The one claim the standard requires

Reverse osmosis reaches further than any other household technology, and it is sold on the one number that carries no health limit.

NSF lists the scope of NSF/ANSI 58 plainly. TDS reduction is the only required claim. Everything else — lead, nitrate, arsenic, cyst, fluoride — is optional.

So a system can be fully certified to the reverse osmosis standard and carry no health claim whatsoever.

How much of the certified field that is

Most of it.

NSF listings returned 371 rows for TDS reduction under NSF/ANSI 58 on 10 September 2026, resolving to 320 distinct models from 54 companies. That is as close to a census of certified reverse osmosis systems as NSF publishes.

192 of those 320 models hold no claim except TDS reduction. Sixty percent of the certified field is certified for the one thing with no health limit.

Twenty-three companies list nothing but TDS-only models. Thirty-five of the 54 list at least one optional claim.

→ What a TDS meter actually measures

The claims are a package, not a menu

This is the part that changes how you read a listing.

Seven optional claims are held by exactly the same 127 models: lead, fluoride, barium, cadmium, trivalent chromium, copper and selenium. Not overlapping sets — one identical set.

Hexavalent chromium and radium 226/228 are held by the same 120 models, all inside that 127. Nitrate and nitrite by 84, all inside the 120. Turbidity (61) and asbestos (10) sit wholly inside the 127 as well.

NSF/ANSI 58 claim Models holding it Where it sits
TDS reduction 320 Required — everything below is optional
Lead, fluoride, barium, cadmium, Cr-III, copper, selenium 127 One identical set of seven claims
Hexavalent chromium, radium 226/228 120 Identical set, inside the 127
Turbidity 61 Inside the 127
Cyst 33 32 inside the 127, one outside
Asbestos 10 Inside the 127
Nitrate and nitrite 84 Inside the 120 — the narrowest

Manufacturers buy a test package, not individual claims. So the question is not which contaminants a membrane can reduce. It is which package this model’s maker paid to have tested.

Cyst reduction is the single exception. One model holds it outside the bundle — the Pentair GRO-2550, carrying cyst and TDS and nothing else.

→ Which claims exist at all, and how populated each is

It is a percentage device

The most useful thing to understand about reverse osmosis, and the least often said.

A rejection rate is the share of a contaminant the membrane holds back. Nebraska Extension states it is calculated separately for each contaminant, and that the incoming concentration is crucial.

Their worked example, at 85 percent rejection:

Nitrate going in Nitrate coming out Against the 10 mg/L limit
40 mg/L 6 mg/L Under it
80 mg/L 12 mg/L Still over it

Same membrane. Same rejection rate. One result is compliant and one is not, because the starting number was different.

This is why a certified claim is tested against a stated challenge concentration. It is also why you read your report before you buy.

→ How to read your water quality report

What it does not reach

Nebraska Extension is specific. Dissolved gases such as hydrogen sulfide pass straight through the membrane. Some pesticides, solvents and volatile organic chemicals are not removed either.

That is why household units are a membrane plus carbon stages. The carbon does the chlorine and several of the organics; the membrane does the dissolved solids and metals.

Minnesota Department of Health puts the general case in one line: no single treatment unit can remove all contaminants in water.

→ Point of use or whole house

The chlorine dependency, which is a city-water problem

Thin film composite membranes have higher rejection rates. They also deteriorate in chlorinated water.

A public supply is chlorinated by design. So where a TFC membrane meets city water, Nebraska Extension states a carbon prefilter is needed to remove the chlorine first.

That prefilter is not an accessory. It is what keeps the membrane alive, and it is the stage most likely to be neglected.

→ Once it is fitted: rated life, running cost, and the listed element

The water it sends to the drain

Recovery is treated water divided by feed water.

Nebraska Extension states most household systems are designed for 20 to 30 percent recovery. At 20 percent, a system fed 100 gallons a day yields 20 gallons treated and sends 80 gallons down the drain.

EPA puts the same point more simply: reverse osmosis units use approximately three times as much water as they treat.

A household membrane typically produces 10 to 35 gallons a day into a 2 to 5 gallon tank. Cold feed water cuts that — output falls 1 to 2 percent per degree below 77°F.

The decision framework

1. Start with a number, not a technology

Reverse osmosis is a percentage device. Without your own figure you cannot say what it would leave behind.

→ Testing city water: what your utility already measured

2. Check whether a narrower certified claim already covers it

Lead has 615 certified filters across standards. Chlorine taste and odour has thousands. A carbon block certified for your contaminant is cheaper, wastes no water and needs no tank.

→ The 615 filters certified for lead

3. If you do want RO, search the claim, not the standard

“NSF/ANSI 58 certified” is compatible with holding only TDS reduction. Search the specific claim in NSF’s listing and confirm the model appears.

→ How to check a certification claim yourself

4. Budget the running cost, not the sticker

MDH estimates 300 to 1,500 dollars initially for point-of-use, plus 100 to 200 dollars every one to two years. Those are 2017 quotes and 2018 research, so treat them as scale.

5. Accept the water and drain load

Three to four gallons out for every gallon you drink. On a septic system, that extra load is a design consideration.

The decision path

Your situation What decides it Where that lands you
No test result, no report read RO leaves a percentage of whatever went in Read the report first
One contaminant, and it has a carbon claim Narrower claim, no water cost Certified carbon block
Nitrate over the limit Only 84 certified RO models hold the claim RO, but search that claim
Fluoride or lead by choice 127 models hold both together RO from the bundled set
Hexavalent chromium 120 models, all inside the bundle RO, claim-checked
High TDS, no health finding TDS has no health limit Nothing is required
Rotten-egg smell Hydrogen sulfide passes through the membrane RO is the wrong tool
Want whole-house RO Nothing certified under 58 is point-of-entry Reconsider the scope

Choose reverse osmosis if

  • Your report or test shows a specific contaminant, and the claim you need is held by a listed model.
  • You need several of the bundled claims at once — the 127-model set covers seven in one device.
  • Nitrate is your problem, which carbon cannot address at all.

Choose something narrower if

  • One contaminant is the concern and a certified carbon claim exists for it.
  • The drain water, tank space or plumbing changes are not acceptable.
  • The only thing prompting the purchase is a meter reading.

Choose nothing if

  • Your report shows no exceedance, you have no staining or odour, and the only evidence offered is a number from a demonstration.

Mistakes that cost the most money

Reading “certified to NSF/ANSI 58” as a health claim. It guarantees TDS reduction. For 192 of 320 models that is the entire claim list.

Buying RO for a contaminant a carbon block already handles. You pay more, wait for a tank and send several gallons to the drain for each one you drink.

Assuming the membrane handles everything. Hydrogen sulfide goes straight through, and so do some pesticides, solvents and VOCs.

Skipping the carbon prefilter on city water. Chlorinated feed degrades a TFC membrane. The prefilter is what protects it.

Trusting a percentage without knowing your starting number. The same membrane can leave you compliant or not, depending on the feed.

Assuming a damaged membrane announces itself. Nebraska Extension states damaged membranes are not easily detected, and advises testing periodically.

Letting it go unmaintained. MDH states unmaintained units lose effectiveness and in some cases can make water quality worse and make you sick.

Three situations, and what changes

City water, clean report, a salesman with a meter. The meter reads total dissolved solids, mostly calcium and magnesium. EPA’s 500 mg/L figure is an unenforced aesthetic guideline. Nothing in the demonstration is a health measurement, and nothing about it requires reverse osmosis.

A municipal nitrate result near the limit. Carbon does not address nitrate at all, so this is the case for RO on a public supply. But only 84 certified models hold the nitrate claim, and the result depends on your starting figure. Search the claim and check the number.

Old house, lead service line, wanting one device for everything. The 127-model bundle covers lead, copper, cadmium and four more in one unit. A certified lead carbon filter is cheaper and wastes no water. If lead is the only concern, the narrower product is the better fit.

The objections worth answering

“Reverse osmosis removes everything.” EPA credits it with eliminating all disease-causing organisms and most chemical contaminants. Nebraska Extension names dissolved gases, some pesticides and some solvents as exceptions, and MDH states no single unit removes all contaminants.

“But it is certified.” To a standard whose only mandatory claim is TDS reduction. Certification is real and worth having; it just says less than the badge implies unless you read which claims are listed.

“The water waste is exaggerated.” EPA says roughly three times as much water as the unit treats. Nebraska Extension puts household design recovery at 20 to 30 percent. Both point the same way.

“It strips out healthy minerals.” It does remove calcium and magnesium — which is most of what made the TDS reading high in the first place. Whether that matters to you is a taste and diet question, not a certification one.

“Home systems must be regulated.” Nebraska Extension states federal, state and local laws do not regulate home RO systems, and that the industry is self-regulated through NSF and WQA. The certification is voluntary.

“I will put one on the whole house.” No model in this listing is point-of-entry. All 320 are point-of-use — 301 plumbed in to a separate tap, 18 plumbed in, one commercial modular.

Who this is for

Households on a public water system deciding whether reverse osmosis is worth the money, the drain water and the plumbing.

It is also for anyone who has been shown a TDS meter and told what the number means.

Private wells are a different case, with higher starting concentrations and no regulated upstream treatment.

→ Well water: the six-parameter test panel

What to settle first

Which claim do I need, and does the model I am looking at actually hold it?

Both are answerable free, in NSF’s listing, in about two minutes. Everything else on the box is marketing.

→ What a TDS meter actually measures → Which claims exist at all, and how populated each is → How to check a certification claim yourself → City water treatment, and the three gaps a utility cannot close → The NSF standards, and what each one covers → Point of use or whole house → Radium and uranium: the one radionuclide with a certified claim → Nitrate: the case where carbon has no certified claim at all → Why reverse osmosis appears on 12 of EPA’s 14 inorganic rows → Distillation, the other broad-spectrum option