Nature and Dynamics of Water Sediment Interface in Combined Sewers

The measurement of suspended solid concentration profiles in dry weather wastewater of the "Le Marais" combined sewer network, Paris, has shown an increase in concentration in the lower part of the sewerʹs flow (1-3 g/L). To obtain a precise description of the composition of the water-sediment interface, an observation system was set up, which showed that an immobile layer composed of fibers and organic matter formed at the bottom of trunks over the coarse mineral deposits. The concentration profiles obtained result from the removal of this layer by aspiration. Samples were taken from this organic layer using a specially adapted system. The accumulation rate of the organic layer is 215 g/(m2·day). The particles of this organic layer are heavily loaded with pollutants (volatile solids to suspended solids = 66-75%). This layer is located at points where the shear stress is <0.1 N/m2. Flush experiments have shown that it can be eroded by small rainfall events. When these results are extrapolated over the whole catchment area by mapping the organic layer, they show that the organic layer is stored in the same quantities as the mass eroded during a rainfall event in the sewer system and it can therefore contribute to pollution in urban storm-water discharge.