How Reverse Osmosis Works: What It Removes and What It Wastes

Short answer: a reverse osmosis (RO) system uses your water pressure to push water through a very fine membrane. Water gets through; most dissolved salts, metals and many other contaminants don’t, and they’re flushed down the drain with part of the water. It can reduce lead, arsenic, nitrate, fluoride, PFAS and total dissolved solids, but only the contaminants a model is certified for. It doesn’t handle dissolved gases or most volatile chemicals on its own, it sends several gallons down the drain for every gallon you drink in a typical system, and it’s often more than you need. Check your water first.

How does a reverse osmosis system work?

The EPA describes RO as a process where “pressure forces water through a semi-permeable membrane”, creating two streams: treated water (called permeate or product) and reject water (concentrate or brine) that carries the contaminants the membrane held back. The concentrate goes to the drain. The membrane’s pores are tiny: about 0.0001 micron, according to the CDC.

A typical under-sink system is a short treatment train. The parts vary by model, but the Connecticut Department of Public Health describes the usual pieces:

• Pre-filter: a sediment filter catches sand, silt and other particles so they don’t foul the membrane. Turbidity, iron, manganese, hydrogen sulfide and chlorine can also affect how well an RO unit works, Connecticut notes, so check what your system’s pre-filters are designed to handle.
• RO membrane: the core of the system, where the separation happens.
• Storage tank: the membrane makes water much more slowly than a normal tap runs, so most systems store treated water in a small tank to have it ready.
• Post-filter: often an activated carbon filter that polishes the water and removes organic chemicals and dissolved gases the membrane didn’t stop.
• Separate faucet: RO water usually comes from its own small faucet at the sink.

Most home systems run on household water pressure, which sometimes needs a boost; some systems use an electric pump.

Tank vs tankless RO

A storage tank is convenient, but it pushes back. As the tank fills, the pressure inside it slows down water production and makes the system less efficient, the EPA explains. Tankless systems make water on demand instead, and the certification standard rates the two types differently for water use (see below).

A permeate pump is a non-electric add-on that reduces back pressure from the tank to improve efficiency, and an automatic shut-off stops water flowing to the membrane (and the drain) when the tank is full. Connecticut’s fact sheet recommends making sure every unit has one.

What does reverse osmosis remove?

According to the CDC, RO systems remove some chemicals including lead, copper, chromium, chloride and sodium, and may reduce arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate and phosphorus. The CDC also lists parasites, bacteria and viruses among what the membrane removes. Connecticut’s list adds dissolved (ferrous) iron, manganese, uranium, total dissolved solids (TDS), some pesticides and some taste, color and odor chemicals.

For PFAS, the EPA tells you to look for a PFAS reduction claim under NSF/ANSI 53 or 58 and then confirm it on the certifier’s site for the type of PFAS you want to remove. The same page adds a caveat: the PFAS certification standards (as of April 2024) did not yet show that a filter removes PFAS down to the levels in EPA’s PFAS drinking water rule, and the EPA is working with standard-setting bodies to update them. For where that rule stands now, see our PFAS guide.

“May reduce” matters: results depend on the membrane, water pressure, the contaminant and its concentration, Connecticut notes, and “RO membranes cannot be certified to remove 100 percent of anything.”

What reverse osmosis doesn’t remove (or doesn’t do well)

• Dissolved gases and most volatile organic chemicals. Connecticut lists dissolved gases and most volatile and semi-volatile organic contaminants, including some pesticides and solvents, as things RO is not effective against. That’s the job of the carbon stages in the system.
• Germs in an unsafe well. The membrane’s pores are small enough to hold back bacteria and viruses, but Connecticut’s private well program says RO units alone “are not recommended for treatment of bacteria and other microscopic organisms.” If a well test shows bacteria, follow your health department’s advice on disinfecting and fixing the well, and any boil-water guidance, rather than relying on an under-sink unit.
• Anything outside its claims. Under certification, a system is only verified for the contaminants on its listing. One thing the NSF listings make clear: arsenic claims under NSF/ANSI 58 are for pentavalent arsenic, and the listings note the product is “intended for use only on water supplies that have been shown to be at or below the referenced concentration and valency of arsenic.” If your well has arsenic, ask your lab or health department whether your water needs pretreatment before RO.

RO also isn’t meant for the whole house. In its RO specification, the EPA notes that point-of-entry RO treats, and wastes, far more water than an under-sink unit, and that most uses of household water (toilets, laundry, bathing) don’t need RO-quality water.

What does NSF/ANSI 58 certify?

NSF/ANSI 58 is the standard for reverse osmosis drinking water treatment systems, and the EPA calls it “the primary standard used in the United States to certify RO systems.” Based on the EPA’s summary of the standard and NSF’s own certification listings:

• TDS reduction is mandatory. NSF/ANSI 58 requires every certified system to reduce total dissolved solids by at least 75 percent.
• Other contaminant claims are optional. On NSF’s listings, each model shows which reductions it’s certified for. Claims available under the standard include lead, arsenic (pentavalent), nitrate/nitrite, fluoride, radium 226/228, hexavalent and trivalent chromium, copper, barium, cadmium, selenium and cysts, among others. A model listed for TDS and lead isn’t certified for nitrate just because it’s an RO system.
• Efficiency rating: the percentage of the water going into the system that comes out as treated water under conditions meant to approximate typical use. Only systems with a storage tank and an automatic shut-off have one.
• Recovery rating: the percentage of water going into the membrane that comes out as treated water when there’s no tank, or the tank is bypassed. Tankless systems only have a recovery rating.

That last pair is where shoppers get misled. A tank system’s recovery rating ignores the tank’s back pressure, so it looks better than real life: “50 percent recovery” on a tank system doesn’t mean one gallon wasted per gallon made, the EPA warns; it wastes more. For a tank system, compare efficiency ratings. For a tankless system, the recovery rating is the number to use. (The EPA notes that an NSF committee task group was set up to remove “recovery rating” from the standard for this reason.)

Certification covers a specific model. Some membranes on the NSF/ANSI 58 listings are certified for “material requirements only”, which says nothing about what they remove. Before you buy, look up the exact model number on the certifier’s own listing (NSF, WQA or IAPMO) and check the contaminant you care about is on it. A claim on the box is not a certificate. We’ve done that for six popular models in our comparison of reverse osmosis systems.

How much water does reverse osmosis waste?

More than most people expect. The EPA says a typical under-sink RO system sends five gallons or more down the drain for every gallon of treated water, and inefficient units up to 10. In its technical review, the EPA found typical systems run at 10 to 20 percent efficiency, or roughly four to nine gallons to the drain per gallon made.

To earn the EPA’s WaterSense label, a system must be certified to NSF/ANSI 58, have an automatic shut-off, reach at least 30 percent efficiency (no more than about 2.3 gallons to the drain per gallon of treated water), pass a membrane life test meant to show the membrane lasts at least a year, and print its efficiency on the box, according to the WaterSense specification.

What that means in a year, using the EPA’s own assumptions (about one gallon of RO water per person per day, 2.57 people per household):

System	Efficiency	Drinking and cooking water made	Water sent to drain
Typical RO system	15%	about 940 gallons	about 5,320 gallons
WaterSense labeled	30%	about 940 gallons	about 2,190 gallons

Based on average US water and sewer rates, the EPA estimates the more efficient system saves a household nearly $50 a year. On a private well with a septic system, Connecticut’s health department considers 30 gallons a day or less of reject water to be incidental and says larger flows could cause septic problems.

Does RO water need remineralization?

RO can also reduce the calcium and magnesium in water along with the contaminants, so RO water tends to be low in minerals. Some systems add a remineralization cartridge after the membrane. Is that necessary?

The World Health Organization’s view, in its Guidelines for Drinking-water Quality:

• Drinking water “can be a contributor to calcium and magnesium intake and could be important for those who are marginal for calcium and magnesium.”
• Epidemiological studies suggest a protective effect of magnesium or water hardness on cardiovascular deaths, but “the evidence is being debated and does not prove causality.”
• There are “insufficient data to suggest either minimum or maximum concentrations of minerals,” so the WHO sets no guideline value for hardness.

The WHO’s advice to add calcium and magnesium back is aimed at public supplies that switch to demineralized water, such as desalinated water. Its 2005 report Nutrients in Drinking Water notes that water demineralized by household treatment usually isn’t remineralized, and its expert meeting called for more research on drinking low-mineral water.

What that means for you: drinking water is one source of these minerals alongside your diet, and the WHO sets no minimum for it. If you’re worried about your intake, talk to your doctor or a dietitian. A remineralization stage is a reasonable choice if you prefer the taste, but the WHO doesn’t say you need one.

One more mineral to think about: RO may reduce fluoride. If your city adds fluoride and you rely on it for your kids’ teeth, mention it to your dentist.

Maintenance and running costs

An RO system only keeps working if you maintain it. The EPA’s PFAS filter guidance puts it plainly: “Filters are only effective if they are maintained according to the manufacturer’s instructions,” and RO units need both their filter cartridges and the membrane replaced.

What to budget for, in general terms:

• The system: the EPA’s 2024 market review found RO systems from about $180 to over $700, before installation.
• Pre- and post-filters: replaced on the maker’s schedule. Connecticut notes that the extra stages add to both purchase and maintenance costs.
• Membrane: the EPA found replacement membranes at about $20 to $100. More efficient systems can foul membranes faster, which is why WaterSense requires a membrane life test.
• Water: a few thousand gallons a year to the drain for a typical system (see above).
• Testing: on a private well, Connecticut recommends testing untreated and treated water at a state-certified lab after installation, then at least yearly. A built-in TDS monitor helps you see when the membrane needs changing.

Our maintenance hub covers cartridge schedules across filter types.

Do you need a reverse osmosis system?

Start with your water, not the filter. The CDC’s advice is to test your water and “choose a system that removes the harmful chemicals or germs you are concerned about”, and it notes you may not need a system at all if your water has none.

If you’re on city water: your utility must send you a Consumer Confidence Report every year by July 1, per the EPA. It lists what was detected and how it compares to the limits. Here’s how to read your water quality report. Keep in mind it describes the water leaving the utility, not what lead pipes or fixtures might add in your home (our lead guide covers how to test for that).

If you’re on a private well: nobody tests it for you. The EPA says private wells aren’t regulated by the federal government and owners are responsible for their water’s safety. Start with well water testing at a certified lab.

RO tends to make sense when a test shows something it’s certified to reduce and simpler filters don’t handle well, for example:

• Nitrate in a well. As of October 2026, the EPA’s legal limit (MCL) for nitrate in public water systems is 10 mg/L as nitrogen. It applies to utilities, not private wells, but it’s the usual yardstick. Real-world data are encouraging: in a Minnesota program that installed 256 RO systems on private wells, none of the follow-up samples taken at the one-year maintenance visit (118 households so far) exceeded 10 mg/L, and the median was 1.5 mg/L nitrate-N, according to the Minnesota Department of Agriculture’s April 2026 report. That’s a state program report, not a peer-reviewed study, and those households got a maintenance visit at the one-year mark.
• Arsenic (pentavalent, within the certified range; the EPA’s legal limit for public water is 0.010 mg/L, as of October 2026).
• PFAS, high TDS, or naturally high fluoride, with a model certified for that claim.
• Lead, though for lead alone a filter certified for lead reduction under NSF/ANSI 53 may be enough (see the lead guide above).

You probably don’t need RO if your report or test shows nothing of concern and your complaint is taste or chlorine smell. The EPA’s own WaterSense page says that “in some cases, other treatment methods that use little or no water, such as filtration, are sufficient to meet consumer treatment needs.” Chlorine and taste reduction are certified under a different standard, NSF/ANSI 42 (aesthetic effects). Compare options in our under-sink filters section. And RO isn’t a fix for hard water across the house; that’s a softener question, covered in hard water.

The bottom line

• RO reduces a long list of contaminants, but a model is only certified for what’s on its NSF/ANSI 58 listing. Check the exact model and the exact contaminant on the certifier’s site.
• Typical systems send four to nine gallons to the drain per gallon made. Compare efficiency ratings on tank systems, or look for the WaterSense label (at least 30 percent).
• RO water is low in minerals. The WHO sets no minimum and calls the heart health evidence debated, so remineralizing is a preference.
• Test first. Many households on city water don’t need RO at all; for others, especially wells with nitrate or arsenic, it’s a well-documented option at the kitchen tap.

Ready to check your water? Start with step one: find out what’s in it.