Filtration is a mechanical or physical operation which is used for the separation of solids from fluids (liquids or gases) by interposing a medium to fluid flow through which the fluid can pass, but the solids (or at least part of the solids) in the fluid are retained. It has to be emphasized that the separation is NOT complete, and it will depend on the pore size and the thickness of the medium as well as the mechanisms that occur during filtration.

 Filtration is used for the purification of fluids: for instance separating dust from the atmosphere to clean ambient air.

 Filtration, as a physical operation is very important in chemistry for the separation of materials of different chemical composition in solution (or solids which can be dissolved) by first using a reagent to precipitate one of the materials and then use a filter to separate the solid from the other material(s).

 Filtration is also important and widely used as one of the unit operations of chemical engineering.

It is important not to confuse filtration with sieving. In sieving there is only a single layer of medium where size separation occurs purely by the fact that the fraction of the particulate solid matter which is too large to be able to pass through the holes of the sieve, scientifically called oversize are retained. In filtration a multilayer medium is involved, where other mechanisms are included as well, for instance direct interception, diffusion and centrifugal action, where in this latter those particles, which are unable to follow the tortuous channels of the filter will also adhere to the structure of the medium and are retained.

Depending on the application, either one or both of the components may be isolated. The filtration process separates particles and fluid from a suspension, and the fluid can be either a liquid or a gas (or a supercritical fluid). To separate a mixture of chemical compounds, a solvent is chosen which dissolves one component, while not dissolving the other. By dissolving the mixture in the chosen solvent, one component will go into the solution and pass through the filter, while the other will be retained. This is one of the most important techniques used by chemists to purify compounds.

Filtration also cleans up water streams or other river streams. Furnaces use filtration to prevent the furnace elements from fouling with particulates. Pneumatic conveying systems often employ filtration to stop or slow the flow of material that is transported, through the use of a bag house.

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