Reverse Osmosis

To understand reverse osmosis, one should have a good understanding of osmosis. Osmosis is the process whereby if we have two solutions separated by a semi-permeable membrane or filter, water will flow to one side or the other to equalize the concentration of solvents on each side of the membrane. It does this because the semi-permeable membrane is actually a filter that only allows the the smaller water molecules to pass through, not the larger solvent molecules.

Reverse Osmosis IllustrationObviously reverse osmosis is making this process work in reverse. It is an artificial process that is done in order to purify water, or separate small molecules from larger ones. To make it work, the same situation is set up as in osmosis where we have pure water and impure water placed side by side seperated by a semi-permeable membrane. However, instead of allowing the pure water to flow in to equalize the concentration of impurities, pressure is placed on the impure side. This pressure must be greater than the osmotic pressure of the pure water molecules. Now instead of the pure water moving to equalize the impure water. The pure water in the impure side moves through the semi-permeable membrane onto the pure side for use, leaving an even higher concentration of impurities on the impure side.

So, what is the difference between reverse osmosis and simply forcing a liquid through a regular filter? Simply this, the built up impure water can now be flushed away. This is done in a process called "crossflow filtration"1. In a filter the impure particles would build up and finally clog the filter. Although reverse osmosis may be a slower process than straight filtration, it may ultimately be more effective and produce higher quality water.

Some of the membranes used in reverse osmosis have pores as small as 0.0005 microns. As a micron is one-millionth of a meter, you can imagine how small the pores must be. Even at this small size, some bacteria and viruses can get through them.

1. Reverse Osmosis