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Worksheet Grade 7 Science

Alder Run's Aeration Tank: Bacteria, Oxygen, and BOD Removal

A grade 7 worksheet on activated-sludge treatment biology, using real EPA effluent standards to calculate percent BOD removal at a fictional facility.

Objectives

  1. Students will explain the role of bacteria and oxygen in the activated sludge treatment process.
  2. Students will calculate the percent of BOD a treatment process removes from wastewater.
  3. Students will apply a constant removal rate to predict effluent BOD from a new raw-wastewater value.

Vocabulary

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.
Clarifier
A clarifier is a big round or rectangular tank where water sits calmly so solids can sink to the bottom and be scraped away. Treatment plants use clarifiers at more than one stage of cleaning.
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: aeration tank, BOD

Teacher overview

This worksheet uses a fictional water reclamation facility called Alder Run to walk through the biological process that treats wastewater before it returns to a stream. In the aeration tank, a controlled community of bacteria consumes the dissolved organic matter in wastewater as food, using pumped-in oxygen to break it down; the used bacteria then settle out in a clarifier before the cleared water moves on to disinfection. The worksheet applies the real federal effluent standard and typical raw-sewage range that treatment plants must work within.

Alder Run's Aeration Tank: Bacteria, Oxygen, and BOD Removal — Grade 7 Science
Name: ____________________________ Date: ______________

Student Worksheet

Use these facts about the fictional Alder Run treatment process for every problem below.

  • In the aeration tank, bacteria eat the organic matter in wastewater as food. They need oxygen pumped into the tank to break that matter down.
  • Federal rules require monthly-average effluent BOD of 30 mg/L or less. Raw, untreated sewage typically measures 200 to 300 mg/L of BOD.
  • Alder Run's raw wastewater today measures 200 mg/L of BOD. After treatment, its effluent (outgoing water) measures 30 mg/L of BOD.
  • The removal efficiency formula is: Removal % = (influent − effluent) ÷ influent × 100.
  1. In the aeration tank, what do the bacteria use as "food"? What do they need pumped in to break it down?
  2. After bacteria (activated sludge) consume the organic matter in the aeration tank, what happens to them? Use the word "clarifier" in your answer.
  3. Calculate the percent of BOD Alder Run's treatment process removes today. Use the influent and effluent values given above.
  4. Does Alder Run's effluent from problem 3 meet the federal standard? By how many mg/L does it clear the standard, if any?
  5. A heavy rainstorm dilutes the raw wastewater, so its BOD drops to 100 mg/L. The process still removes the same percent you found in problem 3. What would the new effluent BOD be?
  6. By how many mg/L would the diluted effluent from problem 5 fall below the 30 mg/L standard?
  7. A classmate claims that adding more bacteria to the aeration tank will always remove more BOD, no matter what else changes. Use what you know about oxygen and organic matter as resources to explain why that claim could be wrong.
  8. Alder Run's aeration tank supports a bacteria population that removes BOD at the rate shown in problem 3. Incoming BOD rises to 300 mg/L, with no change to the bacteria population or oxygen supply. Would you expect the removal percent to stay the same, increase, or decrease? Explain your reasoning.
  9. Using the same removal rate from problem 3, what effluent BOD would result from raw wastewater of 300 mg/L?
  10. Based on problem 9, would that treated water meet the 30 mg/L standard? By how many mg/L would it miss the standard, if at all?

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: organic matter, using pumped-in oxygen.
    The bacteria use the organic matter dissolved in the wastewater as food, and they need oxygen pumped into the aeration tank to break that organic matter down.
  2. Answer: they settle out in the clarifier.
    After consuming the organic matter, the bacteria (now called activated sludge) settle out of the water in a clarifier, a tank where gravity separates the settled sludge from the cleared water above it, which then moves on for further treatment.
  3. Answer: 85 percent.
    (200 − 30) ÷ 200 × 100 = 170 ÷ 200 × 100 = 85%.
  4. Answer: 0 mg/L.
    Alder Run's effluent of 30 mg/L meets the standard, but only exactly at the 30 mg/L limit, leaving 0 mg/L of margin to spare.
  5. Answer: 15 mg/L.
    100 mg/L × (1 − 0.85) = 100 × 0.15 = 15 mg/L of effluent BOD.
  6. Answer: 15 mg/L.
    30 mg/L − 15 mg/L = 15 mg/L below the standard.
  7. Answer: the claim could be wrong because oxygen is also a limited resource.
    Bacteria need both organic matter to eat and oxygen to break it down; if the oxygen supply pumped into the tank does not increase along with the bacteria population, the added bacteria will compete for the same limited oxygen and organic matter, so removal will not necessarily improve.
  8. Answer: decrease.
    With more incoming BOD but the same bacteria population and oxygen supply, the existing bacteria have more organic matter than they can fully process with the oxygen available, so the removal percent would likely decrease.
  9. Answer: 45 mg/L.
    300 mg/L × 0.15 = 45 mg/L of effluent BOD.
  10. Answer: 15 mg/L.
    45 mg/L − 30 mg/L = 15 mg/L over the 30 mg/L standard, so this treated water would not meet the standard.

Differentiation

Support (below grade level)

Provide a labeled diagram of the treatment process (aeration tank → clarifier → disinfection) alongside the text, and work through problem 3 as a class before students attempt problems 5, 6, 9, and 10 independently. Give students the removal-efficiency formula on a printed reference card.

Extension (above grade level)

Ask students to calculate the maximum raw-wastewater BOD Alder Run could accept and still meet the 30 mg/L standard, assuming the same 85% removal rate holds, and to explain in writing what would need to change at the facility if incoming BOD regularly exceeded that maximum.

ELL supports

Pre-teach "aeration," "clarifier," "effluent," and "BOD" with a simple flow diagram of the treatment process and icons for bacteria, bubbles (oxygen), and settling particles. Allow problems 1, 2, and 7 to be answered with labeled diagrams and short phrases rather than full sentences.