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Worksheet Grade 6 Science

Bacteria at Work: Removal Efficiency at Sable Creek Reclamation

A grade 6 worksheet where students calculate how efficiently bacteria remove waste at a fictional wastewater plant and compare results to a real federal standard.

Objectives

  1. Students will calculate removal efficiency percentages for a wastewater treatment scenario.
  2. Students will compare treatment results against a real federal effluent standard.

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: removal efficiency, biological treatment

Teacher overview

Wastewater treatment plants rely on more than pipes and tanks: in the secondary treatment stage, billions of bacteria break down the organic waste in sewage, using it as their own food source, before the water is released back into a river or stream. Federal standards require that treated water leaving a plant carry no more than 30 mg/L of organic waste as a monthly average, measured as biochemical oxygen demand (BOD), while raw sewage entering a plant typically measures 200 to 300 mg/L. This worksheet has students calculate how efficiently a fictional plant's bacteria remove that waste and compare the results with the real federal limit. A common misconception is that "treated" water is completely free of organic material; in practice, treatment removes most of it, and the removal-efficiency percentage is exactly how plant operators track how much remains.

Bacteria at Work: Removal Efficiency at Sable Creek Reclamation — Grade 6 Science
Name: ____________________________ Date: ______________

Student Worksheet

Sable Creek Water Reclamation Facility (a fictional treatment plant) uses bacteria to break down organic waste before returning cleaned water to Sable Creek. Use this formula for every item: Removal efficiency (%) = (influent BOD − effluent BOD) ÷ influent BOD × 100.

  1. In two or three sentences, explain what "biological treatment" means at a wastewater plant. What are the bacteria actually doing to the organic waste?
  2. On a normal day, Sable Creek's influent (incoming) BOD measures 200 mg/L. Its effluent (outgoing) BOD measures 20 mg/L. Calculate the removal efficiency.
  3. Does Sable Creek's normal-day effluent of 20 mg/L meet the federal standard of 30 mg/L or less? By how many mg/L does it beat the limit?
  4. One month, an equipment problem lets more waste through. Effluent BOD averages 35 mg/L. Influent BOD stays at 200 mg/L. Calculate that month's removal efficiency.
  5. Does the item 4 effluent of 35 mg/L meet the federal standard?
  6. A neighboring plant receives influent BOD of 300 mg/L, the high end of the typical range. It removes waste at the same 90% efficiency Sable Creek normally achieves. What would that plant's effluent BOD be?
  7. Would the neighboring plant's effluent from item 6 meet the federal standard of 30 mg/L or less?
  8. In two or three sentences, explain why "removal efficiency" is more useful than effluent BOD alone for comparing two treatment plants. Why does this matter most when the plants receive different influent BOD levels?
  9. Sable Creek wants to keep 90% removal efficiency even during a month when influent BOD averages 260 mg/L. What effluent BOD would that require?
  10. Would the effluent level from item 9 meet the federal standard of 30 mg/L or less? By how many mg/L?

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: Biological treatment means bacteria in the treatment tanks consume the organic waste in sewage as food, breaking it down into simpler substances and reducing the amount of waste left in the water.
  2. Answer: 90 percent.
    (200 − 20) ÷ 200 × 100 = 180 ÷ 200 × 100 = 90%.
  3. Answer: 10 mg/L.
    The standard is 30 mg/L or less; 30 − 20 = 10 mg/L below the limit, so it meets the standard with a 10 mg/L margin.
  4. Answer: 82.5 percent.
    (200 − 35) ÷ 200 × 100 = 165 ÷ 200 × 100 = 82.5%.
  5. Answer: No.
    35 mg/L is above the 30 mg/L limit, so it does not meet the federal standard.
  6. Answer: 30 mg/L.
    90% removal means 10% of the influent remains: 300 × 0.10 = 30 mg/L effluent.
  7. Answer: Yes, at the limit.
    30 mg/L exactly matches the standard of "30 mg/L or less," so it meets the standard with no margin to spare.
  8. Answer: Removal efficiency accounts for how much waste came in, not just how much remains, so two plants can be compared fairly even if one receives much dirtier water than the other; effluent BOD alone would make a plant handling weaker sewage look better without actually treating it more effectively.
  9. Answer: 26 mg/L.
    90% removal means 10% of the influent remains: 260 × 0.10 = 26 mg/L effluent.
  10. Answer: Yes, by 4 mg/L.
    26 mg/L is below the 30 mg/L limit; 30 − 26 = 4 mg/L margin.

Differentiation

Support

Provide the removal-efficiency formula broken into two labeled steps ("Step 1: subtract effluent from influent. Step 2: divide by influent and multiply by 100.") and work through item 2 together as a class model before students try items 4, 6, and 9 independently.

Extension

Ask students to find what effluent BOD Sable Creek would need to reach a stricter, higher removal efficiency of 95% at the normal 200 mg/L influent level, and to state how many fewer mg/L that would be compared with the 90% scenario in item 2.

ELL supports

Pre-teach "influent," "effluent," "removal efficiency," and "biological treatment" using arrows on a simple plant diagram: dirty water in (influent), bacteria at work in the middle tank, cleaner water out (effluent). Allow students to answer items 1 and 8 with a labeled diagram plus a short phrase instead of full sentences.