Osmosis is a physical force. It is the natural tendency of water with a low concentration of dissolved particles to move across a semi-permeable membrane to an area of water with a high concentration of dissolved particles. The water will try to reach an equilibrium on both sides. I.e. both sides of the semi-permeable membrane will have the same concentration of dissolved particles. This is how plants absorb nutrients from the soil. Picture a tea bag placed in a mug of hot water. (The tea bag is the semi-permeable membrane). At first, the water is free of tea. However, with time, the tea will appear to seep from the tea bag into the mug. This is the process of osmosis. If you were to leave the tea bag in the mug for long enough, the concentration of tea inside the teabag would equal the concentration of tea

Raw water enters a module housing the membrane system. The water is forced against the semi-permeable membrane and only clean water molecules pass through the pores in the membrane. Impurities are rejected and flushed away.

While the principles of reverse osmosis are simple the process can not run indefinitely unless steps are taken to ensure the membrane does not become clogged by impurities

In crossflow filtration, two exit streams are generated -- a "concentrate" stream (reject water) containing those material which are rejected or do no pass through the membrane, and the "permeate" stream (product water) which has been pumped through the membrane, and passes to the tank

Various mineral salts, heavy metals, particular matter, some organic molecules, bacteria and even viruses are rejected or repelled by the membrane surface based on their molecular or atomic weight. A second barrier, such as ultraviolet light, should be used if bacteria are present. The ability of the membrane to reject or repel dissolved particles, while allowing water to readily permeate, is based on the incredibly small size of the multitude of pores that penetrate its surface. Such pores are able to reject substances as small as 0.0005 microns. A micron (m) is a metric unit of length equal to a millionth of a meter, or 0.00003937 inch. A human hair is approximately 75 m in diameter. The smallest particle that can be seen with the naked eye is 40 m across The smallest bacteria is about 0.22 m while a virus is even smaller at 0.01 m.

Contaminant % nominal rejection Contaminant % nominal rejection Aluminum 96-98 Ammonium 80-90 Arsenic 98-99% Borate 30-50 Bacteria 99+ Bromide 90-95 Boron 50-70 Calcium 93-98 Cadmium 93-97 Chromate 85-95 Chloride 92-95 Cyanide 85-95 Copper 96-98 Hardness Ca & Mg 93-97 Fluoride 92-95 Lead 95-98 Iron 96-98 Magnesium 93-98 Manganese 96-98 Nickel 96-98 Mercury 94-97 Orthophosphate 96-98 Nitrate 90-95 Polyphosphate 96-98 Phosphate 95-98 Radioactivity 93-97 Potassium 93-97 Silicate 92-95 Silica 80-90 Sodium 92-98 Silver 93-96 Thoisulfate 96-98 Sulfate 96-98     Zinc 96-98

Reverse osmosis is a separation process that uses pressure to force a solvent through a membrane that retains the solute on one side and allows the pure solvent to pass to the other side. More formally, it is the process of forcing a solvent from a region of high solute concentration through a membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. This is the reverse of the normal osmosis process, which is the natural movement of solvent from an area of low solute concentration, through a membrane, to an area of high solute concentration when no external pressure is applied. The membrane here is semipermeable, meaning it allows the passage of solvent but not of solute.

The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases the membrane is designed to allow only water to pass through this dense layer while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2-17 bar (30-250 psi) for fresh and brackish water, and 40-70 bar (600-1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure which must be overcome. This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but has also purified naturally occurring freshwater for medical, industrial process and rinsing applications since the early 1970s

When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. This is achieved by diffusion, in which solutes will move from areas of higher concentration to areas of lower concentrations until the concentration in all the different areas of the resulting mixture are the same, a state called equilibrium.

Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential.

In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure. In the same way as in conventional osmosis, the solute cannot move from areas of high pressure to areas of low pressure because the membrane is not permeable to it. Only the solvent can pass through the membrane. When the effect of the externally applied pressure is greater than that of the concentration difference, net solvent movement will be from areas of high solute concentration to low solute concentration, and reverse osmosis occurs