Drinking water has an osmotic pressure of less than 2 bar, the applied pressure for the reverse osmosis of drinking water is 3 to 30 bar, depending on the membrane used and system configuration. For desalination of seawater, a pressure of 60 to 80 bar is required because sea water at about 30 bar has a much higher osmotic pressure than drinking water. There is even an osmotic pressure of 350 bar in the Dead Sea. In some applications, e.g. for the concentration of landfill leachate, even higher pressures are used.
The osmotic membrane, which allows only the carrier fluid to pass and retain the solutes, must be able to withstand these high pressures. When the pressure difference more than compensates for the osmotic slope, the solvent molecules pass through the membrane like a filter, while retaining the “contaminant molecules”. Unlike a classic membrane filter, osmosis membranes do not have continuous pores. Rather, the ions and molecules migrate through the membrane by diffusing through the membrane material.