A method of cross flow filtration (similar to reverse osmosis but using lower pressures) which uses a membrane to separate small colloids and large molecules from water and other liquids. The ultra-filtration process falls between reverse osmosis and micro-filtration in terms of the size of particles removed, with ultra-filtration removing particles in the 0.002 to 0.1 micron range, and typically rejecting organics over 1,000 molecular weight while passing ions and smaller organics.