This paper presents a model that contributes to finding cost-effective solutions when making decisions about building wastewater treatment plants in the planning process defined in the Framework Directive 2000/60/EC of the European Parliament and of the European Council. The model is useful especially when construction and operation of joint wastewater treatment plants is possible for several (neighbouring) municipalities, where a huge number of theoretical coalitions is possible. The paper presents the model principles for one pollutant and for multiple pollutants, describes the CRAB software used for computing the optimal solutions and presents selected applications. It concludes that the computations can contribute directly to decision-making concerning environmental protection projects and also serve for calculating background models for economic laboratory experiments in the area.