Conventional settling for the treatment of urban stormwaters in separate sewer systems

Abstract : Stormwater from separate sewer networks in two urban and peri-urban watersheds was analysed to establish the concentrations of contaminants it contains, together with the distribution of dissolved and particulate pollutants. Furthermore, sedimentation of suspended solids (SS) and pollutants in these effluents was assessed by using two protocols currently applied in France: VICAS and VICPOL. Concentrations of SS are higher on Grigny (peri-urban site), whereas trace metal concentrations are lower. The distribution between dissolved phase and particulate phase pollutants follows the same trend on both sites. A non-negligible part of organic contamination and zinc and nickel contamination was found in the dissolved fraction. Median settling velocities measured for SS vary between 0.40 and 0.72 m/h for Tolbiac (urban site) and 0.52 and 1.96 m/h for Grigny (peri-urban site). At the two sites, the settling velocities for contaminants differ significantly from one rain event to another and can be very much lower than those of suspended solids. These results contribute to determine the settling times needed, as a function of the height to which settling basins are filled, to attain statutory objectives in SS reduction.