The function of filtration is to remove suspended matter impurities in water thoroughly through appropriate filter layer. It is generally believed that filtration is a comprehensive result of surface adsorption and mechanical retention. Firstly, when water begins to enter the filter layer from the top, some suspended substances in the water are intercepted by the surface of the filter layer due to adsorption and mechanical retention. At this time, the suspended solids will overlap with each other and bridge each other, so after a period of time, the surface of the filter layer seems to form an additional layer of filter membrane. In the future, this membrane will play a major role in filtration. In actual filtration operation, not only the surface of the filter layer, but also the suspended matter can be intercepted by the result of adsorption and coagulation when water flows into the middle of the filter layer.