Occurrence and fate of microplastics in the sewage system and different pathways into the environment : case of Greater Paris area

Plastics have become an integral part of modern human life. Alongside urbanization, metropolitan areas have become hotspots of plastic consumption and disposal. The abundance of microplastics (MPs) in urban wastewater reflects plastic pollution issue in these regions. Since the potential impacts of MPs on humans and ecosystems are of great concern, understanding their pathway in the environment is crucial to support the implementation for mitigation measures. In this context, this PhD project focused on the occurrence and fate of MPs in the Parisian wastewater management system, aiming to investigate and assess potential ways in which MPs from urban areas are released into the surrounding environments.In wastewater treatment plants (WWTPs), MPs separated from water phase are transferred into sewage sludge, making this byproduct a plausible source of MPs when disposed of in the environment. Knowledge of MPs in sewage sludge is essential for gaining a comprehensive understanding of this pollutant within WWTPs and its potential emission from these facilities. By investigating MPs at different sludge treatment stages, the study found that current technologies were inefficient in completely removing MPs. There was no significant reduction in MP abundance observed after all treatment processes, with 8.6×104 to 4.5×105 particles/kg of MPs remaining in the final treated sludge. Some sludge-based MPs were returned back to the system via reject water. Thermal treatment at high temperatures induced the fragmentation of plastic particles. These findings imply sludge disposal as an input of MPs into the environment, for example, via soil application in agricultural land.While MPs in WWTPs have been intensively investigated in the last decade, little attention has been paid for their fate and occurrence during transport within the sewer network before reaching treatment facilities. To address this knowledge gap, an investigation into MPs in sewer sediments, which act as pollutant reserve inside the sewer system, were carried out. The findings indicated that a portion of MPs was temporarily stored in sewer sediments instead of being transported to WWTPs. High concentrations of MPs, ranging from 5×103 to 178×103 particles/kg, were found in these sediments. This highlights a major stock of MPs inside the sewer network and the associated risk of downstream transfer during wet weather events due to the resuspension of in-sewer sediments.Receiving pollutants, including MPs, from various sources such as households, runoff and also the remobilization of in-sewer deposits, combined sewer overflows (CSOs) are expected to release a large amount of MPs into receiving waters. However, research on this emission pathway is still limited. Therefore, a study to evaluate the quality of CSOs in terms of MP contamination and their potential to emit MPs into the environment was carried out. High MP levels were detected in CSOs during different storm events, ranging from 6.7×104 to 3.9×105 particles/m3. At an annual scale, the amount of MPs released with CSOs was equivalent to the massive load from treated wastewater, despite much lower discharge volumes. Thus, these findings confirmed the significant role of CSOs as a land-based source of MPs into the surrounding environment during intense wet weather events.In conclusion, this PhD project has provided data on MP contamination levels in various compartments of the wastewater management system. It has also contributed to the understanding of the different pathways in which MPs are released from the system into the environment. This study emphasizes inadequacy of existing management systems in addressing plastic pollution. Therefore, it calls for urgent actions to mitigate the impact of urban wastewater on nearby water bodies in terms of plastic pollution.