To improve the living environment in Japanese cities, both combined type and separate type sewer systems are being provided to cope with rainwater and sewerage. However, during heavy rain, overflows are causing pollution of the rivers or lakes at the discharge destinations.

In response to this problem, river management authorities in Japan have been studying the discharge load on combined sewer systems during rainfall and have established a simulation method to enable its prediction. As a result, the control of discharge loads by means of detention ponds has been effectively carried out in various cities in Japan.

In addition, in recent years the separate system, that was thought to result in relatively good water quality, has been found to cause adverse effects as a non-point source of pollution in lakes and rivers at discharge destinations. For this reason, simulation models are being developed for the case of separate systems for several cities to accurately determine the discharge loads during rainfall.

In this paper, as well as describing the discharge loads and control during rainfall for combined sewer systems in Japan, a report is given of the results of new research in regard to the discharge loads occurring in the case of separate sewer systems.

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