The process of cross-flow membrane filtration uses pressure to force liquid across the surface of a porous membrane. The size distribution of the pores in the membrane determines which components in the liquid pass through the membrane, and which ones will be retained.

The retained portion is called the retentate – i.e. the high solids fraction. The liquid that passes through the membrane is called the permeate – i.e. the low solids fraction.

Depending on the application it may be the retentate or permeate that is considered the “value” stream.

The following selections denote the various types of filtration – differentiated by the membrane pore size range:

• Microfiltration (MF)
• Ultrafiltration (UF
• Nanofiltration (NF)
• Reverse Osmosis (RO)