Abstract
The Edo Castle is located in central Tokyo and its Outer Moat provides a precious water environment in the urban area. The water sources are limited to precipitation and combined sewer overflow (CSO); therefore, the hydraulic retention time (HRT) becomes very long (tens of days), resulting in the poor water quality. Particularly in summer, serious algal bloom is observed, and it causes the problems of bad landscape and undesirable odor. To develop effective countermeasures for water quality improvement, the estimation of CSO inflow and water-quality prediction by means of runoff and algal growth models were conducted. Furthermore, the actual water level and water quality were regularly measured through field surveys for checking the adequacy of models. The analysis with the models proved that the combination of CSO prevention and introduction of reclaimed water (4,653 m3·day-1) could prevent the algal blooms. The cost of the countermeasure was estimated to be 390 million yen·year-1. The benefit was evaluated by the contingent valuation method (CVM) and estimated to be 750 million yen·year-1. Finally, the cost and benefit were compared and the feasibility of the countermeasures was shown.
