PFAS IN FINAL TREATED SOLIDS (BIOSOLIDS) FROM 190 MICHIGAN WASTEWATER TREATMENT PLANTS

Concentrations, compositions, and variability of PFAS in biosolids are characterized using an extensive dataset of 350 samples from 190 wastewater treatment plants (WWTPs) across Michigan. All samples are comprised of final treated solids generated at the end of the wastewater treatment process. This study represents the second largest study population of biosolids samples to date (the largest study was limited to only 11 PFAS) and reflects modern PFAS signatures for a geographically-diverse range of WWTPs serving communities of various sizes and industrial activity. Concentrations of Σ₂₄ PFAS are lognormally distributed with a range of 1 to 3200 ng/g dry wt. and average 108 ± 277 ng/g dry wt. Compounds with carboxyl and sulfonic functional groups comprised 29% and 71% of Σ₂₄ PFAS concentrations, respectively, on average. Primary sample variability is associated with long-chain PFAS with higher tendency for partitioning to biosolids. Short-chain carboxylic compounds, most notably PFHxA, are responsible for secondary variability and detected in 77% of the samples with average concentrations of 10 ± 30 ng/g dry wt. Despite the voluntary manufacturing phase-out in the early 2000s, PFOS continues to be a dominant compound in biosolids and was detected in 95% of samples. Sulfonamide precursor compounds, N-MeFOSAA in particular, contribute more to total PFAS than PFOS. PFAS enrichment in biosolids (relative to influent concentrations) generally increase with compound hydrophobicity, although the partitioning of PFAS onto biosolids in WWTPs is a complex process not easily described using experimentally-derived partitioning coefficients. Additionally, PFAS enrichment in biosolids confirms the presence of longer-chain PFAS that are typically undetected in WWTP influent. Study trends in PFAS composition and concentration are placed into a global perspective through comparison and contrast with summary statistics extracted from 30 other WWTP biosolids/sludge studies.