PFDA Removal: Strategies for Effective Management of Per- and Polyfluoroalkyl Substances (PFAS)

Introduction

Perfluorodecanoic acid (PFDA) is a long-chain PFAS that appears regularly in water monitoring data but carries no federal drinking water limit. That combination — measured often, regulated nowhere at the national level — makes it a compound utilities need to understand without necessarily needing to act on it separately. This article covers what PFDA is, its actual regulatory position, how it behaves in treatment, and why it is usually addressed as a consequence of treating other compounds rather than as a target in its own right.

PFDA sits within the family of compounds compared in our overview of PFAS removal by compound.

Understanding PFDA

What is PFDA?

PFDA is a perfluoroalkyl carboxylic acid with a ten-carbon chain — two carbons longer than PFOA. It belongs firmly to the long-chain group, and that structure determines its behavior: longer chains adsorb more strongly to treatment media, accumulate more readily in biological tissue, and are less water-soluble than their shorter counterparts.

Sources

PFDA occurs as a manufacturing byproduct and impurity in fluorochemical production, in firefighting foam formulations, and as a degradation product of longer fluorotelomer compounds. Detections are most common near industrial sites, military installations, airports, and downstream of wastewater treatment plants.

Health Considerations

Toxicological research on PFDA is less extensive than for PFOA and PFOS, but available evidence suggests effects consistent with other long-chain perfluoroalkyl acids, including liver effects, immune effects, and developmental effects. As a long-chain compound it is eliminated from the body slowly.

Regulatory Status: What Actually Applies

No Federal Drinking Water Limit

EPA’s April 2024 drinking water regulation covered six PFAS: PFOA, PFOS, PFHxS, PFNA, HFPO-DA, and mixtures via a hazard index. PFDA is not among them. There is no federal maximum contaminant level for PFDA, and the May 2026 proposal — which would retain the PFOA and PFOS limits and rescind the others — does not add one.

Nor has PFDA been designated a hazardous substance under CERCLA; that designation applies to PFOA and PFOS.

Where PFDA Does Appear

• Monitoring programs: PFDA is included in EPA’s national monitoring of unregulated contaminants, so occurrence data exists for many systems.
• Analytical methods: PFDA is on the analyte lists of EPA Methods 533 and 537.1 for drinking water and Method 1633 for other matrices, so utilities testing for regulated PFAS typically receive PFDA results as well.
• State programs: Some states regulate or monitor a broader set of PFAS than the federal list, and requirements should be confirmed with the state primacy agency.

The practical position for most utilities is that PFDA is a compound they have data on, not a compliance obligation.

PFDA in Treatment

Conventional Treatment

Coagulation, sedimentation, filtration, and chlorination provide essentially no PFDA removal, as with all PFAS.

Granular Activated Carbon

GAC removes PFDA well. Because adsorption strength increases with chain length, PFDA adsorbs more strongly than PFOA and considerably more strongly than short-chain compounds. In a system treating mixed PFAS, PFDA is among the last compounds to break through — which is why treatment sized around PFOA and PFOS breakthrough handles PFDA comfortably.

Ion Exchange

PFAS-selective anion exchange resins capture PFDA efficiently, again benefiting from the strong hydrophobic interaction that long chains provide.

Reverse Osmosis and Nanofiltration

High-pressure membranes reject PFDA at very high rates. Membrane performance depends less on chain length than adsorptive media, so PFDA offers no particular challenge here either.

What Does Not Work

Conventional advanced oxidation processes — ozone, UV, UV with hydrogen peroxide — do not degrade PFDA or other perfluorinated acids. The fully fluorinated chain resists hydroxyl radical attack, and these processes can convert precursors into PFDA and similar compounds rather than destroying them.

The Practical Implication

PFDA does not generally require a treatment strategy of its own. Any system installed to meet the PFOA and PFOS limits will remove PFDA at least as effectively. The compound worth designing around is the weakest-adsorbing one present — almost always a short-chain compound — since that governs media replacement frequency and therefore operating cost.

The general principles governing this group of compounds are covered in our discussion of long-chain PFAS treatment.

Implementation Considerations

Monitoring

Where PFDA appears in monitoring results, it is worth tracking for two reasons. It contributes to the total PFAS burden, which matters for risk communication and for understanding sources. And its presence alongside other long-chain compounds helps characterize the contamination — a useful input to source identification.

Designing for Regulatory Change

Utilities designing PFAS treatment should size systems with some allowance for future requirements. Compounds outside today’s list may be regulated later, and systems built with capacity to add vessels or change media adapt at lower cost than those built tightly to current limits.

Residuals

PFDA removed from water ends up on spent media or in membrane concentrate along with every other PFAS. Residuals management is a single question for all compounds, not a separate one for each.

Comparison with Regulated Compounds

Understanding PFDA is easier in relation to compounds with established limits. Its closest regulated analogue is PFOA, a carboxylic acid two carbons shorter — see our coverage of PFOA water treatment. PFOS, a sulfonic acid of eight carbons, adsorbs still more strongly because sulfonates bind more tightly than carboxylates of the same length, as discussed in our guide to PFOS removal technology.

The ordering that results — sulfonates binding more strongly than carboxylates, and longer chains more strongly than shorter — is what determines the sequence in which compounds break through a carbon bed, and therefore what any treatment system should be designed around.

Future Directions

Research on PFDA continues in two areas: toxicology, where the evidence base remains thinner than for PFOA and PFOS, and treatment, where the broader work on selective adsorbents and destruction technologies applies to PFDA as to other long-chain PFAS.

Conclusion

PFDA is a long-chain PFAS with no federal drinking water limit, appearing regularly in monitoring data because it is included in standard analytical methods and national monitoring. Its long chain makes it readily removed by the same technologies used for PFOA and PFOS — granular activated carbon, anion exchange, and high-pressure membranes — and it is typically among the last compounds to break through.

For most utilities, PFDA therefore requires monitoring and understanding rather than dedicated treatment. Systems designed around the short-chain compounds that actually limit performance will handle PFDA as a matter of course. Confirming state requirements remains worthwhile, since some states regulate beyond the federal list.