Environmental factors affecting the colonization of cooling towers by Legionella spp. in South Australia

Legionella colonization in 31 cooling towers in or near Adelaide in South Australia was investigated. All towers were on identical cleaning and chemical treatment prorammes, including treatment with a monomeric quaternary ammonium biocide. Tower construction varied, wood, fibreglass and stainless steel models being included. Over a two-year period, water samples were taken from towers twice weekly and records kept of tower water pH, conductivity, total bacterial count, basin-water temperature, ambient temperature and hours usage of each tower at the time of collection. Temperature and tower usage showed direct correlation with Legionella counts. Between 60 and 75% of cooling tower waters were colonized by Legionella spp. during the summer months. This figure fell to 20-30% during the winter. Basin-water temperatures were shown to be a major determinant in Legionella colonization; regardless of ambient temperature, towers with elevated basin-water temperatures were frequently colonized. Analysis showed that Legionella counts in towers used continuously throughout the year were not influenced by ambient temperature.