Ultrafiltration is a membrane filtration in which forces like pressure or concentration gradients lead to a separation through a semi-permeable membrane. Suspended solids and solutes of high molecular weight are retained in the so-called retentate, while water and low molecular weight solutes pass through the membrane in the permeate.
Ultrafiltration is an effective means of reducing the silt density index of water and removing particulates that can foul reverse osmosis membranes. Ultrafiltration is frequently used to pretreat surface water, seawater and biologically treated municipal water upstream of the reverse osmosis unit. In water treatment, UF is used to recycle and reuse water that contains virtually no physical solids. Industries that consume large volumes of water or discharge highly toxic effluent employ ultrafiltration for water reuse.