When water enters the filter layer from the top, some suspended substances in the water are trapped by the surface of the filter layer due to mechanical retention and adsorption. At this point, the suspended objects overlap each other and bridge. Therefore, after a period of time, it seems to form an additional layer of filter membrane on the surface of the filter layer. In the subsequent filtration, this layer of filter membrane plays a major role, which is called "membrane filtration".
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