Lesson plan Grade 8 Social Studies & Civics
Should Sable Point Build a Desalination Plant?
A grade 8 lesson where teams weigh the costs and benefits of a fictional coastal city building a desalination plant, using real capacity and energy figures.
Objectives
- Students will weigh the costs and benefits of a coastal city building a desalination plant.
- Students will use real capacity and energy figures to support a group recommendation.
- Students will compare desalination's energy cost to conventional water treatment's energy cost.
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.
- Drought
- A long stretch of drier-than-normal weather that leaves too little water for people, farms, and nature. Droughts build slowly and can last for years.
Also: cost-benefit analysis
Teacher overview
Coastal cities facing drought sometimes consider building a desalination plant, a facility that removes salt from seawater to produce drinking water, but the technology carries real tradeoffs in cost and energy use compared to other options such as conservation or water reuse. This lesson asks small groups to weigh those tradeoffs for a fictional coastal city and reach a recommendation, using real figures from an existing large-scale plant as a reference point. A common misconception is that desalination is a simple, unlimited solution to drought; the lesson is built so that the plant's real energy cost becomes part of every group's decision.
Procedure
Hook (5 min): Ask students: "If your city ran out of fresh water but sat right next to the ocean, could you just drink the ocean water?" Take a few answers, then explain that seawater must be treated through a process called desalination before it is safe to drink, and that process is not free.
Direct instruction (10 min): Introduce the real reference point: the Carlsbad, California desalination plant is the nation's largest, producing about 50 million gallons of drinking water per day, roughly 10% of San Diego County's water supply. Explain that seawater desalination through reverse osmosis uses roughly 11 to 15 kilowatt-hours of electricity per 1,000 gallons produced (about 3 to 4 per cubic meter), compared to roughly 1 to 2 kilowatt-hours per 1,000 gallons for conventional water treatment and delivery, meaning desalinated water typically costs more in energy than treating fresh water from a river or reservoir.
Small-group cost-benefit analysis (20 min): Introduce the fictional coastal city of Sable Point, in its third year of drought and considering building a desalination plant similar in scale to Carlsbad's. Divide students into small groups, each given a cost-benefit organizer with two columns: Benefits (a drought-proof water supply, no dependence on rainfall, and jobs during construction and operation) and Costs (higher energy use, higher water bills to cover energy costs, and ocean ecosystem effects at the intake and outflow). Each group ranks the three benefits and three costs from most to least significant to Sable Point specifically, then drafts a one-sentence recommendation: build the plant, do not build it, or pursue an alternative such as conservation or water reuse first.
Class discussion and vote (10 min): Each group shares its recommendation and its top-ranked reason. After hearing all groups, the class votes by show of hands on which option a majority would recommend to Sable Point's city council, and the teacher records the vote breakdown on the board.
Closure / exit ticket (5 min): Students answer one question on an index card: "What is one real number from today's lesson that most influenced your group's recommendation, and why?" Collect and scan for whether students can name a specific figure, such as the 50-million-gallon capacity or the 11-to-15-kilowatt-hour energy cost, and connect it to their reasoning.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Differentiation
Support (below grade level)
Provide the cost-benefit organizer with one benefit and one cost already ranked as an example, and allow groups to complete their recommendation by choosing one sentence from three provided options rather than writing one from scratch.
Extension (above grade level)
Ask groups to work out how Sable Point's energy costs might compare if it instead built a water reuse plant, using the fact that conventional treatment runs roughly 1 to 2 kilowatt-hours per 1,000 gallons versus desalination's 11 to 15 kilowatt-hours (advanced treatment for reuse falls between the two), and to draft a follow-up recommendation comparing the two options.
ELL supports
Pre-teach "desalination," "reverse osmosis," and "energy" with a simple diagram showing seawater entering a plant and clean water leaving it, alongside an electricity symbol representing the energy the process uses. Allow groups to present their recommendation using the cost-benefit organizer itself rather than composing new spoken sentences.
Extensions
Have students find out, using only teacher-approved sources, whether any real coastal community in their own state or a neighboring state has built or debated building a desalination plant, and compare that community's situation to Sable Point's fictional drought scenario.