The quick answer
Reverse osmosis forces water across a semi-permeable membrane. The membrane rejects a large proportion of many dissolved ions and other substances, producing a treated stream (permeate) and a more concentrated reject stream. Real performance depends on the membrane, pressure, temperature, feed chemistry, recovery rate and condition of the system.
| Substance / issue | Typical relevance of RO | What you should verify |
|---|---|---|
| TDS / dissolved salts | Core RO function | Certified TDS reduction and actual rejection under your conditions. |
| Calcium & magnesium / hardness | Strong reduction | Point-of-use RO is not the same job as whole-house softening. |
| Lead | RO can reduce it | Look for a product-specific lead claim; investigate lead plumbing too. |
| Nitrate/nitrite | RO can reduce them | Use a system certified/tested for nitrate/nitrite where this is a health concern. |
| Fluoride | RO can reduce it | Verify the model's fluoride claim rather than assuming from membrane type. |
| Arsenic | Possible, chemistry matters | Arsenic species and pretreatment affect performance; verify the exact claim. |
| PFAS | RO can be effective | Which PFAS were tested, at what concentrations, under what protocol? |
| Microplastics | Fine membrane barrier is relevant | Particle-size range and product test method. |
| Chlorine | Usually a prefilter job | Carbon pretreatment protects many RO membranes and improves taste. |
| Bacteria/viruses | Membranes can provide a barrier | Do not assume a domestic RO unit is a validated disinfectant; downstream hygiene matters. |
Why certification language matters
NSF/ANSI 58 is the principal North American standard for point-of-use RO systems and is widely referenced by manufacturers internationally. TDS reduction is a required performance element under the standard; contaminant claims such as lead, nitrate/nitrite, fluoride, arsenic and others are optional. In plain English: seeing “NSF/ANSI 58” does not automatically mean every optional contaminant claim has been certified. Check the listing for the model and the specific reduction claim.
What the prefilters are doing
A domestic RO machine is usually a treatment train. Sediment filtration protects fine stages from particles. Activated carbon commonly reduces chlorine and organic taste/odour compounds and can protect chlorine-sensitive membranes. The RO membrane then performs the main dissolved-solids separation. Post-carbon can polish taste; remineralisation can deliberately add selected ions back.
Why percentages need context
A statement such as “99% removal” is incomplete without naming the substance, test conditions and point in cartridge life. Rejection can fall as pressure drops or membrane condition changes. Some ions are rejected more readily than others. A broad percentage covering “contaminants” can therefore conceal more than it explains.
Does RO make microbiologically unsafe water safe?
Do not design a private-supply safety system around that assumption. CDC describes RO as capable of removing parasites, bacteria and viruses, but UK private-water-supply practice uses risk assessment and multiple barriers where appropriate. A damaged membrane, seal, storage tank or contaminated dispensing path can defeat an otherwise impressive membrane specification.
What RO deliberately removes that you may want back
RO also reduces calcium, magnesium and other ordinary dissolved ions. That is why some systems include remineralisation. Read our remineralisation guide rather than treating a very low TDS number as a health score.
Deep dives
PFAS · fluoride · lead · nitrate · microplastics · chlorine · arsenic.
Sources and further reading
For factual claims that matter to health, regulation or product performance, we prefer primary and official sources. Useful references for this guide include:
- Drinking Water Inspectorate — Physical removal of contaminants
- CDC — About home water treatment systems
- NSF — NSF/ANSI 58 reverse osmosis systems
- Drinking Water Inspectorate — Domestic water filters and softeners
Related guides
Last reviewed: 6 October 2026. This guide is written for a UK audience. Always follow current supplier, regulator, manufacturer and healthcare advice where it applies.