Resistance of Legionella to disinfection in hot water distribution systems

Abstract

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.

Effects of Disinfection on Legionella spp., Eukarya, and Biofilms in a Hot Water System

“…Our findings here are in perfect agreement with our previous results. Different studies have evaluated Legionella treatments in water (37,38,57), and a few have evaluated treatment in biofilm (49). Most of the studies demonstrated a transient effect on Legionella, where the initial levels were quickly recovered (49,62,69).…”

Longitudinal Evaluation of the Efficacy of Heat Treatment Procedures against Legionella spp. in Hospital Water Systems by Using a Flow Cytometric Assay

“…The percentage of VBNC Legionella cells in environmental samples is likely to be associated with variations of biotic or abiotic factors affecting the ecosystem in which Legionella agents proliferate (2,15,26,33). A further factor that fosters the survival and dissemination of Legionella in aquatic environments is the biofilm (23,29). Even if it is not clear whether the pipe material (37) or the type of disinfection influences the development of a biofilm, it was demonstrated previously that the presence of a biofilm reduces the efficacy of disinfection treatments (36).…”

Peracids in water treatment: A critical review

“…PAA has been used as a cooling water biocide [32,162], as it has a good potential for the removal of Legionella pneumophila [142,[163][164][165]. The continuous doses used for Legionella removal varied considerably (3-1000 mg/L) and re-growth after treatment was reported [142,[163][164][165].…”