Efficacy of methylchloro/methylisothiazolone biocide againstLegionella pneumophila in cooling tower water

Summary Methylchloro/methylisothiazolone biocide was tested for efficacy in cooling tower water againstLegionella pneumophila (strains Flint 1 and 2) andL. gormanii. These analyses are the first reports on the efficacy of isothiazolones againstLegionella in actual cooling tower water; the results of data reported previously obtained from more arbitrary laboratory tests are discussed and compared. The biocide was effective in cooling tower water at the two pH levels tested (pH 8.0 and 6.7). The concentration of biocide required to eliminate theL. pneumophila depended on contact time and pH: 99% of the bacteria were killed after 6 and 3 h of contact by 1.07 and 3.13 ppm active ingredient, respectively, when the pH was 8.0: 99% of the bacteria were killed after 6 and 3 h of contact by 2.23 and 9.43 ppm active ingredient, respectively, when the pH was 6.7. 24 h of contact with 0.35 ppm active ingredient methylchloro/methylisothiazolone biocide reduced the concentration of viable bacterial cells by more than four orders of magnitude (>99.99%), regardless of pH.