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Lesson plan Grade 7 Engineering & Design

Halloway Creek Filter Trials: Evaluating Three Prototype Water Filters

A 50-minute grade 7 lesson where student teams score three prototype filter designs on a weighted rubric and use the totals to choose the strongest design.

Objectives

  1. Students will apply a weighted decision matrix to compare competing filter designs against shared criteria.
  2. Students will calculate weighted totals by multiplying scores by stated weights and summing the results.

Vocabulary

Turbidity
Turbidity is a measure of how cloudy or murky water looks because of tiny particles floating in it. Clear water has low turbidity, and muddy water has high turbidity.

Also: decision matrix, criteria, weighted score

Teacher overview

A fictional small community along Halloway Creek has finished building three prototype filters in a previous class period: a sand-and-gravel column, a charcoal-layered column, and a cloth-and-sand column. Real drinking-water plants must keep finished-water turbidity at or below 0.3 NTU in 95 percent of monthly samples, and never above 1 NTU, so clarity matters most in any real filter comparison. This lesson gives students a systematic way to decide which prototype is genuinely the strongest by scoring all three on clarity, flow rate, and cost, then weighting those scores instead of just picking a favorite. A common misconception in this lesson is assuming that whichever design "wins" the most categories must be the best overall; the weighted-scoring method is built to show that winning the most categories does not guarantee the highest total once a heavily weighted criterion is applied.

Procedure

Hook (5 min): Remind students that all three teams' prototype filters produced water with different levels of clarity, speed, and material cost. Ask: if no single design wins in every category, how should the class decide which one is "best"?

Direct instruction (10 min): Introduce a weighted decision matrix. Explain that each filter is scored from 0 to 5 on three criteria: clarity, flow rate, and cost, and that clarity is weighted three times as heavily as cost because removing cloudiness is a filter's main job, matching the real regulatory focus on turbidity. Point out that clear water can still carry germs, which is why real plants also disinfect after filtering. The weights are: clarity × 3, flow rate × 2, cost × 1. Model computing Filter A's (Sand & Gravel) weighted score step by step on the board: clarity 3, flow rate 4, cost 4, so (3 × 3) + (4 × 2) + (4 × 1) = 9 + 8 + 4 = 21 points. Treat this Filter A computation as the worked model rather than one of the graded items; students then answer the four numbered questions in the steps that follow using the same method.

Guided practice (15 min): Work Question 1 as a class. Question 1: Filter B (Charcoal-Layered) scores clarity 5, flow rate 2, cost 3; find its weighted total. Compute it together: (5 × 3) + (2 × 2) + (3 × 1) = 15 + 4 + 3 = 22 points. Discuss why Filter B can score higher overall despite losing the flow-rate and cost categories to other filters.

Independent practice (10 min): Students answer Questions 2 and 3 on their own. Question 2: Filter C (Cloth & Sand) scores clarity 2, flow rate 5, cost 5; find its weighted total. Question 3: rank all three filters from highest weighted total to lowest.

Closure / exit ticket (10 min): Each student answers Question 4 on an index card. Question 4: A fourth filter design, Filter D, scores clarity 4, flow rate 3, and cost 2. Calculate its weighted total using the same weights, and state whether it would have won the trial. Collect responses and check that students applied all three weights correctly before dismissal.

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: 22 points. Question 1, Filter B (guided practice): (5 × 3) + (2 × 2) + (3 × 1) = 15 + 4 + 3 = 22 points.
  2. Answer: 21 points. Question 2, Filter C (independent practice): (2 × 3) + (5 × 2) + (5 × 1) = 6 + 10 + 5 = 21 points.
  3. Answer: Question 3 ranking: Filter B finishes first with 22 points; Filter A and Filter C tie for second, each with 21 points. Filter B's high clarity score, multiplied by the largest weight, outweighs its lower flow-rate and cost scores.
  4. Exit ticket answer: 20 points. Question 4, Filter D: (4 × 3) + (3 × 2) + (2 × 1) = 12 + 6 + 2 = 20 points, which is below Filter B's 22 points, so Filter D would not have won the trial.

Differentiation

Support

Provide a printed table with the weight multiplication already written as blank equations for each filter (clarity × 3 = ___, flow rate × 2 = ___, cost × 1 = ___), so students only need to fill in and add the products. Allow a calculator throughout.

Extension

Ask students to design a hypothetical Filter E that would tie Filter B's 22-point total using a different combination of clarity, flow rate, and cost scores (each from 0 to 5), and to show the weighted calculation that proves it.

ELL supports

Pre-teach "clarity," "flow rate," "cost," and "weight" with simple icons (a clear glass, a stopwatch, a coin stack, a heavier side of a balance scale) posted next to the scoring table. Allow students to record their calculations using the table format shown in Support instead of full sentences.