First flush storage tanks of transit or capture kind in runoff networks collecting multiple subcatchments

The paper discusses the efficiency, in terms of the reduction of both water volumes and pollution loads discharged to receiving water bodies, of first flush storage tanks in stormwater runoff networks of urban drainage systems, depending on their specific capacity, their kind (transit or capture) and their position inside networks collecting multiple subcatchments (one single tank downstream the whole system rather than multiple tanks, i.e. one per each subcatchment, in series or just in parallel). The numerically achieved results show that the distance between two confluences has a very slight influence on the amount of both water volumes and pollution loads washed off in wet weather from the catchment surfaces. On the contrary, as expected and as already known in literature, the capacity of the tanks has the most relevant role on annual basis in terms of both overflow volumes and discharged pollution loads, while a very smaller influence is related to their position. Moreover, it comes out that the bigger complexity of installation, operation and maintenance required for the capture kind of the first foul flush storage tanks, in comparison to the transit one, does make sense only when their specific capacity is significantly higher than 25 m³/ha