Sewer Sediment and its Relation with the Quality Characteristics of Combined Sewer Flows

Combined sewer lines in the centre of Brussels are laid at very slack gradients, resulting during dry-weather in a systematic deficit of the load of particulate material at the outlet. The first results of a data collection programme are documented, with regard to the influence of in-sewer deposits upon pollution emissions. The monitoring of suspended solids load in the Main Trunk showed the extent of sediment deposition/resuspension processes in the system. Over a long period of time, the accumulation of deposits on sewer inverts is clearly noticeable. The total volume of deposition in the network also appears to be much more influenced by rainfall events than by human sewer-cleaning practices. Particle size analyses of sewer sediment samples have shown that most deposits in running sewers are composed of sandy material, which reduces the hydraulic capacity, but is suspected to have only slight adverse pollutional impact. Samples with finer material and higher organic contents were however found in some protected locations, not submitted to continuous dry-weather flow. From the experience gained in this study, it is strongly believed that the acquisition of detailed field data is essential in order to assess correctly the complex phenomena involved.