Filter paper is a semi-permeable paper barrier placed perpendicular to a liquid or air flow. It is used to separate fine solids from liquids or air.
Filter paper comes in various porosities and grades depending on the applications it is meant for. The important parameters are porosity, particle retention, flow rate, strength, compatibility, efficiency and capacity.
There is two mechanisms of filtration with paper; volume and surface. By volume filtration the particles are caught in the bulk of the filter paper. By surface filtration the particles are caught on the paper surface.