The clarifier is a type of sedimentation basin in which the heavier solids sink to the bottom and the lighter materials float to the surface.  Grease and other scum are skimmed off the surface of the clarifier and are removed.  Sludge settles to the bottom and is sent to waste.  The clear effluent flows out of the clarifier and continues with the treatment process.

A clarifier can be placed at two stages in the treatment process.  The clarifier shown above is a secondary clarifier which treats wastewater flowing from the aeration basin, oxidation ditch, trickling filter, or other type of biological treatment.  

Clarifiers are also found earlier in the treatment process, directly after the grit chamber and comminutor.  These clarifiers are known as primary clarifiers and are similar in design to secondary clarifiers.