Ultrafiltration is a separation process using membranes with pore sizes in the range of 0.1 to 0.001 micron.  Typically, ultrafiltration will remove high molecular-weight substances, colloidal materials, and organic and inorganic polymeric molecules.  Low molecular-weight organics and ions such as sodium, calcium, magnesium chloride, and sulfate are not removed.  Because only high-molecular weight species are removed, the osmotic pressure differential across the membrane surface is negligible.  Low applied pressures are therefore sufficient to achieve high flux rates from an ultrafiltration membrane.  Flux of a membrane is defined as the amount of permeate produced per unit area of membrane surface per unit time.  Generally flux is expressed as gallons per square foot per day (GFD) or as cubic meters per square meters per day.
Ultrafiltration membranes can have extremely high fluxes but in most practical applications the flux varies between 50 and 200 GFD at an operating pressure of about 50 psig in contrast, reverse osmosis membranes only produce between 10 to 30 GFD at 200 to 400 psig.