When two solutions of differing concentrations of dissolved materials are separated by a semi-permeable membrane, the liquid component will tend to flow from the lower to the more highly concentrated side. In a sense, the concentration difference will tend to equilibrate across the membrane. This process is called osmosis.
If the liquid on the more concentrated side is maintained at a higher pressure, however, this process can be reversed: the solvent will flow from the concentrated side to the less concentrated side. Since the membrane blocks the passage of the dissolved waste constituents, the concentrated solution becomes even more concentrated. This process is called reverse osmosis.
Reverse osmosis is often used to remove dissolved organics and metals where
concentrations are less than 300 parts per million. However, special care and
testing must be performed to assure that the wastes don't dissolve or clog the
membrane. Low solubility salts are also prone to precipitate on the membrane
surface.
