Does RO remove minerals from your water?

Reverse osmosis does exactly what it was designed to do. The trouble is that what it was designed to do is broader than most people realise.

Filtering by size

An RO membrane is a physical barrier with pores small enough to block almost anything dissolved in water. Pressure forces water molecules through; most other things do not fit and are left behind.

This works. Heavy metals do not fit. Nitrates do not fit. Neither does much else you would want gone.

But a calcium ion is roughly the same size as the things you are trying to remove. So is magnesium. A membrane sorting purely by size has no mechanism for preferring one over the other — and it does not try to.

How much comes out

In the IAPMO series Crompton commissioned, the competitor RO reduced feed calcium of 62.81 mg/L to 4.95, and magnesium of 34.79 to 1.00. Calcium, magnesium and bicarbonates go the same way as the contaminants, because a membrane sorting by size has no way of telling them apart.

There is a knock-on effect on pH, and it is not the one most people expect. Bicarbonates buffer water against acidity, so stripping them leaves the output unstable — which is why many RO units add an alkaline stage to correct it afterwards. In the same lab series that correction pushed the competitor to pH 9.69–9.77, above the BIS limit of 8.5. The Biofier came out at 6.75–7.21 without correcting anything.

Does that matter?

Food supplies most of your minerals — roughly 85–95% of daily intake. Water quietly provides the rest, sip by sip, without you thinking about it. Around a fifth of daily calcium and magnesium can come from what you drink.

From demineralised water, that contribution is close to zero. It is not a catastrophe. It is a small, steady source of something useful that you may have engineered out of your home without ever deciding to.

Is there an alternative?

There is now. Capacitive deionisation sorts by electrical charge rather than by size, which lets it select what to capture instead of blocking everything above a threshold. That is a genuinely different mechanism, and it is why a biofier can leave the mineral profile intact.