Evaluation of urban runoff volume to be treated at the outlet of separately sewered catchments

The purpose of this study was to estimate the volume of urban stormwater which must be treated in order to achieve a given efficiency in terms of pollutant retention in a storm basin. The data used for this were those from the French national programme (1980-82), collected in four separately sewered catchments whose area is less than 100 hectares. The intercepted loads for one year and for the most polluted events have been calculated for interception volumes varying between 0 and 300 m³ per impervious hectare. It was concluded that 100 m³/imp ha can intercept about 80% of the annual load of SS, BOD₅ and COD. But for the most polluted events, to achieve the same efficiency (80% of the produced load), a reservoir volume of 200 m³/imp ha is needed. The reduction of frequency of the most polluted events was also studied: for the events of which the load is in excess of 5% of the annual load produced, even a storage volume of 200 m³/imp ha is insufficient to entirely prevent these occurring.