Two different measurements
TDS is the weight of everything dissolved in the water: calcium, magnesium, sodium, potassium, chloride, sulphate, bicarbonate, nitrate, fluoride and the rest, added together and expressed in milligrams per litre. It is measured with a conductivity meter that counts charged particles without caring which they are.
Hardness is two of those things only: calcium and magnesium. It is expressed as the equivalent amount of calcium carbonate, which is why you will see "as CaCO₃" after the figure. It is measured by titration in a lab or with a colour-change test kit, not with a TDS pen. A TDS meter cannot report hardness, and a hardness kit cannot report TDS.
So hardness is always a part of TDS, never the other way round. Water cannot have more hardness than TDS. It can have a great deal of TDS and very little hardness, if what is dissolved is mostly sodium and chloride, and that is exactly the case in a lot of coastal groundwater.
What the standard says about each
| Parameter | Acceptablemg/L | Permissibleno alternate source |
|---|---|---|
| TDS | 500 | 2000 |
| Total hardness (as CaCO₃) | 200 | 600 |
| Calcium (as Ca) | 75 | 200 |
| Magnesium (as Mg) | 30 | 100 |
The hardness limit is well below the TDS limit, which tells you something about how the standard sees the two. Dissolved solids in general are tolerated up to 2000 mg/L because the objection to them is largely taste. Hardness is held to 600 because it does practical damage — scale in pipes, geysers, kettles and membranes — long before it tastes of anything. Water can be inside the TDS limit and outside the hardness limit at the same time: 480 ppm TDS made mostly of calcium bicarbonate is legal by one row and not by the other. The wider set of limits is on the TDS reference page.
Why they rise together, and when they do not
In most inland Indian groundwater the two move together because the same rock supplies both. Water that has dissolved a lot of limestone or weathered a lot of granite is both hard and high in TDS, and a high reading on the pen is a fair hint that a kettle will scale. That is why the high TDS guide spends so much time on calcium and magnesium.
They part company where the dissolved solids are mostly salt rather than mineral. Coastal aquifers with seawater working into them read very high on TDS and comparatively low on hardness, because seawater is mostly sodium chloride. A softener does nothing useful for that water. Conversely, some hill supplies are hard but modest in TDS: a few hundred ppm, nearly all of it calcium bicarbonate. That water scales a geyser and passes the TDS limit with room to spare.
The practical test is what the water does. A white crust on the kettle and a film on tea is hardness. A salty or bitter taste is TDS from something other than hardness. Both together, on a borewell, is the usual case.
Why a softener does not lower TDS
A water softener is a bed of resin beads charged with sodium. As hard water passes through, the resin takes calcium and magnesium out of the water and releases sodium in exchange. The hardness drops to nearly nothing, which is the point: the scale stops. But every calcium ion that came out was replaced by sodium ions, so the total of dissolved solids in the water is about the same afterwards as before. A TDS meter reads a softened supply very nearly where it read the hard one, sometimes a touch higher. Anyone who fitted a softener to bring a high TDS reading down will find it did not, and could not.
What a softener does do is protect whatever is downstream from scale, and that includes an RO membrane. On feed above about 2000 ppm with high hardness, the calcium left behind as the membrane concentrates the water precipitates on its surface and blocks it within months. A resin softener ahead of the membrane removes that calcium first. It is a separate unit, plumbed in before the purifier, not a cartridge that fits inside one.
What RO does, and what it does not target
Reverse osmosis removes dissolved ions of every kind, so it lowers TDS and hardness at once. A 96–98% rejection membrane fed hard water at 1200 ppm delivers water that is neither hard nor high in TDS. In that sense RO is the more complete treatment. But it does not target hardness; it removes calcium along with everything else and is damaged by calcium in the process. It also treats only the water that goes through it, which in a domestic purifier is the few litres a day you drink. The geyser, the washing machine and the bathroom taps still get the hard supply. If scale on those is the problem, a softener on the whole-house line is the fix and an RO under the kitchen counter is not.
Put together: RO for the water you drink, when TDS or the trace contaminants call for it; a softener for the water the house uses, when hardness is the problem; and a softener ahead of the RO when the feed is both very high in TDS and hard. The UV vs RO guide covers the other half of the drinking-water decision, which is disinfection.
Where LUXGEN parts fit
LUXGEN does not make a softener; it is a separate category and the site will not pretend otherwise. What it does make is the membrane for water that is both hard and high in TDS: the 100 GPD molded membrane, rated to 3000 ppm feed at 96–98% rejection, for use behind a softener where hardness is high. For hard water inside the 2000 ppm band, the standard 80 GPD 1812 membrane handles it, and the practical advice is to change the sediment cartridge on time so the membrane keeps its pressure.
Questions
Does a water softener reduce TDS?
No. A softener swaps calcium and magnesium for sodium. Hardness falls to nearly nothing, but the weight of dissolved solids stays about the same, so a TDS meter reads roughly what it read before, sometimes slightly higher. Only RO, distillation or deionisation lower TDS.
Can water be hard but low in TDS?
Yes, within limits. Hardness is part of TDS, so it can never exceed it, but water of a few hundred ppm made mostly of calcium bicarbonate is hard enough to scale a kettle while sitting comfortably inside the 500 mg/L TDS limit.
Does RO remove hardness?
Yes, along with everything else dissolved; the output is both low-TDS and soft. But hardness in the feed damages the membrane, so on very hard, very high-TDS water a resin softener ahead of the RO is what keeps the membrane alive.