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Reading passage Grade 10 Science

How the Elmscott Plant Turns Sewage into Water and Energy

A grade 10 science passage on wastewater treatment, tracing the stages a fictional reclamation plant uses to clean sewage and recover energy from sludge.

Objectives

  1. Students will sequence the stages a reclamation plant uses to clean wastewater.
  2. Students will explain how removing organic matter protects dissolved oxygen in the receiving river.
  3. Students will describe how the plant recovers energy from sludge using microbes.

Vocabulary

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.
Activated sludge
Activated sludge is a living brown mixture of helpful bacteria and other microbes that wastewater plants grow on purpose. The microbes clean the water by eating the waste in it.
Biochemical oxygen demand (BOD)
Biochemical oxygen demand, or BOD, measures how much oxygen tiny microbes will use up as they eat the waste in water. Dirtier water has a higher BOD.

Also: anaerobic digestion, biogas

Teacher overview

This passage follows wastewater through a treatment plant, stage by stage, and then shows how the plant turns leftover sludge into energy. It gives students an original, factual account of how a reclamation facility protects a river and recovers power from waste. Teachers can assign it to introduce wastewater treatment or to practice reading a technical process in order.

How the Elmscott Plant Turns Sewage into Water and Energy — Grade 10 Science
Name: ____________________________ Date: ______________

Passage

Every time someone flushes a toilet or drains a sink, the used water flows into a sewer and heads to a treatment plant. The fictional Elmscott Water Reclamation Facility sits at the low end of its town, where gravity carries the sewage. Its job is to clean that water well enough to return it to the river, and to do it every hour of every day.

The water arrives loaded with organic matter: food scraps, human waste, soap, and grease. Scientists measure that load as biochemical oxygen demand, or BOD. BOD tells you how much oxygen bacteria would use as they break the organic matter down. Raw sewage from homes usually carries a BOD of about 100 to 300 mg/L, with medium-strength sewage near 200 mg/L. If that water reached the river untreated, bacteria there would bloom and pull oxygen out of the water, and fish could suffocate. The plant's task is to remove most of that load before the water leaves.

Treatment happens in a set order, and each stage handles one part of the problem. First comes screening. Bar screens catch rags, wipes, and trash so they do not clog the pumps ahead. Next is grit removal, where sand and coffee grounds settle out in a tank built to slow the flow just enough. These first steps protect the equipment and take out the heavy, gritty material.

The water then moves to primary settling. In large, calm tanks, solids sink to the bottom as sludge and grease floats to the top, where a skimmer removes it. Primary settling takes out a good share of the solids, but plenty of dissolved organic matter still remains in the water. That is the job of the next stage.

Secondary treatment is where living microbes do the heavy work. The water flows into aeration tanks, where blowers push air up through it. Billions of bacteria feed on the dissolved organic matter, using the oxygen from the air to break it down, much as they would in the river, but here under control. This mix of water and hungry microbes is called activated sludge. After the aeration tanks, the water passes to a clarifier, a second settling tank where the microbes clump together and sink. Most of them are pumped back to the aeration tanks to keep the process going.

By the end of secondary treatment, the plant has removed at least 85 percent, and usually more than 90 percent, of the original organic load. The treated water must meet a monthly-average standard of 30 mg/L or less for BOD before it leaves. The aeration step is the key to reaching that number. If the blowers fail or the tanks are overloaded with too much sewage, the bacteria run short of oxygen and cannot finish the job, and the water leaving the plant would carry too much BOD.

Before the clean water returns to the river, it is disinfected. The Elmscott plant shines ultraviolet light through the water to kill remaining bacteria and viruses, so the discharge does not spread disease. Only then does the water flow out to the river.

The plant does not throw away the sludge it collected. Instead, it feeds that sludge into large sealed tanks called anaerobic digesters. Inside, with no air, a different set of microbes breaks the sludge down and gives off a gas rich in methane, called biogas. The plant burns the biogas to make electricity and heat, which run the plant's own equipment and warm the digesters. Using living microbes to clean water in one stage and to make energy in another is an example of environmental biotechnology, and it lets the plant recover value from what began as waste.

Questions

  1. Name the first two treatment stages, in order, that remove large debris and grit before biological treatment.
  2. In secondary treatment, what do the microbes in the aeration tank do?
  3. What standard for BOD must the treated water meet before it leaves the plant?
  4. The passage says raw sewage carries a BOD of about 100 to 300 mg/L, but the treated water must be 30 mg/L or less. Explain why removing this organic matter protects fish in the river.
  5. Why would a failure of the blowers or an overloaded aeration tank cause a problem for the treated water?
  6. How does the plant turn the sludge it collects into a resource rather than a waste?
  7. Using the term "environmental biotechnology," explain how the plant uses living microbes in two different stages.
  8. Why does the plant disinfect the water with ultraviolet light before returning it to the river?

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: Screening and then grit removal. The passage lists bar screens first to catch trash, then grit removal to settle out sand and heavy material.
  2. Answer: They feed on the dissolved organic matter and use oxygen from the pumped-in air to break it down. The passage calls this mix of water and microbes activated sludge.
  3. Answer: 30 mg/L or less for BOD. The passage states the treated water must meet this standard before it leaves the plant.
  4. Answer: The organic matter is food for bacteria. If it reached the river, bacteria there would multiply and use up dissolved oxygen as they broke it down, and fish could suffocate. Removing most of the load before discharge keeps the river's oxygen high enough for aquatic life.
  5. Answer: The bacteria in the aeration tank need oxygen to break down the organic matter. If the blowers fail or the tanks are overloaded, the microbes run short of oxygen and cannot finish the job, so the water leaving the plant would carry too much BOD.
  6. Answer: The plant feeds the sludge into sealed anaerobic digesters, where microbes break it down without air and give off biogas rich in methane. The plant burns that biogas to make electricity and heat.
  7. Answer: In secondary treatment, living bacteria (activated sludge) eat the dissolved organic matter to clean the water. In the digesters, a different set of microbes breaks down the sludge to make biogas for energy. Using microbes for both is environmental biotechnology.
  8. Answer: Ultraviolet light kills remaining bacteria and viruses in the water, so the discharge does not spread disease when it enters the river.

Differentiation

Support

Provide a simple flow diagram of the stages (screening, grit removal, primary settling, aeration, clarifier, disinfection, discharge; with sludge branching to the digesters) for students to label while they read, and allow the literal questions to be answered as short phrases.

Extension

Ask students to explain, using only the passage, why returning the settled microbes from the clarifier back to the aeration tanks helps the plant keep up with a steady flow of new sewage.

ELL supports

Pre-teach "sewage," "organic matter," "aeration," and "sludge" with simple labeled pictures in English and, where possible, students' home languages. Allow students to answer the literal questions by pointing to the correct stage on the flow diagram before writing.