This study aims at examining the performance of the ballasted flocculation unit (BFU) on treating combined sewer overflows (CSOs) and the evaluation depends on the values obtained of routine wastewater parameters and on the contents of a large broad spectrum of pollutants. Accordingly, the full-scale BFU at the largest wastewater treatment plant in Europe (Seine Aval plant near Paris, France) is investigated during three sampling campaigns. Of the 97 molecules targeted, 57 substances including 18 priority substances and 11 priority hazardous substances were detected in the BFU influents confirming that wet weather flow (WWF) treatment has definitively proven to be necessary. The WWF treatment by ballasted flocculation appears as a promising but not a fully adapted technology for use in densely urbanized areas to considerably mitigate the CSO impacts. On operating at the optimal chemical and sand doses, this process appears to be a suitable technology to remove particles, carbonaceous and phosphorous pollutants, particulate metals and most of hydrophobic organic compounds whilst nitrogenous pollutants and most of hydrophilic compounds are from poorly (<20%) to moderately (<50%) removed. The BFU appeared less sensitive to the influent concentration fluctuations and hydraulic peak load (at the scale of the campaigns considered) than to the adjustments of chemical doses and sand injection. Investigating the performance of such process, could serve to develop management strategies that enable mitigating the impacts of CSOs on receiving water in compliance with the Water Framework Directive objectives. (C) 2012 Elsevier B.V. All rights reserved.