This paper presents a support module that extracts and reuses the knowledge generated during the sensitivity analyses for design of activated sludge plants. This support module, consists of three steps: 1) Data handling and knowledge extraction, where all the data generated by numerical simulators during the sensitivity analyses is processed in order to extract qualitative knowledge, 2) Knowledge codification, where the extracted knowledge is codified as IF-THEN rules, and 3) Knowledge reutilization, where numerical and qualitative methods are integrated to guidthe designer through the design process. The usefulness of the support module is demonstrated in a case study, where the bioreactor of a denitrifying activated sludge plant is retrofitted to achieve simultaneous carbon, nitrogen and phosphorous removal.