Pressure reverses natural process to achieve fine filtration

To understand "reverse osmosis," it is probably best to define normal osmosis first. In osmosis, a solvent moves through a semi-permeable membrane, like the lining of the intestine or a cell wall, into a solution of higher solute concentration. The pressure that moves the solvent and raises the level of the high-solute liquid is called osmotic pressure, and the process tends to equalize the concentrations of solute on two sides of a membrane. Bodily functions like absorption and elimination operate by osmosis.

To apply the concept of osmosis to water, imagine a semi-permeable membrane separating fresh water and salty water. On the salty side, some of the pores get plugged with salt atoms, but on the pure water side that does not happen. Therefore, more water passes from the pure water side to the salty side, diluting the salt concentration.

In reverse osmosis, the membrane acts like an extremely fine filter to create drinkable water from salty (or otherwise contaminated) water. The salty water is put on one side of the membrane and pressure is applied to stop, and then reverse, the osmotic process.

The process requires a driving force to push the fluid through the membrane, and the most common force is pressure from a pump. The higher the pressure, the larger the driving force. As the concentration of the fluid being rejected increases, the driving force required to concentrate the fluid increases.

As a means of purifying water, reverse osmosis is capable of filtering bacteria, salts, sugars, proteins, particles, dyes and other constituents, thus improving the taste, color and properties of the fluid.