Abstract: A filter bed for separating large quantities of water from granulated blast furnace slag consisting of layers of solid matter, preferably gravel, of various grain sizes is presented. The filter mass (solid matter) is in individual adjacent containers or housings preferably comprised of sheet steel which are closed on all sides and have removable covers. The covers, the bottoms of the containers and the sides of the containers are all perforated to permit filtrate passage. The perforated sheets comprising the covers are all individually replaceable.