11/22/2023 | Press release | Distributed by Public on 11/23/2023 14:07
How does SCWO stack up to alternative PFAS treatment technologies in wastewater treatment?
This article illustrates how supercritical water oxidation (SCWO) compares to, and can be used in conjunction with, existing and emerging technologies for the concentration and destruction of PFAS in wastewater effluents and sludge.
What's in the article?
How PFAS is introduced to and cycled throughout the environment
Per-and polyfluoroalkyl substances (PFAS) are a class of man-made compounds that have an estimated disease attributable cost of $5.52 billion in the United States (US) alone (Obsekov 2023). Direct PFAS exposure comes from the use of PFAS in numerous consumer and commercial products including teflon coated pans, waterproofed fabrics, makeup, cleaning supplies and even bottled water. Indirect PFAS exposure is a result of the re-distribution of PFAS throughout the environment, as shown in Figure 1. The issue of PFAS contamination, and therefore exposure, is widespread; as of August of 2023, PFAS contamination has affected 3,186 locations in all 50 US states, Washington DC, and two US territories (EWG 2023). The occurrence and treatment of PFAS at wastewater treatment plants (WWTPs) has received considerable attention as they play a central role in re-distributing PFAS received in the influent waste stream to sources such as drinking water (see Figure 1). Therefore, eradicating PFAS from wastewater treatment products (effluent and sludge/biosolids) can play a pivotal role in protecting human health.
Figure 1. Lifecycle of PFAS in the environment. Source: 374Water.
Currently recognized PFAS treatment technologies
Understanding the chemical structure of PFAS is vital to developing effective treatment technologies. PFAS has a general formula of R-CnF2n+1 where R is a functional group. PFAS is an organic contaminant and because it contains carbon-fluorine bonds, it is one of the most recalcitrant pollutants (Wee 2023). At the time of writing, recognized treatments include concentration methods such as adsorption (e.g., granular activated carbon (GAC)) and filtration (e.g., nanofiltration) and destruction methods such as incineration. While these concentration technologies remove PFAS from the wastewater effluent streams, they do not destroy PFAS and instead generate a concentrated waste that requires further disposal (Paulsen 2022). The downside to incineration is the volatilization of PFAS, causing the need for further treatment of emissions produced. Therefore, there is a high demand for comprehensive, end-of-the-line solutions to completely mineralize PFAS from wastewater effluents and sludge/biosolids.
Supercritical water oxidation (SCWO) is one potential technology to do such. In this article, the applicability of SCWO to wastewater treatment is highlighted through a comparison of SCWO to other recognized or emerging concentration and destruction technologies. The goal is to identify the features of SCWO that contribute to its ability to meet the demand of the modern wastewater industry.
What supercritical water oxidation (SCWO) is
SCWO is an emerging (but not novel) technology that can be applied to wastewater treatment. SCWO is a process whereby water containing feed is heated and pressured above the supercritical point of water (374oC, 221 bar) to produce distilled water, minerals and energy (Figure 2). SCWO was invented in 1982 by Dr. Michael Modell at MIT (Figueroa, 2021) and since has been applied to various feeds including wastewater sludge, landfill leachate, AFFF, filtration reject and spent filter and resin media.
Figure 2. An example of a SCWO system with the addition of ambient air. Source: 374Water.
How SCWO compares to other technologies
In general there are two approaches to PFAS treatment: (1) concentrate and (2) destroy. Concentration of PFAS means that a wastestream with a dilute concentration of PFAS is converted to a smaller volume of concentrated PFAS. Destruction, for the purposes of this article, is considered as the mineralization of PFAS which means that all carbon-fluorine bonds are broken down into organic fluorine, inorganic fluoride, and carbon dioxide. It is important to note that there is also degradation of PFAS whereby PFAS are broken down into shorter chain PFAS and non-PFAS by-products. In Figure 3 we illustrate technologies that have the potential to concentrate and destroy PFAS, but some of them are, at the time, only known to degrade PFAS. In the next sections SCWO is compared to each of these methods.
Figure 3. Concentration and destruction technologies developed for PFAS treatment in wastewater. Source: 374Water.
SCWO vs PFAS Concentration Methods
Methods to concentrate PFAS from aqueous waste streams such as WWTP effluent include the use of adsorbents (e.g. granular activated carbon (GAC)), ion exchange (IEX) resins, reverse osmosis (RO), nanofiltration (NF), and foam fractionation. All of these methods suffer from the generation of a solid or liquid waste with a high concentration of PFAS and therefore require further treatment or disposal (solids: landfill, SCWO or incineration; liquids: ground injection, SCWO, or direct discharge). No direct comparison can be made between SCWO and these concentration methods as SCWO (1) is not the ideal treatment for wastewater effluents and (2) completely mineralizes (rather than concentrates) PFAS from the feed. Rather, SCWO can be combined with these methods to eliminate PFAS from the spent media, IEX/RO/NF reject and the fractionated foam.
Wastewater sludge undergoes thickening and dewatering processes which may concentrate PFAS levels in the solid phase. Typical disposal of these wastes is to landfill or further treat the sludge using technologies with the potential for PFAS destruction (discussed in the next section). SCWO has the ability to treat sludge directly after thickening and dewatering processes.
SCWO vs PFAS Destruction Methods
Note: The full SWOT analysis of these technologies is available upon request via the form at the end of the article.
There are nine categories of technologies with the potential to partially or completely degrade or mineralize PFAS from wastewater effluent (listed in Figure 3). Technologies that mineralize, rather than degrade, PFAS are the focus of this article as they are essential to producing high quality effluent with the potential to meet changing regulations.These technologies are limited to the application of pre-treated aqueous streams as they are sensitive to background constituents such as natural organic matter and non-PFAS contaminants. Although SCWO is listed because it has the potential to treat effluent wastewater, it is not typically used due to the low calorific value of the effluent.
Sludge (and other solid wastes)
There are six main technologies for the treatment of PFAS in wastewater sludge, which are listed in Figure 3. Unlike the treatment technologies for effluent wastewaters, the sludge treatment technologies listed have all been commercialized and demonstrated on a larger scale. Typically, these technologies are touted for their ability to produce products with beneficial reuse applications, however there is increasing attention to PFAS contamination in these products which may jeopardize their current end uses (e.g. land application). Thus, it is critical to evaluate the capacity of these technologies to destroy PFAS. As these technologies can be directly compared to SCWO, each is described and compared to SCWO below with a summary presented in Table 1. Note: There may be additional references not mentioned here that have tested the destruction of PFAS using these various technologies. This report is not a full literature review and did not cite all relevant literature.
Table 1. Overview of the products produced by each treatment type and which wastes they have been shown to treat PFAS in. Green indicates that there is at least one demonstration of the use of this technology to treat PFAS while grey indicates that no data was found. Acronyms: supercritical water oxidation (SCWO), hydrothermal alkaline treatment (HALT), hydrothermal carbonization (HTC), hydrothermal liquefaction (HTL).
Supercritical water oxidation (SCWO)
The driving principle of supercritical water oxidation (SCWO) is that at a supercritical state (374oC, 221 bar) the properties of water change such that organic constituents are water soluble and inorganics become insoluble. The result is the complete destruction of organic constituents leaving behind distilled water, minerals and heat. Because PFAS (general formula of R-CnF2n+1) contains carbon, it is an organic molecule that is soluble in supercritical water and therefore is non-selectively (i.e. all PFAS) mineralized to CO2 and F containing compounds such as NaF. SCWO has been shown to reduce ≥99% of PFAS in AFFF impacted wastes (Krause 2021; Scheitlin 2023), ≥99.9% in spent IEX and GAC (Chiang 2023), >99% in foam fractionation concentrate (Malovanyy 2023), and >99% in landfill leachate (independent testing by 374Water).
Incineration is a process whereby waste is heated to temperatures ranging from 871oC to 1,371oC (EPA, 2012) in the presence of oxygen. The resulting products are gas and ash. The underlying process to incineration is combustion which does not completely mineralize PFAS and can volatilize some types of PFAS into the gaseous emissions. Incineration is widely used and has been shown to treat PFAS in sludge (Seay 2023), spent GAC and IEX (Patterson 2020), and AFFF (Shields 2023). Studies on the fate of PFAS post incineration of AFFF are lacking although some bans have been put in place due to the uncertainty of PFAS removal in this process. Information on the treatment of landfill leachate using incineration is not available as this is not a common practice. Although SCWO systems can also incorporate high heat and oxygen in the system, SCWO does not use the principle of combustion, is typically operated at a lower temperature, and benefits from a higher water content in the feed. These conditions evades the formation of incomplete combustion products and the incomplete destruction of PFAS.
Gasification + thermal oxidation
During gasification the feed is heated (>800oC (Gao 2020)) and contacted with (limited) oxygen or steam which starts a series of chemical reactions to produce syngas and ash. Generally, gasification degrades (rather than mineralizes) PFAS which forms by-products that are more difficult to remove at the operating temperature (Winchell 2022). This results in syngas and ash products that have to be further treated (e.g. thermally oxidized) for complete PFAS destruction. Gasification for the removal of PFAS has been demonstrated in sludge (Logan City Council 2021) but, to date, no studies show the treatment of PFAS in spent GAC/IEX (Berg 2022) or aqueous wastes such as AFFF, landfill leachate, and foam fractionation foamate. The main difference between gasification and SCWO is that SCWO leverages the properties of supercritical water while gasification relies on the reactions with air or steam.
Pyrolysis + thermal oxidation
Pyrolysis occurs when waste is heated at or above 500oC in the absence of oxygen. The final products are syngas, biochar + ash and bio-oil, the proportion of which depends on the pyrolysis conditions. Although research is still ongoing, pyrolysis of PFAS laden wastes typically degrades (rather than mineralizes) PFAS and therefore must be treated through additional processes such as thermal oxidation. As a general trend, the result is gaseous (Sermo 2023), solid (Kim 2015) and liquid (McNamara 2023) waste streams that contain PFAS. Pyrolysis has been applied to sludge (Kundu 2021), spent GAC (Watanabe 2016), and AFFF (Yao 2022) while no literature was found for the treatment of landfill leachate, spent GAC/IEX resin or foam fractionation foamate. The major difference between SCWO and pyrolysis is that pyrolysis produces biochar which may contain PFAS while SCWO does not produce biochar. Instead, SCWO completely transforms solids in the feed to distilled water and minerals. As biochar can be reused beneficially (e.g. land application) it is important to reuse minerals produced by SCWO to maintain the same benefits.
Hydrothermal alkaline treatment (HALT)
Hydrothermal alkaline treatment (HALT) is when feed is treated at elevated temperatures (350℃), high pressure (16MPa) (Soker 2023), and alkaline chemicals are added in to increase the pH of the wastewater, effectively enabling PFAS degradation. Outputs include benign salts such as sodium fluoride, solid metal rich residue, and filtrate. Due to the high temperature and pressure of the system, generally, HALT can mineralize PFAS regardless of PFAS class, chain length or initial concentration (Pinkard). HALT has been applied for the removal of PFAS in spent GAC (Soker 2023) and AFFF (Pinkard 2023) while no data was found for sludge, spent GAC/IEX resin, landfill leachate or foam fractionation foamate. The main difference between HALT and SCWO is that SCWO does not require any chemical additions (although they can be used) while HALT requires an alkaline input. Additionally, HALT systems operate at lower temperature and pressure conditions than SCWO.
Hydrothermal carbonization (HTC)
Hydrothermal carbonization (HTC) occurs when biomass, or raw feedstock, is
heated in a solution of water at temperatures between 180-300℃ (Nasrollahzadeh 2021), and self generated pressure (2-6 MPa (Yoganandham 2020)). Through a combination of reactions such as hydrolysis, dehydration, and decarboxylation, gasses, water soluble products, water, and hydrochar are produced (Hoekman 2011). Research on how HTC impacts PFAS levels in all matrices discussed here is currently lacking (i.e. no papers were found). Rather, HTC has been used to produce adsorbents with the potential to adsorb PFAS (Izquierdo 2023). The main difference between HTC and SCWO is that SCWO operates at a higher temperature and pressure to utilize water in its supercritical state. Additionally, SCWO does not produce a hydrocharand requires an external pressure source.
Hydrothermal liquefaction (HTL):
Hydrothermal liquefaction (HTL) is a closed system technology that utilizes heat (200℃-400℃ )and pressure (10-25 MPa) (Zhang 2018) to elicit chemical reactions that can break down complex molecules. Through this process wet biomass is transformed into biocrude that contains a complex mixture of acids, alcohols, ethers, hydrocarbons among other components. HTL has the potential to degrade or mineralize PFAS but is not equally effective for all PFAS, resulting in PFAS laden biocrude (Yu 2020). The removal of PFAS using HTL has been demonstrated on sludge (Zhang 2022) but was not found for the other matrices discussed here. Although HTL can be operated at a similar temperature and pressure to SCWO, SCWO does not produce biocrude, and rather, is being studied for its ability to treat HTL biocrude (MicroBio Engineering).
In plasma treatment a voltage is applied to a copper electrode which creates plasma. Next, the feed is pumped upward in the reactor then directed back downward through a gap within the plasma discharge. The remaining products are slag, stack gas, and water. Plasma is able to mineralize PFAS (Fraunhofer 2023) as the electrical discharge between the two electrodes generates highly reactive oxidative and reductive "species" including OH,O,H•,HO2•,O2•‒,H2,O2,H2O2 (Meegoda 2022).Plasma has been shown to treat sludge (Cai 2020), IEX regenerant still bottom samples (Singh 2020) and landfill leachate (Singh 2021) but not AFFF or foam fractionation foamate. Plasma and SCWO are not similar technologies as SCWO utilizes the properties of waterat supercritical temperatures and pressures while plasma uses the properties of plasma generated via an electrode.
Outlook on the use of SCWO in the wastewater industry
SCWO can be a useful technology to pair with treatments for wastewater effluents while it can either be a replacement or addition to treatments for wastewater sludges. SCWO is distinct from nearly every other method in that it exploits the properties of supercritical water to generate benign products (distilled water, minerals, heat). In terms of PFAS removal, SCWO and HALT are ideal options for the mineralization of PFAS that are not PFAS selective. Although this article only explores the PFAS destruction aspect, all technologies should be considered with various economic and environmental factors such as energy usage, footprint, cost and destruction of non-PFAS pollutants. Overall, when considering PFAS treatment, SCWO is a well-rounded technology to treat solids wastes and reject streams produced during wastewater treatment.
This post was written by Naomi Senehi and EJ Mildwurf and was edited by Sudhakar Viswanathan and Doron Gez (374Water).