Reverse osmosis

Reverse osmosis or reverse osmosis is a physical process for the concentration of substances dissolved in liquids, wherein the pressure is reversed by the natural osmosis process.

Action principle

The medium in which the concentration of a particular substance is to be reduced is separated by a semi-permeable ( semipermeable ) membrane from the medium in which the concentration is to be increased. This is exposed to a pressure that must be higher than the pressure generated by the osmotic desire for concentration equilibration. This allows the molecules of the solvent migrate against their "natural" osmotic propagation direction. The process it presses into the compartment present in the less concentrated solutes.

Drinking water has an osmotic pressure of less than 2 bar, the applied pressure for the reverse osmosis drinking water is 3 to 30 bar, depending on the membrane and system configuration used. For seawater desalination, a pressure of 60 to 80 bar is required since the sea water with about 30 bar having a significantly higher osmotic pressure than water. In the Dead Sea even an osmotic pressure of 350 bar is available. In some applications, such as for the concentration of landfill leachate, higher pressures can still be used.

The osmotic membrane that allows only the carrier liquid (solvent ) and solutes restrains ( solutes ) must be able to withstand these high pressures. When the pressure difference more than compensates for the osmotic gradient, the solvent molecules are suitable as a filter through the membrane, while the " contamination " molecules are retained. In contrast to a conventional membrane filter osmosis membranes do not have through- pores. Rather, the ions and molecules migrate through the membrane by diffusion through the membrane material. The solution-diffusion model describes this process.

The osmotic pressure increases with increasing concentration difference. If the osmotic pressure equal to the applied pressure, the process comes to a halt. There is then an osmotic balance. A steady discharge of the concentrate can prevent that. The concentrate outlet of the pressure is controlled either through a pressure regulator, or a pressure exchanger used to build up the required pressure in the inlet of the system. Pressure exchanger energy recovery by reducing the operating costs of a very effective reverse osmosis system. The energy consumption per cubic meter of water is 4-9 kWh.

The crystallization ( precipitation ) of the solutes in the membranes must be avoided. This can be by the addition of anti -plaque agents (English antiscalant ) or acids can be achieved. Anti- plaque agents are polymeric compounds herein to maleic acid or phosphate, which enclose the forming crystallites and thus preventing crystalline precipitation can occur on the membrane. A cleaning of the membrane may still be required.

To prevent damage of the membrane filter can be provided upstream. A fine filter can mechanical, an activated carbon filter to prevent chemical damage ( eg by chlorine ).

Also, it may be necessary to rid the system of biological pollution, especially in the sea water treatment. Here (usually bromine ) intermittently which forms biofilms are removed by means of biocides. Chlorine is used mainly in southern countries for disinfection. Due to the chlorine sensitivity of the membranes, it must be laboriously removed again.

Applications

• Commissioned by the NASA methods have been developed for the application of reverse osmosis on own urine in stays in space.
• Reducing the concentration of dissolved substances: treatment of water for cleaning of industrial water or rinsing waste water or for the production of drinking water, ultrapure water or aquarium water. Water to be treated may be, for example, process water, wastewater, seawater.
• Increasing the concentration of dissolved substances: concentration of the carrier substance, for example for the production of fruit juice concentrates or for compacting must in wine making.
• Production of non-alcoholic beer.