A method, system, and apparatus directed to an innovative approach for the treatment of stormwater utilizing hydrodynamic separator assembly designed to maximize flow movement for more efficient sediment removal and maximize clearance space within assembly to facilitate cleaning and increase storage capacity of trash, debris, and sediment. In particular, the described hydrodynamic separator is an improvement over other hydrodynamic separators known in the art with the inclusion of one or more down pipes which siphon water from the lower portion of the sump chamber. The inclusion of down pipes allows for an improved control of the velocity of the water flow through the device by distributing flow into more than one path.