Ceramics are seldom completely dense, because in most cases pores cannot completely avoided. Actually, the existing of pore is one of the main defeats that affect the life term of the most convectional ceramic products, such as table wares, sanitary wares and tiles. However, pores in ceramics can be used in many important applications on the other hand, to fulfill certain functions, one of which is filtration and separation.

Filtration or separation, is 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. The porous substance that is used in the process is called a “filter”.

Filtration can be divided into two categories according to where the particles are trapped: surface filtration and depth filtration. In surface filtration, any particle smaller than the pore diameter would be swept through the pores, and any particle larger than that would remain on the upstream surface. Whereas in depth filtration, particles enter into pores and move through tortuous pathes in the filter medium until they are trapped on the pore surfaces either because where the pores are too small or by the mechanisms of direct or inertial interception or diffusion.

Where solid or particle concentrations are higher, as is the case with a large number of process separations, “cake filtration”, or fouling and concentration polarization (mostly in ceramic membrane processes) are developed on the upstream surface, creating a resistance to liquid flow. “Cake filtration” can be regarded as a kind of surface filtration, which is formed by the retention particles through jostling with one another at the entrance to each pore, bridging together across the opening to the pore and then acting as the filtration medium. In ceramic membrane separation, cross flow filtration is a typical method employed to reduce fouling and/or concentration polarization, and so increase flux rates.

Cross flow filtration (or sometimes called tangential filtration) is a different filtration mode where the feed stream is perpendicular to the permeate stream, comparing to the dead end filtration (through flow filtration), where the feed steam and retention stream are in the same direction. Cross flow filtration utilizes local shear in the fluid stream to decrease retention particle boundary layer thickness and to increase permeate flux.