Can Desalination Save Us From Drought?

desalination

Credit: CARLSBAD

Perhaps no one captured the vision as well as John F. Kennedy.

“If we could ever competitively, at a cheap rate, get freshwater from saltwater,” he said in a 1961 press conference, “it would be in the long-range interests of humanity, which would really dwarf any other scientific accomplishments. I am hopeful that we will intensify our efforts in that area.”

Kennedy’s vision for reliable, economical water supplies was soon eclipsed by his other calls to action, most notably his Moon Speech the following autumn, but now, more than 50 years later, the world is entering a new age of water—one to be driven by desalination technology on account of great need.

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Though water is abundant on Earth, most of it is unsuitable for human use. Some 97% is saltwater in the oceans, and of the remaining freshwater, over two-thirds, is frozen in glaciers and ice caps. Groundwater and surface water—the primary sources for municipal and industrial supply—make up less than 1% of Earth’s water. The most accessible and economical freshwater sources have already been developed, especially in water-scarce areas. Climate change and growing demands necessitate novel solutions.

Enter desalination. Having advanced tremendously in recent years, desalination—the process of transforming saline water to clean, drinkable water by removing salts—offers new alternatives. It expands the set of potential sources to include seawater, brackish groundwater, and other saline or contaminated water. Rather than be limited to a sliver of Earth’s already-stressed freshwater, water suppliers can use desalination to tap larger and more reliable sources.

“Desalination is one of several tools communities can use in appropriate circumstances to gain greater water security,” says Felicia Marcus, California Water Board Chair, in a news release.

To be sure, desalination is no panacea. In most cases, it is a desperate last option. The process is energy intensive, requires special materials to handle corrosive outcomes of water chemistry, and has certain environmental impacts that must be weighed carefully. Some are concerned that desalination only encourages growth and development when wise resource management is needed most. Still, many are including desalination as part of a strategic water portfolio—a diverse set of water assets that collectively lowers risk and increases reliability.

“Desalination is typically employed when there is water scarcity,” says Ronan McGovern, a researcher at MIT’s Center for Clean Water and Clean Energy. “In such scenarios, the [alternatives] include pumping water over long distances, recycling wastewater, and increasing water conservation. There is no obvious winner as all have benefits and drawbacks. Water conservation measures can be politically difficult. Wastewater recycling is cost effective, but in periods of drought there is less wastewater to recycle. Pumping over long distances may require pipes to be laid in environmentally sensitive locations.” These constraints may leave desalination the most viable option.

“The benefits are that it creates a new source of water that is robust to drought, may be more environmentally acceptable than pumping water from afar, and may be more politically acceptable than water conservation,” says McGovern.

Almost all water has some salinity. Freshwater is defined as having a salt content, measured as total dissolved solids (TDS), of less than 0.05% (500 mg per L) by mass. Brackish water is saltier than freshwater but less salty than seawater, ranging from 0.05–3% (500–30,000 mg per L) TDS. Brackish water is found at freshwater–seawater interfaces such as estuaries and in many aquifers in the southwest United States, especially in Texas and New Mexico. Saline water ranges from 3–5% (30,000–50,000 mg per L) TDS and includes seawater with a typical salinity of 3.5% (35,000 mg per L). Water with salt concentrations over 5% (50,000 mg per L) TDS is called brine. Salinity in Utah’s Great Salt Lake, for example, ranges from 5–27% (50,000–270,000 mg per L) depending on the lake’s level. The higher the salinity, the more effort is required to desalinate.

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About 16,000 desalination plants of various sizes operate around the world. Output has tripled since 2000, and construction is accelerating while technology improves. Global capacity in 2013 was estimated at 21.1 billion gallons per day, and 300 million people in 150 countries rely on desalinated water for some or all their daily needs, according to the International Desalination Association. Saudi Arabia is the largest desalinator, accounting for 17% of global desalination output, followed by the United Arab Emirates and the United States, which each make up 13%. Most desalination in the United States is for inland brackish groundwater, though seawater desalination will be the main source of desalination growth.WE_bug_web

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