In the cake-forming filtration, the particles of a suspension are retained by the filter medium and form a growing layer, the filter cake.
In the initial phase of filtration, a thin top layer must first form on the filter medium. This is usually done by particles which assemble and form bridges over the openings of the filter medium. The first cover layer and the subsequently accumulating filter cake have much smaller pores than the filter medium. The filter fineness is therefore mainly determined by the properties of the filter cake and is generally much lower than the absolute filter fineness of the filter medium.
In the further course of the filtration, the liquid must first flow through the filter cake before it can flow through the filter medium. In view of the energy requirements of the filtration, the flow resistance of the filter cake is therefore of decisive importance.
Fine particles form a filter cake with high flow resistance. Halving the average particle size of the particles to be separated results in roughly a quadrupling of the flow resistance.