A filter that adds minerals is doing the opposite of filtering, and it exists for a specific reason: reverse osmosis and distillation remove dissolved content indiscriminately, and water with nothing dissolved in it tastes flat and is mildly aggressive to the metals it contacts. Putting a controlled small quantity back solves both, and the products that do it fall into three formats.
Inline cartridges after a membrane
The common format is a cartridge of calcium and magnesium bearing media fitted last in an under sink stack, after the membrane and after any polishing carbon, so nothing downstream removes what it has just added. Water passes slowly through the bed and dissolves a small quantity, which raises mineral content and lifts pH from the slightly acidic value typical of fresh permeate. The media is consumed rather than clogged, so it is a consumable that stops working silently rather than blocking.
Pitchers and countertop units
Some pitcher and countertop filters include a mineral stage alongside carbon, which is a different proposition because the water going in was never demineralized. On ordinary tap water that already carries dissolved minerals, the addition is much smaller and the taste change correspondingly modest. These are reasonable products, and the mineral stage in them is doing far less work than the same media does behind a membrane, which is worth knowing when comparing claims between formats.
What the minerals actually change
Taste, which is the honest benefit: the USGS describes hardness as dissolved calcium and magnesium, and those are much of what gives water its character, so restoring a little brings back the flavour people expect in water, tea and coffee. And chemical stability, since demineralized water is aggressive to metals and a mineral content reduces that, which matters more on a whole house system than on a short tubing run to a dedicated tap.
The claims to discount
The strong version is that raised pH water changes body chemistry beneficially. The stomach is strongly acidic and the body regulates blood pH within tight limits regardless of what is drunk, and the quantity of mineral a small cartridge adds is modest against what food supplies. Buy the stage for taste and for less aggressive water, both of which it delivers plainly. Change it on schedule, since exhausted media gives no warning at all. Our whole house filter guide covers the stages ahead of it.
Questions people ask about water filter that adds minerals
Why would a filter add minerals?
Because membranes and distillation strip everything dissolved, leaving water that tastes flat and is mildly aggressive to metals.
Where does the cartridge go?
Last in the stack, after the membrane and any polishing carbon, so nothing downstream removes what it has added.
Do mineral pitchers do the same thing?
Much less, because the water going in was never demineralized. The same media does far more work behind a membrane.
Do the alkaline health claims hold up?
Treat them cautiously. The body regulates its own pH closely, and the mineral quantity added is modest compared with diet.