removal would bring the raw water from 20 ppt down to exactly the 4.0 ppt MCL?

  3. Fresh GAC removes 95 percent of the PFOA. What is the treated-water PFOA concentration, in ppt?

  4. Each GAC vessel holds 20,000 pounds of carbon. How many pounds of carbon do the two vessels hold together?

  5. At 1.0 MGD, how many million gallons does the plant treat over a 30-day month?

  6. A single vessel treats 300 million gallons before breakthrough. At 1.0 MGD, how many days is that?

  7. Run in lead-lag order, the two vessels treat about 600 million gallons before both need change-out. At 1.0 MGD, how many days is that, and how many more days than in item 6?

  8. Explain why PFAS is regulated in parts per trillion, far lower than contaminants limited in parts per million or parts per billion.

  9. Systems must comply by April 2029, and EPA has proposed letting eligible systems apply for a two-year exemption, to April 2031. Give one reason Fenwick might install treatment by April 2029 anyway.

  10. Propose one design change that would extend the time between carbon change-outs, and state one trade-off it creates.

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: 16 ppt.
    20 ppt - 4.0 ppt = 16 ppt above the limit.

  2. Answer: 80%.
    Removal = (20 - 4.0) / 20 x 100 = 16 / 20 x 100 = 80 percent to reach exactly the limit.

  3. Answer: 1.0 ppt.
    Fresh GAC removes 95 percent, leaving 5 percent: 20 x 0.05 = 1.0 ppt, which is well below the 4.0 ppt MCL.

  4. Answer: 40,000 lb.
    Two vessels at 20,000 pounds each: 2 x 20,000 = 40,000 pounds of carbon.

  5. Answer: 30 million gallons.
    1.0 MGD x 30 days = 30 million gallons in the month.

  6. Answer: 300 days.
    300 million gallons / 1.0 MGD = 300 days before a single vessel breaks through.

  7. Answer: 600 days.
    600 million gallons / 1.0 MGD = 600 days, which is 300 days longer than the single vessel in item 6, because the lag vessel keeps polishing after the lead vessel starts to break through.

  8. Answer: PFAS is harmful at very low concentrations and does not break down.
    Because it persists in the body and the environment and poses risks at trace levels, the limit is set in parts per trillion rather than the higher units used for contaminants that are less potent or short-lived.

  9. Answer: Accept any one sound reason.
    Reasonable answers include protecting public health sooner, avoiding a rushed installation near the deadline, locking in construction costs, or reassuring customers, since the exemption is only a proposal, would not be automatic, and the health concern exists now.

  10. Answer: Accept any one workable change with a trade-off.
    Examples include adding more carbon or larger vessels (higher upfront cost and footprint), running lead-lag operation (more piping and complexity), or blending treated water with a cleaner source (needs a second source and careful monitoring).

Differentiation

Support

Give students the removal formula pre-labeled with "influent," "effluent," and "limit," and complete item 1 and item 2 together before releasing the class to work. Provide the bed-life items with the division already set up.

Extension

Have students estimate the yearly carbon cost, using a price per pound that you supply, if a single vessel's 20,000 pounds is replaced every 300 days, then explain how lead-lag operation changes that replacement frequency and why a utility might still prefer it.

ELL supports

Pre-teach "adsorption," "breakthrough," and "maximum contaminant level" with a simple diagram of water passing through a carbon bed that gradually fills up. Provide sentence frames for items 8 through 10, such as "One design change is ___, but a trade-off is ___."

Extensions

Have students research whether their state has begun PFAS monitoring and compare the timeline they find with the April 2029 federal compliance date and the proposed exemption to April 2031. A short written note on what they learn connects the Fenwick scenario to the decisions their own utility faces.