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Lesson plan Grade 10 Science

Sizing Sattersley's Constructed Treatment Wetland

A 60-minute grade 10 lesson where students size a constructed wetland that strips nitrogen from treated wastewater, using loading rate and detention time.

Objectives

  1. Students will calculate nitrogen loading in pounds per day from concentration and flow.
  2. Students will size a constructed wetland using hydraulic loading rate and detention time.
  3. Students will explain how bacteria in a wetland convert nitrate into nitrogen gas.

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, total nitrogen, hydraulic loading rate, removal efficiency, denitrification

Teacher overview

A treatment plant can meet its permit for solids and oxygen demand and still send more nitrogen to a creek than the creek can absorb. One low-energy answer is a constructed wetland, a shallow basin planted with cattails and bulrush where bacteria growing on roots and sediment convert nitrate into nitrogen gas. This lesson uses the fictional Sattersley Water Reclamation Facility and its invented records to size such a wetland. A common misconception this lesson corrects is that the wetland strains nitrogen out the way a filter strains grit; almost all of the nitrogen leaves as a gas produced by bacteria, which is why the process slows sharply when the water turns cold.

Procedure

Hook (5 min): Ask students what happens to a creek when a plant discharges water that is clean enough to swim in but still rich in nitrogen. Take three answers, then name the outcome: algae grow, die, and decay, and the decay pulls oxygen out of the water.

Direct instruction (12 min): Introduce the fictional Sattersley facility and the creek it discharges to. Define total nitrogen, and give the loading formula, pounds per day = concentration in milligrams per liter x flow in million gallons per day x 8.34. Define hydraulic loading rate as flow divided by wetland surface area, and detention time as wetland volume divided by flow. Sketch a wetland cell in cross-section and mark where denitrifying bacteria live.

Worked example, teacher-modeled (8 min): Using numbers that do not appear on the handout, model one loading calculation for the fictional plant: a flow of 1.0 MGD carrying 10 mg/L of total nitrogen delivers 10 x 1.0 x 8.34 = 83.4 pounds per day. Then model an area conversion, 4 acres x 43,560 = 174,240 square feet. Say each unit aloud as you write it.

Guided practice (10 min): In pairs, students predict whether a wider, shallower wetland or a narrow, deeper one of the same volume would remove more nitrogen, and write one sentence of justification. Circulate and confirm that pairs connect removal to contact with plant roots and sediment rather than to volume alone.

Independent practice (20 min): Students complete the numbered handout below on their own.

Closure and exit ticket (5 min): On an index card, students explain in two sentences why the wetland removes less nitrogen in January than in July. Collect the cards and check for a reference to slower bacterial activity in cold water.

Sizing Sattersley's Constructed Treatment Wetland — Grade 10 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose the fictional Sattersley Water Reclamation Facility is adding a constructed wetland before its discharge point. Every figure below is invented for this scenario. Use 8.34 pounds per gallon, 43,560 square feet per acre, and 7.48 gallons per cubic foot.

  1. The plant discharges 1.5 MGD carrying a total nitrogen concentration of 18 mg/L. Calculate the nitrogen load leaving the plant, in pounds per day.
  2. After the wetland, the same 1.5 MGD flow carries a total nitrogen concentration of 8 mg/L. Calculate the nitrogen load leaving the wetland, in pounds per day.
  3. Using an influent concentration of 18 mg/L and an effluent concentration of 8 mg/L, calculate the wetland's nitrogen removal efficiency as a percent.
  4. The wetland covers 20 acres. Calculate its surface area in square feet.
  5. Calculate the hydraulic loading rate, in gallons per day per square foot, for a flow of 1,500,000 gallons per day spread across a wetland surface of 871,200 square feet.
  6. The wetland holds water to an average depth of 1.5 feet across a surface of 871,200 square feet. Calculate the volume of water it holds, in gallons.
  7. Calculate the detention time of the wetland, in days, for a wetland holding 9,774,864 gallons and passing 1,500,000 gallons per day.
  8. Explain why the wetland's nitrogen removal falls in winter even though the flow through it does not change.

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: 225.18 pounds per day. Load = 18 mg/L x 1.5 MGD x 8.34 = 225.18 pounds per day.
  2. Answer: 100.08 pounds per day. Load = 8 mg/L x 1.5 MGD x 8.34 = 100.08 pounds per day.
  3. Answer: 55.6 percent. Removal efficiency = (18 - 8) / 18 x 100 = 55.6 percent, rounded to one decimal place.
  4. Answer: 871,200 square feet. Area = 20 acres x 43,560 square feet per acre = 871,200 square feet.
  5. Answer: 1.72 gallons per day per square foot. Hydraulic loading rate = 1,500,000 / 871,200 = 1.72 gallons per day per square foot.
  6. Answer: 9,774,864 gallons. Volume = 871,200 x 1.5 = 1,306,800 cubic feet, and 1,306,800 x 7.48 = 9,774,864 gallons.
  7. Answer: 6.52 days. Detention time = 9,774,864 / 1,500,000 = 6.52 days.
  8. Answer: Nitrogen leaves the wetland mainly as nitrogen gas released by bacteria, and those bacteria work more slowly at low temperatures. Plants also stop taking up nutrients once they die back, so less nitrogen is held in new growth during the cold months.

Differentiation

Support

Give students a formula card with the loading, removal-efficiency, hydraulic-loading, and detention-time formulas, and work question 1 with them before they begin. Allow calculators and let students answer question 8 as a labeled diagram with short captions.

Extension

Ask students to determine how many acres the wetland would need if the design target were a hydraulic loading rate of one gallon per day per square foot at the same flow, and to explain what a lower loading rate buys the utility. Have them state one drawback of the larger footprint.

ELL supports

Pre-teach "wetland," "nitrogen," "loading," and "detention time" with a labeled cross-section showing water entering, spreading through plants, and leaving. Provide the sentence frame "The load is ___ pounds per day because the concentration is ___ and the flow is ___."