Method for treating a fluid containing ionized particles, by means of an apparatus provided with: - a hydraulic circuit intercepted by at least one immiscible storage tank capable of preventing fluid from being mixed, which said immiscible storage tank is provided with a first inlet connection and with a first outlet connection (2") and is susceptible of containing at least one fluid reserve with variable concentration of ionized particles, and by at least one cyclically regenerating filtering unit (3, 3A, 3B) for purification of water, connected to said immiscible storage tank; wherein said filtering unit is provided with a second inlet connection and with a second outlet connection (3") respectively hydraulically connected to the first outlet connection (2") and to the first inlet connection of said immiscible storage tank by means of a delivery pipe and a return pipe of said hydraulic circuit; wherein said delivery pipe is positioned between the first outlet connection (2") of said immiscible storage tank and the second inlet connection of said filtering unit, and said return pipe is positioned between the second outlet connection (3") of said filtering unit and the first inlet connection of said immiscible storage tank; - a supply pipe connected to said hydraulic circuit upstream of said immiscible storage tank for introducing fluid to be purified; - an extraction pipe connected to said hydraulic circuit downstream of said filtering unit (3, 3A, 3B) for conveying the purified fluid to the user; said method being characterized in that it cyclically comprises: - a circulation step, in which said fluid circulates in said hydraulic circuit through said filtering unit (3, 3A, 3B) and said immiscible storage tank, wherein the fluid flowing through said filtering unit (3, 3A, 3B) is purified of the ionized particles to an increasingly smaller extent due to the progressive exhaustion of the pick-up capacities of said filtering unit (3, 3A, 3B), accumulating in said immiscible storage tank at least one operating amount of fluid with increasing concentration of ionized particles, wherein in at least one zone of said immiscible storage tank said operating amount of fluid is present whose concentration increases in the direction from said first outlet connection (2") to said first inlet connection; - a production step in which one said operating amount of fluid to be purified is introduced into said hydraulic circuit by said supply pipe, and an equivalent operating amount of fluid with increasing concentration of ionized particles is forced to exit from said immiscible storage tank and is drawn by said extraction pipe. - a hydraulic circuit intercepted by at least one immiscible storage tank capable of preventing fluid from being mixed, which said immiscible storage tank is provided with a first inlet connection and with a first outlet connection (2") and is susceptible of containing at least one fluid reserve with variable concentration of ionized particles, and by at least one cyclically regenerating filtering unit (3, 3A, 3B) for purification of water, connected to said immiscible storage tank; wherein said filtering unit is provided with a second inlet connection and with a second outlet connection (3") respectively hydraulically connected to the first outlet connection (2") and to the first inlet connection of said immiscible storage tank by means of a delivery pipe and a return pipe of said hydraulic circuit; wherein said delivery pipe is positioned between the first outlet connection (2") of said immiscible storage tank and the second inlet connection of said filtering unit, and said return pipe is positioned between the second outlet connection (3") of said filtering unit and the first inlet connection of said immiscible storage tank; - a supply pipe connected to said hydraulic circuit upstream of said immiscible storage tank for introducing fluid to be purified; - an extraction pipe connected to said hydraulic circuit downstream of said filtering unit (3, 3A, 3B) for conveying the purified fluid to the user; - a circulation step, in which said fluid circulates in said hydraulic circuit through said filtering unit (3, 3A, 3B) and said immiscible storage tank, wherein the fluid flowing through said filtering unit (3, 3A, 3B) is purified of the ionized particles to an increasingly smaller extent due to the progressive exhaustion of the pick-up capacities of said filtering unit (3, 3A, 3B), accumulating in said immiscible storage tank at least one operating amount of fluid with increasing concentration of ionized particles, wherein in at least one zone of said immiscible storage tank said operating amount of fluid is present whose concentration increases in the direction from said first outlet connection (2") to said first inlet connection; - a production step in which one said operating amount of fluid to be purified is introduced into said hydraulic circuit by said supply pipe, and an equivalent operating amount of fluid with increasing concentration of ionized particles is forced to exit from said immiscible storage tank and is drawn by said extraction pipe.