How Does Reverse Osmosis Work?
A semipermeable membrane, like the membrane of a cell wall or a bladder, is selective about what it allows to pass through, and what it prevents from passing. These membranes in general pass water very easily because of its small molecular size, but also prevent many other contaminants from passing by trapping them. Water will typically be present on both sides of the membrane, with each side having a different concentration of dissolved minerals. Since the water in the less concentrated solution seeks to dilute the more concentrated solution, water will pass through the membrane from the lower concentration side to the greater concentration side. Eventually,
osmotic pressure (seen in the diagram below as the pressure created by the difference in water levels) will counter the diffusion process exactly, and an equilibrium will form.
The process of reverse osmosis forces water with a greater concentration of contaminants (the source water) into a tank containing water with an extremely low concentration of contaminants (the processed water). High water pressure on the source side is used to "reverse" the natural osmotic process, with the semi-permeable membrane still permitting the passage of water while rejecting most of the other contaminants. The specific process through which this occurs is called ion exclusion, in which a concentration of ions at the membrane surface form a barrier that allows other water molecules to pass through while excluding other substances.
Even with these advances, the "reject" water on the source side of a Reverse Osmosis (RO) system must be discarded in order to keep it from becoming so concentrated that it forms a scale on the membrane itself. RO systems also typically require a carbon prefilter for the reduction of chlorine, which can damage an RO membrane. A sediment prefilter is always required to ensure that fine suspended materials in the source water do not permanently clog the membrane. Hardness reduction, either through the use of water softening for residential units or chemical softening for industrial use, may also be desirable in hard water areas.
Water enters the RO membrane under pressure and travels from the feed end to the reject end. Some of this feed water passes through the membrane and becomes purified. This water is called the RO product water. The contaminants that were previously in the product water exit the membrane in the reject water. Several layers of membrane material are sandwiched between spacer material to form leaves with a feed / reject channel and a product channel. These leaves are then rolled around a central product collection tube. This assembly is referred to as a spiral wound membrane element.
The reject water exits the vessel and feed the next vessel or is sent to drain. The product water exits the vessel and is sent to a storage tank or point of use. An o-ring seal prevents the reject water from mixing with the product water.
High pressure commercial / industrial units typically provide from 2 gallons to 400 gallons per minute of water with an efficiency of 0.3 - 6 gallons of reject water per gallon of product water produced. These systems typically remove greater than 95% of the dissolved solids found in the feed water. These systems tend to be much larger and more complicated than low pressure systems.