Membrane technology with reverse osmosis is a physical process for concentrating substances dissolved in liquids, in which pressure is used to reverse the natural osmosis process. This process was developed in the 1960s by NASA, the space agency, to treat urine into ultrapure water during manned space flight. Today, reverse osmosis is used in medicine, the food industry and for desalination of the sea, as this process is excellently suited for turning contaminated water into ultrapure water, i.e. for removing even the smallest molecular substances from the water.
The medium in which the concentration of a particular substance is to be reduced is separated by a semipermeable membrane from the medium in which the concentration is to be increased. The latter is subjected to a pressure that must be higher than the pressure created by the osmotic demand to balance the concentration. This allows the molecules of the solvent to migrate against their “natural” osmotic propagation direction. The process pushes them into the compartment where solutes are less concentrated.
Drinking water has an osmotic pressure of less than 2 bar, the applied pressure for reverse osmosis of drinking water is 3 to 30 bar, depending on the membrane used and the system configuration.
For seawater desalination, a pressure of 60 to 80 bar is required, since seawater has a much higher osmotic pressure of about 30 bar than drinking water. In the Dead Sea, there is even an osmotic pressure of 350 bar. In some applications, e.g. for the concentration of landfill leachate, even higher pressures are used.
The osmotic membrane, which allows only the carrier liquid to pass through and retains the solutes, must be able to withstand these high pressures. When the pressure differential more than compensates for the osmotic gradient, the solvent molecules pass through the membrane, as in a filter, while the “contaminant” molecules are retained. 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.