What is Reverse Osmosis?
Reverse osmosis (RO) is a membrane-technology filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side.
How Reverse Osmosis Works
In the normal osmosis process, the solvent naturally moves from an area of low solute concentration (High Water Potential), through a membrane, to an area of high solute concentration (Low Water Potential). The movement of a pure solvent to equalize solute concentrations on each side of a membrane generates osmotic pressure. Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to other membrane technology applications. However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration.
Reverse osmosis, however, involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure, and water flux rate. Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other substances from the water molecules.
The efficiency of a reverse osmosis water filter is affected by the water pressure coming into the system and the temperature of that water. Membranes are tested at 65 psi of pressure and a temperature of 77 degrees. For each incremental change in either variable, membrane performance changes accordingly. Higher pressures increase production and vice versa. The optimal pressure is 65 PSI and the temperature is 77°.
What are the Stages in a Reverse Osmosis System?
Below is a simplified and exploded diagram of a typical 4-stage reverse osmosis system. The stages have been numbered and the arrows show the directional flow of the water as it moves through the system. A larger diagram of the membrane is featured at the bottom to show its many layers, the component of the system that makes it a ‘reverse osmosis’ system.