A water purification system and method suitable for use in swimming pools, spas, hot tubs, water storage tanks, wells and water cooling towers employs a galvanic cell having a silver or copper or zinc anode electrically connected to a cathode made from a metal of still higher electrochemical potential, normally a platinum group metal and preferably palladium. A galvanic cell of some tens of square centimeters in size and some hundreds of grams in weight liberates sufficient silver or copper ions so as to treat a multi-thousand liter body of water, such as a swimming pool, for, typically under normal contamination, some months until the anode is consumed. Copper and/or silver ions liberated from the galvanic cell suppress bacterial, fungal and/or algae growth, thus, significantly reducing the amount of chlorine, bromine or other chemicals needed to maintain water quality. The invention operates on the current generated by the galvanic action between the dissimilar metals of the anode and cathode, and does not require external electrical power.