A key factor in monitoring cannabis consumption trends through wastewater analysis: Partitioning of THC COOH between the liquid and solid phase of influent wastewater

Current wastewater-based epidemiology (WBE) studies are predominantly focused on the analysis of urinary biomarkers present in the liquid phase of influent wastewater (IWW). This approach systematically underestimates less polar metabolites, such as cannabis biomarkers. These biomarkers can potentially sorb to and desorb from suspended particulate matter (SPM) present in IWW. This study investigates the bidirectional partitioning of THCCOOH between the liquid phase and SPM of IWW by performing multiple sorption experiments using THCCOOH-D₉ as a surrogate due to the unavailability of blank SPM and blank IWW. In addition, this study involves the analysis of IWW collected from eight wastewater treatment plants (WWTP) (n = 56) across four European countries, where raw IWW, the liquid phase and SPM were separately analysed to identify potential trends in the fraction of THCCOOH in the SPM between and withing the examined locations. Based on the performed sorption experiments, bidirectional partitioning of THCCOOH between the liquid phase and SPM was noted_, showing partition between both phases when only one of the two phases was spiked. It was illustrated that the concentration of SPM had a notable influence on the THCCOOH partitioning between both phases. In addition to the inter location variability (average THCCOOH present in the SPM ranged 31-59 %), a substantial intra location variation was also observed, e.g., in one location ranged 17-58 %. While the determination of a correction factor for the amount of THCCOOH present in SPM would be ideal, this is challenging, since the amount of SPM is not fixed in all IWW samples. Although SPM has influence on the THCCOOH partition, no correlation (p value Spearman correlation = 0.3160) was observed between the SPM concentration and the fraction of THCCOOH in the solid phase. Moreover, the collection of homogenized samples is difficult, and the time required to reach an equilibration in partitioning of THCCOOH between both IWW phases remains unclear. Due to i) the large inter-and intra-location variation of THCCOOH present in the SPM, ii) the variability in SPM concentration in IWW samples, and iii) the time required to reach a partitioning equilibration, an analytical procedure based on liquid-liquid extraction (LLE) that considers both the liquid phase and SPM of IWW is recommended to reduce the overall uncertainty for THCCOOH measurement in IWW. It was illustrated that this extraction method is capable to recover the total concentration of THCCOOH in both phases.