Transfert de microplastiques via le réseau d’assainissement unitaire: des dépôts au sein du réseau aux rejets urbains de temps de pluie

Although it is recognized in the literature that urban wastewater carries a significant load of microplastics (MP), the contamination of deposits in sewer networks and combined sewer overflows (CSO) has remained unexplored until now, despite potentially significant contamination. Concentrations of between 5×10³ and 178×10³ particles/kg of dry sediment were observed in deposits, indicating that some of the microplastics carried by wastewater settle. Irrespective of the polymer considered, the sedimentation of some of the microplastics within the network could be linked to wastewater flow conditions, but also to interactions with other particles in the wastewater, and/or to the formation of hetero-aggregates with other mineral or organic particles, whether or not coated with biofilm. These phenomena lead to the formation of a large stock within the network, and the associated risk of transfer during rainy events due to the resuspension of these sediments. MP concentrations in the CSO fluctuate between 67 and 392 particles/L, and no clear relationship between MP concentration and the proportion of wastewater or stormwater or with other physico-chemical parameters, was demonstrated. This suggests that the contamination of CSO is complex and may depend on many other factors. Among these factors, the resuspension of deposits within the network could play a key role. Considering that 21 million m³ of CSO are discharged into the Seine every year and the minimum and maximum concentrations, an annual flow of particles was estimated at between 1.4×10¹² and 8.2×10¹² particles/year, a relatively large flow compared with the flows discharged by wastewater treatment plants (from 2,4×10¹¹ to 2,3×10¹³ particles/an or downstream of the Paris conurbation (1,1×10¹³ particles/an).