Optimization model for BMP selection and placement in a combined sewer

The purpose of this study is to develop an optimization model for the selection and placement of best management practice (BMP) at the watershed level in order to control water quantity and quality. Linear programming (LP) was selected from among several mathematical optimization methods, with the objective function being to minimize the total cost of BMPs within the constraints of guaranteeing a freesurface flow for the whole network during heavy rainfall and limiting the peak flow generated at the outlet of the watershed at the interceptor capacity for frequent rainfalls. The model was used on two scenarios considering a 10-year return rainfall period for the water quantity control, and 25 mm rain for the water quality control. The impact of climate change and urbanization on the combined sewer performances is also discussed for each scenario. The results obtained from the model effectively demonstrate the viability of implementing BMPs in the goal of improving sewer performances and show that an increase of 32% in the overall quantity-control cost is sufficient to achieve also the quality-control objective.