The Doubs river is an ecosystem-rich river which is threatened by several anthropic activities. The three dams placed in the river as well as the discharge of pollutants contribute to the degradation of its natural habitats. The city of La Chaux-de-Fonds is directly concerned by this issue. The combined sewer overflow (CSO) waters generated by the city discharge in la Ronde, an affluent of the Doubs. These waters represent a potential threat to the Doubs water quality, this is why the authorities of La Chaux-de-Fonds need to take measures to limit the impacts of these waters in the natural environment. These measures have to be taken without causing excessive costs. Several solutions have been considered according to the objectives set during the development of the « plan d’évacuation des eaux (PGEE) » of the city. The implementation of a constructed wetland was the selected solution. This system is widely used in a number of European countries and especially in France, but is not yet much developed in Switzerland. Topographic and design constraints have lead the choice of a vertical flow constructed wetland. Its technical feasibility has been deeply studied. The design of the constructed wetland has been examined using the simulation tool Orage (which is currently in phase test in the IRSTEA research institute in Lyon). A second tool, the Rebeka software, allowed to determine the impact of the CSO water on the water quality of the Doubs. The STORM approach helped to integrate these impacts in the design of the system from the beginning of the the project. The costs of the project are detailed in investment costs for the building of the wetland and operation costs which represent essentially functioning and maintenance costs. The treatment system consists of three planted filters which operate in parallel. Simulation results demonstrate that the objectives of the project are fulfilled. The installation has a removal efficiency of 29% on ammonium, 44% on COD and 95% on TSS. The initial investment is 3.6 million Swiss francs. This system offers a solution to resolve pollution issues originating from the CSO waters of the city of La Chaux-de-Fonds with a reasonable initial investment.