Considering Water Quality of Urban Rivers from the Perspectives of Unpleasant Odor

Surface water degradation in urban areas is a common problem in many countries, and degradation hampers ecosystem services provided by rivers, having negative impacts on aquatic organisms and human beings. Unpleasant odor arising from impaired rivers causes annoyance to people, and the study quantitatively analyzed unpleasantness of odor in Tokyo through the application of hedonic tones of odor. Unpleasantness remained or worsened while overall dissolved oxygen (DO) concentration increased downstream. The phenomenon is considered to be caused by multiple external factors, such as combined sewer outflow, hypoxia at Tokyo Bay, and effluent from water reclamation centers (WRC). Converting descriptive odor types into numeric values helped to see changes over time and enabled to depict the relation between DO concentration and the type of odor. In Tokyo, the ratio of effluent from WRC to stream flow increased over time, and it partially contributed to odor becoming more unpleasant. Malodor is no longer a major issue since the situation improved by developing an advanced treatment facility; however, human olfactory sense detects the existence of unpleasant odor in urban rivers. The study presented the complexity of analyzing odor problems in tidal urban areas because these are caused by external factors.