Real-time model predictive and rule-based control with green infrastructures to reduce combined sewer overflows.

The impact of integrating large-scale distribution of green infrastructures (GIs) with different real-time control strategies on combined sewer overflows (CSOs) is assessed for the southern area of the City of Montreal's combined sewer system (Canada). An iterative process involving a synthetic design rainfall event and model predictive control (MPC) of the sewer system is developed to distribute GIs according to cost-efficiency and spatial analysis criteria. The distributed GIs are alternatively integrated with static, rule-based control (RBC) and MPC, for which model simulations are performed for a two-month period. The performance of the three strategies is compared in terms of CSO volume and frequency reductions, fulfillment of the outfall environmental priorities and transfer of runoff capture to CSO volume reduction. A gradual increase in GI implementation levels and an alternative scenario of GIs distribution are also considered to assess the performance of the two real-time control (RTC) strategies. By comparing the scenarios where GIs are uniformly distributed with those where no GIs are implemented and omitting the most extreme rainfall event, average CSO volume reduction is about 65%, 82% and 92%, respectively, for static control, RBC and MPC. Moreover, the scenario integrating GIs with MPC is the only one permitting to avoid almost all CSO events and the fulfilment of the outfall environmental priorities. GIs efficiency performance (the transferability between global runoff capture and CSO volume reduction) is also the highest under MPC, even when considering varying GI implementation levels and spatial distribution schemes.

Reducing CSOs and giving the river back to the public: innovative combined sewer overflow control and riverbanks restoration of the St. Charles River in Quebec City.

After the construction of its wastewater treatment plants, the City of Quebec began to implement overflow control in wet weather to ultimately meet the effluent discharge objectives, i.e. no more than two overflows per summer season in the St. Lawrence River and no more than four in the St-Charles River. After several years of studies to determine which management strategies would best suit the purpose, and to propose optimum solutions, a first project to implement optimal and predictive management in real time, called "Pilot", came to life in 1999. Construction in phases soon followed and the work was completed in the fall of 2009. As a result, requirements with regard to environmental rejects were met in two sectors, namely the St-Charles River and the Jacques-Cartier Beach, and aquatic recreational activities could resume. Meanwhile, the City also worked at giving back access to the water courses to the public by developing sites at the Jacques-Cartier Beach and in the Bay of Beauport, and by rehabilitating the banks of the St-Charles River.