inner layer and its composition is as follows ______________________________________al sub 2 o sub 3 dk 206 martinswerk alumina 50 to 60 al oh sub 3 ol 104 3 to 7 martinswerk aluminum hydrideclay hp 64 ecc clay 10 to 15 kaolin ge ecc kaolin 18 to 25 chalk polcarb 60 ecc 1 to 5 calcium carbonatedispex n40 0 5 to 4 ml kgpolyacrylate dry matterdispersing agentwater 19 to 25 ______________________________________ the pore size of the first filter material layer 1 is about 1 0 to 3 mm bubble point 1 5 bar flow rate of filtered liquid 2 0 mm s and thickness of layer about 0 1 to 1 mm and in the present example 0 3 mm the pore size of the second thicker filter material layer 2 i e the inner layer connected with the said first filter material layer is 4 to 50 u03bcm and preferably substantially 4 to 10 u03bcm the bubble point is 0 5 bar the flow rate of the liquid filtered from the filtered material 2 0 mm s and a preferred thickness of the said layer is within the range of 2 to 6 mm and in the present example 2 7 mm the ratio of the predominant pore sizes in the layers 1 and 2 is at least 2.