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Lesson plan Grade 8 Engineering & Design

Westmark Yard Truck Wash: Choosing a Water Recycling Loop

A 60-minute grade 8 lesson where students score three ways to stop a public works truck wash from draining to a creek, then check the winner against a budget limit.

Objectives

  1. Students will calculate the fresh water a truck wash uses over a season with and without reuse.
  2. Students will score three competing designs on a shared rubric and identify the highest total.
  3. Students will explain how a budget constraint can rule out the highest-scoring design.

Vocabulary

Key words: closed-loop reuse, oil-water separator, criteria, constraint, outfall

Teacher overview

Westmark Yard is a fictional public works garage that washes its plow trucks on a paved apron draining to a storm inlet, which carries road salt, grit, and oil film to a creek behind the property. The town has been told to stop that discharge, and three options are on the table. This lesson has students compute the water each option uses, score all three on a shared rubric, and then test the winner against the project budget. The arithmetic error to watch for is treating an 80 percent reuse rate as an 80 percent fresh water requirement, which reverses the savings; item 3 on the handout is written to surface that mistake early.

Procedure

Framing the problem (8 min): Describe the wash apron at Westmark Yard and where its water goes. Ask students what is actually in the runoff after a February storm route, and list their answers on the board. Add road salt and oil film if no one raises them.

Reading the options (10 min): Present the three options in the handout brief: settle the water and send it to the sanitary sewer, install a closed-loop system that recycles most of the wash water, or stop washing and switch to dry brushing. Have students restate each option in one sentence to a partner before moving on.

Building the scoring rubric (12 min): With the class, agree that a higher score means a better result on each criterion, using the ratings already printed on the handout. Discuss what the town would need to check after the change to know it worked, and settle on a monthly sample at the creek outfall for chloride and a visual check for oil sheen. Write that monitoring step on the board so students can refer to it during the handout.

Student handout (22 min): Students work items 1 through 11 in pairs. Circulate and confirm that pairs working item 3 are multiplying 60 gallons by 0.20 rather than by 0.80.

Formative check (3 min): Ask for a show of hands on item 6, offering 20 percent or 80 percent as the two choices. If more than a quarter of the class chooses 20 percent, work the division 11,520 ÷ 14,400 on the board before continuing.

Close (5 min): Collect the handouts and take two answers to item 11 aloud, so the class hears the difference between a low score and a hard limit.

Westmark Yard Truck Wash: Choosing a Water Recycling Loop — Grade 8 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

The town of Westmark keeps 12 plow trucks at its public works yard. Each truck wash uses 60 gallons of fresh water, and each truck is washed once a week during the 20-week winter season.

Three options are under review, rated on a 1 to 5 scale where 5 is best.

Option Water saved Installation cost Ease of daily use
A: settle and send to the sanitary sewer 1 5 5
B: closed-loop system reusing 80 percent 5 3 4
C: stop washing and dry brush instead 5 4 1
  1. How many gallons of fresh water does one week of washing use?
  2. How many gallons of fresh water does the full 20-week season use?
  3. Option B reuses 80 percent of each wash's water. How many gallons of fresh water does one wash still need?
  4. The yard runs 240 washes in a season. With Option B, how many gallons of fresh water does the season need?
  5. How many gallons of fresh water does Option B save over the season?
  6. What percent of the season's fresh water does Option B save?
  7. Add the three ratings for Option A to find its total score.
  8. Add the three ratings for Option B to find its total score.
  9. Add the three ratings for Option C to find its total score.
  10. Which option has the highest total score?
  11. The town has budgeted $40,000 for this project, and Option B costs $52,000 installed. Explain how that limit changes the recommendation you would give the town.

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: 720 gallons.
    12 trucks × 60 gallons = 720 gallons in one week.
  2. Answer: 14,400 gallons.
    720 gallons × 20 weeks = 14,400 gallons for the season.
  3. Answer: 12 gallons.
    If 80 percent of the water is reused, then 20 percent must be fresh. 60 × 0.20 = 12 gallons per wash.
  4. Answer: 2,880 gallons.
    240 washes × 12 gallons = 2,880 gallons of fresh water for the season.
  5. Answer: 11,520 gallons.
    14,400 - 2,880 = 11,520 gallons saved.
  6. Answer: 80 percent.
    11,520 ÷ 14,400 = 0.80. The share saved matches the reuse rate, which is a useful check on the arithmetic in items 3 through 5.
  7. Answer: 11 points.
    1 + 5 + 5 = 11.
  8. Answer: 12 points.
    5 + 3 + 4 = 12.
  9. Answer: 10 points.
    5 + 4 + 1 = 10.
  10. Answer: Option B.
    Option B's total of 12 points is higher than Option A's 11 and Option C's 10.
  11. Answer: The budget rules Option B out.
    A score compares how well an option performs, while the budget is a limit the design must respect. Option B costs more than the town has, so the recommendation shifts to Option A unless the town finds additional money or the vendor lowers the installed price. Strong answers say plainly that a constraint removes an option from consideration no matter how well it scores.

Differentiation

Support

Give these students items 1 through 5 with the operation named beside each one, such as "multiply" or "subtract," and provide the 240-wash total already computed. Pair them with a partner for the scoring items so the addition is checked aloud.

Extension

Suppose a vendor offers a smaller closed-loop unit that reuses 60 percent of the wash water and costs less than the town's budget. Ask students to recompute the season's fresh water use, assign the unit a water-saved rating, and write a short recommendation that names both the score and the budget.

ELL supports

Provide a labeled diagram of the wash apron, the storm inlet, the creek, and the recycling tank, with each part named in English and in the student's first language. Give sentence frames for item 11, such as "The town cannot choose Option B because ___, so I would recommend ___."