An apparatus and method for killing microbes, including bacteria or yeast-like fungus within or on a conductive medium, is provided including positive and negative electrodes which may be activated to generate antimicrobial agents from within the conductive medium. The agents are any form of anions including atoms or molecules derived from inorganic salts present within the conductive medium. Due to residual killing effect caused by the presence of internally generated antimicrobial agents, constant activation of the electrodes is not necessary thereby allowing intermittent activation in order to prolong the life of the metallic or non-metallic electrodes. Electrode polarity can be reversed periodically to drive precipitation from the electrode surface and to also extend electrode life.