| dc.description.abstract | Per- and polyfluoroalkyl substances (PFAS) have been detected in biosolids, raising concerns regarding their potential leaching into soils, groundwater, and plants when biosolids are land applied. In this study, the fate of PFAS in various sludge handling processes was investigated to determine their fate in individual treatment processes and overall treatment sequences. The results are intended to inform decision-making for the development of effective approaches to biosolids management.
An enhanced analytical method was developed to reliably measure the concentrations of 24 PFAS in the solid streams (i.e., sludges and cakes) of sludge handling processes as the current analytical techniques in the literature were not broadly applicable over a diverse range of samples. The enhancement of existing analytical methods focused on the steps employed to clean the methanolic biosolids extracts. In this study, blends of adsorbents (graphitized non-porous carbon (Envi-Carb), primary secondary amine-functionalized silica (PSA), bare silica and, octadecyl-functionalized silica (C18)) with different adsorptive properties were evaluated for use in the clean-up step. Six different biosolid samples were employed to initially screen and then validate the preferred formulation of the adsorbent blend. A blend consisting of 1,000 mg Envi-Carb, 500 mg PSA and 500 mg C18 was found to effectively eliminate background interferences in extracts of the six biosolids. The cleanup method developed in this study has the potential to facilitate the analysis of PFAS in a broad range of sludge and biosolid matrices.
For the analysis of 24 PFAS in recycled liquid streams (i.e., filtrate and centrate) from sludge handling processes, US EPA Method 1633 was modified, and then used. Modifications of US EPA Method 1633 were necessary because this method was intended for the quantification of PFAS in wastewater samples with a total suspended solids (TSS) value lower than 250 mg/L, whereas the TSS concentration in the liquid samples collected in this study ranged between 100 and 7,000 mg/L. A sample preparation process consisting of ultracentrifugation and extraction steps was developed to first separate solids and extract the PFAS adsorbed on them. The modified method in this study could potentially handle liquid samples with TSS up to 7,000 mg/L.
Following the successful development of the analytical methods, two sampling campaigns were conducted to characterize PFAS fate in the sludge handling systems of two full-scale wastewater treatment plants (WWTPs), namely WWTP-A and WWTP-B. Both systems included gravity drum thickening (GDT), anaerobic digestion (AD), and dewatering (DW). WWTP-B also had dissolved air flotation (DAF), fermentation (FM) and sludge blending (SBT). In both systems, a majority of individual PFAS mass flows did not significantly change through the liquid-solid separation (i.e., GDT, DW and DAF) and the blending processes. It was concluded that these types of physical processes with short sludge retention time (SRT) did not cause major formation or removal of PFAS. In contrast, in both plants, AD resulted in mass flow increases for a majority (13/22) of the detected individual perfluoroalkyl acids (PFAAs), while the mass flows of the detected precursors decreased. More specifically, the mass flows of the precursors decrease ranged between 29-47% and 20-32% in WWTP-A and in WWTP-B, respectively, while the increase in the mass flows of PFAAs ranged between 50% and 70% in WWTP-A, and 30% and 50% in WWTP-B. The degradation of PFAS precursors and formation of PFAAs were attributable to biodegradation. While the FM process at WWTP-B is also a biological process, there was no significant change in a majority (16/24) of mass flows of target PFAS. This lack of precursors degradation and PFAAs formation observed in FM was attributed to the short SRT (4 days) of this process (compared to the longer SRTs of 45 and 20 days of the AD processes in WWTP-A and WWTP-B, respectively). Overall, it was concluded that SRT may be a key factor affecting PFAS fate in sludge handling systems.
With respect to PFAS partition between solid and liquid streams, it was found that PFAS with carbon chain lengths of C3 to C6 were mostly present (between 50% and 86%) in liquid streams, whereas PFAS with chain lengths higher than C6 were mostly present (between 50% and 121%) in solid streams. Furthermore, perfluoro sulfonates (PFSAs) were found to be 1.4 to 4 times more prone to binding to solids as compared to the perfluorocarboxylates (PFCAs) counterparts with the same carbon chain. These results showed that carbon chain length and functional groups are the key factors determining PFAS fate in sludge handling systems. Additionally, of the 24 PFAS measured, perfluorooctane sulfonate (PFOS), which is the compound that has drawn the most attention due to its toxicity and bioaccumulative nature, was found to be the most abundant PFAS. PFOS concentrations were as high as 12.9 µg/kg and 53 ng/L in solid and liquid samples, respectively. | en |
