Glossary · Treatment

What Is Remineralization? Adding Minerals Back to RO Water

Remineralization

A filter stage after a reverse osmosis membrane that adds some calcium and magnesium back to the treated water, sold mainly for taste and pH.

  1. Pre-filterSediment filter: catches sand and silt that would foul the membrane.
  2. RO membranePressure pushes water through; most dissolved contaminants stay behind. Concentrate to the drain: the water that carries away what the membrane held back.
  3. Storage tankThe membrane is slow, so treated water waits here until you open the tap.
  4. Post-filterUsually carbon: polishes taste and catches what the membrane missed.
  5. Remineralization (optional)Adds some minerals back after the membrane, mainly for taste.
  6. RO faucetIts own small faucet at the sink.
Remineralization is an optional last stage: it adds some minerals back after the membrane has taken most of them out.

At a glance

Why it exists
RO removes beneficial minerals such as calcium along with contaminants (EPA)
What it adds
Mainly calcium and magnesium, e.g. from calcite and magnesium oxide media
WHO minimum for minerals
None: the WHO says the data are insufficient to set one
Health effect of pH
The WHO says pH usually has no direct impact on consumers
EPA pH guideline
6.5 to 8.5, for taste and corrosion; not enforceable

Reverse osmosis takes out most minerals along with the contaminants: the EPA’s guide to treatment systems lists the loss of beneficial minerals such as calcium among its limitations. A remineralization cartridge, often sold as a “mineral” or “alkaline” filter, puts some back. On one budget RO system, the maker says its alkaline stage uses calcite and magnesium oxide to add 30 to 60 mg/L of calcium and magnesium. Treat figures like that as the maker’s claim.

Do you need to remineralize RO water?

Probably not for health, based on current guidance. The World Health Organization says drinking water can contribute to calcium and magnesium intake and could matter for people who get too little of them. It also says the studies linking harder water to fewer heart deaths are debated and don’t prove cause and effect, and that there are insufficient data to set a minimum mineral level.

An earlier WHO expert report, Nutrients in Drinking Water (2005), went further, arguing that low-mineral water isn’t ideal and noting that water demineralized by home treatment usually isn’t remineralized. The WHO’s later guidelines still set no minimum. The Minnesota Department of Health makes a practical point: if treatment removes these minerals, you may need to get more of them from your diet. If you’re concerned about your intake, ask your doctor or a dietitian.

Is alkaline water healthier?

The evidence we rely on doesn’t support it. The WHO’s background document on pH says a direct link between the pH of drinking water and health is impossible to establish, that pH usually has no direct impact on consumers, and that no health-based guideline is needed. The EPA’s secondary standard of 6.5 to 8.5 is a non-enforceable guideline about taste and corrosion. Treat health promises for alkaline or mineral cartridges as marketing.

A remineralizer is a reasonable pick if you prefer the taste. It’s one more cartridge to replace, so count it in running costs; our reverse osmosis comparison lists those prices where makers publish them, and our guide to how reverse osmosis works covers the WHO’s view in more detail.

Published by the Checked Water editorial team

Sources

  1. WaterSense Guide to Selecting Water Treatment Systems (Clinical guideline, US EPA, 2024)
  2. Guidelines for Drinking-water Quality, 4th edition: Hardness chemical fact sheet (Clinical guideline, World Health Organization, 2011)
  3. Nutrients in Drinking Water (Review, World Health Organization, 2005)
  4. pH in Drinking-water: Revised background document for development of WHO Guidelines for Drinking-water Quality (Clinical guideline, World Health Organization, 2007)
  5. Secondary Drinking Water Standards: Guidance for Nuisance Chemicals (Clinical guideline, US EPA, 2026)
  6. Home Water Softening Frequently Asked Questions (Clinical guideline, Minnesota Department of Health, 2022)
  7. iSpring RCC7AK Alkaline 6-Stage Reverse Osmosis System (product page) (Source, iSpring Water Systems, 2026)

Where Remineralization comes up