Perfluorooctane sulfonate (PFOS) is one of the most prevalent and studied PFAS compounds found in drinking water. Used extensively in firefighting foam (AFFF) and in manufacturing processes for decades, PFOS has contaminated water supplies near military bases, airports, and industrial sites across the United States.
PFOS Properties Relevant to Filtration
PFOS is an 8-carbon chain sulfonate compound with strong hydrophobic character and high molecular weight. These properties make it highly amenable to removal by activated carbon adsorption and reverse osmosis membrane rejection. PFOS binds strongly to activated carbon surfaces and is effectively excluded by RO membranes based on both size and charge. It is among the most readily removed PFAS compounds by commercial filter technologies.
Removal Performance
NSF-certified carbon block filters typically achieve 96–99% PFOS removal. RO systems commonly achieve 97–99.5%. Ion exchange resins designed for PFAS removal show comparable performance. The challenge with PFOS is not its removability but rather the extremely high contamination levels found near point sources — some well water near military AFFF training sites contains PFOS at hundreds to thousands of ppt — requiring treatment systems sized appropriately for high influent concentrations.
Regulatory Context
PFOS is regulated alongside PFOA at an MCL of 4 ppt under EPA’s 2024 PFAS rule. For communities near AFFF source areas, achieving 4 ppt may require more robust treatment than for areas with ambient contamination. Point-of-use RO supplemented by certified carbon pre-filtration provides the most reliable compliance pathway for households with highly contaminated source water.