As referenced in the history, during the 1950's, the problems of desalting were strongly dealt with. The process of reverse osmosis was designed using special synthetic membranes to separate solutions with even a high concentration of salt. A semipermeable membrane separates a solution from a pure solvent or twosolutions of different concentrations. This is done by a reverse osmotic flow. Actual osmosis is the movement of the phase richer in solvent from the higher concentration to the lesser one.
Pressure is put on the system in the opposite direction to the natural flow of osmosis.
When the flow reaches the point where it does not move it is called equilibrium pressure. The osmotic presssure is the equilibrium pressure difference between the solvent and solution phases. If the pressure is increased from this point then the flow is in the reverse direction. This is the basis for reverse-osmosis desalination. This process does have limiting factors such as the high cost to acquire the pressures needed to reverse the flow.