Resistance of Legionella to disinfection in hot water distribution systems

The efficiency of various disinfection treatments against Legionella was tested on a hot water distribution system (HWDS) pilot unit. The results demonstrated clearly that most Legionella in the networks were fixed in the biofilm at the surface of the pipe (more than 98% for each loop). Chemical treatments (continuous chlorination, hyperchlorination, hydrogen peroxide and peracetic acid mixing) commonly used for the eradication of Legionella in hot water distribution networks appeared to be inadequate for eradicating the bacteria in the biofilm. Unfortunately, the biofilm contained most of the pathogens in an HWDS whereas legislation is only restricted to the Legionella concentration in the water phase. Thermal treatment appeared to be efficient to disinfect most of the biofilm but seemed to promote the biofilm re-growth as well. It was then concluded that the best solution to prevent Legionella contamination in hot water distribution systems would be to have perfect control of the temperature in the networks (temperature > 55 degrees C at all points). Nevertheless, in many cases it is difficult to have such control, so during the time necessary to modify networks, the best solution to control Legionella proliferation appears to be to apply a treatment shock (thermal or chlorination as a function of pipe characteristics). These treatments must be followed by a continuous chlorination that is totally controlled and equipped with alarm systems. This study demonstrates that biofilm sampling devices must be installed in hot water distribution systems to anticipate Legionella contamination and correctly determine the efficiency of the treatments.