There is an extensive bibliography that analyses the structure of a desalination project from the perspective of chemical engineering, but there is another "engineering" that has been much less studied, which is just as decisive for the plant to be able to operate. The aim of this work is to analyse the economic and legal factors that condition the proper operation of a project and, to this end, we will place special emphasis on the usual agents and contractual agreements. We intend to arrive at a general model of operation that we will construct from the study of three particular cases: Algeria, Tunisia and Egypt. These three countries represent different ways of structuring a project, with the involvement of the public sector being the main element that varies between them.