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Draft objective for per- and polyfluoroalkyl substances in Canadian drinking water: Treatment considerations

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Municipal water treatment

Typically, treatment efficacy studies are carried out with a limited suite of PFAS at concentrations much higher than those observed in raw and treated drinking waters (Crone et al., 2019). Removal efficacies may differ greatly for PFAS with different physicochemical properties (for example, carbon chain length) when evaluated at concentrations relevant to drinking water. A vulnerability assessment should be undertaken to identify hazards, including potential sources of contamination and susceptibility of the source water to PFAS contamination (Health Canada, 2021b). Thorough characterization of the source water is necessary to evaluate the presence, identity and concentration of any PFAS; this information is required to establish operational conditions and to estimate how long treatment media will last before breakthrough occurs.

The most effective treatment technologies (> 90% removal efficiencies for certain PFAS) are granular activated carbon (GAC), membrane filtration (reverse osmosis [RO] and nanofiltration [NF]) and anion exchange (AIX) (Appleman et al., 2013, 2014; Dickenson and Higgins, 2016; Sanexen, 2022). Generally, the key issues to consider when selecting treatment technologies for PFAS removal are the presence of competing anions and PFAS species, organic matter and the frequency of regeneration or replacement required for the sorptive medium used (Appleman et al., 2013). The effectiveness of drinking water treatment for PFAS removal will depend on several factors, including source water characteristics, concentration and type of PFAS, treatment goals and proper operation of the system at all times.

Common drinking water treatment technologies (for example, coagulation, flocculation and oxidation) are not effective for PFAS removal.

While there are treatment technologies that can effectively remove certain PFAS, no single treatment can remove a wide range of PFAS under all conditions. Each treatment technology has advantages and disadvantages. Achieving the proposed objective may require a treatment train that includes more than 1 technology, or a technology used multiple times in series to treat the suite of PFAS present in the raw water. To ensure continued and effective removal, each facility should establish operational conditions and parameters based on the selected treatment technology/ies and the characteristics of the raw water, including PFAS type, concentration and treatment goals.

Disposal or manipulation of sorptive media, concentrates or residuals is also a consideration when selecting a treatment technology for PFAS removal. Treatment and/or disposal of the spent GAC filtration media and backwash water; the ion exchange resins and regeneration concentrates; and the membrane concentrates and wash water, which contain elevated PFAS concentrations, are major issues to consider in the selection and operation of PFAS treatment technology. For example, spent filtration (such as GAC) and ion-exchange media will require specialized disposal (for example, high-temperature regeneration/destruction) to avoid release of PFAS back into the environment. Similarly, membrane technologies will require treatment and disposal of the concentrate, wash water or residual stream (U.S. EPA, 2022c). The availability of disposal options for treatment residuals (including media) may also limit the selection of a treatment technology. The selection may also be limited due to disposal requirements of the relevant authority.

A limited number of bench-scale studies have evaluated the removal of perfluorocarboxylates and perfluorosulfonates by powdered activated carbon (PAC). Based on those study results, median removal efficiency for individual PFAS by PAC was 64.5% (Sanexen, 2022). Due to inefficiencies, PAC needs to be combined with other treatment technologies to achieve a removal rate of 90% or more. Also, how the settled sludge containing the PFAS-laden PAC will be disposed of needs to be considered.

GAC technology has the most field-relevant data at full- and pilot-scale (Sanexen, 2022), and has proven to effectively remove PFAS from drinking water at relatively low concentrations (Appleman et al., 2014). Additionally, GAC can maintain its performance across a broad range of water chemistries. However, GAC has demonstrated greater affinity for PFAS with chain lengths greater than 6 carbons compared with shorter chain PFAS (Gagliano et al., 2020). In addition, perfluorinated sulfonates are adsorbed more easily by GAC than their carboxylic acid counterparts due to their higher hydrophobicity (Du et al., 2014). As a result, increased frequency of GAC regeneration or replacement will be required when treating certain PFAS (Rodowa et al., 2020). Operational parameters such as GAC type (for example, bituminous coal), bed size and hydraulic loading rate also influence filter run time (Belkouteb et al., 2020).

Anion-exchange resin properties, such as porosity, functional group and polymer matrix, influence PFAS treatment efficacy (Gagliano et al., 2020). Given many PFAS exist as anions at drinking water pH, strong base AIX resins are capable of removing these PFAS species (Crone et al., 2019). The AIX process also preferentially removes longer chain PFAS and perfluorosulfonates (Appleman et al., 2014). However, adjustments to AIX resin characteristics (for example, hydrophobicity of functional group) can increase the sorption capacity for less hydrophobic PFAS (Chularueangaksorn et al., 2014; Zaggia et al., 2016). Although AIX resins have the advantage of greater adsorption capacity than GAC, they are typically limited to a single use for drinking water applications (Crone et al., 2019; Ross et al., 2018). However, AIX resin regeneration has been achieved in some studies (Crone et al., 2019), albeit utilizing complex or unconventional procedures.

Membrane technologies such as RO and NF are both highly effective for removal of many PFAS. RO effectively removes PFAS of all chain lengths as a function of size exclusion and charge rejection; NF relies principally on electrostatic repulsion and hydrophobicity, particularly for removal of shorter chain PFAS (Dickenson & Higgins, 2016; Zeng et al., 2017). The degree of RO and NF rejection may vary among PFAS and may be substantially lower for charge-neutral PFAS such as FOSA (Steinle-Darling and Reinhard, 2008; Steinle-Darling et al., 2010; Sanexen, 2022). Both membrane technologies are subject to fouling and scaling problems, which limit their wide-scale application.

Treatment achievability

Studies assessing pilot- and full-scale PFAS treatment achievability have demonstrated that GAC, AIX and RO can each effectively reduce concentrations of shorter chain PFCA and PFSA to below detection limits of < 1 to 2 ng/L for individual PFAS. However, to achieve these concentrations, the treatment systems need to be configured and operated properly. Achieving such low concentrations may also lead to challenging operating conditions, such as very long empty bed contact times or frequent media regeneration or replacement, and may not be practically or economically feasible for some water treatment facilities (Sanexen, 2022).

Residential-scale (private well) water treatment technologies

In cases where PFAS removal is desired at a household or small system level, for example, when an individual household obtains their drinking water from a private well, a drinking water treatment device may reduce the concentration of a limited number of PFAS in drinking water. Treatment devices can be certified to NSF Standard 53 (GAC) and NSF Standard 58 (RO) (NSF International, 2021a, b) for the reduction of "total PFAS" in drinking water for the following 7 PFAS:

The revised criteria will be published in NSF Standard 53 and NSF Standard 58 in early 2023. The use of treatment devices certified to the revised criteria will help homeowners further reduce their exposure to PFAS from drinking water.

When certified drinking water treatment systems are not available (such as point of entry systems), Health Canada strongly recommends that chemicals used in treatment systems (such as ion exchange softeners) be certified to NSF/ANSI Standard 60 (additives) and that materials and components be certified to NSF/ANSI Standard 61 (for leaching) and NSF/ANSI Standard 372 (for lead content) (NSF International, 2021c; 2022a, b). These standards ensure that these systems meet health-based requirements and are safe for use in potable water applications. In addition, periodic testing by an accredited laboratory should be conducted on both the water entering the treatment system and the treated water to verify that the treatment system is effective in removing PFAS.

Homeowners should consult with local authorities to determine available options for the disposal of treatment media and/or residuals that may contain elevated PFAS concentrations.

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