The identification of adequate treatment for small communities is a complex problem since it makes it necessary to combine aspects of the community and landscape, the receiving environment, and the available wastewater treatment technologies. This paper presents the development and implementation of a Knowledge-Based Decision Support System (KB-DSS) to tackle this problem. Different knowledge sources have been consulted in order to make up a comprehensive and accurate knowledge base. The core of the KB-DSS embraces two objectives. The first one is to assist in the selection of the treatment level adequate to fulfil the target quality standards for the receiving environment. The second one is to select the specific type of treatment. The KB-DSS is being applied to each one of the 3,482 different small communities comprised in the Small Communities Wastewater Treatment Plan of Catalonia, grouped according to river catchments. This paper also summarizes the different steps involved in the operation of the knowledge-based DSS when solving a real case study.