By Miles Menyhert
Governments around the world are using regulation to reduce the risk of human exposure to the harmful effects of PFAS. By setting limits on the permissible levels of these compounds in drinking water, regulators are forcing water suppliers to undertake advanced treatment to ensure compliance.
To proactively address the threat of PFAS in drinking water, many utilities are upgrading their systems using three primary technologies: activated carbon (AC), ion exchange resin (IER) and reverse osmosis (RO). These technologies can be used independently or collaboratively to mitigate PFAS. Depending on water quality, some of these treatment technologies (mainly AC and RO) can be used to treat additional contaminants as well, increasing their value and cost-effectiveness in water treatment.
Activated carbon
Granular activated carbon (GAC) is widely used in water treatment applications for trace contaminant removal and is effective for mitigating long-chain PFAS to non-detectable levels. It’s a highly porous adsorbent that starts with a carbon-rich raw material such as coal, coconut shell, or wood.
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These materials are converted into char using high heat in an oxygen-starved environment in a process known as pyrolysis. The char is then “activated” with steam to create a highly porous structure. Activated carbon’s range of pore sizes (micropores, mesopores and macropores) impart unique transport and adsorptive characteristics.
This allows for the reduction of organic pollutants by firmly attracting them to the internal surface created by these pores. Once activated, the final product is dried, crushed and sieved into specific-sized granules or powders that are matched to various applications. When activated carbon is used in vessels, water is percolated through the bed to allow for adsorption.
Alternatively, when the activated carbon is applied as a powder, it is more analogous to a chemical dosage. The powder is dosed into the system, mixed with the water, separated, and discarded after the desired contact time.
Powdered activated carbon (PAC) is the pulverized version of GAC and can be used without requiring the installation of pressure vessels or gravity contactors. Because of its smaller particle size, PAC has faster adsorption kinetics than GAC. The smaller particle diameter decreases the diffusion path length by 10 to 100 times when compared to GAC.
PAC is dosed directly into the water as needed to combat contamination spikes, then removed with physical filtration methods such as coagulation/flocculation and sedimentation. This bypasses the capital expenditure and lengthy design/installation process required for GAC pressure vessels, making PAC an attractive, adaptive solution with a low barrier to entry.
Many utilities already use PAC for other purposes, such as taste and odour control. So using it for PFAS treatment may require just a simple dosing adjustment or transition to a higher performance grade. PAC can allow drinking water facilities to start treating for long-chain PFAS right away, while they consider more permanent, long-term treatment strategies.
However, GAC has some advantages over PAC when it comes to PFAS reduction and drinking water treatment in general. Because GAC is applied in a flow-through system, and sometimes in lead-lag systems, a higher utilization of the available capacity on the GAC is possible. This leads to lower carbon usage per treated volume.
Secondly, GAC can be reactivated. The reactivation process takes the spent media and burns off (and in many cases destroys) adsorbed contaminants, thereby refreshing the life of the media. A small portion of GAC is lost in the process. Fresh GAC (also known as make-up carbon) can then be supplied to fill in the lost portion of media. Generally, this requires only 10 – 30% make-up, saving 70 – 90% of the initial media.
Beware of low-cost carbon
When it comes to activated carbon, quality matters. Carbon characteristics can vary significantly among vendors, and the difference in quality can have remarkable implications for the buyer.
High-quality carbon performs more reliably across batches, allows for lower dosages, and lasts longer before needing replacement or reactivation. Lower quality, poorer performing carbon may appear cost-effective on a dollars-per-kilogram basis but can easily end up costing a plant more money. Increased consumption of lower quality carbon leads to increased costs such as more labour, more frequent deliveries/increased site traffic, a larger storage footprint, and greater disposal and/or reactivation costs.
A high-quality carbon has more adsorption capacity and longer life because of a well-balanced pore size distribution that can better transport and capture contaminants. Special care is taken when manufacturing certain brands of GAC, like Jacobi’s AquaSorb™ line, to ensure that it is safe and suitable for use in water treatment.
This special care can include sourcing of high purity raw materials or pre-treatment, such as acid washing, to limit the introduction of unwanted extractable content into the water. Outside of inadequate porosity, improper sourcing or insufficient washing can cause low-quality carbon to contain more impurities like heavy metals, which can pass into the treated drinking water.
To ensure limited levels of extractables are leached into the drinking water from low-quality carbon, excessive backwashing of the media during commissioning could be required. Depending on extractable constituents, all backwash water could require specialized disposal.
With specific regard to PAC, pore distribution is a key performance aspect. Due to the relatively short contact time of the PAC with the treatment water, the adsorption kinetics and capacity must be tuned to the contaminants in the source water.
This is most properly explored through jar testing. Jar testing can be done to compare dosage and contact time requirements for different carbon grades, allowing for fine tuning of the carbon to your process. This exercise can help a utility save money by generating a price vs performance metric for each carbon.
Anion ion exchange
Ion exchange resin (IER) is another leading filtration technology commonly used in PFAS treatment. IER beads are tiny, polymeric, highly porous beads. For PFAS removal, specialized functionality is applied to the media to provide selectivity for trace amounts. The positively charged functionality attracts the negatively charged functional groups of the PFAS compounds, pulling them out of the solution. IER reduces both short- and long-chain PFAS effectively, while utilizing a shorter contact time than activated carbon.
When identifying whether activated carbon or ion exchange is the right choice for a facility, the first step must be to understand the specific water quality to be treated. In the water quality analysis, other substances that can interfere with PFAS removal must be identified as well as PFAS compounds that must be targeted.
For ion exchange, these other substances would be sulfate, chloride/chlorine, nitrate and TOC. For activated carbon, these compounds would include organics (TOC, DOC, BOD). If possible, speciation of these compounds in the influent stream is useful for the selection of the appropriate media.
High molecular weight components will be more readily adsorbable onto the carbon’s surface and push off any previously adsorbed PFAS, while blocking adsorption sites for additional PFAS adsorption. If the analysis of PFAS compounds indicates mainly long-chain and sulfonated PFAS species, carbon may be a more viable option.
If the PFAS contamination is short-chain and tending towards carboxylic acid functionality, ion exchange should be considered a stronger performer. Of course, rapid small scale column testing or pilot scale testing, would better highlight true performance and operational differences between the media types.
Using activated carbon and ion exchange together
Using activated carbon followed by ion exchange resin can be a robust approach for treating PFAS. Activated carbon treatment in front of a PFAS-specific ion exchange media will extend the life of the ion exchange resin. It can protect the resin from fouling prematurely and the IER can protect the system from early short-chain PFAS breakthrough.
As Canada’s PFAS regulations turn greater attention to drinking water, municipal water treatment operations should start planning now to explore treatment technologies that fit their needs and budgets.
Miles Menyhert is with Jacobi Carbons Inc. For more information, visit: www.jacobi.net
This article appeared in the February 2025 edition of Environmental Science & Engineering Magazine. Read the full issue below.
