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Influence Of Rainfall Patterns On The Efficiency Of First Flush Storage Tanks


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S. Mambretti & U. Sanfilippo


The environmental impact caused by urban drainage overflows is related to both water quantity and water quality. About water quality, it has been widely recognized that usually catchment runoff is much more polluted during the initial part of a rainfall event, because of the deposits built up in the antecedent dry periods on the catchment surface and in the combined sewer conduits which are washed off in wet weather. To the aim of reducing such a first foul flush impact on the receiving water bodies, as the capacity of wastewater treatment plants is usually limited to flow rates not larger than 3–5 times the mean wastewater flow, a traditional structural work is the construction of storage tanks positioned immediately upstream the main sewer overflows of the network. Nevertheless, it is very difficult to design their volume, because the pattern of rainfall events has a relevant influence on the pollutograph, and synthetic hyetographs seem not to be adequately representative. This paper describes a check of the performances, in terms of reduction of the pollution discharged by the overflows to the receiving water bodies, of first foul flush storage tanks designed using synthetic hyetographs with different patterns (rectangular and Chicago). Such a check is carried out numerically through a comparison with the results given by the simulation of a historical rainfall series. The results show that synthetic hyetographs are able to represent convectivetype events, but not stratiform events. Therefore, in general, a proper design of first flush storage tanks should be carried out through historical series simulation, which is by far more complex to obtain but which ensures the correct representation of the influence of rainfall patterns, including also minor events which are very important from the point of view of water quality. Keywords: sewer overflows, environmental impact, first foul flush, storage tanks, synthetic hyetographs.


sewer overflows, environmental impact, first foul flush, storage tanks, synthetic hyetographs.