PFAS Assessment of a Rotary Drum Dryer

Introduction Biosolids drying is gaining interest as a volume reduction strategy in regions where biosolids management outlets are becoming increasingly constrained due to concerns over per- and polyfluoroalkyl substances (PFAS). However, no studies as of yet have been conducted on the fate of PFAS through a thermal dryer system. Synagro partnered with Brown and Caldwell to conduct a PFAS mass balance through a centrifuge dewatering and rotary drum dryer facility to evaluate the fate of PFAS through the system and assess potential points of PFAS transfer to the environment. Methodology A three-day PFAS sampling event was conducted the week of April 17, 2023. Process samples were taken at all inputs and outputs to the system with composite sampling over a four hour period each day. An overview of the dewatering and dryer system and PFAS sampling points is provided in Figure 1. The dewatering and dryer system was operated at roughly 2,300 dry pounds of total solids per hour for the duration of the test. Gas-phase sampling was conducted by a commercial stack sampler using an OTM-45 PFAS sampling train at the stack and prior to the regenerative thermal oxidizer emissions (RTO) control device. In addition, a fourth run was conducted on the final day of the week when the dryer was down, but the scrubber was kept in operation to evaluate if PFAS were released from cooling water in the scrubber. The PFAS data was received in October 2023 and are currently being evaluated using the data quality analysis plan developed under the Brown and Caldwell lead Water Research Foundation Tailored Collaboration program project (WRF 5111). Results and Discussion PFAS sampling data is under review and will be available for detailed comparison by 2024 WEF RBC. An initial review of preliminary findings is provided in Tables 1, 2 and 3 attached with the following findings below. - Individual PFAS were transferred from biosolids to the gas phase at a transfer rate of 50% to 92% through the dryer. - No widespread trend of PFAS accumulation in the scrubber water was observed. Individual PFAS showed a modest increase from 10% to 20%. - Eighteen individual PFAS were detected in the gas phase from the dryer, prior to treatment in the RTO, - HFPO-DA and trace amounts of three PFAS were detected in the emissions from the RTO. Field QC and prior study findings indicate HFPO-DA detection may be due to contamination/interference, more work is required to verify. - More work required to refine liquid sludge analysis and develop system-wide PFAS mass balance. Conclusions This study is the first to present data demonstrating PFAS transfer from solids phase to the process air during thermal drying of biosolids. The data suggests the thermal oxidizer is capable of providing effective PFAS destruction or removal from the gas phase. More analysis will be conducted to present PFAS partitioning through the dewatering system and evaluate the aggregate PFAS mass balance through the dewatering and drying process.