Legionaires’ Disease
Legionaires’ disease is a dangerous, potentially fatal form of pneumonia. Certain groups, such as old or sick people, are known to be at higher risk of contracting Legionaires’ disease. Men appear more susceptible than woman, as do those over 45 years of age, smokers, alcolholics and diabetics. Infection by Legionella is also responsible for a feverish form of illness known as Pontiac fever. We don’t know how many deaths are caused by Legionella infection because pneumonia is very often accepted without further inquest as a “natural” cause of death in older persons. For the USA 15.000 deaths/year are estimated!
The Cause
Main determined pathogen is Legionella pneumophila. Not only the best known sero-group 1, but all 40 sero-groups as well as other Legionella types are pathogenous. Legionella have optimal growth conditions between 30 and 37 °C, they stagnate between 40 and 45 °C and will die above 70 °C. Characteristic to Legionella is that they live and proliferate in other microorganism such as amoebae. Biofilm which is present in every water pipe and tank provides optimal living conditions for Legionella at appropriate temperatures.
Risk Areas
Legionella bacteria are common and can be found in every natural lake, in slop and in wet soil, without becoming dangerous for humans. Infection will appear after inhalation of tiny droplets of water (aerosols) contaminated with Legionella. For that reason, the following risky areas can be specified:
- showers in hotels, hospitals, nursing homes, swimming-pools and other sport facilities
- spa baths and swimming pools with other sparkling water attractions
- ventilation plants with wet gas scrubbers
- cooling towers
- areas, where water is sprayed for cooling and moistening purposes (e.g. coolants/lubricants in metal-cutting manufacturing, wetting of carbon powder with river water)
Biofilm
Biofilms consist of microorganism and extracellular subtances . Legionella colonize these often slimy and fungoid films and proliferate. The main problem: Biofilms are extremely resistant to disinfectants. For that reason Legionella are extremely resistant against common disinfectants because they are protected against chemical attack by the biofilm. Any effective measures or propylaxis against Legionella has to start at the root cause, which is the biofilm. All measures that focus on only the direct distruction of the Legionella are not longterm and therefore do not solve the problem.
Effective Control Measures
The following measures and methodologies have proven effective and especially cost efficient for Legionella destruction and prevention.
Appropriate Design of Piping and Fittings
- correct selection of material
- correct hydraulics (high flow velocity, no „dead“ corners)
- avoidance of risky areas
- no parallel connection of boilers
- no heat recovery via pre-heaters
Maintenance and Control
- regular maintenance with checkup and cleaning of the boilers
- regular inspection regarding Legionella infestation
Treatment with Chlorine Dioxide
- high disinfectant effect independent of the pH-value
- decomposition of the biofilm in the piping system, thus protection against re-infection
- deposit action longer than with chlorine
- less danger of corrosion than with chlorine
Treatment with Ozone
- strong disinfectant, reliably removes biofilms
- reacts without resulting residuals in oxygen
- in water, short half life period, for that reason no deposit effect
- good application in cooling towers and air scrubbers
Other Measures
High Temperature Water (> 70 °C)
- high energy consumption
- danger of scalding at the taps
- hardly practical with long pipelines
- Boiler temperature of > 55 °C avoids proliferation of Legionella only in the boiler, not in the system
UV-Radiation
- high radiation dose necessary in case of infestation with amoebae
- no deposit effect
- no protection against re-infection in biofilms
UV-Radiation + Ultrasonic
- Legionella are released when treating amoebae and pieces of biofilm with ultrasonic
- no deposit effect
- no protection against re-infestation in biofilms
- high investment costs, large space required
Ultrafiltration
- no deposit effect
- no decomposition of the biofilm, thus no protection against re-infection
Chlorination (bleach, electrolyses)
- no sufficient disinfectant effect without simultaneous adjustment of the pH-value
- no decomposition of the biofilm, thus no protection against re-infection
- chlorine odour due to oxidation byproducts such as chloramine is possible
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