A process for transforming sludge into npk type granulated fertilizer in which the energy consume is reduced a 90% thanks to the addition of a special filtration step in which the water content is reduced a 50% and the remaining water is evaporated by the exothermic reaction occurring in the process. Additionally, the process of the present invention includes mechanisms that allow some of the by products generated by some of the reactions such as gypsum, to adsorb crystallization water and reduce the humidity of the mass without using external energy.