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Reading passage Grade 7 Science

From Sludge to Fertilizer: Kenniston Recovers Its Phosphorus

An informational passage about a fictional treatment plant that pulls phosphorus out of wastewater, turns it into fertilizer pellets, and protects a river's oxygen.

Objectives

  1. Students will cite evidence from the passage about how phosphorus affects a river.
  2. Students will explain how a treatment plant removes and recovers phosphorus.
  3. Students will describe why one element can be both a pollutant and a resource.

Vocabulary

Dissolved oxygen
Dissolved oxygen is the oxygen gas mixed into water that fish and other underwater animals breathe. Cold, moving water holds more oxygen than warm, still water.

Also: phosphorus, nutrient, struvite, sludge

Teacher overview

This passage follows phosphorus through a fictional wastewater plant, from the sewer pipe to a bag of fertilizer pellets. It gives students a clear case of one element acting as a pollutant in a river and as a resource on a farm. Teachers can assign it before or after a unit on nutrients and algal blooms, and the questions ask students to cite stated details and to draw inferences across paragraphs.

From Sludge to Fertilizer: Kenniston Recovers Its Phosphorus — Grade 7 Science
Name: ____________________________ Date: ______________

Passage

Dana Ruiz starts her shift at six in the morning. She works at the water resource recovery facility in the town of Kenniston, on the bank of the Vessy River. Everything that goes down a drain in town arrives here. Food scraps, human waste, and some cleaning products all carry an element called phosphorus.

Phosphorus is a nutrient. Nutrients help plants grow, and phosphorus helps build the cells of every living thing. Farmers spread it on fields on purpose. In a river, though, extra phosphorus feeds algae. The algae spread across the surface in thick green mats. Then they die, and bacteria break them down. Those bacteria use up oxygen while they work. Most fish need dissolved oxygen above 5 mg/L to stay healthy. When a bloom dies, the oxygen can drop below that line, and fish die with it.

That is why the state gives Kenniston a permit with limits in it. Federal rules already require secondary treatment, which holds monthly average BOD and solids to 30 mg/L or less. Kenniston's permit adds a limit of 1.0 mg/L for phosphorus in the water it returns to the Vessy River. The wastewater arriving at the plant carries about 6 mg/L of phosphorus. The water leaving the plant carries about 0.8 mg/L. That means the plant removes roughly 87 percent of the phosphorus it receives.

Two methods do that work. In one tank, the plant grows bacteria that pack phosphorus into their own cells. Operators move those bacteria between tanks with oxygen and tanks without it, which makes them store far more than they need. In another step, the plant adds a metal salt that makes phosphorus clump and settle. Either way, the phosphorus leaves the water and ends up in the thick mixture at the bottom, which operators call sludge.

For years, that was the end of the story. The sludge went to a landfill or onto a field, and the phosphorus went with it. Then Kenniston added a recovery reactor. Inside it, phosphorus joins with magnesium and ammonium and forms small hard crystals. The mineral is called struvite. Workers dry the crystals into pellets that look like coarse gray sand. A nearby farm buys them as slow-release fertilizer.

Dana likes this part of the plant for a second reason. Struvite used to form on its own inside pipes and pumps, in places nobody wanted it. The crusty buildup narrowed pipes and wore out equipment. Crews had to cut sections out and replace them. Now the plant grows the crystals on purpose in one tank, so less of the mineral forms in the pipes.

There is a larger reason to recover phosphorus. The phosphorus in fertilizer comes mostly from mined rock. Mines do not refill, and the rock is not spread evenly around the world. Every pellet of struvite is phosphorus that does not have to be dug out of the ground.

Dana checks the reactor twice a shift and logs the numbers. She describes the work as deciding where the town's phosphorus ends up. Sent down the river it feeds algae and pulls oxygen away from fish, while packed into a pellet it feeds a farm field that would otherwise buy mined phosphate.

Questions

  1. According to the passage, what happens to the oxygen in a river after an algal bloom dies?
  2. What limit does Kenniston's permit set for phosphorus in the water returned to the river?
  3. The plant takes in water with about 6 mg/L of phosphorus and sends out water with about 0.8 mg/L. Does that meet the permit? Explain how you know.
  4. The passage describes struvite as both a problem and a product. Explain how it can be both.
  5. The passage calls phosphorus a nutrient. Using the passage, explain what a nutrient does.
  6. Explain why the same element can be called a pollutant in the Vessy River and a resource on a farm field.
  7. Name two reasons Kenniston recovers phosphorus instead of only removing it from 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
  1. Answer: The oxygen drops as bacteria break down the dead algae.
    The passage states that bacteria use up oxygen while they decompose the bloom, and that most fish need dissolved oxygen above 5 mg/L, so the drop can kill fish.
  2. Answer: 1.0 mg/L of phosphorus.
    The passage states that the state permit adds a phosphorus limit of 1.0 mg/L for water returned to the Vessy River.
  3. Answer: Yes, the plant meets the permit.
    The water leaving carries about 0.8 mg/L, which is below the 1.0 mg/L limit named in the passage.
  4. Answer: It clogs pipes when it forms by itself and becomes fertilizer when it is grown on purpose.
    The passage explains that struvite once built up inside pipes and pumps and had to be cut out, while the recovery reactor now forms the same mineral in one tank, where it is dried into pellets.
  5. Answer: A nutrient helps living things grow.
    The passage says nutrients help plants grow and that phosphorus helps build the cells of every living thing, which is why farmers spread it on fields.
  6. Answer: The same element helps or harms depending on where it ends up.
    In the river it feeds algae, whose decay pulls oxygen out of the water and kills fish. On a field it feeds crops, which is the reason a farm buys the pellets.
  7. Answer: It keeps struvite out of the pipes and it replaces mined phosphate rock.
    The passage says the crystals used to build up in pipes and pumps until crews replaced them, and that phosphorus for fertilizer is otherwise mined from rock that does not refill. Accept the sale of pellets to a nearby farm as a third reason.

Differentiation

Support

Read the third paragraph aloud with students and mark the three numbers in the margin: the incoming concentration, the outgoing concentration, and the permit limit for the town of Kenniston. Provide a two-column chart labeled "in the river" and "on a field" for students to fill in before item 6.

Extension

Have students write a short paragraph from the point of view of a farmer near Kenniston, explaining why the struvite pellets are worth buying and what questions the farmer would ask the plant first.

ELL supports

Preteach nutrient, sludge, and crystal with images, and supply a labeled flow diagram of the Kenniston plant showing water in, sludge, reactor, and pellets. Allow students to answer items 4 and 6 orally or with a labeled drawing.

Extensions

Students can compare this passage with their own community's wastewater plant page or annual report, then list which steps described at Kenniston appear there and which do not.