Reverse osmosis membrane technology is a physical process for concentrating substances dissolved in liquids, which reverses the natural osmosis process with pressure. This method was developed in the 1960s by the NASA space agency to treat urine in ultrapure water in human spaceflight. Reverse osmosis is currently used in the medical, food and marine desalination industries, as this process is well suited to treat contaminated water in ultrapure water, i. to remove even the smallest molecular substances from the water.

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The medium in which the concentration of a certain substance is to be reduced is separated from the medium by a semipermeable membrane in which the concentration is to be increased. This is subjected to a pressure which must be higher than the pressure which is created by the osmotic desire to balance the concentration. This allows the molecules of the solvent to migrate towards their “natural” osmotic propagation direction. The process pushes them into the compartment where dissolved matter is less concentrated.

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.

Reverse osmosis removes even drug residues from the water

The osmotic pressure increases with increasing concentration difference. When the osmotic pressure becomes equal to the applied pressure, the process comes to a halt. There is then an osmotic balance. A steady outflow of the concentrate can prevent this. At the concentrate outlet, the pressure is either controlled by a pressure regulator or used via a pressure exchanger to build up the pressure required in the system inlet. Energy exchangers very effectively reduce the operating costs of a reverse osmosis system through energy recovery. The energy required per cubic meter of water is 4 to 9 kWh.