By Stephen Rosansky, P.E.
Per- and polyfluoroalkyl substances (PFAS) are a class of more than 4,700 chemicals that have been used for many commercial applications, including metal plating, plastic molds, photographic films, semiconductors, textile manufacturing, and firefighting foam. Their pervasive adoption has led to a range of potential multiple source contamination scenarios. Because most products end up in wastewater treatment plants and landfills, these waste-related facilities also serve as a potential source of PFAS contamination.
Of particular concern to the Department of Defense (DOD) is the historical use of aqueous film-forming foams (AFFF), which also are used by airports, oil and gas companies, and fire training facilities to fight fuel-based fires. Over the past 50 years, DOD has used different formulations of approved AFFF for fire-fighting and training operations, resulting in hundreds of PFAS-impacted sites across the country, with many known and unknown PFAS constituents.
Many of the unknown compounds are often PFAS precursors that could potentially form some of the terminal perfluoroalkylated acids, which are the primary PFAS chemicals of concern. These PFAS also have an increasing potential for regulation.
DOD is currently completing site assessments and initiating remedial investigations at PFAS-impacted sites throughout the United States and territories in the South Pacific, generating substantial volumes of investigation waste that often cannot be disposed at conventional treatment facilities. Furthermore, it is anticipated that many of these sites will require some form of active remediation, which may not be achievable using conventional technologies. As a result, DOD has an immediate need for on-site PFAS destruction technologies capable of destroying PFAS in contaminated groundwater to meet, or exceed, current or future federal or state guidelines.
RAPID DESTRUCTION PROCESS
A supercritical fluid is a substance that exists as neither a liquid nor a gas at a temperature and pressure above its critical point. For water, this occurs at a temperature of 374°-C (705°-F) and a pressure of about 221-bar (3,200-lbs/in2).
Above the critical point, there is no distinction between gas and liquid, and the properties of the fluid change dramatically. Polar substances, such as salts, become immiscible, whereas non-polar substances, such as petroleum fuels and their constituents, become completely miscible. Water becomes compressible and expandable and importantly, because of the absence of liquid and gas phases, mass transfer is unrestricted, which facilitates reaction kinetics.
A technology developed by Battelle leverages the properties of supercritical water to rapidly destroy PFAS, using readily available oxidants such as air or hydrogen peroxide. Similar supercritical water oxidation processes have been used to destroy other recalcitrant compounds like polychlorinated biphenyls, radioactive wastes, and nerve agents.
This process differentiates itself from other PFAS destruction technologies because it achieves near complete destruction of PFAS regardless of contaminant properties such as carbon chain length or structure. It also generates a minimal amount of waste products and it is not impeded by the presence of co-contaminants like petroleum hydrocarbons or chlorinated solvents, which are easily oxidized at supercritical conditions.
TUBULAR REACTOR TECHNOLOGY
This process differentiates itself from other PFAS destruction technologies because it achieves near complete destruction of PFAS regardless of contaminant properties such as carbon chain length or structure. It also generates a minimal amount of waste products and it is not impeded by the presence of co-contaminants like petroleum hydrocarbons or chlorinated solvents, which are easily oxidized at supercritical conditions.
At the core of the supercritical water oxidation process is a tubular reactor in which the feed and oxidant react. The desired reaction temperature is achieved using one or more heaters. The amount of heat required varies depending on the concentration of total organic carbon in the feed stream. When a sufficiently high concentration of total organic carbon is present, little to no additional heat may be required due to the heat that is liberated during oxidation. Heat exchangers are used to transfer heat between the influent and effluent streams to improve reaction efficiency and reduce operating cost. Because the defluorination reaction generates hydrofluoric acid, it is necessary to also add a neutralizing agent before discharging it to the environment.
A bench-scale system of this process was constructed and operated for over a year. Having processed more than 30 different types of PFAS-contaminated samples, the technique demonstrated greater than 99.99 percent destruction of total PFAS. Samples have included investigation waste and AFFF that have been stockpiled at sites around the United States. Supercritical water oxidation has been demonstrated to destroy PFAS to levels less than 5-ppt in under 10 seconds, leaving inert salts and PFAS-free water.
The steady-state energy requirements to run the reactor are minimal due to the oxidation reactions being exothermic and the implementation of reactor heat recovery by the influent stream. This low energy input, along with inexpensive oxygenation and neutralization chemicals, yield an economically feasible and scalable PFAS destruction process.
MOBILE TRAILER SYSTEM
The next step is the deployment of a mobile trailer system to demonstrate the efficacy of supercritical water oxidation to treat various feed streams for an extended period. To accommodate a wide range of feed solutions, the mobile unit incorporates a number of pretreatment and concentration steps.
In addition, the oxidation process generates a vapor stream, which changes in composition based on the mass of oxygen added to the reactor and the concentration of organic matter present in the feed that is converted to carbon dioxide and water. Although bench-scale data indicates that minimal, if any, contaminants are present in the vapor stream, the system currently includes activated carbon to help ensure clean air is discharged to the environment while waiting to receive confirmatory analytical results from samples.
The construction of a second larger mobile system that will be enclosed in a shipping container is underway and will be completed in 2021. This unit will be able to treat up to 3,500-gal/day of contaminated media. It will be used to treat stockpiled aqueous investigation waste, AFFF, and other concentrated waste streams. In this way the technology will further support DOD’s need for on-site PFAS destruction technologies as the department continues to address these contaminants of concern.
Stephen Rosansky, P.E., is Senior Chemical Engineer, Battelle; rosansky@battelle.org.
[This article first published in the January-February 2021 issue of The Military Engineer.]
