S5: Legionella control practices in cooling towers

Chemical biocide control in cooling towers is a critical part of the overall prevention strategy against L. pneumophila and Legionnaires disease. The purpose of the current study was to analyze a large population of cooling towers (3,829) for the presence and levels of L. pneumophila, and to correlate with specific biocide treatment (eight different biocides in 28 different combinations).  On average L. pneumophila was found to be present (≥10 cfu/ml) in 12% of the cooling towers, while only 2% of the towers had high levels of L. pneumophila (>1,000 cfu/ml).  No biocide combinations were 100% effective in preventing Legionella colonization, and also none always failed. Oxidizing biocide (primarily bromine) combinations with non-oxidizers were not significantly better than two non-oxidizers, and in the case of bromine alone, bromine/glutaraldehyde, and bromine/DBNPA, were significantly worse. The non-oxidizing biocide THPS was particularly effective, alone and in most combinations, as were quat/bromine, quat/hydroperoxide, quat/isothiazoline, and carbamate/isothiazoline.  Testing of a small group of cooling towers using a non-chemical device showed it to be less effective than the average chemical control.  Monitoring of L. pneumophila levels in cooling towers is an essential part of validating whether the control practices are effective for a particular cooling tower system.