Project Grade 7 Engineering & Design
Varnholt Flats Brackish Wells: Designing an RO Pretreatment
A four-session grade 7 project where teams order and justify the treatment steps ahead of a fictional brackish-well membrane plant, then defend the train to the board.
Objectives
- Students will match each raw-water problem to a treatment step that addresses it.
- Students will order treatment steps into a train and justify the order.
- Students will score two competing trains against stated criteria and recommend one.
- Students will name the first measurements an operator should take at start-up.
Vocabulary
Key words: brackish, membrane, fouling, pretreatment, recovery, cartridge filter, greensand
Teacher overview
A fictional desert town has drilled three salty wells and bought a reverse osmosis skid, and the membranes keep plugging within months. Teams design the treatment that goes in front of the membranes, choosing from a set of unit-process cards, ordering them, and defending the order against a raw-water report. Over four sessions each team builds a process flow diagram, scores it against a rival train, and presents to a fictional water board. The misconception to confront in the first session is that a membrane is a filter that catches everything, so nothing needs to happen upstream. Students who hold it will put the skid first and add steps behind it. Show the raw-water report early and ask what happens to iron and silt that arrive at a membrane surface with nowhere to go.
Project Brief
Driving question. What treatment should stand in front of the membranes at the fictional Varnholt Flats plant, and in what order?
The client. You work for the fictional Varnholt Flats Water Board. The board bought a reverse osmosis skid last year. The membranes now plug within four months. Replacing them is expensive and the plant shuts down each time.
The raw-water report. The three wells share these problems.
| What is in the water | Amount | Why a membrane cares |
|---|---|---|
| Fine silt and sand | Heavy after any pump start | Grit scratches and blocks the membrane surface |
| Iron | High enough to stain fixtures | Iron turns to rust flakes when it meets air, and the flakes cake on the membrane |
| Hardness, meaning calcium and magnesium | High | Hardness forms hard scale on the membrane as water is squeezed out |
| Bacteria | Present at low levels | Bacteria grow into a slime layer on the membrane |
The stated limits. The board wrote these down.
- Membranes must last at least three years between changes.
- The plant must recover at least 70 percent of the water it pumps.
- Total pretreatment cost may not pass 150,000 dollars.
- One operator, working four hours a day, must be able to run it.
- The plant may not use any chemical that needs a locked storage room, because the site has none.
Your unit-process cards. You may use any of these, once each.
| Step | What it removes or does | Cost | Operator time |
|---|---|---|---|
| Settling basin | Drops out heavy silt and sand | 22,000 dollars | Low |
| Greensand filter | Removes iron after air is added | 48,000 dollars | Medium |
| Cartridge filter | Catches fine grit right before the membranes | 9,000 dollars | Medium, cartridges are changed often |
| Antiscalant feed | Keeps hardness from forming scale | 31,000 dollars | Low |
| Softener | Removes hardness with resin and salt | 74,000 dollars | High, salt must be hauled |
| Chlorine feed with a locked room | Controls bacteria | 40,000 dollars | Medium |
| Ultraviolet unit | Controls bacteria with light | 26,000 dollars | Low |
What you turn in. Your team submits three things.
- A process flow diagram on poster paper. Show every step in order, from well to finished water. Label what each step removes.
- A memo of one page to the board. Name your train, its total cost, and the limit it comes closest to breaking.
- A start-up test plan. Name the first three things the operator should measure, where each is measured, and how often.
Milestones
Session 1. Match problems to steps. Read the raw-water report. For each of the four problems, list every card that could address it. Some problems have more than one answer. Turn in the matching chart and a one-line note on any problem with no card at all.
Session 2. Build and price two trains. Build two different trains that address all four problems. Order the steps and explain each placement in one sentence. Add up the cost of each train. Check both against the five limits. Turn in the two trains with their totals.
Session 3. Score and choose. Build a scoring table with at least four criteria. Cost, operator time, how well each problem is covered, and risk if one step fails are a good start. Score both trains. Total the columns. Pick one. Turn in the table and a paragraph saying which limit decided it.
Session 4. Diagram, memo, and board meeting. Draw the final process flow diagram in ink. Write the one-page memo. Present for five minutes to the board. Take two challenge questions. Turn in the diagram, the memo, and the start-up test plan.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Rubric
| Criterion | 4, exceeds | 3, meets | 2, approaching | 1, beginning |
|---|---|---|---|---|
| Problems matched to steps | Every raw-water problem is matched to all cards that address it, and the gaps are named | All four problems are matched to at least one workable step | Two or three problems are matched and one is left uncovered | Steps are listed without reference to the raw-water report |
| Order of the train | Every placement is justified, including why grit removal precedes the membranes and why nothing is placed after them that belongs before | The order is workable and most placements carry a reason | The order is mostly workable but reasons are missing or generic | Steps appear in an order that would not function |
| Constraints respected | The chosen train meets all five limits and the memo names which limit is closest to breaking | The chosen train meets all five limits | The train breaks one limit and the team does not notice | Cost or the storage-room limit is ignored |
| Scored comparison | Four or more criteria are scored with totals shown, and the recommendation follows the totals with a stated decisive limit | Four criteria are scored and the recommendation follows the table | Criteria are listed but scoring is partial | One train is named with no comparison |
| Diagram and start-up plan | The diagram is complete and readable, and all three start-up measurements name a location and a frequency | The diagram shows every step in order and three measurements are named | The diagram is missing labels or only one measurement is named | No usable diagram or test plan |
Differentiation
Support
Give these teams the problem-to-card matching chart already completed so they begin at ordering. Provide the unit-process cards as physical strips that can be laid out and rearranged on a desk before anything is drawn. Reduce the memo to a fill-in form with four headings, and let the start-up plan be answered orally while a partner writes.
Extension
Ask these teams to design a third train under the assumption that the board finds money for a locked chemical room, then argue whether that money would be better spent on pretreatment or on a spare membrane set. Have them mark on their diagram the one step whose failure would shut the plant down fastest and propose a way to detect that failure early.
ELL supports
Pre-teach membrane, scale, and fouling with a labeled sketch of a membrane surface before and after a month of use. Provide the frames "This step goes before ____ because ____" and "We must measure ____ at ____." Allow the process flow diagram to carry picture labels alongside words, and permit the memo in bullet form.
Extensions
Have teams write a short maintenance calendar for the fictional plant, listing what the operator does daily, weekly, and yearly for each step in their train, and then check that calendar against the four-hour-a-day staffing limit. Teams often find that a train that passed on cost fails on labor once the calendar is written out, which is a useful lesson about limits that only appear when a design is made concrete.