An apparatus for purifying water, the apparatus including - a cathode arrangement, which includes • a first cathode having the shape of a cylindrical shell, thus the first cathode having the shape of a longitudinal (z) profile and - an anode arrangement, which is electrically isolated from the cathode arrangement, the anode arrangement including • a first anode having the shape of a bar or a cylindrical shell, the longitudinal direction of which is the same as the longitudinal direction of the first cathode, • an electrically conductive flange, • an electrically conducting anode support, which is connected to the first anode and the flange and arranged to supply electricity to the first anode, and • a connection point for connecting an electric wire to the flange, in which apparatus - the first cathode encircles said first anode in the transverse directions perpendicular to said longitudinal direction ( z) in such a way that a first gap remains in the transverse direction between the first anode and the first cathode for transporting water, and - said flange is mechanically supported to the first cathode in the longitudinal direction (z), - the first cathode includes cathode material, - the first anode includes anode material, which is selected from the group of aluminium (Al), iron (Fe), magnesium (Mg), carbon, chromium (Cr), copper (Cu), manganese (Mn), tin (Sn), lead (Pb) and bismuth (Bi), characterised in that - said connection point is at least as far from the longitudinal centre axis of the cathode arrangement as such a point of the first cathode that is closest to said connection point, and - in the anode arrangement, at least said anode support and the connection point of said anode support and the first anode are coated with a water-insoluble and electrically insulating coating and the first anode is uncoated for their other part. - a cathode arrangement, which includes • a first cathode having the shape of a cylindrical shell, thus the first cathode having the shape of a longitudinal (z) profile and • a first cathode having the shape of a cylindrical shell, thus the first cathode having the shape of a longitudinal (z) profile and - an anode arrangement, which is electrically isolated from the cathode arrangement, the anode arrangement including • a first anode having the shape of a bar or a cylindrical shell, the longitudinal direction of which is the same as the longitudinal direction of the first cathode, • an electrically conductive flange, • an electrically conducting anode support, which is connected to the first anode and the flange and arranged to supply electricity to the first anode, and • a connection point for connecting an electric wire to the flange, in which apparatus • a first anode having the shape of a bar or a cylindrical shell, the longitudinal direction of which is the same as the longitudinal direction of the first cathode, • an electrically conductive flange, • an electrically conducting anode support, which is connected to the first anode and the flange and arranged to supply electricity to the first anode, and • a connection point for connecting an electric wire to the flange, in which apparatus - the first cathode encircles said first anode in the transverse directions perpendicular to said longitudinal direction ( z) in such a way that a first gap remains in the transverse direction between the first anode and the first cathode for transporting water, and - said flange is mechanically supported to the first cathode in the longitudinal direction (z), - the first cathode includes cathode material, - the first anode includes anode material, which is selected from the group of aluminium (Al), iron (Fe), magnesium (Mg), carbon, chromium (Cr), copper (Cu), manganese (Mn), tin (Sn), lead (Pb) and bismuth (Bi), characterised in that - said connection point is at least as far from the longitudinal centre axis of the cathode arrangement as such a point of the first cathode that is closest to said connection point, and - in the anode arrangement, at least said anode support and the connection point of said anode support and the first anode are coated with a water-insoluble and electrically insulating coating and the first anode is uncoated for their other part.