Any thick, viscous mass, usually a sedimented or filtered waste product. Examples are (1) aluminum ore tailings (red mud), (2) asphaltic petroleum residues, and (3) municipal waste (Imhoff and activated sludge). All these may be used as components of useful products, e.g., (1) in steel manufacturing, (2) in roofing and road treatment compositions, and (3) as a base for fertilizers.See Sewage Sludge.