| 摘要 |
PURPOSE:To enhance filtration capacity and to reduce back washing time by charging a floating granular material having a diameter larger than the particle diameter of a granular filter medium at thickness of two-times or more of the particles diameter thereof into the upper part of the filter layer of the granular filter medium. CONSTITUTION:A floating granular material 7 (especially 20-100mm particles diameter, 0.05-1.5 apparent specific gravity, a spherical shape, a rectangular parallelopiped and a columnar shape, etc., are preferable) which has a diameter larger than the particle diameter of a granular filter medium and can be freely moved is charged at mean packing thickness of two-times or more of the particle diameter of the granular material 7 into the upper part 6 of a filter layer 2 consisting of the granular filter medium such as sand. When the flow of raw liquid is passed through the layer of the floating granular material 7, it is broadly dispersed and rectified and therefore the filter layer can be uniformly utilized and filtration capacity is enhanced. On the other hand, at a time of back washing, the granular material 7 is mixed together with the filter medium and floated and dirt stripping of the filter medium is promoted by its contact, collision and rolling and thereby back washing time can be shortened. When the back washing is finished, the granular material 7 is built up to a layer to the upper part of the filter layer 2 in its former condition by means of the difference of settling velocity for the filter medium.
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