Cost efficiency in water supply systems: An applied review on optimization models for the pump scheduling problem

The need for efficient pump operation in water supply systems (WSS) has become increasingly important over time, driven by the growing energy consumption and the associated energy costs. Forecasts for 2050 anticipate a global increase in water demand by 55%, indicating an increasing surge in WSS energy consumption. Control of pumping stations, which consume 70% of the energy in WSS, is the most critical area for optimization. This optimization challenge is commonly referred as the pump scheduling problem (PSP), and can be addressed using a variety of mathematical formulations. While numerous formulations exist to solve this optimization problem, the large majority of the studies are focus on the optimization techniques, sidelining the problem formulation. Due to the unique physical characteristics of each WSS, individual mathematical formulations may exhibit different levels of performance. In addition to general pumps’ operation optimization, the employment of variable speed pumps (VSP) can lead to significant energy savings compared to fixed speed pumps (FSP). However, despite their apparent benefits, many established optimization models for the PSP have not yet incorporated VSP decision variables into their formulations. Therefore, this work aims to review the main mathematical formulations for the pump scheduling problem for WSS with VSP and to present a quantitative comparative study of three mathematical formulations applied to a case study in the literature. The comparative analysis here presented revealed that the optimization model based on duty cycles is more cost-efficient when compared to alternative approaches discussed in the literature.