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Worksheet Grade 7 Science

Sludge Microbes at Wickford Water Reclamation: Grade 7 Worksheet

A grade 7 science worksheet on the microbes that clean wastewater: students reason about food and oxygen supply and compute the percent of BOD an aeration tank removes.

Objectives

  1. Students will explain what the microbes in an aeration tank use for food and why.
  2. Students will predict how changing food or oxygen affects the microbe population.
  3. Students will compute the percent of BOD removed by the treatment step.

Vocabulary

Aeration
Aeration means mixing air into water. At a wastewater plant, blowers bubble huge amounts of air through tanks so the helpful microbes have the oxygen they need to eat the waste.
Wastewater
Wastewater is used water from sinks, showers, toilets, and businesses that goes down the drain. It travels through sewer pipes to a treatment plant to be cleaned before returning to nature.

Also: microbe, BOD, removal efficiency

Teacher overview

Most of the cleaning at a wastewater plant is done by living microbes. In the aeration tank, billions of tiny microbes eat the organic waste in sewage, which lowers a measure called BOD (biochemical oxygen demand). This worksheet uses readings from the fictional Wickford Water Reclamation Facility, where treated water leaves the tank at 30 mg/L, to have students reason about the microbes and calculate how much waste they remove.

Sludge Microbes at Wickford Water Reclamation: Grade 7 Worksheet — Grade 7 Science
Name: ____________________________ Date: ______________

Student Worksheet

Wickford Water Reclamation is a fictional plant. Its aeration tank is full of microbes that eat organic waste. Raw sewage enters the tank with a BOD of 240 mg/L. The treated water leaving the tank has a BOD of 30 mg/L.

  1. In the aeration tank, what do the microbes use as their food?
  2. The plant blows air into the tank all day. Why do the microbes need that air?
  3. Find the percent of BOD removed. Use (incoming − outgoing) ÷ incoming × 100, with 240 mg/L incoming and 30 mg/L outgoing.
  4. Secondary treatment must bring the BOD down to 30 mg/L or less. Did Wickford's outgoing water meet that limit?
  5. Suppose the food (BOD) coming into the tank dropped by half for several days. Predict what happens to the size of the microbe population, and explain why.
  6. One night the air blowers were switched off by mistake. Predict what happens to the air-breathing microbes without oxygen.
  7. How many mg/L of BOD did the microbes remove? Show your subtraction.
  8. Raw sewage BOD usually runs between 200 and 300 mg/L. Is Wickford's incoming 240 mg/L inside that usual range?
  9. A worker says a bigger tank could hold more microbes. Explain how tank size is connected to how much waste the plant can treat.
  10. In one or two sentences, explain why the microbes are called the real workers of the plant.

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: The organic waste in the sewage.
    The microbes feed on the dissolved and floating organic material, which is measured as BOD.
  2. Answer: The microbes need the oxygen in the air to break down the waste.
    These are air-breathing (aerobic) microbes, so the blowers keep oxygen in the tank.
  3. Answer: 87.5 percent.
    Incoming minus outgoing is 240 − 30 = 210 mg/L. Then 210 ÷ 240 = 0.875, which is 87.5 percent removed.
  4. Answer: Yes.
    The outgoing water was 30 mg/L, which meets the "30 mg/L or less" limit for secondary treatment.
  5. Answer: The population would shrink.
    With less food coming in, fewer microbes can be supported, so the population would drop until the food and the population are back in balance.
  6. Answer: The air-breathing microbes would die off or go inactive.
    Oxygen is a resource these microbes need, so without the blowers the tank runs low on oxygen and the aerobic microbes cannot survive well.
  7. Answer: 210 mg/L.
    240 mg/L minus 30 mg/L equals 210 mg/L of BOD removed.
  8. Answer: Yes.
    240 mg/L falls between 200 and 300 mg/L, so it is inside the usual range for raw sewage.
  9. Answer: A bigger tank can hold more microbes and hold the water longer, so more waste can be treated.
    Accept answers noting that more tank volume means more microbes and more contact time to eat the waste.
  10. Answer: The microbes do the actual cleaning by eating the waste.
    Accept answers explaining that the microbes lower the BOD by consuming the organic pollution, which is what makes the water clean enough to release.

Differentiation

Support

Provide the subtraction for item 3 already set up (240 − 30) and a calculator for the division. Give a labeled sketch of the tank with arrows for "waste in," "air in," and "clean water out" so students can point to each part.

Extension

Ask students to find the percent removed if the incoming BOD were 300 mg/L and the outgoing stayed at 30 mg/L, then explain whether a higher incoming load makes the microbes' job easier or harder.

ELL supports

Pre-teach "microbe," "waste," and "remove" with simple pictures. Allow students to answer items 1, 2, and 4 with short phrases or by labeling a diagram rather than full sentences.