Actual Conditions and Factors of Scum in Hirano River Flowing through Urbanized Area of Osaka

Abstract

Floating sludge (scum) is frequently generated after rainfall in the Hirano River which flows through the urban area of Osaka City. Scum degrades the living environment, such as it causes bad odor; therefore, it is necessary to understand the mechanism of scum generation to consider its countermeasures. In this study, scum generation was monitored by installing observation cameras on the river revetment, and the river water and sediment quality during rainfall were investigated. In addition, the river flow rate during rainfall was simulated by two-dimensional flow analysis. Our results showed that scum was mainly generated within 1 to 3 days after combined sewer overflow (CSO) discharge. No resurfacing of settled scum was observed during the study. The river flow tends to decelerate owing to the effect of ocean tides, and organic suspended solids can be easily deposited during CSO runoff. Moreover, the bottom layer of the river was anaerobic; thus, anaerobic bacteria could easily generate gases to encourage the generation of floating sludge. Our results also suggest that the amount and location of scum generation depend on rainfall patterns.