The main objective of primary treat is removal of organic and inorganic large materials and solids that are found in raw wastewater by sedimentation and skimming. This preliminary treatment is important since it enhances subsequent treatment’s operation and maintenance. the primary treatment involves grit removal, coarse screening and fragmentation of larger objects. It also involved reducing the Biochemical Oxygen Demand(BOD), which is the amount of oxygen required to be consumed by the microorganisms to break down the organic materials in wastewater. approximately 50 to 70% of solids,some oraganic phosphorus and nitrogen, heavy metals, 25 to 50% of BOD and 65% of grease and oil are removed at this stage.