Published on Mar 13, 2020
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.
In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure. In the same way as in conventional osmosis, the solute cannot move from areas of high pressure to areas of low pressure because the membrane is not permeable to it. Only the solvent can pass through the membrane. When the effect of the externally applied pressure is greater than that of the concentration difference, net solvent movement will be from areas of high solute concentration to low solute concentration, and reverse osmosis occurs.
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