Hydrodynamic separator

A separator unit includes a tank defining an internal volume and having an inlet and an outlet. An insert is provided within the tank, the insert including a down cylinder substantially centrally disposed within the tank and a baffle assembly at an external side of the down cylinder. The baffle assembly defines first and second inlet flumes for flowing incoming water from an external side of the down cylinder to an internal volume within the down cylinder. Inward ends of the first and second inlet flumes are positioned and oriented to create first and second vortex flows that progress downward within the internal volume, wherein a rotational direction of the first vortex flow is opposite a rotational direction of the second vortex flow in top plan view.