Les dépôts en réseau d'assainissement unitaire, une importante source de pollution des rejets urbains de temps de pluie

Agence de l'Eau Seine-Normandie, Travail réalisé au CEREVE

Abstract

Previous research has shown that combined sewer systems are an important source of particle and organic pollution during rainfall events contributing to combined sewer overflow. The aim of this article is to identify in an urban catchment area called "Le Marais", in the center of Paris, the types of sediments that are eroded and contribute to the pollution of combined sewer overflow. Three sediment types are considered: granular material found in the inverts of pipes, organic biofilms and organic sediment at the water bed interface, identified as an immobile layer in the "Le Marais" catchment area. The method used consists, firstly, of sampling and assessing the pollutant loads (Volatile Solids, COD, BOD₅) and metallic loads (Cu, Zn, Pb, Cd) of the particles in each type of sediment. Then, the mass of each type of sediment is assessed. The mass and the characteristics of each type of sediment is finally compared to the mass and characteristics of the particles eroded in the catchment area, in order to find the source of eroded particles. The only identified type of deposit that can contribute to combined sewer overflows is the organic layer. Indeed, the solids of this layer have mean organic (Volatile solids: 64 %, BOD₅: 0,28 g/g) and metallic loads that are of the same order of magnitude as the eroded particles. Moreover, the mass of the organic layer considered over different time scales is of the same order of magnitude as the eroded masses during rainfall events and an erosion experiment showed that the organic layer is actually eroded. Technical solutions can be imagined so as to erode the organic layer during dry weather and so limit the pollution eroded during wet weather: the use of the Hydrass valve, changing the shape of the sewers, improving the organization of the cleaning of the streets. Those technical solutions should be tested in situ to improve our knowledge on the management of sewer deposit that has to take into account not only hydraulic considerations but also consideration on the potential of pollution of the deposits.