Per- and polyfluoroalkyl substances, better known as PFAS, have become a serious challenge for the water sector as they are difficult to remove and even harder to destroy.
As innovations continue to emerge, Aquatech Online looks at five solutions to tackling these harmful chemicals.
What is PFAS?
Per- and polyfluoroalkyl substances (PFAS) are a group of man-made chemicals that includes PFOA, PFOS, GenX, and many other chemicals, according to the Environmental Protection Agency.
Often referred to as 'forever chemicals', these chemicals have been manufactured and used in industries across the world since the 1940s.
As of July 2020, there are over 2,200 confirmed sites of PFAS contamination across all 50 states in the US.
PFAS has been found in food packaging, commercial household products, workplaces, drinking water and even in living organisms, such as fish.
As a result, PFAS is extremely difficult to remove and destroy but there are some solutions out there that have proven effective.
Carbon filtration
To date, the most widely used technique in drinking water remediation at the utility level is granular activated carbon (GAC).
GAC is a well-known and well-used technology for other water treatment purposes. As a result, it often means there is little more than a more frequent bed changeout to the investment already made in a location.
In the US, utility Orange County Water District (OCWD) has tested GAC and ion exchange products, as well as novel absorbents.
While utilities are focused on GAC at present, this is likely to shift over time due to innovation and economies of scale.
Previously writing for Aquatech Online, Keith Hays, vice president and co-founder at Bluefield Research, said: "While utilities are focused on GAC at present, this is likely to shift over time due to innovation and economies of scale.”
Ion-exchange resins
Ion exchange (IX) resins are made up of highly porous, polymeric material that is acid, base, and water-insoluble.
The tiny beads that make up the resin are made from hydrocarbons and there are two broad categories of ion exchange resins: cationic and anionic.
IX resins are an adsorptive technology where media can be formulated to target specific PFAS compounds.
Though typically more costly per unit of volume, IX resins often have longer bed contact lifetimes, reducing operation costs (OPEX), and usually require less space. This makes them an option for smaller operations with less available space, or for mobile remediation techniques as well.
This approach is more effective towards eliminating emerging short-chain PFAS, which are not removed by carbon-based adsorption processes.
Ion exchange resins work like tiny powerful magnets that attract and hold the contaminated materials from passing through the water system. Negatively charged ions of PFAS are attracted to the positively charged resins.
Plasmatron
A study in the US has shown that rather than filtering out the chemicals using activated carbon or reverse osmosis, in fact, the best way is to destroy them.
Researchers from Drexel University in Pennsylvania have developed what is being called a ‘plasmatron’ technology that they claim breaks down PFAS contaminants, rather than filtering them out.
They said that with current filtration methods, such as carbon filters, PFAS are merely collected, not destroyed, so “unless the filters are incinerated at high temperatures”, the used filters “become a new source of PFAS”.
The study saw the development of a device called a “gliding arc plasmatron”, creating a rotating electromagnetic field that splits the chemicals apart in the water, the process was described as the chemical equivalent of “using a blender to make a smoothie”.
Researchers claim the process takes one hour and uses less energy than it takes to boil a kettle, while removing more than 90 per cent of PFAS from the water.
The team said previous plasma treatment methods on PFAS did not lend themselves to being easily scaled up for use at large treatment facilities.
Alexander Fridman, PhD and director of the Nyheim Plasma Institute, said the technology could be adjusted to treat contaminated soil, achieving "near-complete defluorination of PFAS compounds".
Supercritical water oxidation
Supercritical water oxidation is a technology that's been in practice since the 1980s to deal with difficult-to-treat compounds.
First proposed by MIT, essentially supercritical water oxidation is an advanced oxidation technology that can be used to destruct organic matter by taking advantage of the distinctive properties of supercritical water under the typical operating conditions of 450–600°C.
The technology involves increasing the temperature and the pressure so that it's in a unique state. That will enable the breaking of the carbon-fluorine bond, which is the backbone of the PFAS molecules.
A closed-loop on-site destruction solution demonstrated its capability to mobilise and destroy PFAS chemicals that were present in the contaminated water.
A closed-loop on-site destruction solution demonstrated its capability to mobilise and destroy PFAS chemicals that were present in the contaminated water.
To break down these complex chemicals, the solution’s pumps draw the contaminated wastewater into the system, where it is mixed with hydrogen peroxide, isopropanol as a co-fuel and sodium hydroxide as a neutralizing agent.
After passing through a heat exchanger and removing salts, the water goes into the reactor at a designed temperature and pressure with an oxidant to break the carbon-fluorine bond.
Reverse osmosis
Reverse osmosis (RO) is known for the removal of ions, chemicals, and micro-sediment filtration via a semipermeable membrane.
High-pressure membranes, such as nanofiltration or RO have been effective at removing PFAS.
According to the EPA, RO is typically more than 90 per cent effective at removing a wide range of PFAS, including shorter-chain PFAS.
With both high-pressure membrane types, approximately 80 per cent of the feed water, the water coming into the membrane, passes through the membrane to the effluent.
This technology is best suited as a point-of-use technology for a homeowner due to the volume of water being treated being much smaller and the waste stream could be disposed of easily with less cause for concern.
