Screening also removes large pieces. That are smaller than the debris removed in pre-treatment including plastic packets, cans, rags and sticks. This is usually done with a raking bar but can be done manually. Removal of grit includes removing sand, small stones and broken glass. This is mainly done in large systems, because in small systems the grit may be removed during the screening. In larger systems fat and grease is removed with skimmers that collect what is floating on the surface. This is done in most plants with mechanical surface skimmers as primary clarifiers. The solids that fall to the bottom of the clarifier are know as sludge and pumped out regularly to ensure it doesn’t impact the process of separation. The sludge is then discarded after any water is removed and commonly used as fertilizer.