Reverse osmosis removes the vast majority of contaminants from water, including heavy metals, dissolved salts, bacteria, viruses, fluoride, and “forever chemicals” like PFAS. It works by forcing water through a membrane with pores approximately 0.0001 microns in size, small enough to block almost everything except water molecules and a handful of dissolved gases and volatile organic compounds.
How the Membrane Works
A reverse osmosis (RO) membrane acts as an ultra-fine physical barrier. At 0.0001 microns, the pores are roughly 500,000 times smaller than the diameter of a human hair. Water molecules pass through under pressure, while larger molecules, ions, and particles get left behind and flushed away as wastewater.
The result is a dramatic reduction in total dissolved solids (TDS). According to the FDA, a typical RO system achieves a 90% to 95% rejection rate for dissolved solids. If your tap water starts at 300 parts per million TDS, the filtered water will contain roughly 15 to 30 parts per million.
Heavy Metals and Inorganic Contaminants
This is where reverse osmosis truly excels. Lead rejection rates consistently hit 99% or higher across multiple studies. Arsenic removal ranges from 91% to over 99%, depending on the form of arsenic present and the specific membrane used. Chromium-6, a carcinogen found in some municipal water supplies, sees rejection rates between 88% and 99.8%.
RO also strips out minerals you might actually want: calcium, magnesium, sodium, and potassium all get reduced by 60% to 98%. Fluoride, nitrates, and sulfates are removed at similarly high rates. Overall, inorganic contaminant removal typically falls between 95% and 100%.
Bacteria, Viruses, and Parasites
Bacteria like E. coli are typically 0.2 to 5 microns in size. Viruses are smaller, around 0.02 to 0.3 microns. Protozoan cysts like Giardia and Cryptosporidium range from 1 to 15 microns. All of these are dramatically larger than the 0.0001-micron RO membrane pore, so they’re physically blocked from passing through. In practice, a well-maintained RO system is one of the most effective point-of-use methods for removing microbiological contamination from drinking water.
PFAS and “Forever Chemicals”
Per- and polyfluoroalkyl substances (PFAS) have become one of the most talked-about water contaminants, and reverse osmosis handles them well. EPA research tested residential RO systems against six different PFAS compounds, including PFOA and PFOS, and found that most systems reduced all tested PFAS to below detection limits (less than three parts per trillion). Even the lowest-performing system in the study still removed more than 75% of individual PFAS compounds. That underperformance was linked to intermittent use, which suggests running your RO system regularly helps maintain its effectiveness.
What Reverse Osmosis Does Not Remove
RO has real blind spots, and they matter. Dissolved gases pass right through the membrane because they’re small enough to slip between the pores. Hydrogen sulfide, the gas responsible for a rotten-egg smell in some well water, is the most common example.
Certain volatile organic compounds (VOCs) and pesticides also bypass the membrane. These include benzene, carbon tetrachloride, toluene, trichloroethylene, and total trihalomethanes, along with pesticides like atrazine and 2,4-D. Their molecular structures allow them to dissolve into and permeate through the membrane material rather than being blocked by it.
This is why most home RO systems pair the membrane with an activated carbon filter. The carbon stage catches chlorine, VOCs, pesticides, and organic solvents that the membrane misses. If you’re shopping for an RO system, look for one with both pre- and post-carbon filtration stages rather than a standalone membrane.
What Happens to the Water’s pH and Taste
Freshly produced RO water has a neutral pH of about 7. But within an hour of sitting in a glass exposed to air, it can drop to a pH of 5 to 5.5. That’s mildly acidic, comparable to black coffee. The explanation is simple: very pure water absorbs carbon dioxide from the air, which forms carbonic acid. The fewer minerals present to buffer that reaction, the more the pH drops.
The mineral removal also affects taste. Water with nearly zero mineral content can taste flat or slightly metallic to some people. This is why many RO systems include a remineralization stage that adds small amounts of calcium and magnesium back into the water after filtration, improving both the taste and the pH stability.
Water Waste and Efficiency
Every RO system produces wastewater because the rejected contaminants have to go somewhere. In residential systems, the typical ratio is about 4 gallons of wastewater for every 1 gallon of purified water. Some newer models improve on this, and industrial systems can reach a 1:1 ratio, though at significantly higher cost and complexity.
If water waste concerns you, some homeowners route the reject water to their garden, washing machine, or storage tank for non-drinking uses. Just keep in mind that this concentrate contains everything the membrane removed, so it will have elevated levels of whatever contaminants were in your source water.
RO Compared to Other Filtration Methods
- Activated carbon filters are effective for chlorine, taste, odor, and many organic chemicals, but they don’t remove dissolved salts, heavy metals at high rates, or fluoride. RO handles all of these.
- Distillation produces similarly pure water and also removes heavy metals and dissolved solids. It’s slower and more energy-intensive than RO, but it handles volatile organics better because the boiling step can evaporate some compounds that an RO membrane lets through.
- UV purification kills bacteria and viruses but doesn’t remove any chemical contaminants at all. It’s often paired with RO as an extra layer of biological protection.
For most households dealing with a broad range of potential contaminants, a multi-stage RO system with carbon filtration offers the widest coverage in a single setup. If your primary concern is a specific contaminant like VOCs or hydrogen sulfide, a carbon filter or aeration system may actually be more appropriate than RO alone.