Getting fresh with the West’s groundwater

  That shot of hot air coming from the bottom of the refrigerator may soon serve a greater purpose than just warming your feet. A new saltwater distillation technique that uses solar energy and waste heat from appliances could provide remote Southwestern communities with clean drinking water.

Researchers Nirmala Khandan and Veera Gude of New Mexico State University in Las Cruces have created a desalination system that could provide almost a million gallons of fresh water annually – enough to supply 25 homes with up to 100 gallons of water each per day.

Salty water has long plagued states like New Mexico, Arizona and Nevada, where high levels of naturally occurring minerals, as well as industrial waste and agricultural runoff, have made most groundwater too salty for drinking or agriculture. The U.S. Geological Survey estimates that 75 percent of New Mexico’s groundwater has salt levels above 1,000 micrograms per liter, the point at which water becomes saline.

But that briny reserve has a lot of potential, if technology can convert it to freshwater. “We live in a pretty dry area, and more people are looking to live here every year,” says Thomas Mayer, project manager for Sandia National Laboratories’ geochemical department. “We’re just plain out of (fresh)water. But desalination is relatively drought-proof.”

So far, a major challenge of desalination has been the enormous amount of energy it requires, something this new method addresses.

Traditional distillation technologies, which require boiling saltwater, take as much as 11,000 kilowatt-hours of electricity to produce 36,500 gallons of freshwater (the amount of water per house per year produced by Khandan and Gude’s project). Their prototype creates a natural vacuum, which makes water boil at lower temperatures. The new technology saves about half of the energy used by traditional methods; it’s roughly equivalent to running a household water heater.

Unlike other desalination methods, the prototype runs almost entirely on renewable energy: solar power and waste heat, like that emitted by air conditioners. Other desalination methods -- such as reverse osmosis, a method that filters water through special membranes -- use even less energy, but that efficiency is offset by their reliance on fossil fuels.

The prototype uses a more sophisticated version of a 3,000-year-old method. A vacuum sucks water from a saltwater tank into a depressurized container. This evaporation tank acts much like a glass jar left out in the sun, turning water into steam and leaving a more concentrated salt solution behind. Those salty leftovers collect in a waste bin, while the freshwater steam travels through tubes and condenses in a final tank. From there, pure water can then be pumped away to fill empty glasses in nearby communities. Because it produces water more slowly than other methods, solar desalination works best for small communities.

Regardless of the desalination method used, researchers must contend with the waste that’s left behind. “There’s salts left over, and there are not very good options for disposing of it,” says Mayer. One technique for treating waste is to evaporate leftover water in ponds, package the salts left behind and ship it to industrial waste facilities. However, doing this exposes wildlife and the public to potential environmental hazards.

Another uncertainty is the cost of building a solar desalinator. “Even if (a solar desalinator) was cheap to run, the capital cost of building it would be astronomical,” says Mayer. But Gude contends that “desalination is like it’s almost free” because the energy is free, and Khandan estimates that building one of the plants would cost about $50,000.

Meanwhile, Khandan and Gude are testing their prototype, have already applied for a patent license, and are fielding offers from water utilities and private contractors interested in their invention. They hope to have a final design available next year.

Despite some skepticism, competing researchers admire the possibilities. Mayer says, “The New Mexico prototype is a very ingenious device.”
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