Ultra filtration, also known as UF, is a class of filtration that uses a membrane, either in the form of a spiral wound element similar to a reverse osmosis membrane, or more often, a tubular element known as a hollow fiber. Other types of membrane filters are Microfiltration (MF), Nano filtration (NF), and Reverse Osmosis (RO). These differ primarily in the sizes of particles that are excluded from the filtered water. Ultra filtration and Microfiltration are size-exclusion processes that reject particles, pathogens, and high molecular weight species. UF has pore sizes in the range of 0.001 to 0.1 micron, with a 0.03 nominal micron rating being typical.
UF is often used as pretreatment to an RO. The UF removes the majority of particulates that fouls an RO, as well as colloids, harmful bacteria, most viruses, and parasites such as cryptosporidium. The RO removes the remaining bacteria and viruses, dissolved salts, dissolved organics, color, taste, and odor.