Erosion mechanisms in combined sewers and the potential for pollutant release to receiving waters and water treatment plants

The problems associated with solids in sewerage systems result in common difficulties such as blockages and flooding and the subsequent maintenance requirements have been well documented. Concerns regarding pollutant release have also been demonstrated, with the contribution from in-sewer solids to the quality of the flow during a storm event being especially significant. These events known as “foul flushes” in combined sewers typically occur in the initial period of storm flows, when the concentration of suspended sediments and other pollutants are significantly higher than at other times. Traditionally impacts from these events have been related to the suspended solids phase of the flow passing through a CSO structure. It is now apparent that much of the suspended load originates from solids eroded from the bed. The “near bed solids” which are re-entrained into the flow, together with solids eroded from the bulk bed, account for large changes in the suspended sediment concentration under time varying flow conditions. The influence of these eroded solids and their potential impact on receiving waters and treatment plants will be reviewed using data obtained from field studies carried out in the main Dundee interceptor sewer in Scotland. This paper describes some of the methods employed to investigate the characteristics of the pollutants associated with solids erosion in combined sewers.