Variability of sewage bacterial quality in a large urban area [Variabilité da la qualité microhiologique des eaux usées brutes dans una grande agglomération]

National audienceMicrobial quality of urban wastewater relates to several environmental, sanitary and political issues. However, there is a lack of knowledge concerning the variability of raw wastewater. Our study aims at evaluating the microbial variability of the influent of several wastewater treatment plants in the Paris area, as well as estimating this variability impact on the treatment efficiency. Faecal bacterial indicators (E. coli et intestinal enterococci) and their partitioning on settleable and free phases were measured at three wastewater treatment plants (Marne Aval, Seine Amont et Seine Centre) during dry and storm periods. Our results show that faecal indicator levels fluctuate according to the type of sewer system and the hydrological conditions. During storm events, a significant dilution of faecal indicators can be observed, as well as an increase in the percentage of settleable bacteria. Enterococci removal by primary and secondary treatment is linked to the influent level of enterococci, however the removal of both indicators is also influenced by the management operation of the wastewater treatment plants