Method of clarifying liquids

Abstract

A longitudinally elongated settling tank with a converging longitudinal hopper bottom along the length thereof has a longitudinally elongated travelling bridge reciprocating continuously or intermittently through a relatively short stroke between the inlet and outlet ends of the tank from which depends a plurality of longitudinally spaced sludge pick-up heads each sucking sludge from a zone in the bottom of the tank during a preset cycle. Liquid fed into one end of the tank forms a settling pond from which clarified liquid flows over longitudinal weirs at the opposite end of the tank. In a preferred embodiment a first pick-up head serves the inlet end zone of the tank in which the heavier sludge settles from the pond, a second pick-up head serves the middle zone of the tank while a third pick-up head serves the outlet end zone of the tank. The first head is evacuated as the bridge advances toward the outlet end of the tank until the head reaches the end of the inlet end zone which it serves whereupon the bridge is retracted and the now advanced second head is evacuated during the retraction to remove lighter sludge from the bottom of the middle zone of the pond. Then, the first head is again evacuated during the next advance of the bridge to pick up the heavy sludge that has settled in the interim in the bottom of the inlet end zone of the pond. The bridge is next again retracted and the third suction head is evacuated to remove the lightest sludge at the outlet end zone of the pond. Thus the heavier sludge is removed twice during two full stroke cycles of the bridge while the lighter sludges are successively removed only once during these two cycles and the length of the stroke is much less than the length of the pond. Because of the short stroke of the bridge, the tank can be reinforced with internal framework between the paths of the suction heads permitting use of lighter sheet metal for the tank. A skimmer is automatically lowered into the top of the pond as the bridge advances to sweep oil, scum, and other material floating on the pond into an outlet and is retracted above the pond on the return stroke of the bridge.