Graphic organizer Grade 7 Engineering & Design
Fernlow Lagoon Polishing Filter: Combining Test Results
A grade 7 organizer for reading three media-bed test results, naming the feature worth keeping from each, and stacking those features into one combined filter design.
Objectives
- Students will read a test table and name one strength and one weakness for each media bed.
- Students will build a layered design that borrows a named feature from each tested bed.
- Students will state what the combined design must still be tested for before it is built.
Vocabulary
Key words: polishing filter, media, biofilm, clogging, loading rate, ammonia
Teacher overview
This organizer is a three-stage merge frame. Students first read a table of test results from three prototype media beds, then fill a strengths-and-weaknesses grid that ends in a single feature worth keeping from each, and finally stack those features into a layered design with a reason attached to every layer. The setting is a fictional small town called Fernlow whose wastewater lagoon needs a polishing step before discharge. The misconception to address is that testing produces a winner. A student who believes that will circle the best column and stop, when the bed with the best solids and ammonia results is also the bed that was matting shut at three weeks, so the column that treats best is not the column that could run for a year. Say plainly that the combined design is allowed to be a design nobody tested, which is why stage three asks what still has to be checked.
Organizer
Test file from the fictional Fernlow lagoon trials. Each team ran a barrel-sized media bed for three weeks on lagoon water. The permit limit is 30 mg/L for suspended solids. Incoming lagoon water carried 52 mg/L of suspended solids and 14 mg/L of ammonia.
| Media bed | Flow it handled | Suspended solids leaving | Ammonia leaving | Condition after three weeks | Cost per cubic foot |
|---|---|---|---|---|---|
| Pea gravel | Highest of the three | 34 mg/L | 12 mg/L | Clean, no clogging | 9 dollars |
| Wood chips | Lowest of the three | 19 mg/L | 5 mg/L | Surface matted, flow slowing | 4 dollars |
| Lava rock | Middle of the three | 25 mg/L | 7 mg/L | Clean, light biofilm | 21 dollars |
Stage 1. Strengths and weaknesses.
| Media bed | What it did well | Where it fell short | The one feature worth keeping |
|---|---|---|---|
| Pea gravel | |||
| Wood chips | |||
| Lava rock |
Stage 2. Combined design. Stack the layers from the top of the filter down.
| Layer, top to bottom | Feature borrowed from | Why this layer is here |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 |
Stage 3. Still to test. Finish each line for the combined design.
- The measurement we would take first: ____
- The failure we are most worried about: ____
- How long the test must run before we trust it: ____
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Completed Exemplar
Stage 1. Strengths and weaknesses.
| Media bed | What it did well | Where it fell short | The one feature worth keeping |
|---|---|---|---|
| Pea gravel | Passed the most flow and stayed clean for the full three weeks | Left 34 mg/L of solids, above the 30 mg/L permit limit, and barely touched ammonia | Large open spaces between stones that resist clogging |
| Wood chips | Best on both solids and ammonia, and cheapest per cubic foot | Matted at the surface and the flow was already slowing at three weeks | A surface that grew the biofilm behind the best ammonia result |
| Lava rock | Clean after three weeks with a light biofilm and middling results on both tests | Costs more than twice the gravel per cubic foot | A rough, pitted surface that holds biofilm without matting shut |
Stage 2. Combined design.
| Layer, top to bottom | Feature borrowed from | Why this layer is here |
|---|---|---|
| 1 | Pea gravel, the open spacing | A coarse top layer spreads the flow and catches the heaviest solids without matting, which is what closed the wood chip bed |
| 2 | Lava rock, the rough surface | A thick middle layer carries most of the biofilm and keeps working after the top layer loads up |
| 3 | Wood chips, the biofilm surface | A thin bottom layer adds the biofilm that gave the best ammonia result, and putting it last keeps the matting away from the inlet |
Stage 3. Still to test.
- Suspended solids leaving the combined bed, measured weekly against the permit limit of 30 mg/L.
- That the thin wood chip layer mats shut anyway once solids work their way down to it.
- At least six weeks, since the wood chip bed only began slowing near the end of three weeks.
Accept any layer order students can defend from the data, including a design that drops the wood chips entirely, as long as the reason column cites a number or an observation from the test file. What should not be accepted is a combined design that names a feature no bed actually showed, or a stage-three answer that reports a result rather than naming a test.
Differentiation
Support
Give these students the strengths column filled in so their work starts at the weaknesses and the feature to keep. Provide a cut-and-paste strip of the three features so the stacking step becomes an ordering task. Read the test table aloud row by row and ask which number in the row is the one that matters.
Extension
Have students write the fictional town a short memo explaining why a design that no team tested can still be the right one to build, and what would have to go wrong in the six-week test for the town to fall back on the lava rock bed alone. Ask them to propose a second combined design that uses only two of the three media and to say what it gives up.
ELL supports
Pre-teach media, clogging, and biofilm with a labeled sketch of a filter cut in half, showing water entering the top and leaving the bottom. Provide the frames "This bed was good at ____ but weak at ____" and "I put this layer here because ____." Allow one-word entries in the feature column with an oral explanation.
Extensions
Ask students to sketch the combined design at a scale of their choosing on grid paper, label each layer's depth, and mark where they would install a sample tap so the fictional operators could measure the water between layers rather than only at the end. Discuss why a sample point between layers tells an operator something the outlet sample cannot.