When a great amount of the raw water containing the SS at a high concentration as mentioned above is to be filtered, an amount of the SS to be captured in a filtering layer needs to be increased. For this purpose, the filtration is commonly carried out in a down-flow system by using a precipitatable granular filter medium having particle diameters of around 10 mm, which is greater than those employed in an ordinary sand filtration. However, if the diameters of the filter medium are large, the SS-removing percentage inevitably decreases. Further, since the SS is captured by the entire filtering layer by back washing, there are problems in that it is difficult to discharge the SS captured, and a great amount of wash water is required in the back washing.
When an up-flow filtration system is employed, filter media made of polypropylene or polyethylene having a density of 0.9 to 0.95 g/cm3 each in the form of cylindrical pieces is often used (In this application, the term “medium” means a single small piece, and the term “media” means a mass of a number of such small pieces or mediums). However, since such filter media have a small difference in density between water, there is a fear that the filter media having cylindrical forms flow out. In view of this, a screen is provided under a filter media-filled layer. In this case, the screen is clogged after being repeatedly subjected to the filtration and back washing operations, which disenables the filtration. Furthermore, it is very difficult from the standpoint of the structure of the apparatus to eliminate the clogging of the lower screen.
Further, since the screen exists during back washing, the contaminant clogging the screen cannot be discharged and removed through gaps being formed among the filtering media due to their expansion. Therefore, the filter media need to be repeatedly washed with air and water, which requires a long-time back washing step. In addition, the mesh of the screen is often generally increased to as much as 10 mm or so in order to prevent the lower screen from being clogged, the grain diameters of the filter media need to be greater than that mesh, and the thickness of the filter media needs to be larger than 2 m or so in order to enhance the SS-capturing efficiency. In this case, the apparatus unfavorably becomes very large.