This is a process that exposes water under pressure‚ to a semi-permeable membrane with a very fine pore structure. Because most inorganic contaminants are of a larger molecular size than water‚ the membrane rejects certain contaminants‚ minerals and a large part of the water. The portion of water that passes through the membrane is stripped of inorganic compounds and trace minerals.
Because many synthetic chemicals like herbicides and pesticides are smaller‚ molecularly‚ than water... an R.O. system must also be used in conjunction with a carbon filter.

R.O. systems require adequate water pressure and extensive maintenance. Because most point-of-use R.O. systems produce less than 1 gal. per hour‚ they require a diaphragmed storage tank. Reverse osmosis typically wastes 2 to 3 gallons of water for every gallon it produces and has an 18 to 24 cents per gallon usage cost.