Chapter 128 : Efficacy of Monochloramine against Surface-Associated Legionella pneumophila in a Cooling Tower Model System

Abstract:

Efficacy of monochloramine against mature biofilms formed in a model recirculating water system was compared on different surfaces under identical conditions. A laboratory-scale cooling tower was designed to simulate a wet evaporative cooling system, which was experimentally seeded with Legionella pneumophila. Rather low inoculum was added to the model system to mimic the natural entry of L. pneumophila from the water supply. Biofilm had been allowed to grow on coupons, and the model system was disinfected after a 180-day period with monochloramine. Three coupons of each material were removed from the basin and dip-rinsed in sterile phosphate buffer to remove unattached cells. The bacterial densities in biofilms from different materials after disinfection (180 min, 1.5 ppm dose) suggested a material-dependent activity of monochloramine. Monochloramine was found ineffective against microorganisms on polypropylene (PP) and galvanized steel surfaces. Results indicated that monochloramine shows material-dependent activity in the cooling tower model system and it has long residual activity at high temperatures and pH levels, leading to improved performance in recirculating water. This observation supports the material-dependent activity of monochloramine and it could be explained by the formation of dissimilar biofilms on different surfaces, which affects the architecture of the biofilm matrix. Chemical disinfection is strongly recommended from the beginning of the tower operation to prevent or reduce the occurrence of Legionnaires’ disease. Monochloramine disinfection is inexpensive and could be used in automatic injection devices instead of chlorine.