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Worksheet Grade 9 Science

Ellery Point's Combined Sewer: Overflow Volume and Tunnel Storage

A grade 9 worksheet where students calculate combined sewer overflow volumes for a fictional coastal town, size a storage tunnel, and compare it with green retrofits.

Objectives

  1. Students will calculate the volume of sewage and stormwater that a combined sewer releases when flow exceeds plant capacity.
  2. Students will use the cylinder volume formula to find the capacity of a proposed storage tunnel in gallons.
  3. Students will compare a storage tunnel and a green infrastructure retrofit using captured volume and pollutant load.

Vocabulary

Combined sewer overflow
In some older cities, one pipe carries both sewage and rainwater. A combined sewer overflow happens when a big storm fills the pipe past what the plant can handle, and the extra mix spills into a river untreated.
Biochemical oxygen demand (BOD)
Biochemical oxygen demand, or BOD, measures how much oxygen tiny microbes will use up as they eat the waste in water. Dirtier water has a higher BOD.
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: storage tunnel

Teacher overview

Older cities often carry sewage and street runoff in the same pipe. In dry weather that pipe delivers everything to the treatment plant, yet during a storm the combined flow can rise past what the plant can treat, and the excess is released to a river or bay through an outfall. This worksheet uses the fictional coastal town of Ellery Point, whose plant treats 15 MGD and whose combined sewer discharges into Gullwing Bay. Students calculate how much untreated water a storm releases, size a proposed storage tunnel with the cylinder volume formula, and then compare the tunnel against a green infrastructure retrofit. A common student misconception is that an overflow is pure sewage; in fact it is heavily diluted storm water carrying a sewage fraction, which is why the pollutant load, not the volume alone, decides how much harm it does.

Ellery Point's Combined Sewer: Overflow Volume and Tunnel Storage — Grade 9 Science
Name: ____________________________ Date: ______________

Student Worksheet

The fictional town of Ellery Point runs a combined sewer. In dry weather it carries 6.0 MGD of sewage to a plant that can treat 15 MGD. Anything above 15 MGD leaves through an outfall into Gullwing Bay without treatment. The town has proposed a storage tunnel that holds storm flow until the plant can catch up. The tunnel would be a cylinder 14 feet in diameter and 4,200 feet long. Use 0.785 times diameter squared times length for the volume in cubic feet, and 7.48 gallons per cubic foot. Round gallon answers to the nearest gallon.

  1. During a summer storm, combined flow peaks at 42 MGD for 3 hours. How many MGD of that flow are above the plant capacity of 15 MGD?
  2. Find the volume released to the bay during those 3 hours, in gallons. Multiply the excess flow in millions of gallons per day by 3/24 of a day.
  3. Find the volume of the proposed tunnel in cubic feet.
  4. Convert your answer from item 3 into gallons.
  5. Could the tunnel hold the entire release you calculated in item 2? Give the number of gallons of tunnel capacity that would still be empty.
  6. A larger autumn storm pushes combined flow to 60 MGD for 4 hours. How many gallons rise above the plant capacity of 15 MGD during that storm?
  7. If the tunnel is empty when the autumn storm begins, how many gallons still reach the bay?
  8. What percent of the autumn storm's excess would the tunnel capture? Round to the nearest tenth of a percent.
  9. Suppose the water that reaches the bay in item 7 carries 120 mg/L of biochemical oxygen demand. Use pounds equals concentration in mg/L times volume in millions of gallons times 8.34 to find the pounds of oxygen demand released to the bay. Round to the nearest pound.
  10. After a storm, the tunnel is pumped back to the plant at 2,000 gpm. How many hours would it take to empty the full tunnel you sized in item 4? Round to the nearest tenth of an hour.
  11. Instead of the tunnel, the town could spend on green infrastructure, meaning permeable pavement, rain gardens, and disconnected downspouts, which the design team estimates would cut the excess storm volume by 20 percent. Find the new excess volume for the autumn storm from item 6.
  12. If the town built the tunnel as well as the 20 percent green infrastructure reduction, how many gallons would reach the bay during the autumn storm?
  13. The tunnel would cost $92 million and the green infrastructure program would cost $61 million, and the green program takes about twelve years to build out across many small sites. Name one advantage and one disadvantage of each option, using your numbers from items 7, 8, and 12 as support.

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: 27 MGD. 42 MGD minus the 15 MGD plant capacity equals 27 MGD above capacity.
  2. Answer: 3,375,000 gallons. 27 million gallons per day times 3/24 of a day equals 3.375 million gallons.
  3. Answer: 646,212 cubic feet. 0.785 times 14 squared equals 0.785 times 196, which is 153.86, and 153.86 times 4,200 ft equals 646,212 cubic feet.
  4. Answer: 4,833,666 gallons. 646,212 cubic feet times 7.48 gallons per cubic foot equals 4,833,665.76, which rounds to 4,833,666 gallons.
  5. Answer: 1,458,666 gallons. The tunnel holds 4,833,666 gallons, which is more than the 3,375,000 gallons released, so it captures the whole event and 4,833,666 minus 3,375,000 equals 1,458,666 gallons of capacity left empty.
  6. Answer: 7,500,000 gallons. 60 MGD minus 15 MGD equals 45 MGD, and 45 million gallons per day times 4/24 of a day equals 7.5 million gallons.
  7. Answer: 2,666,334 gallons. 7,500,000 minus the 4,833,666 gallons the tunnel holds equals 2,666,334 gallons reaching the bay.
  8. Answer: 64.4 percent. 4,833,666 divided by 7,500,000 equals 0.6445, or 64.4 percent captured.
  9. Answer: 2,668 pounds. 2,666,334 gallons is 2.666334 million gallons, and 120 mg/L times 2.666334 times 8.34 equals about 2,668.47 pounds, which rounds to 2,668 pounds.
  10. Answer: 40.3 hours. 4,833,666 gallons divided by 2,000 gpm equals 2,416.8 minutes, and 2,416.8 divided by 60 equals 40.3 hours.
  11. Answer: 6,000,000 gallons. 7,500,000 times 0.80 equals 6,000,000 gallons of excess after a 20 percent reduction.
  12. Answer: 1,166,334 gallons. 6,000,000 minus the 4,833,666 gallons the tunnel holds equals 1,166,334 gallons reaching the bay.
  13. Answer: Accept any response that pairs one supported advantage with one supported disadvantage for each option. The tunnel captures 64.4 percent of the autumn storm on its own and works the day it opens, yet it costs $92 million, treats no water by itself, and still lets 2,666,334 gallons through in that storm. The green program costs $61 million, keeps rain out of the sewer in the first place, and combined with the tunnel cuts the release to 1,166,334 gallons, yet it depends on hundreds of small sites and takes about twelve years to finish.

Differentiation

Support

Work item 3 with the class on the board, writing out 0.785 times 14 times 14 times 4,200 step by step so students see the squared term handled before the length. Provide a filled conversion card for the fictional Ellery Point tunnel showing gallons per cubic foot and minutes per hour, and reduce the assigned set to items 1 through 8.

Extension

Have students find the tunnel diameter that would capture the entire autumn storm excess from item 6, holding the length at 4,200 feet, and then comment on whether the fictional town should build to that size or accept some releases in the largest storms.

ELL supports

Pre-teach combined sewer, overflow, capacity, and retrofit with a two-panel diagram of the fictional Ellery Point system showing dry weather flow and storm flow through the same pipe. Provide sentence frames for item 13: "One advantage of the tunnel is ___ because ___. One disadvantage is ___ because ___."