Comparative evaluation of activated sludge and biofilm processes for per- and polyfluoroalkyl substances (PFASs) removal in a full-scale wastewater treatment plant

Ran Yu; Rong Shen; Peng Li

doi:10.4491/eer.2026.109

Environ Eng Res > Volume 31(6); 2026 > Article

Yu, Shen, and Li: Comparative evaluation of activated sludge and biofilm processes for per- and polyfluoroalkyl substances (PFASs) removal in a full-scale wastewater treatment plant

Research

Environmental Engineering Research 2026; 31(6): 260109.

Published online: April 16, 2026

DOI: https://doi.org/10.4491/eer.2026.109

Comparative evaluation of activated sludge and biofilm processes for per- and polyfluoroalkyl substances (PFASs) removal in a full-scale wastewater treatment plant

Ran Yu¹, Rong Shen¹, Peng Li^2,^†

¹School of Bioengineering, Beijing Polytechnic University, Daxing District, Beijing, China

²Beijing Zhiyu Tiancheng Design and consulting Co., Ltd, Tongzhou District, Beijing, China

^†Corresponding author: E-mail: ronnie2323@sina.com, Tel: +86 10 56862600, ORCID: 0000-0002-9445-506X

Received February 23, 2025 Revised April 8, 2026 Accepted April 14, 2026

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

The increasing use of short-chain per- and polyfluoroalkyl substances (PFASs) and PFASs alternatives has led the widespread environmental distribution and new pollution profiles, posing new challenges for the efficient removal of PFASs. This study systematically compared the occurrence, removal, and sludge enrichment characteristics of PFASs in two parallel wastewater treatment processes (activated sludge and biofilm). A total of 21 PFASs were detected in the influent, with short-chain PFASs accounting for 60.8%. The total PFASs removal efficiency of biofilm process (58.9%) was higher than that of activated sludge process (53.6%), especially under the high proportion of short-chain PFASs in the influent. Among the 15 PFASs detected in sludge, biofilm process sludge exhibited significantly higher total PFASs concentration and a more complex composition, with notably increased proportion of short-chain PFASs (particularly PFBS). In contrast, the sludge of activated sludge process was dominated by long-chain PFASs with negligible short-chain components. Biofilm processes exhibit superior removal performance for wastewater containing a high proportion of short-chain PFASs and their alternatives. The study provides comprehensive insights into the effective removal of PFASs in different wastewater treatment configurations, offering valuable guidance for PFASs control in wastewater treatment plants.

Keywords: PFASs, PFOA, PFOS, Conventional activated sludge processes, Membrane bioreactor, WWTPs

Graphical Abstract

Keywords: PFASs, PFOA, PFOS, Conventional activated sludge processes, Membrane bioreactor, WWTPs

1. Introduction

Per- and polyfluoroalkyl substances (PFASs) are synthetic fluorinated chemicals characterized by extraordinary chemical stability and resistance to degradation. Owing to their unique surfactant properties, they are extensively applied in industrial production, food processing, chemical synthesis, and numerous consumer products. Classified as persistent organic pollutants (POPs), PFASs persist and bioaccumulate in the environment, transfer through trophic levels, and eventually enter the human food chain. Exposure to PFASs [1]has been associated with diverse adverse health outcomes, including metabolic disorders, reproductive dysfunction, and immunotoxicity [2].

Moreover, PFASs exert toxic effects on aquatic organisms and microbial communities, reducing bacterial density [3], lowering microbial diversity [4], and posing significant ecological risks [5].

Perfluorooctanoic acid (PFOA) and hexafluoropropylene oxide dimer acid (HFPO-DA) were the predominant PFASs detected [6]. Perfluorooctane sulfonate (PFOS), PFOA and perfluorohexane sulfonate (PFHxS) have been listed as controlled substances [7]. Based on the number of CF₂ moieties, PFASs can be broadly categorized into long-chain PFASs, short-chain PFASs, and ultra-short-chain PFASs [8]. Long-chain perfluoroalkyl sulfonic acids (PFSAs) showed a marked tendency to accumulate [9]. The hexafluoropropylene oxide dimer acid (HFPO-DA) and short-chain perfluoroalkyl carboxylic acids are emerging as alternatives to PFOA, while 6:2 fluorotelomer sulfonic acid (6:2 FTSA) and sodium p-perfluorous nonenoxybenzene sulfonate (OBS) are becoming key substitute substances for PFOS [10]. Although considered safer, these alternatives retain high persistence and exhibit higher water solubility [11], weaker sorption, and stronger environmental mobility, leading to more extensive dispersion in surface water and groundwater [12]. As a result, short-chain PFASs have become ubiquitous environmental contaminants of growing global concern.

PFASs primarily originated from industrial effluents, municipal wastewater, food packaging, AFFFs, metal plating, and surface runoff [13]. Wastewater treatment plants (WWTPs) are important point sources of PFASs discharge [14].While conventional processes can partially remove long-chain PFASs, they often show limited removal or even net generation of short-chain congeners. PFASs concentrations frequently increase during treatment due to precursor transformation. For example, perfluorohexanoic acid (PFHxA), perfluorooctanoic acid (PFOA), perfluorohexanesulfonic acid (PFHxS), and perfluorooctanesulfonic acid (PFOS) all showed net mass gains after being processed in wastewater treatment plants (WWTPs), with average increases of 83%, 28%, 37%, and 58% respectively [15]. The environmental fate and persistence of PFASs are largely determined by their partitioning behavior between aqueous phases, suspended particulate matter, and sediments [16]. Short-chain PFASS demonstrate higher mobility and greater potential for atmospheric deposition, while long-chain compounds tend to adsorb more readily to solid matrices [17]. Conventional technologies including activated carbon, coagulation, membrane filtration, and ion exchange perform well for long-chain PFASs but are generally less efficient for short-chain species [18]. Adsorption represents a highly promising strategy owing to its high operational efficiency and favorable cost-effectiveness [19]. Coagulation is one of the common water treatment processes that demonstrates effectiveness in PFASs removal, however, its removal efficiency depends on the basicity value of the coagulants used [20]. Advanced treatment methods [21] also show promising PFASs removal efficacy in controlled studies [22]. Membrane treatment [23] can effectively decrease the concentration of PFASs in wastewater [24]. And about 18.1–30.8% of PFASs can be removed by microbiota [25]. Pressure-driven membrane processes (reverse osmosis, nanofiltration) [26] and coagulation-flocculation systems [27] are widely implemented in full-scale applications, yet their efficiency depends strongly on PFAS chain length.

In this study, the PFASs removal performance of two biological processes (activated sludge and biofilm) in full scale WWTP was systematically investigated. The efficiencies toward short-chain, long-chain, and alternative PFASs were compared to identify chain-length-dependent differences. This work aims to provide scientific support for optimizing WWTPs to better control the release of short-chain and emerging PFASs.

2. Materials and Methods

2.1 Wastewater Treatment Plants and Sample Collection

Samples were collected from a WWTP in Chengdu, Sichuan, China. The WWTP has a sewage treatment capacity of 100,000 metric tons per day and a service area covering 55 square kilometers. The system adopts a comprehensive tertiary treatment process that sequentially incorporates wastewater pretreatment (including screening and grit removal), biological secondary treatment using activated sludge and biofilm system, and tertiary treatment comprising coagulation, denitrification, and ultraviolet disinfection.

Two parallel biological systems, activated sludge (Phase I) and biofilm (Phase II), were operated under identical influent conditions to enable direct comparison of PFASs removal performance. Both systems shared the same upstream pretreatment (screening, grit removal, and membrane filtration) to ensure consistent water quality.

Phase I was an anaerobic-anoxic-oxic membrane bioreactor (A²O activated sludge), designed for simultaneous organic degradation, nitrogen removal, and phosphorus removal. Solid-liquid separation was achieved via ultrafiltration membranes, followed by ultraviolet disinfection.

Phase II was an anoxic-oxic-oxic biofilm reactor (AOO-biofilm) using polyethylene suspended carriers. Phase II focused on advanced organic degradation and nitrogen removal. The effluent was further treated by magnetic coagulation-sedimentation and a denitrification filter, with final ultraviolet disinfection. Detailed description of wastewater treatment processes can be referred to Text S2.

Influent wastewater samples were collected in October 2025 using a 24-hour composite sampling methodology. During sampling, an automated sampler was employed to collect 200 mL of subsamples every 2 hours for 24 consecutive hours, and the samples obtained at different time points were mixed thoroughly to form the composite sample. Sludge samples were collected as composite samples during one day in October 2025 from the WWTP. Specifically, Sludge-Phase I was sampled from the membrane tank, while Sludge-Phase II was collected from the biofilm carrier-associated sludge. Sludge samples were freeze–dried and stored at −18°C until analysis.

2.2. Chemicals and Materials

This study analyzed 29 PFASs including 13 perfluoroalkyl carboxylates (C4 PFBA, C5 PFPeA, C6 PFHxA, C7 PFHpA, C8 PFOA, C9 PFNA, C10 PFDA, C11 PFUnA, C12 PFDoA, C13 PFTrDA, C14 PFTeDA, C16 PFHxDA, C18 PFOcDA), 5 perfluoroalkyl sulfonates (C4 PFBS, C6 PFHxS, C7 PFHpS, C8 PFOS, C10 PFDS), 4 fluorotelomer carboxylic acids (2-Perfluorohexylacetic Acid, 2-Perfluorooctylacetic Acid, 3-Perfluoropropylpropionic Acid, 3-Perfluoropentylpropionic Acid), 4 heterocyclic, oxygenated and halogenated PFASs (11-Chlorodecafluoro-3-oxetane-1-sulfonic Acid, 9-Chlorohexafluoro-3-oxacyclohexane-1-sulfonic Acid, 4,8-Dioxo-3H-perfluorononanoic Acid, Hexafluoropropylene Oxide Dimer Acid (HFPO-DA)), and 3 hydrogen-substituted perfluoroalkyl sulfonates (H-PFSAs: 1H,1H,2H,2H-Perfluorohexane Sulfonic Acid, 1H,1H,2H,2H-Perfluorooctane Sulfonic Acid (PFOS), 1H,1H,2H,2H-Perfluorodecane Sulfonic Acid). All target analytes were sourced from Wellington Laboratories, Inc. (Canada). Surrogate standard (SS) solution including ¹³C₂-PFHxA, ¹³C₅-PFNA, M3HFPO-DA, and d₅-N-EtFOSAA. Quantitative internal standard (IS) solution including ¹³C₂-PFOA, d₃-N-MeFOSAA, and ¹³C₄-PFOS. Both SS and IS solutions were also acquired from Wellington Laboratories, Inc. (Canada). Chromatographic grade methanol and other reagents, including ammonium acetate, acetic acid, aqueous ammonia and analytical grade sodium acetate, were all purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., China. The ultrapure water used in the experiments was Milli-Q water with a resistivity of no more than 18.2 MΩ·cm. The full names of abbreviations and chemical structures were presented in Table S1.

2.3. Pretreatment

2.3.1. Wastewater sample pretreatment

All samples were filtered through a membrane to remove suspended particles in water prior to treatment. Take 500 mL of filtered water sample, add 10–40 ng of surrogates (50 μL of 200–800 ng/mL solution), and age for 30 min. Install a WAX cartridge on the SPE system and then activate the cartridge with methanol solution containing 0.5% ammonia, methanol, and ultrapure water sequentially. Connect the sample to the pre-rinsed SPE cartridge using a guide tube, turn on the vacuum pump, and adjust the flow rate to 3–5 mL/min to allow the water sample to pass through the SPE cartridge. After the sample has passed through the cartridge, rinse the cartridge with 25 mM sodium acetate solution, then dry under 10 atm to remove moisture. Elute the target analytes with methanol and methanol solution containing 0.5% ammonia sequentially. The eluent was then blow-dried to near dryness with nitrogen in a 40 °C water bath. Add 0.25 mL of methanol-water (V/V, 1:1) solution to constant volume, then add 10–40 ng of IS (50 μL of 200–800 ng/mL solution) and vortex. Filter the sample through a 0.22 μm microporous membrane and finally perform LC-MS/MS analysis. Meanwhile, prepare blank samples, blank spiked samples, and parallel samples according to the above steps.

2.3.2. Sludge sample pretreatment

An accurately weighed amount (≤ 2 g) of uniformly freeze-dried sludge sample was transferred into a 15 mL polypropylene (PP) centrifuge tube. Subsequently, 10–40 ng of SS (50 μL of a 200–800 ng/mL stock solution) were added, and the mixture was equilibrated for 30 min. 8 mL methanol was then added, followed by vortex mixing for 5 min, ultrasonic extraction at 30 °C for 20 min in an ultrasonic cleaner, and centrifugation at 3500 rpm for 5 min. The supernatant was carefully transferred to a 500 mL PP sample bottle. This extraction procedure was repeated twice, and the three extracts were combined. Ultrapure water was then added to bring the total volume to approximately 500 mL. Glacial acetic acid was added to the resulting methanol-water extract to achieve a final concentration of 0.1% acetic acid [28]. The subsequent SPE purification and sample preparation steps were identical to those for water samples.

2.4. UPLC-MS/MS Analysis

Chromatographic separation was achieved using an Agilent ZORBAX Eclipse XDB-C18 column (3.5 μm particle size, 2.1 mm × 150 mm i.d.). The column temperature was maintained at 30 °C, with an injection volume of 10 μL and a flow rate of 0.3 mL/min. Mobile phase A was composed of a 2 mM ammonium acetate aqueous solution, while mobile phase B was methanol. The gradient elution program was optimized as follows, the initial mobile phase composition was 40% B, which was linearly increased to 100% B from 1 min to 7 min. The 100% B composition was held constant for 3.5 min, followed by an immediate reversion to 40% B. The system was then equilibrated for 3 min prior to the next injection to ensure analytical reproducibility. Mass spectrometric analysis was performed with an electrospray ionization (ESI) source operated in negative ionization mode, and detection was conducted using multiple reaction monitoring (MRM) mode. The key mass spectrometry (MS) parameters were as follows, ion spray voltage was −4500 V, and the source temperature was maintained at 450°C. For the preparation of standard series, a certain volume of methanol and ultrapure water was first added to adjust the methanol/water ratio of each working solution to 1:1 (v/v), which was consistent with the sample matrix. Subsequently, 10–40 ng of SS and IS were added [29].

2.5. Statistical Analysis

All experiments in this study were performed in triplicate. Results are expressed as the mean ± standard deviation (SD). Data was analyzed by one-way analysis of variance (ANOVA) using IBM SPSS Statistics software, with statistical significance set at P < 0.05. All statistical graphs were plotted using Origin 2023 software.

3. Results and Discussion

This study provides a systematic characterization of the occurrence and removal of PFASs in two typical wastewater treatment processes. A total of 21 PFASs were detected in the influent of the WWTP (Fig. 2). The dominant compounds of concentration were PFOcDA (23 ng/L), HFPO-DA (19 ng/L), and PFPeA (18 ng/L). The distribution profile of PFASs in the influent suggests that long-chain PFASs remain important components in local domestic sewage. Meanwhile, the application of long-chain PFASs alternatives and short-chain PFASs has gradually increased, making them key constituents of PFASs in wastewater. A widely accepted classification criterion was adopted in this study. Perfluorocarboxylic acids (PFCAs) with ≥ 7 perfluorinated carbon atoms and perfluorosulfonic acids (PFSAs) with ≥ 6 perfluorinated carbon atoms were defined as long-chain PFASs, while the remaining homologs were classified as short-chain PFASs [30]. Based on mass concentration analysis, short-chain PFASs accounted for approximately 60.8% of the total PFASs in the influent, exceeding the proportion of long-chain PFASs (Fig. 3).

3.1. PFASs Removal Efficiency

The removal efficiency of PFASs varied across different treatment stages and between the two processes, with distinct performance observed for different PFAS homologs. During the preliminary stage, the overall PFASs removal efficiency via grid filtration and similar processes was approximately 6.5%. Specifically, 6:2 FTCA and 8:2 FTS showed relatively high removal efficiencies, both exceeding 30%, followed by PFHxS, PFOA, and 3:3 FTCA with removal rates above 20% (Fig. 4). In contrast, a negative removal efficiency was observed for 5:3 FTCA. During the adsorption process, the removal rates estimated from individual adsorption coefficients were higher than the measured values, which could be ascribed to the transformation of certain precursors that caused an increase in the concentrations of some PFASs [31].

Phase I achieved an overall PFASs removal efficiency of approximately 53.6%, with highly efficient removal of PFUnA, 6:2 FTCA, and HFPO-DA, the removal rates of PFPeA and PFOcDA also exceeded 60%. However, this phase exhibited low removal efficiencies for PFBA (4.9%), PFHxA (3.7%), PFBS (0.8%), and PFDS (0.9%), and even a negative removal efficiency for 8:2 FTS (−19.9%).

Phase II exhibited a higher overall removal efficiency of 58.9% compared to Phase I. In Phase II, the PFASs removal efficiency in the biological treatment stage reached approximately 36.9% although increased effluent concentrations were observed for PFOA, PFNA, PFHxDA, and 6:2 FTCA during this stage. It is mainly due to the biotransformation of precursor compounds into corresponding PFASs. Nevertheless, the subsequent magnetic coagulation high-density sedimentation unit effectively eliminated the transformation products of PFOA, PFHxDA, and 8:2 FTS. In the final effluent of Phase II, high removal proportions were achieved for Cl-PFOSOXA, 4:2 FTS, and HFPO-DA. This process also presented obvious advantages in the removal of PFHxDA and PFOcDA, although the removal efficiencies for 5:3 FTCA and PFDS were relatively low. Under the condition of a high proportion of short-chain PFASs, the removal performance of Phase II was superior to that of Phase I, with the magnetic coagulation high-density sedimentation unit making an effective contribution to the removal of PFASs from the aqueous phase.

3.2. Differences in PFASs Chain Length Distribution

The chain length of PFASs significantly influenced their occurrence, removal, and enrichment patterns in both treatment processes, reflecting regional shifts in PFAS usage and process-specific characteristics. The predominance of short-chain PFASs (60.8%) in the influent of the wastewater treatment plant reflects a distinct regional shift in PFAS usage patterns. Amid the global restriction and phase-out of long-chain PFASs, short-chain PFASs and their alternatives have been extensively manufactured and applied, gradually emerging as the dominant PFAS components in aquatic environments. The high detection frequencies and concentrations of emerging PFAS alternatives (e.g., HFPO-DA, Cl-PFOSOXA) further demonstrate that the substitution of legacy PFASs has not fully eliminated environmental risks, but rather redirected the pollution profile toward novel fluorinated alternatives. These findings align with the global evolutionary trend of PFAS contamination, underscoring the urgency of enhancing the monitoring and risk management of emerging PFAS compounds.

The two treatment processes exhibited distinct responses to PFASs of different chain lengths. Phase I achieved high removal rates for specific long-chain PFASs but demonstrated limited efficiency for short-chain homologs, which can be attributed to the weak hydrophobic adsorption of short-chain PFASs onto sludge flocs. In contrast, Phase II attained superior overall PFASs removal, particularly under high short-chain PFASs loading conditions, primarily due to the enhanced particle interception and adsorption capabilities afforded by the magnetic coagulation high-density sedimentation unit. The unremoved short-chain PFASs in Phase I were discharged with the effluent, posing potential risks to the receiving water body.

3.3. PFASs Enrichment in Sludge

Sludge serves as a crucial sink for PFASs in wastewater treatment systems, with significant differences in PFASs composition and enrichment levels observed between the two treatment processes. A total of 15 PFASs were identified in the sludge samples, including legacy PFCAs, PFSAs, and several emerging alternatives (6:2 FTCA, 3:3 FTCA, 5:3 FTCA, Cl-PFOSOXA, Cl-PFCHSOXA, and 8:2 FTS).

Sludge from Phase I was dominated by long-chain PFASs, with PFTrDA as the most abundant congener, followed by PFHxS and 3:3 FTCA. Emerging alternatives (e.g., 8:2 FTS) and certain medium-chain PFASs also accounted for considerable proportions, whereas the contribution of short-chain PFASs (e.g., PFBS) was negligible. The total PFASs concentration in Phase I sludge was relatively low compared to Phase II.

In comparison, Phase II sludge exhibited a more complex PFAS composition, with a markedly increased proportion of short-chain PFASs, particularly PFBS. Meanwhile, medium- and long-chain PFASs (including PFPeA, PFHxA, PFHpA, PFNA, PFUnA, and PFTrDA) and emerging alternatives (e.g., Cl-PFOSOXA, Cl-PFCHSOXA) were also highly enriched. The total PFASs concentration in Phase II sludge was significantly higher, reaching more than 1.3 times that in Phase I sludge (Fig. 5), suggesting stronger adsorption and interception capacity and a higher treatment load in Phase II.

The compositional differences between the two sludge samples were consistent with the high proportion of short-chain PFASs in the influent and further reflect the distinct enrichment characteristics of each treatment process. Overall, Phase I sludge was dominated by long-chain PFASs and partial emerging alternatives with relatively low total enrichment. In contrast, Phase II sludge showed significantly enhanced enrichment of short-chain PFASs, a more diversified profile, and a higher total PFAS level, demonstrating its superior capacity to accumulate various PFAS categories, especially short-chain homologs and emerging alternatives. The high abundance of short-chain and emerging PFASs in sludge also poses potential challenges to subsequent sludge disposal, including leaching risks during land application, incineration, or landfill. These findings underline the importance of safe management and disposal of PFASs-contaminated sludge.

3.4. PFASs Removal Mechanism

The removal and enrichment of PFASs in the two treatment processes are governed by their physicochemical properties, process configurations, and microbial interactions, with distinct mechanisms observed for different chain lengths and treatment units. The number of unique carbon-fluorine (C–F) bonds in PFASs is a key determinant of their lipophilicity: specifically, the longer the perfluorinated carbon chain, the stronger the lipophilicity of the PFAS congener. The physicochemical properties of PFASs, especially pKa and lipid-water partition coefficient, regulate the phase partitioning of PFASs as well as their removal rules and adsorption properties [32].With increasing carbon chain length of PFAS, the pKa increases accordingly. Under the actual operating pH values (6.5–8.0), short-chain PFAS show more complete ionization and exist predominantly as negatively charged species. Thus, they are more significantly influenced by the zeta potential of microbial cells and biofilms. The preferential adsorption of PFSAs in sedimentary environments was ascribed to their enhanced hydrophobic characteristics [33]. Short-chain PFCAs displayed relatively lower retention rates, which explains their poor removal in Phase I.

Biofilm plays a significant role in PFASs adsorption and removal [34, 35], and this effect increases with the increase of the biofilm’s zeta potential and hydrophobicity [36]. PFASs promote the synthesis of proteins and polysaccharides by up-regulating amino acid metabolism and membrane transport functions, thereby accelerating microbial aggregation, promoting biofilm formation, and the formed biofilm further adsorbs PFASs [37].

The correlation between biofilm zeta-potential changes and adsorption was closely associated with biofilm formation [38]. The total hydraulic retention time (HRT) of Phase I was 16–20 h, including 3–3.5 h of anaerobic phase, 3–3.5 h of anoxic phase, and 10–13 h of aerobic phase. In Phase I, the aerobic tank maintained a dissolved oxygen (DO) concentration of 2–4 mg/L, under which the biofilms grown had relatively weak formation ability. For these weak biofilm formers, the zeta-potential showed a trend of first becoming more negative and then less negative in the next stage. The weakened electronegativity of the biofilm in the mature stage slightly enhanced the electrostatic attraction to anionic short-chain PFAS, but the lack of a synergistic effect with magnetic coagulation and the relatively low secretion of extracellular polymeric substances (EPS) under this DO condition limited the adsorption capacity, ultimately resulting in a lower solid-phase partitioning ratio of short-chain PFAS.

The total HRT of Phase II was also 16–20 h, with the distribution of each reaction zone adjusted to 6–7 h of anoxic phase and 10–13 h of aerobic phase. The DO concentration in the membrane tank of Phase II was higher (4–5 mg/L), and this higher DO condition promoted the growth of strong biofilm formers. The increased secretion of EPS induced compensated for the adverse effect of increased electronegativity on PFAS adsorption. The solid-phase partitioning ratio of short-chain PFAS in Phase II was significantly higher than that in Phase I, especially under the influent condition with a high proportion of short-chain PFAS. The mean zeta-potential of weak biofilm formers was significantly more negative than that of strong biofilm formers. The biofilm in Phase II had a more favorable surface charge state for PFAS adsorption when combined with EPS and magnetic coagulation.

The solid-phase partitioning ratio of short-chain PFASs in Phase II was significantly higher than that in Phase I, primarily due to the enhanced adsorption capacity of the biofilm and magnetic coagulation unit in Phase II, which effectively increased their actual solid-phase partitioning ratio, resulting in higher solid-phase concentrations and lower aqueous-phase residues. This difference in adsorption affinity directly led to a higher concentration of short-chain PFASs (e.g., PFBS) in the solid phase of Phase II, especially under the influent condition with a high proportion of short-chain PFASs.

Certain PFAS undergo biotransformation within organisms, yielding short-chain PFAS that may exhibit higher toxicity. Specifically, these PFAS can be converted into novel PFAS, such as PFOA and PFOS, through processes including defluorination, oxidation, decarbonylation, and desulfonamidation. Enzymes present in organisms, such as oxidoreductases and transferases, serve as the primary mediators of PFAS biotransformation [39]. The biotransformation of PFASs precursors during the biological treatment stage leads to an increase in PFASs concentrations, which underestimates the actual removal performance of conventional treatment processes and elevates the ecological risk of the effluent. The magnetic coagulation process in Phase II effectively removed these transformation products, indicating that advanced physicochemical treatment is indispensable for the simultaneous control of both legacy and newly formed PFASs in wastewater treatment systems.

In terms of removal pathways, long-chain PFASs (e.g., PFTrDA) in Phase I were mainly removed by adsorption onto activated sludge, relying on their lipid-water partition coefficients and strong adsorption affinity to sludge [40]. Short-chain PFASs (e.g., PFBS) and emerging alternatives (e.g., HFPO-DA) in Phase II were primarily removed via the synergistic effect of biofilm adsorption and magnetic coagulation precipitation (Fig. 6).

4. Conclusions

This study systematically investigated the occurrence, removal, and sludge enrichment characteristics of PFASs in two distinct wastewater treatment processes under identical influent conditions to avoid interference from influent PFAS variability. Short-chain PFASs dominated in the influent, accounting for approximately 60.8%, indicating the increasing application of short-chain PFASs and their alternatives in local domestic sewage.

Comparative results demonstrated that Phase II exhibited superior overall PFASs removal (58.9%) compared with Phase I (53.6%), especially under high proportions of short-chain PFASs. Precursor transformation to PFOA, PFNA, PFHxDA and other products occurred in the biological unit of Phase II, while the subsequent magnetic coagulation-high-density sedimentation tank effectively removed these transformation products, confirming its key role in PFAS control.

Sludge analysis revealed significantly higher total PFAS enrichment in Phase II (over 130% of Phase I), with a more diverse profile including increased short-chain PFASs (especially PFBS) and strong accumulation of long-chain congeners and emerging alternatives. In contrast, Phase I sludge was dominated by long-chain PFASs with minimal short-chain enrichment.

These differences were primarily attributed to the distinct microbial community structures and sludge properties between the two processes. The stronger adsorption capacity of Phase II sludge facilitated greater uptake of short-chain and alternative PFASs, which are generally recalcitrant in conventional wastewater treatment systems. By contrast, long-chain PFASs preferentially accumulated in Phase I sludge due to their higher hydrophobicity, which is consistent with well-established adsorption mechanisms. Nonetheless, the long-term stability and potential re-release of PFASs from sludge were not evaluated in this study.

Overall, process configuration significantly determines the fate, removal, and sludge enrichment of both legacy and emerging PFASs. These findings offer critical implications for optimizing advanced treatment processes to strengthen PFAS control in full-scale wastewater treatment plants. Further research should focus on long-term sludge risk assessment and key operational parameter optimization toward practical engineering applications.

Supplementary Information

eer-2026-109-Supplementary.pdf

Notes

Acknowledgements

Authors wish to thank key projects of Beijing Polytechnic University (2024X007-KXZ) and plan for outstanding young talent (2023R008-JFQB) for financial support.

Conflicts of Interest

The authors declare that they have no conflict of interest.

Author Contributions

R.Y. (Associate Professor) conducted all the experiments and wrote the manuscript. R.S. (Professor) wrote and revised the manuscript. P.L. (Professor) revised the manuscript.

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