Author(s): Shuzo Nishida; Daiki Nei; Yusuke Nakatani; Daichi Imafuku
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Keywords: Combined sewer overflow; Urban tidal river; Pollution load; Nutrients; Numerical simulation
Abstract: Combined sewer overflows (CSOs) have a major impact on the water and benthic environment of rivers in urban areas. Osaka City is located at the head of Osaka Bay, and many tidal rivers and channels are located around the city environs. Eutrophication due to the inflow load from rivers is a serious problem in Osaka Bay. Most of the sewage system in Osaka City is a combined system; hence, CSO loads greatly affect the water in the rivers and the bay. To clarify the influence of CSO on the water quality of the Osaka City urban tidal river system, we conducted field surveys and laboratory analyses of water and sediments. Water samples were collected in and after rainfall events using an autosampler at the riversides near the CSO outlets. Analyses of PO 4 -P, NH 4 -N, NO 2 -N, NO 3 -N, TP, TN, and TOC in the water were performed using an autoanalyzer. We also measured river velocity, water level, temperature, salinity, turbidity, and chlorophyll a using memory-type instruments. Numerical simulations were performed using a three-dimensional hydrodynamic model to understand the behavior of the CSO pollution in the rivers. The flux calculation suggested that dissolved matter of CSO loads exhibits storage stability, whereas particle matters delivered with CSO are deposited immediately near the CSO outlet. The numerical results suggested that the suspended sediment in the CSO load was immediately deposited near the CSO outlet, causing hypoxia and the degradation of the channel water quality.
Year: 2014