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Reading passage Grade 8 Science

Cape Verity Weighs Desalination: How Reverse Osmosis Works

A grade 8 science reading passage on how reverse osmosis turns seawater into drinking water, with the energy, brine, and cost trade-offs a fictional town must weigh.

Objectives

  1. Students will explain how reverse osmosis separates fresh water from seawater.
  2. Students will weigh the energy, brine, and cost trade-offs of desalination against water reuse.

Vocabulary

Desalination
Turning salty water, like ocean water, into fresh water people can use. It works, but it takes a lot of energy and leaves behind very salty water that must be handled carefully.
Reverse osmosis
Reverse osmosis cleans water by pushing it through a membrane with holes so tiny that salt and most other dissolved things cannot fit through. It can even turn seawater into fresh water.
Brine
The extra-salty water left over after desalination pulls fresh water out of seawater. Plants must dispose of it carefully so it does not harm sea life.

Also: membrane

Teacher overview

This informational passage explains how reverse osmosis turns salty seawater into drinking water and lays out the trade-offs a coastal town must weigh before building a desalination plant. It uses a fictional town, Cape Verity, as the frame while describing the real Carlsbad plant and Orange County's reuse system factually. Teachers can assign it to build science literacy about water supply and to practice citing textual evidence on a topic with genuine competing considerations.

Cape Verity Weighs Desalination: How Reverse Osmosis Works — Grade 8 Science
Name: ____________________________ Date: ______________

Passage

Cape Verity is a fictional coastal town that has lived through several dry years in a row. Its main reservoir is low, and rain has not refilled it. The ocean sits right at the edge of town, full of water, yet no one can drink it because it is far too salty. Town leaders have begun to study a technology that could turn that seawater into a steady supply: desalination.

Most large desalination plants use a process called reverse osmosis. The heart of the process is a thin sheet called a membrane. The membrane has openings so small that water molecules can slip through them, but salt and other dissolved solids cannot. To push the water through, the plant uses powerful pumps that press the seawater against the membrane at very high pressure. Clean water collects on the far side, and the salt is left behind.

That high pressure is the reason desalination takes so much energy. Pushing seawater through a membrane uses about 11 to 15 kilowatt-hours of electricity for every 1,000 gallons produced. Treating fresh water from a river or lake usually uses only about 1 to 2 kilowatt-hours for the same amount. More energy means higher costs and, unless the power is clean, more pollution from the electricity itself.

Energy is not the only concern. The salt that the membrane holds back does not disappear. It leaves the plant as a thick, extra-salty liquid called brine. Brine is heavier and saltier than ordinary seawater, so if a plant dumps it carelessly, it can sink and harm the plants and animals living on the seafloor near the outfall. Plants must plan how to blend and release brine so that it spreads out and does less damage.

Even with these trade-offs, desalination can work at a large scale. In California, the Carlsbad plant is the largest in the nation. It produces about 50 million gallons of drinking water each day, which is roughly 10 percent of the water used across San Diego County. Because the ocean does not shrink during a drought, a plant like Carlsbad gives a region a supply that does not depend on rain. That reliability is the main reason a dry coastal region considers building one.

Cape Verity's leaders are also looking at a different idea: reusing water the town already has. Orange County runs the world's largest system of its kind, cleaning treated wastewater until it is pure enough to refill the groundwater basin that supplies homes. That system produces about 130 million gallons a day. Reuse usually takes less energy than seawater desalination, because wastewater is far less salty than the ocean and needs less pressure to clean.

In the end, Cape Verity faces a choice with no perfect answer. Desalination promises a drought-proof supply but costs the most in energy and raises the brine problem. Reuse costs less energy but depends on how much wastewater the town collects. Science can describe what each option would cost and how each would affect the environment, yet the decision about which path to take belongs to the community.

Questions

  1. What does a reverse osmosis membrane do to seawater?
  2. According to the passage, about how much water does the Carlsbad plant produce, and roughly what share of San Diego County's water is that?
  3. How does the energy use of seawater desalination compare with treating fresh water?
  4. Why might a coastal town facing drought still hesitate to build a desalination plant?
  5. Which detail in the passage most strongly supports the idea that desalination uses a lot of energy?
  6. What is brine, and why is disposing of it a concern?
  7. How is water reuse both similar to and different from desalination?
  8. Based on the passage, why is ocean desalination described as a supply that does not depend on rain?

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: It lets water molecules pass through while holding back the salt and other dissolved solids. The passage explains the membrane's openings are too small for salt to slip through.
  2. Answer: About 50 million gallons a day, roughly 10 percent of San Diego County's water. The passage gives these figures for the Carlsbad plant.
  3. Answer: Desalination uses about 11 to 15 kilowatt-hours per 1,000 gallons, while treating fresh water uses only about 1 to 2. The passage contrasts the two directly.
  4. Answer: Because it costs the most in energy, raises electricity costs and possible pollution, and creates brine that can harm sea life. The passage names each of these trade-offs.
  5. Answer: The comparison showing desalination uses about 11 to 15 kilowatt-hours per 1,000 gallons versus 1 to 2 for fresh water. That side-by-side figure is the strongest evidence of high energy use.
  6. Answer: Brine is the thick, extra-salty leftover the membrane holds back, and disposing of it is a concern because it can sink and harm seafloor life near the outfall, as the passage describes.
  7. Answer: Both create a new supply that does not depend on local rain, but reuse cleans the town's wastewater, which is less salty and takes less energy, while desalination treats far saltier seawater.
  8. Answer: Because the ocean does not shrink during a drought, so a plant drawing from it keeps producing water even when rain is scarce, unlike a reservoir that depends on rainfall.

Differentiation

Support

Provide a labeled diagram of the reverse osmosis step (seawater in, high-pressure pump, membrane, clean water out, brine out) for students to reference while reading. Allow the literal questions to be answered with short phrases pulled straight from the passage.

Extension

Ask students to write a short recommendation to Cape Verity's leaders arguing for either desalination or reuse, citing at least two specific details from the passage as evidence for their choice.

ELL supports

Pre-teach "desalination," "membrane," "brine," and "reuse" with simple definitions and a picture for each before reading. Let students mark the paragraph where they found each answer, and accept answers given orally or as labeled sketches.