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Using a case study on the Lambro River and the Stream Bevera, this paper compares the use of dynamic modelling with the “normative criteria” to evaluate the effects of Combined Sewer Overflows (CSOs). The normative criteria is the method currently used in Italy to design first flush water tanks to reduce the pollutant discharges. The dynamic modelling technique compares the natural river conditions and the conditions in the river when a CSO occurs. This study finds that the current law, which is based on an effluent standard approach, can significantly underestimate the size of the CSO storage tanks needed to prevent pollution. An alternative is to change the law to use a stream standard approach and to require the CSO tanks to be sized based on the volume and concentration of the pollutants at the CSO. By using a dynamic model of the river, it was found possible to provide a more accurate estimate of the size of the CSO storage tanks required to prevent pollution. Furthermore the paper finds that without a model, it is almost impossible to evaluate the effect of CSOs and changes to the size of the CSO storage tanks. [ABSTRACT FROM AUTHOR]