Formatted Title
Technoeconomic Analysis of a Regional Supercritical Water Oxidation (SCWO) Facility for Handling PFAS-Laden Wastes
Background/Objectives
The ongoing production, use, and disposal of products containing per- and poly-fluoro alkyl substances (PFAS) causes their continual release into the environment through air emissions, treated wastewater, and other sources. Because PFAS are chemically stable, their destruction requires specialized conditions not typically present in waste management and disposal facilities. Regional management of waste material containing PFAS has the potential to provide cost savings to multiple customers by centralizing PFAS destruction rather than having onsite destruction at multiple sites. While these conditions for destruction vary and are evolving due to ongoing research, they include high temperatures, high pH, high pressure, and other forms of energy input. Supercritical water oxidation (SCWO) has several advantageous features for a regional PFAS destruction facility implementation, including: (1) an ability to capture energy from the destruction process for energy production, providing the opportunity to turn PFAS destruction from a huge user of electrical to an energy-neutral or even energy-positive endeavor, and (2) an ability to operate self-sustaining on multiple high-energy waste streams, including granular activated carbon (GAC) media, and anion exchange resins (AER), and biosolids.
A regional SCWO facility concept accepting 200 wet tons per day (wtpd) of PFAS waste was developed for this techno-economic analysis. Feedstock selection and dilution was developed to balance energy content and supply a minimum of 2.3 MJ/kg (1,000 BTU/lb) to keep SCWO self-sustaining. This concept includes the potential to use a combination of waste feedstocks as listed above.
SCWO is a physical-thermal process that relies on the unique reactivity and transport properties of water above its critical point of 374°C and 218 atm (705°F and 3200 PSI). At these conditions, organics are fully soluble in supercritical water, and with the addition of oxygen, all organics, including PFAS, rapidly and completely oxidize to form carbon dioxide, treated water, and inorganic salts. The highly oxidizing SCWO environment coupled with the presence of halogens can lead to rapid degradation.
Depending on the concentration and calorific value of the waste feedstock, SCWO can be operated auto-thermally (i.e., no outside input of heat). The highly oxidizing environment makes it possible to effectively treat a wide variety of organic wastes and organic micropollutants, including industrial sludges, slurries of biosolids, waste oil, food wastes, plastics, 1,4-dioxane, and PFAS. SCWO has been used to successfully treat spent GAC and AER, PFAS rinsates, landfill leachate, aqueous film-forming foam (AFFF) dilutions, and wastewater sludges. Data from various tests treating PFAS laden wastes have resulted in 99% or greater removal of targeted PFAS.
Regulatory guidance and requirements for PFAS wastes continues to evolve, putting pressure on industries, waste management facilities, and WRRFs. A moratorium on PFAS incineration by the Department of Defense (DoD) and some states (Illinois, Massachusetts and New York), makes any destruction of PFAS using existing infrastructure nearly impossible in some areas. PFAS in WRRF effluent and biosolids are beginning to be scrutinized and regulated. WRRF effluent permit limits for PFAS have been implemented in Michigan, and Maine has banned land application of biosolids due to concerns about PFAS application and migration.
The current network of available destruction venues for PFAS-laden wastes include hazardous waste incinerators and GAC reactivation facilities, of which there are less than 50 total in the United States. These facilities are currently the primary mechanism by which PFAS is destroyed. A new regional SCWO facility could be implemented either as a greenfield site or as part of an existing waste management facility. The potential benefit of regionalization for smaller utilities and waste producers relative to onsite small PFAS destruction systems include reduced share of capital cost due to improved economy of scale for larger facilities and lower downtime. Potential benefits relative to more distant hazardous waste incinerators include reduced hauling costs.
Approach/Activities
This study focused on a 200 wtpd SCWO facility that accepts spent GAC and AER media, municipal biosolids, as well as PFAS concentrates (e.g., foamates or AFFF). The facility developed could evolve to accept other types of high-carbon wastes (e.g., plastics) with limited contributions of low-carbon wastes, with feedstock selected to maintain a self-sustaining, energy-positive SCWO reaction. A 200 wtpd SCWO facility is expected to be able to manage GAC/AER resin from industrial pretreatment as well as WRRF biosolids for sewershed receiving approximately 15 MGD of water a day, serving roughly 150,000 residents.
Capital costs were estimated based on scaling up smaller SCWO systems and estimating building footprint needed and include allowances for pumping and piping, electrical, instrumentation, installation, engineering and design, contractor overhead and profit, and construction contingency. Estimated total capital costs are between $62 million and $134 million, which reflects a cost uncertainty range of -30%/+50%, as listed in Table 1. This uncertainty range reflects a Class 5 capital cost estimate for a project less than 1% developed (per AACE International).
Operating costs were developed based on operation of existing SCWO systems. The proposed system is expected to produce about 400 kWh/day of excess energy that can be used to heat the building and offset electrical costs. Income was estimated based on recent EPA reports that hazardous PFAS disposal rates range between $1,000 and $1,500 per ton (U.S. EPA 2020). The estimated O&M cost is between $3 million and $13 million, with $33 million to $50 million of net income (minus taxes). These composite costs and income reflect a simple payback of about 5 years or less.
Results/Lessons Learned
This study outlines conceptual design and economic analysis for regional SCWO infrastructure for PFAS destruction that has the potential to be: (1) economically viable with an estimated payback time of five years or less depending, (2) energy-neutral on a process basis, with the potential to produce electricity and heat that could offset facility operation needs, (3) and able to accept multiple concentrated waste streams including GAC, AER, biosolids, foamate, and AFFF.