Desalinization is already with about a factor of 2 or 3 of the thermodynamic optimum, so improvements can't be revolutionary. Desalinization mostly needs cheap clean energy.
Fluids can remain subbornly un-mixed, over large distances and for long times. Given that brine is heavier than seawater, it will almost certainly tend to sink and flow along bathymetric contours, perhaps pooling in local low spots. And sealife can be exceedingly sensitive to changes in temperature, ion density, salt content, etc.
This reminds me of a book in which an ecologist described a discussion with a chemist over the concentration of some pollutant in seawater. Roughly, "Assuming a well-mixed solution..." starts the chemist. "How are you planning to stir the oceans?" asks the ecologist....
> But that doesn’t mean that there are no other ways of getting better output for our energy. The total process of desalination turns out to require three to four times the theoretical minimal energy use, since the salt water must be pumped and pretreated, the membranes maintained, and the resulting brine handled afterwards. Some of these things might be amenable to further improvements, and there has been work put into developing membranes that don’t clog up as easily or better pre-filtering of biological materials.
http://arstechnica.com/science/2011/08/desalinization-is-thi...
More: http://science.sciencemag.org/content/333/6043/712
https://www.researchgate.net/profile/Menachem_Elimelech/publ...
> Based on thermodynamic principles and practical kinetic requirements, the theoretical minimal energy for desalination with FO is always higher than that without FO. In other words, using FO cannot reduce the minimum energy of separation.
https://www.researchgate.net/profile/Shihong_Lin/publication...
The idea behind forward osmosis is to replace the salt in water with some other solute, like sugar. A strict 1-particle-to-1-particle replacement can (I think) have arbitrarily low free energy cost, but then you're just stuck with sugar water instead of salt water. In certain special cases this is fine (like for emergency water generation at sea, when you don't mind drinking sugar water). Insofar as low levels of sugar can't be tasted, and insofar as you have access to free sugar, you can avoid the energetic cost of removing some of the salt by replacing it with sugar. But I don't think this is very much before it becomes noticeable, and sugar isn't free.
If you happen to have excess thermal energy from some source, there are also ways to harvest some of this using forward osmosis, but this is really just another way to smuggle in free energy. In principle, you could use the heat gradient to generate electricity and then use that to power normal reverse osmosis, although which is more efficient will depend on the details of your equipment losses.
Even better, it can have a (theoretical) energy gain if you use a substance with a lower heat of mixing than (sea) salt and water.