THE CHOICE OF A REAL-TIME CONTROL STRATEGY FOR COMBINED SEWER OVERFLOW CONTROL

This paper focuses on the strategies used to operate a collection system in real time control (RTC) in order to optimize use of system capacity and to reduce the cost of long-term combined sewer overflow (CSO) control. Three RTC strategies were developed and analyzed based on the Quebec Urban Community (QUC) case study. QUC operates an Optimal Global and Predictive (OGP) RTC system since the summer of 1999, which constitutes one of the strategies evaluated.Simulation of each RTC strategy using 32 real back-to-back rainfalls showed that the existing system performs better under RTC than with conventional control. The best strategy in terms of CSO control, flooding protection, and operational benefits is the OGP-RTC. The high level of performance reached by OGP-RTC results from the centralization of the decision making process in order to constantly readjust the system's control set points according to global and updated system-wide information. Efficient in-system storage and treatment capacities and the elimination of flooding risks are the main benefits of this type of real-time management.Since each strategy reduces to a certain degree CSO volumes to be controlled, it is not surprising that the cost of the CSO long-term control plan (LTCP) for each RTC scenario is less than for the conventional scenario. This is also due to the fact that the cost of implementing the RTC system is low compared to the total cost of the LTCP using conventional CSO storage tanks. In this way, the OGP-RTC strategy, with an LTCP cost of 32,264,232, is the most economical scenario, especially when compared to the 41,285,493 required by the conventional scenario; the result is a capital savings of 22%.