Reverse osmosis (RO) is a water purification technology that uses a semipermeable membrane to remove ions, molecules, and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property, that is driven by chemical potential differences of the solvent, a thermodynamic parameter. Reverse osmosis can remove many types of dissolved and suspended species from water, including bacteria, and is used in both industrial processes and the production of potable water. 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. To be "selective", this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as solvent molecules) to pass freely.
The reverse osmosis water filter process
Around the world, household drinking water purification systems, including a reverse osmosis step, are commonly used for improving water for drinking and cooking.
Such systems typically include a number of steps:
a sediment filter to trap particles, including rust and calcium carbonate
optionally, a second sediment filter with smaller pores
an activated carbon filter to trap organic chemicals and chlorine, which will attack and degrade thin film composite membrane reverse osmosis membranes
a reverse osmosis filter, which is a thin film composite membrane
optionally, a second carbon filter to capture those chemicals not removed by the reverse osmosis membrane
optionally an ultraviolet lamp for sterilizing any microbes that may escape filtering by the reverse osmosis membrane
latest developments in the sphere include nano materials and membranes
In some systems, the carbon prefilter is omitted, and cellulose triacetate membrane is used. CTA (Cellulose Triacetate) membrane is a paper by-product membrane bonded to a synthetic layer and are made to allow contact with chlorine in the water. These require a small amount of chlorine in the water source to prevent bacteria from forming on it. The typical rejection rate for CTA membranes is 85-95%.
The cellulose triacetate membrane is prone to rotting unless protected by chlorinated water, while the thin film composite membrane is prone to breaking down under the influence of chlorine. A thin film composite (TFC) membranes are made of synthetic material, and require chlorine to be removed before the water enters the membrane. To protect the TFC membrane elements from chlorine damage, carbon filters are used as pre-treatment in all residential reverse osmosis systems. TFC membranes have a higher rejection rate of 95-98% and a longer life than CTA membranes.
