The fight against Legionella

How keeping Legionella out of water systems can reduce the risk of infection.


The first major outbreak of Legionnaires’ Disease occurred in July of 1976 at an American Legion convention in Philadelphia at the Bellevue-Stratford Hotel. The source of the outbreak was cooling tower water infected with high concentrations of Legionella that became airborne on wind currents. The airborne bacteria were inhaled by a number of convention participants. In the end, 130 convention attendees were hospitalized with pneumonia-like symptoms, resulting in 25 fatalities.

Last year, the Centers for Disease Control and Prevention estimated that between 8,000 and 18,000 people were hospitalized with Legionnaires’ Disease, which is caused by exposure to Legionella Pneumophilia (CDC, 2013). Legionnaires’ Disease can be a fatal form of pneumonia acquired by inhaling the Legionella bacteria through the lungs. The bacteria is not spread from person to person and is naturally found in the environment including cooling towers, decorative fountains, evaporative condensers, hot tubs, and hot water systems. The key to reducing the risk of infection is keeping Legionella out of water systems.

By November of 1976, The Bellevue-Stratford, one of Philadelphia’s premier hotels, closed their doors for good. Legionella Pneumophilia are dormant at temperatures ranging from 0˚ F. to 68˚ F. At temperatures ranging between 68˚F to 121˚ F., Legionella thrive and multiply. At temperature above 121˚F, Legionella are killed within the following time frame: 100% are instantly killed at 160˚ F.; 90% are killed within 2 minutes at 140˚ F.; 90% are killed within 2 hours at 122˚ F. Legionella also thrive in environments rich with nutrients such as algae, flavobacteria, pseudomonas, biofilms, and scales that can be found in hot water tanks and dead legs in domestic hot water systems.

Water testing and emergency disinfection

The key to determining the present risk of Legionella infection is regular and routine water testing. Water samples from different areas of the domestic hot water system should be sent to a qualified laboratory for analysis. A Legionella mitigation strategy should be formed based on the results of regular water testing. Assuming that a significant Legionella Colony Forming Unit (CFU) exists within a domestic hot water system, steps must be taken to eradicate and disinfect the system.

There are two recommended types of methods for emergency disinfection; Hyper Halogenation and Thermal Eradication. Hyper Halogenation involves isolating the infected water side piping system and introducing high levels of a halogen that kill waterborne organisms. Once the hyper halogenation process is complete, the piping must be flushed and water samples must be taken at different locations to ensure that concentrations of the halogen are below safe drinking water levels.

High concentration levels of halogens, however, can be destructive to piping and fixtures. Damage to water systems from halogens can be expensive. Furthermore, high concentrations of halogens are not altogether reliable due to the ineffectiveness in penetrating bio films.

Another method for emergency disinfection is Thermal Eradication. This method requires the isolation of the water side piping system and increasing the hot water system temperature to greater than 160˚ F. While circulation of this extreme hot water continues throughout the system, each distal tap is then flushed by an operator for a pre-determined period of time.

Domestic hot water heaters must have the ability to generate these high temperatures in an emergency disinfection mode without the interference of temperature safety systems at a prescribed flow rate suitable for system coverage. Once the thermal eradication process is complete, water system temperatures must be returned to safe levels before the water system is recommissioned. Once the water system is disinfected, routine water treatment maintenance must continue.

One way of treatment is silver copper ionization. Positively charged silver and copper ions are introduced into the water system that attach themselves to the negatively charged Legionella organisms causing cell death. This is a highly effective treatment process but concentrated levels of heavy metals can be toxic to humans so regular testing for unsafe levels of heavy metals is recommended.

Another form of treatment is ultraviolet light. Exposure of the domestic water flow to an ultraviolet light source (254nM) renders the suspended Legionella bacteria unable to reproduce. Pre-filtration is essential with this highly effective treatment method as wavelengths can be distorted by the presence of debris on the light source. Unless the water flows past the UV light source, however, it will remain untreated.

Hyperchlorinization is another method for water treatment. This method introduces higher levels of chlorine into water systems while keeping below safe drinking water standards. As stated earlier, chlorine can be destructive to water systems and is not always effective as higher Legionella CFUs can avoid exposure through biofilms with some strains showing resistance to chlorine contact. Chlorine also decays at higher temperatures.

Finally, the heat treatment method involves increasing the outlet water temperature of domestic hot water heating systems to temperatures above a 140˚ F. Of course scalding is a danger and so the hot water must be tempered with cooler water before usage. However, Legionella growth can be limited as long as system temperatures remain above the Legionella “kill zone.” This method is perhaps the most cost effective as hot water is already created and is not non-toxic and not destructive to piping systems.

- Content provided by Spirax Sarco, originally published in Steam News Magazine.

- Edited by Jessica DuBois-Maahs, Associate Content Manager, CFE Media, Plant Engineering, Control Engineering

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