A retention treatment basin system and method 10 that handles an influent of combined wastewater and sewer overflows. The influent includes floatable 12 and settlable solids 14. The basin includes a ceiling 16. The retention treatment basin has one or more influent channels 20 that duct the influent into the basin. A weir assembly 22 extends upwardly from the floor, over which the influent flows. A cell 24 receives the influent passing over the weir. At least some of the settlable solids are retained in the cell. A baffle 26 traps at least some of the floatable solids in the cell. Part of the influent passes under the baffle. One or more up-flow screen systems 28 with bars having a triangular cross-section extend from the baffle that are pervious to fluids for receiving the influent and arresting movement of floatable solids therethrough. The screen system creates a floatable-solids depleted effluent.