Settling velocity of particulate pollutants from combined sewer wet weather discharges

Settling velocities of TSS and of particulate pollutants (COP, PDCO, PTKN, PCu, PPb, PZn, PPAH) measured on a wide range of wet weather flow (WWF) samples collected at different levels of the Parisian combined sewer system are reported. The recorded V30 (0.01 to 0.1 mm.s^-1) and V50 (0.09 to 0.6 mm.s ^-1) values exceed by a factor 10 those of dry weather sewage and also exceed the values measured for pavement runoff. These values lie however often below the 0.28 mm.s^-1 reference value considered in France for the design of WWF settling facilities. A decrease in settleability is observed between a small upstream catchment and larger scaled downstream catchments. The settling behaviour of particulate pollutants varies depending on the considered parameter and can differ significantly from the TSS behaviour, due to a non homogeneous distribution of micropolluants over the different classes of particles. PZn and PTKN appear far less settleable than TSS, whereas PPAH show higher settleability.