This paper presents the application of an optimization model developed for Lake Rozkoš, which is located in a sub-basin of the Elbe River basin in Bohemia. Two scenarios for achieving environmental targets (required status of the lake water) were formulated by specialists, together with an assessment of the investment and operating costs of the projects. The optimal solution (investment program) for 41 pollutermunicipalities, where 166 coalitions can be considered (41 individual and 125 joint wastewater treatment plants), was computed. The results suggest that the modelling method presented in this paper has the potential to increase the role of economic considerations in the early stages of planning and decision-making about water treatment programs. In this case, more than 20% of the costs could be saved when selected joint wastewater treatment plants are realized. The approach has the potential to contribute to increasing costeffectiveness of allocation of financial sources from national and European Union sources.