Control of Legionella contamination and risk of corrosion in hospital water networks following various disinfection procedures

Physical and chemical disinfection methods have been proposed with the aim of controlling Legionella water contamination. To date, the most effective procedures for reducing bacterial contamination have not yet been defined. The aim of this study is to assess the long-term effectiveness of various disinfection procedures in order to reduce both culturable and not culturable (NC) legionellae in different hospital water networks treated with heat, chlorine dioxide, monochloramine and hydrogen peroxide. The temperature levels and the biocide concentrations that proved to obtain reliable results were analysed. In order to study the possible effects on the water pipes, we verified the extent of corrosion on experimental coupons after applying each method for six months. The percentage of positive points was at its lowest after treatment with monochloramine, followed by chlorine dioxide, hydrogen peroxide and hyperthermia. A different selection of Legionella spp was observed, as networks treated with chlorine-based disinfectants were mainly contaminated by L. pneumophila serogroup 1, hyperthermia was associated with the serogroups 2-14 and hydrogen peroxide mainly with non-pneumophila species. NC cells were only detected in heat-treated waters, also when the temperature was approximately 60°C. The corrosion rate of the coupons was within a satisfactory limit for water networks, but the morphology changed. We confirm that chemical disinfection controls Legionella colonization more effectively than hyperthermia. Monochloramine was the most effective treatment, while hydrogen peroxide could be a promising alternative to chlorine-based disinfectants due to its ability to select for other less virulent or non-pathogenic species.