837,595. Gravity separation apparatus. DORR-OLIVER Inc. April 13, 1956 [April 29, 1955], No. 11278/56. Class 46. A multitray clarifier comprises a cylindrical tank 10 which is divided by trays 12, 16, 18, 22 into a feed compartment 29 and at least one subjacent set of superposed settling compartments. The embodiment shows two such sets where the upper compartments 15 and 21 of each set form clarification compartments and the lower compartments 17, 23 form clarification and thickening compartments, each set being hydraulically sealed from the other. The mixture to be clarified is fed tangentially to the feed compartment 29 and passes through ports 47, 49 of a rotatable shaft 40 to feed wells 19, 25 formed by cylinders 20, 26 extending from the trays 16, 22. The shafts 40 is divided by a partition plate 42 to provide separate feeds to each feed well and scrapers 63 in the feed compartment 29 ensure an equal feed of solids which readily settle in this compartment. Clarified liquid overflows through outlets 92, 94, 96, 98 at the top of each compartment to a tank 100, the outlets being adjustable in height to regulate the overflow and thus control the feed to each set. Scrapers such as 67 are provided in each compartment to impel settled solids from the upper compartment of each set to the lower compartment and the solids in the lower compartments to the centre where they are drawn off through pipes 78, 82 by a mud pump 84. The scum in the feed compartment is also removed by the pump 84 through a conduit 36. Means are provided to vent the apparatus to atmosphere in filling the tank prior to operation.