What is Osmosis?

Osmosis is based upon the fundamental pursuit for balance. Two fluids containing different concentrations of dissolved solids that come in contact with each other will mix until the concentration is uniform. When these two fluids are separated by a semi permeable membrane (which lets the fluid flow through, while dissolved solids stay behind), the fluid containing the lower concentration will move through the membrane into the fluid containing the higher concentration of dissolved solids (Binnie e.a., 2002).
After a while the water level will be higher on one side of the membrane. The difference in height is called the osmotic pressure.
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What is Reverse Osmosis?

By applying a pressure that exceeds the osmotic pressure, the? reverse effect occurs. Fluids are pressed back through the membrane, while dissolved solids stay behind.

To purify water by Reverse Osmosis membrane, the natural osmosis effect must be reversed. In order to force the water of the brine stream (high salt concentration) to flow towards the fresh stream (low salt concentration), the water must be pressurized at an operating pressure greater than the osmotic pressure. As a result, the brine side will get more concentrated.
The operating pressure of seawater is around 60 bar.