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. 2025 Feb 23;97(2):e70039. doi: 10.1002/wer.70039

Per‐ and polyfluoroalkyl substances in untreated and treated sludge/biosolids from 27 water resource recovery facilities across the United States and Canada

Shubhashini Oza 1,✉, Hui Li 2, Qingguo Huang 3, John W Norton 4, Lloyd J Winchell 5, Martha J M Wells 6, Thomas Nangle 7, Natalia Perez 8, Dan DeLaughter 9, Jan M Hauser 10, Malcolm Taylor 11, Zonetta E English 12, Mike Melnyk 13, Phuong Truong 14, Katherine Y Bell 15
PMCID: PMC11847621  PMID: 39988323

Abstract

Per‐ and polyfluoroalkyl substances (PFAS) are being studied in all environmental matrices because of their ubiquitous presence and adverse human health impacts. This study conducted a surveillance of 27 water resource recovery facilities throughout the United States and Canada to screen the range of PFAS concentrations in pre‐stabilized sludge and post‐stabilized product. Among the 27 water resource recovery facilities, 82% use anaerobic digestion and the rest use chemical stabilization and/or incineration for sludge stabilization. Forty PFAS compounds were evaluated by US Environmental Protection Agency Method SW846/537.1, and four and nine compounds were reported in the pre‐stabilized sludge and post‐stabilized product, respectively. Concentrations of reported compounds in pre‐stabilized sludge and post‐stabilized product varied from 5 to 33 ng/g dry basis and 2 to 220 ng/g dry basis, respectively. 3‐Perfluoropentylpropanoic acid (5:3 FTCA) and perfluorooctanesulfonic acid (PFOS) were the most frequently observed compounds, and PFAS concentrations in the post‐stabilized products were generally higher than the corresponding pre‐stabilized sludge.

Practitioner Points

  • Among the 40 target PFAS, four were above reporting limit in the pre‐stabilized sludge and nine in the post‐stabilized product.

  • Incineration ash (post‐stabilized product) samples did not have any reportable PFAS.

  • 5:3 FTCA and PFOS were the two frequently observed compounds; concentrations were higher in the post‐stabilized product compared to the pre‐stabilized sludge.

  • PFPeA and PFHxA were the only two short chain perfluoroalkyl carboxylic acids reported.

  • PFOA was reported in only one of the 54 samples evaluated.

Keywords: 5:3 FTCA, anaerobic digestion, emerging contaminants, method SW846/537.1, PFOS, survey

This surveillance of 27 WRRFs throughout the United States and Canada provides information on the range of PFAS concentrations in pre‐stabilized sludge and post‐stabilized product. Of the few PFAS compounds reported, concentrations in post‐stabilized product were generally higher than in the corresponding pre‐stabilized sludge.

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INTRODUCTION

Per‐ and polyfluoroalkyl substances (PFAS) are a family of fluorinated organic compounds that do not occur naturally in the environment and are synthesized for their unique chemical and physical properties which include thermal stability, and water and grease resistance. They are widely used in industrial and commercial products such as stain‐resistant and water‐resistant textiles, fire‐extinguishing products, pesticides, paints, personal care products, and surfactants (Lau et al., 2007). Because of their ubiquitous use, they are found in air, ground water, surface water, biota, soil, sediment, sludge, and biosolids (ITRC, 2023). Currently, the “CompTox Chemicals Dashboard v2.4.1” lists 12,039 PFAS compounds (OECD, 2021; US EPA, 2021) of which approximately 4,700 have a “Chemical Abstracts Service Registry Number; however, fewer than 100 compounds can be measured with commercially available analytical laboratory methods.

Considering the widespread use and occurrence of these compounds, PFAS are of interest from a potential human health and environmental risk perspective, particularly because of their toxicity at very low concentrations and propensity to bioaccumulate (Conder et al., 2008; De Silva et al., 2020; Kennedy et al., 2004; Martin et al., 2004; Pérez et al., 2013; U. S. Department of Health and Human Services, 2021). PFAS exposure has been linked to reproductive and developmental, liver and kidney, and immunological effects, as well as tumors in laboratory animals, and PFAS can cause acute lung toxicity (DoTS, 2021; Sørli et al., 2020). In addition to human health risks, PFAS have also been linked to phytotoxicity, aquatic toxicity, and terrestrial ecotoxicity (Adu et al., 2023; Mahoney et al., 2022; Melo et al., 2022).

The persistence of PFAS in the environment, leading to human and ecological exposures, is due to their chemical composition. The foundational carbon‐fluorine bond makes them chemically and thermally stable, which inhibits their degradation and treatment in municipal water resource recovery facilities (WRRFs) (Arvaniti & Stasinakis, 2015; Chen et al., 2018; Szabo et al., 2023). PFAS in a WRRF partition between the liquid and solid phases; solid residuals from the treatment process (primary and secondary) are segregated as sludge, which typically undergoes stabilization prior to its final disposition. Stabilized sludge and/or biosolids (United States Environmental Protection Agency [USEPA] defines biosolids as a nutrient‐rich product of the wastewater treatment process obtained by treating the solids separated during the wastewater treatment process; US EPA, 2023a) are commonly land applied for beneficial reuse because it is sustainable means of managing the carbon and associated plant nutrients contained in wastewater. Currently, 53% of the six million dry metric tons of biosolids generated by WRRFs in the United States of America (USA) are land applied (National Biosolids Data Project, 2020). Several recent studies indicate that biosolids are a sink for long‐chain PFAS compounds (Pozzebon & Seifert, 2023), and other reports have shown that biosolids containing PFAS have resulted in contamination of soil, ground, and surface waters (Lindstrom et al., 2011; Röhler et al., 2021; Sepulvado et al., 2011). Researchers have documented the presence and behavior of agricultural soils with land applied biosolids and observed that long‐term tailing could be caused by transformation of precursors into perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) (Röhler et al., 2021). Their results, summarized a 12‐year study, indicating that PFAS leaching from agricultural topsoil will occur over periods of several years.

Currently, there are no federal rules regulating PFAS in biosolids; however, the USEPA has a generic framework for assessing pollutants in biosolids through risk assessment. The agency is working toward the goals outlined in its PFAS Strategic Roadmap, which includes development of an updated biosolids risk assessment for two PFAS compounds, PFOA, and PFOS (US EPA, 2023c). Similarly, in Canada, there are no federal regulations for PFAS in biosolids, although recently, the Canadian Food Inspection Agency (CFIA) issued an interim standard that limits the amount of PFAS in biosolids that can be sold or imported into Canada (Canadian Food Inspection Agency, 2024).

In the absence of federal regulations for PFAS in biosolids, and the concerns around potential adverse human health impacts, several United States and Canadian provinces have already passed or drafted legislation for PFAS impacted biosolids. Maine is the first state that banned land application of sludge or sludge‐derived products (Huges, 2023); however, recently, Vermont has severely restricted land application of biosolids in its April 2024 interim strategy (Vermont Residuals Management Working Group, 2024), and more recently, the state of Connecticut has proposed bans on land application of biosolids (Substitute Senate Bill No. 292, 2024). New Hampshire has requirements for facilities to monitor PFAS in biosolids and requires all Sludge Quality Certificate biosolids (Class A or B) to be monitored (Anthony Drouin, 2024). The Massachusetts Department of Environmental Protection has required PFAS monitoring in biosolids on a quarterly basis since 2021. Michigan has had monitoring requirements for PFAS in land‐applied biosolids since 2017 and has prohibited application of biosolids deemed to be industrially impacted (Huges, 2023); the state has also set thresholds that trigger additional monitoring and application requirements. A similar threshold‐based approach has been published in New York (September 2023), where the New York State Department of Environmental Conservation published an interim strategy for control of PFAS in biosolids in their Division of Material Management (DDM) – 7 documents (NYDEC, 2023), with PFOA and PFOS being addressed as described in Table 1.

TABLE 1.

DMM 7 New York state interim guidelines for PFOA and PFOS in biosolids.

PFOS in biosolids, dry weight or μg/kga PFOA in biosolids, dry weight (μg/kg)a Action required for biosolids that are recycled
20 or less 20 or less No action required
>20 but < 50 >20 but < 50 Additional sampling required. NYDEC will take appropriate steps to restrict recycling after 1 year if the PFOS or PFOA levels are not reduced to below 20 ppb.
>50 >50 NYDEC will take action to prohibit recycling until PFOS or PFOA concentration is below 20 ppb.
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a

In addition to dry weight results, NYDEC may require analyses using the Synthetic Precipitation Leaching Procedure (SPLP) and use those results to determine whether the biosolids source can be recycled. Microgram per kilogram (μg/kg).

In Canada, Quebec announced a temporary ban on the import of biosolids from the United States because of concerns over PFAS in March 2023, which establishes controls and thresholds for PFAS. Quebec has proposed making the ban permanent (Williams et al., 2024).

As research continues to identify issues around risks, other states and provinces including Arizona, Colorado, North Carolina, Tennessee, Texas, and Quebec (Canada) already have counties, municipalities, or other jurisdictions that have established their own standards, bans, and monitoring regimes, related to biosolids disposal or land application (Huges, 2023). Recognizing that there is currently a patchwork of regulatory standards related to PFAS management in biosolids across North America, it is important to acknowledge that completion of a risk assessment for PFOA and PFOS in biosolids by USEPA, targeted for completion before 2025, could trigger additional regional and state regulations in the United States. This aligns with the announcement by CFIA regarding collaborations with the Environment and Climate Change Canada, Health Canada, and provincial partners. These Canadian entities are still assessing the level of risk to the food and feed chains posed by biosolids contaminated with PFAS and are developing a coordinated approach to protect human health and the safety of Canadian agriculture (ESE, 2024).

In the United States, the Clean Water Act (40 CFR Part 503) provides specific authorization for USEPA to set pollutant limits and monitoring and reporting requirements for contaminants in biosolids if sufficient scientific evidence shows that there is potential harm to human health or the environment (USEPA, 1992). Thus, the ongoing risk assessment will likely serve as the basis for future federal regulation of PFOA and PFOS in biosolids. While the risk assessment process is ongoing, there is yet limited data for PFAS in biosolids. USEPA conducted the first ever National Sewage Sludge Survey (NSSS) in 2001, surveilling 13 PFAS compounds (Venkatesan & Halden, 2013). Since that survey, a nationwide survey has not been conducted. However, in 2024, USEPA announced its intent to initiate a Publicly Owned Treatment Works (POTW) (analogous to WRRF) study of PFAS, which will focus on collecting nationwide data on wastewater influent, effluent, biosolids, and industrial discharges of PFAS to WRRFs (USEPA, 2024b). USEPA described this study in the Effluent Guidelines Program Plan 15 with aims of verifying sources of PFAS in wastewater and to aid WRRFs in implementing source control, instead of only focusing on treatment technologies (US EPA, 2023b). While USEPA has announced its intent to conduct these surveillance studies, it could be years before this information is available for public review or for use in guiding regulations and management options. In this uncertain regulatory landscape, WRRFs will be expected to plan for future regulations; thus, there is great interest in obtaining data that can help WRRFs assess potential management options which may require years to plan, fund, and implement.

The most immediate challenge is that currently recorded investigations of PFAS in biosolids have been limited in scope to specific sample populations and/or analyte suites. One of the largest surveys reports included 4981 sewage sludge samples from 1165 WRRFs, but the analytical suite was limited to only 11 PFAS compounds (Ulrich et al., 2016). However, the study did not specify the type of sludges evaluated. Other studies sampled biosolids from storage repositories (facility wherein biosolids are stored for extended period for future evaluation), which do not reflect the PFAS compositions and signatures of modern biosolids (Munoz et al., 2021). The industries need for data quantifying PFAS in biosolids is reflected by other studies, including the recent survey of PFAS in final treated solids from 190 Michigan WRRFs (Link et al., 2024). Link et al. evaluated PFAS at 190 WRRFs, analyzing 24 PFAS compounds by Modified Method 537 and American Society for Testing and Materials (ASTM) D‐7968, but the study was geographically limited to Michigan, although the study provides a compilation of global PFAS investigations in biosolids to provide data comparative to their study. Among the 30 studies reviewed, eight included samples collected in the United States; however, these studies were conducted in the early 2000s and included a limited number of PFAS analytes (minimum of eight and maximum of 16), except for one study that evaluated 92 PFAS compounds from eight biosolids samples collected in 2021 (Thompson et al., 2023).

Along with the variation in the number of compounds evaluated by these studies, the methods used for PFAS analysis also differed because a final USEPA approved method for PFAS analyses in biosolids was not available until 2024. The USEPA and ASTM PFAS testing methods have evolved over time for various environmental media as depicted in Figures 1 and 2. Thus, studies conducted prior to 2024 evaluating PFAS in sludges and biosolids used either an unregulated method or have adapted other methods to extract and analyze various numbers of targeted PFAS compounds.

FIGURE 1.

FIGURE 1

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USEPA PFAS analysis method development timeline. PFAS, per‐ and polyfluoroalkyl substances; USEPA, United States Environmental Protection Agency.

FIGURE 2.

FIGURE 2

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ASTM PFAS analysis method development timeline. ASTM, American Society for Testing and Materials; PFAS, per‐ and polyfluoroalkyl substances.

To address the data gap in availability of PFAS concentrations in sludge and biosolids in the United States and Canada, this study extended efforts of an USEPA funded investigation led by Michigan State University, in collaboration with Colorado State University, the University of Georgia, the Great Lakes Water Authority (GLWA), and Brown and Caldwell, to improve knowledge of occurrence, transport, fate, plant uptake, livestock, and human exposure to pollutants in biosolids. The study was expanded to include samples from 27 WRRFs across the United States and Canada (Figure 3) to develop high‐quality data and inform future policies, regulations, and management options. At the time PFAS analyses were performed, only the second Draft Method 1633 was available for analysis of sludge and biosolids, and discussions with the commercial laboratory staff indicated that an alternative method (Method SW846‐3500C, Modified Method 537.1) was more reliable for sludge and biosolids samples. The challenges of the Draft Method 1633 for analysis of biosolids have also been documented by others (Ozelcaglayan et al., 2023). Ozelcaglayan et al. demonstrated that when using Draft Method 1633, precipitates were formed when methanolic extracts were reconstituted with water, preventing direct analysis by liquid chromatography tandem mass spectrometry (LC–MS/MS) and centrifugation/filtration of reconstituted extracts to remove precipitates that did not sufficiently eliminate interferences to allow PFAS quantification.

FIGURE 3.

FIGURE 3

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Location of WRRFs sampled for sludge and biosolids in the United States and Canada. WRRFs, water resource recovery facilities. (Note: The number after the state abbreviation indicates the number of WRRFs sampled in that state).

The objective of this study was to surveille and document the concentration, distribution, and variability of PFAS in final post‐stabilized product (sludge/biosolid/ash) from a sample population across the United States and Canada, representing a geographically diverse range of WRRFs serving communities of various sizes and industrial activity.

METHODOLOGY

Sample locations

Wastewater flows to WRRFs go through preliminary, primary, secondary, and tertiary or advanced treatment processes prior to being discharged to a water body. Each of these treatment processes produce a solid or sludge stream that needs to be further treated. The sludge generated during primary and secondary treatment are called primary sludge and secondary sludge or waste activated sludge, respectively. In most conventional WRRFs, these two sludge streams are blended in a certain ratio, thickened or concentrated to reduce water content, and treated before disposal or reuse.

In this study, samples were collected at two locations from each participating WRRF as shown in Figure 4: (i) the blended and concentrated sludge prior to treatment (labeled as ‘pre‐stabilized sludge’ or pre‐stabilized) and (ii) the blended sludge undergoes various stabilization process, for example, anaerobic digestion, chemical stabilization, incineration, and compost. A sample was collected after the stabilization process (this sample location was labeled as ‘post‐stabilized sludge’ or post‐stabilized). Pre‐stabilized sludge was sampled to represent sludge prior to stabilization, and post‐stabilized product was sampled to represent the sample after stabilization (the product could be categorized as sludge or biosolids or ash based on the stabilization process). The sample collection was performed only once at each of the 27 facilities. At most WRRFs, the pre‐stabilized sludge sample was typically a blend of primary and secondary sludge; however, there were less than 15% WRRFs for which the sample was either only primary or only secondary sludge. All post‐stabilized sludge samples were collected post‐dewatering of the stabilized sludge (mostly biosolids, but there were facilities with incinerators where ash was analyzed for PFAS). To maintain anonymity of the samples collected from the WRRFs across the United States and Canada, the WRRFs were assigned site identification numbers (IDs), and results are presented by their respective IDs.

FIGURE 4.

FIGURE 4

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Typical WRRF schematic indicating the sample locations. WRRF, water resource recovery facility.

Sample Collection

Sampling at 25 WRRFs was conducted between September 19 and December 12, 2022, and two additional WRRFs were sampled on April 11, 2023. Sampling at each site was performed only once. Samples were collected by WRRF personnel according to the PFAS sampling protocol using a PFAS‐free sampling kit as described in Figures S1 and S2. The sampling protocol included a sampling checklist addressing appropriate field clothing, personal protection equipment, field equipment, equipment decontamination, and food packaging contamination consideration (Michigan Dept. EGLE, 2020). Samples were shipped overnight, on ice to a centralized facility (Brown and Caldwell Treatability Laboratory, Nashville, TN). Samples were homogenized and added to PFAS‐free sample bottles and shipped overnight to the commercial laboratory, Eurofins Lancaster Laboratories Environment Testing, LLC, Lancaster, PA (Eurofins). Upon receipt of samples, the laboratory analyzed samples as described in Oza, Bell, Xu, Wang, Wells, Norton, Winchell, and Huang (2024).

Sample preparation, extraction, detection, and quantification

Samples were extracted using Method SW846‐3500C and analyzed according to Modified Method 537.1; additional details regarding the modifications of the method are reported by Oza, Bell, Xu, Wang, Wells, Norton, Winchell, Huang, and Li, 2024. The modified method targeted the same 40 compounds as Method 1633. The PFAS standards, used for matrix fortification, native standards, and isotope labeled PFAS analogs used by the laboratory were purchased from Wellington Laboratories Inc. (Guelph, Canada). The list of PFAS compounds analyzed (same list of compounds as of Method 1633) and the isotopes used for the internal dilution method are summarized in Tables S1 and S2, respectively.

The detailed procedure adopted for quality assurance and quality control (QA/QC) for the results used in the study is documented in a previous publication (Oza, Bell, Xu, Wang, Wells, Norton, Winchell, Huang, & Li, 2024).

RESULTS AND DISCUSSION

PFAS concentrations at 27 WRRFs

Because sampling was conducted only once at each WRRF site, no statistical analysis specific to the site was performed. Of the 40 compounds assessed, only four were reported in pre‐stabilized sludge samples (5:3 FTCA, NMeFOSA, PFHxA, and PFOS); nine compounds were reported in post‐stabilized product (5:3 FTCA, NMeFOSAA, NMeFOSE, PFPeA, PFHxA, PFOA, PFDA, PFDoA, and PFOS). All the available results for the 27 WRRFs with reporting limits (RLs), method detection limits (MDL), and laboratory qualifiers are presented in Table S3.

The number of detections among the WRRF sites sampled versus the number of PFAS compounds reported is summarized in Figure 5. The observed concentrations of total PFAS compounds for the WRRFs in pre‐stabilized sludge and post‐stabilized product are presented in Figures 6 and 7, respectively. Because the molecular weights of the analyzed PFAS compounds range from 300.1 to 614.1 g/mol and the fact that analytical results are reported on a dry mass basis, comparisons are only made among sites for individual compounds. Comparison using summations of multiple compounds are presented with the mass being converted to molar concentrations. While regulatory frameworks typically leverage mass concentrations, this can only be used in context of evaluating individual compounds.

FIGURE 5.

FIGURE 5

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Counts of PFAS compounds reported in pre‐stabilized sludge and post‐stabilized product among the 27 WRRFs evaluated. PFAS, per‐ and polyfluoroalkyl substances; WRRFs, water resource recovery facilities.

FIGURE 6.

FIGURE 6

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Total PFAS concentration in the pre‐stabilized sludge where PFAS was reported. PFAS, per‐ and polyfluoroalkyl substances.

FIGURE 7.

FIGURE 7

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Total PFAS concentration in the post‐stabilized product where PFAS was reported. PFAS, per‐ and polyfluoroalkyl substances.

The PFAS compounds in the pre‐stabilized sludge were reported in only seven of the 27 WRRF sites (Site 1, 2, 3, 6, 7, 8, and 13). 5:3 FTCA was the most frequently reported compound at five out of seven, and PFOS was reported at two out of seven sites. NMeFOSA and PFHxA were only reported at one out of seven sites. In terms of reported concentrations, 5:3 FTCA was reported at a higher concentration, 13 to 33 ng/g (in pre‐stabilized sludge), while the other three PFAS were reported at concentrations below 10 ng/g. To understand this variation in type and concentration of PFAS, the type of influent to the plant (residential, commercial, industrial, and others) and upstream treatment was reviewed. The percentage of the type of influent for each plant varied significantly (Figure 8), and no meaningful conclusions could be derived from this evaluation, indicating additional sampling at similar WRRFs is required. Also, no meaningful treatment train conclusions were possible for these WRRFs because the plants had various type of secondary treatment processes such as conventional activated sludge, high purity oxygen activated sludge, and five‐stage Bardenpho system. Additional targeted sampling of plant influent is required to capture the impact of community and industrial activities to provide clarity on the observed higher concentration of PFAS compounds specifically at WRRF sites 1, 2, 3, 6, and 7.

FIGURE 8.

FIGURE 8

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Percentage contribution of the type of influent flow to the WRRF where PFAS compounds were reported in the pre‐stabilized sludge. PFAS, per‐ and polyfluoroalkyl substances; WRRFs, water resource recovery facilities.

Reported PFAS compounds

5:3 FTCA

Belonging to the non‐polymer class of PFAS compounds (IRTC, 2022), this compound is a fluorotelomer and a potential precursor of perfluoroalkyl carboxylic acids (PFCA) (Zhang et al., 2016). In this study, it was observed that 5:3 FTCA increased in post‐stabilized sample in all WRRFs, except WRRFs 8 and 18, which have an incinerator (only ash was measured, no air samples were evaluated). Figure 9 depicts the 5:3 FTCA concentrations in pre‐stabilized sludge and post‐stabilized product. This observation is consistent with other reports of increases in 5:3 FTCA following anaerobic treatment, which has been attributed to anaerobic degradation of 6:2 fluorotelomer alcohols (FTOH) (Zhang et al., 2013). In addition to increasing in concentration through stabilization, 5:3 FTCA generally had higher concentrations compared to other reported PFAS compounds, similar to observations by Fredriksson et al. (2022). Other studies have also shown that the terminal end products of 6:2 fluorotelomer sulfonate (FTSA) and 6:2 FTOH comprise FTCAs, including 5:3 FTCA, which does not degrade to PFOA or PFOS because of their resistance to biological transformation (Kabadi et al., 2020).

FIGURE 9.

FIGURE 9

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5:3 FTCA concentration in pre‐stabilized and post‐stabilized product.

PFOS

This long chain perfluoroalkanesulfonic acids (PFSA) have been in use since 1940. Several studies have reported that this compound tends to accumulate in the human body for extended periods, resulting in negative health impacts (USEPA, 2024a). The 3M Company, historically one of the largest manufacturers of this compound, phased out manufacturing of this product by 2008, yet it is still observed in WRRFs because of its continued manufacture in other countries (USEPA, 2021). Like 5:3 FTCA, PFOS concentrations increased in post‐stabilized products compared to pre‐stabilized sludge samples (Table 2). The highlighted data indicate that half the WRRF samples resulted in higher concentration of PFOS in the post‐stabilized product, comparatively. Table 2 includes qualifiers, and while some qualified data were not used in the data analysis, these results are included in Table 2 to demonstrate the MDL variation between pre‐stabilized sludge and post‐stabilized product. Notably, the range of PFOS concentrations (5 to 41 ng/g dry basis) observed in this study was less than studies conducted in early the 2000s. For example, Venkatesan and Halden (2013) observed 403 ± 127 ng/g dry basis. In more recent studies, PFOS concentrations were reported to be substantially lower than those observed in other earlier studies, at 40 ± 179 ng/g dry basis (Link et al., 2024). This decrease in PFAS concentrations over time is well documented, wherein several researchers noted a decline from 2004 to 2017 (Fredriksson et al., 2022; Lakshminarasimman et al., 2021; Link et al., 2024; Venkatesan & Halden, 2013). This is likely attributable to the bans and/or voluntary phase out of manufacturing and use of the compound. The 27 WRRFs evaluated in this study had concentrations from below the reporting limit to 25 ng/g dry basis, consistent with the more recent findings by Link et al., 2024.

TABLE 2.

PFOS concentration (ng/g dry basis.)

Site ID. No. Pre‐stabilized Post‐stabilized Site ID. No. Pre‐stabilized Post‐stabilized
1 9.7 38 (I) 15 7.9 (J) 7.4
2 <11 8.2 (J) 16 <3.3 4.0
3 4.8 (J) 20 (J, I) 17 <4.9 (cn) <0.73
4 4.9 (J) 8.7 18 <4.4 <0.2
5 <9.3 2.6 (J) 19 <3.1 10
6 4.5 (J) 6.3 20 7.7 (J) 6.3
7 11 (J) 9.5 21 <3.9 <0.63
8 5.9 (J) <0.6 22 <5.9 13
9 <11 9.8 23 24 (J) 13
10 <37 16 24 <4.2 6.9 (I)
11 <7.7 8.8 25 <2 7.9 (J, I)
12 41 (I) 4.8I cn 26 <3.6 (cn) 30
13 5.3 <9.8 (cn) 27 40 (J) <0.88
14 < 9.9 6.5
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Note: Qualifiers: J – result is less than the reporting limit, but greater than or equal to the minimum detection limit and the concentration is an approximate value; I – value is an estimated maximum possible concentration; cn – refer to detailed case narrative.

The highlighted orange color indicates the variability in the data. Laboratory data with qualifiers were used in data analysis as described in Oza, Bell, Xu, Wang, Wells, Norton, Winchell, and Huang, 2024.

PFOA

This long chain PFCAs has been in use since 1940 and was phased out of production in the United States by 2015. Like PFOS, PFOA is one of the most studied PFAS compounds because of its previous ubiquity in the environment and toxicity at low concentrations. Notably, PFOA was reported only at one site and at a low concentration of 1.9 ng/g dry basis (Table S3). This finding of PFOA concentrations being lower than PFOS concentrations in pre‐stabilized sludge samples and post‐stabilized product is consistent with observations reported in other studies (Guerra et al., 2014; Link et al., 2024). The few detections of this compound may also point to the importance of bans and voluntary phase outs of these compounds in a long‐term regulatory strategy.

NMeFOSA, NMeFOSAA, and NMeFOSE

These are nonpolymeric polyfluoroalkyl substances in the perfluorooctane sulfonamido group of compounds and are produced for applications related to firefighting foams and furniture coatings. Compared to PFOA and PFOS, these compounds have received relatively less attention in published literature (Ricolfi et al., 2024). NMeFOSA was reported at one site in the pre‐stabilized sludge at a concentration of 10 ng/g dry basis but not observed in post‐stabilized product. NMeFOSAA and NMeFOSE were not reported in the pre‐stabilized sludge samples but were reported in post‐stabilized product with concentration ranging between 10 and 17 ng/g dry basis and 9 to 15 ng/g dry basis, respectively. NMeFOSAA was reported at three WRRFs and NMeFOSE at five WRRFs. Similar concentration findings have previously been reported for NMeFOSAA (13 ± 18 ng/g dry basis; Link et al., 2024). While concentration ranges were similar to previous studies, mechanisms responsible for the apparent disappearance in the post‐stabilized product is an area that require further research.

PFPeA and PFHxA

These are short chain PFCAs with five and six carbons, respectively. These compounds form as terminal transformation products of precursors like FTOHs (Buck et al., 2011; Butt et al., 2014; Liu & Mejia Avendaño, 2013). PFPeA was only observed in post‐stabilized product at one WRRF at a concentration of 15 ng/g dry basis (Table S3). However, PFHxA was observed in both pre‐stabilized sludge (1 WRRF) and post‐stabilized product (2 WRRF) with concentration ranging from 5 to 9 ng/g dry basis. Link et. al observed concentrations for PFPeA in the range of 3.4 ± 11 ng/g dry basis and PFHxA in the range 5.2 ± 12 ng/g dry basis. The concentrations observed in this study was similar for PFHxA and marginally higher for PFPeA (Link et al., 2024).

PFDA and PFDoA

These long chain PFCAs have 10 and 12 carbon chain lengths, respectively. According to the USEPA New Chemicals Program risk assessment, both PFDA and PFDoA were among the nine substances with significant new use notices, indicating uncertainty in the amount of these compounds manufactured as byproducts during the fluorination process (USEPA, 2023). Both compounds were only reported in the post‐stabilized product (Table S3) with concentrations in the range of 2 to 9 ng/g dry basis for PFDA and 3 to 5 ng/g dry basis for PFDoA, which is consistent with reported concentrations in other studies (Lakshminarasimman et al., 2021; Link et al., 2024). Link et al., observed PFDA concentrations at 6.8 ± 13 ng/g dry basis and PFDoA at 2.4 ± 4.8 ng/g dry basis.

PFAS concentration and WRRF operation parameters

The project team conducted a survey prior to sample collection to understand the operations at each WRRF. The questionnaire sent to the WRRF is presented in Appendix S1. Among the parameters collected, several were evaluated closely, including plant influent flow, characteristics of the sludge entering the stabilization process, stabilization process, and sludge retention time. The plant influent flow was provided as an annual average daily flow (AADF) for the WRRFs. The smallest WRRF had an AADF of 8 million gallons per day (MGD), and the largest WRRF had an AADF of 575 MGD. The sludge retention time (SRT) for WRRFs with anaerobic digestion ranged from 10 to 40 days (Figure 10). Notably, in Figure 10, WRRFs with stabilization processes other than anaerobic digestion are labeled as SRT of ‘zero.’

FIGURE 10.

FIGURE 10

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Annual average daily flow and solids retention time at the 27 WRRFs. WRRFs, water resource recovery facilities.

The sludge stabilization process varied among WRRFs and can be broadly grouped into three categories: anaerobic digestion (mesophilic, thermophilic. etc.,), chemical stabilization, and incineration. The individual percentage of each is depicted in Figure 11. The characteristics of the pre‐stabilized sludge entering these stabilization processes also varied. Nineteen WRRFs received blended primary and secondary sludge (waste‐activated sludge [WAS]; blend ratio varied between 0.2 and 0.6 primary to WAS), five did not have a primary treatment, and three of them only treated their secondary sludge/WAS.

FIGURE 11.

FIGURE 11

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Type of stabilization process among the 27 WRRFs. WRRFs, water resource recovery facilities.

Table 3 summarizes key treatment information for the 27 WRRFs. No statistical correlations were identified between reported PFAS compounds concentrations and individual key parameters for each facility (influent flow, sludge characteristics to the stabilization process, stabilization process, and sludge retention time). However, this surveillance study represents a single sample of pre‐stabilized sludge and post‐stabilized product from 27 WRRFs with various treatment processes, and further research is needed to validate the findings of others. For example, Ozelcaglayan et al. recently reported 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, although their study only focused on two WRRFs only (Ozelcaglayan, Pham, & Parker, 2024).

TABLE 3.

Treatment processes at the 27 WRRFs.

Site id. No. Influent flow Primary treatment Secondary treatment Primary sludge: secondary sludge blend Solids stabilization method Solids dewatering method Type of biosolid generated Biosolids ultimate use
1 58% residential, 0.5% commercial, 1.3% industrial, 40.2% other Stack rectangular clarifiers Pure oxygen activated sludge No Mesophilic anaerobic digestion (AD) Centrifuge, thermal drying Class A Land application
2 50% residential, 4.2% commercial, 0.1% industrial, 45.5% other Gravity settling tanks Activated sludge 49%: 51% Mesophilic AD Centrifuge N/A (SRT requirement not met per part 503 Landfill and partially for beneficial reuse
3 39.4% residential, 2.9% commercial, 57.7% other Gravity settling tanks Activated sludge 57%: 43% Mesophilic AD Centrifuge
4 90% residential, 9% industrial, 1% landfill Primary tanks Aeration tanks 10:1 Mesophilic AD Belt press Class B Landfill
5 99% residential, 1% industrial Primary clarifiers Activated sludge 1:0.77 Mesophilic AD Screw press Class B Fertilizer
6 98% residential, 2% industrial Primary tanks Aeration tanks 1: 1.8 Mesophilic AD Centrifuge Class A
7 65% residential, 25% commercial, 10% industrial Rectangular covered tanks Five‐stage Bardenpho activated sludge BRN system 55%:45% Mesophilic AD Centrifuge Class B Land application
8 86% residential, 7% commercial, 6% industrial, 40.2 1% other Rectangular tanks Aeration tanks

Varies

(5.5:1)

75% lime stabilization

25% incineration

 Class AEQ Land application
9 29% residential, 17% commercial, 1% industrial, 53%other Primary clarifiers High purity oxygen activated sludge Varies by season Mesophilic AD Centrifuge Class B Land application
10 97.4% residential, 2.6% commercial Primary clarifiers One‐pass and two‐pass aeration tanks No Mesophilic AD Lagoons Class A, B, AEQ Farmland, local parks
11 87% residential, 9% commercial, 3% industrial, 1% other Primary clarifiers Rectangular bioreactors 53%: 47% Mesophilic AD Centrifuge Unclassified Landfill
12 89.4% residential, 10.1% other Primary tanks High purity oxygen‐activated sludge ‐‐ Mesophilic AD Centrifuge Class AEQ Beneficial reuse
13 60% residential, 10% commercial, 25% industrial, 5% other Primary clarifiers Tricking filters + activated sludge Yes Mesophilic AD Belt filter presses Class B Landfill
14 87% residential, 11% commercial, 2% industrial Primary clarifiers Tricking filters + activated sludge 64%: 36% Mesophilic AD Centrifuge Class B Land application
15 85% residential, 11.6% commercial, 2.6% industrial, 0.8% landfill Primary clarifiers MLE, activated sludge 0.6 WAS to PS Mesophilic AD Belt filter press Class B Land application, landfill
16 97%resedential, 1% commercial, 1% industrial, 1% landfill Primary clarifiers Trickling filter/solids contact Yes, 25% Lime stabilization Belt filter press Class B Land application
17 86% residential, 11% commercial, 3% industrial Primary clarifiers Trickling filters and aeration basins No Mesophilic AD Centrifuge Class B Land application
18 70% residential, 20% commercial, 10% industrial Primary clarifiers Aeration basins 60:40 Incineration ‐‐ ‐‐
19 Not available Primary clarifiers Aeration basins 45%:55% Mesophilic AD Centrifuge Class B Land application
20 63% residential, 17% commercial, 5% industrial, 15% other Rectangular tanks Trickling filter/solids contact

N/A

 Thermophilic AD Centrifuge Class A Land application
21 52% residential, 10% commercial, 4% industrial, 34% other Rectangular tanks None N/A Thermophilic AD Centrifuge Class B Land application
22 80% residential, 5% commercial, 10% industrial, 5% landfill Primary clarifiers High purity oxygen‐activated sludge 1:1 Thermophilic AD Screw press Class A Soil amendment
23 80% residential, 16% commercial, 4% industrial No primary tanks Bioreactors No primary Composting Screw press Class AEQ Composting
24 68% residential, 27% commercial, 2% industrial Rectangular tanks Trickling filter/solids contact tank Mesophilic AD Centrifuge Class B Land application
25 32% residential, 14% commercial, 2% industrial, 52% other Rectangular tanks None N/A Mesophilic AD Lagoons Class B Land application
26 Not available Primary clarifiers Step‐feed biological nutrient removal system 45% PS:55%WAS Thermophilic followed by mesophilic Rotary press Class A Land application, compost
27 95% residential, 4.5% commercial, 0.5% industrial Primary clarifiers Step‐feed activated sludge 1.5 PS:1 WAS Lime stabilization Belt filter press Class B Land application
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Note: PS – primary sludge; WAS – waste activated sludge.

Based on this surveillance study, three key considerations are suggested for future studies:

  • Multiple sampling event: Additional sampling (replicates) is required to further elucidate the potential transformation of PFAS compounds. For example, Site No. 1 WRRF, three PFAS (5:3 FTCA, PFHxA, and PFOS) compounds were reported in the pre‐stabilized sludge, but only one PFAS (5:3 FTCA) compound was reported in the post‐stabilized product. The 5:3 FTCA concentration increased from 22 to 220 ng/g from pre‐stabilized to post‐stabilized product. As observed by other researchers, this transformation could be attributed to the microbial transformation of larger molecules such as PFOS, but to provide statistical confidence that this mechanism is occurring, additional sampling data at this WRRF are required.

  • Sampling at WRRF with similar process treatment train: In this study, 82% of the 27 WRRFs had some form of anaerobic digestion as the stabilization process. But the digester operations among these WRRFs varied with respect to solid content of the digester feed, digester operating temperature, digester configuration (acid and methanogenesis phase separated), solids retention time in the digester, microbial community composition, etc. All these factors may have potential impact on the transformation products of PFAS and the mechanisms. Comparing WRRFs with similar process treatment train will provide insight into understanding the transformation of PFAS compounds through the sludge stabilization treatment process.

  • Sludge matrix and its impact on reporting limits: Although all samples were analyzed by the same laboratory, using the same method, the detection limit could play a vital role to the conclusion derived from the available qualified data. Studies have indicated that sludge matrices are generally homogeneous, but for PFAS analysis because the concentrations are so close to the reporting limits, data interpretation could be difficult without additional data collection to provide statistically relevant conclusions.

Although generally accepted that sludge treatment processes clean up the sludge matrix by reduction of organic matter between pre‐ and post‐stabilization, it is an important finding of this research that this was not to be the case at all sites sampled. At 22% of the sites in this study (i.e., 1, 2, 3, 13, 25, and 26), the analytical RLs and MDLs for PFAS as recorded in Table S3 were higher for post‐stabilized product than to the pre‐stabilized sludge. The observation is site specific. The data indicate that the opposite was noted for 78% of the sites in which the RLs and correspondingly the MDLs for PFAS were higher in pre‐stabilized samples than in post‐stabilized samples. This observation may result from the extremely low analytical limits to which PFAS analyses are subjected. Also, at one‐fourth of the sites sampled, the composition of organics remaining in the treated matrix may contain interferences particularly occurring under the same extraction and chromatographic conditions as the analytical methods used for PFAS.

No commonalities were yet determined among sites 1, 2, 3, 13, 25, and 26. Nevertheless, for all 27 sites, appropriate QA/QC measures were applied irrespective of the matrix background was greater in the pre‐stabilized samples compared to the post‐stabilized samples or vice versa. Data are appropriately compared between pre‐ and post‐stabilized samples and among sites only after QA/QC is considered for each of the 40 PFAS compound at each location at each of the 27 WRRF site.

CONCLUSIONS

Given the widespread occurrence of PFAS and regulatory uncertainty related to how this group of compounds could impact long‐term biosolids management strategies, there is a clear need to better understand contemporary PFAS concentrations in biosolids. There are several studies documenting PFAS concentrations in biosolids, including the 2001 USEPA NSSS; however, this study is the first to sample and analyze 40 PFAS in contemporary samples from WRRFs across the United States and Canada that represent various sizes, geographies, treatment trains, and practices for ultimate disposition of wastewater solids.

The 2001 NSSS survey evaluated 13 PFAS in US WRRF biosolids composites from 32 States and the District of Columbia. It showed that the most abundant compounds were PFOS and PFOA, with concentrations of 403 ± 127 and 34 ± 22 ng/g, respectively (Venkatesan & Halden, 2013). Considering that the primary US manufacturer of PFOS voluntarily phased out production of PFOS in 2002, and eight other companies voluntarily agreed to phase out production of PFOS and PFOA‐related chemicals by 2015 (USEPA, 2017), it is not surprising that PFOS and PFOA concentrations in this study were lower than those reported for the 2001 NSSS in this surveillance study.

Further, of the 40 compounds in the modified Method (and Method 1633), only 10 were reported (four compounds in the pre‐stabilized sludge and nine compounds in the post‐stabilized product), which may indicate that the perceived risks of PFAS in land applied biosolids may be less than originally thought considering previous data. These data are critical to utilities for long‐term planning around disposition of biosolids. Considering that currently, over 50% of biosolids produced in the United States are land applied and the USEPA risk assessment for PFOA and PFOS in biosolids is targeted for early 2025, which could prompt additional regional and state legislation, data documented in this surveillance study could support utilities with decisions about their long‐term management strategies moving forward—such as considering alternative disposal technologies, incineration, and pyrolysis—that have been demonstrated to destroy PFAS in biosolids (Winchell et al., 2024; Winchell et al., 2020; Winchell, Ross, et al., 2022; Winchell, Wells, et al., 2022).

AUTHOR CONTRIBUTIONS

Shubhashini Oza: Conceptualization; data curation; formal analysis; investigation; validation; writing—original draft; methodology; project administration; supervision. Hui Li: Writing—review and editing; funding acquisition. Qingguo Huang: Funding acquisition; writing—review and editing. John W. Norton: Funding acquisition; visualization. Lloyd J. Winchell: Writing—review and editing. Martha J. M. Wells: Writing—review and editing. Thomas Nangle: Writing—review and editing. Natalia Perez: Writing—review and editing. Dan DeLaughter: Writing—review and editing. Jan M. Hauser: Writing—review and editing. Malcolm Taylor: Writing—review and editing. Zonetta E. English: Writing—review and editing. Mike Melnyk: Writing—review and editing. Phuong Truong: Writing—review and editing. Katherine Y. Bell: Writing—review and editing; conceptualization; funding acquisition; resources; supervision; visualization.

CONFLICT OF INTEREST STATEMENT

Martha J.M. Wells is a chemical consultant to Brown and Caldwell and served in that role while assisting with the preparation of this manuscript. John Norton is the Director of Energy, Research, and Innovation at Great Lakes Water Authority who provided funding for this work.

Supporting information

Figure S1: Page one of Sampling Guideline Document shared with 27 WRRF.

Figure S2: Page three of Sampling Guideline shared with 27 WRRF.

Table S1: Summary of the forty PFAS compounds evaluated.

Table S2. Summary of PFAS Isotopes used by the laboratory.

Table S3: Measured PFAS concentrations for all 27 WRRFs (ng/g dry basis) [Analyzed as per EPA Methods SW846 and Modified 537.1].

Table S4: Percentage recovery of isotopes.

WER-97-e70039-s001.docx (499.1KB, docx)

ACKNOWLEDGMENTS

This work was supported in part by the USEPA National Priorities Program (No. 84025201). This document has not been formally reviewed by the funding agencies. The views expressed in this document are solely from the authors and do not necessarily reflect those from the funding agencies. The agencies do not endorse any products or commercial services mentioned in this paper. The authors would like to acknowledge the Great Lakes Water Authority and other Water Resource Recovery Facilities who provided additional financial support. The authors would also like to acknowledge all the 27 Water Resource Recovery Facilities and their staff who aided in sampling coordination activities. Further, the authors would like to acknowledge the financial support provided by Brown and Caldwell through their Research and Innovation program, as well as the sampling coordination activities provided by the Brown and Caldwell Treatability Laboratory, Nashville, TN.

Oza, S. , Li, H. , Huang, Q. , Norton, J. W. , Winchell, L. J. , Wells, M. J. M. , Nangle, T. , Perez, N. , DeLaughter, D. , Hauser, J. M. , Taylor, M. , English, Z. E. , Melnyk, M. , Truong, P. , & Bell, K. Y. (2025). Per‐ and polyfluoroalkyl substances in untreated and treated sludge/biosolids from 27 water resource recovery facilities across the United States and Canada. Water Environment Research, 97(2), e70039. 10.1002/wer.70039

WEF Member: All authors are WEF members.

Shubhashini Oza, Hui Li, Qingguo Huang, John W. Norton, Lloyd J. Winchell, Martha J. M. Wells, Thomas Nangle, Natalia Perez, Dan DeLaughter, Jan M. Hauser, Malcolm Taylor, Zonetta E. English, Mike Melnyk, Phuong Truong, and Katherine Y. Bell are WEF members.

DATA AVAILABILITY STATEMENT

The data that supports the findings of this study are available in the supplementary material of this article

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Figure S1: Page one of Sampling Guideline Document shared with 27 WRRF.

Figure S2: Page three of Sampling Guideline shared with 27 WRRF.

Table S1: Summary of the forty PFAS compounds evaluated.

Table S2. Summary of PFAS Isotopes used by the laboratory.

Table S3: Measured PFAS concentrations for all 27 WRRFs (ng/g dry basis) [Analyzed as per EPA Methods SW846 and Modified 537.1].

Table S4: Percentage recovery of isotopes.

WER-97-e70039-s001.docx (499.1KB, docx)

Data Availability Statement

The data that supports the findings of this study are available in the supplementary material of this article

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