Water treatment with (reverse) osmosis plants

Reverse osmosis is a physical process of membrane technology for the concentration of substances dissolved in liquids, in which pressure is used to reverse the natural osmosis process.

 

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 concentrating landfill leachate, even higher pressures are used.

The osmotic membrane, which allows only the carrier liquid (solvent) to pass through and retains the solutes (solute), 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 like a filter, while the "contaminant" molecules are retained. Unlike a classic membrane filter, osmosis membranes do not have continuous pores. Rather, ions and molecules migrate through the membrane by diffusing through the membrane material. The solution-diffusion model describes this process.

The osmotic pressure increases with increasing concentration difference. When the osmotic pressure becomes equal to the applied pressure, the process comes to a halt. Osmotic equilibrium then exists. A steady outflow of the concentrate can prevent this. At the concentrate outlet, the pressure is either controlled by a pressure regulator or used by a pressure exchanger to build up the pressure required in the system inlet. Pressure exchangers are very effective in reducing the operating costs of a reverse osmosis system through energy recovery. The energy consumption per cubic meter of water is 4-9 kWh.

Osmosis plants or reverse osmosis plants are used

  • in military and aerospace applications,
  • in domestic and industrial drinking water treatment,
  • in industrial water treatment such as boiler water treatment.

Reverse osmosis can also be used to increase the concentration of dissolved substances, in which case the carrier substance is concentrated e.g. in the production of fruit juice concentrates or for the compression of must in wine production. Reverse osmosis is also used in the production of non-alcoholic beer, milk concentrates and protein powders.

 

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