R.L. Brigmon, C. D. Smith, S. F. Oppenheimer
Clemson University & Augusta University,
United States
Keywords: Legionella pneumophila, cooling towers, biofilm control, water chemistry
Summary:
Biofilms, a viscous layer of microorganisms combined with organic and inorganic matter in aquatic systems can provide a conducive environment where pathogenic Legionella pneumophila can proliferate. L. pneumophila is the etiological agent of Legionnaires’ disease, a pneumonia caused by the inhalation of the bacterium. The disease has been linked with cooling towers contaminated with L. pneumophila resulting in human exposure via aerosolization. Since elevated Legionella in cooling tower water are a potential health concern questions have been raised as to the best control methodology. L. pneumophila concentrations >107 cells/L in cooling tower water have been correlated with Legionnaire’s disease outbreaks. Analysis of chemical, biological, and atmospheric data was performed to determine the best method or parameter to control Legionella densities. Evaluation of 14 cooling towers, seven water quality parameters, and five Legionella serogroups over three years demonstrated that most significant factors (p<0.005) were related to suspended particulates, conductivity, pH, and dissolved oxygen, not chlorine or bromine as expected. In biofilms, L. pneumophila is protected from biocides and other cleaning methods. To model the cooling tower environment we have constructed a system of five nonlinear ordinary differential equations. The five-time dependent variables are the concentration of nutrient, the concentration of Legionella in solution, the concentration of Legionella in the biofilm relative to the volume of fluid in the tower, the concentration of Bromine in the solution, and the concentration of chlorine in the solution. Our goal is twofold. First of all, we seek to capture the gross qualitative behavior of the biological system. Second, since the mass rate of bromine being added and the mass rate of chlorine being added are included in the model, we will seek to use these as time-dependent control variables to develop optimal strategies to manage the growth of Legionella within cooling tower biofilms.