Abstract: A method and apparatus for treating water or wastewater for drinking and/or industrial use. The method and apparatus comprises of a plurality of vertically positioned electrodes, which are placed in a treating chamber and wherein the electrodes are interconnected to one another. The positive and negative electrodes are insulated there-between. The polarity of the direct current supply is changeable at regular intervals in order to prevent passivation of the electrodes when reaching an even abrasion. The current can preferably be pulsatory. In order to be able to keep the current density between the electrodes at a desired value, the most efficient possible electrolysis is achieved by means of a minimum total current and wherein the spacing between the electrodes are adjustable.