Optimizing a water distribution network design on water age. Comparison between implicit and explicit approaches

A common practice in Dutch and Flemish water utilities is to make masterplans for their network revisions and revise them on a regular basis, approximately every five years. The masterplans represent the ideal redesigns of their networks in terms of company specific objectives and constraints related to existing network infrastructure. The masterplans are used as a guideline when rehabilitating the networks. Among others, one of the objectives is to minimize the residence time, i.e., water age. However, the recurring assessment of water age with traditional methods, within an optimization procedure could take years for convergence for a large network of several thousand nodes. Consequently, the modellers often try to improve the residence time implicitly by minimizing network’s volume via layout optimization and diameter minimization, thus leading to increased velocities in pipes. Recently a graph theory model for estimating water age with satisfying accuracy has been proposed in the literature. The proposed model is estimated to be more than a hundred thousand times faster than the assessment of water age using Epanet, thus enabling the assessment of water age within optimization procedures. This research proposes a novel optimization methodology for simultaneous layout and pipe sizing optimization, employing the proposed graph-based model to explicitly assess the water age objective. To secure the reliability of the optimal solutions the methodology introduces a penalty to limit the size of the branched sections by setting a maximum to the number of customers connected to branched sections. The proposed methodology is applied to a real-world Dutch network. The aim of the research is to compare the optimal designs obtained using implicit (minimizing network’s volume) and new explicit (minimizing maximum water age) approaches.