Reverse osmosis is one of the most effective ways to remove PFOA and PFOS from drinking water. RO membranes typically achieve over 90% removal of these contaminants, and lab testing of commercial RO membranes has shown removal rates above 99% for both PFOA and PFOS. For most households concerned about PFAS in their tap water, an under-the-sink reverse osmosis system is a reliable option.
How Reverse Osmosis Filters Out PFAS
RO membranes work through a combination of physical size exclusion and electrical charge repulsion. PFOA and PFOS molecules are roughly 1 nanometer in size, while RO membrane pores range from 0.1 to 1 nanometer. That tight fit means most PFAS molecules simply can’t pass through. PFOS, with a molecular weight of 538 g/mol, is large enough to be reliably trapped by the membrane’s structure. The membrane surface also carries a slight negative charge that repels PFAS molecules, adding a second layer of rejection beyond size alone.
This is what distinguishes reverse osmosis from simpler filtration methods. Standard water filters and even ultrafiltration membranes have pore sizes of 2 to 100 nanometers, far too large to catch PFAS. Only RO and nanofiltration membranes are tight enough to block these molecules effectively.
Removal Rates for Long-Chain and Short-Chain PFAS
PFOA and PFOS are “long-chain” PFAS, meaning they have longer molecular structures that are easier for RO membranes to catch. Testing of a widely used commercial RO membrane (the Dow BW30) showed over 99% removal of PFOA, PFOS, and nine other PFAS compounds ranging from four to eleven carbon atoms in chain length. RO membranes consistently achieved over 99% removal across all tested conditions in comparative studies.
Short-chain PFAS, like PFBA, are smaller molecules and slightly harder to remove. They require higher operating pressures and tighter membranes. Still, RO handles them far better than other home treatment options. Research from NC State University found that under-the-sink RO systems removed 57 to 72% of short-chain PFAS, compared to just 29 to 65% for activated carbon fridge filters. For long-chain PFAS like PFOA and PFOS, activated carbon filters ranged from 88 to 100% removal, but RO remains more consistent across the full spectrum of PFAS types.
How RO Compares to Carbon Filters
Activated carbon filters can reduce PFOA and PFOS, particularly at higher concentrations and when the filter is fresh. But their performance drops as the carbon becomes saturated, and they struggle with shorter-chain PFAS compounds. RO systems don’t rely on chemical adsorption the way carbon does, so their performance is more stable over time. NC State’s research concluded that reverse osmosis filters remove PFAS better than activated carbon filters overall.
One practical note: dual-stage carbon filters (two carbon cartridges in sequence) performed similarly to reverse osmosis in some tests. If installing an RO system isn’t feasible, a high-quality dual-stage carbon filter can still provide meaningful PFAS reduction, though it will need more frequent filter changes to maintain performance.
What the EPA Now Requires
In April 2024, the EPA finalized enforceable limits for PFOA and PFOS in public drinking water at 4.0 parts per trillion (nanograms per liter) for each compound individually. That’s an extremely low threshold. To put it in perspective, NSF/ANSI certification standards for home water treatment systems test whether a product can reduce PFOA and PFOS to a combined concentration of 20 parts per trillion. A certified RO system should get you well below the EPA’s new limits, assuming your source water isn’t extraordinarily contaminated.
If you’re shopping for an RO system, look for one certified under NSF/ANSI Standard 58 with specific claims for “PFOA Reduction” and “PFOS Reduction.” Not all RO systems have been tested for PFAS. The certification means the system has been independently verified to meet that 20 parts per trillion combined threshold under standardized test conditions.
What Happens to the Removed PFAS
Reverse osmosis works by pushing water through the membrane while diverting contaminants into a separate waste stream called reject water. The PFAS your system catches doesn’t disappear. It gets flushed down the drain in concentrated form along with other rejected minerals and contaminants. For a home system, this reject water enters your municipal wastewater system. RO systems also produce 2 to 4 gallons of waste water for every gallon of filtered water, which is worth considering if water conservation is a concern in your area.
Keeping Your System Effective
Most residential RO systems include pre-filters (often a sediment filter and an activated carbon cartridge) that protect the RO membrane from chlorine damage and particulate fouling. These pre-filters need regular replacement, typically every 6 to 12 months, depending on your water quality. If a carbon pre-filter becomes saturated, it stops protecting the membrane, which can shorten the membrane’s lifespan and reduce overall PFAS removal.
The RO membrane itself generally lasts 2 to 3 years before needing replacement. Factors that shorten membrane life include high sediment loads, high chlorine levels (which is why the carbon pre-filter matters), and very hard water. Following the manufacturer’s replacement schedule is the simplest way to ensure your system continues removing PFAS at the rates it was certified for.