The proposed model is based on the hydration theory of dissolved chemical species in water. The molecular dehydration occurs when the solution system is subjected to critical water loss associated with re-displacement of ionic charges from attached water molecules towards the dissolved molecule. This mechanism allows the formation of solid phase, i.e. fouling. In order to estimate the true fouling load, it is necessary to define the type and molar concentration of the hard dissolved compound individually, then estimate the fraction available for deposition in a given chloride content. The mathematical model is illustrated and tested for six RO plants, and the obtained results are in agreement with field observation.