The selection of a dual purpose desalination plant depend mainly on many factors such as the required power to water ratio, cost of fuel energy charged to the desalting process, electricity sales, capital costs, and local requirements. Alternative configurations must be analyzed in order to select the optimal configuration to satisfy electricity and fresh water demands. This paper presents an optimization mathematical model of a superstructure of alternative configurations of dual purpose desalination plants (DPP). In fact, the mathematical model recently proposed by [1] has been properly modified in order to include other alternative arrangements. The proposed alternative configurations reduce significantly the total levelled cost of the DPP. The superstructure of optional arrangements (optimization problem) is modelled as a Mixed Integer Nonlinear Programming (MINLP) Model where binary variables (0-1) are used to select the equipments for the cogeneration plant. The resulting MINLP mathematical model can be used for synthesis as well as for analyzing different design alternatives. The solution obtained by the proposed mathematical model provides the basic design of the DPP. The model has been implemented in General Algebraic Modelling System [2]. Several study cases were successfully solved by applying the MINLP model. In this paper a study case is presented in order to illustrate the model capabilities.