To alleviate the pollution in the Oslofjord region it is planned to transport wastewater to a central treatment plant by a 35 km tunnel system. The objective of the study was to analyze the consequences of a reduced treatment plant capacity vs. the utilization of the tunnel as a retention basin. The analysis was performed by the use of a recently developed mathematical model for urban runoff and drainage. Simulations were performed for large rain events and for all rains during a summer season. The most important result of the study was that the treatment plant could be reduced to half its original capacity if the tunnel was utilized as a retention basin without any increase, in fact a decrease, in the total pollution load on the fjord.

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