Aerobic waste sludge digester-thickener orbital system and method

A thickener well (25) is provided in one end of a digester orbital ditch (10) having a pair of channels (17,18) and an elongated center partition (19) spaced from a pair of ditch endwalls (14,15), the overall ditch forming a basin (16) for digestion of wasted sludge. A curved endwall (15) of the ditch provides a common curved top outer peripheral wall of the thickener well. The remainder of the well has a cylindrical configuration integral with the orbital ditch bottom (29) and extending downwardly therefrom to a position below the horizontal plane of the ditch bottom. A sludge raking structure (28) is positioned at the bottom of the well for periodically raking settled sludge into a sludge outlet (36) at the bottom of the well. A surface aerator (20) is provided between and spaced from the other ditch endwall (14) and a juxtaposed end of the partition (19) or other aeration apparatus mounted in a channel(s) to aerate and propel a liquid-solids sludge mixture wasted from a secondary treatment apparatus and fed into the orbital ditch. The mixture is circulated in the ditch and across and through a top portion of the well. The orbital ditch (10) may be decanted by turning off the aerator (20), allowing a clear liquid top layer to form in the ditch and decanting the clear liquid from the tank by lowering a weir (46) or by adding additional activated sludge to the ditch to displace clear liquid over a fixed weir.