A case study showed the advantages of using a design support system during the design and retrofit of wastewater treatment plants (WWTP). The Manresa WWTP was chosen to demonstrate these advantages. While the records of the decision-making process used during the Manresa WWTP design were not available to the authors, its original design objectives and the resulting structure and values for its more important variables were. A simplified but plausible design history focusing on the design decisions and not on the detailed modelling of the WWTP was generated with this information. Using such a design history it is possible (1) to evaluate automatically the compliance of alternative design proposals with respect to the design objectives; (2) to study the influence of the weight of the arguments in the selection of the most adequate proposal; (3) to document the decision-making process; and (4) to assist the designer in the search of specific items within the historical records during the review of the current design or during the retrofit of the plant.