How does a filter medium allow fluid to pass while trapping solid particles?

In filtration, a filter medium acts as a barrier that permits the passage of a fluid while retaining suspended solid particles. The mechanism by which this occurs depends on the type of filter medium used:

• Thin Barriers: Media such as filter paper, cloth, or screens function primarily through size exclusion. These barriers contain openings smaller than the particles being removed. As the fluid passes through, the solids are trapped on the surface, often forming a porous layer known as a filter cake.1
• Thick or En Masse Barriers: Media such as sand beds or porous ceramics utilize a more complex process. Even if the pores within the medium are larger than the particles, the fluid follows a tortuous path through the material. As the fluid moves, the particles are eventually entrapped within the finer interstices of the filter bed. This method allows solids to be distributed throughout the volume of the medium, which helps prevent the surface from becoming clogged or blinded.12

To facilitate this process, a driving force—typically a pressure difference—is applied to move the fluid through the medium.34