The Movement of Total Suspended Solids in Combined Sewers

Studies have shown that a large number of parameters influence the quality of the sewer flow in stormwater and sanitary systems. One primary process to influence quality is the deposition, erosion and transport of sewer sediments. It is generally accepted that a high proportion of the pollutants found in sanitary sewers are attached to or entrapped within deposited sediments and that these are released into the sewer flow as the sediments are eroded during storms. Many authors report that such erosion is the primary cause of the first flush of pollutant loads where the concentration of pollutants such as BOD, COD and TSS are significantly higher in the early part of the storm. This paper presents a review of work completed at the University of Sheffield in an attempt to describe the concepts and processes associated with the movement of deposited cohesive-like sediment beds in sewers and of the way in which this may be related to the prediction of the first flush. Attention has been focussed on the movement of Total Suspended Solids through the system. The paper contrasts an approach based on regressional relationships established from a large number of recorded events with that of a model to describe the erosion of a sediment layer of increasing erosional resistance with depth of erosion. Both methodologies are shown to predict pollutographs which correspond to observed field data. Further refinements of each model are required. The final section of the paper discusses the current avenues of further work.