Reverse Osmosis

Reverse osmosis describes a process of filtration that results in the removal of large molecules and ions from a variety of solutions by means of pressure applied on the solution it self towards a selective membrane. As a result of the reverse osmosis process, the solute with the unwanted large molecules and ions stays in the side that was pressurized and the now pure solvent passes through to the other side – This way it’s possible to use the solvent to a diverse variety of uses, such as filtered drinking water.

Reverse osmosis is called this way as it’s the opposite process of normal osmosis that occurs naturally al the time. In normal osmosis solvent pass freely from a low solution concentration to a high solution concentration, this is also done via a membrane. The pure solvent moves (in normal osmosis) from one side of the membrane to the other to equalize the concentrations, this movement created pressure that is called the “Osmotic pressure”. Hence, when applying pressure artificially for the purpose of reversing this natural process, this is reverse osmosis.

Reverse osmosis is most widely known for its uses in purifying drinking water and desalination (removing of salt) of sea water for drinking or irrigation purposes.

Technical details about the reverse osmosis process

Reverse osmosis is different than a normal filtration process that is based on straining out large molecules. In reverse osmosis the process is achieved through a diffusive mechanism – thus the efficiency of the process depends on the solution concentration, pressure level and the water flux rate. The process of reverse osmosis is performed with the selective membrane made with microscopical thickness of a dense polymer; in most cases only water can pass the membrane in reverse osmosis.

The solvent that is referred to a lot in regards to reverse osmosis is the substance in the solution that is the largest in terms of quantity. In water for instance, the water molecules are the solvent and all the other materials and molecules (salts and pollutants for example) are the solutes. The separation of the solvent, water in this case, from the rest of the solution is done through a semi permeable membrane; this allows the transfer of water and prevents all the rest from passing beyond it.

The pressure in reverse osmosis is normally between 2-17 bar (30-250 PSI) for fresh water and 40-82 bar (600-1200 PSI) for sea water. In order to reach the pressures needed for reverse osmosis the systems use electricity, industrial desalination plants use a great deal of energy for the reverse osmosis process.

Uses of the technology

  • Desalination of water: used on sea water and water with a lower salt level. The water from reverse osmosis can be used for drinking and agriculture. Desalination and reverse osmosis are considered a major factor in overcoming current and future global water shortages.
  • Drinking water purification: this type of reverse osmosis exists in household purification systems. In most cases reverse osmosis in only one part of the purification process alongside a mechanical filter, a carbon filter and other technologies.  

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