This work aims at determining the efficiency of the Seine Centre wastewater treatment plant during wet weather periods, regarding conventional wastewater parameters and two organic pollutant families, alkylphenols (AP) and polybromodiphenylethers (PBDE). During dry weather periods, the treatment in Seine-Centre consists of a physicochemical lamella settler and a 3-stage submerged trickling filter to treat carbonaceous and nitrogenous pollution. Under wet weather flows, the biological treatment is modified: 30% of the influent is treated on one stage only, while 70% is treated through two stages. In this study, four rain events have been sampled. The results gave some relevant data regarding: - raw water quality under dry and wet weather flows; - and efficiency of the treatment facilities. Regarding raw water quality, concentrations of conventional wastewater parameters are diluted; except for total suspend solids, whereas AP and PBDE concentrations are 1.5 to 5 fold higher under wet weather flows. Regarding efficiency, the lamella settler performs high removal of conventional wastewater parameters, AP and PBDE under wet weather flows, slightly higher than under dry weather flows. An important proportion of the particulate phase of AP (> 84%) and PBDE |> 87%) is eliminated in the settler. The biological treatment enhances the removal of these pollutants, for both dissolved and particulate phases. Except for nitrates and BDE-209, the modified biological treatment seems to have no influence on the treatment of conventional wastewater parameters, AP and PBDE.