Alternative seawater reverse osmosis

The comparison between the two major seawater desalting alternatives, reverse osmosis and distillation, is more complex then ever. The location, system size, time of implementation and economic parameters, especially the price of conventional energy and also the possibility of use of non-conventional energy in the future, such as solar or geothermal energy sources, may greatly affect the final decision. 

An interesting alternative development is that of forward osmosis. Whereas in reverse osmosis a high pressure is required to oppose the natural tendency of freshwater to move across such a membrane via osmosis to dilute the seawater, in forward osmosis the system takes advantage of this natural tendency. Here, salt water sits on one side of the membrane, but the freshwater on the opposite side is transformed into a high-concentration solution by adding NH3 and CO2. Water naturally flows from the salt water to what is now the draw solution, which can have a solute concentration as high as 10 times that of the salt water. There is no need for an external pressure. The diluted draw solution is then heated to evaporate off the CO2 and NH3 for reuse, leaving behind freshwater. (See Patel-Predd, 2006). 

Membrane separations began in the 1960s as an alternative means to distillation for the desalination of salt (i.e. sea) and brackish waters. This was called reverse osmosis because it works by applying a transmembrane pressnre greater than the natural osmotic pressure between the two solutions (seawater, say, and desalted water). 

The mixing of fresh water from estuaries with seawater has the potential to produce more than 2.5 TW of power globally [138]. Recovering this energy has been discussed for the past half century using desalination processes operated in a reverse mode. Proposed alternatives include the use of reverse electro-dialysis (RED) [139], pressure retarded osmosis PRO [140-141], and vapor pressure differences [142]. 

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