Understanding Gasification for PFAS Removal in Residuals: Full-Scale Study Results from WRF Project 5107

This abstract reports on results from the first PFAS testing in a full-scale operating municipal biosolids processing gasification facility, including air emission testing. The presence of PFAS in solids or biosolids has affected the beneficial use and disposal options in the US. The land application of biosolids reintroduces PFAS into the environment, potentially contaminating surface water and/or groundwater. Gasification/pyrolysis followed by thermal oxidation are processes that were considered successful in removing PFAS in biosolids.

In addition to the ability to remove PFAS from solids due to high temperature during the thermal oxidation part, gasification significantly reduces solids and recovers valuable energy from the solids. However, no full-scale facility has reported PFAS removal data from gasification operations, and full-scale PFAS air emission testing and data is also lacking.

This WRF funded project aimed to evaluate the performance and feasibility of a full-scale thermal drying and gasification facility for processing municipal sludge as feedstock, with a focus on its effectiveness in removing/destroying PFAS. Key objective of this project includes assessing the fate of selected PFAS through the thermal drying and gasification unit processes, performing mass and energy balances on metals and PFAS around the system and the various unit processes, and comparing the process's life-cycle cost to conventional solids treatment and beneficial use or disposal technologies. This study seeks to provide comprehensive insights into the potential of gasification for managing PFAS in biosolids effectively.

Figure 1- 1 shows a general schematic diagram of gasification/pyrolysis processes, which are thermochemical processes that decompose carbonaceous materials at elevated temperatures, typically ranging from 1,100-1,800°F, using stoichiometrically insufficient oxygen or no oxygen. Higher temperatures convert most carbon in the biosolids to syngas, resulting in an inert, sand-like material. Conversely, lower temperatures yield carbon-rich biochar suitable for various applications. The generated syngas is passed through the thermal oxidation process at temperatures >2000°F thus removing PFAS and generating energy that can be used in drying, and the gasification process itself. While most gasification technologies require feedstock greater than 75% TS, some technologies prefer feedstock between 50% - 70% TS. Furthermore, these technologies do not require stabilized biosolids.

The gasifier studied in this research is operating at the Edmonds facility in Washington and processing 30 wet tons per day dewatered combined and waste activated sludge solids. Figure 1-2 shows the process flow diagram of the tested facility that includes direct drum drying, gasification, thermal oxidation, air treatment and ancillary processes. The gasification operates at higher than usually practiced temperature, thus generating sand like material termed by the vendor as FLGSand. Sampling was conducted at nine (9) key points for PFAS and other solids constituents. Sampling included stack air sampling (Figure 1-3) for PFAS analysis-a first for such studies at two locations, before and after an activated carbon filter for air treatment. Collected samples from the FLGSand were analyzed by two independent laboratories for accuracy.

Mass and energy balance were conducted based on operational conditions and analytical data. As shown in Figure 1-4, the system processed approximately 550 dry lbs/hr) of dewatered cake, producing about 92 lbs/hr of inert product/sand, representing over a 95% mass reduction. The gasifier produced approximately 4.2 mbtu/hr of energy in the syngas, which is subsequently recovered as heat for the dryer. The dryer used approximately 2.5 mbtu/hr. The operation and inherent energy value of the solids resulted in a net positive energy balance.

The targeted PFAS load (g F/d) in the entire and split system are presented in Figure 1-5. Results from two different laboratories confirmed no PFAS was detected in the FLGSand. The overall process achieved a significant mass and volume reduction compared to the input. The heated air from the gasifier, after passing through the thermal oxidizer and heat exchanger, was used in the thermal dryer. This air has been reported to contain very little to no PFAS. However, the drying process caused the air to pick up trace concentrations of PFAS precursors and terminal products, which were released in the scrubber water and stack air at very low levels. On a mass loading basis, about 5% of the total input measured PFAS was released in the air and about 15% is recovered in the scrubber water. Granular activated carbon (GAC) can be practiced capturing ~ 90% of the PFAS concentration from the scrubber water, thus increasing the system overall PFAS removal efficiency. Also, it has been demonstrated that using regenerative thermal oxidation (RTO) can remove more than Also, it has been demonstrated that using regenerative thermal oxidation (RTO) on air can remove more than 95% of the measured PFAS.

In addition, the study measured PFOA and PFOS due to their regulatory significance. Data showed no detectable levels of these compounds in the solids output, trace PFOA in scrubber water, and negligible PFOA, PFOS in stack air. Additional analyses of TF and TOF on solids and liquids corroborated the findings of target PFAS analysis.