Worksheet Grade 9 Science

Corvale's PFAS Pilot: GAC Removal Efficiency Worksheet

A grade 9 science worksheet on a fictional town's activated-carbon pilot, where students calculate PFAS removal efficiency and compare treated water to the federal limit.

Grade: Grade 9
Subject: Science
Material: Worksheet
Time: 40 min
Prep: No prep
Materials: Calculator

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: Item 10 has students weigh carbon treatment against a blending plan on cost, reliability, and safety, the prioritized trade-off evaluation this standard requires, and item 8 applies the reliability criterion by judging the pilot against its 90 percent replacement threshold.

All materials for HS-ETS1-3 →

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: Items 2 through 6 and 9 chain parts-per-trillion readings, percent removal, and flow conversions through several steps, requiring the consistent unit tracking this standard specifies.

All materials for HSN-Q.A.1 →

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

Objectives

Students will calculate the removal efficiency of a treatment step from influent and effluent concentrations.
Students will compare a treated-water result against a federal contaminant limit and interpret the margin.
Students will weigh two treatment options using cost, reliability, and safety.

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.
Activated carbon: Activated carbon is a special charcoal full of microscopic holes that grab and hold chemicals as water flows past. It is the same material used in many home water filter pitchers.
Influent: Influent is the incoming dirty water that arrives at a treatment plant, ready to be cleaned. Operators test it to see how much work the plant has to do.
Effluent: Effluent is the cleaned water that leaves a treatment plant at the end of the process. It is tested carefully before being released to a river, lake, or the ocean.

Also: removal efficiency

Teacher overview

PFAS are a group of long-lasting chemicals that show up in some drinking water sources at very low levels, measured in parts per trillion. The federal limit for PFOA and PFOS is 4.0 parts per trillion each, a rule that is still being phased in. This worksheet follows the fictional town of Corvale as it tests a granular activated carbon (GAC) unit on one of its wells. Students calculate how much of the contaminant the carbon removes, check the result against the limit, and compare carbon treatment with a simpler blending plan.

A common student error is to report a removal efficiency without dividing by the starting amount. Removal efficiency is always the amount removed as a fraction of the influent, not the raw difference in parts per trillion.

Corvale's PFAS Pilot: GAC Removal Efficiency Worksheet — Grade 9 Science

Name: ____________________________ Date: ______________

Student Worksheet

The town of Corvale (a fictional town) draws part of its drinking water from Well 6. Recent testing found PFAS in the raw well water. Corvale is piloting a granular activated carbon unit, which pulls the chemicals out of the water as it passes through the carbon. Use the numbers in each item to answer the questions. The removal efficiency formula is: removal efficiency (%) = (influent − effluent) ÷ influent × 100.

In one or two sentences, explain what "influent" and "effluent" mean for the carbon unit.
Well 6 raw water tests at 28 parts per trillion of PFOA. The federal limit for PFOA is 4.0 parts per trillion. By how many parts per trillion does the raw water exceed the limit?
After the carbon unit, the PFOA level drops to 2 parts per trillion. Calculate the removal efficiency of the carbon unit.
Does the treated water meet the 4.0 parts per trillion limit? State whether it is above or below the limit, and by how much.
The pilot unit treats 0.5 MGD. Convert this flow to gallons per minute, using 1 MGD = 694 gpm.
Over a 30-day pilot run at 0.5 million gallons per day, how many total gallons pass through the carbon unit?
Carbon removes PFAS by adsorption, meaning the chemicals stick to the surface of the carbon rather than reacting with it. Explain why this means the carbon eventually stops working and must be replaced.
Corvale plans to replace the carbon once it can no longer remove at least 90 percent of the PFAS. Based on your answer to item 3, is the carbon still doing its job? Explain.
A second well, Well 3, tests at 6 parts per trillion of PFOA and drops to 1 part per trillion after carbon. Calculate Well 3's removal efficiency, and state which well's carbon removed a greater percentage.
Corvale could instead blend Well 6 water with cleaner water to lower the PFAS level by dilution. Give one advantage and one drawback of carbon treatment compared with blending, considering cost, reliability, and safety.

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: Influent is the water entering the carbon unit, and effluent is the water leaving it. In this scenario the influent is the raw Well 6 water and the effluent is the treated water.
Answer: 24 ppt. 28 − 4.0 = 24 parts per trillion above the limit.
Answer: 92.9 percent. (28 − 2) ÷ 28 × 100 = 26 ÷ 28 × 100 = 92.857, which rounds to 92.9 percent.
Answer: Below the limit, by 2.0 ppt. The treated water is 2 parts per trillion, which is 4.0 − 2 = 2.0 parts per trillion below the 4.0 limit.
Answer: 347 gpm. 0.5 MGD × 694 gpm per MGD = 347 gallons per minute.
Answer: 15,000,000 gallons. 0.5 million gallons per day × 30 days = 15 million gallons.
Answer: Because the chemicals collect on the carbon's surface, the surface slowly fills up. Once the available surface is used, the carbon can no longer hold more PFAS, so it must be replaced or regenerated.
Answer: Yes. The carbon is removing 92.9 percent, which is above the 90 percent threshold, so it is still performing well enough to keep in service.
Answer: 83.3 percent; Well 6 removed a greater percentage. Well 3: (6 − 1) ÷ 6 × 100 = 83.33, which rounds to 83.3 percent, lower than Well 6's 92.9 percent.
Answer: Accept any reasoned response. Carbon reliably lowers PFAS at the source but costs more to install and maintain and the carbon must be replaced over time; blending is cheaper but depends on having enough clean water and does not remove the PFAS, only spreads it out.

Differentiation

Support

Provide the removal efficiency formula worked through on a separate practice example (influent 40, effluent 10) before students begin, and give a filled-in unit key listing "1 MGD = 694 gpm" so students do not need to recall the conversion.

Extension

Have students find the effluent concentration Well 6 would need in order to reach a 95 percent removal efficiency, working backward from the formula using the 28 parts-per-trillion influent.

ELL supports

Pre-teach "influent," "effluent," "adsorption," and "removal efficiency" with a simple labeled diagram of water flowing into and out of a carbon unit. Allow items 1 and 7 to be answered with a labeled sketch plus a short phrase rather than full sentences.