Sludge Removal – Sludge which settles in primary tanks is collected mechanically and pumped to further processing.
Scum Removal – Scum which floats in primary tanks is collected by skimmers or sprays and digested, burned, or buried.
Dissolved Air Flotation (DAF) – Dissolved air is sometimes used to float particles to the surface, where they are skimmed off.
Vacuum Flotation – A vacuum is maintained above the liquid level in a DAF tank to aid in floating the solids.
Imhoff Tanks – Rectangular or circular settling tanks have a sludge digestion compartment in the bottom and baffles to contain scum.