The development of the Wallingford Procedure WASSP (1981) and the MOSQITO (Moys 1987) models will provide the sewerage engineer with design tools to assess the quantitative and qualitative performance of sewer systems. As part of the development of such a quality model, the University of Manchester, financed by the Water Research centre and North West Water, have undertaken a fieldwork program of research to monitor the hydraulic performance and the temporal variation of pollutants in the inflow and the overflow at five combined sewage overflows in the North West of England. These projects are integrated into the program of research co-ordinated by the River Basin Management group at WRc Engineering.

This paper describes the instrumentation used at a typical field site and illustrates the monitored temporal variation of pollutants for a number of storm events. These results show the complexity of the monitored pollutographs and highlight the large number of variables which influence combined sewer quality. Using data monitored in 1986, it is hypothesised that the long term impact of combined sewer discharges on receiving waters may be estimated from the flow retention performance and some estimate of annual average pollutant concentration. To predict the short term impact on river quality it is necessary to consider the complex processes associated with combined sewer flow and to include the separation performance of the overflow structure.

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