What Does Reverse Osmosis Remove, and What Does It Leave Behind?

If you've searched for what reverse osmosis actually removes, you've probably run into two very different kinds of answers: hedged, careful language from health agencies, and marketing pages that simply say a membrane takes out "everything" or "99.99% of contaminants." The honest answer sits in between, and it sorts into three groups. One group of contaminants is reduced reliably because of how the membrane physically works. A second group is only partially and inconsistently reduced, and the result depends on the specific system and whether it carries a real certification for that exact contaminant. A third group was never the membrane's job at all; a different stage, or a different kind of trade-off, handles it instead. Placing your own concern into one of these three tiers is more useful than trusting any single flat number.

• Reliably reduced: dissolved salts, a specific set of metals, and microbes, because size and electrical charge catch them consistently rather than through a reaction that can run out.
• Depends on the system: arsenic, fluoride, nitrate and PFAS, where the outcome depends on the particular membrane and a genuine, named certification, not the phrase "reverse osmosis" by itself.
• Not the membrane's job: chlorine and taste, the beneficial minerals that leave with the contaminants, and the reject water the process produces along the way.

Why "what does reverse osmosis remove" doesn't have one single answer

Part of the disagreement between sources comes down to where the information originates. The CDC's guidance on household water treatment systems doesn't give reverse osmosis one blanket rating. It separates the contaminants it says RO removes from the contaminants it says RO may also reduce, and that distinction is doing real work, not just hedging for its own sake. A seller page has every reason to round that qualifier up into a single confident number, because a tiered answer is harder to put on a label than a flat one. This article follows the CDC's tiered language instead of flattening it, because the membrane genuinely behaves differently depending on what it's filtering out.

What reverse osmosis reliably reduces

The CDC lists parasites, bacteria and viruses plainly under what reverse osmosis removes, alongside a specific group of chemicals: lead, copper, chromium, chloride and sodium (CDC — About Household Water Treatment Systems). These are different kinds of problems, but they land in the same tier for a shared reason: a tight enough membrane blocks them by size and charge rather than by a chemical reaction, so the result doesn't depend on dosing or on contact time running out partway through a filter's life. That's also why, in a system like the RO600, it's specifically the 0.0001-micron membrane stage doing this work; the carbon stages on either side of it exist for a different purpose, covered later in this article.

What depends on the specific system and its certification

A second group of contaminants gets a noticeably softer verb in the CDC's guidance: reverse osmosis "may also reduce levels of" arsenic, fluoride, radium, sulfate, calcium, magnesium, potassium, nitrate and phosphorus (CDC — About Household Water Treatment Systems). "May also reduce" isn't a promise, and how much reduction you actually get depends on the specific membrane, the water pressure it runs at, and the chemical form the contaminant takes in your water, not just the words printed on a box.

Two of these come up often enough in searches to be worth a closer look. Nitrate carries a federal limit of 10 mg/L as nitrogen in public drinking water, set largely because elevated levels pose a real risk to infants, sometimes called blue baby syndrome (see the EPA's overview of well water contaminants and their health impacts; the limit itself comes from the agency's national primary drinking water regulations). Fluoride carries a limit of 4.0 mg/L, and naturally elevated levels in some well water have been linked to skeletal and dental fluorosis. Neither number describes the RO600 specifically; both are the regulatory limits the EPA sets for the water itself, which is a reminder that testing your own water comes before deciding what a filter needs to solve. If fluoride is the reason you're reading this, a dedicated look at reverse osmosis and fluoride goes into the certification question in the depth a single paragraph here can't.

PFAS sits in this same tier, but the evidence is specific about scale in a way that's easy to misread. At the level of an entire water system, the EPA names reverse osmosis, alongside granular activated carbon, anion exchange and nanofiltration, as one of the "Best Available Technologies" for meeting its PFAS limits, according to its own fact sheet on PFAS treatment options. That's a statement about municipal-scale treatment, not an automatic pass for any home filter that happens to contain a membrane. For a filter under a kitchen sink, the EPA's own advice is to check the packaging for a specific certification, NSF/ANSI 53 or 58, rather than assume the phrase "reverse osmosis" already covers it, as the agency's own guidance on reducing PFAS with a home filter explains. The same fact sheet adds a caution worth knowing: as of its 2024 guidance, filters already carrying one of those certifications were not yet certified against the newer, stricter PFAS limits the EPA had just set, so the date on a certificate can matter as much as the certificate itself. None of this confirms or denies what the RO600 does with PFAS; its product page describes the membrane stage as reducing it among other things, and that description is the seller's own claim rather than an independent test result, which is exactly why it stays in this tier until a specific, current certificate says otherwise.

What a membrane alone was never built to handle

Chlorine, and the taste or odor it leaves behind, is a common complaint after a reverse osmosis system goes in, but it's rarely a membrane problem. The RO600's own product description attributes chlorine reduction to its first cartridge, the PPC composite stage that pairs a sediment layer with a carbon block, and keeps the reduction of dissolved solids, heavy metals and fluoride described as the membrane's job in the cartridge after it. If water still tastes or smells of chlorine once a system like this is running, that points toward the carbon stage rather than the membrane, a distinction this closer look at reverse osmosis and chlorine walks through in more detail.

The same size-and-charge exclusion that catches heavy metals can't tell a contaminant from a mineral the body actually benefits from, so calcium, magnesium and potassium leave the water along with everything else. That's exactly why an alkaline or mineral post-filter exists as its own separate stage rather than being designed into the membrane. The Hydrion TAM3 is built for that role: according to its product description, it sits after the RO stage and reintroduces calcium, magnesium, potassium and zinc while raising pH above 7.5, rather than asking a single membrane to filter contaminants out and put minerals back in at the same time.

There's a third thing the membrane doesn't hand back cleanly: all of the water that went in. The EPA describes reverse osmosis and nanofiltration as splitting incoming water into a clean stream and a reject, or concentrate, stream that generally runs around a fifth of the water that entered the system, in the same fact sheet on PFAS treatment options cited above. The RO600's stated 2:1 pure-to-drain ratio is the seller's version of that same trade-off, and it's worth knowing before installation rather than after noticing how much the drain line is running.

None of this turns reverse osmosis into a stand-in for knowing what's in your water supply to begin with. The CDC and EPA both frame home treatment as something added on top of a water source, not proof that the source has already been tested, which matters most for anyone on a private well deciding what to treat for in the first place.

How to check what your own system actually claims

The practical habit that follows from all of this is reading past the headline claim. Look for the specific standard named for the specific contaminant that matters to you, for example NSF/ANSI 58 for the RO membrane itself or NSF/ANSI 53 for certain organic contaminants, rather than settling for a general "certified" badge with no standard attached to it. A live TDS display, like the one built into the RO600, is a genuinely useful signal of an overall dissolved-solids trend, and our guide to what a TDS reading actually means explains how to read one properly, but a single number can't confirm removal of any one named contaminant by itself, since dissolved solids lump dozens of different substances together. When a product page doesn't name a specific, current certification for the thing you're trying to solve, the honest approach is to treat its description as the seller's own claim rather than filling in a certification that was never stated.

The short version

Reverse osmosis doesn't deserve either extreme: not the claim that it removes everything, and not the assumption that it only handles the obvious. It reliably reduces dissolved salts, a specific set of metals, and microbes because of how the membrane physically works. It partially reduces arsenic, fluoride, nitrate and PFAS, to the extent the specific system and a current certification actually state. And it was never built to handle chlorine, taste, or the reject water the process produces; those are other stages' jobs, or trade-offs worth planning around rather than membrane shortcomings. For the full mechanism behind all three stages, the complete guide to how reverse osmosis works is the next read, and this piece on remineralizing RO water picks up exactly where this one leaves off, with what adding minerals back actually involves.