The specially structured material of the form-cut filter mats takes over the cleaning process. They are penetrated by the water & mechanically trap large & small particles. After a while the particles settle in the special sponge that intensively filters out biological contaminants. The set consists of: 1 x pre-filter mat (blue) 4 x filter membranes (white) The blue pre-filter can be washed out & re-used, but the white filter membranes should be switched everytime you clean your filtering unit.