the present invention comprises an improved method and apparatus for treating water in heat exchange systems . the present invention automatically and inexpensively provides an effective chemical to process water systems at times and in proportion to the need for water treatment in such systems . when used in evaporative cooling systems , the present invention adds the treatment chemical to the system via the make - up water . the make - up water acquires a substantially constant concentration of treatment chemical as it is passed through a device containing a treatment chemical . the constant concentration of chemical is achieved by dividing the make - up flow into two streams within the device , saturating the smaller of the two streams with chemical , and re - mixing the two streams . the biocide agent used in the preferred embodiment of the invention is prilled elemental iodine .