Reading passage Grade 8 English Language Arts
Port Alden's Desalination Plant: Reverse Osmosis Explained
A grade 8 informational passage about a coastal town's decision to build a desalination plant, explaining reverse osmosis using real capacity and energy figures.
Objectives
- Students will explain how a reverse osmosis membrane removes salt from seawater.
- Students will cite specific evidence from the passage to support an analysis of a real-world water-supply tradeoff.
- Students will compare desalination with water recycling using details stated in the passage.
Teacher overview
A coastal town that has run short of fresh water has an option that inland towns do not: the ocean sitting right beside it. This original passage follows a fictional town council weighing whether to build a desalination plant, and it uses the technology's real energy costs and a real, currently operating plant to ground the discussion in facts rather than speculation. The questions move from locating stated facts to comparing two real water-supply strategies described in the same passage.
Passage
Salt From the Sea
Port Alden (a fictional coastal town used for this passage) had not seen a normal rainy season in four years. The reservoir that once supplied most of its drinking water was down to a fraction of its usual level, and the town council had run out of easy answers. At their spring meeting, the town's water engineer proposed something Port Alden had never tried: pulling drinking water directly from the ocean.
The process is called desalination. The most common method today is reverse osmosis. Seawater is pushed through a membrane under high pressure. The membrane's pores are so small that water molecules can squeeze through, while dissolved salt and minerals cannot. What comes out the other side is fresh water. What does not pass through is a concentrated saltwater byproduct called brine. Brine must be returned to the ocean carefully, so it does not harm marine life near the outflow. The high pressure is the key to the whole process. Without it, seawater would not force its way through such a fine membrane at any useful rate.
The engineer pointed to a real example already operating in California: the desalination plant in Carlsbad. It is the largest in the country. It produces about 50 million gallons of drinking water every day, supplying roughly 10 percent of San Diego County's total water needs. If a plant that size could supply a tenth of one large county's needs, she argued, a much smaller version could reliably supply a town the size of Port Alden, drought or no drought.
Not every council member was convinced. One member raised the plant's biggest drawback: energy. Removing salt from seawater through reverse osmosis uses roughly 11 to 15 kilowatt-hours of energy for every 1,000 gallons treated. Treating and delivering the same amount of fresh water from a river or reservoir uses only about one to two kilowatt-hours. Desalinated water, she pointed out, would cost Port Alden noticeably more to produce than the water the town used to pull from its reservoir. That higher cost would eventually reach residents' bills.
The engineer had an answer for that concern too, though not the one some members expected. She described a different approach used in Orange County, California. A facility there called the Groundwater Replenishment System purifies wastewater to a drinking-water standard and returns it to the local water supply instead of discharging it. That system is the world's largest of its kind. It produces about 130 million gallons of purified water a day, more than the Carlsbad desalination plant. It also uses less energy per gallon. Purifying wastewater takes less pressure than purifying salty ocean water.
By the end of the meeting, the council had not made a final decision. But they had a clearer choice in front of them. They could build a desalination plant that draws on an ocean supply that will never run dry. Or they could invest instead in a water-recycling system that costs less to run, but requires convincing residents to accept purified wastewater as part of their drinking supply. Both options, the engineer reminded them, beat the alternative of simply running out of water.
Questions
According to the passage, why did Port Alden's town council begin considering desalination?
What does a reverse osmosis membrane do to seawater as it passes through, and why must the water be pushed through under high pressure?
What happens to the concentrated salty byproduct left behind after reverse osmosis, according to the passage?
Name the real desalination plant the passage uses as an example, and state how much water it produces each day.
According to the passage, how does the energy cost of desalinating seawater compare with the energy cost of treating fresh water from a river or reservoir?
Based on the passage, why did the engineer believe a much smaller desalination plant could still reliably supply a town the size of Port Alden?
Compare the Orange County Groundwater Replenishment System to a desalination plant, using at least two details stated in the passage.
Writing task. In one paragraph, explain why removing salt from seawater requires more energy than treating fresh water from a river or reservoir. Use at least two facts 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
Answer: The town had gone four years without a normal rainy season, and its reservoir had fallen to a fraction of its usual level.
This is stated directly in the passage's first paragraph.Answer: The membrane lets water molecules pass through its tiny pores while blocking dissolved salt and minerals; high pressure is required because seawater will not force its way through such a fine membrane at any useful rate without it.
Both parts of the answer come from the second paragraph.Answer: It is returned to the ocean carefully, since it must not harm marine life near the outflow.
This detail follows immediately after the passage explains what brine is.Answer: The Carlsbad, California desalination plant, which produces about 50 million gallons of drinking water per day.
This figure appears in the paragraph where the engineer cites a real example.Answer: Desalinating seawater uses roughly 11 to 15 kilowatt-hours of energy per 1,000 gallons, compared with about one to two kilowatt-hours for treating fresh water.
This comparison is the core of the council member's energy concern in the fourth paragraph.Answer: She reasoned that if a plant could supply roughly 10 percent of a large county's water needs, a much smaller version could reliably meet the needs of a single small town.
This is an inference built from the same paragraph that states the Carlsbad plant's output and share of county supply.Answer: Any two of: the Groundwater Replenishment System purifies wastewater rather than seawater; it produces about 130 million gallons a day, more than Carlsbad's desalination plant; and it uses less energy per gallon because purifying wastewater requires less pressure than purifying seawater.
All three comparison points appear in the passage's next-to-last paragraph.
Writing task exemplar. Removing salt from seawater takes more energy than treating fresh water because seawater must be forced through a very fine membrane under high pressure to separate out the dissolved salt, while fresh water from a river or reservoir already lacks that salt and needs a far less forceful process. The passage states that reverse osmosis uses roughly 11 to 15 kilowatt-hours of energy per 1,000 gallons treated, compared with only about one to two kilowatt-hours for conventional fresh-water treatment. That extra pressure is the reason desalinated water costs more to produce than water from a river or reservoir.
Differentiation
Support (below grade level)
Read the passage aloud once before independent rereading, and provide a simple two-column reference sheet listing "Desalination" and "Water Recycling" with blank space for students to jot the key numbers and tradeoffs mentioned in the passage as they read. Reduce the question set to items 1 through 5 plus the writing task as a shared class discussion.
Extension (above grade level)
Ask students to write a one-paragraph recommendation to Port Alden's council, taking a position on which option the town should pursue and defending it with at least three details from the passage, including at least one counterargument a council member might raise against their recommendation.
ELL supports
Pre-teach desalination, membrane, brine, and reverse osmosis with a simple diagram showing seawater entering a filter, salt being blocked, and fresh water coming out the other side. Allow literal questions (1 through 5) to be answered with short phrases or by underlining the supporting sentence directly in the passage.