Step 1: Normal osmosis direction.
In normal osmosis, solvent (water) flows spontaneously from a dilute solution (or pure water) across a semipermeable membrane into a concentrated solution such as sea water.
Step 2: Reversing the direction.
If an external pressure greater than the osmotic pressure of sea water is applied on the sea water side, it reverses the natural direction of flow. Water is forced to move from the concentrated solution to the dilute side.
Step 3: What passes through the membrane.
Only water molecules can pass through the semipermeable membrane. Dissolved salts, ions, and other impurities are too large and are retained on the seawater side.
Step 4: Application.
Pure water is collected on the other side, leaving behind the concentrated brine. This process is called reverse osmosis (RO). Reverse osmosis is the phenomenon responsible for the desalination of sea water.