Impacts des épisodes pluvieux sur les émissions d'un système d'assainissement: cas du Grand Paris

International audienceIn typical life cycle assessment (LCA) studies of urban wastewater systems (UWS), average conditions are modelled but there are many annual flooding events with releases of untreated sewage. Such peak conditions are not considered and present a high temporal variability which is not currently accounted for. In addition, the aggregation of the loads from several storm events could bring an issue for the impact assessment on the aquatic categories of eutrophication and ecotoxicity. Hence we are investigating the contributions of these wet weather-induced discharges relative to average conditions along with the inclusion of temporal variability in the life cycle inventory (LCI) for UWS. In collaboration with the Paris public sanitation service (SIAAP), this work aimed at identifying and comparing contributing flows from the UWS in the Paris area by a selection of routine wastewater parameters and priority pollutants. This collected data is organized according to archetypal weather days during a reference year. Secondly, for each archetypal weather day and its associated flows to the receiving river waters (Seine) the parameters of pollutant loads (statistical distribution of concentrations and volumes) are determined statistically. The resulting inventory flows (i.e. the potential loads from the UWS) can be used as inputs in a classical LCA to investigate the relative importance of episodic wet weather versus "continuous" dry weather loads coupled to uncertainty analysis using a Monte Carlo method. Results analysis showed that all storm events in 2013 contribute to one third of the total annual nutrient load from wastewater effluents on routine wastewater parameters. Regarding priority pollutants, results on the freshwater ecotoxicity category tend to be dominated by heavy metals, but further consolidation on the extent of included substances is required. With such significant contributions of pollutant loads at the LCIA level, further research is required on how to include temporally-differentiated emissions in the methodological framework of the impact categories of eutrophication and ecotoxicity, to better understand how the performance of an UWS system affects the receiving environment for given local weather conditions