Cooling Tower - Legionnaires' Disease

Legionnaires' Disease

Another very important reason for using biocides in cooling towers is to prevent the growth of Legionella, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila, or Mycobacterium avium. The various Legionella species are the cause of Legionnaires' disease in humans and transmission is via exposure to aerosols—the inhalation of mist droplets containing the bacteria. Common sources of Legionella include cooling towers used in open recirculating evaporative cooling water systems, domestic hot water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources include freshwater ponds and creeks.

French researchers found that Legionella bacteria travelled up to 6 kilometres through the air from a large contaminated cooling tower at a petrochemical plant in Pas-de-Calais, France. That outbreak killed 21 of the 86 people who had a laboratory-confirmed infection.

Drift (or windage) is the term for water droplets of the process flow allowed to escape in the cooling tower discharge. Drift eliminators are used in order to hold drift rates typically to 0.001%-0.005% of the circulating flow rate. A typical drift eliminator provides multiple directional changes of airflow while preventing the escape of water droplets. A well-designed and well-fitted drift eliminator can greatly reduce water loss and potential for Legionella or other chemical exposure.

Many governmental agencies, cooling tower manufacturers and industrial trade organizations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers. Below is a list of sources for such guidelines:

• Centers for Disease Control and Prevention PDF (1.35 MB) - Procedure for Cleaning Cooling Towers and Related Equipment (pages 239 and 240 of 249)
• Cooling Technology Institute PDF (240 KB) - Best Practices for Control of Legionella, July, 2006
• Association of Water Technologies PDF (964 KB) - Legionella 2003: An Update and Statement
• California Energy Commission PDF (194 KB) - Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants
• SPX Cooling Technologies PDF (119 KB) - Cooling Towers Maintenance Procedures
• SPX Cooling Technologies PDF (789 KB) - ASHRAE Guideline 12-2000 - Minimizing the Risk of Legionellosis
• SPX Cooling Technologies PDF (83.1 KB) - Cooling Tower Inspection Tips {especially page 3 of 7}
• Tower Tech Modular Cooling Towers PDF (109 KB) - Legionella Control
• GE Infrastructure Water & Process Technologies Betz Dearborn PDF (195 KB) - Chemical Water Treatment Recommendations For Reduction of Risks Associated with Legionella in Open Recirculating Cooling Water Systems