Process for the treatment and decontamination of acid waters which contain dissolved metals and their conversion into fertilizers, comprising the combination of three specific processes: a) selective removal of iron and of the acidity of the medium through biooxidation of Fe (II) and precipitation with alkalis (Ca(OH2), NaOH, KOH or NH4OH) of Fe (III) compounds; b) taking advantage of the high sulphate content of the acid waters for the synthesis, through ionic exchange, of K2SO4 or NH4SO4, free of chlorides which may be used as fertilizers or commercialised in other applications; c) recovery of the metals contained in such waters: Cu , Al , Zn and Mg . The process may be applied to acid waters originating from various sources: active or inactive mine installations, river basins, water courses, lagoons, etc. which, due to their contact with numeral masses, are contaminated with sulphuric acid and heavy metals, as well as industrial plants which produce acid effluents (for example galvanotechnical industry).