What “low” means here
There are two kinds of low TDS water and they are worth separating. The first is a naturally soft supply: some hill sources, rainwater, a few surface reservoirs. These read under 150 ppm and have always done so. The second is RO output. A membrane removes most of the dissolved solids from whatever it is fed, so a purifier on 600 ppm borewell water can deliver 20 ppm at the tap. Nearly everyone who asks whether low TDS is bad is asking about the second kind, because it is water they made, and this page is mostly about that.
For the scale that puts these numbers in context, the TDS reference page has the full chart. The short version is that the Indian standard, IS 10500:2012, has an upper limit of 500 mg/L for TDS and no lower limit at all.
What RO actually strips out
Reverse osmosis pushes water through a membrane fine enough to hold back dissolved ions. It does not choose. Calcium and magnesium go, and so do sodium, chloride, sulphate, bicarbonate, fluoride, nitrate and the trace metals. That is the point: the membrane is fitted to remove things that should not be drunk, and it removes the things that are harmless along with them. The output is close to the water chemists call demineralised.
Two of what it removes are noticed. Calcium and magnesium give water body and a faintly sweet edge, and bicarbonate buffers it. Take them out and the water tastes flat, sometimes slightly sour, and feels thinner in the mouth. That is not imagination and it is not harmful; it is what water without minerals tastes like.
Is it a health problem?
Here is the honest position. IS 10500 sets no minimum TDS, which means the Indian standard does not regard low mineral water as unsafe. The World Health Organization has published discussion of demineralised water and possible effects of drinking it for long periods, and the question remains argued over rather than settled. What is not argued over is that most of the calcium and magnesium in a normal diet comes from food; water is a minor source for most people, and a lower share of a minor source is a small change.
So very low TDS water is not dangerous in the way that water over the nitrate limit is dangerous. It is water that tastes less good than it could and delivers a little less mineral than it could. Both are fixable, and neither is a reason to remove an RO where the feed needed one. If your feed did not need one — under 300 ppm, clean lab report — the better fix is not to run RO at all, and the UV vs RO guide explains when that applies.
Why the taste goes flat
Taste in water is mostly the minerals and the dissolved gases. RO leaves the gases largely alone but takes the minerals, and the result is water that is technically very clean and noticeably dull. A second effect adds to it: with the bicarbonate gone, the small amount of carbon dioxide in the water is no longer buffered, so the pH drifts slightly down and some people pick up a faint sourness. Neither is a fault in the purifier. A membrane that produces flat water is a membrane doing its job.
The fix is to put a little mineral back, and there are two ways of doing it. They are not interchangeable, and the difference matters on a borewell.
What a TDS controller does
A TDS controller is a small valve on the purifier that lets a measured amount of water skip the membrane. That water has been through the sediment and carbon stages but not RO, so it still carries the feed's dissolved solids. Mixed back into the membrane output, it raises the TDS to a level that tastes right. Turn it one way for more mineral, the other for less. It costs nothing to run and it is why the output of a controller-fitted purifier is not a small fraction of the feed; the RO output guide explains how to read it.
The catch is what the bypass water brings with it. On a municipal feed with a clean lab report, the controller is simply adding back harmless mineral. On a borewell feed with fluoride or nitrate above the limit, it is adding back a proportion of the fluoride and nitrate too. On such a feed, keep the controller low and get the taste from the second method.
What a mineral cartridge does
A mineral cartridge sits after the membrane and adds minerals to the finished water as it passes through a bed of mineral media. Nothing bypasses the RO, so whatever the feed carried stays removed; what is added comes from the cartridge, not the feed. Output TDS rises a little, taste improves, and the cartridge needs replacing when its media is spent. It is the right choice where the feed is not one you would want to drink a share of untreated, and it is the only choice on a purifier that has no controller.
Some purifiers add a specific mineral rather than a mix; a copper stage is the common Indian example. In the LUXGEN range the Royale Copper has both an adjustable TDS controller and a copper cartridge, the Aura has a controller, and the Curve has neither, so its output is the membrane's output. If your feed is low-TDS to begin with, a purifier with a controller lets you keep most of the mineral you started with. The safe TDS guide gives the range to aim for once you can set it.
Questions
Is 20 ppm TDS water safe to drink?
Yes. IS 10500:2012 sets no lower limit for TDS. Water at 20 ppm is typical RO output and is safe; the objection most people have to it is flat taste, which a TDS controller or a mineral cartridge corrects.
Does RO water remove all minerals?
Most of them. A membrane holds back the large majority of dissolved ions, including calcium and magnesium, leaving output that reads a small fraction of the feed. It does not remove dissolved gases, which is why RO water is flat rather than tasteless.
Should I turn the TDS controller up?
On a municipal feed with a clean lab report, yes, to the point where the water tastes right. On a borewell feed that has fluoride or nitrate above the limit, no, because the bypass water carries them back in; use a mineral cartridge for taste instead and keep the controller low.