Ultrafiltration (UF) is similar to RO and NF, but is defined as a crossflow process that does not reject ions. UF rejects solutes above 1000 daltons (molecular weight). Because of the larger pore size in the membrane, UF requires a much lower differential operating pressure: 10 to 100 psig (0.7 to 6.9 bar). UF removes larger organics, colloids, bacteria, and pyrogens while allowing most ions and small organics such as sucrose to permeate the porous structure.
The cross-flow filtration membrane technology is used to physically separate from an aqueous solution even extremely small soluble substances (ions, molecules), whose size is measurable in nm (nanometers).
The degree of filtration depends on many factors:
- the size of the pores of the membrane and the material of which it is made;
- the chemical characteristics of the fluid to be treated;
- operating parameters (temperature, pressure, pH, etc.);
- the fluid dynamic regime (increasing or decreasing the circulation velocity of the fluid that flows tangentially to the surface it is possible to alter the performance of the membrane in terms of rejection).
Ultrafiltration membranes are true porous membranes, capable of retaining medium molecular weight organic substances (in the range of 1,000-500,000 D - from narrow ultrafiltration up to the microfiltration thresholds).