Osmosis occurs when pure water and contaminated water is separated only by a semipermeable membrane (the cell wall is one - but we can also manufacture these membranes).

Osmosis means that nature tries to achieve the same concentration of substances in both the water masses. The only way this can happen is by pure water leaking through the membrane and diluting the contaminated water.

Reverse osmosis (RO) is when we use the osmosis phenomenon "backwards" by increasing the pressure of the unfiltered water, it will be forced through a semipermeable membrane which will let through only the water molecules. A major advantage with reverse osmosis water filtration is that no chemicals are used in the drinking water.

Osmosis was known as a natural phenomenon as early as in the 18th century. In the nature, water is purified and transported through osmosis - in the body of mammals as well as in plants. In a living organism, the cell wall is the membrane through which nutrients and foods are transported via osmosis. Reverse osmosis for water purification and other uses was invented much later.

If you drink salty water it will eventually kill you. Osmosis is to blame for this - since the salty water on one side (in your stomach) is separated from pure water in your body by the stomach wall, the osmosis process will lead to your body being dehydrated in an attempt to "dilute" the salty water in your stomach.

When using reverse osmosis, the general idea is to use the membrane like a very fine filter for the creation of drinking water from contaminated (salty - or other contaminants) water. By applying pressure on the contaminated side, it is possible to reverse the osmosis process and achieve filtration. It is a slow process but it works well.

Membranes used in the reverse osmosis purification process are not using pores but are instead using very dense layers of polymers of microscopic thickness. The membrane used will allow only water to pass through. For the whole thing to work, it is necessary to use a fairly high pressure on the contaminated side of the membrane. Many reverse osmosis water filters need extra pressure but the GE Merlin reverse osmosis water filter manages well with only the line pressure which makes it ideal for small installations as household water filtration systems.

Reverse osmosis is otherwise best known as a desalination technique.