Some Possible Process Configurations for Modern Wastewater Treatment Plants for Per- and Polyfluoroalkyl Substances (PFASs) Removal

Abstract

Per- and polyfluoroalkyl substances (PFASs) have been detected in the influent, effluent, and sludge/biosolids of wastewater treatment plants (WWTPs) globally. Due to their potential to bioaccumulate in humans, wildlife, and the environment over time owing to their seriously persistent nature and/or strong C-F bonds, PFASs can cause public health concerns. Conventional processes in full-scale WWTPs are usually inefficient in PFASs removal from wastewater and sludge, and advanced treatment technologies are needed for PFASs removal. This study intends to briefly (i) summarize the technologies for PFASs remediation in wastewater and sludge; (ii) review PFASs removal in full-scale WWTPs; (iii) discuss some possible theoretical configurations for the wastewater processing train of modern WWTPs for PFASs remediation; and finally (iv) provide future directions. Further research regarding the techno-economic assessment and optimization of treatment technologies in removing PFASs (especially short-chain PFASs) from real wastewater as well as the performance of full-scale WWTPs consisting of advanced innovative efficient treatment technologies for PFASs removal and associated costs (i.e., construction, operation, maintenance, chemical, energy, and amortization) is still required.

1. Introduction

Per- and polyfluoroalkyl substances (PFASs) represent a large group (i.e., over 4000 [1,2] or more than 12,000 [3]) of manufactured organofluorine chemical compounds which have been utilized in different industries and consumer products such as aqueous film-forming foams, household products, and food packaging, among others [1,4,5,6] since the 1940s [1,2,7]. The terminology and classification of PFASs have been summarized/discussed by Buck et al. [4]. Releasing PFASs into the environment can result in water and soil contamination [7,8,9]. The United States Environmental Protection Agency (US EPA) has classified PFASs as “emerging contaminants” or “contaminants of emerging concern” because of their environmental impacts [9,10]. Due to their potential to bioaccumulate in humans, wildlife, and the environment over time owing to their seriously persistent nature and/or strong C-F bonds, PFASs can cause public health concerns [1,6,9,11]. For instance, it has been reported that exposure to certain levels of PFASs may lead to high cholesterol levels, thyroid disease, pregnancy-induced hypertension, high risk of some cancers (e.g., kidney, testicular), etc. [1,2,5,7].

PFASs have been widely detected in the aquatic environment (e.g., groundwater, surface water, drinking water, and wastewater) as well as sludge [1,2,12,13,14,15,16]. Schaefer et al. [16] reported 98 ± 28 ng/L, 80 ± 24 ng/L, and 160,000 ± 46,000 ng/kg (dry weight basis) as means of the sums of detected, quantifiable PFASs concentrations for the influent, effluent, and biosolids (respectively) of 38 United States wastewater treatment plants (WWTPs) [16]. Fredriksson et al. [15] reported the total levels of 79 measured PFASs in sludge from four WWTPs in Sweden in the range of 50 to 1124 ng/g (dry weight basis) [15].

The global occurrence and distribution of PFASs in water and wastewater were reviewed by Kurwadkar et al. [14], and detection of PFASs in the environment was reported despite their being phased out [14]. The occurrence, transformation, fate, and migration of PFASs in water and wastewater as well as remediation technologies have been discussed by Vo et al. [1]. Araújo et al. [17] reviewed the detection and treatment technologies of PFASs in wastewater [17]. Lenka et al. [2] reviewed the fate of PFASs in WWTPs and reported up to 1000 ng/L and 15 - >1500 ng/L of short- and long-chain perfluoroalkyl acids (PFAAs) in the influents and effluents of WWTPs, respectively [2]. Zhou et al. [18] surveyed occurrence, fate, and remediation techniques for PFASs in biosolids and sludge from WWTPs and reported concentrations of PFASs in the range of hundreds of ng/g (dry weight) in developed countries [18]. Wanninayake [19] compared PFASs remediation technologies in terms of simplicity, efficiency, energy consumption, cost, and sustainability and reported on techniques including advanced oxidation processes (AOPs), sonochemical, and plasma as effective approaches [19]. Lu et al. [5] reviewed various synergetic technologies and treatment strategies for the remediation of PFASs and suggested a new system consisting of nanofiltration (NF), electrochemical anodic oxidation, and electro-Fenton degradation as an optimal solution for PFASs remediation [5].

Conventional full-scale WWTPs are typically inefficient in PFASs removal from wastewater and sludge. For PFASs removal, there may be a need to modify/upgrade these WWTPs by applying advanced treatment technologies [20]. In recent years, the focus of most studies has been on the performance/efficiency of different advanced treatment techniques in PFASs removal from wastewater and sludge at lab and pilot scales, and full-scale WWTP studies and integrating advanced treatment technologies into full-scale WWTPs needs further attention. Thus, the main aim of this study is to briefly review PFASs removal in full-scale WWTPs and explore possible WWTP designs/configurations for PFASs removal.

2. Methodology

To conduct literature review and analysis focusing on the selected research topic, this study used the most relevant identified/collected papers from different databases (e.g., Google Scholar, Scopus, ResearchGate, Web of Science, etc.). Based on synthesis and analysis of the available information/data in collected sources, the study (i) summarizes the technologies for PFASs remediation in wastewater and sludge; (ii) reviews PFASs removal in full-scale WWTPs; (iii) discusses some possible theoretical designs/configurations for the wastewater processing train of modern WWTPs for PFASs remediation; and finally, (iv) provides conclusions and future perspectives.

3. A Summary of Treatment Technologies for PFASs in Wastewater and Sludge

Several treatment technologies are available to remove PFASs from wastewater and sludge. A summary of treatment technologies reported for PFASs remediation in wastewater and sludge at the laboratory to the full scale is listed in

Table 1. The classification of technologies for PFASs removal from wastewater and sludge.

Media	Treatment Type	Treatment Technologies	References
Wastewater	Separation/concentration	- Adsorption (using natural adsorbents (e.g., biomaterials and minerals), activated carbon adsorbents, ion exchange resins or polymers, and nanomaterials) - Membrane processes (nanofiltration and reverse osmosis) - Ozofractionation and foam fractionation	Vo et al. [1] and Wanninayake [19]
	Destruction	- Advanced oxidation processes - Advanced reduction processes - Sonochemical degradation - Plasma - Biodegradation
Sludge	Physical	- Adsorption	Zhou et al. [18]
	Chemical	- Sonochemical degradation - Pyrolysis - Hydrothermal liquefaction
	Biological	- Aerobic digestion - Anaerobic digestion

. The technologies for PFASs remediation in wastewater can be classified into (i) separation/concentration technologies and (ii) destruction technologies [1,19]. The first-category technologies can be applied to separate and concentrate PFAS to condensed concentration; meanwhile, the second-category technologies can be used to defluorinate the condensed PFAS solute [1]. It is necessary to note that advanced treatment technologies (e.g., AOPs, membrane processes, etc.) are more effective than conventional technologies in PFASs removal from water/wastewater [2,19]. However, the required extreme operating conditions and high energy and chemical uses of most of these technologies increase their cost and difficulty, which leads to reductions in their practicability and applicability in full-scale applications [5]. It is obvious that simplicity, efficiency, energy consumption, cost, and sustainability are important factors in treatment technology selection. Wanninayake [19] reported techniques such as AOPs, sonochemical, and plasma as effective approaches for PFASs removal [19]. Also, Nzeribe et al. [21] compared treatment technologies in terms of defluorination yield, PFASs treated range, energy use, and cost, which reported electrochemical oxidation > advanced reduction processes (ARPs) > plasma > sonolysis > heat-activated persulfate > photochemical oxidation [21]. Although some destruction technology such as plasma can be very effective in PFASs removal from water/wastewater, it is a high-cost technique and not yet ready for full-scale application in the marketplace [1].

The technologies to remediate PFASs in sludge may be classified into (i) physical treatment, such as adsorption; (ii) chemical treatment, including sonochemical degradation, pyrolysis, and hydrothermal liquefaction; and (iii) biological treatment, including aerobic biodegradation and anaerobic biodegradation. Zhou et al. [18] reported that biological treatment processes can be considered high-potential technologies for PFASs remediation due to their low-cost and eco-friendly nature. However, physical/chemical treatment technologies use high amounts of energy or are associated with further challenges in terms of PFASs contamination and disposal [18].

4. PFASs Removal in Full-Scale Wastewater Treatment Plants

PFASs removal in full-scale WWTPs has been reported on/discussed in various studies (

Table 2. A summary of PFASs removal studies on some full-scale wastewater treatment plants.

WWTP Processes	Type	PFASs Removal Results	References
Wastewater processing train: grids, screening, aerated sand traps, primary sedimentation with pre-precipitation of ferric chloride addition, biological treatment with activated sludge in aerated and anoxic basins, and chemical treatment with ferric chloride Sludge processing train: thickening, anaerobic digestion, and dewatering (by centrifuges)	Domestic (may receive a mixture of domestic, industrial, and other commercial wastewater)	Average removal efficiency of 10 ± 68% for individual PFAS	Gobelius et al. [20]
Wastewater processing train: bar rack, grit chamber, primary clarifiers, aeration basins, secondary clarifiers, and chlorine contact tanks Sludge processing train: gravity thickener, dewatering by centrifuge, sewage sludge incinerator, ventury/tray scrubber, and wet ash lagoon	Domestic	PFASs destruction and removal efficiency of 51% at the sewage sludge incinerator	Seay et al. [27]
Screening, grit removal, primary clarifier, biological treatment (anaerobic/anoxic/aerobic), secondary clarifier, ultrafiltration/granular media filter, pre-ozonation, biological activated carbon filter, and post-ozonation	Domestic	191.3% increase of perfluoroalkyl acids (PFAAs) after biological treatment	Kim et al. [26]
Screening, grit removal, primary clarifier, biological treatment (anoxic/anaerobic/aerobic), aerobic membrane bioreactors, and UVdisinfection	Domestic	185.1% increase of PFAAs after biological treatment	Kim et al. [26]
Wastewater processing train: screening, grit removal, primary clarifier, aeration tank, secondary clarifier, and UV disinfection Sludge processing train: sludge storage tanks and centrifuges	Domestic	Total PFASs of 97 ng/L in final effluent and total PFASs of 104 µg/kg in cake	Bogdan and Curran [25]
Screening, grit removal, primary clarifier, activated sludge aeration with nitrogen removal, secondary clarifier, dual-media pressure filter, and chlorine contact tanks	Domestic	Total PFASs of 78–136 ng/L in final effluent	Bogdan and Curran [25]
Primary sedimentation, activated sludge aeration with NDH process, secondary sedimentation, insert media gravity filtration, and chlorine contact tanks	Domestic	Total PFASs of 56–96 ng/L in final effluent	Bogdan and Curran [25]
Wastewater processing train: equalization tanks, screening, grit removal, sequencing batch reactors, post-equalization, disk filters, and UVdisinfection Sludge processing train: sludge storage tanks, rotary drum thickener, and thickened sludge tanks	Domestic	Total PFASs of 44 ng/L in final effluent and total PFASs of 10 µg/kg in thickened sludge	Bogdan and Curran [25]
Wastewater processing train: screening, grit removal, primary clarification, trickling filters, SCT tanks, secondary clarification, nitrifying trickling filters, denitrification filters, and disinfection Sludge processing train: dissolved aeration flotation tanks, anaerobic digestion, sludge storage tanks, and centrifuges	Domestic	Total PFASs of 80 ng/L in final effluent and total PFASs of 65–66 µg/kg in cake	Bogdan and Curran [25]
Coarse and fine grid, aerated grit chamber, first sedimentation, anaerobic–anoxic–oxic (A2O), second sedimentation, flocculation, third sedimentation, filtration, and UVdisinfection	Domestic	Removal efficiency of 69% for PFBA, 54% for PFBS, and 43% for all the 12 PFAAs; while PFOA and PFOS all increased	Wang et al. [23]
Coarse and fine grid, aerated grit chamber, first sedimentation, A2O (enhanced by adding carbon), second sedimentation, flocculation, third sedimentation, filtration, and UVdisinfection	Domestic	No removal efficiency for ∑PFAAs	Wang et al. [23]
Coarse grid, fine grid, main reaction tank and first sedimentation, upflow hydrolysis tank, A2O, moving bed biofilm reactor, second sedimentation, ozone contact tank, biological aerated filter, upflow sludge bed, filtration, and UVdisinfection	Industrial	Removal efficiency of 55% for ∑PFAAs, including 45% for PFBA, 58% for PFOA, 65% for PFBS, and 93% for PFOS	Wang et al. [23]

Abbreviations: PFOA: perfluorooctanoic acid, PFOS: perfluorooctane sulfonate, PFBA: perfluorobutanoic acid, PFBS: perfluorobutane sulfonic acid, PFAAs: perfluoalkyl acids.

) [20,22,23,24,25,26,27,28]. Generally, the removal efficiencies and fate of PFASs in WWTPs may be influenced by influent wastewater source (i.e., industrial, domestic, urban runoff and/or agricultural) and characteristics (e.g., PFASs’ physicochemical characteristics, concentrations), design and/or type of applied treatment techniques, and process operating conditions (including temperature, flow rate, hydraulic and sludge retention time, mixed liquor suspended solids, etc.) [2,24]. According to Kim et al. [26], the transformation/conversion of precursors into short-chain PFAAs by biodegradation and the partitioning of long-chain PFASs into sludge streams can be important factors specifying PFASs fate in WWTPs [26]. Within the WWTPs, short-chain PFASs have a tendency to remain in aqueous streams, whereas long-chain PFASs dominate in sludge/biosolids [20]. Using mass flow analysis, Schaefer et al. [16] concluded that majority of PFASs exited WWTPs via the aqueous effluent in comparison with the biosolids stream [16].

Chen et al. [22] investigated the performance of treatment technologies in removing PFASs from wastewater and reported 32.2, 17.5, and −1.49% as annual average PFASs removal efficiencies for the cyclic activated sludge system, orbal oxidation ditch, and anaerobic–anoxic–oxic (A²O) processes, respectively [22]. Also, Kibambe et al. [24] reported 84, 94, and 80% as the highest removal efficiencies for PFOS in the activated sludge unit of the Daspoort WWTP, the activated sludge unit of the Zeekoegat WWTP, and the anaerobic pond of the Phola WWTP, respectively [24]. Ilieva et al. [28] studied the removal of 13 PFASs, analyzed mainly in 161 worldwide WWTPs, including treatment technologies such as activated sludge (e.g., conventional activated sludge, constant sequencing batch reactor), oxidation ditch, lagoon (e.g., aerated, facultative), membrane bioreactor, biofilm/attached growth (e.g., trickling filter, moving bed biofilm reactor, aerated biofilter), biological nutrient removal processes (e.g., A²O, modified Ludzack-Ettinger), combinations of two or more biological treatments (e.g., trickling filter + conventional activated sludge, A²O + oxidation ditch, oxidation ditch + bioreactor), and biological + tertiary/advanced physical/chemical treatment (conventional activated sludge + mixed media filter, trickling filter + ultrafiltration membrane, conventional activated sludge + dissolved air flotation, biological nutrient removal + chemical phosphorus removal with alum, conventional activated sludge + sand filter, A²O + cloth media filter, A²O + deep bed filtration). Among the studied treatment techniques, membrane bioreactor, biofilm processes, and combinations of two or more biological treatments were reported to have the highest PFASs removal efficiencies; however, the addition of the studied tertiary treatment was reported to have a non-beneficial effect on PFASs removal. It is necessary to note that membrane processes such as NF and reverse osmosis (RO) are effective in PFASs removal; however, fouling, cost, and maintenance issues present a barrier to their extensive application in full-scale WWTPs worldwide [28].

It is necessary to techno-economically assess/evaluate and optimize new emerging and available advanced technologies for PFASs remediation in real wastewater for deployment in the marketplace in the future. Also, there is a need to focus on full-scale WWTP studies and integrate advanced efficient treatment technologies into full-scale WWTPs for PFASs removal.

5. Wastewater Treatment Plant Configurations for PFASs Removal

The wastewater processing train in a municipal WWTP may consist of preliminary, primary, secondary, and tertiary (advanced or polishing) treatment steps, whereas the sludge processing train may include sludge thickening, sludge digestion, sludge dewatering, and sludge disposal (Figure 1) [29,30,31,32,33]. In a WWTP, a tertiary/advanced treatment step may be needed to remove non-biodegradable substances as well as persistent contaminants such as PFASs from wastewater (which secondary treatment is not effective in cleaning up) to produce treated effluent clean enough to meet strict discharge criteria or for water reuse. For instance, Rizzo et al. [34] discussed different technologies and options of treatment trains for urban wastewater reuse for crop irrigation, especially to remove contaminants of emerging concern, including antibiotics, antibiotic resistant bacteria and antibiotic resistance genes [34]. The concept of their study is useful for design purposes if one can identify efficient and cost-effective remediation technologies for PFASs in wastewater. A literature review was conducted to identify reported effective remediation technologies for PFASs in wastewater to discuss some possible configurations for the wastewater processing train of modern WWTPs for PFASs removal in the future (when PFASs removal is needed).

As mentioned before, some secondary treatment techniques (including membrane bioreactor, biofilm processes, and combination of two or more biological treatments) (Note that according to [28], none of these treatment types actually resulted in substantial PFASs removal) [28] and several established and emerging tertiary treatment technologies (including adsorption, membrane processes (NF and RO), ozofractionation and foam fractionation [19], ARPs, AOPs, sonochemical degradation, and plasma) [1,19] have been reported to be effective in PFASs removal from wastewater. It is necessary to note that some of these advanced technologies, such as plasma, are high-cost techniques and not yet ready for full-scale application in the marketplace [1]. On the other hand, some of these technologies, such as membrane processes (NF and RO), which may be applied in the marketplace; factors such as fouling, cost, and maintenance issues present a barrier to their extensive application in full-scale WWTPs worldwide [28].

Some possible theoretical designs/configurations for the wastewater processing train of modern WWTPs (Figure 2A–C) are shown here as potentially applying some advanced technologies for PFASs removal in the future (when PFASs removal is needed in WWTPs). The first configuration (Figure 2A) for the wastewater processing train of a WWTP may consist of (i) a preliminary treatment step, including screening and grit removal; (ii) a primary treatment step using primary clarifier; (iii) a secondary treatment step using membrane bioreactor or biofilm processes (e.g., trickling filter, aerated biofilter, moving bed biofilm reactor) or a combination of two biological treatment processes (e.g., trickling filter + conventional activated sludge, A²O + oxidation ditch) (Note that according to [28], none of these treatment types resulted in substantial PFASs removal; thus further investigations are still needed) [28]; and (iv) a tertiary/advanced treatment step including granular media filtration (filtration may not be necessary if a membrane bioreactor is used as a secondary treatment, which has been shown with dotted lines in Figure 2A), adsorption (i.e., GAC adsorption), and disinfection (i.e., UV-disinfection). Adsorption process (e.g., using activated carbon (either granular or powdered) or anion-exchange resins) has shown promising results in treating PFASs-contaminated water; however, this technology requires appropriate regeneration (chemical or thermal) of spent adsorbent and/or disposal which is a major drawback. Gagliano et al. [13] reviewed PFASs removal from water by adsorption process and mainly concluded the following: (i) the adsorption capacity of long-chain PFASs is greater than that of short-chain PFASs, which implies a challenge for removal of short-chain PFASs from water; (ii) anion-exchange resins are more effective than activated carbon in removal of both long- and short-chain PFASs from water; and (iii) the effect of co-existing organic matter during adsorption process (e.g., negative effect on short-chain PFASs adsorption) should be considered [13]. Also, Ross et al. [35], in their review of PFASs remediation technologies, concluded that GAC can be an effective adsorbent to remove long-chain PFAAs; however, its performance for short-chain PFAAs and precursors is not good (i.e., less effective). Meanwhile, anion-exchange resins may treat both long- and short-chain PFAAs; however, they struggle to remove the shortest-chain PFAAs [35].

In the second configuration (Figure 2B), granular media filtration (filtration may not be necessary if a membrane bioreactor is used as a secondary treatment, which has been shown with dotted lines in Figure 2B), hybrid AOPs, and biological filtration (e.g., biological activated carbon filtration) may be applied as selected technologies in the tertiary/advanced treatment step. According to reviews conducted by Merino et al. [36] and Ahmed et al. [37], several AOPs, including electrochemical oxidation, photocatalysis, activated persulfate oxidation, and UV-induced oxidation, may be successful in degrading PFASs, especially PFOA and PFOS; however, further research on the influence of co-contaminants, PFASs mixtures, and environmental matrices as well as research in real WWTPs is required [36,37]. Mojiri et al. [38] reported that integrated/combined AOPs (e.g., UV/sulfate radical as a hybrid reactor) can effectively degrade and remove PFASs [38]. After AOPs, a biological treatment unit (e.g., biological sand or activated carbon filtration) is usually needed to eliminate biodegradable oxidation by-products and transformation products [34]. Zhong et al. [39] reported that the ozone–biological activated carbon process is an effective treatment technique for PFASs removal in drinking water treatment plants [39]. In treating municipal wastewater effluent, Vatankhah et al. [40] reported that a combined ozone–biologically active filtration process improved PFAA removal by GAC compared to standalone ozone and biologically active filtration [40]. Disinfection may not be necessary because AOPs can be effective for pathogens inactivation/elimination, as shown in Chen et al. [41]’s review of the features and mechanisms of various AOPs (i.e., Fenton and Fenton-like processes, electrochemical, photocatalysis, ozonation, sonolysis, and persulfate-based AOPs) for water disinfection [41].

In the third configuration (Figure 2C), filtration (granular or microfiltration or ultrafiltration), membrane (NF or RO), electrochemical oxidation for treatment of membrane concentrate, and disinfection (i.e., UV disinfection) may be applied as selected technologies in the tertiary/advanced treatment step. Pre-treatment by filtration (granular or microfiltration or ultrafiltration) (which is shown with dotted lines in Figure 2C) may be applied to eliminate suspended solids for controlling membrane fouling; note that microfiltration and ultrafiltration are appropriate pre-treatment techniques before NF or RO. When this case (microfiltration or ultrafiltration as pre-treatment plus NF or RO step) is used, disinfection is not necessary [34]; also, electrochemical oxidation effluent may not need disinfection (as electrochemical oxidation can be a promising approach to disinfection [42]); however, disinfection may be applied under strict regulation or as a safety measure for water reuse (which is shown with dotted lines in Figure 2C). A suitable treatment technique for generated concentrate/retentate by membrane (NF or RO) is required, for which electrochemical oxidation has been suggested; however, its wide-scale application is still challenging due to its high energy use and high capital cost [43,44]. High removal efficiencies for PFASs from water (on average 99%) [45] and industrial process waters (99.6%) [46] by NF (Dow Filmtec NF270 membrane) have been reported. Tow et al. [47] reviewed available technologies for treating/managing PFASs in membrane concentrates [47]. Soriano et al. [46] used a system combining NF (Dow Filmtec NF270 membrane) followed by electrochemical oxidation with boron-doped diamond electrodes to effectively remove perfluorohexanoic acid (PFHxA) from industrial process waters. A volume reduction factor of 5 for NF and PFHxA degradation rate of 98% and total organic carbon mineralization of more than 95% by electrochemical oxidation were reported [46]. Veciana et al. [44] reviewed the removal of PFASs from contaminated water and wastewater using electrochemical oxidation and concluded that this process can effectively treat long-chain PFASs, while short-chain PFASs are more recalcitrant to electrochemical oxidation. Also, generation/formation of short-chain PFASs during treatment/degradation and/or presence of precursors should be considered [44]. Based on the quality of electrochemical oxidation effluent, it may be combined with the membrane permeate (to send to the next unit); otherwise, it may be recycled/recirculated for further treatment (which is shown with dotted lines in Figure 2C). It is necessary to note that the full-scale application of electrochemical oxidation is still challenging [43,44] and future studies should focus on the techno-economic evaluation/optimization of this technology (to reduce energy use and capital cost) for PFASs removal from real wastewater or membrane concentrate streams at full scale. In other words, finding an efficient and cost-effective technology for managing PFASs in real membrane concentrates is still of special interest.

It is obvious that each treatment train can provide a certain degree of PFASs removal. The feasibility, efficiency/performance, and costs (i.e., construction, chemical, energy, operation, maintenance, and amortization) [30,31,32,33] of these theoretical designs/configurations should be evaluated/assessed and compared before selection/implementation. Also, there is still the need to develop and suggest innovative designs/configurations for WWTPs for effective removal of PFASs.

As mentioned before, some treatment technologies, including adsorption, sonochemical degradation, pyrolysis, hydrothermal liquefaction, and biodegradation (aerobic and anaerobic), have been reported to remediate PFASs in sludge. As physical/chemical treatment technologies use high amounts of energy or are associated with further challenges with PFASs contamination and disposal, biological treatment processes may be considered high-potential technologies for PFASs remediation in sludge due to their low-cost and eco-friendly nature [18].

Figure 2. Simplified theoretical configurations for the wastewater processing train of modern wastewater treatment plants for PFASs remediation. Notes: (i) For more information on the process justification and dotted lines, the readers are encouraged to see the text. (ii) The concepts of the references [1,34] for the sequence of processes in Figures (A–C); the results/conclusions of the reference [28] for selecting the type of biological processes in Figures (A–C); the concept of reference [46] for selecting the technology for treating membrane concentrate in Figure (C); etc. were applied. (iii) Each treatment train can provide a certain degree of PFASs removal. (iv) The performance and techno-economic assessments and optimization are required before implementation.

Figure 2. Simplified theoretical configurations for the wastewater processing train of modern wastewater treatment plants for PFASs remediation. Notes: (i) For more information on the process justification and dotted lines, the readers are encouraged to see the text. (ii) The concepts of the references [1,34] for the sequence of processes in Figures (A–C); the results/conclusions of the reference [28] for selecting the type of biological processes in Figures (A–C); the concept of reference [46] for selecting the technology for treating membrane concentrate in Figure (C); etc. were applied. (iii) Each treatment train can provide a certain degree of PFASs removal. (iv) The performance and techno-economic assessments and optimization are required before implementation.

6. Conclusions and Future Perspectives

This study briefly reviewed PFASs removal in full-scale WWTPs as well as some possible WWTP designs/configurations for PFASs remediation. Major findings of this study include but are not limited to:

• The removal efficiencies and fate of PFASs in WWTPs may be influenced by influent wastewater source (i.e., industrial, domestic, urban runoff and/or agricultural) and characteristics (e.g., PFASs’ physicochemical characteristics, concentrations), design and/or type of applied treatment techniques, and process operating conditions (including temperature, flow rate, hydraulic and sludge retention time, mixed liquor suspended solids, etc.).
• Within the WWTPs, short-chain PFASs have a tendency to remain in aqueous streams, whereas long-chain PFASs dominate in sludge/biosolids.
• Biological treatment processes may be considered as high-potential technologies for PFASs remediation in sludge due to their low-cost and eco-friendly nature.
• Three theoretical configurations for the wastewater processing train of modern WWTPs (Figure 2A–C) were presented to remove PFASs. The tertiary/advanced treatment steps were in configuration A (filtration, GAC adsorption, and disinfection), configuration B (filtration, hybrid AOPs, and biological filtration), and configuration C (filtration, membrane (NF or RO), electrochemical oxidation for treatment of membrane concentrate, and disinfection (if applicable)). Note that each treatment train can provide a certain degree of PFASs removal, and performance and techno-economic assessments and optimization are required before selection/implementation.

It is suggested to consider a continued periodic monitoring of PFASs in WWTPs to track their level and class [48]. There is still the need to further study (i) the fate, transformation, and distribution of historical and emerging PFASs [26] as well as precursors [16] in WWTPs; (ii) the performance of specific microorganisms capable of degrading PFASs [2], biodegradation mechanisms and optimization [49], and innovative biological/combined (with other physicochemical processes) techniques for complete PFASs degradation [50]; (iii) the remediation technologies for short-chain PFASs and precursors [1]; and (iv) evaluation and optimization of treatment technologies in removing PFASs from real wastewater containing organic matter and inorganic ions [2].

All emerging technologies should be techno-economically assessed and optimized at pilot to full scale for deployment in the marketplace in the future. Also, suggesting innovative configurations for WWTPs for effective removal of PFASs is of interest. Furthermore, the efficiency/performance of full-scale WWTPs consisting of advanced treatment technologies for PFASs removal and associated costs (i.e., construction, operation, maintenance, chemical, energy, and amortization) should be assessed.