Filtration is the process of separating material, usually a solid, from a substrate (liquid or gas) by passage of the substrate through a septum or membrane which retains most of the materials on or within itself. The septum is called a filter medium, and the equipment assembly that holds the medium and provides space for the accumulated materials is called a filter. The filtration is a physical process; any chemical reaction is inadvertent and normally unwanted. The object of filtration may be to purify the fluid by clarification or to recover clean, fluid-free particles, or both.

There are two kinds of filters: surface filters and depth filters. With the surface filter, filtration is essentially an exclusion process. Particles larger than the filter's pore or mesh dimensions are retained on the surface of the filter; all other matter passes through. Examples are filter papers, membranes, mesh sieves, and the...