Reverse osmosis strips out nearly everything dissolved, including the calcium and magnesium that give water body and flavor, which is why the result tastes flat to a lot of people. Remineralizing puts a controlled amount back. The usual method is a small cartridge after the tank, and there are two simpler alternatives.
Why the water tastes flat in the first place
Taste in drinking water comes largely from dissolved minerals. Hardness is dissolved calcium and magnesium, and a membrane rejects both along with sodium, sulfate and bicarbonate, so what arrives at the faucet is close to pure water. Pure water has almost no buffering, so it also sits at a lower pH than the tap water it came from and can pick up flavor from whatever it touches on the way to the glass. None of that makes it unsafe to drink. It makes it taste thin, and it is why coffee brewed with straight permeate often comes out sour and hollow compared with the same beans on treated tap water.
The inline remineralizing cartridge
This is the standard answer and the one most makers sell as an add-on stage. It is a slim cartridge filled with calcium and magnesium mineral media, usually calcite plus a magnesium source or a blend of trace minerals, and water dissolves a little of it on the way past. Fit it after the storage tank and after the post-carbon, so it treats only the water heading to the faucet: put it before the tank and you are dosing everything the system produces, the media exhausts faster and you can encourage growth in the tank. Expect to replace it roughly on the same schedule as the post-carbon.
Mineral drops and the pitcher route
If your system has no spare port, or the cabinet is full, trace mineral drops added to the jug are a legitimate answer. You control the dose, there is nothing to install, and it costs nothing to try before committing to a cartridge. The drawback is that it is a manual step at every refill, so households abandon it. A third route is a remineralizing pitcher: you run permeate into a pitcher with a mineral cartridge in the lid and pour from that. This suits renters and anyone with a countertop reverse osmosis unit that has no plumbing to modify at all.
How much to add and how to judge it
The target is taste, not a number. Add mineral back until the water tastes like water rather than like nothing, and stop: overshoot and you are putting hardness back into the very water you paid to purify, with spotting on glassware and scale in the kettle following. A cheap total dissolved solids meter is the practical tool here, not because a specific reading is correct but because it lets you reproduce a setting you liked and lets you see when the cartridge has stopped contributing. If you use the reverse osmosis line to feed an ice maker, taste-test the ice too; our reverse osmosis maintenance guide covers the cartridge order.
Questions people ask about how to remineralize reverse osmosis water
Is unremineralized reverse osmosis water bad for you?
It is a taste and cooking question for most households rather than a safety one, since diet supplies minerals. People remineralize because flat water is unpleasant and makes poor coffee and tea.
Where does the remineralizing cartridge go?
After the storage tank and after the post-carbon stage, so it treats only the water going to the faucet rather than everything the membrane produces into the tank.
Will remineralizing make my water hard again?
Only if you overdo it. The cartridge adds a small controlled amount of calcium and magnesium, but an oversized dose or mineral drops used heavily will bring back spotting and kettle scale.
Can I just add a pinch of salt?
It changes the taste but it is not remineralizing in any useful sense, since the minerals people are after are calcium and magnesium rather than sodium. Use a proper mineral blend.
Does remineralizing raise the pH?
Dissolving alkaline mineral media does lift pH from the low value permeate typically sits at. That is a side effect of adding minerals rather than a separate health benefit to chase.