Membrane filters are surface filters with a precise micro-porous structure. They are used to separate, remove particles or collect micro-organisms for analysis from a fluid. Particles bigger than the nominal porosity remain on the filter surface, whilst smaller particles goes though the filter unless other interactions into the filter retain them into the filter matrix. The microfiltration is slower than the filtration with the filter papers (depth filters). Membranes are made of different polymers and are available in several diameters and pore sizes. Membrane filters are used in microbiological quality control procedures for a wide range of industries; food, beverage, pharmaceutical, cosmetics, etc.