Microfiltration (MF) - Filters solids of .1 - 10 micrometers in size, like bacteria
MF is used on process streams to remove small-diameter dispersed solids such as bacteria, fat and oil globules. Pressure is used to force water through a semi-permeable membrane with small pores. MF is often a pretreatment before ultrafiltration.
Ultrafiltration (UF) - Filters solids of .001 - .1 micrometers in size, like viruses
UF uses membranes in which the pores are somewhat smaller than MF. Salts, sugars, organic acids and smaller peptides pass through the pores of the membrane, but viruses, proteins, fats and polysaccharides do not. A cross-flow of water prevents build up on the membrane and makes it possible to divide the feed stream into two others: a dirty water stream (retentate) and a clean water stream (permeate). UF is often used before nanofiltration or reverse osmosis.
Nanofiltration (NF) uses membranes with pores that are even smaller than UF.
Small ions pass through, but larger ions and most organic components do not. This technology is ideal for filtering out micropollutants, large salts and coloring agents, while small monovalent salts pass through. NF is used for the high-yield concentration and demineralization of products like whey. They’re also used in producing low-alcohol beverages, because the alcohol component passes through the membrane, while the color and aroma remain.
Reverse osmosis (RO) uses membranes with pores so tiny only small fractions of salts can pass through.
Certain organic compounds with low molecular weights can also pass through to a limited extent, but it is impossible for other components suspended or dissolved within the liquid (salts, sugars, etc.) to do so. RO membranes are often used to further filter streams that have already been ultra- or nanofiltered.