Numerical modelling of microorganisms dispersion in Urban area: Application to Legionella

Dispersion modelling is often used to estimate potentially contaminated areas in case of accidental release of microorganisms in the atmosphere. In the specific case of Legionella, accidental spread in the atmosphere due to contaminated cooling towers system may occur over distance larger than 10km. In addition, most cooling towers are located in Urban areas where dispersion due to obstacles is complex. In this case, dispersion models have to take into account complex flows and microphysical processes that occur within the plume and may have an impact on the survival of the microorganisms. To estimate the concentration of microorganisms in these areas, a specific module has been developed within the lagrangian dispersion model Micro Swift Spray (MSS, Aria technologies). This module takes into account microorganisms outside or inside water liquid droplets and microphysical interaction inside the plume. A simple biological module governing the survival of airborne microorganisms has also been implemented in the dispersion model. In order to evaluate this model, a field campaign of biological aerosols dispersion was performed by CSTB (Champs-sur-Marne, France) on June 23rd, 2009. Spores of bacillus atrophaeus (usually referred to as BG) initially contained in a water tank were disseminated in a subUrban area from a source at 3 meters above ground level. Air was sampled by DGA MNRBC (Vert-Le-Petit, France) at 4 various locations from 50 to 300 meters from the source to monitor NG concentration. Direct impaction onto Petri dishes was performed with slit-samplers and sixstage Andersen impactors. Wetted-wall cyclones and SKC Biosamplers were also implemented in order to sample air and generate liquid samples. Dispersion modelling for this campaign has been carried out by INERIS using the microorganism module developed in MSS. The results show that predicted concentrations and in situ measurements are in agreement. MSS Model was implemented to simulate legionella airborne dispersion from a virtual cooling tower at the same location. The biological model has been activated. Results show that the impact of biological model on airborne concentration is significant.