How it Works – The difference between osmosis and reverse osmosis
Osmosis is a natural process in which a solvent (e.g. water) moves from an area of low solute concentration through a membrane to an area of high solute concentration. This movement of solvent aims to equalize the solute concentration on both sides of the membrane.
Concept Diagram – Osmosis
Reverse osmosis (RO) is a water purification technology that uses pressure to force water molecules from an area of high solute concentration (contaminated water) through a special membrane to an area of low solute concentration – hence RO. The end result is that impurities, including salt, calcium, magnesium and bacteria, which are of a larger diameter than the water molecules are retained on the pressurised side of the membrane and pure water accumulates on the other. The build-up of impurities on the membrane is flushed away by turbulent flow created within the RO cartridge.
How RO works in the cartridge
Impure water enters the cartridge under pressure and is forced through the layers of membrane wrap separated by a fibrous support layer. The movement of water over the fibrous layer sets up eddies within the membrane layers and this turbulence:
- Evenly disperses the water over the membrane surface, and
- Keeps the pores of the membrane clean to prevent performance drop.
Concept Diagram – RO Cartridge
There are two classes of how RO works in the membranes:
- Low-energy membranes with a pore size of .0005 micron which require an operating pressure of 38-52 psi provided by municipal water supplies. These are normally used in domestic reverse osmosis water purification systems, and
- Medium-energy membranes with a pore size of .0001 micron which require a pump to provide an operating pressure of 150-500 psi. These are normally used in commercial/agricultural reverse osmosis water purification systems.
The reverse osmosis operating principle is that dissolved calcium, magnesium, and sodium salts, organic material, pathogens and other contaminants with a size in excess of the membrane pore size cannot pass through the membrane. Water molecules, on the other hand, are sufficiently small to be able to pass through the membrane.
Comparison of RO Membrane Pore Size and Size of Contaminants in Water