Legionellosis prevention

Controlling the potential growth of Legionella in cooling towers
Various studies have shown that some 40 to 60% of cooling towers tested contained Legionella. A recent research study provided evidence that Legionella pneumophila, the causative agent of Legionnaires' disease, can travel at least 6 km from its source by airborne spread. It was previously believed that transmission of the bacterium was restricted to much shorter distances. A team of French scientists reviewed the details of an epidemic of Legionnaires' disease that took place in Pas-de-Calais in northern France in 2003–2004. There were 86 confirmed cases during the outbreak, of whom 18 perished. The source of infection was identified as a cooling tower in a petrochemical plant, and an analysis of those affected in the outbreak revealed that some infected people lived as far as 6–7 km from the plant.

Temperature affects the survival of Legionellae as follows:


 * 70 to 80 °C (158 to 176 °F): Disinfection range
 * At 66 °C (151 °F): Legionellae die within 2 minutes
 * At 60 °C (140 °F): Legionellae die within 32 minutes
 * At 55 °C (131 °F): Legionellae die within 5 to 6 hours
 * Above 50 °C (122 °F): They can survive but do not multiply
 * 35 to 46 °C (95 to 115 °F): Ideal growth range
 * 20 to 50 °C (68 to 122 °F): Legionellae growth range
 * Below 20 °C (68 °F): Legionellae can survive but are dormant

Removing slime is an effective control process, because up to 1,500 times more chemicals are needed to kill bacteria in slime than free floating bacteria.

Action levels following microbial monitoring for cooling towers
The European Working Group for Legionella Infections (EWGLI) was established in 1986 within the European Union framework to share knowledge and experience about potential sources of Legionella and their control. This group has published guidelines ) about the actions to be taken to limit the number of colony forming units (i.e., the aerobic count) of micro-organisms per mL at 30 °C (minimum 48 hours incubation):

Aerobic count, cfu/ml at 30°C (minimum 48 hours incubation). Colony count determined by pour plate method according to ISO 6222(21) or by spread plate method on yeast extract agar

Legionella, bacteria cfu/litre. Determined in accordance with ISO 11731(20).

Expect to find Legionella CFUs because almost all natural water sources, contain Legionella. Legionella samples need to be run within 24 hours. Samples should be kept cold with ice packs, but not frozen, and should remain up right.

Guidelines for control of Legionella in cooling towers
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:


 * SPX (Marley) Cooling Technologies - ASHRAE Guideline 12-2000 - Minimizing the Risk of Legionellosis
 * EWGLI Technical Guidelines for the Control and Prevention of Legionella in Water Systems
 * Centers for Disease Control and Prevention - Procedure for Cleaning Cooling Towers and Related Equipment (pages 239 and 240 of 249)
 * Cooling Technology Institute - Best Practices for Control of Legionella
 * OSHA - How to maintain these systems - Frequency of cleaning
 * OSHA page on Legionellosis - US Occupational Safety and Health Administration
 * Association of Water Technologies - Legionella 2003
 * California Energy Commission - Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants
 * SPX (Marley) Cooling Technologies - Cooling Towers Maintenance Procedures
 * Marley Cooling Technologies - Cooling Tower Inspection Tips {especially page 3 of 7}
 * Tower Tech Modular Cooling Towers - Legionella Control
 * GE Infrastructure Water & Process Technologies Betz Dearborn - Chemical Water Treatment Recommendations For Reduction of Risks Associated with Legionella in Open Recirculating Cooling Water Systems
 * Wisconsin Division of Health, Control of Legionella in Cooling Towers: Summary Guidelines, June 1987, Wisconsin Department of Health and Social Sciences.

Regulations & Ordinances
The guidance issued by the UK government's Health and Safety Executive (HSE) now recommends that microbiological monitoring for wet cooling systems, using a dip slide, should be performed weekly. The guidance now also recommends that routine testing for legionella bacteria in wet cooling systems be carried out at least quarterly, and more frequently when a system is being commissioned, or if the bacteria has been identified on a previous occasion.

The City of Garland, TX requires yearly testing for legionella bacteria at Cooling towers at apartment buildings.

Malta requires twice yearly testing for legionella bacteria at Cooling towers and water fountains. Malta prohibits the installation of new cooling towers and evaporative condensers at health care facilities and schools.