Ultrafiltration: Between Conventional Filters and Reverse Osmosis

by Gene Franks

In water treatment, the term “ultrafiltration” is used to describe a filtration process that separates out particles down into the 0.1 to 0.001 micron range.

That’s extremely small when compared with conventional filtration, but it’s large when compared with nanofiltration and reverse osmosis. Ultrafiltration is tight enough to strain out pesky colloidal particles that conventional filters can’t hold, and it rejects both organic and inorganic large molecule substances. It cannot, however, remove ions and organics with low molecular weights (sodium, calcium, sulfate, for example), which are readily removed by reverse osmosis.

Molecular weight, in fact, is the yardstick by which ultrafiltration systems are usually measured. For example, an ultrafiltration membrane that removes dissolved solids with molecular weights of 10,000 is said to have a molecular weight cutoff of 10,000. Such a membrane has a nominal pore size of about 0.003 micron.

Compared with reverse osmosis, ultrafiltration membranes have extremely high flux rates. (Think of flux as the speed that the product water goes through the membrane.) They can also be operated at much lower pressure. As with reverse osmosis, temperature can have a great effect on performance, with lower temperature resulting in reduced flux rate.

Unlike conventional filters, ultrafiltration membranes do not trap and hold contaminants but like the reverse osmosis membranes they act as a barrier, holding contaminants until they are washed away. Ultrafiltration works in the same cross-flow separation method as reverse osmosis.

One great advantage of ultrafiltration membranes is that they can operate at pressures much lower than those required for reverse osmosis. In fact, UF systems usually operate at pressures below 100 psi, and 50 psi operation is common.