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Lesson plan Grade 7 Science

Torbrook's Combined Sewer: Counting the Storms That Overflow

A 55-minute grade 7 science lesson comparing storm flow with plant capacity at a fictional town's combined sewer, then sizing the overflow a green plan prevents.

Objectives

  1. Students will calculate how many gallons overflow when combined sewer flow passes a plant's treating capacity.
  2. Students will express a change in overflow volume as a percent decrease.
  3. Students will explain why a combined sewer spills during storms but not on dry days.

Vocabulary

Green infrastructure
Green infrastructure uses soil and plants, like rain gardens, green roofs, and street trees, to soak up rainwater where it falls instead of piping it away. It copies the way nature handles rain.

Also: combined sewer, overflow, treating capacity, percent decrease

Teacher overview

Older neighborhoods in many cities are drained by a single pipe that carries both sewage and street runoff to the treatment plant, an arrangement called a combined sewer. In the fictional town of Torbrook that pipe works well until a storm arrives, and this lesson has students compare the storm flow against what the plant can actually treat. A common misconception in this lesson is that an overflow means a pipe has broken; the arithmetic corrects that by showing the outfall is a designed relief point that opens whenever flow passes capacity, which is why reducing the flow matters more than repairing the pipe.

Procedure

Hook (5 min): Describe the pipe under the oldest streets in the town of Torbrook, which carries wastewater from homes and runoff from every gutter in the same channel. Ask students to predict what happens to that pipe during a long thunderstorm, and record predictions without correcting them.

Direct instruction (12 min): Define combined sewer, treating capacity, and overflow. Explain that a treatment plant is built for a maximum daily flow, and that when more water arrives than the plant can treat, the extra volume is released through an outfall to a creek without full treatment. Model one calculation on the board using numbers that do not appear on the handout: if a plant treats up to 4,000,000 gallons per day and a storm delivers 5,200,000 gallons, the overflow is 1,200,000 gallons, which is about 23 percent of the day's flow. Then introduce green infrastructure, meaning rain gardens, porous pavement, and disconnected downspouts that hold runoff on the land instead of sending it into the pipe.

Guided practice (13 min): Work handout items 1 through 3 together. Have students state each subtraction and division aloud before computing, so the difference between "how much spilled" and "what percent spilled" stays clear. Ask two students to explain why item 3 divides by the day's total flow rather than by the plant's capacity.

Independent practice (15 min): Students complete handout items 4 through 8 on their own, then compare answers with a partner and settle any disagreements by rechecking the numbers on the handout rather than by voting.

Closure and exit ticket (10 min): Each student answers one question on a half sheet: "A storm brings 7,000,000 gallons to a plant that treats 6,000,000 gallons per day. How many gallons overflow, and what would cut that number?" Collect the sheets and sort them into correct and incorrect piles. If more than a third of the class subtracts in the wrong order, reteach the capacity comparison with a labeled bar diagram at the start of the next session.

Torbrook's Combined Sewer: Counting the Storms That Overflow — Grade 7 Science
Name: ____________________________ Date: ______________

Student Handout

Torbrook is a fictional town, and all of the numbers below were written for this exercise. Show your work for every item.

  1. Torbrook's rain gauge recorded 92 storms last year. The combined sewer overflowed during 23 of them. What percent of the storms caused an overflow?
  2. The Torbrook plant can treat up to 6,000,000 gallons per day. During one storm day the combined sewer delivered 9,000,000 gallons. How many gallons more than the plant could treat arrived that day?
  3. On that same storm day, 9,000,000 gallons arrived and 3,000,000 gallons overflowed to the creek. What percent of the day's water overflowed? Round to the nearest whole percent.
  4. Torbrook plans rain gardens and porous pavement that would keep 1,500,000 gallons of runoff out of the pipe on a day like that one. If 9,000,000 gallons arrived before the plan, how many gallons would arrive after it?
  5. After the plan, 7,500,000 gallons arrive at a plant that treats 6,000,000 gallons per day. How many gallons would overflow?
  6. The overflow falls from 3,000,000 gallons to 1,500,000 gallons. What is the percent decrease?
  7. A sample of Torbrook's overflow water held 180 mg/L of suspended solids. Water leaving the plant after full treatment held 12 mg/L. How many times more suspended solids does the overflow water carry?
  8. Explain why the Torbrook pipe overflows during a storm but never on a dry summer day. Use the words capacity and runoff in your answer.

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

Answer Key
  1. Answer: 25 percent.
    23 storms divided by 92 storms equals 0.25, which is 25 percent of the storms.
  2. Answer: 3,000,000 gallons.
    9,000,000 gallons arrived and the plant could treat 6,000,000 gallons, so 9,000,000 minus 6,000,000 leaves 3,000,000 gallons with nowhere to go.
  3. Answer: 33 percent.
    3,000,000 divided by 9,000,000 equals 0.333, which rounds to 33 percent of the day's water.
  4. Answer: 7,500,000 gallons.
    9,000,000 gallons minus the 1,500,000 gallons held back by the rain gardens and porous pavement equals 7,500,000 gallons arriving at the plant.
  5. Answer: 1,500,000 gallons.
    7,500,000 gallons arrive and 6,000,000 gallons can be treated, so 7,500,000 minus 6,000,000 leaves 1,500,000 gallons of overflow.
  6. Answer: 50 percent.
    The overflow drops by 3,000,000 minus 1,500,000, or 1,500,000 gallons. Dividing 1,500,000 by the original 3,000,000 gives 0.5, a 50 percent decrease.
  7. Answer: 15 times.
    180 mg/L divided by 12 mg/L equals 15, so the overflow water carries 15 times the suspended solids of fully treated water.
  8. Answer: Storm runoff pushes the flow past the plant's capacity.
    On a dry day the pipe carries only wastewater from homes and businesses, which stays well under the plant's treating capacity. During a storm, runoff from streets and roofs enters the same pipe, the total flow passes capacity, and the extra water is released at the outfall. Accept any answer that connects added runoff to flow passing capacity.

Differentiation

Support

Give students a labeled bar diagram for items 2 and 5 with the capacity line already drawn, so the subtraction is visible before it is written. Provide a calculator and a worked model of one percent-decrease problem with different numbers.

Extension

Ask students to find the smallest amount of runoff the green infrastructure plan would have to keep out of the pipe to prevent any overflow at all on the storm day in item 2, and to explain why a plan sized for that storm might still fail during a larger one.

ELL supports

Pre-teach "capacity," "overflow," and "runoff" using a clear pitcher and a measuring cup: pour past the cup's line so the excess spills, then name each part of the demonstration. Provide sentence frames for item 8: "On a dry day the pipe carries only ___. During a storm the pipe also carries ___, so ___."

Extensions

Have students sketch a map of their own school grounds, mark every surface that sheds water quickly, and propose two places where a rain garden could intercept runoff before it reaches a storm drain. Ask them to state what they would measure to know whether the garden was working.