A multi-objective approach to incorporate indirect costs into optimisation models of waterborne sewer systems

[摘要] ENGLISH ABSTRACT: Waterborne sewage system design and expansion objectives are often focused onminimising initial investment while increasing system capacity and meetinghydraulic requirements. Although these objectives make good sense in the shortterm, the solutions obtained might not represent the optimal cost-effectivesolution to the complete useful life of the system. Maintenance and operation ofany system can have a significant impact on the life-cycle cost. The costingprocess needs to be better understood, which include maintenance and operationcriteria in the design of a sewer system. Together with increasing publicawareness regarding global warming and environmental degradation,environmental impact, or carbon cost, is also an important factor in decisionmakingfor municipal authorities. This results in a multiplicity of differentobjectives, which can complicate the decisions faced by waterborne sewageutilities.Human settlement and migration is seen as the starting point of expansionproblems. An investigation was conducted into the current growth predictionmodels for municipal areas in order to determine their impact on future planningand to assess similarities between the models available. This information was usedas a platform to develop a new method incorporating indirect costs into modelsfor planning waterborne sewage systems.The need to balance competing objectives such as minimum cost, optimalreliability, and minimum environmental impact was identified. Different modelswere developed to define the necessary criteria, thus minimising initialinvestment, operating cost and environmental impact, while meeting hydraulicconstraints. A non-dominated sorting genetic algorithm (NSGA-II) was applied tocertain waterborne sewage system (WSS) scenarios that simulated theevolutionary processes of genetic selection, crossover, and mutation to find anumber of suitable solutions that balance all of the given objectives. Stakeholderscould in future apply optimisation results derived in this thesis in the decision making process to find a solution that best fits their concerns and priorities.Different models for each of the above-mentioned objectives were installed into amulti-objective NSGA and applied to a hypothetical baseline sewer systemproblem. The results show that the triple-objective optimisation approach suppliesthe best solution to the problem. This approach is currently not applied in practicedue to its inherent complexities. However, in the future this approach may becomethe norm.

[发布机构] Stellenbosch University