a separator tank for separating and trapping contaminants in rainwater and runoff is disclosed . according to one embodiment of the present invention , the separator tank comprises a container having a bottom wall , side wall , and top wall defining an internal chamber ; an insert located inside of the internal chamber , the insert comprising a weir defining an intake area between the weir and the side wall ; and a round - edged orifice positioned within the intake area ; an inlet conduit for introducing an influent liquid into the intake area ; wherein the weir is positioned such that the weir induces the influent liquid to flow in a swirling motion within the intake area . according to another embodiment of the present invention an insert for a separator tank is disclosed . the insert includes a weir defining an intake area for receiving an influent liquid ; and a round - edged orifice positioned within the intake area .