An oil and grit separator system having twin vortex which uses gravity providing a twin vortex effect to remove sediment and oil from contaminated water. The system is designed to control storm water runoff and reduce the annual suspended solids loading of the granulometric particles. The conception and configuration of the system has a double action vortex. It ensures an optimal treatment level in either light or heavy rain and avoids re-suspension of particles. It also removes, separates and stores floating oil without any possible re-suspension of the matter.