After passing through preliminary treatment, the water arrives at Primary Treatment. The purpose of primary treatment is to settle material by gravity, removing floatable objects,and reducing the pollution to ease secondary treatment. Primary Treatment aims to reduce the Biochemical Oxygen Demand (BOD) and Total Suspended Solids (TSS) in the wastewater. BOD, in short, is the soluble pollution in the wastewater and TSS are the solids that are in suspension in the wastewater. Typically, primary treatment can remove 25-40% of the BOD and 50-70% of the TSS before the water goes to secondary treatment. The WRF uses circular clarifiers in the primary treatment process, the solids settle to the bottom, the floatables collect and store in a tank, and then both are pumped to the anaerobic digestion process. The remaining water then flows to secondary treatment for further treatment.