The filtration of coarse particles with a size of up to several microns can be carried out by the so-called. simple filtration. The particles, retained by the filter during normal filtration, over time they accumulate as a layer of sediment, which causes an increase in filtration resistance. This requires periodic cleaning or replacement of the filters.
In cross-flow filtration, the main phase of the treated solution due to the applied pressure moves parallel to the membrane surface, thus washing away the accumulated (detained) particles, which keeps the sludge layer relatively thin and the filtration resistance low. This allows relatively high flux values to be maintained for a long period of time. Cross-flow filtration is used to separate particles with a size of approx 10 microns to dissolved molecules several angstroms in size.
Microfiltration is the coarsest of all filtration types. Its purpose is to separate small particles, suspended in liquids. Microfiltration membranes are classified according to the diameter of the retaining holes, which is actually the diameter of the smallest particles, which are retained in the membranes, usually in the boundaries between 0,1 and 10 the micron. Microfiltration requires the lowest operating pressure of all, is usually within the limits of 0,7 to 3,5 asks.
Ultrafiltration is used to separate large macromolecules such as protein and starch. Ultrafiltration membranes are classified according to the molecular weight of the retained particles, which is defined as the molecular weight of the smallest molecules, 90 % of which are retained by the membrane. The molecular weight of the retained particles during ultrafiltration is within the range of 1000 to 500000. Ultrafiltration requires a slightly higher operating pressure than microfiltration, usually within the limits of 1,5 to 15 asks.
Nanofiltration membranes retain dissolved molecules with a molecular weight of 100 to 1000. Nanofiltration membranes are classified according to the molecular weight of the retained particles, like ultrafiltration membranes, or according to the percentage of sodium chloride transmittance, like reverse osmosis membranes. Nanofiltration is used at an operating pressure in the range of 7 to 35 asks.
Reverse osmosis involves the membranes with the smallest openings, which are able to separate even the smallest dissolved molecules. Reverse osmosis membranes are classified according to the percentage of sodium chloride permeation in an aqueous solution under specific conditions and ranges from 99,95 to 95 the percentage. To overcome the high osmotic pressure of solutions of small molecules, it is necessary to apply a high working pressure within the limits of 15 to 100 asks.