Project Grade 6 Engineering & Design
Greywater Biofilter Design for the Fernwood Community Garden
A grade 6 engineering project where teams design a small layered biofilter to clean handwashing greywater for garden irrigation, within a fixed budget and footprint.
Objectives
- Students will design a layered biofilter that fits a stated budget and footprint.
- Students will explain which filter layer or plant targets which type of contaminant in greywater.
- Students will write and present a proposal justifying their design's cost and layout choices.
Vocabulary
- Greywater
- Gently used water from showers, bathtubs, bathroom sinks, and washing machines. With the right setup, it can be reused to water plants instead of going down the drain.
- Wetland
- A wetland is an area like a marsh or swamp where the ground is soaked with water for much of the year. Wetlands soak up floodwater, help clean it, and shelter many kinds of animals.
- Filtration
- Filtration is the step where water passes through layers of sand, gravel, or other materials that trap the small particles still left in it. It works like a very fine strainer.
Teacher overview
The Fernwood Community Garden (a fictional garden used for this project) has an outdoor hand-washing sink whose greywater, water used for washing hands, currently drains straight onto bare gravel and goes to waste. Over four sessions, teams design a small layered biofilter, a miniature version of a constructed wetland, that could clean that greywater well enough to reuse it for watering the garden's non-edible landscaping, working within a fixed materials budget and a fixed footprint. Teams present their final design and a written proposal to the garden's (fictional) volunteer board.
Project Brief
Driving question: How can we design a layered biofilter that cleans handwashing greywater well enough to water non-edible plants? The design has to fit a budget of $150 and a space 3 feet by 2 feet.
Scenario: The Fernwood Community Garden has an outdoor hand-washing station. Greywater is the used water that drains from a sink like that one. A biofilter cleans greywater by sending it down through layers of material, with plants growing on top. The garden's volunteer board has asked student teams to design one. It would be built into the planting bed next to the sink, which measures 3 feet by 2 feet.
Constraints:
- Budget: $150 in all. That covers gravel, sand, mulch, wetland-tolerant plants, a container, and tubing. The budget is fictional, set for this project only. Your teacher will hand out a price sheet.
- Footprint: the finished biofilter must fit inside the garden's planting bed. The bed is 3 feet by 2 feet, an area of 6 square feet.
- The design needs at least three separate layers. Gravel, sand, and mulch or wood chips are examples. It also needs at least one wetland-tolerant plant, because plant roots take up nutrients from the greywater.
- For each layer and each plant, the proposal must say what it is mainly expected to remove or break down. Larger particles, soap residue, and dissolved nutrients are three examples.
Final deliverable: Each team turns in two pieces of work. The first is a cross-section diagram of the biofilter, drawn to scale on grid paper. Label every layer and every plant on it. The second is a one-page written proposal to the garden's volunteer board. In the proposal, explain how the design meets the budget and the footprint. Explain as well why your team chose each layer and each plant.
Milestones
Session 1 -- Research and constraints (45 minutes): Teams study how a constructed wetland or biofilter cleans greywater in stages. Gravel and sand trap the larger particles. Mulch feeds the microbes that break down soap residue and other organic matter. Plant roots take up the dissolved nutrients. Teams then write the $150 budget and the 3-by-2-foot footprint on a planning sheet. Output: a completed research and constraints sheet.
Session 2 -- Draft two design options (45 minutes): Teams sketch two different layered cross-sections on grid paper. Each sketch has to fit the 3-by-2-foot footprint. Teams then use the teacher's price sheet to estimate a rough cost for each one. Output: two labeled draft sketches with rough cost estimates.
Session 3 -- Choose and refine one design (45 minutes): Teams compare their two drafts against the budget and the footprint. They choose one draft and refine it into a final scaled cross-section diagram. Output: one final labeled cross-section diagram with itemized costs totaling $150 or less.
Session 4 -- Write and present the proposal (35 minutes): Teams write the one-page proposal, explaining their layer choices and their cost. They then present the diagram and the proposal to the class. Classmates role-play the garden's volunteer board. Output: a completed written proposal and a short team presentation.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Rubric
| Criterion | Emerging | Developing | Proficient | Advanced |
|---|---|---|---|---|
| Filter design & layer reasoning | Layers are listed with no explanation of what each targets | Layers are listed with a vague explanation of purpose | Each layer's purpose (particles, soap residue, or nutrients) is clearly and correctly explained | Each layer's purpose is clearly explained, and the team justifies the order of the layers |
| Budget & footprint constraint | Design exceeds the $150 budget, the 3-by-2-foot footprint, or both, with no adjustment | Design is close to both constraints but costs are not itemized | Design fits the footprint with an itemized cost list at or under $150 | Design fits the footprint, is itemized at or under $150, and explains one tradeoff the team made to stay within budget |
| Proposal & presentation | Proposal is incomplete or difficult to follow | Proposal covers the design but with unclear reasoning | Proposal clearly explains how the design meets both constraints | Proposal clearly explains both constraints and anticipates a likely question from the garden's volunteer board |
Differentiation
Support (below grade level)
Provide a completed sample cross-section (three layers plus one plant) as a model teams can adapt rather than design entirely from scratch, and provide a shortened price sheet with fewer plant choices.
Extension (above grade level)
Ask teams to calculate what percent of their project's fictional $150 budget went toward plants versus toward gravel, sand, and mulch combined, and to explain whether shifting money from one category to the other would improve the design.
ELL supports
Provide a labeled diagram of a biofilter cross-section (gravel, sand, mulch, plant roots, with arrows showing water flowing downward through each layer) before Session 1. Allow the written proposal to include labeled sketches alongside shorter sentences rather than full paragraphs.