The primary sedimentation process aims to remove the settleable fractions of inorganic and organic fractions. To clarify, the settleable fraction is removed. There are also precipitates among the substances that make up the BOD0 fraction. The dissolved BOD0 then passes to the next treatment stage. There is no biological or chemical treatment in primary sedimentation; only physical treatment. The basic principle of this process is to utilize the sedimentation properties of the fractions to help the process function correctly in the subsequent treatment steps.