This paper has the intention of to establish a method that combines the economic criteria and the environmental ones for the selection of technology of water purification in localities with less than 30,000 inhabitants and to implement an alternative win-win (economic and environmental gains) in this process. For the development of the study a hypothetical environmental scene has been constructed, which makes possible to make the evaluation of environmental impact and this form to raise a previous situation to the implementation of a project of treatment plant. Also, the methods of environmental economic valuation are used to quantify the identified impacts, which correspond to direct methods as they are the cost of opportunity and defensive expenses. Once the environmental costs have been found through these methods and the financial costs (Investment, Or & M and Administration), can be appreciated an important correlation between these costs since the technology of smaller financial cost is the technology of smaller environmental impact, aspect that fortifies the selection of such technologies.