A Study on Environmental Impacts of Combined Sewer Overflow in Japan

The City of Nagoya and the Japan Institute of Wastewater Engineering Technology have been carrying out a joint study to quantitatively clarify the dispersion phenomena in the receiving waters into which pollutants included in Combined Sewer Overflow (CSO) are discharged and the impacts on receiving waters designated by combined sewer system improvement measures by researching a pollutant dispersion simulation model for fair current rivers and tidal rivers that receive discharged water and by applying this model to analyze the impact of CSO on receiving waters.This report introduces the pollutant dispersion simulation model for tidal rivers, provides some examples, and results of the analysis done by using this model.The concrete analysis results provided the following information.The results of the monitoring survey and the analysis have shown that in areas that are affected by tides, the impact of CSO concentrates on the freshwater with a low chloride ion concentration (surface layer) wanes gradually.It was also confirmed that seawater areas with a high chloride ion concentration (lower layer) are resistant to the impact of CSO.In regards to water quality items, the monitoring survey and analysis results showed that pathogenic microorganisms, such as E. coli in particular, tend to remain to affect the water quality of effluent streams for 48 hours after the end of CSO discharge. Also, a certain degree of reproducibility in the observed values and the analytic values was verified in each water quality item. Based on these findings, it is concluded that this model is practical as a tool for analyzing the impact of CSO on receiving waters. In the future, the reproducibility of the pollutant load dispersion simulation model for pathogenic microorganisms such as T-N, T-S, E. coli and so on will be improved. This model will be used to analyze the river conditions impacted by CSO, and to research on the effectiveness of combined sewer system improvement measures.