There are four gravity thickener basins. The basins receive sludge and scum (stuff floating on top of the water in each basin) flow from the four primary clarifiers along with the scum from the ten final clarifiers.

The gravity thickeners provide a quiescent condition where the primary sludge can settle to the bottom of the basin further thickening the sludge prior to pumping to the sludge holding tanks (SHT) in the solids processing building. The quiescent condition in the gravity thickeners also allows the scum and grease to float to the surface of the basin.

A rotating collector mechanism scrapes the bottom sludge toward a center sump for removal and skims the floating material into a scum manhole. Scum pumps transfer the scum from the manholes to the scum concentrator in the solids building. This scum is then further thickened and then pumped to the Incinerator as a fuel source.