Reading passage Grade 6 Science
Feeding the Ellersby Digesters: Sludge, Microbes, and Gas
A grade 6 reading passage about the microbes inside the sludge digesters at Ellersby, with a five-week data table and seven comprehension questions.
Objectives
- Students will cite details from an informational passage to explain how a sludge digester works.
- Students will interpret a five-week data table to compare two different causes of falling gas output.
- Students will explain why the microbes in a digester need a steady food supply and a steady temperature.
Vocabulary
- Biosolids
- Biosolids are the nutrient-rich solid material left over after sewage sludge has been treated and made safe. Many farms use treated biosolids as fertilizer for their fields.
Also: sludge, digester, microbe, methane
Teacher overview
This passage introduces anaerobic digestion at a wastewater plant through the daily routine of one operator in the town of Ellersby, and it pairs the text with a five-week table of feed, tank temperature, and gas output. Teachers can assign it as a science reading that requires students to move between prose and data, since two of the questions can only be answered by comparing rows of the table with claims made in the text. The misconception it addresses is that treatment plants kill everything living in the water. The passage answers that idea with a plant that keeps a population alive on purpose and measures its health by how much gas it produces.
Passage
Every day the town of Ellersby sends its used water to a plant at the edge of town. The plant is called the Ellersby Water Reclamation Facility. Water arrives there from sinks, showers, washing machines, and toilets. It carries bits of solid waste along with it. The first job at the plant is to let those solids settle to the bottom of a large tank. What settles is a thick gray sludge. At most plants that sludge is a problem to get rid of, while at Ellersby it is food.
The plant pumps the sludge into two tall steel tanks. Each tank is called a digester. A digester is sealed shut, and no air gets inside. Billions of tiny living things live in that dark, warm space. They are called microbes. These microbes cannot use oxygen. Air would kill many of them. They eat the solids in the sludge and break them into simpler substances.
As the microbes eat, they give off gas. Most of that gas is methane. Methane is the same gas that burns on a kitchen stove. A pipe at the top of each digester catches it. The plant burns the gas in an engine. The engine turns a generator, and the generator makes electricity. Waste heat from the engine flows back into the tanks and keeps them warm. On a good week the digesters make enough power to run much of the plant.
Rosa Delgado has run the Ellersby digesters for nine years. She checks two numbers every morning. The first is how much sludge went into the tank the day before. The second is how warm the tank is. She writes both in a log book before she does anything else.
"The microbes are a herd," Rosa says. "I have to feed them about the same amount every single day. If I skip a day, they get thin. If I let the tank go cold, they slow down and stop eating."
Last spring the plant kept a careful record for five weeks. The numbers below come from Rosa's log book.
| Week | Sludge fed each day (gallons) | Tank temperature (degrees F) | Gas made each day (cubic feet) |
|---|---|---|---|
| 1 | 8,000 | 98 | 24,000 |
| 2 | 8,000 | 98 | 24,500 |
| 3 | 4,000 | 98 | 13,000 |
| 4 | 8,000 | 84 | 11,000 |
| 5 | 8,000 | 98 | 23,800 |
Week 3 was a planned test. The crew cut the feed in half to see what would happen. Week 4 was not planned at all. A heater failed on a Sunday night, and the tank cooled off before anyone noticed. Rosa says the two weeks looked about the same on the gas meter but were nothing alike inside the tank. In week 3 the microbes were healthy and simply had less to eat. In week 4 they had plenty of food and were too cold to eat it.
When the microbes finish, what is left is called biosolids. Biosolids are much smaller in volume than the sludge that went in, and they no longer smell the way raw sludge does. Farms near Ellersby take the biosolids and spread them on hay fields. The material adds nitrogen and other nutrients to the soil.
Rosa says the part people find hardest to believe is that her job is mostly about keeping something alive. The pumps and the pipes matter, yet the tank only works when the herd inside it is fed and warm.
Questions
- According to the passage, what do the microbes inside the Ellersby digesters use as food?
- What does the plant do with the gas that the digesters make?
- In week 3 the crew fed the digester 4,000 gallons a day instead of 8,000. What happened to the gas the digester made that week?
- In week 4 the feed stayed at 8,000 gallons a day, but the tank cooled to 84 degrees. Explain why the gas dropped even though the amount of food did not change.
- Rosa calls the microbes "a herd" that she has to feed. Use two details from the passage to explain what she means by that.
- The passage says the digester is sealed and that no air gets inside. Cite the detail from the passage that explains why that matters.
- Weeks 3 and 4 both made less gas than week 1. Explain how the cause in week 3 was different from the cause in week 4.
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: The solids in the sludge that settle out of the used water.
The passage says the settled sludge is a problem at most plants but is food at Ellersby, and later says the microbes eat the solids in the sludge. - Answer: The plant burns it in an engine to make electricity and heat.
The passage states that a pipe catches the gas, an engine burns it, a generator makes electricity, and waste heat returns to the tanks. - Answer: The gas dropped to about 13,000 cubic feet a day, roughly half of week 1.
The table shows 24,000 cubic feet in week 1 and 13,000 in week 3. Accept any answer stating that the gas fell by about half. - Answer: The cold slowed the microbes down, so they ate less.
The passage quotes Rosa saying that a cold tank makes the microbes slow down and stop eating, and the passage later says the week 4 microbes had plenty of food but were too cold to use it. - Answer: She means the microbes are a living population that needs steady daily feeding.
Look for two supporting details, such as her statement that she has to feed them the same amount every day, her warning that skipping a day makes them thin, or her closing remark that the job is mostly about keeping something alive. - Answer: The microbes cannot use oxygen, and air would kill many of them.
That sentence appears directly after the passage describes the sealed tank, and it explains why the seal is necessary rather than optional. - Answer: Week 3 was a food shortage, while week 4 was a temperature problem.
In week 3 the crew cut the feed in half on purpose and the microbes stayed healthy. In week 4 a heater failed, so the microbes had full rations but were too cold to eat them. Full credit requires naming both causes and using the table or the text as support.
Differentiation
Support
Read the first three paragraphs aloud while students follow along, then have them underline the sentence that states what the microbes eat before continuing on their own. Provide the table with weeks 3 and 4 already highlighted so the comparison in question 7 has a visual starting point.
Extension
Ask students to write the short note Rosa would leave for the night crew after the week 4 heater failure, explaining what happened, what it did to the gas output, and what the crew should check each night from now on at the Ellersby plant.
ELL supports
Pre-teach "sludge," "digester," "microbe," and "biosolids" with a labeled flow diagram running from the house to the settling tank, into the digester, and out to the hay field. Provide sentence frames for questions 5 and 7, such as "Rosa calls them a herd because ___," and "Week 3 was different from week 4 because ___."
Extensions
Have students find out whether the wastewater plant serving your community captures gas from its digesters, then compare what they learn with the practice described at the Ellersby plant. A short paragraph naming one similarity and one difference works well as an exit assignment.