Pollution concentrations in wet weather discharges from combined sewer systems.

The national authorities recommend three levels of complexity in the calculations of urban runoff loads from CSO structures during rain. The methods range from unit loads to long-term simulation with the SAMBA module in the MOUSE software package. Common for all these methods are the concept of event mean concentrations (EMC), i.e. that the pollutant concentrations in the runoff are assumed to be independent of the size and distribution of the rain volume and also independent of characteristics of the catchment such as size, slope, storage capacity, maintenance etc. All variation of measured pollutant concentrations is assumed to be random. On this basis the national authorities recommend EMC-values for calculating annual pollution loads.
Recent data question the present concept. During two summer periods runoff from six catchments have been measured. The preliminary analysis shows significant systematic differences between the hydrograph and the pollutograph, possibly caused by flushing of the sewer systems. More troublesome is the fact that the calculated event mean concentrations are significantly higher than what has previously been observed. The present work elaborates the data more closely and evaluates the results in light of the many data sets that are available now throughout the world.
The aim is to identify a few parameters that can describe pollution runoff from urban areas with sufficient accuracy. At present three parameters are used, i.e. an initial losses, a hydrological reduction factor and an event mean concentration. Detailed models that incorporate pollutant washout are most often extremely complex and use a vast number of parameters. It will be attempted to reach a better description then currently by using one or two extra parameters and perhaps re-parameterising the present runoff model. The findings are meant to improve the basis for estimating pollution loads from combined sewer systems in the future.