Graphic organizer Grade 5 Engineering & Design
Oldencote Microbe Bed Choice Board: A Grade 5 Trade-Off Chart
A grade 5 trade-off organizer comparing three microbe-powered cleaning beds for a fictional town's small wastewater plant against four stated criteria.
Objectives
- Students will restate a town's stated limits as criteria and constraints on a design choice.
- Students will rate three treatment options against the same four criteria in one grid.
- Students will choose one option and name the criterion that decided it.
Vocabulary
- Trickling filter
- A trickling filter is a wastewater treatment method where sewage trickles over a bed of rocks or plastic media covered in a living film of bacteria that eat the pollutants as the water passes over them. It is an older but still widely used way to provide secondary treatment.
Also: microbe, reed bed, lagoon, trade-off
Teacher overview
This organizer is a trade-off grid: three design options down the side, four criteria across the top, and a decision box underneath. It is built around the fictional town of Oldencote, which is replacing the worn-out cleaning stage at its small wastewater plant. All three options on the grid do the same biological job, which is to feed dirty water to tiny living things that eat the waste in it, so the choice between them turns entirely on how each one fits the town rather than on which one cleans best.
A misconception this chart is designed to correct: fifth graders often decide that the option with the most boxes marked "good" is the right one, counting favorable ratings rather than weighing them. A single constraint can rule out an option that scored well everywhere else, and Oldencote is written so that this happens. The town owns only a small field, so an option needing many acres is out even if it is cheap and simple. Say this out loud before students fill the decision box, and ask them to point at the one square that ended the argument. Naming the criterion that decided the case is the habit this organizer is built to teach.
Teachers should also expect students to assume that microbes are a problem in water rather than a tool. Every option here uses living things on purpose, and it helps to say plainly that a wastewater plant is a place where operators keep helpful microbes alive, fed, and supplied with air.
Organizer
Rule a sheet into five columns and four rows, then draw one wide decision box underneath.
Header strip, above the grid. Write these three facts about Oldencote in your own words, and label each one C for criterion or L for limit: the town owns one small field beside the creek; winters are long and cold; the plant has one operator who works part time.
Column headings, left to right.
- Option
- How much land does it need?
- Does it need electricity?
- Does it keep working in a cold winter?
- How much daily attention does the operator give it?
Row 1 — Reed bed. A shallow gravel bed planted with reeds. Water moves slowly through the gravel, where microbes on the stones and roots eat the waste.
Row 2 — Trickling filter. A round tank filled with rough stones or plastic shapes. Water is sprayed over the top and trickles down past microbes growing on every surface.
Row 3 — Lagoon. A wide, shallow pond. Water sits for weeks while microbes and sunlight break the waste down.
Leave every cell in columns 2 through 5 blank. Write a short phrase in each one, not a single word, so your reason travels with your rating.
Decision box. Finish these three stems. "We chose the ___ ." Then, "The criterion that decided it was ___ ." Then, "The option we gave up was ___ , and what we gave up with it was ___ ."
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Completed Exemplar
| Option | Land needed | Electricity | Cold winter | Daily attention |
|---|---|---|---|---|
| Reed bed | A large flat area, because water must move slowly through a wide bed | Little or none, since water can flow downhill through the gravel | Slows down, because the reeds die back and the microbes work more slowly when cold | Low, mostly cutting reeds and checking for clogs |
| Trickling filter | A small round footprint, because the tank is built upward instead of outward | Yes, pumps lift the water to the sprayer above the stones | Keeps working if the tank is covered, since the water stays moving and warm | Medium, checking the sprayer arm and clearing the nozzles |
| Lagoon | The most land of all three, because water must sit for weeks | Little, unless paddle aerators are installed to add air | Slows a great deal, and a surface layer can freeze | Low, mostly watching the level and the outlet |
We chose the trickling filter. The criterion that decided it was the small field, because a reed bed and a lagoon both need far more land than Oldencote owns. The option we gave up was the reed bed, and what we gave up with it was its very low electricity use, since the trickling filter needs pumps running every day.
A teacher checking this grid should look for reasons in the cells, not ratings alone. A student who wrote only "good," "bad," and "okay" has recorded an opinion, while a student who wrote "needs a lot of land because water moves slowly" has recorded a trade-off that can be argued with.
Differentiation
Support
Give these students the three option descriptions already pasted into the left column and a word bank for the cells: a lot, a little, none, slows down, keeps working, every day, once a week. Ask them to complete only columns 2 and 4, which are the two that decide the case, and then fill the decision box.
Extension
Ask these students to add a fifth column of their own choosing, such as how the option smells to nearby neighbors or how easy it would be to make bigger in ten years, and to fill it for all three rows. They should then write a short paragraph on whether their new column changes the decision, and explain why a town might weigh one criterion more heavily than the others.
ELL supports
Pre-teach "reed," "trickle," "lagoon," and "microbe" with a simple labeled sketch of each option, drawn side by side on the board so the shapes are easy to tell apart. Allow students to fill cells with a drawing plus one or two words. Provide the decision box stems as printed strips, and let students say their answer aloud to a partner before writing.
Extensions
Many small towns post photographs or a plant tour on their utility website, and finding one that uses a lagoon and one that uses a trickling filter gives students two real versions of the options on this grid. Students can also look at a nearby marsh or wet meadow, in photographs or on an adult-supervised visit, and describe how it does the same work a reed bed does, without anyone having designed it.