How Reverse Osmosis Works

Reverse Osmosis is an affordable and very effective way to purify water for a variety of applications.  But what is Reverse Osmosis and how does it purify water?  Wikipedia defines Reverse Osmosis as "a water filtration method that removes many types of large molecules and ions".  But to best understand reverse osmosis one needs to first understand the concept of regular osmosis.  First, a couple of handy definitions:

Solvent:  a liquid that dissolves something (think of water, alcohol, turpentine)

Solute:  the thing that gets dissolved by the solvent (could be salt, dye, laundry detergent)

Semi-Permeable:  allows the passage of some molecules but not others (usually allows the movement of water)

Osmosis is the movement of a solvent through a semi-permeable membrane from an area of low solute concentration to an area of high solute concentration.  The best example to illustrate this concept in every day terms is by looking at a bowl of salad.  In this example our solvent is the water that is trapped inside the leaves of lettuce.  Our semi-permeable membrane is the cell walls of the lettuce that is holding this water.  Our solute is the salty salad dressing that is coating the lettuce.

Water will naturally move from an area where the solute concentration is low to where it is high.  Water wants to dissolve as much solute as possible.  In our example, the water wants to move from where the solute (salad dressing) concentration is low on the inside of the lettuce leaf, to where it is high on the outside of the leaf.  Osmosis is the process where the water moves through the membrane (wall of the leaf) to the outside of leaf to dissolve the salad dressing.  You know this happens, because the longer you leave salad dressing on your salad the more wilted the lettuce becomes.  This is because the water inside the leaves of lettuce is moving byosmosis to the outside of the leaves.

A Reverse Osmosis system uses this same principle but in reverse.  Instead of moving the water from where the solute concentration is low to where it is high, a reverse osmosis system moves the water from where the solute concentration is high to where it is low.  This allows a reverse osmosis system to remove only the pure water from a mix of water and dissolved contaminants.

Just as with regular osmosis, reverse osmosis is accomplished with a semi-permeable membrane.  The membrane used in a reverse osmosis system allows only water to pass through.  Contaminant molecules are too large to make it through.  In order to push the pure water through the membrane, the water must be pressurized.  Typically, the water pressure found in a home is sufficient to make this process work.

The membrane used in a residential reverse osmosis system is damaged if it is subjected to chlorine or too much sediment.  So a typical home reverse osmosis system utilizes several stages of pre-filtration before the water and contaminants are separated by the reverse osmosis membrane.  The first stage is usually a 5-micron sediment filter followed by a carbon filter for the removal of chlorine.  Some reverse osmosis systems will also incorporate a carbon filter after the reverse osmosis membrane stage as a final polishing filter.