The specially structured material of the form cut filter mats takes over the cleaning process they are penetrated by the water and mechanically trap large and 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 and re used but the white filter membranes should be switched everytime you clean your filtering