An RO system with remineralization adds a final stage that returns some calcium and magnesium to water the membrane has just stripped bare. There are two real reasons to want it: taste, because mineral free water reads as flat, and chemistry, because very low mineral water is corrosive to the metal it then travels through. Neither reason is a health claim, and you should be suspicious of any seller who makes one.
Why the membrane takes minerals out at all
A reverse osmosis membrane does not sort contaminants from nutrients. It rejects dissolved ions by size and charge, so the calcium and magnesium that give water its body leave with the sodium, nitrate and dissolved metals you were targeting. The USGS defines hardness as dissolved calcium and magnesium, classing water as soft up to 60 mg/L as calcium carbonate, moderately hard at 61 to 120, hard at 121 to 180 and very hard above 180. Permeate sits far below the bottom of that scale, which is exactly why it tastes different from what you are used to.
How a remineralization cartridge works
The usual design is a small post filter packed with calcite, a granular calcium carbonate, sometimes blended with a magnesium mineral such as corosex. Water passing through dissolves a small, self limiting amount of the media, which raises calcium, magnesium and pH slightly. The media is consumed as it works, so it is a consumable, not a permanent component. Some units instead blend a controlled trickle of pretreated feed water back into the permeate, which achieves a similar result and costs nothing to run but reintroduces whatever else that feed water carries.
Where the stage has to sit
Remineralization always goes after the membrane and, in a tanked system, after the storage tank rather than between membrane and tank. Putting mineral media upstream of the membrane simply hands the membrane more hardness to reject and scale on. In practice the cartridge is the last thing the water meets before the faucet, often paired with a polishing carbon stage that smooths taste. If you are retrofitting, check there is a spare port and enough tubing run under the cabinet, because a post stage adds length and a little pressure drop at the dedicated faucet.
The corrosion argument, which matters most
Water with almost no dissolved minerals is chemically hungry and will pick up metal from whatever it flows through. On an under sink unit the permeate only travels a short run of plastic tubing to a dedicated faucet, so the risk is small. On a whole house system the permeate goes into the household copper, brass and soldered joints, and there remineralization or blending is not a taste upgrade, it is protection for the plumbing. That is the single clearest case where the stage is not optional.
What it does not fix
A remineralization stage does not improve contaminant removal, does not extend membrane life and does not make water alkaline in any therapeutic sense. It slightly raises pH because dissolving carbonate does that, and it adds back a small amount of hardness, which is why a kettle fed from a remineralized system may show light scale again. If your complaint is that RO water tastes flat, this fixes it. If your complaint is anything else, the answer is somewhere else in the chain, and our under sink filter comparison guide covers the alternatives.
Questions people ask about ro system with remineralization
Does remineralized RO water still remove contaminants?
Yes, the membrane does the removal and the mineral stage sits downstream, so it changes taste and chemistry without affecting what was rejected.
How often does a remineralization cartridge need replacing?
The media dissolves as it works, so it depletes on a schedule similar to the other post filters and should be changed at the same service visit.
Will remineralization make my RO water hard again?
It adds back only a small amount of calcium and magnesium, far below typical tap hardness, but it is enough to bring back a faint kettle deposit.
Do I need remineralization on an under sink system?
It is optional there and mostly a taste preference, because the permeate only crosses plastic tubing rather than the household copper plumbing.