A device for purifying water, the device comprising - an outer casing which limits an inner part of the device, - a cathode electrode having the shape of a generalized cylinder sleeve, the cathode electrode thus having a profile shape extending in a longitudinal direction (+z) of the cathode electrode, the cross-section of the profile shape forming a closed curve, the said longitudinal direction (+z) being arranged to be essentially vertical during the use of the device, which - cathode electrode is arranged in the inner part of the device, the device further comprising - an anode electrode having the shape of a generalized cylinder sleeve in such a way that - between the anode electrode and cathode electrode remains, in a transverse direction that is perpendicular to the said longitudinal direction (+z), a gap for conveying the said water, - the said anode electrode comprising on its outer surface at least a first anode material. characterised in that - the anode electrode in addition comprises on its outer surface at least a second anode material, which second anode material is a different material than the said first anode material and the anode electrode comprises a first area or first areas which comprise the first anode material, and a second area or second areas which comprise the second anode material, the said first area or first areas not comprising the same part of the outer surface of the anode electrode directed towards the cathode electrode as the said second area or second areas, and - the anode electrode remains inside the said cathode electrode in the said transverse direction, and the device further comprises - an anode electrode support extending in the said longitudinal direction (+z), at least a part of the anode electrode support being provided inside the said anode electrode in the said transverse direction, which - anode electrode support comprises in its lower part, protruding transversely from its body, a first projection arranged to support the anode electrode from its lower part upwards in the said longitudinal direction ( z), whereupon - the anode electrode can be lifted by means of the said anode electrode support out of the cathode electrode and is thus replaceable. - an outer casing which limits an inner part of the device, - a cathode electrode having the shape of a generalized cylinder sleeve, the cathode electrode thus having a profile shape extending in a longitudinal direction (+z) of the cathode electrode, the cross-section of the profile shape forming a closed curve, the said longitudinal direction (+z) being arranged to be essentially vertical during the use of the device, which - cathode electrode is arranged in the inner part of the device, the device further comprising - an anode electrode having the shape of a generalized cylinder sleeve in such a way that - between the anode electrode and cathode electrode remains, in a transverse direction that is perpendicular to the said longitudinal direction (+z), a gap for conveying the said water, - the said anode electrode comprising on its outer surface at least a first anode material. - the anode electrode in addition comprises on its outer surface at least a second anode material, which second anode material is a different material than the said first anode material and the anode electrode comprises a first area or first areas which comprise the first anode material, and a second area or second areas which comprise the second anode material, the said first area or first areas not comprising the same part of the outer surface of the anode electrode directed towards the cathode electrode as the said second area or second areas, and - the anode electrode remains inside the said cathode electrode in the said transverse direction, and the device further comprises - an anode electrode support extending in the said longitudinal direction (+z), at least a part of the anode electrode support being provided inside the said anode electrode in the said transverse direction, which - anode electrode support comprises in its lower part, protruding transversely from its body, a first projection arranged to support the anode electrode from its lower part upwards in the said longitudinal direction ( z), whereupon - the anode electrode can be lifted by means of the said anode electrode support out of the cathode electrode and is thus replaceable.