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Reading passage Grade 10 English Language Arts

Ninebark's Activated Sludge Process: A Reading Passage

A grade 10 informational passage on how a fictional plant uses microbes and aeration to clean wastewater, with a wet-weather exception and comprehension questions.

Objectives

  1. Students will identify the sequential stages of an activated sludge treatment process.
  2. Students will explain how microbes and oxygen work together to remove pollutants from wastewater.
  3. Students will make inferences about a treatment plant's design using evidence from an informational text.

Vocabulary

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.
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.
Clarifier
A clarifier is a big round or rectangular tank where water sits calmly so solids can sink to the bottom and be scraped away. Treatment plants use clarifiers at more than one stage of cleaning.
Effluent
Effluent is the cleaned water that leaves a treatment plant at the end of the process. It is tested carefully before being released to a river, lake, or the ocean.

Also: microorganism

Teacher overview

After wastewater leaves a drain, it flows to a treatment plant, where cleaning it is one of the largest biological jobs a community runs every day. This passage describes activated sludge, the process at the heart of many wastewater plants, using a fictional facility so students can follow the biology without confusing it with a real plant's exact numbers. The passage traces a multistep process and builds in a wet-weather exception, giving students a chance to read technical writing for both sequence and special cases. It works as an introduction to environmental biotechnology, where living organisms do the treatment work.

Ninebark's Activated Sludge Process: A Reading Passage — Grade 10 English Language Arts
Name: ____________________________ Date: ______________

Passage

The fictional Ninebark Water Reclamation Facility cleans the wastewater from a town of about twelve thousand people. What arrives at the plant is mostly water, but it carries dissolved and suspended waste. Engineers measure that waste in part as biochemical oxygen demand, or BOD, which is the amount of oxygen that microbes would use as they break the waste down. Raw household sewage usually carries a BOD of roughly 100 to 300 milligrams per liter, and sewage of medium strength runs near 200. Before the plant can return water to Ninebark Creek, it must lower that BOD sharply. Federal secondary treatment standards require the treated water leaving the plant to average 30 milligrams per liter or less over 30 days, and they require the plant to remove at least 85 percent of the incoming load.

Treatment happens in a set order, and each stage prepares the water for the next. First, the water passes through screens and a grit chamber. Screens catch rags, plastic, and other large solids, while the grit chamber slows the flow just enough for sand and coffee grounds to settle out. Removing grit early protects the pumps and pipes downstream from wear. Second, the water flows into primary clarifiers, which are large, quiet tanks. Here the flow slows so much that heavier solids sink to the bottom as sludge, and grease floats to the top, where a skimmer removes it. Primary treatment is purely physical: nothing is alive yet doing the work. It removes a share of the solids but leaves most of the dissolved waste behind.

The biological stage is where activated sludge earns its name. The water enters long aeration tanks, where blowers push air up through the liquid in a steady stream of bubbles. That air feeds a dense community of microorganisms, mostly bacteria, that live in the tank and feed on the dissolved waste. Given enough oxygen, these microbes consume the organic pollutants and multiply, clumping together into small brown flakes called floc. The mixture of water and living floc is the activated sludge itself. The longer the microbes work, and the more oxygen they receive, the more waste they remove, so operators watch the air supply closely. Too little air starves the microbes and treatment falls off; far more air than needed wastes electricity, which is one of a plant's largest costs.

From the aeration tanks, the mixture flows into secondary clarifiers. These tanks are quiet again, like the primary ones, so the floc can settle. The clumps of microbes sink to the bottom, leaving clearer water above. This step is why the microbes were encouraged to clump: floc that settles can be separated from the water, while microbes that stayed loose would flow out with it. Operators return most of the settled sludge to the aeration tanks, seeding them with a fresh, hungry population for the next batch of incoming water. The rest is removed as waste sludge and handled separately. The clear water on top, now called effluent, moves on to disinfection before it is released to the creek.

The process is tuned for ordinary conditions, but it is not built to handle every flow the same way. Heavy rain can flood a collection system with stormwater, sending far more water to the plant than the aeration tanks can properly treat. Ninebark's engineers planned for this. When incoming flow rises above the plant's rated capacity, a portion of the excess is routed to a large holding basin instead of forcing it through the aeration tanks too fast. Pushing water through faster than the microbes can work would wash the floc out of the tanks, sending poorly treated water to the creek and stripping the plant of the very organisms it depends on. The holding basin stores the surge until the storm passes. Then the stored water is fed back through the plant slowly, at a rate the microbes can handle. The exception protects the biology, so the plant keeps meeting its limits once normal flows return.

Questions

  1. What is biochemical oxygen demand (BOD), and why does the plant measure it?
  2. List, in order, the four main stages water passes through at Ninebark, and state what each one does.
  3. What happens at Ninebark when incoming flow rises above the plant's rated capacity, and why was this exception designed into the plant?
  4. Why are the microbes encouraged to clump into floc rather than stay loose in the water?
  5. The passage says operators watch the air supply closely. Using details from the passage, explain the two problems that come from supplying the wrong amount of air.
  6. Why does the passage call primary treatment "purely physical," and how is the biological stage different?
  7. Explain how microorganisms and oxygen work together to clean the water, based on details from the passage.

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: BOD is the amount of oxygen microbes would use to break down the waste in the water. The plant measures it to know how much pollutant load is coming in and whether the treated water is clean enough to release. The passage defines BOD in its first paragraph.
  2. Answer: (1) Screens and grit chamber remove large solids and sand; (2) primary clarifiers let heavier solids settle and grease be skimmed; (3) aeration tanks let air-fed microbes eat the dissolved waste and form floc; (4) secondary clarifiers let the floc settle, leaving clear effluent. The passage describes these stages in order.
  3. Answer: Excess flow above the plant's rated capacity is routed to a holding basin instead of being forced through the aeration tanks too fast. This exception prevents the fast flow from washing the floc out of the tanks, which would send poorly treated water to the creek and strip the plant of its microbes. The passage explains this in its final paragraph.
  4. Answer: Floc that clumps together can settle out in the secondary clarifiers and be separated from the water, while loose microbes would flow out with the effluent. The passage makes this point when describing the secondary clarifiers.
  5. Answer: Too little air starves the microbes, so treatment falls off and less waste is removed. Far more air than needed wastes electricity, one of the plant's largest costs. The passage states both problems when discussing the air supply.
  6. Answer: Primary treatment is purely physical because it only lets solids settle and grease float, with nothing alive doing the work. The biological stage is different because living microbes actively consume the dissolved waste. The passage draws this contrast across its second and third paragraphs.
  7. Answer: Answers will vary but should explain that blowers supply oxygen to a community of microbes in the aeration tanks, and that with enough oxygen the microbes consume the dissolved organic pollutants and multiply into floc, removing the waste the physical stages left behind. The passage's third paragraph supports this.

Differentiation

Support

Provide a four-box flow diagram labeled "Screens and grit," "Primary clarifier," "Aeration," and "Secondary clarifier" for students to annotate while reading, and allow question 7 to be answered with a labeled sketch plus one or two sentences.

Extension

Ask students to predict, using only the passage, what a plant operator would observe in the secondary clarifier if the aeration blowers failed for several hours, and to explain the reasoning in a short paragraph.

ELL supports

Pre-teach activated sludge, aeration, microorganism, clarifier, and effluent with simple definitions and a labeled diagram of the treatment stages before reading. Allow questions 1, 2, and 4 to be answered with short phrases rather than full sentences.