What is the principle of sand filter?

A certain scale of quartz sand is generally placed in the sand tank. The size and specifications of the quartz sand are different within a certain range. In the sand tank, the quartz sand is arranged from top to bottom, from small to large, and the water is from the sand during normal filtration. The upper level enters and the lower level exits. When the water flows through the filter layer from above, part of the solid suspended matter in the water enters the tiny holes formed by the upper filter material and is trapped by the surface layer of the filter material by adsorption and mechanical retention. At the same time, these suspended solids are trapped Overlapping and bridging effects occur again, as if a thin film is formed on the surface of the filter layer, which continues to filter suspended substances in the water. This is the so-called membrane filtration of the surface layer of the filter material. This filtering effect is not only on the surface of the filter layer, but also has this interception effect when water enters the middle filter layer. In order to distinguish it from the filtration of the surface layer, this effect is called osmotic filtration.
In addition, because the sand particles are closely arranged with each other, when the suspended particles in the water flow through the tortuous pores in the sand layer, they will have more opportunities to collide and contact with the surface of the filter material and time. The suspended matter on the particle surface of the sand layer and the flocs adhere to each other to cause a contact coagulation process.
In summary, the filtration of the sand tank is to further purify the water through the process of membrane filtration, permeation filtration and contact filtration.

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