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Worksheet Grade 9 Engineering & Design

Hollowbrook Treatment Wetland: Sizing a Natural System

A grade 9 worksheet where students size a constructed treatment wetland for a fictional small community, finding area, removal efficiency, and detention time.

Objectives

  1. Students will size a constructed treatment wetland from a design flow and a loading rate.
  2. Students will calculate BOD removal efficiency and detention time for the wetland.

Vocabulary

Detention time
Detention time is how long water stays inside a tank or basin before flowing out. Water must stay long enough for each treatment step, like settling or disinfecting, to finish its job.

Also: constructed wetland, hydraulic loading rate, removal efficiency

Teacher overview

Hollowbrook is a fictional small community that wants to treat its wastewater with a constructed wetland instead of a large mechanical plant. A constructed wetland is a shallow bed of gravel and plants where microbes clean the water as it passes through slowly. This worksheet has students size that wetland and check how well it lowers the pollution load.

Hollowbrook Treatment Wetland: Sizing a Natural System — Grade 9 Engineering & Design
Name: ____________________________ Date: ______________

Student Worksheet

Use these facts about the fictional Hollowbrook treatment wetland.

  • The design flow is 60,000 gpd (gallons per day).
  • The hydraulic loading rate is 1.5 gallons per day per square foot of wetland.
  • The influent BOD is 120 mg/L. The target effluent BOD is 30 mg/L, the secondary treatment limit.
  • The gravel bed is 1.5 feet deep, and gravel holds water in about 0.4 of its volume. One cubic foot holds 7.48 gallons.

  1. What wetland surface area is needed, in square feet? (Flow ÷ loading rate.)

  2. Convert that area to acres. One acre is 43,560 square feet. Round to the nearest hundredth of an acre.

  3. How much BOD, in mg/L, must the wetland remove? (Influent − effluent.)

  4. What is the BOD removal efficiency, as a percent? (Removed ÷ influent × 100.) Round to the nearest whole percent.

  5. If the community can only clear 30,000 square feet of land, what is the largest flow, in gpd, the wetland can treat at the same loading rate?

  6. Find the wetland's detention time in days. First find the water volume: area × depth × 0.4. Convert cubic feet to gallons, then divide by the daily flow. Round to the nearest tenth of a day.

  7. In one or two sentences, explain the job the plants and microbes do in the wetland.

  8. In one or two sentences, name one trade-off of choosing a wetland over a mechanical plant.

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: 40,000 square feet.
    Area = 60,000 gpd ÷ 1.5 gpd per sq ft = 40,000 square feet.

  2. Answer: 0.92 acres.
    40,000 ÷ 43,560 = 0.918, which rounds to 0.92 acres.

  3. Answer: 90 mg/L.
    Removed BOD = 120 − 30 = 90 mg/L.

  4. Answer: 75 percent.
    Removal % = 90 ÷ 120 × 100 = 75%. Federal secondary standards also require at least 85 percent removal, so with this weak influent a permit would call for about 18 mg/L or less unless the state grants an exception for dilute wastewater.

  5. Answer: 45,000 gpd.
    Largest flow = 30,000 sq ft × 1.5 gpd per sq ft = 45,000 gpd.

  6. Answer: 3.0 days.
    Volume = 40,000 × 1.5 × 0.4 = 24,000 cubic feet. In gallons that is 24,000 × 7.48 = 179,520 gallons. Detention time = 179,520 ÷ 60,000 = 2.99, which rounds to 3.0 days.

  7. Answer: The plants supply roots and surfaces where microbes grow, and the microbes break down the organic matter that makes up the BOD, cleaning the water as it moves slowly through the gravel.

  8. Answer: Accept any reasonable trade-off, such as a wetland needing much more land than a mechanical plant, or working more slowly in cold weather, in exchange for lower energy use and simpler operation.

Differentiation

Support

Give students the area from item 1 already found, so they can focus on the removal and detention-time steps. Provide the detention-time formula broken into three labeled steps (volume, gallons, days) with one blank each.

Extension

Ask students to compare the wetland's land need to a mechanical plant that fits on 2,000 square feet, expressing the wetland footprint as a multiple of the plant's, and to explain why land cost matters to this choice.

ELL supports

Pre-teach "wetland," "gravel," "microbe," and "detention time" with a labeled cross-section showing water entering, moving through gravel and roots, and leaving cleaner. Allow items 7 and 8 to be answered with a labeled sketch plus a short phrase.