Cost Implications of PFAS Destruction in Biosolids: A Case Study from the Minnesota Pollution Control Agency

Introduction and Objectives Our scientific understanding of per- and poly-fluoroalkyl substances (PFAS) in our environment has evolved rapidly in the last several years and PFAS have increasingly gained attention from the public, scientific, and regulatory sectors due to their carcinogenic and reproductive and endocrine disruptive nature. Previous research has illustrated that water resources recovery facilities (WRRF) are notable PFAS emission routes to the environment, both liquid and solid discharge routes. Despite industrial phasing-out of some PFAS and increasing source management of industrial discharges, PFAS are expected to be present in wastewater solids for the foreseeable future. According to Northeast Biosolid and Residual Association (NEBRA), more than seven million dry tons of wastewater solids were produced at WRRFs in 2004 and solids management practices have resulted in the release of approximately 3000 kg PFAS per year to agricultural lands and landfills (Venkatesan and Halden, 2013). Much work has been done to develop and demonstrate wastewater treatment technologies to destroy PFAS compounds (mineralize to non-fluorinated end-products) in wastewater solids, but few studies have documented the practical application of PFAS destruction technologies for wastewater solids or estimated the cost of implementing those technologies. Our team conducted an evaluation for the Minnesota Pollution Control Agency (MPCA) to document the technology options that could be implemented today to remove and destroy PFAS within municipal wastewater effluent, compost contact water, landfill leachate, and municipal wastewater solids. The objectives of this project were to: -establish a framework for determining which technologies meet the definition of commercially available and effective for PFAS destruction -conceptualize upgrades to destroy PFAS at existing WRRFs -estimate the cost of completing these upgrades for a range of WRRF capacities. Systems were conceptualized for facilities ranging from 0.1-10 MGD. For capacity greater than 10 MGD, a regionalized option was considered. This paper will highlight the findings from wastewater solids portion of the study. This information will be valuable for utilities looking to implement PFAS destruction technologies as regulations around PFAS evolve rapidly. Target PFAS Initial work for the study involved selecting a set of PFAS to represent the range of PFAS known or predicted to be in wastewater solids. Selected PFAS are well-described in PFAS treatment literature, frequently detected in wastewater solids, and diverse in chain length and functional groups. Table 1 lists the selected compounds, assumed typical and high concentrations in wastewater biosolids, and associated target post-treatment. The selected PFAS are generally long-chain perfluoroalkyl acids (PFAAs) or are precursors to PFAA formation, which research has shown to partition into solids. Representative concentrations of PFAS were selected from available literature. Technology Screening PFAS destruction technologies were screened for their ability to consistently remove at least one identified PFAS to below the currently accepted laboratory detection limit, as outlined in Table 1. Technologies were further screened by their readiness for full-scale implementation. Wastewater solids PFAS destruction technologies passing the screening included pyrolysis with thermal oxidation of the process gas, gasification with thermal oxidation, and supercritical water oxidation (SCWO). Pyrolysis and gasification systems were considered to use similar enough equipment to be evaluated as a single alternative for conceptual design. Pyrolysis/gasification systems included a thermal dryer as part of the equipment assembly since both technologies require high solids content. Conceptual Design and Cost Estimating A conceptual design was created for both short-listed PFAS destruction alternatives and for solids production rates of 1, 5, and 10 dry tons per day at a given WRRF, corresponding roughly with the plant influent flow rates used for the liquids portion of the study. The feed solids to the pyrolysis/gasification process were assumed to be dewatered to 25 percent total solids. For SCWO, the feed solids were assumed to be screened and dewatered to 15 percent total solids. Construction cost estimates (AACE Class 5) were prepared from recent project estimates and input from vendors. The construction costs were plotted against the system feed rate to create cost curves for each alternative. One example of a cost curve for pyrolysis/gasification is provided as Figure 1. The construction cost curves for pyrolysis/gasification and SCWO, and parameters used to develop each, will be included in the paper. Additionally, a conceptual design for a regional pyrolysis/gasification facility for wastewater solids was developed to represent another approach for utilities. Our findings showed that due to high costs associated with PFAS treatment, regional approach might become viable for smaller facilities (facilities less than 10 MGD). A regional facility would receive dewatered solids from two or more individual WRRFs within a reasonable hauling distance. Fifty dry tons per day was assigned as a representative capacity for a regional PFAS destruction facility. Operation and maintenance costs estimated for recent pyrolysis/gasification projects were assembled to select representative 2022 O&M costs for a 50 dry ton per day facility. A 20-year lifecycle cost analysis was prepared and tested for sensitivity to variations of the tipping fees and interest rates. This paper will include the results and conclusions from the lifecycle cost analysis of the regional facility concept. Conclusion This study documents the cost of implementing currently available technologies to destroy PFAS in wastewater solids. For pyrolysis/gasification, the O&M costs per dry ton trended lower as the system capacity increased, indicating that smaller utilities could reduce costs if they formed a partnership and developed a regional PFAS destruction facility. Demonstration testing of SCWO systems is expected to provide evidence to clarify if similar results can be expected for SCWO systems.