When I began designing and selling Reverse Osmosis (RO) systems circa 1975, very few people ever heard of the technology let alone understood it. Today, RO is a common technology found in many homes but many still do not understand why it is called Reverse Osmosis. So, lets take a closer look. The accompanying figure will help with the description.

The key to an RO system is something called a semipermeable membrane (SPM). It allows water to pass in both directions, but it does not allow contaminants to pass through it. By using this membrane and some engineering, we can pass clean water through and collect it on the other side, with the contaminants being held back on the other side. Before we talk about RO let’s talk about Osmosis.

If we were to put two solutions with different concentrations of salts into a container with two compartments separated by a SPM we would find water would flow from the lower concentrated side into the less concentrated side. The water level on the less concentrated side would actually rise up to a higher level and the water in the more concentrated side would lower. This process is called osmosis.

The force of the water transfer that causes this water level rise is called osmotic pressure.

In water purification this process would not be useful to us because it simply is diluting the salts on the higher concentrated side, but still, that water has salts/contaminants in it, so it is not desirable.