Implications of Climate Change Mitigation and Household Water Saving in Existing Water Networks: Upgrade Requirements and Hydraulic Perspectives

Abstract

Climate change and the ever-increasing water demands have led to the introduction of mitigation and adaptation interventions in water systems, which need to be understood. The effects of these countermeasures on the upgrade requirements and hydraulic performances of existing water distribution systems (WDSs) and sanitary sewers (SSs) are analyzed in this study. In addition to the common two-objective problems, a three-objective problem that provides cost-effective water-saving schemes (WSSs) that also reduce greenhouse gas (GHG) emissions have been proposed. In order to obtain and analyze solutions for all the aforementioned optimizations, the tasks of selecting WSSs considering different performances were formulated and solved as multiobjective optimization problems. The objectives considered are the minimization of the overall cost (i.e., less savings) versus minimization of water demand or maximization of network upgrade postponement introduced by WSS measures. Optimizing the GHG emissions and reductions to any of these two-objective problems completes a three-objective problem. The decision variables are the domestic water saving appliance and fitting capacities, and the tank sizes of the rainwater harvesting systems (RWHSs). The method provides Pareto fronts, which are the outputs from a nondominated sorting genetic algorithm. The proposed method was applied to the subsystems of the Tsholofelo WDS and sanitary sewer (SS) in Gaborone, Botswana. The results indicate that three-objective climate change counteractive WSSs are more desirable than two-objective WSSs. Solutions obtained considering WDS demand or the WDS only excluding the corresponding SS would undesirably impact the hydraulics and SS upgrade requirements and vice versa.