Lesson plan Grade 7 Social Studies & Civics
Desalination, Reuse, or Leak Repair: A Water Investment Lesson
A grade 7 civics lesson where pairs use real case studies and a fictional state's cost, time, and energy estimates to rank three water investments.
Objectives
- Students will compare the costs and benefits of desalination, water reuse, and leak-repair investments.
- Students will explain how each investment option affects different groups differently.
- Students will use case-study data to support a recommendation for a fictional region.
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.
- Indirect potable reuse
- Recycling water into drinking water with a natural stop along the way. Highly cleaned water is put into the ground or a reservoir, then drawn back out later and treated again for drinking.
- Non-revenue water
- Water that a utility treats and pumps but never gets paid for. Most of it escapes through leaks in old pipes, and some is lost to broken meters or theft.
- Infrastructure funding gap
- The difference between the money needed to fix and replace the country's water systems and the money actually being spent on them.
Teacher overview
Regions facing both water scarcity and aging pipes have to choose among competing investments. This lesson uses three real examples (a large desalination plant, a large water-reuse plant, and the nationwide need to repair aging pipes) to show students the scale of each approach, then asks them to apply that knowledge to a fictional coastal state, Palmetto Bay, whose cost, construction-time, water, and energy estimates are printed in Part B of the Student Handout. The real case studies do not give cost or construction-time figures, so every entry in the Cost and Time to build columns of the decision matrix comes from the fictional Palmetto Bay estimates rather than from the real plants. A common misconception this lesson addresses is the idea that a single "best" fix exists for every water-stressed region; the Palmetto Bay options are built so that each one comes out strongest on a different measure (desalination on water gained, reuse on cost, and leak repair on speed of first results and energy use), so students must weigh trade-offs rather than pick an obvious winner.
Procedure
Hook (5 min): Ask students to imagine their household's tap suddenly running dry for a week during a drought. Take a quick poll: would they rather their region spend money making ocean water drinkable, reusing wastewater, or fixing old leaky pipes first? Record the class's initial vote without discussion.
Direct instruction (15 min): Hand each pair the Student Handout. Walk through Part A, the real case studies. First, the Carlsbad desalination plant in California produces about 50 million gallons of drinking water per day from ocean water, supplying roughly 10 percent of San Diego County's water. Second, Orange County's Groundwater Replenishment System, the world's largest indirect potable reuse facility, purifies wastewater into drinking-quality water at 130 million gallons per day. Third, nationwide, EPA estimates that $625 billion in drinking-water infrastructure investment is needed over the next 20 years. The part of that need that current funding is not expected to cover is often called the infrastructure funding gap. Aging distribution pipes account for about two-thirds of that total need. Old pipes leak treated water into the ground before it ever reaches a customer's meter, a loss that counts as non-revenue water. Point out the energy figures in Part A: industry engineering references put seawater desalination at roughly 11-15 kWh per 1,000 gallons produced, compared with about 1-2 kWh per 1,000 gallons for conventional treatment and delivery. Purifying wastewater for reuse also uses reverse osmosis, so it takes more energy than conventional treatment but less than desalting seawater, while leak repair saves water that has already been treated. Then turn to Part B and read the Palmetto Bay situation aloud: the state expects a drought shortfall of 35 MGD within 10 years, the legislature has approved $900 million for this round of funding, and three options are on the table. Make clear that the Palmetto Bay figures are fictional planning estimates written for this lesson, which are realistic in scale but do not describe any real project.
Guided practice (10 min): As a whole class, fill in the desalination row of the decision matrix in Part C, thinking aloud. For each column, copy the figure from Part B (Option A) and then rate it Strongest, Middle, or Weakest compared with the other two options. Use Carlsbad to check whether the figures are reasonable: Option A's 40 MGD plant would be somewhat smaller than Carlsbad, which supplies roughly a tenth of a large county's water. Point out that Option A costs more than the $900 million available, so choosing it means starting a project this round that a later round must finish. Fill in the Who benefits and who pays cell together using the notes in Part B. As a check for understanding before pairs begin, ask each pair to show on fingers how they would rate leak repair's energy use (one finger for Strongest, two for Middle, three for Weakest). If fewer than about three-quarters of pairs show one finger, reread the leak-repair energy row of Part B with the class before moving on.
Independent/pair practice (15 min): In pairs, students complete the water reuse and leak repair rows of the decision matrix, using Part B for the figures. Each pair then completes Part D: a recommendation of two to three sentences that names which option Palmetto Bay should fund first, cites at least one figure from Part A or Part B, and names one trade-off, meaning something the state gives up or a group that is worse off under that choice.
Closure / exit ticket (5 min): Each student answers one question independently: "Name one group that benefits most from fixing aging pipes, and one group that might prefer a new desalination plant instead. Use a reason, not just a name." Collect responses and re-poll the class's initial hook question to see whether opinions changed.
Student Handout
Part A: Real case studies
These three examples are real. They show how large each kind of water project can be.
Desalination (Carlsbad, California). Desalination removes salt from ocean water to make drinking water. The Carlsbad plant is the largest in the nation. It produces about 50 million gallons of drinking water per day (MGD), which is roughly 10 percent of San Diego County's water supply.
Water reuse (Orange County, California). Orange County's Groundwater Replenishment System cleans treated wastewater until it meets drinking-water quality. The purified water is then added to an underground water supply, called an aquifer, and pumped up again later for drinking. This is called indirect potable reuse. It is the largest system of its kind in the world, at 130 MGD.
Aging pipes (United States). EPA estimates that $625 billion in drinking-water infrastructure investment is needed over 20 years. About two-thirds of that need is for distribution pipes, the mains that carry water under streets to homes and businesses. Old pipes leak treated water into the ground before it reaches a customer's meter. Water that is treated but never billed is called non-revenue water. The part of the needed investment that current funding will not cover is called the infrastructure funding gap.
Energy use. Industry engineering references estimate these amounts of electricity for each 1,000 gallons of water:
| Process | Electricity per 1,000 gallons |
|---|---|
| Seawater desalination | about 11-15 kWh |
| Conventional treatment and delivery of river, lake, or well water | about 1-2 kWh |
Water reuse also pushes water through reverse-osmosis filters, so it uses more energy than conventional treatment but less than desalination. Fixing leaks needs no new treatment, because the water saved has already been treated.
Part B: Palmetto Bay's three options
Palmetto Bay is a fictional coastal state. Its utilities lose about 12 of every 100 gallons they treat through leaking pipes. State planners expect that, in a severe drought within the next 10 years, the state will be short about 35 MGD of water. The legislature has approved $900 million for this round of water funding. Whichever option is chosen, its cost is repaid over time through water bills, so the table also shows how each option would change an average household's monthly bill. The state water board's planners prepared the estimates below. All figures are fictional estimates for this lesson.
| Measure | Option A: Seawater desalination plant | Option B: Water reuse plant | Option C: Leak repair program |
|---|---|---|---|
| Water gained or saved | 40 MGD of new supply | 25 MGD of new supply | 15 MGD saved once all work is done |
| Estimated cost | $1.4 billion | $600 million | $800 million, spent over 10 years |
| Time to build | 8 years before any water (4 years of permits and design, then 4 years of construction) | 5 years before any water | First savings in about 1 year; full 15 MGD after 10 years |
| Energy use | About 13 kWh per 1,000 gallons produced | About 4 kWh per 1,000 gallons produced | No new treatment energy; saves the energy already spent treating water that now leaks |
| Who pays | All water customers in the state, through a bond repaid on water bills; about $9 more per month for an average household | Customers of the coastal regional utility that builds it; about $5 more per month | Customers of every utility doing repairs; about $6 more per month |
| Other notes | Supply does not depend on rain. Built on the coast, so inland towns benefit only if new pipelines are added later. Salty brine must be piped back into the ocean under a permit. | Supply depends on a steady flow of wastewater, which shrinks when people conserve hard. Refills the aquifer that inland farms and towns pump from. Some residents are uneasy about drinking purified wastewater. | Fewer main breaks, street floods, and service outages in every city. Digging up streets slows traffic and hurts nearby shops while crews work. |
Part C: Decision matrix
For each cell, write the figure from Part B. Then rate the option against the other two as Strongest, Middle, or Weakest. In the last column, write in words who benefits most and who pays or is harmed.
| Option | Cost | Water gained or saved | Time to build | Energy use | Who benefits and who pays |
|---|---|---|---|---|---|
| A: Desalination | |||||
| B: Water reuse | |||||
| C: Leak repair |
Part D: Your recommendation
Write two to three sentences that answer the question: Which option should Palmetto Bay fund first with its $900 million? Your answer must include:
- The option you recommend.
- At least one figure from Part A or Part B that supports your choice.
- One trade-off: something the state gives up, or a group that is worse off, under your choice.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
Part C, sample completed decision matrix. Ratings compare the three Palmetto Bay options with one another.
| Option | Cost | Water gained or saved | Time to build | Energy use | Who benefits and who pays |
|---|---|---|---|---|---|
| A: Desalination | $1.4 billion; Weakest (also more than the $900 million available) | 40 MGD; Strongest | 8 years; Weakest | About 13 kWh per 1,000 gallons; Weakest | Benefits coastal cities and businesses that need a supply that does not depend on rain. All customers statewide pay about $9 more per month, including inland towns that may not receive the water. |
| B: Water reuse | $600 million; Strongest | 25 MGD; Middle | 5 years; Middle | About 4 kWh per 1,000 gallons; Middle | Benefits coastal customers and inland farms and towns that pump from the refilled aquifer. Regional utility customers pay about $5 more per month. |
| C: Leak repair | $800 million over 10 years; Middle | 15 MGD; Weakest | First savings in about 1 year; Strongest | No new treatment energy; Strongest | Benefits all current customers in every city, through less wasted water and fewer main breaks. Customers pay about $6 more per month, and drivers and shops on streets under repair bear the disruption. |
Accept Middle or Weakest for leak repair's Time to build if the pair explains that the full 15 MGD takes 10 years, which is longer than either plant takes to build. Accept other reasonable entries in the last column if they name a specific group and a reason drawn from Part B.
Part D, scoring guide. A complete recommendation names one option, cites at least one figure from the handout correctly, and states a real trade-off of that choice. Any of the three options can earn full credit. Sample responses:
- Leak repair: "Palmetto Bay should fund the leak repair program first, because it costs $800 million, fits within the $900 million budget, and starts saving water in about a year. The trade-off is that it saves only 15 MGD, which is less than half of the 35 MGD drought shortfall, so the state will still need a new supply later."
- Water reuse: "Palmetto Bay should build the reuse plant first, because at $600 million it is the cheapest option and leaves $300 million to begin fixing leaks. The trade-off is that its 25 MGD depends on steady wastewater flow, which drops when people conserve during a drought."
- Desalination: "Palmetto Bay should start the desalination plant, because it is the only option that covers the whole 35 MGD shortfall with 40 MGD and does not depend on rain. The trade-off is that it costs $1.4 billion, more than this round's $900 million, and uses about 13 kWh per 1,000 gallons, so customers statewide will pay more for both construction and electricity."
A response that names an option but cites no figure, or that lists only benefits with no trade-off, is incomplete.
Exit ticket, expected reasoning: Repairing aging pipes benefits current customers broadly, since leaks waste water that customers ultimately pay for through rates, and fixing pipes does not require new energy-intensive treatment. A new desalination plant might be preferred by coastal cities or businesses facing a drought emergency, because it is the only option that adds a large supply (40 MGD in Palmetto Bay) that does not depend on rain, even though it takes the longest of the three to deliver any water and uses more energy per gallon than any other option. Accept any answer that names a specific group (current ratepayers, drought-vulnerable communities, inland farms, coastal businesses, future residents, taxpayers funding a bond) and ties that group's preference to a reason drawn from the handout, rather than a guess with no supporting detail.
Differentiation
Support (below grade level)
Provide the decision matrix with the desalination and reuse rows already completed from the Answer Key, so students only need to fill in the leak-repair row using Part B, and provide a word bank (ratepayers, drought, energy, pipes, cost, trade-off) and a sentence frame for Part D: "Palmetto Bay should fund ___ first because ___. The trade-off is ___."
Extension (above grade level)
Ask students to propose a fourth option not covered in the case studies, such as a public conservation-rebate program, and add a row to the decision matrix rating how it might compare to the other three on each column, even without exact figures. Students then explain how the fourth option would change the best way for the fictional state to spend its $900 million, including whether combining two smaller options is better than one large one.
ELL supports
Pre-teach desalination, reuse, and aging infrastructure with simple images (ocean water turning into a glass of water; wastewater turning into a glass of water; a cracked pipe). Read the column headings of Part B aloud and pair each with a gesture or symbol (a dollar sign for cost, a clock for time, a lightning bolt for energy). Allow the exit ticket to be answered as a single sentence using a provided sentence frame: "___ benefits most from fixing pipes because ___."
Extensions
Have students find out which of the three approaches, if any, their own local water utility uses or is considering, either by checking the utility's website or writing a short letter asking the utility directly.