The function of combined sewer overflows (CSO) from both a quantitative and a qualitative point of view is often little known in the Nordic communities. Since large pollution loads are discharged from urban areas by CSO, it is important to consider this problem when rehabilitation plans are established.

A study aiming at the estimation of yearly volumes of CSO and yearly quantities of pollution load from CSO has been carried out in the city of Malmö, Sweden. The CSO discharges into four different types of receiving waters: the channel, the harbour, the Sege River and the Öresund. Registration of frequency, duration and water volume was made at about 30 weirs at different time periods.

Storm Water Management Model (SWMM) was used both to simulate single SCO events and for continuous simulation. The results from these simulations were compared with actual measurements.

The total pollution load was estimated from mean concentration of pollutants from more than 100 samples taken at different weirs and CSO events.

Through analysis of the occurrence of CSO in time and space, a program of priorities for the rehabilitation of the CSO-system was established.

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