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Reading passage Grade 10 English Language Arts

Turning Salty Groundwater into Tap Water: A Reverse Osmosis Passage

A grade 10 informational passage on how a fictional inland town uses reverse osmosis to treat brackish groundwater, with comprehension and vocabulary questions.

Objectives

  1. Students will identify the sequential stages of a reverse osmosis treatment process.
  2. Students will explain why an inland desalination plant disposes of its reject stream differently than a coastal one.
  3. Students will use context clues to determine the meaning of technical vocabulary in an informational text.

Vocabulary

Key words: brackish, semipermeable membrane, permeate, concentrate, salinity

Teacher overview

Reverse osmosis is best known for turning seawater into drinking water at large coastal plants, but the same technology also treats brackish groundwater, water that is saltier than fresh water but far less salty than the ocean, found in aquifers under many inland communities. This passage describes how the fictional inland town of Prairie Bend uses reverse osmosis to treat its brackish groundwater supply, tracing the process from the well to the tap and explaining why an inland plant has to solve a problem coastal plants do not: what to do with the salty water the process leaves behind.

Turning Salty Groundwater into Tap Water: A Reverse Osmosis Passage — Grade 10 English Language Arts
Name: ____________________________ Date: ______________

Passage

Beneath the fictional town of Prairie Bend sits a deep aquifer. It has supplied the town with groundwater for generations. Unlike the fresh aquifers many towns rely on, Prairie Bend's aquifer is brackish. That means its water carries a noticeable amount of dissolved salt, too much to drink comfortably but far less than seawater. For decades, this salt content made the aquifer a backup source at best. Then Prairie Bend built a reverse osmosis plant to treat it as a full-time supply.

The treatment process begins when water is pumped up from the aquifer. It passes through pre-filters that strip out sediment, sand, and any larger particles before the water reaches the reverse osmosis membranes. This pre-filtration step matters because a membrane clogged with grit fails faster and treats less water over its lifetime. Protecting it is worth the extra step. From there, high-pressure pumps push the pre-filtered water against a semipermeable membrane. This is a thin barrier with pores small enough to let water molecules pass through, while blocking most of the dissolved salt. The water that makes it through the membrane is called the permeate. It is nearly salt-free and becomes the plant's finished drinking water after final disinfection. The salty water left behind is called the concentrate. It carries away the salt the membrane removed, now at a higher concentration than it started.

A coastal reverse osmosis plant can return its concentrate to the ocean, since the ocean is already salty and can absorb a small, carefully managed discharge. Prairie Bend cannot do this, since it sits far from the coast. Its concentrate has to go somewhere else. The town disposes of its concentrate through a deep injection well. This pumps it far below the level of the aquifer the town draws from, into a rock layer already unsuitable for drinking water. Some inland plants instead use evaporation ponds, shallow basins where the water evaporates over time and leaves the salt behind as a dry residue for disposal. Prairie Bend's engineers chose deep injection instead, because the geology beneath the town includes a suitable layer of rock at the right depth.

Prairie Bend's plant also has a written exception, for when the aquifer's salinity rises above what the membrane system is rated to treat. This has happened during past droughts, when less rain reduced the fresh water normally recharging the aquifer. When incoming salinity crosses that threshold, operators respond in one of two ways. They blend in water from a second, less salty well to dilute the feed, or they shut down membrane trains until the salinity falls, rather than risk damaging them with a feed saltier than they were designed to handle. This exception exists because a membrane damaged by an overly salty feed is far more expensive to replace than the temporary loss of treatment capacity, while operators wait for the aquifer to recover.

Reverse osmosis membranes generally use less energy to treat brackish groundwater than they do to treat seawater. Brackish water starts out far less salty, so it needs less pressure to force it through the membrane. That lower energy demand, combined with an aquifer already sitting under the town, is part of why an inland community like Prairie Bend can consider the technology at a smaller scale than a large coastal desalination plant would require.

Questions

  1. What does "brackish" mean, based on how the passage uses the word?
  2. List, in order, the main steps water passes through at Prairie Bend's treatment plant, from the aquifer to the finished drinking water.
  3. Why does the plant pre-filter the water before it reaches the reverse osmosis membranes?
  4. Explain, using details from the passage, what a semipermeable membrane does and how the permeate differs from the concentrate.
  5. What exception does Prairie Bend's plant have for when the aquifer's salinity rises too high, and why does this exception exist?
  6. According to the passage, how does Prairie Bend's disposal of its concentrate differ from a coastal plant's, and why?
  7. Based on the passage, why might reverse osmosis be a more realistic option for an inland town treating brackish groundwater than for one considering seawater desalination?

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: Water that carries a noticeable amount of dissolved salt, more than fresh water but far less than seawater.
    The passage defines the term directly in its first paragraph when describing Prairie Bend's aquifer.
  2. Answer: Pumping from the aquifer, pre-filtration, high-pressure reverse osmosis through the membrane, then final disinfection of the permeate into finished drinking water.
    The passage describes these steps in order in its second paragraph.
  3. Answer: To strip out sediment, sand, and larger particles so the membrane does not clog, since a clogged membrane fails faster and treats less water over its lifetime.
    The passage states this reason directly in its second paragraph.
  4. Answer: The semipermeable membrane has pores small enough to let water molecules through while blocking most dissolved salt. The permeate is the nearly salt-free water that passes through and becomes drinking water, while the concentrate is the salty water left behind at a higher concentration than it started.
    The passage explains both terms together in its second paragraph.
  5. Answer: When the aquifer's salinity rises above what the membrane system is rated to treat, operators blend in water from a less salty well or shut down membrane trains until salinity falls. This exception exists because a membrane damaged by an overly salty feed costs far more to replace than the temporary loss of treatment capacity.
    The passage explains this exception and its reasoning in its fourth paragraph.
  6. Answer: A coastal plant can return its concentrate to the ocean, but Prairie Bend, being far from the coast, disposes of its concentrate through a deep injection well instead.
    The passage draws this comparison directly in its third paragraph.
  7. Answer: Answers will vary, but a strong response notes that brackish groundwater needs less pressure and energy to treat than seawater, and that an inland town may already have a usable aquifer nearby, making the technology more practical at a smaller scale than a large coastal plant would need.
    The passage supports this reasoning in its final paragraph.

Differentiation

Support (below grade level)

Provide a simple flow diagram with four labeled boxes (Aquifer, Pre-filter, Membrane, Tap) for students to fill in with a short phrase from the passage as they read, and allow question 4 to be answered with a labeled diagram instead of full sentences.

Extension (above grade level)

Ask students to research, using only the passage's details, why a deep injection well is considered a safer disposal method than simply discharging concentrate into a nearby stream, and write a short paragraph explaining the risk that method would create.

ELL supports

Pre-teach "brackish," "semipermeable membrane," "permeate," "concentrate," and "salinity" with simple definitions and a labeled diagram of the reverse osmosis process before reading. Allow questions 1, 3, and 6 to be answered with short phrases rather than full sentences.