Water for Schools
Menu

Lesson plan Grade 11 Science

Sable Cove's Drought Plan: Reverse Osmosis and Its Energy Cost

A grade 11 lesson pairing the real Carlsbad desalination plant with a fictional coastal town's energy-cost comparison to teach reverse osmosis and drought trade-offs.

Objectives

  1. Students will explain how reverse osmosis uses applied pressure to reverse the natural direction of osmotic flow across a semipermeable membrane.
  2. Students will compare the energy cost of desalination to conventional treatment using a stated per-1,000-gallon range.
  3. Students will evaluate desalination as one drought-adaptation strategy given its cost and energy trade-offs.

Vocabulary

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.

Also: osmosis, semipermeable membrane, concentration gradient, osmotic pressure

Teacher overview

Reverse osmosis lets a coastal community turn seawater, a supply that does not shrink during drought, into drinking water, but the process costs several times as much energy as treating fresh water from a river or reservoir. This lesson uses the real Carlsbad, California desalination plant as an anchor and then has students apply the underlying energy trade-off to Sable Cove (a fictional coastal town created for this lesson) that is weighing whether to build a desalination plant of its own. A common misconception this lesson is built to correct: students often assume reverse osmosis works like a filter that simply strains salt out, when it actually depends on applying pressure strong enough to overcome and reverse water's natural tendency to flow toward the more concentrated (saltier) side of a membrane.

Procedure

Hook (5 min): Remind students that about 97 percent of Earth's water is in the oceans, while less than 1 percent is easily accessible fresh water. Ask: if a coastal town runs low on fresh water during a drought, why can't people just drink from the ocean next door?

Direct instruction (15 min): Explain osmosis first: water naturally moves across a semipermeable membrane from the side with fewer dissolved particles toward the side with more, trying to equalize concentration. Reverse osmosis pushes water the opposite direction, from salty seawater toward fresh water, by applying mechanical pressure greater than the seawater's natural osmotic pressure, forcing water molecules through the membrane while leaving dissolved salts behind. Introduce the Carlsbad, California plant: the nation's largest desalination facility, producing about 50 million gallons per day, roughly 10 percent of San Diego County's water supply. Note that this scale of production requires a matching scale of energy input.

Guided practice (15 min): Present the fictional town of Sable Cove's situation: it needs 1,000,000 gallons per day during a drought emergency and is comparing reverse osmosis to conventional treatment of a smaller remaining fresh-water source. Work through the calculation as a class using the middle of each treatment's typical energy range (seawater reverse osmosis: 11–15 kWh per 1,000 gallons, about 3–4 kWh per cubic meter; conventional treatment: 1–2 kWh per 1,000 gallons): assuming 13 kWh per 1,000 gallons for RO and 1.5 kWh per 1,000 gallons for conventional treatment, RO would use 1,000 × 13 = 13,000 kWh/day, while conventional treatment would use 1,000 × 1.5 = 1,500 kWh/day, a difference of 11,500 kWh/day.

Independent practice (10 min): Students answer two follow-up problems on their own: (1) if Sable Cove's need grew to 1,500,000 gallons per day, how many kWh/day would reverse osmosis require at 13 kWh per 1,000 gallons? (2) What percent more energy does reverse osmosis use than conventional treatment for the original 1,000,000-gallon need?

Closure / exit ticket (5 min): Students answer one question on an index card: "Give one reason a coastal town facing drought might still choose desalination even though it costs more energy per gallon than conventional treatment."

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: 19,500 kWh/day. 1,500,000 gallons ÷ 1,000 × 13 kWh = 1,500 × 13 = 19,500 kWh/day.
  2. Answer: about 767 percent more. (13,000 − 1,500) ÷ 1,500 × 100 = 11,500 ÷ 1,500 × 100 ≈ 767%, so reverse osmosis uses roughly 767 percent more energy than conventional treatment for the same 1,000,000-gallon need.
  3. Exit ticket answer: Answers will vary. A strong answer notes that desalination draws on the ocean, a supply that does not shrink the way rivers and reservoirs do during drought, so it can keep providing water reliably even though it costs more energy per gallon than conventional treatment.

Differentiation

Support (below grade level)

Provide the energy-comparison formula already set up with the 1,000,000-gallon figure and both midpoint energy values filled in, so students only need to complete the multiplication and subtraction. Work through the Sable Cove example fully on the board before independent practice.

Extension (above grade level)

Ask students to explain, in terms of osmotic pressure, why seawater reverse osmosis needs more energy than reverse osmosis used on less-salty brackish groundwater.

ELL supports

Pre-teach "membrane," "pressure," and "concentration" using a simple diagram of a membrane with salt particles on one side, an arrow showing the natural direction of osmosis, and a second arrow (labeled with an applied-pressure symbol) showing the reversed direction. Allow the exit ticket to be answered orally or with a labeled diagram instead of full sentences.

Extensions

Have students look up how their own state or region sources its drinking water and whether desalination, reuse, or another drought strategy is under discussion locally, then write two sentences comparing that real strategy to Sable Cove's situation in this lesson.