Does RO remove everything from water?

Mostly falseSettled by the underlying physicsLast reviewed 2026-08-08

The ruling

Mostly false, and the exceptions are not trivia. RO rejects dissolved salts, most metals, fluoride, nitrate and hardness very effectively. It handles arsenite — As(III) — poorly without pre-oxidation, and As(III) is the dominant form in exactly the reducing groundwater where arsenic is a problem. Some low-molecular-weight dissolved gases pass through. And the storage tank downstream of the membrane is a real recontamination point, which is why RO+UV exists as a configuration at all.

How this was checked

Settled by the underlying physics

RO membrane rejection depends on molecular size and charge. That single mechanism explains every case where RO underperforms its reputation.

1A thin-film composite RO membrane rejects ions strongly because they are both larger than water molecules and electrically charged. Divalent ions like magnesium and sulphate are rejected best; monovalent ions like sodium and nitrate slightly less well.
2Arsenate — As(V) — carries a negative charge at drinking-water pH and is rejected above 95%. Arsenite — As(III) — is electrically neutral, and rejection typically falls to 50–70%. The same membrane, the same water, a different chemical species.
3Reducing groundwater is low in oxygen, which is precisely the condition under which arsenic exists as As(III). So the form RO handles worst is the form most likely to be present where arsenic is a problem.
4Small uncharged molecules — including some dissolved gases — pass through more readily than their size alone suggests.
5The membrane excludes bacteria comfortably. What it cannot do is protect the storage tank downstream of it, which is warm, wet, dark and rarely sanitised.
6Iron is not a rejection problem but a fouling one: it deposits on the membrane surface and ends its life. Most manufacturers specify maximum feed iron around 0.1–0.3 mg/L.

What is true in the claim

Every durable myth has a grain of truth, and pretending otherwise teaches nobody anything.

For the great majority of Indian supplies, RO removes the great majority of what matters, and does it well. The claim is not absurd — it is an overgeneralisation of something largely true, which is why it survives.

What it means for your purchase

If you are buying RO specifically for arsenic, ask in writing which species the reduction claim was tested against and whether the system includes pre-oxidation.
On borewell water, test iron before installing RO. It is the single most useful pre-installation test, because getting it wrong destroys the membrane.
Sanitise the storage tank as part of servicing. It is a standard step and it is routinely skipped.
Do not treat RO as a substitute for disinfection where the tank is unprotected — that is what the UV stage in RO+UV is for.

What this page does not establish

This page describes membrane behaviour in general. Individual membranes and system designs vary, and a certified system tested to NSF/ANSI 58 carries verified claims that a general argument cannot.
It does not establish that any particular RO system underperforms. It establishes where the technology's limits are.

Frequently asked questions

Does RO remove arsenic?

It removes arsenate — As(V) — very effectively, typically over 95%. It removes arsenite — As(III) — far less reliably, often 50 to 70%. Since As(III) dominates in the reducing groundwater where arsenic problems occur, an RO system intended for arsenic should include a pre-oxidation stage that converts As(III) to As(V) before the membrane.

Does RO remove bacteria and viruses?

The membrane excludes both comfortably on size. The weakness is downstream: the storage tank is warm, wet and rarely sanitised, and it can recontaminate treated water. That is precisely why RO+UV configurations exist — the UV protects the stored water rather than the incoming water.

Sources

Every limit and standard quoted on this page comes from one of the documents below. Where a figure is not in one of these, we have not published it.

IS 10500:2012 Drinking Water — Specification (Second Revision)
Bureau of Indian Standards · published 2012-05-01 · checked 2026-08-01
Guidelines for Drinking-water Quality, 4th edition incorporating the 1st addendum
World Health Organization · published 2017-04-01 · checked 2026-08-01

These pages check claims against published standards, the underlying physics, or our own testing where it exists — and each page says which. We do not emit ClaimReview structured data, because that markup is reserved for accredited fact-checking organisations and we are a commercial publisher. Last reviewed 2026-08-08.

Related claims

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