UF membranes perform separations based on solute size. They have retention ratings of 1–5,000 K for roughly spherical solutes and can retain solutes of 10–1,000 Å or larger in diameter (roughly 300–1,000 KDa) such as colloids, large molecules, and nanoparticles. Smaller solutes and solvents pass through the ultrafilter. UF membranes retain smaller solutes than microfiltration membranes and larger solutes than reverse osmosis membranes.
Most commercial operations are run as tangential flow filtration (TFF), but dilute solutions such as water treatment or small-scale sample preparation are run as normal flow filtration (NFF). Virus-retaining filters, sometimes referred to as nanofilters, are on the most open end of UF and can be run as NFF or TFF. The first large commercial application of UF was paint recycling, followed by dairy whey recovery in the mid-1970s. Applications can be described as clarification of a permeate product (e.g., water purification), concentration of a retentate...
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