A New Desalination Technique

December 26, 2010

In many arid regions of the world, removing the salt from sea water – desalination – is an important, although expensive, source of fresh water.  Currently, there are two techniques that are widely used.  The first is thermal evaporation or distillation: sea water is heated until it evaporates, leaving the salt behind; the water vapor is then condensed to get fresh water.  The second is called reverse osmosis: water is forced, under pressure, through a membrane that allow water molecules to pass, but blocks the salt ions.   Both of these processes are fairly energy-intensive.

An article in Technology Review reports that OASYS Water, based in Cambridge, Massachusetts, is working on a new desalination technology, which it expects to bring to market next year.  The OASYS method uses ordinary (forward) osmosis and heat to produce fresh water.  The technique is clever, based on the fact that osmosis through  a permeable membrane will tend toward an equilibrium with equal concentrations of dissolved substances on both sides of the membrane.

On one side of a membrane is sea water; on the other is a solution containing high concentrations of carbon dioxide and ammonia. Water naturally moves toward this more concentrated “draw” solution, and the membrane blocks salt and other impurities as it does so.

Since the membrane blocks movement of salt, the result is that solution on the “draw” side has high concentrations of carbon dioxide and ammonia, but no salt.  Since both ammonia and carbon dioxide, although soluble in water, are gases at normal temperatures, they can then be driven from the solution by heat.  (The solubility of gases in water decreases with increasing temperature.  That is why fizzy drinks, which contain dissolved carbon dioxide, keep better in the refrigerator than at room temperature.)  The gases are then captured and reused.

OASYS claims that this process can produce fresh water more economically than current technology, and might use waste heat from a power plant, since the water/gas solution does not need to be heated as much as sea water in a plant using a thermal process.

The system uses far less energy than thermal desalination because the draw solution has to be heated only to 40 to 50°C, McGinnis [OASYS cofounder and chief technology officer Robert McGinnis] says, whereas thermal systems heat water to 70 to 100 °C. These low temperatures can be achieved using waste heat from power plants.

Although the savings from the new technology could be substantial, they are unlikely to be enough to make the process economically viable for water production in agriculture (which is by far the single largest use of fresh water).  Still, it might be a welcome development for coastal cities in arid climates.

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