What Does Reverse Osmosis Remove? (Complete Contaminant List)
Drinking Water Filtration
What Does Reverse Osmosis Remove? (Complete Contaminant List)
Reverse osmosis is one of the most effective drinking water treatment methods available. It removes contaminants that most other filters cannot touch, including lead, arsenic, nitrates, PFAS, and fluoride. But it has real limitations, too. After 30+ years helping homeowners solve water quality problems, I've put together this complete reference covering what RO actually removes, the real removal rates, and which contaminants require a different approach entirely.
Looking for the best under-sink system? Start with our Best Under-Sink Water Filter Guide.
The Short Version
Reverse osmosis forces water through a semipermeable membrane with pores around 0.0001 microns (for a full explanation of the process, see how reverse osmosis works). That's small enough to block the vast majority of dissolved contaminants. Here's the quick breakdown:
- Excellent removal (95-99%+): Lead, arsenic (As V), chromium, fluoride, nitrates, PFAS/forever chemicals, radium, uranium, microplastics, TDS, sodium, barium, cadmium, copper, selenium
- Good removal (85-95%): Arsenic (As III), chloride, sulfate, phosphate, silica
- Requires pre-filter: Chlorine (carbon pre-filter), chloramine (catalytic carbon pre-filter), sediment (sediment pre-filter)
- Not effective for: Dissolved gases (hydrogen sulfide, radon), most VOCs (need activated carbon), some pesticides, dissolved oxygen
- Better handled by whole-house systems (see RO vs other water filters for when RO is and isn’t the right choice): Iron (use an iron filter), hardness (use a water softener), low pH (use an acid neutralizer)
RO is best used as a point-of-use drinking water system at one tap, not a substitute for whole-house water treatment. For the full overview of under-sink filtration options, see our under-sink water filter guide.
What Contaminant Are You Concerned About?
Select your concern to see if reverse osmosis is the right solution.
Reverse osmosis is one of the most effective methods for removing lead from drinking water. NSF/ANSI 58-certified RO systems remove up to 99% of lead. The EPA specifically recommends point-of-use RO for homes with lead service lines or lead solder in plumbing.
If lead is your primary concern, an under-sink RO system at your kitchen tap is the right solution.
RO is highly effective against arsenate (As V), removing 95% or more. For arsenite (As III), removal drops to around 60-80%. If your water contains As III, an oxidation pre-treatment step converts it to As V before the membrane.
For well water, whole-house iron filters using Katalox Light media also remove arsenic alongside iron and manganese, protecting every tap in the house.
RO is one of the few residential methods that effectively removes nitrates from drinking water. Removal rates typically range from 90-95%. This is critical because the EPA MCL for nitrates is 10 mg/L, and levels above that pose serious health risks, especially for infants.
Standard carbon filters and water softeners do NOT remove nitrates. If nitrates are your concern, RO or a dedicated whole-house nitrate filter are your options. See our complete nitrate guide for health risks and treatment details.
Reverse osmosis is one of the few filtration methods that reliably removes fluoride. Standard carbon filters and water softeners do not. RO systems typically reduce fluoride by 90-95%.
Whether your fluoride is naturally occurring (common in certain well water regions) or added by your municipality, an under-sink RO handles it effectively.
Reverse osmosis is one of the most effective residential methods for removing PFAS (per- and polyfluoroalkyl substances), commonly called "forever chemicals." Studies show removal rates of 95% or higher for both PFOA and PFOS.
Activated carbon also reduces PFAS levels, and a quality whole-house carbon filter combined with an under-sink RO provides a strong two-layer approach. Learn more in our PFAS water filter guide.
The RO membrane itself is actually damaged by chlorine. That's why every RO system includes a carbon pre-filter that removes chlorine before water reaches the membrane. The carbon stage removes 95%+ of free chlorine.
For chloramine (used by many municipalities), you need a catalytic carbon pre-filter instead of standard activated carbon. If you're on city water and want chlorine/chloramine removed from every tap, a whole-house carbon filter is the better approach.
An intact RO membrane blocks bacteria and most viruses due to its extremely small pore size. However, a tiny tear or worn O-ring can allow pathogens through without any visible sign of failure. You would never know.
For biological safety, UV disinfection is the industry standard. It provides a reliable kill step with no membrane integrity concerns. If your well water tests positive for coliform bacteria, a UV purifier should be your first line of defense.
While RO membranes technically reject dissolved iron, iron will foul and destroy the membrane rapidly. Iron staining also affects your entire plumbing system, appliances, laundry, and fixtures, none of which a single-tap RO system protects.
The correct solution for iron in well water is a whole-house iron filter. It treats every drop of water entering your home and requires no annual maintenance with the right media.
RO does remove calcium and magnesium (the minerals that cause hard water). But hard water causes scale buildup in your water heater, pipes, dishwasher, and every fixture in your house. A single-tap RO system cannot protect any of that.
A water softener treats your entire home, protects your plumbing, and extends appliance life. Use a softener for hardness, and an RO at the kitchen sink for drinking water purity.
Hydrogen sulfide is a dissolved gas. It passes right through an RO membrane. If your water has a rotten egg smell, you need a whole-house treatment system that oxidizes and filters out the sulfur before it reaches any tap in your home.
An AIO (air injection oxidation) iron filter removes hydrogen sulfide along with iron and manganese in a single tank, with no chemicals required.
Microplastics are far larger than the RO membrane's pore size. Studies show reverse osmosis removes over 99% of microplastics from drinking water. Even the sediment pre-filter catches many of them before water reaches the membrane.
If microplastics in your drinking water are a concern, RO is one of the most reliable solutions available.
Total dissolved solids (TDS) is a measure of everything dissolved in your water: minerals, salts, metals, and other inorganic compounds. RO is the most effective residential method for reducing TDS, typically achieving 90-98% removal.
Keep in mind that not all TDS is bad. Calcium and magnesium contribute to TDS but are beneficial minerals. If your TDS is high due to harmful contaminants (lead, arsenic, nitrates), RO is the right answer. If it's just natural mineral content, high TDS doesn't necessarily mean unhealthy water.
How Reverse Osmosis Removes Contaminants
Reverse osmosis works by pushing water under pressure through a semipermeable membrane. The membrane's pores are approximately 0.0001 microns, roughly 500,000 times smaller than a human hair. Water molecules pass through; most dissolved contaminants are too large and get rejected into a waste stream.
A typical residential RO system has multiple stages:
- Sediment pre-filter (5 microns): Removes sand, silt, and particulate matter that would clog the membrane
- Carbon pre-filter: Removes chlorine, which damages the RO membrane, along with some organic chemicals and taste/odor compounds
- RO membrane: The core stage. Rejects dissolved solids, heavy metals, salts, and most contaminants
- Carbon post-filter: Final polishing step for taste and any remaining trace organics
The effectiveness of each stage matters. The RO membrane handles the heavy lifting on dissolved contaminants, but the carbon stages are essential for chlorine, VOCs, and taste. That's why a quality multi-stage system outperforms a membrane alone.
RO Removal Effectiveness Scale
Green = excellent • Blue = good • Yellow = partial • Red = poor or no removal
Complete Contaminant Removal Table
This table covers the most common contaminants homeowners ask about. Removal rates are based on NSF/ANSI 58 certification testing, EPA data, and peer-reviewed research. Actual results vary by membrane condition, water pressure, temperature, and TDS levels.
| Contaminant | Category | RO Removal Rate | Notes |
|---|---|---|---|
| Lead | NSF 58 certified. One of the best residential methods for lead removal. | ||
| Arsenic (As V) | Arsenate form. Pre-oxidation converts As III to As V for better removal. | ||
| Arsenic (As III) | Arsenite form. Lower rejection rate; oxidation pre-treatment recommended. | ||
| Nitrates (NO₃) | Chemical | One of the few residential methods effective against nitrates. EPA MCL: 10 mg/L. | |
| Fluoride | Mineral | Standard carbon filters do NOT remove fluoride. RO is the most accessible option. | |
| PFAS (PFOA/PFOS) | Chemical | EPA finalized PFAS MCLs at 4 ppt (2024). RO is a top residential method. | |
| Chromium-6 | Hexavalent chromium. Few other residential methods are effective. | ||
| Radium 226/228 | Radioactive contaminant found in some well water. RO is highly effective. | ||
| Uranium | Common in granite bedrock regions. EPA MCL: 30 μg/L. | ||
| Cadmium | EPA MCL: 0.005 mg/L. RO removes it effectively. | ||
| Copper | Often leaches from plumbing. Acid neutralizer fixes the root cause (low pH). | ||
| Mercury | Inorganic mercury. Organic mercury (methylmercury) removal is lower. | ||
| Barium | EPA MCL: 2 mg/L. | ||
| Selenium | EPA MCL: 0.05 mg/L. | ||
| Sodium | Mineral | Good for reducing sodium after a water softener (ion exchange adds sodium). | |
| TDS (Total Dissolved Solids) | Mineral | Measures total dissolved content. High TDS isn't always harmful. | |
| Chloride | Mineral | Monovalent ion; slightly lower rejection than divalent contaminants. | |
| Sulfate | Mineral | EPA secondary standard: 250 mg/L. | |
| Phosphate | Mineral | Effectively removed by the membrane. | |
| Silica | Mineral | Can cause scaling on the membrane if levels are very high. | |
| Microplastics | Organic | Microplastics are far larger than the membrane pore size. | |
| Cysts (Giardia, Cryptosporidium) | Biological | Protozoan cysts are large enough to be fully blocked by the membrane. | |
| Bacteria (E. coli, Coliform) | Biological | Membrane blocks bacteria, but UV is more reliable (no integrity risk). | |
| Viruses | Biological | Some very small viruses may pass through a damaged membrane. UV recommended. | |
| Chlorine (free) | Chemical | Removed by the carbon pre-filter stage, not the membrane itself. | |
| Chloramine | Chemical | Standard carbon is slow with chloramine. Catalytic carbon pre-filter recommended. | |
| Hardness (Ca/Mg) | Mineral | Removes at the tap, but a water softener protects your entire home. | |
| Iron (dissolved) | Mineral | Technically removes it but iron fouls the membrane. Use an iron filter instead. | |
| Manganese | Mineral | Same as iron: technically removed, but will damage the membrane over time. | |
| Hydrogen Sulfide (H₂S) | Chemical | Dissolved gas passes through the membrane. Use an AIO iron/sulfur filter. | |
| Radon | Chemical | Dissolved gas. Aeration or GAC (granular activated carbon) is effective. | |
| VOCs (benzene, toluene, etc.) | Organic | Some low-molecular-weight VOCs pass through. Carbon filtration is better. | |
| Pesticides/Herbicides | Organic | Varies by compound. Carbon post-filter helps; combined approach is best. | |
| Pharmaceuticals | Organic | Most pharmaceutical compounds are well-rejected by quality membranes. |
A Note on Removal Rates
These percentages represent typical performance under standard test conditions (77°F, 40-60 psi feed pressure, new membrane). Real-world removal rates can drop if water pressure is low, the membrane is aged, water temperature is cold, or TDS is very high. Replacing filters on schedule and maintaining adequate water pressure are the two most important factors for consistent performance.
Lead
RO removal rate: 97-99%
Lead in drinking water is a serious health concern. There is no safe level of lead exposure, according to the CDC. It's particularly dangerous for children, where even low levels can affect brain development, and for pregnant women.
Lead typically enters water through aging pipes, solder, and fixtures, not from the water source itself. That means your water can test clean at the well or treatment plant and still contain lead by the time it reaches your kitchen faucet.
Reverse osmosis is one of the best residential solutions for lead. NSF/ANSI 58-certified systems are independently verified to reduce lead to safe levels. Because lead contamination happens in the plumbing, a point-of-use system at the kitchen sink (where you drink and cook) is actually the most strategic placement.
The EPA recommends RO as a point-of-use treatment specifically for lead. A whole-house system would be overkill since you don't need to remove lead from shower or toilet water.
Bottom Line
An under-sink RO system is the gold standard for lead removal at the drinking water tap. Our Pure-75 Reverse Osmosis system ($595) handles lead effectively with a higher flow rate for busy households.
Arsenic
RO removal rate: 95-99% (As V) / 60-80% (As III)
Arsenic in well water is more common than many homeowners realize, particularly in areas with certain geological formations. The EPA MCL is 0.010 mg/L (10 ppb), and long-term exposure above this level increases the risk of cancer and other health effects.
Arsenic exists in two forms in water:
- Arsenate (As V): The oxidized form. RO removes 95-99% of it.
- Arsenite (As III): The reduced form, commonly found in well water with low dissolved oxygen. RO only removes 60-80% without pre-treatment. An oxidation step (like an AIO filter upstream) converts As III to As V, dramatically improving removal.
For well water with arsenic alongside iron and other common well water issues, a whole-house approach often makes more sense. Iron filters using Katalox Light media can remove arsenic alongside iron and manganese from every tap. In fact, from working with hundreds of customers dealing with arsenic in well water, we've found that addressing iron and pH first with whole-house filtration often brings arsenic levels down significantly.
RO at the kitchen sink provides an excellent final safety net for drinking water, especially when combined with whole-house treatment upstream.
Nitrates
RO removal rate: 90-95%
Nitrates are one of the most important contaminants to know about because they're dangerous, common in agricultural areas, and very few treatment methods actually remove them.
The EPA MCL for nitrates is 10 mg/L. Above this level, nitrates pose a serious risk to infants (blue baby syndrome) and pregnant women. High nitrates in well water typically come from agricultural runoff, fertilizers, or septic system proximity.
Here's what does NOT remove nitrates:
- Standard carbon filters
- Water softeners
- Sediment filters
- UV purifiers
- Boiling (actually concentrates them)
Your options for nitrate removal are limited to reverse osmosis or a dedicated ion-exchange nitrate resin system. For drinking water, an under-sink RO is the most practical solution. For whole-house nitrate treatment, a whole-house nitrate filter using nitrate-selective resin treats every tap in the home.
For a deep dive on nitrate contamination, health risks, testing, and all treatment options, see our Nitrates in Well Water guide. If your well water tests above 5 mg/L for nitrates, call Aidan at 800-460-5810 to discuss the right approach for your situation.
Fluoride
RO removal rate: 90-95%
Fluoride is one of the hardest contaminants to remove from drinking water. Most standard filters don't touch it. That's because fluoride ions are small and don't adsorb well onto typical filter media.
Sources of fluoride in water:
- Municipal water: Many cities add fluoride at 0.7 mg/L for dental health
- Well water: Naturally occurring fluoride from geological deposits can range from trace levels to over 4 mg/L in some regions
The EPA MCL for fluoride is 4 mg/L, with a secondary standard of 2 mg/L. Whether you want to reduce municipally-added fluoride or address naturally high levels in well water, RO is the most reliable and accessible residential solution.
Other options for fluoride removal include activated alumina and bone char filters, but these are less common and require more frequent media replacement. RO remains the practical choice for most homeowners. For more on fluoride, see our fluoride in drinking water guide.
PFAS (Forever Chemicals)
RO removal rate: 95-99%
PFAS (per- and polyfluoroalkyl substances) have become one of the most talked-about water contaminants in recent years. These synthetic chemicals, used in nonstick cookware, firefighting foam, food packaging, and hundreds of other products, don't break down in the environment. That's why they're called "forever chemicals."
In 2024, the EPA finalized the first enforceable drinking water standards for six PFAS compounds, setting MCLs as low as 4 parts per trillion (ppt) for PFOA and PFOS. That's an extremely low threshold, and it means even trace levels matter.
Reverse osmosis is one of the most effective residential methods for PFAS removal. Studies consistently show 95%+ reduction in both PFOA and PFOS concentrations. Activated carbon also provides meaningful PFAS reduction, which means the carbon pre-filter and post-filter stages in an RO system contribute additional removal beyond the membrane itself.
For well water homes concerned about PFAS, a two-layer approach works well: a whole-house carbon filter to reduce PFAS throughout the home, combined with an under-sink RO for maximum drinking water purity. Learn more about PFAS treatment options in our PFAS in drinking water guide.
Chlorine and Chloramine
Chlorine removal: 95%+ (via carbon pre-filter) | Chloramine: partial (needs catalytic carbon)
An important distinction: the RO membrane itself doesn't remove chlorine. In fact, chlorine damages polyamide RO membranes. Every quality RO system includes a carbon pre-filter specifically to remove chlorine before water reaches the membrane.
This matters because it means the carbon stage is doing the chlorine removal, not the RO process. If chlorine in your water is your main concern and you don't need heavy metal or nitrate removal, a whole-house carbon filter removes chlorine from every tap in your house, including showers, where chlorine exposure through steam inhalation and skin absorption is significant.
Chloramine Requires Special Attention
Many municipalities have switched from chlorine to chloramine (a combination of chlorine and ammonia). Standard activated carbon filters remove chloramine much more slowly than free chlorine. If your city uses chloramine, make sure your RO system (or whole-house filter) uses catalytic carbon, which is specifically designed for chloramine removal. Check your city's water quality report to find out which disinfectant is used.
Bacteria and Viruses
RO removal rate: 99%+ (but UV is more reliable)
An intact RO membrane is small enough to physically block bacteria (like E. coli and coliform), protozoan cysts (Giardia, Cryptosporidium), and most viruses. Under ideal conditions, removal rates exceed 99%.
However, I do not recommend relying on RO as your primary defense against biological contamination. Here's why:
- Membrane integrity: A microscopic tear, a worn O-ring, or a poor seal can allow pathogens through. You would have no way to detect this failure without laboratory testing.
- Single-tap protection: An under-sink RO only protects one faucet. Bacteria can enter through any tap, shower, or water-using appliance in your home.
- No residual protection: Unlike UV or chlorination, RO provides no ongoing disinfection in your plumbing downstream of the system.
For well water homes with bacterial concerns, a whole-house UV purifier is the correct first line of defense. UV provides a reliable kill step (99.99% inactivation of bacteria and viruses) for your entire home, with no chemicals and no membrane integrity risk.
The ideal setup: UV for whole-house biological safety, with an RO at the kitchen sink for maximum drinking water purity. That gives you two independent layers of protection at the tap where it matters most.
What Reverse Osmosis Does NOT Remove
No single treatment method removes everything. RO has clear blind spots that are important to understand:
Dissolved Gases (H₂S, Radon, CO₂)
Dissolved gases pass through the RO membrane because they exist as gas molecules, not dissolved solids. This means RO will not fix rotten egg smell (hydrogen sulfide), radon in water, or excess carbon dioxide.
What to use instead: For H₂S, an AIO iron/sulfur filter oxidizes and removes it. For radon, aeration or a whole-house granular activated carbon (GAC) filter is effective.
Volatile Organic Compounds (VOCs)
Some VOCs (benzene, toluene, MTBE, TCE) have small molecular weights that allow partial passage through the RO membrane. Removal varies from 40-80% depending on the specific compound. The carbon pre-filter and post-filter stages in multi-stage RO systems help, but activated carbon filtration is the primary treatment method for VOCs.
What to use instead: A quality activated carbon filter (either whole-house or as part of a multi-stage system) is the standard treatment for VOCs.
Chloramine
Standard activated carbon removes chloramine slowly because the chloramine molecule is more stable than free chlorine. A standard RO system with a basic carbon pre-filter may not fully remove chloramine, which can then damage the membrane over time.
What to use instead: Catalytic carbon (often sold as Centaur or similar trade names) is specifically designed for chloramine removal. If your municipality uses chloramine, make sure any RO or whole-house system uses catalytic carbon.
Whole-House Problems (Iron, Hardness, pH, Sulfur)
RO technically removes many of these contaminants. But iron fouls the membrane, hardness causes scale throughout your plumbing, low pH corrodes pipes in every room, and sulfur smells come from every faucet. An under-sink RO that treats one tap cannot address whole-house water quality problems.
What to use instead: Iron filter, water softener, acid neutralizer, or the correct combination in the right order.
When a Whole-House System Is the Better Answer
Reverse osmosis is excellent at what it does: producing ultra-pure drinking water at a single tap. But many of the water quality problems homeowners deal with require a fundamentally different approach. Here's when to look beyond RO:
| Problem | Why RO Isn't Enough | Better Solution |
|---|---|---|
| Iron in well water | Iron stains fixtures, clogs pipes, ruins laundry, and fouls RO membranes | Whole-house iron filter |
| Hard water | Scale builds up in water heater, pipes, dishwasher; shortens appliance life | Water softener |
| Low pH / acidic water | Corrodes copper pipes throughout your house; causes blue-green stains and pinhole leaks | Acid neutralizer |
| Rotten egg smell (H₂S) | Dissolved gas passes through the RO membrane. The smell comes from every tap. | AIO iron/sulfur filter |
| Bacteria / Coliform | RO protects one tap; bacteria enter through every faucet, shower, and appliance | Whole-house UV purifier |
| Sediment / sand / silt | Clogs everything upstream of the RO. Needs to be removed before it reaches any filter. | Sediment filter |
| Chlorine / taste / odor | Affects showers, cooking, ice, and every glass of water. RO only fixes one tap. | Whole-house carbon filter |
The most effective approach for many homes: solve whole-house problems first with the right combination of treatment systems, then add an under-sink RO at the kitchen sink as a final polishing step for drinking water. This is what we recommend to most of our customers dealing with well water.
Not sure what your water needs? Call Aidan at 800-460-5810 or send your water test results to support@midatlanticwater.net. We'll tell you exactly what you need, in the right order, with no pressure to buy anything you don't need.
The Right Treatment Sequence Matters
If you have multiple water quality issues, the order of your treatment systems is critical. The wrong sequence can damage equipment or reduce effectiveness. Our guide on the correct order of well water treatment systems explains exactly how to set up a multi-system approach.
📊 Infographic: What Does Reverse Osmosis Remove?
Visual breakdown showing contaminants sorted by removal effectiveness. Green zone (95%+), blue zone (85-95%), yellow zone (partial), and red zone (not removed). Each contaminant shown as a labeled bar with percentage.
🔧 Infographic: 4-Stage RO System Diagram
Cross-section showing water flow through sediment pre-filter, carbon pre-filter, RO membrane, and carbon post-filter. Labels showing what each stage removes and why the order matters.
🏠 Infographic: RO vs. Whole-House Treatment
Side-by-side showing what an under-sink RO protects (one tap) vs. what a whole-house system protects (every tap, appliance, pipe, and fixture). Visual highlighting when each is the right choice.
Frequently Asked Questions
Does reverse osmosis remove lead from water?
Yes. Reverse osmosis removes 97-99% of lead from drinking water. NSF/ANSI 58-certified RO systems are independently verified for lead removal. The EPA recommends point-of-use RO as a treatment for homes with lead in plumbing. An under-sink RO system at the kitchen tap is one of the most effective and practical residential solutions for lead.
Does reverse osmosis remove fluoride?
Yes, reverse osmosis removes 90-95% of fluoride. This applies to both naturally occurring fluoride in well water and fluoride added to municipal water supplies. RO is one of the few residential filtration methods that effectively reduces fluoride. Standard carbon filters and water softeners do not remove it.
Does reverse osmosis remove PFAS (forever chemicals)?
Yes. Reverse osmosis removes 95%+ of PFAS compounds, including PFOA and PFOS. The EPA set drinking water standards for PFAS as low as 4 parts per trillion in 2024, making effective removal critical. RO is one of the most reliable point-of-use methods. Activated carbon also reduces PFAS, so the carbon stages in a multi-stage RO system provide additional protection. See our PFAS water filter guide for more details.
Does reverse osmosis remove bacteria and viruses?
An intact RO membrane blocks 99%+ of bacteria and most viruses due to its extremely small pore size (0.0001 microns). However, a microscopic tear or worn seal can allow pathogens through undetected. For biological safety, a UV water purifier is more reliable because it provides a definitive kill step without depending on membrane integrity. The best approach is whole-house UV plus an under-sink RO for redundant protection at the drinking water tap.
Does reverse osmosis remove chlorine?
Yes, but the carbon pre-filter stage removes chlorine, not the RO membrane itself. In fact, chlorine damages the RO membrane, which is why every RO system includes a carbon pre-filter to strip chlorine before water reaches the membrane. If you want chlorine-free water from every tap in your house (not just the kitchen sink), a whole-house carbon filter is the better solution.
Does reverse osmosis remove nitrates?
Yes. RO removes 90-95% of nitrates. This is significant because very few residential treatment methods are effective against nitrates. Carbon filters, water softeners, sediment filters, and UV purifiers do NOT remove nitrates. Boiling water actually concentrates them. If your well water tests above the EPA limit of 10 mg/L, an under-sink RO system or a dedicated ion-exchange nitrate filter are your options.
Does reverse osmosis remove iron from water?
Technically, yes. An RO membrane rejects 95-99% of dissolved iron. But using RO for iron removal is a bad idea: iron fouls the membrane rapidly, reducing its life and effectiveness. More importantly, iron staining affects your entire house (fixtures, laundry, pipes, appliances), and a single-tap RO doesn't protect any of that. The right solution for iron in well water is a whole-house iron filter.
Does reverse osmosis remove arsenic?
Yes, but effectiveness depends on the form. RO removes 95-99% of arsenate (As V) but only 60-80% of arsenite (As III). Many well water sources contain As III, which requires an oxidation pre-treatment step to convert it to As V for maximum removal. For well water with arsenic alongside iron, a whole-house iron filter using Katalox Light media removes both contaminants from every tap.
Does reverse osmosis remove microplastics?
Yes, RO removes 99%+ of microplastics. Microplastic particles are far larger than the RO membrane's pore size, so they are easily blocked. Even the sediment pre-filter stage captures many microplastics before water reaches the membrane. If microplastics in drinking water are a concern for you, RO is one of the most effective solutions available.
What is the disadvantage of reverse osmosis water?
The main drawbacks of RO are: (1) water waste, as typical systems send 2-4 gallons down the drain for every gallon produced; (2) mineral removal, since RO strips beneficial minerals like calcium and magnesium along with harmful contaminants; (3) slow production rate, typically 50-75 gallons per day with a storage tank; (4) maintenance, as filters need replacement every 6-12 months and the membrane every 2-3 years; and (5) single-tap coverage, meaning it only treats one faucet while whole-house problems need whole-house solutions.
Does reverse osmosis remove E. coli?
An intact RO membrane blocks E. coli and other bacteria (99%+ removal). However, a membrane defect can allow bacteria through undetected. For well water with E. coli or coliform bacteria, a UV water purifier is the recommended primary defense because it provides a reliable kill step without depending on membrane integrity. UV treats your whole house; RO only covers one tap.
Does reverse osmosis remove hard water minerals?
Yes, RO removes 95-99% of calcium and magnesium (the minerals that cause hard water). However, hard water causes problems throughout your entire plumbing system: scale in the water heater, buildup in pipes, spots on fixtures, reduced soap efficiency. A single-tap RO can't prevent any of that. A water softener treats your entire home and is the correct solution for hard water.
Does reverse osmosis remove radon from water?
No. Radon is a dissolved gas, and dissolved gases pass through the RO membrane. For radon in well water, effective treatment options include aeration systems or whole-house granular activated carbon (GAC) filtration. A whole-house carbon tank is often the most practical residential approach for radon removal.
More in Our Under-Sink & RO Guide Series
Aidan Walsh has been in the water treatment industry for over 30 years, helping homeowners across the Mid Atlantic region solve complex water quality problems. From iron and sulfur in well water to lead and PFAS in city water, Aidan has diagnosed and fixed thousands of water systems. Have a question about what's in your water? Call Aidan directly at 800-460-5810 or email your water test to support@midatlanticwater.net.