What Does Reverse Osmosis Remove? (Complete Contaminant List)
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Reverse osmosis takes out most of the dissolved metals and salts in your water: lead, arsenic (one form may need treating first), uranium, fluoride and sodium, and part of the nitrate. What it does not take out matters just as much. Gases like hydrogen sulfide, the rotten egg smell, go right through the membrane. Some pesticides and solvents get past it. It is not a way to treat bacteria, and it treats one faucet, not the house.
A caller from Tennessee had heard that an RO adds something back to the water. I told him, "No, it takes out. It doesn't add, it takes out contaminants." The real question is which ones, and what to use for the rest. That is what this page lays out, contaminant by contaminant.
Quick Answer
Reverse osmosis does not remove gases like hydrogen sulfide, and some pesticides, solvents and volatile organic chemicals (VOCs) get past it. It is also not meant for water with bacteria in it.
- Hydrogen sulfide goes through. It is a gas, and the membrane does not stop it. The maker of our MAW-50 allows none in the water going in. See what reverse osmosis does not remove.
- Some chemicals get past it. Some pesticides, solvents and VOCs get through the membrane. The carbon filters catch some of them.
- Chlorine is the carbon filter's job. The carbon filter takes chlorine out before it can damage the membrane. Chloramine, which some cities use instead, is a different chemical: neither of our two RO units' makers lists chloramine reduction, so ask your utility which one it uses.
- Bacteria need a UV light first. See does reverse osmosis remove bacteria and viruses.
- What it does take out. Most dissolved metals and salts, and part of the nitrate. See what reverse osmosis removes and which filter removes each heavy metal.
- Iron and hardness come out first. They ruin the membrane. See what ruins an RO membrane.
What's in This Guide
What reverse osmosis does not remove
Gases like hydrogen sulfide go right through an RO membrane, and so do some pesticides, solvents and VOCs. The University of Nebraska's guide to home RO puts it plainly: hydrogen sulfide "passes through the RO membrane," and "some pesticides, solvents and volatile organic chemicals (VOCs) are not removed by RO." Chlorine and bacteria are two more things an RO is not built to handle on its own. Here is each one and what to use instead.
| In your water | What the RO does | What to use instead |
|---|---|---|
| Hydrogen sulfide (rotten egg smell) | It is a gas, so it goes right through the membrane. Our MAW-50's maker allows none (0.0 ppm) in the water going in. | A whole-house filter for the sulfur smell, picked by your level. See the hydrogen sulfide guide. |
| Radon | The membrane does not stop it. The University of Nebraska lists radon among what the carbon filter in an RO system removes. | Test for it first. See how to test for radon in well water. |
| Some pesticides, solvents and VOCs | Some get through the membrane. The carbon filters catch some of them, like benzene, toluene and atrazine. | Activated carbon. A VOC lab test tells you which ones you have. |
| Chlorine | The carbon filter takes it out before the membrane. Chlorine would damage the membrane. | For chlorine at every tap and in the shower, a whole-house carbon filter. |
| Bacteria and viruses | Not meant for it. See the next section. | A UV light. See the UV guide. |
| Iron, hardness and sediment | They clog and ruin the membrane. | Take them out before the RO. See what ruins an RO membrane. |
Does reverse osmosis remove bacteria and viruses?
An intact RO membrane can stop bacteria, but reverse osmosis is not meant for water that has bacteria in it. The Nebraska guide says only coliform-free water should be fed to an RO, because bacteria can damage the membrane and "contamination may occur through pinhole leaks." The maker of our MAW-50 says the same thing in its own words: "Do not use where water is micro-biologically unsafe or with water of unknown quality without adequate disinfection before or after the unit."
So when a well test shows coliform or E. coli, the RO is not the answer to it. What I tell people is simple: "If you have bacteria in the water, you have to have a UV light first." The UV treats the whole house; see how to get rid of coliform bacteria in well water.
What reverse osmosis removes
Reverse osmosis removes most dissolved metals and salts, and it lowers nitrate. The Nebraska guide's table of what household RO units remove includes lead, arsenic, uranium, radium, chromium, cadmium, mercury, copper, fluoride, sodium and sulfate. The membrane does that work. The filters around it are there to protect it and to catch what it misses.
The membrane is also the difference between an RO and a carbon filter. "One of the things that reverse osmosis will remove that a regular carbon filter doesn't is TDS or total dissolved solids." TDS is all the minerals and salts dissolved in the water, and when it runs 500 ppm or more, an RO is the way to bring it down in your drinking water.
| Contaminant | Does an RO take it out? | What to know |
|---|---|---|
| Heavy metals (lead, arsenic, uranium, copper, chromium, cadmium, mercury) | Yes | Arsenic has one form that may need treating first. See which filter removes each heavy metal. |
| Radium | Yes | EPA lists reverse osmosis among the treatments for radium. |
| Nitrate | Part of it | I tell nitrate callers to expect about 50 to 70 percent less. Bigger numbers come from tests run "under laboratory conditions." See the nitrate guide. |
| Fluoride | Yes | Fluoride is on Nebraska's list. See fluoride in drinking water. |
| PFAS (forever chemicals) | Many kinds | EPA says RO membranes are "typically more than 90 percent effective at removing a wide range of PFAS." The two we sell are not certified for PFAS. See the PFAS guide. |
| Sodium, sulfate and other salts | Yes | Salts are part of your TDS reading. |
How much an RO takes out varies
How much an RO takes out depends on how much is in the water, which contaminant it is, the membrane's condition, and the water pressure and temperature. Nebraska's guide shows why that matters with nitrate. A membrane that rejects 85 percent turns 40 mg/L into 6, which is under the 10 mg/L limit. The same membrane turns 80 mg/L into 12, which is still over it.
Some RO systems are certified to NSF/ANSI 58, which tests a system against specific claims such as lead, arsenic V, fluoride or nitrate, and requires at least 75 percent of the dissolved solids to come out. The two systems we sell are not certified as systems. Their maker rates the membranes at 91 to 98 percent of dissolved solids. Whatever RO you buy, test the water from its faucet after it goes in, then check it with a TDS meter now and then. A torn membrane does not announce itself.
Which filter removes each heavy metal
An under-sink RO lowers every heavy metal in this table at the kitchen tap. The whole-house answer is different for each metal. Of the 361 well water lab tests my customers sent in that came back between May 20 and October 2, 2026, lead came back over EPA's action level on 20 and arsenic over its limit on 11. Chromium and mercury were never over.
| Metal | EPA limit | Well tests over the limit | Kitchen tap | Whole house |
|---|---|---|---|---|
| Lead | 0.015 mg/L (15 ppb), action level | 20 of 361 | RO, or a filter certified to reduce lead | A whole-house filter cannot catch lead from the faucets and pipes after it. EPA says skin does not absorb lead in water, so the drinking tap is where it matters. On a well, an acid neutralizer stops the acidic water that eats the pipes. |
| Copper | 1.3 mg/L, action level | 18 of 361 | RO | An acid neutralizer when the water is acidic |
| Arsenic | 0.010 mg/L (10 ppb) | 11 of 361 | RO, though one form (arsenic III) may need treating first. See the arsenic guide. | Our whole house arsenic removal system, $3,495, with arsenic-selective resin |
| Uranium | 0.030 mg/L (30 ppb) | 2 of 361 | RO | EPA lists ion exchange. Its maker says the resin in our arsenic system also removes uranium. Ask before buying it for uranium alone. |
| Cadmium | 0.005 mg/L (5 ppb) | 1 of 361 | RO | EPA lists ion exchange. We do not sell a system for cadmium. |
| Chromium | 0.1 mg/L (100 ppb), total | 0 of 361 | RO | EPA lists ion exchange. We do not sell a system for chromium. |
| Mercury | 0.002 mg/L (2 ppb) | 0 of 361 | RO | EPA lists granular activated carbon |
EPA's lists are the treatments it names for water systems to meet each limit (40 CFR 141.62 and 141.66). For a kitchen tap, EPA's own advice on lead is a filter certified by an accredited third party to reduce it. Ours are not certified for lead, so test the RO water after it goes in.
Text Aidan a photo of your report
Send the page with the numbers. Aidan will tell you whether an RO at the kitchen sink covers it or the house needs more.
8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.What ruins an RO membrane
Iron, hardness and sediment ruin an RO membrane. On a well, they come out before the water reaches the RO. A Maryland caller with an RO already under her sink asked about adding more. I told her that where there is a lot of iron, "that has to be taken out, and certain levels of hard water have to be taken out before it can go through the RO because it'll foul the membrane."
The makers put numbers on it. These are the limits on the water going into the two systems we sell:
| Water going in | Our MAW-50 | Our Pure-75 |
|---|---|---|
| Iron | 0.3 ppm or less | 1 ppm or less |
| Hardness | Under 15 grains | Under 300 ppm (about 17.5 grains) |
| Hydrogen sulfide | 0.0 ppm | Not listed |
| Manganese | Under 0.05 ppm | Not listed |
| Chlorine | Not listed | 3 mg/L or less |
| Water pressure | 30 to 100 psi | 30 to 100 psi |
The number I give callers is lower than the MAW-50 sheet's. When a nitrate caller asked about an RO, I told him to make sure the test checks iron and hardness, "'cause if you have like 10 grains of hardness, it's-- or you have iron in it, it's gonna fail the membrane." For iron, that means an iron filter; for hardness, a water softener. Both treat the whole house, and the RO gets clean water to work with.
Sediment is the easy one. A caller on a well with an old under-sink RO and nothing ahead of it asked if he needed a pre-filter. I told him a 20-inch Big Blue with a 5 micron cartridge, for the whole house, after the well tank: "every well should have one." See the Big Blue filter guide.
One lab test covers 53 contaminants
One certified lab test, mailed to you, run by an accredited lab, reports iron, hardness, lead, arsenic, uranium, nitrate and TDS from the same sample. If you already have a lab report, skip this.
An RO treats one faucet
An under-sink RO is connected to one faucet, usually at the kitchen sink. It does nothing for the shower, the laundry or the water heater. As I told the Tennessee caller at the top of this page, "People wanna filter the whole house and different types of contaminants like hard water, iron, sulfur, manganese, arsenic."
A whole-house RO exists, but it is a different machine. When the Maryland caller above asked about one, I told her you have to work out how many gallons a day the house uses, then size a commercial unit, "like a 350 gallon a day, a 500 gallon a day," with "storage tanks in the basement next to where the RO goes." On city water, a California caller asked about adding our Pure-75 to a carbon filter and a softener. I told him the carbon filter and the softener "really takes care of everything. If you wanna add the reverse osmosis, that's fine, but they don't make a whole lot of water." The RO vs other water filters page compares the two.
Frequently asked questions
What is the downside of reverse osmosis?
It treats one faucet. It also sends water down the drain while it makes the water you drink. EPA says a typical under-sink unit sends five gallons or more down the drain for every gallon it makes. The filters need changing about every six months. The membrane needs changing when a TDS meter shows it fading; the Pure-75's maker rates its membrane at 24 months. RO water can also be more corrosive than untreated water, which is why the system comes with its own faucet and tubing.
Is it unhealthy to drink reverse osmosis water?
Not because the minerals are gone. As I put it, "It's clean, very clean, very safe water, and the minerals don't really matter because in order to get any benefit from the minerals in the water, you'd probably have to, what they estimate, is drink about 800 glasses of water a day." What keeps RO water safe is the rest of the setup: the right pre-treatment for your water, filters changed on time, and a UV light ahead of it if the well has bacteria.
Aidan Walsh
Co-owner & Residential Water Treatment Specialist, Mid Atlantic WaterAidan Walsh has spent 32 years in residential water treatment, the first stretch of it installing and servicing well systems before Mid Atlantic Water became an online supplier. Mid Atlantic Water is a family company; every recommendation on this site is based on what actually failed in the field and what fixed it. If you have a question this page did not answer, call 800-460-5810 or text him at 443-277-2204.
Aidan answers calls and texts 8 AM to 5 PM Eastern, every day.Number copied. Text it from your phone: 443-277-2204.