This research is based on CSO emissions from Dutch sewer systems. During the years 1982 to 1989 research was done on several sewer systems, all of them equiped with a single overflow weir. Pollutant emissions were calculated from the measurements, whereby each storm was considered as a single event. Extreme emissions have a detrimental, sometimes even desastrous effect on water quality. Such extreme emissions are the result of heavy storms, giving it a low frequency of occurrence. From the measurements a statistical model was developed enabling the user to forecast extreme waste emissions with a certain return period in a range of 2 to 10 years. Five pollutants are put in the model: BOD, COD, Kjeldahl nitrogen, total phosphate and suspended solids. The model operates with standardized emission values in kg per ha of impervious area. When the model is used in practice the runoff area to the specific overflow under consideration has to be known.

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