What RO strips
A reverse-osmosis membrane works by pressure-pushing water through a semipermeable layer that blocks particles, dissolved salts, and organic molecules above a certain size. The water that comes through carries roughly one to four percent of the original dissolved solids. Hardness drops to near zero. TDS, in most residential installations, drops from feed levels of 300 to 800 ppm down to 20 to 60 ppm.
The minerals that get removed are not just inconvenient hardness. They are calcium, magnesium, potassium — minerals the body uses, in trace quantities, from drinking water. They are also the minerals that give water its taste. Below about 80 ppm TDS, water starts to taste flat. Below 50, it tastes empty.
What re-mineralisation does
A re-mineralisation cartridge sits downstream of the RO membrane and adds back a controlled, small quantity of calcium and magnesium. The output TDS comes up to a comfortable 100 to 150 ppm — well within drinking range, well within taste range, and with the dietary minerals the body expects from water.
The cartridge is small, low-pressure, and inexpensive. It does not add chemicals. It uses calcium carbonate beads that slowly dissolve as water passes; the dissolution rate is metered by the contact time and the cartridge's internal structure. Replacement is annual or bi-annual depending on water volume.
Why it matters
Two reasons. The first is taste — RO water without re-mineralisation tastes noticeably worse than tap water. Most homeowners do not stay loyal to a system that produces water they do not enjoy drinking, even if the chemistry is technically purer. The second is corrosion: very-low-TDS water is mildly aggressive on metal plumbing downstream of the unit. Re-mineralised water sits in a more stable range.
Re-mineralised. Doesn't taste flat. The cartridge is small. The difference is large.
What to ask
When evaluating a kitchen RO, two questions matter. Does the unit include a re-mineralisation stage as standard? And is the output TDS controlled to a target range, or is it just whatever the membrane produces? A well-engineered residential RO targets 80 to 150 ppm output. A unit that produces 20 ppm and considers that a feature has misunderstood what residential drinking water is for.