Lesson plan Grade 8 Engineering & Design
Mossbank's Campus Pond: Designing a Floating Treatment Wetland
A grade 8 engineering lesson where students size a floating wetland raft to pull nitrogen from a green campus pond, working within a materials budget.
Objectives
- Students will use a per-square-foot removal rate to size a floating treatment wetland for a given nitrogen load.
- Students will explain how nitrogen acts as both a pollutant in the pond and a resource once the plants are harvested.
Vocabulary
Key words: nitrogen, nutrient, algae, floating treatment wetland, runoff
Teacher overview
Mossbank Middle School has a fictional stormwater pond that turns bright green with algae every summer because extra nitrogen washes in from the lawn and parking lot. Too much nitrogen feeds algae, and when the algae die and rot they pull oxygen out of the water, which can drop below the 5 mg/L that most fish need. In this lesson students design a floating treatment wetland, a raft of water plants whose roots take up nitrogen, and size it to the pond's load within a budget. A common misconception is that a bigger raft is always better; students should see that the raft only needs to match the weekly nitrogen load, and that the harvested plants turn the nitrogen into a compost resource rather than a waste.
Procedure
Hook (5 min): Show the image of an algae-covered pond and ask students why the water is green. Collect ideas without correcting, then name the cause: extra nitrogen feeding algae.
Direct instruction (12 min): Explain that nitrogen is a nutrient plants need, but too much in a pond acts as a pollutant because it feeds algae blooms that lower dissolved oxygen below the 5 mg/L fish need. Introduce the floating treatment wetland as an engineered fix: plant roots take up nitrogen, and the plants are later harvested and composted, so the nitrogen becomes a resource on land. Model one sizing example on the board using different numbers than the handout, for a fictional pond that gets 60 grams of nitrogen each week with a raft that removes 5 grams per square foot: 60 divided by 5 = 12 square feet. This board example is for teaching only and is not part of the graded handout.
Guided practice (13 min): As a class, size a raft for a pond that gets 45 grams of nitrogen each week using the same 5 grams-per-square-foot rate, then discuss what happens to the leftover load if the raft is built too small.
Independent practice (17 min): Students complete the numbered handout, sizing a raft for the Mossbank pond and checking it against a budget.
Closure and exit ticket (8 min): Students answer one question on an index card: "Which raft size fits both the weekly nitrogen load and the budget, and why?" Collect and scan for students who correctly compare grams removed with grams loaded before dismissal. If more than a third miss the comparison, reteach the check step next class.
Student Handout
Suppose the Mossbank pond, a fictional campus pond, receives 84 grams of nitrogen each week. A floating wetland raft removes about 6 grams of nitrogen for each square foot of raft each week. Raft material costs 8 dollars per square foot, and the class budget is 150 dollars.
How many square feet of raft are needed to remove all 84 grams of nitrogen each week?
A raft of 10 square feet removes how many grams of nitrogen each week?
What percent of the 84 grams does the 10-square-foot raft remove? Round to the nearest whole percent.
After a heavy storm the pond receives 168 grams of nitrogen in one week. How many square feet of raft would remove all of it?
The class budget is 150 dollars and material costs 8 dollars per square foot. What is the largest whole-number raft area the class can afford?
Can the raft from question 5 remove all 84 grams in a normal week? Show the grams it removes and compare with the load.
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: 14 sq ft.
84 grams divided by 6 grams per square foot = 14 square feet.Answer: 60 g.
10 square feet x 6 grams per square foot = 60 grams removed each week.Answer: 71%.
60 grams removed divided by 84 grams loaded = 0.714, which rounds to about 71 percent.Answer: 28 sq ft.
168 grams divided by 6 grams per square foot = 28 square feet.Answer: 18 sq ft.
150 dollars divided by 8 dollars per square foot = 18.75, and the class cannot buy part of a square foot, so it rounds down to 18 square feet.Answer: Yes; 108 g removed.
18 square feet x 6 grams per square foot = 108 grams, which is more than the 84-gram weekly load, so the affordable raft can handle a normal week.
Differentiation
Support
Give students a partially worked handout with the removal rate already set up as "grams divided by 6" for question 1, and provide a calculator. Pair students so one reads the number sentence aloud while the other computes.
Extension
Ask students to design a two-raft system for a pond whose load changes with the season, 84 grams in summer and 40 grams in spring, and to explain how they would move or store rafts so material is not wasted in the low season.
ELL supports
Provide a labeled picture of a floating wetland raft with roots reaching into the water, and pre-teach nitrogen, nutrient, and algae with the pond image. Offer the sentence frame "The raft removes ___ grams, and the pond gets ___ grams, so the raft is big enough or too small."
Extensions
If your school has a pond, rain garden, or retention basin, arrange for the class to observe it from the bank with adult supervision, staying back from the water's edge and not touching the water, and note signs of algae or runoff. Students can sketch where a floating wetland might sit and write a short proposal to the facilities office describing the raft size their math supports.