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Lesson plan Grade 8 Science

Driftwood Marsh's Wastewater Choice: Treatment Plant or Wetland

A 50-minute grade 8 lesson where students calculate BOD removal efficiency for a treatment plant and a constructed wetland, then compare both to a real standard.

Objectives

  1. Students will calculate removal efficiency for two wastewater treatment options using influent and effluent BOD data.
  2. Students will compare a conventional treatment plant and a constructed wetland against a real federal effluent standard.
  3. Students will evaluate two design solutions against removal efficiency, land, and cost criteria to make a recommendation.

Vocabulary

Influent
Influent is the incoming dirty water that arrives at a treatment plant, ready to be cleaned. Operators test it to see how much work the plant has to do.
Effluent
Effluent is the cleaned water that leaves a treatment plant at the end of the process. It is tested carefully before being released to a river, lake, or the ocean.

Also: BOD, removal efficiency, constructed wetland

Teacher overview

Driftwood Marsh (a fictional community used for this lesson) needs a way to treat wastewater from a new subdivision before it reaches the nearby creek, and the town council is weighing two options: expanding its conventional aerobic treatment plant or building a constructed wetland that relies on plants, soil, and microbes instead of powered tanks. This lesson has students calculate how well each option removes organic waste, using the real removal-efficiency formula and the real federal standard for treated wastewater, then evaluate both options against cost and land-use criteria the way an engineer would. A common misconception is that a wetland's plants directly "eat" the pollution; the actual cleaning is mostly done by microorganisms living in the wetland's soil and root systems, with the plants supplying oxygen and habitat that let those microorganisms thrive.

Procedure

Hook (5 min): Ask students, "Wastewater treatment plants use bacteria to clean water. Could a marsh full of plants do the same job?" Take initial reactions without confirming or correcting them.

Direct instruction (12 min): Introduce the fictional town of Driftwood Marsh's situation: a new subdivision's wastewater needs treatment before release into the nearby creek. Present the real secondary-treatment standard (finished effluent must average 30 mg/L BOD or less, with typical raw sewage running 200 to 300 mg/L BOD) and the removal-efficiency formula: removal percent = (influent − effluent) ÷ influent × 100. Explain how both a conventional aerobic treatment plant, using bacteria in tanks, and a constructed wetland, using plants, soil microbes, and gravel beds, can lower BOD, since both rely on microorganisms breaking down organic matter in different physical settings.

Guided practice (13 min): In pairs, students calculate the removal efficiency for Option A, a conventional plant expansion with 250 mg/L influent and 28 mg/L effluent, and Option B, a constructed wetland with 250 mg/L influent and 22 mg/L effluent, then check whether each option's effluent meets the real 30 mg/L secondary-treatment standard.

  1. What is Option A's removal efficiency?
  2. What is Option B's removal efficiency?
  3. Does each option's effluent meet the real 30 mg/L secondary-treatment standard?

Independent practice (12 min): Each student completes a comparison organizer scoring Option A and Option B against three criteria the town council cares about: removal efficiency, calculated above; estimated land area needed, 2 acres for the plant expansion versus 6 acres for the wetland; and estimated annual operating cost, about $80,000 a year for the plant expansion versus about $20,000 a year for the wetland, mainly for monitoring, since the wetland needs no chemicals or powered aeration. All land and cost figures are fictional, created for this lesson.

  1. Comparing all three criteria, how do Option A and Option B trade off against each other?

Closure / exit ticket (8 min): Students write one paragraph recommending Option A or Option B to Driftwood Marsh's town council, citing at least two of the three criteria from the organizer to support their recommendation.

  1. Which option do you recommend, and why?

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: 88.8 percent.
    Option A, conventional plant expansion: removal percent = (250 − 28) ÷ 250 × 100 = 222 ÷ 250 × 100 = 88.8 percent.
  2. Answer: 91.2 percent.
    Option B, constructed wetland: removal percent = (250 − 22) ÷ 250 × 100 = 228 ÷ 250 × 100 = 91.2 percent.
  3. Answer: Both meet the standard.
    Option A's effluent (28 mg/L) and Option B's effluent (22 mg/L) both fall at or below the real secondary-treatment standard of 30 mg/L, so both options meet the standard, with Option B's result further below the limit.
  4. Answer: Option B removes more and costs less; Option A needs less land.
    Comparing the organizer's three criteria: Option B has higher removal efficiency (91.2 percent versus 88.8 percent) and a lower estimated annual operating cost, while Option A needs less land, a genuine trade-off for the town council to weigh.
  5. Answer: Answers will vary.
    A strong closure response names a specific criterion, such as removal efficiency, cost, or land area, and explains how it supports the chosen option, rather than simply stating a preference.

Differentiation

Support (below grade level)

Provide the removal-efficiency formula already set up with the influent and effluent numbers plugged in for Option A, so students complete only the subtraction, division, and multiplication steps, then repeat the full process independently for Option B.

Extension (above grade level)

Ask students to calculate what effluent BOD, in mg/L, Option A's fictional plant would need to reach 95 percent removal instead of 88.8 percent, using the same fictional 250 mg/L influent value from Option A. (0.95 = (250 − x) ÷ 250, so 250 − x = 237.5, and x = 12.5 mg/L.)

ELL supports

Pre-teach "influent," "effluent," and "removal efficiency" with a simple before-and-after diagram, dirty water entering and cleaner water leaving, labeled with the two BOD numbers before students calculate. Allow the closure paragraph to be delivered as a short spoken recommendation to a partner instead of written.

Extensions

Invite a local wastewater or stormwater professional, or use a recorded interview if available, to describe how their community's treatment system compares to the conventional-plant and constructed-wetland options students evaluated in this lesson.