MIT Technology Review – The Oasys forward osmosis desalination system requires just one-tenth as much electricity as a reverse-osmosis system because water doesn’t have to be forced through a membrane at high pressure. That’s a crucial source of savings, since electricity can account for nearly half the cost of reverse-osmosis technology. Not working with pressurized water also decreases the cost of building the plant—there is no need for expensive pipes that can withstand high pressures. The combination of lower power consumption and cheaper equipment results in lower overall costs.
Oasys Water has been demonstrating commercial-scale components of its system in recent months, plans to begin testing a complete system early next year and to start selling the systems by the end of 2011.
The system uses far less energy than thermal desalination because the draw solution has to be heated only to 40 to 50 °C, McGinnis says, whereas thermal systems heat water to 70 to 100 °C. These low temperatures can be achieved using waste heat from power plants. Thermal-desalination plants are often located at power plants now, but it takes extra fuel to generate enough heat for them. The new system, on the other hand, could run on heat that otherwise would have been released into the atmosphere.
Currently, desalination is done mainly in one of two ways: water is either heated until it evaporates (called a thermal process) or forced through a membrane that allows water molecules but not salt ions to pass (known as reverse osmosis). Oasys’s method uses a combination of ordinary (or forward) osmosis and heat to turn sea water into drinking water.
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. The resulting mixture is then heated, causing the carbon dioxide and ammonia to evaporate. Fresh water is left behind, and the ammonia and carbon dioxide are captured and reused.
Oasys says the technology could make desalination economically attractive not only in arid regions where there are no alternatives to desalination, but also in places where fresh water must be transported long distances.
* The cost will be low enough to make aqueduct and dam projects look expensive in comparison.
* The fuel consumption and carbon emissions will be lower than those of almost any other water source besides a local lake or aquifer.
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