Filtration is an operation that separate solids from fluids (liquids or gases) by adding a medium through which only the fluid can pass. The fluid that passes through is called the filtrate. A filter is basically a device for separating one substance from another, and to do that it requires the placing of a filter medium in the way of the fluid flow, so as to trap the solids in some way. The filter then becomes any contrivance that is able to hold the filter medium in the best way to achieve the purpose of the filter process. Filtration is also important and widely used as one of the unit operations of chemical engineering. It may be simultaneously combined with other unit operations to process the feed stream, as in the biofilter, which is a combined filter and biological digestion device.
There is hardly a human activity, industrial, commercial or domestic, that is not affected by filtration. It is a very widely used process, from the kitchen counter top water filter to the enormous wastewater treatment plants, or from the delicate membrane ultrafilter to the rugged tipping pan filter of a mineral processing works. It has a major processing role in many industries, and all service applications, such as hydraulic control systems, would literally come to a halt without it.
The reason behind filtration is removal of fluid contaminants and collection of suspended solids. The major filtration variables are Flow Rate, filtration rating, amount of suspended solids, differential pressure, Area, Fluid compatibility.
Two main types of filter media are employed in laboratories: a surface filter, a solid sieve which traps the solid particles, with or without the aid of filter paper (e.g. Büchner funnel, Belt filter, Rotary vacuum-drum filter, Cross-flow filters, Screen filter); and a depth filter, a bed of granular material which retains the solid particles as it passes (e.g. sand filter). The first type allows the solid particles, i.e. the residue, to be collected intact; the second type does not permit this. However, the second type is less prone to clogging due to the greater surface area where the particles can be trapped. Also, when the solid particles are very fine, it is often cheaper and easier to discard the contaminated granules than to clean the solid sieve.