bar (30–250 psi) for fresh and brackish water, and 40–82 bar (600–1200 psi) for seawater, which has around 27 bar (390 psi)^[7] natural osmotic pressure that must be overcome. This process is best known for its use in desalination (removing the salt and other minerals from sea water to produce fresh water), but since the early 1970s, it has also been used to purify fresh water for medical, industrial, and domestic applications.

Fresh water applications[edit]

Drinking water purification[edit]

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

The latest developments in the sphere include nano materials and membranes.

In some systems, the carbon prefilter is omitted, and a cellulose triacetate membrane is used. CTA (cellulose triacetate) is a paper by-product membrane bonded to a synthetic layer and is 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) membrane is made of synthetic material, and requires 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.