Source: Water Industry Carbon Neutrality Information
Author: Wu Yunfeng, Hao Xiaodi, etc.
Introduction: Per- and polyfluoroalkyl substances, commonly known as PFAS, have become a serious challenge to the water industry for they are difficult to be removed and degraded. By constantly learning about new methods and technologies, Aquatech Online has summarized five treatment methods for removing these chemicals that are harmful to the environment and human body.
What are PFAS?
According to the definition given by the U.S. Environmental Protection Agency, per- and polyfluoroalkyl substances (PFAS) are man-made chemicals composed of chemicals such as perfluorooctanoic acid (PFOA), perfluorooctane sulfonate (PFOS), and GenX . Although such chemicals are known as "persistent chemicals" (once they enter the body, they remain for a long time and are difficult to remove), they have been made and used in factories around the world since the 1940s. As of July 2020, more than 2,200 scenes in 50 states in the United States are contaminated by PFAS, covering food packaging, household products, workplaces, drinking water and other fields. PFAS has even been found in living organisms (fish) . It can be seen that PFAS is ubiquitous and difficult to remove and degrade. Fortunately, there are some effective solutions.
1. Carbon Filtration
From an efficiency level perspective, the most widely used drinking water remediation technology so far is Granular Activated Carbon (GAC). As we all know, GAC is a material that can be effectively used in water treatment technology and is widespread and efficient. Therefore, for sewage treatment plants that have been invested and built, GAC materials do not need to frequently replace the reaction bed. As for the removal effect of GAC on PFAS, the Orange County Water District (OCWD) in the United States has also tested the treatment performance of granular activated carbon, ion exchangers and new adsorbents on PFAS.
Although GAC technology currently has relatively high performance, as technology continues to innovate and is affected by economies of scale, GAC may be replaced by better technology in the future.
Because the operation of replacing the filter element is very simple, carbon filtration technology has been applied in its applicable fields. Keith Hays, vice president and co-founder of Bluefield Research, once wrote for Water Technology Online. He said: "GAC technology currently has high efficiency, but as technology continues to innovate and is affected by economies of scale, there is likely to be better technology in the future. technology to replace GAC."
2. Ion-Exchange Resins
Ion exchange (IX) resin is composed of high-porosity polymeric materials, mainly composed of acids, alkalis and water-insoluble substances, forming resin beads. IX resins include two major categories: cation exchange resins and anion exchange resins. By selecting resin media for specific PFAS, IX resin becomes a technology with adsorption capabilities. Although the resin cost per unit volume of IX resin is higher, it has a long service life and does not require a large reaction space, so its operating cost is not very high. It can be seen that when faced with small reaction spaces or small-scale processes such as mobile repair technology, using IX resin is an efficient and economical choice. For PFAS, using IX resin can more effectively remove short-chain PFAS that cannot be adsorbed by activated carbon. Ion exchange resin is like a powerful small magnet. The cations in the resin attract the anions in PFAS, thereby absorbing and containing contaminated materials from the water to achieve the effect of removing PFAS.
3. Plasmatron
A study in the United States shows that to deal with chemical pollutants, rather than using activated carbon or reverse osmosis technology to filter them, it is better to break them down directly. Researchers from Drexel University in Pennsylvania have developed a technology called a "plasmatron" that does not filter PFAS but directly breaks it down. They believe that current filtration methods such as carbon filtration can only collect PFAS, but cannot destroy and decompose it. In other words, used filters will become a new source of PFAS unless the filters are incinerated at high temperatures. The research looked at the development of a device called a sliding arc plasma device, which generates a rotating electromagnetic field that separates chemicals in water, a process much like using a smoothie blender to make a smoothie. Researchers say this process takes 1 hour and can remove more than 90% of PFAS in water, and it consumes less energy than people boiling a pot of boiling water. The research team said that previous PFAS plasma treatment methods have been difficult to promote in large-scale treatment facilities. Dr. Alexander Fridman, director of the Nyheim Plasma Institute, said that the technology can be adapted to treat soil pollution. , to achieve "almost complete defluorination of PFAS compounds."
4. Supercritical water oxidation
Supercritical water oxidation is a technology that has been used to treat refractory compounds since the 1980s. Supercritical water oxidation technology was first proposed by the Massachusetts Institute of Technology (MIT). It is essentially an advanced oxidation technology that uses the characteristics of supercritical water to destroy organic matter under operating conditions of 450~600°C. By increasing the temperature and pressure of supercritical water, it becomes a special state that can break the carbon-fluorine bonds in the PFAS molecular skeleton, thereby destroying PFAS.
Through closed-loop on-site PFAS destruction experiments, this technology has been proven to have the ability to collect and destroy PFAS in water.
Battelle, an independent nonprofit technology research and development organization, recently used supercritical water oxidation technology to treat PFAS at a wastewater treatment plant in Michigan. Through closed-loop on-site PFAS destruction experiments, this technology has been proven to have the ability to collect and destroy PFAS in water. The specific method is: first, the contaminated wastewater is lifted to the system through a liquid pump. In the system, the wastewater is mixed with the mixed fuel hydrogen peroxide, isopropyl alcohol and sodium hydroxide as the neutralizing agent; then, the water enters the hot water After the exchanger and desalination operation, it enters the reactor together with the oxidant at a specific temperature and pressure, and performs a supercritical water oxidation reaction to break the carbon-fluorine bond of PFAS, and ultimately achieve the effect of destroying PFAS.
5. Reverse Osmosis (RO)
Reverse osmosis (RO) technology is known for its semi-permeable membrane that removes ions, chemicals and micro-deposits. Among them, high-pressure membranes such as nanofiltration membranes and reverse osmosis membranes can effectively remove PFAS. According to data provided by the US Environmental Protection Agency, RO's removal rate of various PFAS, including short-chain PFAS, can usually exceed 90%. In this technology, about 80% of the incoming water can pass through the high-pressure membrane and flow out, achieving the purpose of water purification. Reverse osmosis technology is suitable for urban residents, because the volume of water residents need to treat is small, and domestic sewage is easy to treat, so there is no need to worry too much about its treatment efficiency and other issues.
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