Resurrecting an Old Desalination Technology by Brett Walton

Back when Athens was the center of the world, Aristotle wrote in Meteorologica, his examination of the natural sciences, that the processes of evaporation and condensation, when applied to salt water, turned the foul liquid into sweet. “This I know by experiment,” he wrote.

Since then, those same processes have been incorporated in the solar still, a device that uses sunlight to produce freshwater. For centuries, the water-purifying apparatus has been used to supply drinking water to small communities around the world.

But now, crafty engineers both in the U.S. and abroad who recognize global constraints on land, food production and water resources are laying out bigger plans. Besides producing fresh drinking water, they envision using evaporation and condensation to resurrect the solar greenhouse, an idea that, according to Hill Kemp — the CEO of the desalination start-up Suns River — has been “tossed in the boneyard of technology” but could turn tracts of coastal desert into productive land.

Last September, his company received one of nine grants for water purification research handed out by the U.S. Bureau of Reclamation. This month, Suns River will begin testing one of its solar stills at the bureau’s Brackish Groundwater National Desalination Research Facility in Alamogordo, N.M.

Solar stills remove most contaminants from water — all salts, heavy metals, bacteria and microbes — by passing a thin film of feed water across a dark surface tilted toward the sun. As the water evaporates, it condenses into droplets on a clear plate above the black floor and rolls downward to a collection trough. A carbon filter then removes any remaining organic compounds.

A still’s productivity increases with the amount of incoming solar energy, or insolation. With the solar conditions in the U.S. Southwest, an average still could produce about 0.8 liters (0.2 gallons) per sun hour per square meter, according to a demonstration project funded by the U.S. Environmental Protection Agency (EPA). Larger surface areas generate higher output.

The stills have been a blessing in regions that don’t have easy access to freshwater. The EPA-funded project tested hundreds of solar stills in the unincorporated communities, or colonias, along the U.S.-Mexico border.

The colonias often are not connected to a public water supply system and are located in areas without clean groundwater. They rely on bottled water or supplies that are trucked in — expensive options compared to the stills. Surveys taken as part of the project found widespread satisfaction with the units.

read more>>>>