Abstract: An interceptor tank for primary treatment of rainwater runoff which may contain a variety of pollutants. The tank is a multi-chambered device which provides a partial separation of the pollutants contained in the effluent rainwater. A bypass system prevents overload of the tank during periods of heavy inflow conditions. Under low inlet flow conditions, an orifice in the bypass system functions to allow the effluent to drop into the first chamber which serves as a trash and debris collector. The effluent passes from the first chamber to a second chamber where free petroleum based products are separated and held. The effluent from the second chamber passes into a third chamber where sediment is separated from the effluent. The effluent from the third chamber may be fed back into the bypass conduit to be fed into a sewer system or alternately through an aperture in one of the exterior walls of the last chamber to a sewer.