Pollution loads from stormwater overflows

The knowledge of the volume of combined effluents outflowing from overflows is not enough to allow a direct evaluation of polluting loads discharged into final receptors; the hypothesis of complete mixing between sewage and stormwater flow, fed in at a pollutant concentration level equal to zero, hasn't proved to be successful. The amount of the outflowing loads largely depends on the contamination of the stormwater runoff before inflow into the drainage system and on sedimentation and resuspension phenomena. This paper reports the main aspects connected with wet and dry atmospheric deposition of pollutants and with paved surface wash-out phenomena. The origin of pollutants flush, due to the resuspension and mass transport of polluting substances stored up in the sewer during draughts, is also described. Attention is drawn to the importance of the behaviour of the different pollutants with respect to the sedimentation phenomena. Reference is made to evaluations conducted on a drainage system for the recovery of a small pre-alpine lake