Lab / experiment Grade 7 Engineering & Design

Braycott Basin Baffles: Testing Three Layouts for Clearer Water

A grade 7 lab where teams test three baffle layouts in a model settling basin, then build a fourth that combines features from their best layout and a partner team's.

Grade: Grade 7
Subject: Engineering & Design
Material: Lab / experiment
Time: 2 sessions, ~90 min total
Prep: Moderate prep
Materials: Clear plastic storage bin, about 30 cm by 45 cm, one per team, Foam board or cardboard strips for baffles, six per team, Waterproof tape, Potting soil, about 240 g per team (60 g for each of four rounds), Kitchen scale (or soil pre-weighed into 60 g portions), Pitcher marked at 1 liter, Spoon or stirring stick, Tray to set the bin on, one per team, Paper towels, Clear plastic sample cups, four per team, Stopwatch or phone timer, Ruler marked in millimeters, Printed 1-to-5 cloudiness card, one per team, Safety goggles

Standards

MS-ETS1-4 NGSS

Develop a model to generate data for iterative testing and modification of a proposed object, tool, or process such that an optimal design can be achieved.

How this material aligns: Teams run a model basin through four timed rounds and change the baffle layout between rounds, generating the data this standard requires for iterative optimization.

All materials for MS-ETS1-4 →

MS-ETS1-3 NGSS

Analyze data from tests to determine similarities and differences among several design solutions to identify the best characteristics of each that can be combined into a new solution to better meet the criteria for success.

How this material aligns: Analysis questions 4 and 5 make teams name the feature they borrowed from each of two tested layouts and check, against their own data, whether the combined build beat the best earlier round.

All materials for MS-ETS1-3 →

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This material is in the public domain (CC0) and may be copied, edited, and shared freely, without attribution. Updated 2026-09-23.

Objectives

Students will build a model settling basin and test three baffle layouts under the same conditions.
Students will record cloudiness scores and settled sediment depth for each layout.
Students will design a fourth layout that combines a named feature from their own best layout with one from a partner team's best layout.
Students will judge whether the combined layout beat the layouts it borrowed from.

Vocabulary

Key words: baffle, settling basin, short circuiting, sediment, cloudiness, iteration

Teacher overview

A settling basin gives dirt time to fall out of water before the water reaches a filter, and the walls inside it, called baffles, decide how long the water actually spends in the tank. This lab asks which baffle layout leaves the clearest water at the outlet of a model basin built for the fictional Braycott water plant. Real plants care because the finished water has to meet a turbidity limit of 0.3 NTU in most monthly samples, and a basin that lets water rush straight through leaves that whole job to the filters. The misconception to name before students build is that a bigger basin always settles more. Volume is only part of the picture, because a large basin with a straight path can pass water from inlet to outlet in less time than a small basin with a winding one. That shortcut is called short circuiting, and preventing it is what baffles are for.

Braycott Basin Baffles: Testing Three Layouts for Clearer Water — Grade 7 Engineering & Design

Name: ____________________________ Date: ______________

Procedure

Put on goggles before mixing the muddy water. Do not drink or taste any water in this lab. Wipe spills right away so the floor stays dry.
Set the bin on a tray. Prop one short end up about 2 cm with a folded paper towel. The low end is the outlet end. Mark both ends with tape.
Mix the test water. Put 60 g of potting soil into the pitcher. Add water to the 1 liter mark. Stir for 20 seconds so the soil is spread through the water.
Run Round 1 with no baffles. Pour the full liter into the high end over about 20 seconds. Start the timer as you pour.
At 2 minutes, dip one sample cup at the outlet end. Hold it against the cloudiness card. Write down the score from 1 to 5. A 1 means clear and a 5 means very cloudy.
Let the bin sit for 10 more minutes. Then measure the settled soil at the outlet end with the ruler. Record the depth in millimeters.
Empty and rinse the bin. Build Round 2. Tape one cardboard strip across the middle of the bin. Leave a gap of about 3 mm under it so water can pass beneath. The water is less than 1 cm deep at the middle, so a wider gap would let it flow under without touching the baffle.
Repeat the same pour, the same 2 minute sample, and the same 10 minute settling measurement. Use a fresh liter of muddy water mixed the same way.
Empty and rinse the bin. Build Round 3. Tape three shorter strips in a staggered line, so water must weave left, right, then left again.
Repeat the test one more time with a fresh liter. Record both numbers.
Trade tables with a partner team. Find the one feature of their best layout that beat yours. Find the one feature of yours that beat theirs.
Build Round 4 using both borrowed features. Stay under six strips. Run the same test a fourth time and record both numbers.

Data & Analysis

Record every round in this table before you answer the questions.

Round	Layout	Cloudiness score at 2 minutes	Settled soil depth at 12 minutes
1	No baffles
2	One full baffle with a gap under it
3	Three staggered baffles
4	Combined design

Which of Rounds 1, 2, and 3 gave the lowest cloudiness score at the outlet? Name the round and give its score.
Subtract the cloudiness score of your best layout from the score of the no-baffle round. Show the subtraction and the answer.
Baffles make the water travel a longer path from inlet to outlet. Explain how a longer path gives soil more chance to settle.
Name the feature you borrowed from your own layout and the feature you borrowed from the partner team for Round 4. Say why you picked each one.
Did Round 4 beat the best of Rounds 1, 2, and 3? Use two numbers from your table to support your answer.
Name one way this model basin is not like a real one. Say how that difference could change your answer to question 1.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key

Answer: Accept the round with the lowest recorded score.
Most teams report Round 3 with a score of 2 or 3, against a Round 1 score of 4 or 5. Accept any round the team's own table supports. Check that the round named matches the lowest number in the cloudiness column, since students sometimes read the scale backward and pick the highest.
Answer: Accept the correct subtraction of the team's own two scores.
A team with 5 in Round 1 and 2 in Round 3 writes 5 minus 2 equals 3. The expected difference is between 1 and 3 points. Accept a difference of 0 if the table shows it, but ask that team to check whether the gap under their baffle was too large.
Answer: A longer path means more time in the basin, and time is what lets soil sink.
Look for the idea that soil falls at its own slow rate, so the water has to stay in the tank long enough for that fall to finish. Accept answers that say the baffles slow the water down, since the slowing and the lengthening both raise the travel time. Do not accept the idea that the baffles catch the soil like a screen, which is a different mechanism.
Answer: Two named features with reasons.
Accept any two concrete features, such as the staggered spacing, the gap height under a baffle, a baffle placed close to the inlet, or a baffle angled across the flow. The reason should point at a number in one of the two tables rather than at a preference. Do not accept an answer that names the whole layout instead of a feature.
Answer: Accept either a yes or a no that the team's own table supports.
The claim must be backed by two figures, usually the Round 4 cloudiness score against the best earlier score, or the two settled depths. Roughly half of teams find no improvement, which is a valid result worth discussing. Push those teams to say which borrowed feature they suspect did not carry over.
Answer: A named difference plus its effect on the conclusion.
Common correct answers include the model running one batch rather than a steady flow, the soil being coarser than real basin solids, the short 2 minute wait, the bin being far shallower than a basin, and the absence of the chemicals a plant adds to make particles clump. Each should be paired with an effect, such as a steady flow making the no-baffle short circuit worse and widening the gap between rounds.

Differentiation

Support

Pre-build the Round 2 and Round 3 baffle sets so these teams spend their time testing rather than taping. Give them a data table with the round and layout columns already filled and the units written into the headings. Provide the cloudiness card with each number labeled in words as well as digits, and let one partner call out readings while the other records.

Extension

Have these teams add a fifth round that tests one feature at two settings, such as a 2 mm gap against a 5 mm gap under the same baffle, so they can separate the effect of the gap from the effect of the layout. Ask them to sketch the path water most likely takes in each of their four layouts and mark where they think the short circuit forms.

ELL supports

Pre-teach baffle, settle, and outlet with a labeled side-view sketch of the bin showing the pour end, the baffles, and the sample point. Provide the frames "Round ____ was clearest because ____" and "We borrowed ____ from ____." Allow question 3 and question 6 to be answered with a labeled drawing plus one short caption each.

Extensions

Ask teams to write a short note to the fictional plant's operators explaining, in plain language, why a basin can be the right size and still perform badly. Have them include one thing an operator could watch for, such as cloudiness at the outlet weir rising after a storm, that would hint at short circuiting inside a tank nobody can see into.