Reading passage Grade 4 Science
The Tiny Cleaners at Cedarbrook's Wastewater Plant
A grade 4 field-trip narrative where a class learns how billions of bacteria clean wastewater at a fictional plant.
Objectives
- Students will identify the sequence of treatment steps water passes through at a wastewater plant, from screening to bacteria treatment.
- Students will explain why plant workers add air to the tank where bacteria clean the water.
- Students will make an inference about why a treatment plant needs so many bacteria working together.
Vocabulary
Key words: bacteria, settling tank, oxygen, disinfect
Teacher overview
This original passage follows a fourth-grade class on a field trip to the fictional Cedarbrook Wastewater Treatment Plant, where an operator explains what happens to water after it swirls down a drain. Many students picture used water simply disappearing, so the passage walks through each real treatment step, screening, settling, and biological treatment, ending with the surprising idea that living bacteria do much of the cleaning work. The comprehension questions move from locating stated details to drawing inferences, matching the two halves of RI.4.1, and a short research task extends the passage into new territory for W.4.7.
Passage
The Tiny Cleaners
Our class visited the Cedarbrook Wastewater Plant in October. Mr. Alvarez is one of the plant workers. He met us at the gate. He asked a question first. "Where does the water from your sink go?" Most of us guessed it just disappeared. Mr. Alvarez smiled. He said the water takes a long trip before it can return to the river.
He walked us to the first stop. This room had a large metal screen. Water from homes and stores flows through this screen first, he said. The screen catches big things, like wipes and bits of trash. This keeps the pipes and tanks ahead from clogging. Next, he showed us a wide, calm tank called a settling tank. In this tank, the water moves slowly. Heavy grit and sand sink to the bottom, and the clearer water on top moves ahead.
Even after the screen and the settling tank, the water still is not clean, Mr. Alvarez said. Tiny bits of waste too small to see are still floating in it. That is where the tiny cleaners come in.
He led us to a tank that looked like it was boiling, even though no one had turned on a stove. Bubbles rose everywhere. "This tank is full of living things," he said, "so small that one drop of this water holds millions of them." These living things are bacteria. They are not the kind that make people sick. This kind of bacteria eats the leftover waste as its food. Billions of these tiny cleaners eat every day, and the water gets clearer and cleaner.
Mr. Alvarez told us why workers add air to this tank. The bubbles were not just for show. Bacteria need oxygen to do their work well, the same way people need to breathe. Without enough air, the tiny cleaners would slow down, and the water would not get as clean.
After the bacteria finish their work, the plant lets the water settle once more so the bacteria sink out of it. Then one more step disinfects the water before it can safely return to the river.
On the bus ride back, one classmate said the trip changed how she thinks about her sink at home. Now she knows that the water going down her drain travels to a plant where tiny cleaners help make it clean again.
Questions
What is the first thing that happens to water when it reaches the plant?
What does the settling tank do to the water passing through it?
What are the "tiny cleaners" in the passage, and what job do they do?
Why do plant workers add air to the tank full of bacteria?
Why couldn't the plant go straight from the metal screen to the river?
A single drop of water in the bacteria tank holds millions of tiny living things. Why might the plant need so many bacteria at once?
Research task. Wastewater plants use one more step after the bacteria tank, called disinfection. The passage only mentions it briefly. Research this step using a teacher-provided source. Write two or three sentences explaining what disinfection does and why a plant still needs it after bacteria have cleaned the water.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Answer: It flows through a large metal screen.
The passage states that water from homes and businesses flows through the screen first, which catches big objects like wipes and trash.Answer: It lets heavy grit and sand sink to the bottom while clearer water moves ahead.
Stated directly in the passage's description of the settling tank, where the water moves slowly enough for heavy particles to settle out.Answer: Bacteria that eat the leftover waste in the water.
The passage describes billions of tiny living things, identified as bacteria, that eat the waste still floating in the water after screening and settling.Answer: Bacteria need oxygen from the air to do their work well.
The passage compares this directly to how people need to breathe, explaining that without enough air, the bacteria would slow down and the water would not get as clean.Answer: Because tiny bits of waste too small to see were still floating in the water.
The passage states this directly right before introducing the bacteria tank, explaining why screening and settling alone were not enough.Answer: Answers will vary.
A strong answer notes that a whole town's wastewater carries a large amount of waste, so many bacteria working together are needed to clean that much water for the whole community, not just a small sample.
Research task exemplar. Disinfection is the treatment step that kills germs left in the water after the bacteria have removed most of the waste. Plants still need this step because bacteria clean the water of waste, but they do not remove every germ, so disinfection makes sure the water is safe before it returns to the river.
Differentiation
Support (below grade level)
Read the passage aloud once before students reread it independently, and provide a simple four-box sequence chart (Screen → Settling tank → Bacteria tank → Disinfection) for students to fill in before answering the questions. Reduce the question set to questions 1, 2, 3, and 4.
Extension (above grade level)
After question 6, ask students to research (using a teacher-provided source, not open internet search) what happens to the settled material, called sludge, that sinks out of the water during treatment, and to write two sentences explaining where it goes.
ELL supports
Pre-teach bacteria, settling tank, oxygen, and disinfect with a simple diagram showing the plant's steps in order, each labeled with a small icon. Allow students to answer questions 1 through 4 with short phrases pointed to directly in the text rather than full sentences.