In the first step of treatment debris, suspended and floating material is removed physically from the water. When the wastewater enters a sedimentation tank, it slows down and sand, grit, cinders, small stones and the suspended solids sink to the bottom. This mass of solids is called primary sludge. Various methods have been devised to remove primary sludge from the tanks. Floating materials like oil and grease are skimmed or filtered. Some plants add chemicals to the primary process (chemical coagulation) to enhance the removal of suspended and to diminish the extent of dissolved solids. (Tchobanoglous et al. 2003 & EPA 2004)