Do suspended particles matter for wastewater-based epidemiology?

As wastewater-based epidemiology (WBE) continues to evolve and expand the range of targeted compounds, some limitations remain underexplored, particularly the sorption of targeted markers onto suspended particulate matter (SPM) in raw wastewater. This issue is crucial as it could lead to underestimations in retrospective calculations. While previous studies have addressed this topic, they have primarily focused on a limited range of analytes (e.g., illicit drugs and selected pharmaceuticals) and relied on small sample sizes, highlighting the need for further research. This study aims to bridge these gaps through a six-month monitoring campaign analyzing a broad range of WBE markers (pharmaceuticals, illicit drugs, and lifestyle biomarkers) in both the dissolved and the particulate phases. A dedicated analytical method based on pressurized liquid extraction and liquid chromatography coupled with tandem mass spectrometry allowed the assessment of daily loads and distribution behavior of these compounds, providing new insights into their relevance for WBE estimates. The results demonstrated that most compounds exhibited low sorption to SPM, confirming their reliability for WBE monitoring based solely on dissolved-phase measurements. Notably, this study provides the first clear assessment of the minimal sorption of tobacco and coffee biomarkers. However, significant sorption was observed for 11 molecules, including fluoxetine, THCCOOH, and methadone, revealing a "hidden load" that could bias estimates without proper correction. Log D proved to be a useful and better than log Kow predictor of sorption potential for ionized compounds but failed to accurately predict sorption for neutral species. Additionally, wastewater dilution due to urban runoff appeared to influence compound partitioning, potentially increasing their affinity for the solid phase. Nonetheless, potential changes in the organic composition of SPM do not appear to be the driving factor, as the studied material retained stable total organic carbon (TOC) levels, most likely due to in-sewer remobilization of organic deposits which share a similar signature with domestic wastewater effluent. Further investigations are needed to identify the key parameters influencing compound affinity for SPM and their potential implications for WBE applications.