Reverse Osmosis is the mechanism whereby water is pressurised through a mambrane to exclude contaminants. 

Originally developed by the U.S. Navy to remove the "salt" from salt water so that the missions on which warships were sent could exceed the amount of fresh water that could be carried.

Today's domestic R.O. units are quite compact. They typically are made up of 4 or more likely 5 stages.

The first three stages are the pre-filters. These are in place to remove chlorine and sediment and other impurities which would otherwise contaminate the membrane and require more maintenamnce.

The membrane is next. This is a winding of a material with very fine pores. These pores allow molecules of water to pass, while "rejecting" molecules larger than that of water. These rejected continants are then washed away to drain, cleaning the surface of the membrane and extending it's life.

R.O. is a very slow means of filtration due to these small pores. Therefore to make R.O. a convenient and more practical proposition, a tank or reserviour is used to collect the purified water. This tank is pressurised by the mains water or typically by a pump. When the tap is turned on, the water travels from the tank, through the fifth stage and to the tap for drinking.

Membrane manufacturers rate their membranes for the exclusion of contaminants in the high 90% range.

Some contaminants that R.O. is used to remove include: Bacteria, Cysts such as Cryptosporidium, heavy metals such as lead, and mercury, pesticides, herbicides, sodium. In addition Aluminium, Arsenic and many other undesireables from drinking water, making it safe and secure for your family.