Before disposal, sewage is treated in sewage treatment plants (STPs) to make it less polluting. The initial steps basically involve the physical removal of particles - large and small - from the sewage through filtration and sedimentation, which is known as the Primary Treatment. These are removed in stages: initially, floating debris is removed by sequential filtration. Then the grit (soil and small pebbles) are removed by sedimentation. All solids that settle form the primary sludge, and the supernatant forms the effluent. The effluent from the primary settling tank is taken for secondary treatment.