Fate of 15 PFAS in Two Full-Scale Wastewater Sludge-Handling Systems: An Interstage Mass Balance Analysis

Abstract

Interstage mass balance analyses were performed to assess the fate of 15 PFAS in two full-scale wastewater sludgehandling systems. Both systems consisted of thickening (rotary drum, gravity, and dissolved air flotation), anaerobic digestion, and dewatering, while one of them also included primary sludge fermentation. On a mass concentration basis, PFOS was the most abundant compound in both systems, with concentrations as high as 12.9 μg/kg and 53 ng/L in solid and liquid samples, respectively. On a molar basis, PFOS was the most abundant compound in solid samples (up to ∼26,000 picomol/kg), while PFBA was the most abundant compound in liquid samples (up to 140 picomol/L). Fermentation (a sludge retention time of 4 days) did not cause a significant change in PFAS mass flows. In contrast, anaerobic digestion (sludge retention times of 45 and 20 days) resulted in mass flow increases for 12 PFAAs and mass flow decreases for three precursors. Precursor transformation and PFAA formation increased with the sludge retention time of the digesters. In the liquid−solid separation processes (thickening and dewatering), PFAS with carbon chain lengths of <C7 were mostly present (between 50 and 86%) in the liquid recycle streams, whereas PFAS with chain lengths of ≥C7 were mostly present (between 50 and 121%) in the solid streams. In terms of the PFAS fate at a system-wide scale, over 50% of the mass of each <C6 compound exited both systems via liquid streams, while over 50% of the mass of each ≥C6 compound exited the systems via the biosolids stream. Collectively, the results demonstrated that the fate of PFAS in sludge-handling systems is likely dependent upon the processes employed and their retention time, as well as the physicochemical properties of PFAS. The results also demonstrated that a thorough interstage mass balance analysis of an entire sludge-handling system could provide insights into the impact of individual processes on the PFAS fate and also identify opportunities where interventions might be introduced to remove PFAS from the urban water cycle.