by the cooperative use of the copper and silver ions at safe concentrations far below those which would be required in order to achieve a similar effect using copper and silver which were dispersed separately in a bactericidic or algicidic system respectively the system of the invention is operated so as to provide cu ions at a level in the range of approximately 350 to 500 part per billion of water ppb and ag ions at a level in the range of approximately 25 45 ppb in other words the water being purified must be given a combination of 350 500 ppb cu and 25 45 ag in order to reach potable water quality if one were to attempt to use only cu to produce acceptable bacteria free pool water it would be necessary to have 2 500 ppb cu in the water however if cu were present at 2 500 ppb in pool water there would be staining of the walls of the pool and the level of cu ions in the water would be too high for safe human consumption one thousand 1 000 ppb cu is the maxiumum allowable level of copper in drinking water it is impossible to reach an effective concentration of ag alone which would be satisfactory for drinking quality 34 potable 34 pool water this is because it would require at least 400 to 500 ppb ag to achieve appropriate quality drinking water at such a level of concentration however the ag would combine.