Estimating the bacterial source strength and the downwind influence of outdoor composting platforms

National audience; Active and passive emission processes at composting sites result in the release of microorganisms that can then be transported by the atmospheric flow over some distance before they settle. Determining the impact of compost operations on bacterial source strength and on the distance from the site at which air concentration returns to its background value (the ‘return distance’) is a topic of importance for risk assessment. To this purpose, experimental campaigns were conducted in France at two outdoor composting sites, where air concentration measurements were performed at several locations downwind from the source, along horizontal transects aligned with the mean wind direction. All relevant microclimatic variables were also registered. Quantitative PCR was used to monitor the microbial indicators selected in a previous study in order to trace specifically the bioaerosols emitted from compost piles and evaluate their dynamics in the atmosphere. A simple Gaussian-type modelling approach was designed to estimate the source strength and the return distance from the measured concentration transects. The main factors of variation in terms of air concentration, source strength and return distance were found to be the site itself, then the operation type: overturning generates the largest amount of bacteria, followed by loading and screening. This affects the return distance, which also shows a weak variation with mean wind velocity. The generic character of this method combining atmospheric transport modelling with air concentration measurements makes it applicable to other cases of atmospheric dispersal from point or area sources.