PFAS Ex Situ Water Treatment
Well-developed ex situ treatment technologies applicable to treatment of perfluoroalkyl and polyfluoroalkyl substances (PFAS) in drinking water and non-potable groundwater include membrane filtration (reverse osmosis or RO and nanofiltration or NF), activated carbon adsorption (granular and powdered), and anion exchange. However, these technologies are less demonstrated for removal of PFAS from more complex matrices such as wastewater and leachate. There are also a variety of separation and destructive technologies in various stages of development. Some of these processes may also be applicable to more complex matrices including wastewater and landfill leachate.
- Perfluoroalkyl and Polyfluoroalkyl Substances (PFAS)
- PFAS Soil Remediation Technologies
- PFAS Sources
- PFAS Transport and Fate
- PFAS Treatment by Electrical Discharge Plasma
- Supercritical Water Oxidation (SCWO)
Established PFAS Treatment Technologies
Three technologies are well demonstrated for removal of PFAS from drinking water and non-potable groundwater (as described below):
- membrane filtration including reverse osmosis (RO) and nanofiltration (NF)
- granular activated carbon (GAC) and powdered activated carbon (PAC) adsorption
- anion exchange (IX)
However, these technologies are less demonstrated for removal of PFAS from more complex matrices such as wastewater and leachate. Site-specific considerations that affect the selection of optimum treatment technologies for a given site include water chemistry, required flow rate, treatment criteria, waste residual generation, residual disposal options, and operational complexity. Treatability studies with site water are highly recommended because every site has different factors that may affect engineering design for these technologies.
Given their ability to remove dissolved contaminants at a molecular size level, RO and some NF membranes can be highly effective for PFAS removal. For RO systems (Figure 1), several studies have demonstrated effective removal of perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS) (see PFAS for nomenclature) from drinking water with removal rates well above 90%. RO potable water reuse treatment systems implemented in California have also demonstrated effective PFOS and PFOA removal as reported by the Water Research Foundation (WRF). Analysis of permeate at both sites referenced by the WRF confirmed that short and long chain PFAS concentrations in the treated water were reduced to levels below test method reporting limits.
Full-scale studies using larger effective pore size NF membranes for PFAS removal are limited in number but are promising since NF systems are somewhat less costly than RO and may be nearly as effective in removing PFAS. Recent laboratory or pilot studies have shown good performance of NF membranes.
Although membrane RO and NF processes are generally capable of providing uniform removal rates relative to short and long chain PFAS compounds (see PFAS for nomenclature), other aspects of these treatment technologies are more challenging:
- Membranes must be flushed and cleaned periodically, such that overall water recovery rates (process water volumes consumed, wasted, and lost vs. treated water volumes produced) are much lower than those for GAC and IX processes. Membrane fouling can be slowed or avoided depending on operating conditions, membrane modifications, and feed modifications. Typically, 70-90% of the water supplied into a membrane RO process is recoverable as treated water. The remaining 10-30% is reject containing approximately 4 to 8 times the initial PFAS concentration (depending on recovery rate).
- These cleaning and flushing processes create a continuous liquid waste stream, which periodically includes harsh membrane cleaning chemicals as well as a continuous flow of concentrated membrane reject chemicals (i.e., PFAS) that must be properly managed and disposed of. Management often includes further treatment to remove PFAS from the liquid waste.
- RO and NF systems are inherently more expensive and complicated systems to implement, operate, and maintain compared to adsorption processes. Treatment system operator certification and process monitoring requirements are correspondingly markedly higher for RO and NF than they are for GAC and IX.
- Water feed pressures required to drive flow through membrane RO and NF processes are considerably higher than those involved with GAC and IX processes. This results in reduced process efficiency and higher pumping and electrical operating costs.
- Membrane systems can also be subject to issues with irreversible membrane fouling, clogging, and scaling or other physical membrane damage and failures. Additional water pretreatment and higher levels of monitoring and maintenance are then required, further adding to the higher costs of such systems.
Activated Carbon Adsorption
Activated carbon is a form of carbon processed to have small pores that increase the surface area available for adsorption of constituents from water. Activated carbon is derived from many source materials, including coconut shells, wood, lignite, and bituminous coal. Different types of activated carbon base materials have varied adsorption characteristics such that some may be better suited to removing certain contaminant compounds than others. Results from laboratory testing, pilot evaluations, and full-scale system operations suggest that bituminous coal-based GAC is generally the best performing carbon for PFAS removal.
The removal efficiency of individual PFAS compounds using GAC is a function of both the PFAS functional group (carboxylic acid versus sulfonic acid) and also the perfluoro-carbon chain length(see PFAS for nomenclature):
- perfluoro-sulfonate acids (PFSAs) are more efficiently removed than perfluoro-carboxylic acids (PFCAs) of the same chain length
- long chain compounds of the same functional group are removed better than the shorter chains
Activated carbon may be applied in drinking water systems as GAC or PAC. GAC has larger granules and is reusable, while PAC has much smaller granules and is not typically reused. PAC has most often been used as a temporary treatment because costs associated with disposal and replacement of the used PAC tend to preclude using it for long-term treatment. A typical GAC installation for a private drinking water well is shown in Figure 2. Contrary to PAC, GAC used to treat PFAS can be reactivated by the manufacturer, driving the PFAS from the GAC and into off-gas. The extracted gas is then treated with thermal oxidation (temperatures often 1200°C to 1400°C). The reactivated GAC is then brought back to the site and reused. Thus, GAC can ultimately be a destructive treatment technology.
Anion exchange has also been demonstrated for the adsorption of PFAS, and published results note higher sorption per pound than GAC. The higher capacity is believed to be due to combined hydrophobic and ion exchange adsorption mechanisms, whereas GAC mainly relies on hydrophobic attraction. Anion exchange resins can be highly selective, or they can also remove other contaminants based on design requirements and water chemistry. Resins have greater affinity for PFAS subgroup PFSA than for PFCA, and affinity increases with carbon chain length. Anion exchange resins are a viable alternative to GAC for ex situ treatment of PFAS anions, and several venders sell resins capable of removing PFAS. Resins available for treating PFAS include regenerable resins that can be used multiple times (Figure 3) and single-use resins that must be disposed or destroyed after use. Regenerable resins generate a solvent and brine solution, which is distilled to recover the solvent prior to the brine being adsorbed onto a small quantity of GAC or resin for ultimate disposal. This use of one treatment technology (GAC, IX) to support another (RO) is sometimes referred to as a “treatment train” approach. Single-use resins can be more fully exhausted than regenerable resins can and may be a more cost-effective solution for low concentration PFAS contamination, while regenerable resins may be more cost effective for higher concentration contamination.
Developing PFAS Treatment Technologies
|* There are several other destructive technologies such as alternative oxidants, and activation|
methods of oxidants, but for the purpose of this article, the main categories are presented here.
Numerous separation and destructive technologies are in the developmental stages of bench-scale testing or limited field-scale demonstrations. Some of these are listed in Table 1 and defined below.
- Biochar is charcoal produced from cellulosic biomass by pyrolysis in an oxygen-free environment. Without additional chemical or physical treatment, biochar is not quite as effective at adsorbing some contaminants as granular activated carbon (GAC). However, GAC is typically produced from high quality hardwood charcoal or coal while biochar can be made from locally available feedstocks or waste streams such as paper mill waste or agricultural waste. Biochar has a long history of agricultural use as a soil amendment.
- Zeolites are microporous, aluminosilicate minerals such as thomsonite and stilbite which can remove contaminants through adsorbent, molecular sieve, and/or ion exchange interactions. Some types of zeolites occur naturally but many more are synthetically produced for a wide variety of industrial uses.
- Specialty adsorbents such as cross-linked chitosan beads may offer improved adsorption capacity or kinetics, while bench-scale studies suggest TiO2 nanotubes can adsorb PFAS and subsequently serve as a photocatalyst for their destruction.
- Chemical coagulation refers to the addition of a chemical such as alum or ferric chloride to neutralize the unbalanced charges that would otherwise help keep heavy metal ions and colloidal solids in suspension through electrostatic repulsion. Once the electrostatic force is neutralized, particles begin to aggregate into flocs and settle out of suspension. Bench-scale studies suggest that 25% to 30% of the polar molecules perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) could be removed by using approximately twice the usual dose of traditional coagulants. Initial treatment of higher concentrations of polar PFAS by coagulation may prove useful to reduce loading on subsequent removal processes such as activated carbon adsorption or anion exchange which can achieve much lower effluent PFAS concentrations.
- Electrocoagulation neutralizes electrostatic repulsion using DC power rather than chemical coagulants. As current passes between submerged electrodes, metal ions are released from the sacrificial anode that counter the unbalanced charges of very small particles in suspension which can then coagulate into flocs. Some contaminants are captured in the flocs, while others may be removed by ionization, hydrolysis, or attack by free radicals as they move through the applied electric field.
- Foam fractionation is used to separate surface active agents (i.e. surfactants) and hydrophobic particles from water. Many PFAS have been valued commercially and industrially because of their surfactant or hydrophobic properties. In this technique, small air bubbles are introduced at the bottom of a narrow column of contaminated water. As the bubbles rise through the column, surfactants and hydrophobic materials partition to the air/water interface of the bubbles. As these materials accumulate at the interfaces, a PFAS-rich foam develops which accumulates at the top of the column where it can be easily removed and further processed or disposed.
- Electro-oxidation relies on submerged electrodes but uses greater current densities than electrocoagulation with the goal of producing highly reactive hydroxyl radicals (*OH) at the anode surface to destroy PFAS and many other contaminants. A wide variety of anode materials have been studied at the bench scale, each with different PFAS removal efficiencies and durabilities. Very low salinity waters may require addition of salts to have sufficient ion concentrations to support the electrolytic reaction.
- Heat activated persulfate has a long history of use to oxidize a variety of organic contaminants in soil. A bench scale feasibility study simulating in situ treatment of PFAS impacted groundwater found that perfluorooctanoic acid (PFOA) was oxidized within 72 hours at 50°C and that the rate of oxidation increased with temperature. However, perfluorooctanesulfonic acid (PFOS) was reportedly unaffected by this treatment.
- Alkaline peroxone treatment has been used to remove PFOA and PFOS from electronics fabrication industry wastewater in Taiwan. Degradation rates of 85% to 100% were reported using peroxone, a combination of ozone (O3) and peroxide (H2O2), with pH elevated to 11. Best results were obtained by ozonating for 15 minutes followed by pH adjustment and 4 more hours of ozonation.
- Sonolysis of PFAS is believed to occur primarily at the vapor/water interface of cavitation bubbles caused by application of sonic energy to the contaminated water. Higher degradation rates have been reported for the more hydrophobic PFAS which are more likely to partition to the bubble interface. Degradation rates also increase with applied sonic power density. The presence of other organic or inorganic constituents in the water being treated may drastically reduce the rate of PFOS and PFOA destruction. Sonolysis of PFAS may be most effective at pH of 3 to 4 and when using two frequencies of sonic input simultaneously in a single reactor.
- Supercritical Water Oxidation (SCWO) takes advantage of the unique properties of water in the supercritical phase (temperature ≥ 374°C and pressure ≥ 218 atm) which acts like a dense non-polar solvent with the transport qualities of a gas. Because oxygen is fully miscible in supercritical water, organic contaminants can be fully oxidized quickly. The oxidation of organics is an exothermic reaction, and the released heat energy can be harnessed to make a SCWO system self-sustaining after startup, if the influent waste stream is sufficiently concentrated.
- When a normally neutral and non-conductive fluid is heated sufficiently or subjected to a strong enough electromagnetic field, some electrons are stripped from their nucleus creating a highly charged and electrically conductive gas of ions and free electrons known as a plasma. Low temperature plasma treatment relies on a continuous electric discharge (i.e. spark) to create a localized plasma that contaminated water can be circulated through to destroy many organic compounds. One bench scale study reported removing 90% of PFOA after 30 minutes of plasma treatment. In another study, a pilot scale plasma reactor treating moderately to highly PFAS-impacted water samples reportedly reduced PFOS and PFOA concentrations to below the US EPA’s health advisory limits (HALs) in two thirds of the samples with less than one minute of treatment. The most contaminated samples and the most conductive samples required up to 50 minutes of treatment.
The well established processes for removing PFAS from water all produce residuals that require management, and it is likely that newer processes under development will also produce some residuals. Often, it is the residuals that limit the usefulness of the process. For instance, RO and NF may currently provide the most complete treatment of water, but the production of a relatively high volume of PFAS-containing liquid reject (the portion of the liquid that retains the contaminants and is “rejected” from the process) limits their application. Often, a second treatment technology such as an adsorbent is required to support the main technology by concentrating or treating the residuals.
As more testing and operational data on adsorbents are generated, it is becoming evident that no adsorbent technology outperforms the others in all cases. Whether GAC, ion exchange or another technology is the most technically efficient and cost effective long term option for a given site depends on influent water geochemistry and contaminant concentrations, treatment standards, co-contaminants, duration of treatment, and required flow rates. New generation adsorbents are rapidly being introduced into the market at “evaluation scale” which may provide advantages over commercially available adsorbents.
Several newer technologies are being evaluated in the lab and in the field which include electro-oxidation, heat-activated persulfate, sonolysis, electrocoagulation, low temperature plasma, supercritical water oxidation, and foam fractionation. These and other potential treatments for PFAS are still largely in the developmental stage. Several technologies show promise for improved management of PFAS sites. However, it is unlikely that a single technology will be adequate for full remediation at many sites. A multi-technology treatment train approach may be necessary for effective treatment of this complicated group of compounds.
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