Fate of PFAS Through a Biosolids Drum Dryer with Regenerative Thermal Oxidizer Emissions Control

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) have become an important environmental contaminant of concern, particularly for water resource recovery facilities (WRRFs) that face increasing scrutiny regarding disposition of wastewater solids (biosolids when stabilized). In the United States, concerns regarding PFAS release to soils (Munoz et al., 2022), groundwater (Pepper et al., 2023; Pepper et al., 2021) and agricultural crops (Ghisi et al., 2019) from land application have increased scrutiny on traditional solids management outlets. Thermal dryers are commonly used for biosolids volume reduction and stabilization, but the fate of PFAS in these systems is not well understood. This study presents findings from a full-scale PFAS fate study through a municipal biosolids rotary drum dryer with a regenerative thermal oxidizer (RTO), which is the most widely applied air pollution control device for biosolids drum drying. To the author's knowledge, this is the first study evaluating the fate of PFAS through a biosolids rotary drum dryer considering gas-phase emissions.

The facility selected for this study includes centrifuge dewatering and a conventional rotary drum dryer equipped with a condenser, venturi scrubber, and RTO. All major inputs and outputs sampled are shown in Figure 1 using the following numbering: (1) liquid sewage sludge or solids, (2) dewatering centrate or liquid return, (3) dewatered sewage sludge or solids, (4) furnace combustion air, (5) dried products or pellets, (6a) treated WRRF effluent or process water scrubber supply, (6b) potable water scrubber supply, (7) condenser drain, (8) venturi scrubber drain), (9) dryer process gas exhaust or off-gas, and (10) RTO flue gas or exhaust stack

Triplicate samples from solid, liquid, and gas inputs and outputs were analyzed, including dryer emissions before and after the RTO. PFAS in solid and aqueous were analyzed using the commercial lab Eurofins' proprietary standard operating procedures (SOPs) based on Methods SW846 and Method 537 (modified) adapted from EPA Method 537.1 for targeted PFAS analysis. Gas phase sampled were according to 'Other Test Method 45 (OTM-45). Liquid and solids samples were analyzed for total organic fluorine (TOF) methods using combustion ion chromatography (CIC).

Substantial PFAS removal in the solids (60% on a total molar basis for targeted PFAS) was observed in the dryer without a statistically significant increase in targeted PFAS in exhaust scrubbing system process drains. Total organic fluorine analyses were also performed on the solid and liquid samples and generally concurred with the targeted PFAS analysis, but was complicated by high reporting limits. PFAS were detected in the dryer exhaust using OTM-45, but at a total mass six orders of magnitude less than that measured in the dewatered feed solids. Destruction or removal efficiency (DRE) of the gas-phase PFAS detected in the dryer exhaust using OTM-45 through the RTO was complicated by detection of comparatively high levels of HFPO-DA in the RTO exhaust sample and field blank train (FBT). When including HFPO-DA found in the non-contaminated OTM-45 analytical fraction in the gas-phase emissions sampling data, the DRE through the RTO using OTM-45 was 93.5%. However, its relatively low thermal stability, and high levels of contamination in the other three FBT analytical fractions, casts doubt on its presence in the emissions samples. If considered without HFPO-DA, the DRE was 99.3%. PFAS transformation could be a potential reason for gap in the targeted PFAS mass balance through the dryer. Findings from this study demonstrate that the RTO is capable of substantially removing targeted PFAS detected by OTM-45.

Although limitations in analytical techniques, and contamination of the gas phase sampling train limited the study's ability to complete a PFAS mass balance, several key findings were made. First, when the targeted PFAS and TOF values for the dryer inputs and outputs are considered, it appears likely that a considerable amount of PFAS can transfer from the dewatered solids (and cake water) to the gas phase within the dryer. Second, only a small fraction of targeted PFAS were detected in the gas phase, indicating PFAS may transfer within the dryer, or within the exhaust recycle loop through the furnace. Finally, the RTO demonstrates high levels (>90%) of PFAS destruction or removal, but the actual DRE value is called into question due to HFPO-DA contamination. If the HFPO-DA readings are disregarded, the RTO achieves a DRE of 99.3% for all PFAS on a sum molar mass basis. Further work is recommended to better understand drivers behind PFAS phase transfer within the drying system to identify the extent that PFAS may be undergoing biotransformation within the solids mass or volatilized to the gas stream and transformed within the recycle line passed through the dryer furnace using OTM-50 in addition to OTM-45 gas-phase sampling. Finally, greater understanding into the cause for HFPO-DA contamination in this study, as well as other similar PFAS stack sampling studies, will be required to advance the state of the science and verify PFAS destruction through high-temperature processes such as the RTO.