Reverse osmosis is the opposite process, except for the water to move in the opposite direction, pressure must be applied to the solution with the higher concentration in an excess of osmotic pressure. When pressure is applied to the feed water (concentrate solution), the flow of the liquid reverses, creating permeate water. This permeate water contains very low TDS levels and is ready for immediate use. There is residual source water that contains an elevated percentage of contaminants. This is referred to as wastewater and is either flushed down the drain or sometimes recycled back into the system.

Semi-permeable membranes are vital for reverse osmosis to be successful. The membrane acts as a barrier to all dissolved salts, organic molecules, and inorganic molecules that have a molecular weight larger than 100. The membrane allows water molecules to pass through the membrane, which creates a purified product stream. Generally, the rejection rate of these dissolved salts is 95% to over 99%. The most common membranes are made from aromatic polyamide resins. Aromatic polyamide is made from 1,3 phenylene diamine and the tri acid chloride of benzene. This is chemically resistant and structurally strong polymer that contains carboxylic acid and free (not reacted) amines at different levels. High chemical stability makes it the most durable and easy to clean membrane material available. Membranes today are strong enough to handle extremely high-water pressure that is needed for the greatest contaminant removal.