A water purification apparatus used to prevent the formation of scale around electrodes which form part of the apparatus. The apparatus comprises at least one electrode containing a proportion of an ion-emitting electrode such as copper, silver or a copper and silver alloy and at least one electrode containing a proportion of a non-emissive metal such as titanium. The electrodes are connected to a DC power supply which has switching means to reverse the polarity of the electrodes at predetermined intervals. The apparatus ensures accurate control of disinfection and minimal equipment maintenance.