Multi-objective model for regional wastewater systems planning

The planning problems posed by regional wastewater systems consist of determining an efficient solution for the layout of the sewer networks, and for the location, type, and size of the pump stations and treatment plants to be included in the systems. These problems have typically been addressed through optimisation models with a cost-minimisation objective. However, in a world increasingly more concerned with sustainable development, factors other than economic must be considered. In this paper, we describe a multi-objective model for regional wastewater systems planning. Through a model of this type, it is possible to identify solutions that are a good compromise with regard to conflicting objectives. For presentation purposes, we chose to focus on three objectives: minimisation of capital costs; minimisation of operating and maintenance costs; and maximisation of dissolved oxygen. The multi-objective model is handled through the weighting method and solved through a simulated annealing algorithm. Its application is illustrated for three test instances designed to replicate real-world problems.