Scale model tests were used to determine the efficiency relationships for a hydrodynamic separator (Storm KingTM) and a high-side weir overflow. With the help of these relationships optimal dimensions of an improved high-side weir overflow chamber could be found. The steady state efficiency for this improved overflow chamber seems to be comparable to that of a hydrodynamic separator, when both devices are subjected to the same surface load. On the other hand, during real storm events, the storage inside the overflow structure can play an important role. This role is investigated by calculating the removal efficiencies for both structures when 4 monitored storm events are used as input. When higher removal efficiencies are wanted, the storage effect gets more and more important and as a consequence the hydrodynamic separator can technically as well as economically compete with simpler structures like e.g. a high-side weir overflow. These findings are confirmed by a simple construction cost estimation.

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