The process of passing liquid or gas through a porous substance, and separating, removing, and collecting particulate matter from suspended solids and suspended substances (fine particles, microbes, polymer molecules, mist, aerosol, etc.) from the fluid is called "filtration," and the porous substance used at this time is called a "filter."
In the filtration method, there are natural filtration performed by gravitational force, pressure filtration performed by positive pressure, vacuum filtration performed by negative-pressure, centrifugal filtration performed by centrifugal force.

Filtration mechanisms

There are three mechanisms to the filtration.

��	Surface filtration	:	This filtration mechanism mainly retains particulate matter from the fluid on the surface of the filter.
��	Depth filtration	:	This filtration mechanism retains particulate matter not only on the surface of the filter but also at the inside of the filter.
��	Cake filtration	:	By this filtration mechanism, the cake accumulated on the surface of the filter is itself used as a filter.

The applicable filtration mechanism is different depending on the size and character of the substances to be filtrated, and the filtration method.
However, the actual mechanism of almost all the filtrations performed in combination of the three mentioned mechanisms.

Three major factors of filtration

There are three major factors of filtration efficiency, flow rate and filtration life.
Filtration efficiency is the ratio of the upstream suspended solids concentration compared to the down stream suspended solid concentration which has passed through the filter. The flow rate is the volume or weight of the fluid which has passed through the filter at the unit area at the unit time. The life of the filter is the time it takes until the filter becomes clogged with particles and other suspended matter so that it can no longer function at a certain flow volume, it is also defined as the time it takes until a certain pressure drop occurs (the difference between the inlet side pressure and the outlet side pressure).
High filtration efficiency, high flow rate and long filtration life are required to perform an efficient filtration.
In order to perform an efficient filtration, it is necessary to select an appropriate filter to suit the fluid to be filtered.