During primary treatment, wastewater is temporarily held in a settling tank where heavier solids sink to the bottom while lighter solids float to the surface.
Once settled, these materials are held back while the remaining liquid is discharged or moved through to the more rigorous secondary phase of wastewater treatment.
These large tanks are also often equipped with mechanical scrapers that continually drive collected sludge in the base of the tank to a hopper which pumps it to sludge treatment facilities.