The geothermal waters exploitation infrastructures in Chott El Fejjej (Tunisia) are constantly being damaged by a scaling phenomenon. Experimental work has been carried out on a new kind of decarbonation reactor: the open double partition reactor (RODP), which may be used in the construction of a new plant at the site of Chott El Fejjej. The objective of this paper is to propose a model, which may contribute to the design and optimisation of the process of decarbonation. Our model is based on the thermodynamic laws, which describe the physical and chemical phenomena occurring in an electrolytic three-phase continuous reactor. However, due to electrostatic forces, these kinds of systems cannot be successfully described by the concept of an ideal solution. Indeed, the calculation of activity coefficients is required. So, by using a group contribution method to calculate the activity coefficient, the model remains user-friendly. The non-linear system obtained is solved by using Newton-Raphson's method. Confrontation with experimental results shows that the model correctly describes the ODP reactor.