Apparatus for purifying a fluid, which comprises electrode layers facing each other, power supplied to different polarities with spacer layers interposed, and traversed by a flow of fluid to be treated containing ionized particles. The electrode layers and the spacer layers are wound as a spiral around a longitudinal extension axis inside the suitably cylindrical containment structure. The power supply comprises a pair of collectors that are extended with portions inside the winding connected to the corresponding first internal sides of the electrode layers at the longitudinal extension axis.