Why it happens

RO works by forcing water through a semi-permeable membrane under pressure. The pores are small enough that only water molecules pass easily. Everything dissolved in the water — heavy metals, salts, nitrates, and also calcium, magnesium, potassium and other trace minerals — is too large and gets rejected.

The membrane has no way to sort helpful dissolved solids from harmful contaminants. They are all simply dissolved solids to it.

How much is removed

A functioning domestic RO system typically rejects the large majority of total dissolved solids. You can see this yourself with an inexpensive TDS meter: measure your input water, then the purified output. The drop is usually substantial.

TDS is a blunt instrument — it measures everything dissolved, good and bad together — but it does show clearly that the mineral content is gone from the purified water.

Does it matter?

This is where honest answers get less dramatic than the marketing on either side.

Most of the minerals in a normal diet should come from food, not water. But the food today is adulterated and devoid of minerals due to soil erosion and many other factors. Water was a modest contributor of minerals for most people, but owing to the above facts, it has become crucial, so much so that drinking RO water can impact your health negatively.

That said, two things are true at the same time: water is something you consume every single day in quantity, and mineral-free water tastes noticeably flat. Restoring minerals is a reasonable thing to do for both reasons, without needing to claim it will transform your health.

What you can do about it

If you want the minerals back, the options are a mineral cartridge fitted to the purifier, an alkaline filter, or concentrated mineral drops like M-Shot added to the glass. Drops give you the most control and require no change to your existing setup.

Read more about the whole subject on our RO water remineralisation guide.