We present an ontology (named WaWO - Waste Water Ontology), integrated into an existent environmental decision-support system (named DAI-DEPUR+), designed to model the information about a wastewater treatment process through the definition of the basic terms and relations comprising the vocabulary of the wastewater treatment area. The WaWO ontology is built following the ideas of Uschold and Grüninger and is a hierarchically structured set of terms and a set of axioms for describing the domain of wastewater treatment. It is the manifestation of a shared understanding of the wastewater domain that is agreed among a number of agents: mainly, experts in environmental and chemical engineering. This ontology incorporates some new features for the wastewater domain, such as: semi-automatic generation of knowledge bases, modeling of the cause-effect relations existing within a wastewater treatment plant, integration with predictive reasoning based on transition networks.