A method and apparatus for treating a body of water to kill microorganisms by enhancing the concentration metal ions therein. The apparatus comprising a dispenser with a first housing having a water accessible compartment containing a source of 5,5-dimethylhydantoin for releasing the 5,5-dimethylhydantoin when contacted by the body of water and a second housing having a water accessible compartment containing an insoluble metal ion donor for releasing metal ions when contacted by the body of water containing the 5,5-dimethylhydantoin
