Specific relationships for the first flush load in combined sewer flows

Many studies have identified the first flush phenomenon as being a relatively high load of pollutants in the initial phases of combined sewer flow. In systems without storage, this first flush of pollutants may be discharged from the system and result in the heavy pollution of the receiving watercourse. However, by the inclusion of a storage tank, this first flush can be retained and the effluent 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 like QSIM and MOUSETRAP to predict the pollutants in urban sewer flows 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 the development of site specific regressional relationships to predict the first flush load of suspended solids in combined sewer flow and these may be used for storage tank design. Data from two sites at Great Harwood and Clayton-le-Moors in the Northwest of England has been used to develop predictive equations which relate the first flush load of suspended solids and the hydrological parameters most likely to influence sewer flow quality. A multiple stepwise linear regression technique has been utilised for this purpose. The maximum rainfall intensity, maximum inflow, rainfall duration and the antecedent dry weather period were found to be the most important parameters influencing the first flush load of suspended solids. The equations were verified using an independent set of data and gave good predictions of the first flush load for the sites considered. This study has the limitation that the equations are catchment specific. However, as more data for different catchments becomes available, it may be possible to establish standard coefficients for application to a wide range of catchment conditions.