Urban discharges during wet weather: what volumes have to be treated?

To treat urban wet weather discharges, storage tanks seem to be efficient tools. Using 17 French catchments with an average of one year of measurements, we have estimated the volumes required, at the outlet of the catchments and at the inlet of a tank, to intercept the suspended solids load discharged during several months and during the most polluted events. The results vary widely from one basin to another, between 30 and 150 m³/active ha to intercept 80% of the annual suspended solids load. This does not allow us to draw conclusions about the volumes required in an average situation because rainfall depths and return periods of the rainfall events are different between the catchments. These factors seem to have a big influence on the interception efficiency of the storage tanks. The results can only be seen as representative of the year during which the measurements were taken.