Many studies have identified the first flush phenomenon as being the relatively high concentration of pollutants in the initial phases of combined sewer flow following a storm. One way of controlling the first flush is by the provision of a storage tank such that the effluent can be discharged in a controlled manner. To optimise the storage volume, both the total pollutant load discharged and the temporal variation in pollutant concentration within an event need to be predicted. Sophisticated models to predict the pollutant concentrations in urban sewer flows, for example QSIM and MOUSETRAP are already available. However, the data requirements for these models are extensive, which usually limit their application to major or environmentally sensitive schemes.

This paper describes attempts to relate the peak concentration of suspended solids in combined sewer flows to observed storm characteristics. In this study, it was hypothesised that the peak concentrations of suspended solids could be related to the hydrological parameters of maximum rainfall intensity, storm duration, and antecedent dry weather period prior to the storm which are commonly used as the basic parameters for urban sewer design. Data from two sites at Great Harwood and Clayton-le-Moors in the North-west of England has been used in the study and an attempt has been made to define an upper limit of the first flush concentration of suspended solids corresponding to storms which have been categorised into bands defined by their peak rainfall intensity.

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