Two consecutive storms and optimal control of urban sewer networks to minimize the pollution load of combined sewer systems

Abstract

Combined sewer overflows (CSOs) are a burdened environmental issue. Structural measures are not the best solution to cope with or to minimize these adverse impacts from CSOs. Non-structural measures, if possible to implement, would be the best solution in sustainable development. Controlling of existing urban sewer networks is a potential non-structural measure to minimize the adverse impacts of CSOs. Several algorithms to control urban sewer networks are in literature; however, there is little literature in minimizing the environmental impacts from CSOs. Rathnayake and Tanyimboh (Management 29:2715–2273, 2015) have successfully developed a control algorithm to minimize the environmental impacts or to enhance the quality of receiving water in an event of CSOs. However, this control algorithm is based on single-peaked runoff hydrographs. Not only for the research in control of urban sewer networks, but also in most other researches, single-peaked runoff hydrographs are generally applied. This is due to the modeling simplicity. However, in real world, these conditions may not be applicable. It is very common to have a second peak after the first peak in the hydrograph. The second peak may or may not be high as the first; however, it is important to consider these peak flows, when it comes to design and control of combined sewer systems. Therefore, this reach was carried out to improve Rathnayake and Tanyimboh’s optimal control algorithm (2015) for two consecutive storms. Runoff hydrographs due to two consecutive storms and pollutographs were developed in improving the Rathnayake and Tanyimboh’s control algorithm. Results manifest the benefits of using multi-objective optimization in controlling combined sewer networks under two consecutive storms where many sets of feasible control settings can be obtained. A desired control settings can be implemented to the sewer system according to the available resources.