Water for Schools
Menu

Reading passage Grade 7 Science

Tiny Cleaners: How Microbes Break Down Water Pollution

A grade 7 science reading passage on bioremediation, tracing how microbes clean sewage and a fictional fuel-leak site, and why PFAS resist the same living cleanup.

Objectives

  1. Students will explain how bioremediation uses living microbes to clean polluted water and soil.
  2. Students will cite passage details to support an inference about why PFAS resist microbial cleanup.

Vocabulary

Contaminant
A contaminant is anything in water that should not be there, whether it is a germ, a chemical, or too much of a natural mineral, that could make the water unsafe or unpleasant to use. Treatment plants are built specifically to find and remove contaminants before water reaches the tap.
Emerging contaminant
A pollutant that scientists have recently learned to detect or worry about, but that may not be regulated yet. PFAS, some medicines, and microplastics are examples.

Also: bioremediation, microbe, biostimulation

Teacher overview

Some of the hardest cleanup work in water treatment is done by microbes far too small to see. This passage explains bioremediation, the use of living microbes to break down pollution in water and soil. It follows the idea from a wastewater plant to a fictional fuel-leak site, then to PFAS, a group of chemicals that resist this kind of cleanup. Teachers can assign it to introduce how biology, chemistry, and engineering meet in real water problems.

Tiny Cleaners: How Microbes Break Down Water Pollution — Grade 7 Science
Name: ____________________________ Date: ______________

Passage

Some of the most useful workers in water treatment are microbes, the tiny living things you need a microscope to see. Using microbes to clean pollution is called bioremediation. Certain bacteria feed on certain pollutants. As they feed, they break the pollutants into simpler, safer substances, often carbon dioxide and water.

The clearest example is a wastewater plant. Raw sewage carries a large load of organic waste. That load is usually about 100 to 300 mg/L when measured as the oxygen the waste would use up. If that water reached a river untreated, bacteria in the river would use up the oxygen fish need. So the plant grows its own bacteria in tanks and lets them eat the waste first. Secondary treatment must bring the waste down to 30 mg/L or less. The microbes do most of that work.

Bioremediation also cleans polluted ground. Imagine an old rail yard in the fictional town of Cordera. Years ago, fuel leaked from storage tanks into the soil and groundwater. Some natural bacteria in the ground can eat that fuel, but they work slowly. Engineers speed them up by adding oxygen and nutrients, which help the bacteria grow and feed faster. Helping the microbes along this way is called biostimulation.

Not every pollutant can be cleaned this way. PFAS are human-made chemicals used in nonstick and waterproof products. People call them forever chemicals because they break down very slowly. Microbes that easily eat fuel cannot break the strong bonds in PFAS. The legal limit for two common PFAS is only 4.0 parts per trillion, an extremely small amount. Scientists are now searching for microbes or enzymes that can finally break PFAS apart. Because the science and the rules are both still new, PFAS is called an emerging contaminant.

The people who guide this work include environmental scientists, microbiologists, and treatment plant operators. They study which microbes to use and what conditions those microbes need. Their work shows that cleaning water does not always depend on machines and filters, because in some cases living organisms do the work instead.

Questions

  1. What is bioremediation, in your own words?
  2. According to the passage, what do certain bacteria break pollutants into as they feed?
  3. What waste level must secondary treatment bring the water down to? Cite the number from the passage.
  4. Why do engineers add oxygen and nutrients at the fictional Cordera site?
  5. The passage says PFAS break down very slowly. Explain why that makes cleaning PFAS harder than cleaning the fuel leak.
  6. Using the passage, give one advantage and one limit of using microbes to clean water.
  7. The passage calls PFAS an emerging contaminant. Why does that label fit PFAS but not the organic waste in sewage?

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: Using living microbes to break down pollution.
    The passage defines bioremediation as using microbes to clean pollution in water and soil.
  2. Answer: Simpler, safer substances such as carbon dioxide and water.
    The first paragraph states the bacteria break pollutants into simpler substances, often carbon dioxide and water.
  3. Answer: 30 mg/L or less.
    The passage says secondary treatment must bring the waste down to 30 mg/L or less, from a raw load of about 100 to 300 mg/L.
  4. Answer: To help the natural bacteria grow and feed faster.
    The passage explains that adding oxygen and nutrients speeds up the bacteria that eat the leaked fuel, a step called biostimulation.
  5. Answer: Microbes cannot break the strong bonds in PFAS.
    The passage says microbes that easily eat fuel cannot break PFAS apart, so PFAS stay in the water while fuel can be broken down. Accept answers that cite the slow breakdown or the strong bonds.
  6. Answer: Advantage: microbes can break some pollutants into safe substances. Limit: they cannot break down PFAS.
    Accept any advantage and limit drawn from the passage, such as cleanup done by living organisms instead of machines and filters as the advantage and slow or impossible breakdown of certain chemicals as the limit.
  7. Answer: Because the science and the rules about PFAS are both still new.
    The passage says PFAS is called emerging because the science and the rules are still new, while sewage waste is a long-understood pollutant that microbes already handle.

Differentiation

Support

Provide a labeled diagram showing bacteria in a tank eating waste and turning it into carbon dioxide and water, and let students answer questions 1 through 3 orally before writing. Highlight the sentence that answers each literal question.

Extension

Ask students to explain, using only the passage, why a plant would keep testing its water for PFAS even after bacteria have removed the sewage waste, and to cite the detail about PFAS that supports their reasoning.

ELL supports

Pre-teach "microbe," "pollutant," "bioremediation," and "emerging contaminant" with simple definitions and images. Allow literal questions to be answered with a short phrase or by pointing to the sentence in the passage that gives the answer.