Sources and erosion of organic solids in a combined sewer

Sewer deposits play a significant role as a source of pollution in combined sewer overflows. The objective of this paper is to identify, using a direct approach on a sewer length, the main source of organic solids among the different deposits and their erosion processes. The different deposits studied were large inorganic solids at the bottom of the sewer, biofilms on the walls of the sewer and organic solids at the water–bed interface. The method applied consisted firstly of analysing very precisely (mass and pollutant loads) the solids in the three types of deposits along a sewer line with a length of 150 m. Then, the deposits were eroded by in situ flushing experiments using tap water. Finally the nature and the mass of particles eroded along the length of the sewer were compared with the nature and the mass of solids in each type of sewer sediment in order to identify the main source of organic solids in the sewers. This comparison indicates that the organic layer at water–bed interface is the main source of eroded solids. An erosion test carried out under continuous observation confirms that the organic layer is actually eroded.