According to the retention of solid particles on the filter medium, filtration is divided into several typical types include surface filtration, depth filtration, and cake filtration.

Surface filtration


The surface filter media has large pores; it will capture the particles more significant than its orifice on the surface, while the smaller particles pass through the filter media along with the fluid. This filtration theory plays a significant role in the bar screen, plain weave mesh and filter membrane.

Depth filtration

Depth filtration theory is a complex mixture. This type of filter media has a three-dimensional pore structure that traps solid particles smaller than the pores, and even far smaller can be trapped in the deep layers of the media. Under the action of inertial force, hydraulic pressure or Brownian motion, the solid particles first contact the wall of the flow channel, and then attached on the wall, or the particles agglomerate together under the action of van der Waals or other force. The depth filtration theory is shown in the filtration of micron felts and nonwovens and is especially crucial in high-efficiency air filters and depth sand filters.

Cake filtration

Simply the cake filtration refers to a filter cake formed on the surface of the filter media at the beginning during the filtration process, and the filter cake itself acts as a filter media then. The solid particles are trapped by the filter cake, while the fluid passes through the filter cake, and the medium gets into the filtrate. The filter cake gradually thickens, and the liquid slowly clears.

In the initial section of filtration, the filter media trap the larger particles than its pores, and the fine particles are passed through the filter media with the fluid. Now the fluid is not clear enough and should be recycled back to the suspension tank. While large particles are trapped by the filter media, much smaller particles “bridge” on the pores. The “bridge” will retain the fine particles so that the trapped large particles and small particles gradually accumulate to be a pancake even a thick filter cake. After that, the filtration is cake filtration. The filter media is the crucial part at the beginning of the filtration, and will permanently affect the structure of the filter cake and the entire filtration process.

The cake filtration theory is particularly distinct in the belt filter, and press filter.

Another new filtration method – Cross-flow filtration

To increase the filtration efficiency for the tiny particles or metal hydroxides, The cross-flow filtration method which is entirely different from the conventional cake filtration has been introduced.

The difference between Cross Flow Filtration and traditional cake filtration or dead-end filtration is that during cross-flow filtration, the slurry flows rapidly parallel to the surface of the filter medium, and the filtrate flows out at a low-speed perpendicular to the filter medium surface. The directions of the two flows are vertically interlaced, which called cross-flow filtration.

The cross-flow filtration rate is almost constant, and the filtration pressure does not rise rapidly over time, since the rapid flow of the filter exerts a shear sweep on the particles accumulated on the medium, thereby suppressing the thickness of the filter cake increase.

Cross-flow filtration is mainly used for filtration and concentration of the filtered slurry, cannot obtain a relatively dry filter cake.

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.

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