A capacitive deionization device (10, 20, 30, 40, 50, 60) comprising: at least one flow path (11, 21, 31, 41, 51, 61) configured to receive influent fluid; at least one pair of electrodes (13a; 13b, 23a; 23b; 33a; 33b, 43a; 43b, 53a; 53b, 63a; 63b); at least one charge barrier (12a; 12b, 22a; 22b; 32a; 32b, 42a; 42b, 52a; 52b, 62a; 63b) disposed between the at least one flow path and a corresponding electrode of the at least one pair of electrodes, the at least one charge barrier ionically separating the corresponding electrode from the at least one flow path; and at least one electrolyte solution (14a; 14b, 24a; 24b; 34a; 34b, 44a; 44b, 54a; 54b, 64a; 64b) disposed between at least one electrode of the at least one pair of electrodes and a corresponding charge barrier of the at least one charge barrier; characterized by: at least one electrolyte compensation device (17, 27, 37, 47, 57, 67) in fluid communication with the at least one electrolyte solution, the at least one electrolyte compensation device comprising a compensation timing determination unit and a compensation unit; wherein the compensation timing determination unit is arranged to directly or indirectly measure a total concentration of ionic species contained in the at least one electrolyte solution, and start operation of the compensation unit if the total concentration of the ionic species is out of a reference concentration range; wherein the compensation unit is arranged to compensate for a difference between the total concentration of ionic species contained in the at least one electrolyte solution (14a; 14b, 24a; 24b; 34a; 34b, 44a; 44b, 54a; 54b, 64a; 64b) and a reference concentration by supplementing the at least one electrolyte solution with an external electrolyte solution or by removing a solvent component from the at least one electrolyte solution; and wherein the at least one electrolyte solution differs from the influent fluid. at least one flow path (11, 21, 31, 41, 51, 61) configured to receive influent fluid; at least one pair of electrodes (13a; 13b, 23a; 23b; 33a; 33b, 43a; 43b, 53a; 53b, 63a; 63b); at least one charge barrier (12a; 12b, 22a; 22b; 32a; 32b, 42a; 42b, 52a; 52b, 62a; 63b) disposed between the at least one flow path and a corresponding electrode of the at least one pair of electrodes, the at least one charge barrier ionically separating the corresponding electrode from the at least one flow path; and at least one electrolyte solution (14a; 14b, 24a; 24b; 34a; 34b, 44a; 44b, 54a; 54b, 64a; 64b) disposed between at least one electrode of the at least one pair of electrodes and a corresponding charge barrier of the at least one charge barrier; characterized by: at least one electrolyte compensation device (17, 27, 37, 47, 57, 67) in fluid communication with the at least one electrolyte solution, the at least one electrolyte compensation device comprising a compensation timing determination unit and a compensation unit; wherein the compensation timing determination unit is arranged to directly or indirectly measure a total concentration of ionic species contained in the at least one electrolyte solution, and start operation of the compensation unit if the total concentration of the ionic species is out of a reference concentration range; wherein the compensation unit is arranged to compensate for a difference between the total concentration of ionic species contained in the at least one electrolyte solution (14a; 14b, 24a; 24b; 34a; 34b, 44a; 44b, 54a; 54b, 64a; 64b) and a reference concentration by supplementing the at least one electrolyte solution with an external electrolyte solution or by removing a solvent component from the at least one electrolyte solution; and wherein the at least one electrolyte solution differs from the influent fluid. at least one electrolyte compensation device (17, 27, 37, 47, 57, 67) in fluid communication with the at least one electrolyte solution, the at least one electrolyte compensation device comprising a compensation timing determination unit and a compensation unit; wherein the compensation timing determination unit is arranged to directly or indirectly measure a total concentration of ionic species contained in the at least one electrolyte solution, and start operation of the compensation unit if the total concentration of the ionic species is out of a reference concentration range; wherein the compensation unit is arranged to compensate for a difference between the total concentration of ionic species contained in the at least one electrolyte solution (14a; 14b, 24a; 24b; 34a; 34b, 44a; 44b, 54a; 54b, 64a; 64b) and a reference concentration by supplementing the at least one electrolyte solution with an external electrolyte solution or by removing a solvent component from the at least one electrolyte solution; and wherein the at least one electrolyte solution differs from the influent fluid.