Reverse osmosis is a certain type of filtration methods which is designed to remove various sorts of ions and molecules from solutions by utilizing pressure on the solution when it is on one side of the selective membrane. The pressurized side of the membrane retains the solute while the pure solvent passes to the other side of the membrane. The membrane is selective because it does not let any large ions or molecules to pass through but it still does let smaller components of the solution to pass through.
The normal process of osmosis involves the moving of solvent from an area of low solute concentration towards the area of high solute concentration. The movement required for the equalization of solute concentrations on each respective side of the membrane generates a certain type of pressure which is known as the osmotic pressure. The reverse osmosis involves the application of external pressure in order to reverse the natural flow of the solvent.
The process may be similar to ordinary filtration but in truth it is actually completely different. Membrane filtration relies on size exclusion or straining, so in theory it is capable of perfect exclusion of particles. Reverse osmosis is much different because it utilizes a diffusive mechanism whose separation efficiency relies on completely different parameters such as water flux rate, pressure and solute concentration. Reverse osmosis is commonly used for the drinking water purification from seawater as it is very efficient in removing the salt and other unwanted substances from the water molecules.
Ordinary osmosis is a completely natural process in which two different liquids characterized by different concentrations get separated by a semi permeable membrane. In this process the fluid tends to move from low to high concentrations in order to achieve the chemical equilibrium.
Reverse osmosis is the total opposite of the process described above. Due to the application of high pressure the liquid tends to move from high to low concentrations. This process is commonly applied for the filtration of water because the system is very efficient in rejecting salt and other unwanted ions which may be harmful for the health of humans. The outlet water filtered by the reverse osmosis system is referred to as permeate.
The membranes used in the process are characterized by a dense layer in the polymer matrix. It was originally used for the desalination of water but it is nowadays also used for the purification of water for domestic industrial and medical purposes.