The purpose of a primary sedimentation basin is to remove settleable organic solids. It is achieved in large basins under relatively quiescent conditions. The settled solids are collected by mechanical scrapers into a hopper, from which they are pumped to a sludge processing area. Oil, grease, and other floating materials are skimmed off from the surface. The effluent is discharged over weirs into a collection trough. Often, chemicals are added to enhance the removal efficiency of a plain sedimentation basin. The colloidal particles are flocculated and removed with the settling floc. These particles would otherwise not be removed in plain sedimentation. Chemicals may also be added to precipitate soluble organics, phosphorous, and heavy metals. The objectives of this chapter are to present the basic design and selection criteria for plain and enhanced primary sedimentation basins.