AWWA WQTC58911

Efficiency of various treatments of disinfection against Legionella were tested on a Hot Water Distribution System (HWDS) pilot unit. The results demonstrated clearly that a large majority of 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 a large majority 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 the majority 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 a hot water distribution system would be to have perfect control of the temperature in the networks (temperature > 55C at all points). Nevertheless, in many cases it is difficult to have such control, so during the time necessary to do the network modifications, the best solution to control the Legionella proliferation appeared to be applying a treatment shock (thermal or chlorination in function of pipe characteristics). These treatments must be followed by a continuous chlorination totally controlled and equipped with alarm systems. This study demonstrated that biofilm sampling devices must be installed in hot water distribution systems to anticipate Legionella contamination and correctly determine the efficiency of the treatments. Includes 21 references, table, figures.