Filtration is a process for the removal of solid particles from a suspension or two-phase system of particles in a fluid. The separation is achieved by some form of interaction between the particles to be removed and a filtering medium.
The simplest method of filtration is to pass a fluid with suspended particles through a porous media such that the solid is retained, while allowing the fluid to pass through. This method usually relies on the principle of size exclusion, which depends on the difference in size of the suspended particles and the void spaces in the filter medium. According to ancient Sanskrit and Greek writings, this simple method was used circa 4000 BC as a means to clean “impure” water by passing it through sand and gravel.
A disadvantage of filtration is the tendency for fouling which decreases the efficiency of the filter media over time. Fouling occurs when the suspended particles cover or clog the channels and pores of the media, such as dust in a vacuum cleaner bag. To mitigate the effects of fouling, filters must be regularly changed or regenerated. This can be problematic in the operation of continuous filtration systems.
For separations involving very small size differences between particles and media, chemical filters may be used. Chemical filters operate on the principle of adsorption, adhering the suspended particles to the surface of the filtration media. Similarly to physical filters, chemical filters also are limited by fouling. Even when a chemical filter is not physically obstructed, the active adsorption sites may still be blocked due to the adhesion of particles. This reduces the efficiency of the filter, requiring it to undergo cleaning or replacement. Most chemical filters are designed to be cleaned chemically or thermally to remove the undesirable particles and allow the filter to be re-used.
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