Infographic by Carollo.
Coverage of PFAS is supported by Carollo.
The toxicity and widespread detectability of certain per- and polyfluoroalkyl substances (PFAS) have raised concern about their potential health impacts or changes to water and wastewater practices in response. PFAS are commonly present in water, human blood, environmental samples, household products, and biosolids (Ducatman et al., 2022; Hu et al., 2019, 2016; Vedagiri et al., 2018; Zheng and Salamova, 2020).
California’s GeoTracker PFAS Map reports at least one type of PFAS has been detected in solids at 94% of the 156 water resource reclamation facilities (WRRFs) sampled (CWB, 2022). As part of their comprehensive PFAS strategy, the USEPA is conducting a risk assessment of two PFAS—perfluorooctanoic acid (PFOA) and perfluorosulfonic acid (PFOS)—in biosolids, to be completed in 2024.
The assessment will be the basis for any following federal regulation addressing PFAS in biosolids for land application (USEPA, 2021).
Recent actions by the USEPA and state governments have raised concerns that future regulatory thresholds for PFOA and PFOS in biosolids could be low enough to cause widespread challenges to land application. In June 2022, the USEPA announced new health advisory levels (HALs) for PFOA and PFOS (USEPA, 2022). While these HALs are non-enforceable guidelines and specific to drinking water they were over a thousand times below the PFOA & PFOS HALs announced in 2016, and more than one hundred times below lab detection limits.
Meanwhile, in April 2022, the state of Maine banned biosolids land application in response to PFAS concerns.
Several potential pathways exist for chemicals in biosolids to reach people or animals (top image), yet the actual health risk depends on the existence of these pathways, the chemical’s concentration in biosolids, toxicity, and tendency to move through each pathway step.
To answer the many questions involved in comprehensive biosolids risk assessment, scientists and engineers continue to conduct extensive long-term research on the occurrence, transport, and fate of PFAS in biosolids and the agricultural environment.
This article is a brief state-of-the-science review of PFAS in biosolids. This information is important for:
PFOA and PFOS have been linked to many adverse human health effects. A USEPA Science Advisory Board stated the PFOA and PFOS are likely associated with “liver disease, immune system dysfunction, serum lipid aberration, impaired fetal growth, and cancer,” (Science Advisory Board, 2022). Ducatman et al. (2022) found substantial scientific evidence for links between PFAS and immune suppression, kidney cancer, diminished breastfeeding, high cholesterol, and altered liver enzymes. A meta-analysis of 24 human epidemiological studies concluded with over 99.9% confidence that higher alanine aminotransferase in serum was associated with PFOA and PFOS exposure. Alanine aminotransferase is an enzyme used as a biomarker for liver health.
For the June 2022 drinking water HALs, the EPA set guidance levels based on antibody response after tetanus and diphtheria vaccines (USEPA, 2022). These HALs were calculated based on a series of studies from children born at the same hospital in the Faroe Islands, where PFAS exposure is presumed to occur through diet, especially seafood (Budtz-Jørgensen and Grandjean, 2018; Grandjean et al., 2017b, 2017a, 2012). The HALs also factored in bioconcentration in the breastmilk of exposed mothers, since infants would be the key population affected by vaccine inhibition (Verner et al., 2016). Experts acknowledge the uncertainty in estimating any resulting disease incidence or mortality from PFAS impacts on the immune system.
Many studies have measured PFAS in biosolids and indicated that mean biosolids concentrations are often dominated by a few industrially impacted outliers. More recent studies found generally lower PFOA and PFOS concentrations in biosolids, suggesting industrial phaseouts in the 2000s are having beneficial downstream impacts. On one hand, the monitoring data suggests PFOA or PFOS biosolids concentrations below 10 ppb are achievable through source control alone. On the other hand, some PFAS detection in biosolids is likely unavoidable due to the abundance similar compounds being developed as replacements.
In 2020, California required WRRFs treating over 1 MGD to measure PFAS in influent, effluent, and biosolids. Based on data from 156 WRRFs (CWB, 2022), the majority of California biosolids results were equal to or below their reporting limits for PFOA and PFOS (8 ppb; Figure 2). Reported median biosolids concentrations were 1.7 ppb PFOA and 6.7 ppb PFOS when including values between the detection and reporting limits.
A meta-analysis of PFAS in biosolids in the USA found PFOA and PFOS data available through the year 2020 for 36 individual WRRFs (Thompson et al., 2022). The mean concentrations of PFOA and PFOS were 24±7 ppb and 233±107 ppb, respectively. Median PFOA and PFOS were much lower at 5.5 ppb and 59 ppb, respectively. Some of the biosolids sampled in that meta-analysis were collected as long ago as 1998, well before the industrial phaseout (Higgins et al., 2005). Furthermore, the aggregated studies said that over 20% of the WRRFs had suspected industrial sources of PFAS; however, a much lower percentage of WRRFs likely have significant PFOA and PFOS industrial sources, especially in recent years. Omitting likely industrially impacted outliers, median PFOA and PFOS would have been 2.1 ppb and 31 ppb, respectively.
In 2018, Michigan’s sampling program of 42 WRRFs (EGLE, 2021) demonstrated six WRRFs were industrially impacted based on having biosolids PFOS concentrations over 150 ppb and a plausible source. These results reiterated the importance of source identification and control.
Boxplot of California biosolids PFAS monitoring results for PFOA and PFOS. Values below their detection limit were assumed to be half their detection limit.
PFAS concentrations in land‑applied soil largely depend upon the concentrations entering WRRFs and application rates. Several studies have looked at soils amended with land-applied biosolids from industrially impacted WRRFs versus non-industrially impacted biosolids. Two key takeaways are (1) PFAS do not degrade in soil under realistic conditions in a meaningful timeframe and (2) they can transport through the soil to lower depths depending up PFAS chain length and soil properties (e.g., porosity, organic content).
Industrial PFAS sources in the WRRF collection systems lead to higher PFAS concentrations in land-applied biosolids. PFAS in land-applied biosolids from an industry-impacted WRRF in Alabama remained at double- to triple-digit ppb levels in the soils even 5 years after the PFAS source was eliminated (Figure 3; Washington et al., 2010). These concentrations in Alabama were over 100 times higher than found in land-applied biosolids in Arizona with no industrial source (Pepper et al., (2021). Even without major industrial sources, Pepper et al. (2021) still found somewhat higher PFAS concentrations in soils amended with biosolids relative to unamended soils. Nonetheless, PFOA and PFOS concentrations were essentially attenuated to low single digit ppb concentrations at 6-ft soil depths, suggesting long-chain PFAS would including PFOS and PFOS have little movement to deep groundwater (Pepper et al., 2021).
Comparison of soil concentrations at sites amended with known point sources of PFAS (Decatur) and those without known point sources (Southern Arizona).
PFAS in the environment have a variety of potential pathways, including from soils to plants and earthworms, from plants to livestock and humans, and from contaminated meat and dairy products to humans. A major factor in PFAS fate in the environment is chain-length. Several studies have shown long-chain PFAS (i.e., eight or more carbons) have higher concentrations at the soil surface, while short-chain PFAS can transport to significant depths in soils (Pepper et al., 2021; Sepulvado et al., 2011; John W. Washington et al., 2010). Washington et al. (2010) found long-chain PFAS like PFOA and PFOS dominated topsoil concentrations, but shorter chain compounds like PFHxA, PFHpA, and PFHxS tended to increase with soil depth, likely due to their hydrophilicity.
Plant uptake studies have demonstrated PFAS bioaccumulation in a variety of crops ranging from grains like wheat, leafy vegetables like lettuce, and root vegetables like carrots and onions (Blaine et al., 2013; Liu et al., 2019). While long and short chain PFAS can be taken up by plants, studies have demonstrated that shorter chain PFAS are more likely to bioaccumulate in plants, likely due to their hydrophilicity allowing for transport to roots for uptake (Costello and Lee, 2020).
Several researchers have shown that beef and dairy cows exposed to contaminated waters and feeds bioaccumulate PFAS compounds (Lupton et al., 2022; Vestergren et al., 2013). Tissue samples from 180 cattle with long‑term PFAS exposure found bioaccumulation of sulfonated PFAS including 56-96 ppb PFOS (Lupton et al. (2022)). Kowalczyk et al. (2013) focused on the role of feed consumption in PFAS uptake and accumulation in dairy cows. After being fed contaminated feed for 28 days, dairy cows secreted PFOS and PFHxS through the milk, and PFBS and PFOA accumulated in body tissues. A month after the feeding of contaminated feed was discontinued, PFOS continued to accumulate in cow muscle, and traces of PFBS were still found in dairy milk. Transference through the food chain will be an important part of the EPA’s risk assessment. Nonetheless, less intuitive pathways—namely children directly consuming land-applied biosolids—have been the regulatory driver for several metals in biosolids.
Source control is the most proven, cost-effective way to reduce PFAS concentrations in biosolids. Multiple Michigan WRRFs have successfully reduced biosolids PFOS concentrations by over 90% through source control (EGLE, 2021). Guidance on PFAS source control is available in other CWEA articles and in the forthcoming Water Research Foundation Project #5082 Investigation of Alternative Management Strategies to Prevent PFAS from Entering Drinking Water Supplies and Wastewater.
PFAS destruction in biosolids may be required if source control alone is not enough to comply with future regulations governing land application and landfilling is not feasible due to capacity constraints, greenhouse gas emissions concerns, public pressure, or regulations placed upon the landfills. To minimize long-term risk, utilities may seek to mineralize PFAS, i.e., destroy it completely to carbon dioxide and hydrofluoric acid (HF).
Conventional biological processes like aerobic or anaerobic digestion or pre-digestion treatments like thermal hydrolysis have shown no impact on PFAS destruction, though they may transform PFAS precursors into other PFAS compounds. While long-chain PFAS can decompose at temperatures over 400 °C, complete mineralization does not occur until temperatures exceed 1,000 °C (Zhang et al., 2023).
High temperature treatment technologies exist at the commercial (incineration, pyrolysis, gasification) and demonstration scales (supercritical water oxidation (SCWO), hydrothermal liquefaction (HTL)) that show potential for partial or complete PFAS destruction. Temperatures over 1,000 °C can be achieved and maintained with some incineration processes, although conventional sewage sludge incinerators do not operate at those temperatures.
Pyrolysis and gasification have shown high rates of removal, though they generally operate at temperatures lower than incineration. One pyrolysis study removed 9 PFAS (including PFOA and PFOS) in biosolids to non-detection in biochar with 500-600°C (Kundu et al., 2021). Another biosolids pyrolysis study found 94% removal of 18 PFAS at 650 °C (Wang et al., 2022). Potential emission of PFAS or their thermal transformation products to air is a key consideration and an ongoing research area for these thermal technologies (Wang et al., 2020). Since exhaust may still contain volatilized PFAS, they require high temperature regenerative thermal oxidizers for full destruction and scrubbers for produced HF, which is already a regulated compound.
Other demonstration-scale technologies like HTL and SCWO can treat biosolids at somewhat lower temperatures but higher pressures. Great Lakes Water Authority is testing PFAS destruction with HTL at pilot scale. A SCWO demonstration unit is being manufactured for installation at the Orange County Sanitation Districts plant, scheduled for startup later this year.
Due to the relative newness of these high-temperature systems, questions remain regarding capital and operating costs, permitting, long-term operations and maintenance, scalability, reliability, and overall system sophistication. All of these characteristics are currently being studied and will continue to be assessed as more installations and operating experience are gained. Meanwhile, regulators and researchers continue to study fate, transport, and risks imposed by PFAS in biosolids with near-term focus on reducing PFAS use in commercial and industrial products.
We would like to thank Trinity River Authority, who funded a detailed literature review from which this article was derived.
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