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Lesson plan Grade 9 Science

Selby's Water Reclamation Plant: BOD Removal by Microbes

A 50-minute grade 9 lesson where students calculate biochemical oxygen demand removal at a fictional plant and link microbial treatment to a healthy river.

Objectives

  1. Students will calculate the removal efficiency of a treatment step from influent and effluent BOD.
  2. Students will use the pounds formula to find the mass of BOD removed per day.
  3. Students will explain how removing BOD protects dissolved oxygen in a receiving river.

Vocabulary

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.
Activated sludge
Activated sludge is a living brown mixture of helpful bacteria and other microbes that wastewater plants grow on purpose. The microbes clean the water by eating the waste in it.
Secondary treatment
Secondary treatment uses billions of helpful microbes that eat the dissolved waste in the water. The plant keeps the microbes supplied with air, then settles them out, leaving much cleaner water.
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: removal efficiency

Teacher overview

Raw sewage is full of organic matter that bacteria in a river would eat, using up the oxygen fish need. Secondary treatment solves this by growing helpful microbes in a controlled tank, so they consume most of that organic matter before the water is released. Engineers measure the load as biochemical oxygen demand, or BOD, and federal rules require monthly-average effluent BOD of 30 mg/L or less, with raw sewage usually running about 100 to 300 mg/L. This lesson uses the fictional town of Selby so students can calculate BOD removal and connect it to keeping river oxygen above the 5 mg/L most fish need. A common misconception is that sewage treatment works mainly by killing microbes; in fact the process relies on microbes to do the cleaning, and the plant settles most of them out at the end.

Procedure

Hook (5 min): Ask students what would happen to a stream if a town poured its untreated kitchen and bathroom waste straight into it. Guide the discussion toward the idea that bacteria would feed on the waste and use up oxygen. Introduce BOD as a way to measure how much oxygen that waste would demand.

Direct instruction with worked example (15 min): Present the two tools on the board. Removal efficiency (%) = (influent − effluent) ÷ influent × 100, and the pounds formula: pounds per day = concentration (mg/L) × flow (MGD) × 8.34. Work the fictional town of Selby's plant. Influent BOD is 240 mg/L, effluent BOD is 24 mg/L, and the plant treats 3 MGD. Show removal efficiency = (240 − 24) ÷ 240 × 100 = 90 percent, and note that 24 mg/L meets the 30 mg/L limit. Then find the BOD removed per day: (240 − 24) mg/L × 3 MGD × 8.34 = 216 × 3 × 8.34 = 5,404.32 pounds per day.

Guided practice (15 min): Hand out the printed page and work item 1 together. Then have pairs complete items 2 through 6, which run the same math for a different plant loading. Circulate and check that pairs subtract effluent from influent before dividing, and that they multiply by 8.34 only once.

Independent practice (10 min): Students complete any remaining handout items independently and compare their pounds-per-day answer with a partner.

Closure and exit ticket (5 min): Formative check. On an index card, each student answers: "A plant's influent BOD is 200 mg/L and its effluent BOD is 20 mg/L. What is the removal efficiency?" Collect and scan for the correct method, (200 − 20) ÷ 200 × 100 = 90 percent. If several students forget to divide by the influent, reteach the formula next class.

Selby's Water Reclamation Plant: BOD Removal by Microbes — Grade 9 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose a second fictional plant treats 2 MGD. Its influent BOD is 260 mg/L and its effluent BOD is 26 mg/L. Use removal efficiency (%) = (influent − effluent) ÷ influent × 100 and pounds per day = concentration × flow (MGD) × 8.34.

  1. Find the concentration of BOD removed, in mg/L (influent minus effluent).
  2. Find the removal efficiency as a percent.
  3. Does the 26 mg/L effluent meet the 30 mg/L limit? State whether it is above or below, and by how much.
  4. Find the pounds of BOD removed per day, using the removed concentration from item 1, the 2 MGD flow, and 8.34.
  5. Raw sewage BOD usually runs about 100 to 300 mg/L. Is this plant's 260 mg/L influent within that range?
  6. The river below the plant needs dissolved oxygen above 5 mg/L for fish. Explain how removing most of the BOD before discharge helps keep the river's oxygen above that level.

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: 234 mg/L. 260 − 26 = 234 milligrams per liter removed.
  2. Answer: 90 percent. 234 ÷ 260 × 100 = 90 percent.
  3. Answer: Below the limit, by 4 mg/L. 26 mg/L is 30 − 26 = 4 milligrams per liter below the 30 mg/L limit.
  4. Answer: 3,903.12 pounds per day. 234 mg/L × 2 MGD × 8.34 = 468 × 8.34 = 3,903.12 pounds per day.
  5. Answer: Yes. 260 mg/L falls within the usual 100 to 300 mg/L range for raw sewage.
  6. Answer: Removing most of the BOD means far less organic matter reaches the river, so the bacteria there use up much less oxygen. That leaves the dissolved oxygen higher, helping it stay above the 5 mg/L that fish need.

Differentiation

Support

Provide the pounds formula and removal-efficiency formula on a reference card with one fully worked example, and give students the subtraction step (influent minus effluent) already completed for item 1.

Extension

Have students refine the design: find what effluent BOD the second plant would need in order to reach 95 percent removal, and explain one change to the process (such as longer aeration) that could push removal that high.

ELL supports

Pre-teach "influent," "effluent," "biochemical oxygen demand," and "removal efficiency" with a labeled diagram showing dirty water in, microbes at work, and cleaner water out. Provide a sentence frame: "The plant removed ___ percent of the BOD, so the river keeps ___ oxygen."