A Study of Why the Number of Coliform Bacteria Did Not Decrease in Higashi-Yokobori River—Dotonbori River Despite the Completion of the New Stormwater Storage Pipe “Heisei no Taiko Gesui”

Sewer stormwater overflow, one of the causes of water pollution in Higashi-Yokobori River–Dotonbori River that flow through central Osaka, has been stored in a rainwater storage pipe called “ Heisei no Taiko Gesui ” since April 2015, but the number of coliform bacteria in both rivers did not decrease significantly. If the pipe is working, the number of coliform bacteria at a downstream point should depend on the number at the upstream point, flowing time, and water temperature. Thus, the decay rates of coliform bacteria were determined experimentally, and the predicted numbers of coliform bacteria at the downstream point were compared with the expected values obtained by the Monte Carlo method. The predicted values tended to be underestimated because the obtained decay rates did not reflect the effects of sunlight and other factors. However, the differences between the expected and predicted values after completion become close regardless of the weather, indicating that the pipe is functioning. It was considered that the number of coliform bacteria has not declined in both rivers because the increased load from the upstream cancelled the reduction effect of the pipe.