Abstract: A separator tank for rainwater and waste-water includes a container defining an upwardly open chamber, with a lid to close the chamber. An inlet is at a first location and an outlet is at a second location. A passageway forms part of the container and communicates the inlet with the outlet. A first opening is adjacent the inlet and communicates the passageway with the chamber, while a second opening is adjacent the outlet, also communicating the passageway with the chamber. The passageway itself is sealed from communication with the chamber, except for the openings. A weir is provided within the passageway, disposed with respect to the first opening such that, under relatively low entry flow rates, all entering materials are contained by the weir and flow through the first opening and into the chamber. Under relatively high entry flow rates, the entering materials overflow the weir and are delivered by the passageway to the outlet.