distributed over the upper surface of the filter again in typical fashion three sieve layers 81 82 and 83 are provided whereby a first sieve layer 81 represents the largest sieve and is formed in general of lava tuff rocks a layer 82 consisting of porous synthetic for example hard foam rubber or a mesh of polyamide fibers polyacrylonitrile cellulose acetate polyester or other synthetic fibers is provided below the upper layer this second layer 82 represents a sieve with a medium sieve opening the lowest of the three layers namely layer 83 is formed of a porous fleece with relatively fine openings so that the water passing through the three layers is practically freed of suspended particles glass wool or a porous hard foam rubber with correspondingly fine openings can also be provided at this location instead of porous fleece a filter layer 84 to be passed through following the sieve layers consists of a mixture of granular matter and namely preferably of coal in a piece size from 1 to 2 mm diameter silica sand of the same size as well as gravel scree and or of synthetic granulates of the same size for a biofiltration a biological breakdown then again occurs in this area of the filter whereby a further mechanical filtering occurs in conjunction with the biological breakdown by means of the employment of the silica sand and the scree the water then flows through a synthetic sieve 85 into the second part of the double filter container this synthetic sieve is again formed of polyamide fibers beneath sieve 85 is a layer 86 consisting of non acitve hydracrabon finally a lower space 87 is filled with activated carbon which then removes the very last milligram of.