Desalination: How eco-friendly is it?

Illuminating answers to environmental questions.
Jan. 11 2011 6:58 AM

A Glass of Seawater. Hold the Salt.

How eco-friendly is desalination?

Desalination plant in Britan. Click image to expand.
A desalination plant in Britain

Climate-change scientists say that rising sea levels endanger our fresh-water supply. You've explained why conserving freshwater matters, but what if we run out of the stuff? Can we just turn to desalination?

Water is the new oil. Everyone's talking about impending water wars, and there's some science behind the doomsday predictions. Climate change will affect water supplies in complicated ways. People who get their water from melting glaciers, like those living in the shadow of the Andes or Himalayas, might see a surge in runoff in the near term. Areas that rely on seasonal monsoons will see increasing fluctuations in water supply, as rainy seasons get rainier and droughts get drier. Then there are the coastal areas. More than one-half of the world's population lives within 6 0 miles of the ocean. Their water supply will shrink as the sea rises as much as 23 inches over the next century. Seawater will contaminate coastal rivers, lakes, and groundwater, turning potable water into salty, brackish water.

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Humans have been distilling freshwater from the ocean for centuries, although early desalinators were looking for salt, with freshwater being just a byproduct. Most technological advancement, however, has happened in the last couple of decades.

Today, there are two main ways to desalinate water: With heat or with high-tech membranes. It's pretty easy to understand the basics of thermal distillation, as the heat-based method is known. Take some seawater, boil it, and then catch the salt-free vapors that rise from the pot. Most desalinators relied on this method in the 1980s, when desalination experienced its first major surge and global output tripled to 4 billion gallons per day. Thermal distillation still accounts for 43 percent of global production.

Membrane desalination is the more-modern technique. It works by forcing saltwater through a semipermeable material that blocks salt and other dissolved solids. Membrane desalination became popular in the 1990s, and now represents 56 percent of global capacity. (A third method, known as ion exchange, accounts for the final 1 percent.)

Because the plant designs are so diverse, it's impossible to say how much energy thermal distillation "typically" requires. However, according to a 2008 report sponsored by the National Research Council, membrane technology is usually more efficient

That said, thermal desalination plants can take steps to reduce their energy footprint. In Middle Eastern countries, where thermal processes became popular in the 1970s, desalination plants often piggyback off power plants. The fossil-fuel and nuclear plants use steam to power turbines. Desalinators run on the extra heat that exits the turbines. Aside from cruise ships, which use heat from their engines to extract freshwater from the sea, desalinators in the United States generally haven't been good about using waste heat.

More-efficient desalination plants can also use vacuums to decrease energy requirements. Just like your kitchen's pressure cooker drives up water's boiling temperature by applying pressure, putting seawater in a vacuum decreases its boiling point.

It takes, on average, between three and seven kilowatt-hours of energy to produce one cubic meter (around 264 gallons) of potable water from the sea using the more-efficient membrane technology. That's less than one-quarter the energy needed in 1975, but researchers think there's room for improvement. Under laboratory conditions, it's possible to use just 1.6 kilowatt-hours per cubic meter of water.

What does that mean on a population-level scale? Few climate scientists are willing to guess how much freshwater we'll lose over the next century—there's simply too much variability in terms of how far seawater will encroach on groundwater, how much snow-pack runoff will change, and other factors. But if you insist on having numbers, consider California. California uses 15.2 billion gallons of water per day. Let's say, hypothetically, that the state had to find a replacement for one-quarter of that total—a terrifying, but not altogether impossible (PDF), scenario. That means California would have to expend 23 million kilowatt-hours per day to satisfy its water needs using today's cutting-edge laboratory technology.

At prevailing emission rates, that would send nearly 15,000 tons of carbon dioxide into the atmosphere every day. That's the same as putting almost 1 million extra cars on the road, or about 3 percent of the state's current fleet.

Desalination raises some additional environmental challenges, aside from energy use. Saline water has to come from somewhere, and some critters may call that somewhere a home. If we try to get it from the ocean, we risk killing a lot of fish in the process. Power plants, which also suck in water in huge volumes, kill billions of immature fish every year, and one study estimated that a single power plant would have the same ecological impact as eliminating thousands of acres of wild habitat.

If, on the other hand, desalination efforts focus on brackish groundwater, we risk causing earthquakes, sinkholes, and land subsidence. Eighty percent of land subsidence in the U.S. is due to groundwater withdrawal. And disposing of the super-salty runoff, which would also contain some chemicals that aid in the desalination process, risks polluting what precious groundwater still remains.

To a certain extent, there's little we can do anymore to halt near-term climate change. Desalination, then, may become a necessary tool, alongside water conservation. Fortunately, if we continue to improve desalination efficiency and runoff disposal, there's enough saltwater to last a great while.

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Brian Palmer is Slate's chief explainer. He also writes How and Why and Ecologic for the Washington Post. Email him at explainerbrian@gmail.com. Follow him on Twitter.