Reverse osmosis filters remove 95 to 99% of dissolved solids from water, including heavy metals, salts, fluoride, and most microorganisms. They are one of the most thorough filtration methods available for home use, but they do have blind spots, particularly with certain dissolved gases and some small organic chemicals.
How the Membrane Works
Unlike most water filters, which trap contaminants by catching them in tiny pores, reverse osmosis membranes are effectively non-porous. Water molecules don’t flow through holes. Instead, they absorb into one side of the membrane, diffuse through it, and emerge on the other side. This is called the solution-diffusion model, and it’s what makes RO so effective: contaminants are rejected not just by size but by their inability to dissolve into and travel through the membrane material.
The result is a filter that blocks particles, bacteria, dissolved minerals, and even individual salt ions. Most contaminants are flushed away with the rejected water that drains out of the system.
Dissolved Solids and Minerals
The headline number for any RO system is its TDS rejection rate. TDS, or total dissolved solids, includes everything dissolved in your water: minerals, salts, metals, and other ions. A well-functioning RO membrane typically rejects 95 to 99% of TDS. High-performance membranes can hit 98.5% rejection under optimal conditions.
This means RO removes the vast majority of calcium, magnesium, sodium, potassium, and other minerals from your water. It also strips out dissolved metals like iron, manganese, and zinc. For people on well water with high mineral content or anyone dealing with hard, salty, or metallic-tasting tap water, this is the primary benefit.
The tradeoff is that beneficial minerals get removed along with harmful ones. Some RO systems include a remineralization stage that adds back small amounts of calcium and magnesium to improve taste and slightly raise the pH of the filtered water.
Heavy Metals and Toxic Contaminants
RO is particularly effective at reducing contaminants that pose health risks at low concentrations. Under the NSF/ANSI 58 certification standard, the only required test is TDS reduction, but manufacturers can also certify their systems for optional contaminant categories. These include:
- Lead
- Arsenic (both trivalent and pentavalent forms)
- Cadmium
- Chromium (hexavalent and trivalent)
- Barium
- Selenium
- Copper
- Radium 226/228
- Nitrate and nitrite
- Perchlorate
- Asbestos
If your concern is a specific contaminant like lead or arsenic, look for a system that carries the NSF/ANSI 58 certification for that particular substance, not just for general TDS reduction.
Fluoride
RO systems remove 85 to 95% or more of fluoride from tap water, depending on membrane quality, water pressure, and how well the system is maintained. This makes reverse osmosis the most reliable home method for reducing fluoride, since standard carbon filters and most pitcher filters barely touch it. If you’re specifically trying to lower fluoride levels, an RO system with NSF/ANSI 58 certification for fluoride reduction is the most straightforward option.
Bacteria, Viruses, and Parasites
Because the membrane blocks contaminants at the molecular level, bacteria and viruses are far too large to pass through. Testing on RO membranes has consistently shown greater than 4-log removal of microorganisms, which translates to over 99.99% reduction. This applies to bacteria, viruses, and protozoan cysts like Cryptosporidium and Giardia.
That said, RO systems in homes aren’t typically relied on as a primary disinfection method the way they might be in emergency or off-grid situations. Municipal water is already disinfected before it reaches your tap. The microbiological barrier of an RO membrane is more of an added safety layer than the main reason most people install one.
PFAS (Forever Chemicals)
PFAS are synthetic chemicals that don’t break down in the environment and have been linked to health concerns at very low exposure levels. The EPA has confirmed that reverse osmosis point-of-use systems can greatly reduce PFAS levels in drinking water, and RO systems certified under NSF/ANSI 58 are one of the agency’s recommended options for home PFAS reduction.
One important caveat: as of early 2024, the certification standards for PFAS filters have not yet been updated to reflect the EPA’s newest, stricter drinking water limits for these chemicals. That means a certified system will reduce PFAS significantly, but the certification label alone doesn’t guarantee reduction down to the latest federal standards. The EPA is working with standards organizations to close that gap.
What RO Doesn’t Remove Well
The membrane’s solution-diffusion mechanism has a weakness: dissolved gases. Because small gas molecules can dissolve into the membrane material and diffuse through it just like water does, they aren’t effectively blocked. This includes radon, hydrogen sulfide (the compound behind a rotten egg smell), and methane. If your water has a sulfur odor, an RO system alone won’t solve it. Activated carbon or aeration systems are better suited for dissolved gas problems.
Certain volatile organic compounds (VOCs), particularly small solvent molecules, can also slip through the membrane. The same goes for some pesticides and herbicides with very low molecular weights. Most multi-stage RO systems address this by pairing the membrane with an activated carbon filter, which is effective at trapping organic chemicals. If your system has a carbon pre-filter or post-filter (most residential units do), the combination covers a much wider range of contaminants than the membrane alone.
Factors That Affect Performance
The rejection rates quoted above assume a system that’s working properly. Several things can degrade performance over time. Water pressure is the driving force behind reverse osmosis. If your home’s water pressure is low, the membrane won’t reject contaminants as efficiently, and some systems include a booster pump to compensate. Water temperature also matters: colder water moves through the membrane more slowly, which can affect output but generally doesn’t reduce contaminant rejection.
Membrane age is the biggest variable. As the membrane fouls or degrades, rejection rates drop. Most manufacturers recommend replacing the RO membrane every two to three years, with pre-filters and post-filters changed more frequently, often every six to twelve months. A system that hasn’t been maintained will still produce water, but it won’t deliver the 95-plus percent rejection rates it’s capable of. If you notice a change in taste or your TDS meter readings start climbing, the membrane is likely due for replacement.