Lesson plan Grade 6 Engineering & Design

Choosing a Wet Well Size for the Wrayside Lift Station

Students size the working volume a fictional sewage lift station needs, then test three proposed wet wells against a pump-start rule and a width limit.

Grade: Grade 6
Subject: Engineering & Design
Material: Lesson plan
Time: 50 min
Prep: Low prep
Materials: One printed copy of the student handout per student, Calculators

Standards

MS-ETS1-2 NGSS

Evaluate competing design solutions using a systematic process to determine how well they meet the criteria and constraints of the problem.

How this material aligns: Students run three proposed wet wells through the same two-test process, a stored-volume rule and a width limit, and select the one design that clears both tests.

All materials for MS-ETS1-2 →

6.G.A.2 Common Core Math

Find the volume of a right rectangular prism with fractional edge lengths by packing it with unit cubes of the appropriate unit fraction edge lengths, and apply the formulas V = l w h and V = b h to find volumes of right rectangular prisms with fractional edge lengths in the context of solving real-world and mathematical problems.

How this material aligns: Handout items 3, 5, and 7 apply the volume formula for a right rectangular prism, including Option 1, whose working depth of 2.5 feet is a fractional edge length.

All materials for 6.G.A.2 →

6.NS.B.3 Common Core Math

Fluently add, subtract, multiply, and divide multi-digit decimals using the standard algorithm for each operation.

How this material aligns: Items 4, 6, and 8 multiply by the multi-digit decimal 7.48 gallons per cubic foot, and items 9 and 12 subtract decimals to find how far an option sits from the target.

All materials for 6.NS.B.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 calculate the storage a lift station needs from an inflow rate and a pump-start rule.
Students will find the working volume of three rectangular wet wells in cubic feet and in gallons.
Students will select the one design that satisfies both the volume rule and the width limit.

Vocabulary

Lift station: A lift station is a pumping setup that raises wastewater from a low spot so it can keep flowing toward the treatment plant. Sewers mostly flow downhill, so low areas need a lift.
Easement: The legal right to use a strip of someone else's land for a specific purpose, such as burying and maintaining a water pipe underneath it.

Also: wet well, working volume, pump start

Teacher overview

Sewer pipes run downhill, so a neighborhood in a low spot needs a pumped station that lifts its sewage up to the next gravity line. The tank that holds sewage between pump runs is the wet well, and its size is a design decision with a rule on each side. The lesson gives students a fictional station on Wrayside Street in the town of Ivesdale, three proposed wet wells, a limit on how often the pump may start, and a width limit set by the easement, and asks which proposal the town should build. The misconception worth naming out loud is that a larger tank is always the safer choice. An oversized wet well holds sewage so long that it goes stale and smells, and grease cakes on the walls, so operators size these tanks to be just large enough rather than as large as the site allows.

Procedure

Framing the problem (7 min). Draw a sewer line dropping downhill into a buried tank with a pump at the bottom. Ask why the pump cannot simply run all the time at the speed sewage arrives, and steer the answer toward the fact that flow rises and falls all day while a pump has one speed. Introduce the term working volume as the water held between the level that starts the pump and the level that stops it.
Reading the design brief (8 min). Distribute the handout and read the brief aloud. In pairs, students state the two constraints, which are the six-starts-per-hour limit (which sets the smallest storage allowed) and the 4.5-foot width limit, and the criterion, which is a well no larger than it needs to be so sewage does not sit and go stale. Add the criterion yourself if no pair names it. Record the list on the board and leave it up.
Modeling the conversion (10 min). Work a practice tank at the board that is not on the handout: a well 3 feet long, 3 feet wide, with a working depth of 2 feet holds 18 cubic feet, and 18 times 7.48 gives 134.64 gallons. Show the multiplication of the decimal in full. Ask students to say which number in the product is a rate and what its units are.
Student handout (20 min). Students complete items 1 through 12. Circulate and check item 2 early, because students who divide the hourly inflow by 60 minutes instead of by the 6 starts will carry that error through the rest of the page.
Design decision and exit check (5 min). Take a show of hands for each option, then call on one student per option to state which tests their option passed or failed. Close with an exit ticket: "Name the option you chose and the test that removed each of the other two." If fewer than 80 percent of tickets name both removed options correctly, revisit items 10 and 11 the next day.

Choosing a Wet Well Size for the Wrayside Lift Station — Grade 6 Engineering & Design

Name: ____________________________ Date: ______________

Student Handout

Design brief: the Wrayside Street lift station (fictional)

The town of Ivesdale is rebuilding a small sewage lift station. Use only the numbers given in each item.

During the busy hour, sewage arrives at 30 gallons per minute.
The pump may start no more than 6 times in that hour.
One cubic foot of water is 7.48 gallons.
The easement is narrow, so the well may be no wider than 4.5 feet.
Option 1 is 4 feet long, 4 feet wide, with a working depth of 2.5 feet.
Option 2 is 5 feet long, 5 feet wide, with a working depth of 2 feet.
Option 3 is 4 feet long, 4 feet wide, with a working depth of 3 feet.

Sewage arrives at 30 gallons per minute. How many gallons arrive in one hour of 60 minutes?
In that hour, 1,800 gallons arrive and the pump may start only 6 times. How many gallons must the well hold between one start and the next?
Option 1 is 4 feet long, 4 feet wide, and 2.5 feet deep in its working range. What is its working volume in cubic feet?
One cubic foot is 7.48 gallons. How many gallons does Option 1's 40 cubic feet hold?
Option 2 is 5 feet long, 5 feet wide, and 2 feet deep in its working range. What is its working volume in cubic feet?
How many gallons does Option 2's 50 cubic feet hold, at 7.48 gallons per cubic foot?
Option 3 is 4 feet long, 4 feet wide, and 3 feet deep in its working range. What is its working volume in cubic feet?
How many gallons does Option 3's 48 cubic feet hold, at 7.48 gallons per cubic foot?
Option 1 holds 299.2 gallons and the rule asks for 300 gallons. How many gallons short is Option 1?
The well may be no wider than 4.5 feet. Option 1 is 4 feet wide, Option 2 is 5 feet wide, and Option 3 is 4 feet wide. Which option does the width limit remove?
Option 1 holds 299.2 gallons, Option 2 holds 374 gallons, and Option 3 holds 359.04 gallons. The well must hold at least 300 gallons and must be no wider than 4.5 feet. Option 1 is 4 feet wide, Option 2 is 5 feet wide, and Option 3 is 4 feet wide. Which option should the town build?
Option 3 holds 359.04 gallons and the target is 300 gallons. How many gallons above the target is that?

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: 1,800 gallons.
30 gallons per minute times 60 minutes gives 1,800 gallons.
Answer: 300 gallons.
1,800 gallons divided by 6 starts gives 300 gallons for each start.
Answer: 40 cubic feet.
4 times 4 times 2.5 gives 40 cubic feet.
Answer: 299.2 gallons.
40 cubic feet times 7.48 gallons per cubic foot gives 299.2 gallons.
Answer: 50 cubic feet.
5 times 5 times 2 gives 50 cubic feet.
Answer: 374 gallons.
50 cubic feet times 7.48 gallons per cubic foot gives 374 gallons.
Answer: 48 cubic feet.
4 times 4 times 3 gives 48 cubic feet.
Answer: 359.04 gallons.
48 cubic feet times 7.48 gallons per cubic foot gives 359.04 gallons.
Answer: 0.8 gallon.
300 gallons minus 299.2 gallons leaves 0.8 gallon. Option 1 misses the rule by less than one gallon, which is still a miss.
Answer: Option 2.
Option 2 is 5 feet wide, and 5 feet is more than the 4.5-foot limit, so it cannot be built in the easement even though it stores the most water.
Answer: Option 3.
Option 2 fails the width limit at 5 feet. Option 1 fits the easement but holds 299.2 gallons, which is under the 300-gallon rule. Option 3 fits at 4 feet wide and holds 359.04 gallons, so it is the only option that clears both tests.
Answer: 59.04 gallons.
359.04 gallons minus 300 gallons leaves 59.04 gallons of extra storage.

Differentiation

Support

Provide a handout with items 1 and 3 worked, and supply a drawing of one wet well labeled with length, width, and working depth so the three dimensions are visible before the multiplication starts. Allow calculators throughout and accept answers rounded to the nearest tenth of a gallon.

Extension

Ask students to find the working depth that would let a well 4 feet by 4 feet hold exactly the required storage, then explain in writing why a designer would still build the well slightly deeper than that answer.

ELL supports

Pre-teach "lift station," "wet well," "working depth," and "easement" with a labeled cutaway drawing, and post the volume formula with each letter matched to a labeled edge in the drawing. Allow students to answer items 10 and 11 by circling the option and pointing to the failed rule.

Extensions

Have students look up whether their own town lists any pump or lift stations on a public utility map, and write a short paragraph on why a station is usually placed at the lowest point of a neighborhood rather than at the highest.