Usually, the actual filtration is not work in a single mechanism, but two or more mechanisms are working together. For example, cake filtration occurs after surface filtration, which large particles retained by the screen at the beginning of filtration, and these larger particles are one of the components of the filter cake. At the same time, smaller particles form a “bridge” over the pores of the medium, and that the particle even smaller than the pore of the filter media maybe cannot pass through the medium and become part of the filter cake as well. Thus in the cake filtration, it’s surface filtration at initial, and depth filtration subsequently.
Also, in the depth filtration progresses, the small particles are deposited in the large pores of the filter medium, as shown in Figure 1-1. When the pores are fulfilled by the particles gradually, it turns into surface filtration. It means, in the depth filtration coupled with other filtration mechanisms.
Filtration could also be divided into five categories according to the processed particle size: particle filtration (F), micron filtration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis filtration (RO). The latter four are collectively called membrane filtration so that they can be classified into general filtration and membrane filtration (precision filtration). Usually, the particle size treated by general filtration is greater than 1/20 mm, up to 1 mm, and membrane filtration is to remove solid particles less than 10 um.