Forward osmosis, alternative

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). 

A. AchUh, T.Y. Cath, E.A. Marchand, A.E. ChUdress, The forward osmosis membrane bioreactor A low fouling alternative to MBR processes. Desalination 2009, 239, 10-21. 

A number of alternatives to reverse osmosis are being considered. Two promising alternatives are membrane distillation [97] and forward osmosis [98]. Membrane distillation relies on vapor pressure differences across a membrane, arising from a temperature difference, to drive water transport. The process utilizes low temperature heat sources and operates at low pressure which can reduce operating costs relative to reverse osmosis. Forward osmosis relies on water permeation across a water selective membrane to a draw solution - the reverse of reverse osmosis. The water must then be separated from the draw solution but this may be less expense than reverse osmosis because the process operates at low pressure. 

For forward osmosis or pressure-retarded osmosis, since the ICP effects occurring inside the PSL of permeable membrane are also investigated, an alternative momentum governing equation for porous media, Brinkman equation, should be applied for porous region of the membrane [8] ... 

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