Sand tank filtration principle:
Generally, a certain scale of quartz sand is placed in the sand tank. Within a certain range, the sizes and specifications of quartz sand are different. In the sand tank, quartz sand is arranged from top to bottom, from small to large. In normal filtration, water enters from the upper layer of sand and comes out from the lower layer. When the water flows through the filter layer from the top, part of the solid suspended matter in the water enters into the tiny empty hole formed by the upper filter material, which is blocked by the adsorption and mechanical retention by the surface layer of the filter material. At the same time, there is overlapping and bridging between these suspended matters. It is like forming a film on the surface of the filter layer to continue filtering the suspended matter in the water, which is called the filter material Membrane filtration of the surface layer. This kind of filtration is not only on the surface of the filter layer, but also on the middle filter layer when water enters. For filtration different from that on the surface layer, this kind of filtration is called infiltration filtration.
In addition, because the sand particles are closely arranged with each other, when the suspended particles in the water flow through those curved channels in the sand layer, there will be more opportunities to collide and contact with the filter material surface with time. Therefore, the suspended particles in the water adhere to the flocculent on the particle surface of the sand layer and contact coagulation occurs.
To sum up, the filtration of sand tank is the process of membrane filtration, infiltration filtration and contact filtration to further purify the water.