Nested Optimization Approach for the Capacity Expansion of Multiquality Water Supply Systems under Uncertainty

A nested optimization approach is proposed to solve capacity expansion problems of multiquality water supply systems. The problem to be solved consists of determining the infrastructure that should be built and/or rehabilitated at a specific time. This decision should be taken in a long-term planning perspective. It should consider how the operation will be performed to satisfy demand and water quality requirements by using multiple sources with different water quality at the source, take into account the temporal and spatial distribution of the water resources available and remain aware of the environmental impacts. In addition, decision processes which do not appropriately consider inherent uncertainties (e.g., hydrological, demographic, and technological uncertainties) can lead to suboptimal solutions. Here, uncertainty is handled using scenario planning with the aim of finding expansion solutions that can be expected to perform well under a set of possible future situations (or scenarios). The solution method combines simulated annealing with nonlinear programming to determine the solution to the nested optimization problem.