Feasibility Study on the Real-time Control System of the Pumps for the Reduction of Combined Sewer Overflows

The sewage system for Metropolitan Tokyo has reached 100% target fulfillment for the Tokyo Wards over the long period of more than a century. Because of the hastiness with which the effluent sewage system and the flood prevention measures for the low-lying areas have been developed simultaneously, approximately 80% of the present facilities are of the combined sewer type collecting both effluents and rainwater in the same sewage pipe.The problem with the combined sewer system is that water pollution occurs in the catchment areas into which the sewer empties as a result of the combined sewer overflow (CSO) in rainy weather conditions. A few years ago, a white solid mass (consisting of oil and fat congealing in the shape of balls that adhere to the pipe walls and are discharged with the CSO) was spotted in a place on the manmade shore of the Bay of Tokyo, a discovery that aroused great concern among the public. In response to this event, the Study Committee for Combined Sewer Improvement was appointed to look into the problem. As a result, policy proposals for action on a national level were formulated.With regard to flood problems, there has also been an increase in the volume of rainwater runoff as a result of the rapid pace of urbanization in recent years, and sudden intensive outbursts of torrential rain brought about by the heat island effect are occurring more frequently. These problems have created a need for countermeasures at a higher level.While it is necessary to expand the infrastructure hardware in terms of a reinforcement of the facilities and a shift to the separate sewer system in order to address these issues, the problem is that it would take a long time and a major investment commitment to complete the work. Tokyo Metropolitan government has therefore decided to make the maximum possible use of the existing facilities while ensuring safety against inundation and to promote measures also from a software approach by introducing a system capable of minimizing combined sewer overflow, the real-time control system (RTC).A pilot RTC system was introduced in August of last year for the Shinozaki Pumping Station's catchment area which covers a rainwater runoff area of approximately 2100 hectares. The two trunk sewers, that is, the Shinozaki and Shibamata trunk sewers, are interconnected by bypass pipes and rainwater is drained off by the three Pumping Stations connected to the Shinozaki trunk sewer while the remainder is removed at the Shinozaki Pumping Station at the downstream end. The RTC system monitors the precipitation volume in the catchment area and the water level in the pipe. On the basis of these data, the gates and drainage pumps are operated while making optimum use of the water storage capacity of the pipes to achieve the effective removal of rainwater. In the development of this system, extensive use was made of state-of-the-art technology, including the use of optic fiber water level gauges suited for measuring the water level inside pipes and the combination with a rainfall data system by radar raingauges.Simulations were carried out on the basis of past rainfall data. From the results, it was found that with the use of the RTC it is possible to reduce CSO by roughly 50% for small rainfalls with a total precipitation level of 20mm or less by storing rainwater in the pipe routes at the beginning of the rain. It has also been shown that CSO can be reduced by about 80% through the use of rainfall forecasting.This article presents the methodology used for the implementation of a fact-finding study of the rain volumes and water levels in the sewer and gives an overview of the results. It also includes a description of the RTC system structure, the method of developing and calibrating hydraulic models, the development of the operational support and operating roles and the effect achieved in terms of reducing CSO.