In engineering practice, more and more frequently innovative installations are being introduced for the removal of hazardous, light liquids in order to make their penetration to surface waters impossible. Those installations are often integrated with by-pass systems, where storm weirs are built into the main body of the separator, instead of being installed in separate chambers. Such technical solution provides many economical profits and is related with easier installation of the system and its’ further exploitation. However, they quite frequently do not fulfill the adopted hydraulic criteria, and because of this, fail to acknowledge technical feasibility. They simply do not guarantee the assumed flow partitioning inside the devices, which causes general installation overload. In well -designed systems, nominal discharge capacity Qn is a defined percentage of maximum capacity (from 10 to 20% of Qmaxmax > Q > Qn in an open channel to a degree , that Qn travels to the separator, and the rest goes into the by-pass system. In consequence, when the flow in the main supply channel Q exceeds Qn, contemporary separating structures (weirs) are likely to discharge contaminants excess into the by-pass system. The improvement of this situation requires research on physical models in a real scale, that was undertaken in this paper.