Nowadays, most industries are required by law to treat their wastewater, but the selection of the most appropriate wastewater treatment technologies involves a complex decision making process. Environmental decision support systems can be useful to reduce the time needed to make decisions, to improve the consistency and quality of the decisions and to provide direct access to experience, expertise and human-kind reasoning. This study aims to obtain an environmental decision support system to select the most suitable alternative for industrial wastewater treatment in the food, drink and milk sector. The process developed is described, and a set of case studies showing the capabilities of the tool is presented in order to validate the system procedure.