Abstract: A hydrodynamic separator to separate floatable material from a liquid includes a separating vessel having a cylindrical outer wall; an inlet means for introducing the liquid into the vessel in a manner to promote a low energy rotational flow of liquid in the vessel; a base at one end; a top wall at the end opposite the base, the top wall including an axial outlet opening for receiving a flow containing floatable material separated from the liquid flow to the vessel; a flow modifying member provided within the vessel to define with the top wall an annular opening which is spaced from the outer wall; an annular dip plate spaced from the outer wall of the vessel, the dip plate comprising an upper edge spaced from the top wall to define a gap for the flow of liquid containing floatable matter; and an outlet separate from the axial outlet opening in the top wall, comprising a conduit for removing a primary liquid flow from the vessel, which conduit communicates with the interior of the vessel at a substantially ax