TDS in Drinking Water: What's Safe, and What the Number Doesn't Tell You

A salesman puts a little meter into a glass of your water, it flashes a number, and a decision worth tens of thousands of rupees gets made on the spot. High number, buy the purifier. Low number, you are fine. Almost everything about that ritual is misleading, because the number — TDS — is real, useful, and routinely asked to answer questions it cannot.

TDS, total dissolved solids, is the combined weight of everything dissolved in your water: the calcium and magnesium that make it hard, the sodium and chlorides that make it salty, sulphates, nitrates, and any dissolved metals, all added together and reported in milligrams per litre. It is a bulk figure. That is its strength and, as we will see, its blind spot.

What "safe" actually means

India's drinking-water standard, BIS IS 10500, puts the acceptable TDS limit at 500 mg/L. Above that, in the absence of a better source, it allows up to 2,000 mg/L as permissible — water you can drink without an alternative, not water anyone would call ideal. Most municipal supply lands comfortably under 500. Borewell water across much of the country does not; in hard-water belts it runs 800–1,500, and in coastal or over-exploited pockets it turns frankly saline, well past 2,000.

So as a first-pass screen, TDS is genuinely useful. A reading of 1,800 mg/L tells you the water is heavily mineralised and probably hard, salty, or both, and that it will scale your geysers and taste of the ground it came from. That is worth knowing.

Is low TDS bad for you?

This is where the myth-making runs in the other direction. A common claim is that very low TDS water — the kind an RO produces, often 20–50 mg/L — is unhealthy because it has been "stripped of minerals." The honest answer is more boring. The mineral contribution of drinking water to your diet is small compared with food, and demineralised water is not dangerous to drink. What low-TDS water mostly is, is flat-tasting; the minerals that carry flavour are gone. Some RO systems add a remineralisation stage back precisely for taste, not survival.

The real problem with reflexively driving TDS to near-zero is not health. It is that you may be solving a problem you do not have, throwing away a quarter or more of your water as RO reject to remove minerals that were never the issue, and paying to run and service a membrane you did not need. Low TDS is not a virtue in itself. It is only worth the cost when the dissolved load was genuinely too high for the use.

The number's blind spot

Here is the part the meter ritual never mentions, and it is the whole point. TDS is a sum. It cannot tell you what is dissolved.

Two glasses of water can both read 450 mg/L — comfortably "safe" by the meter — and be completely different. One is benign calcium and bicarbonate from a limestone aquifer. The other hides 60 mg/L of nitrate, or arsenic, or fluoride above the safe limit, with the rest made up of harmless salts. The TDS meter shows the same friendly green number for both, because those contaminants weigh almost nothing against the bulk of dissolved solids. A dangerous water can have a perfectly acceptable TDS.

And TDS says nothing at all about the two things most likely to make you ill. It does not see bacteria — E. coli can be thriving in water with a beautiful 200 mg/L reading. It does not see turbidity, or organic contamination, or pesticides. TDS is a measure of dissolved minerals, not of safety.

This is exactly why we keep telling people that a TDS meter is a screening tool, not a verdict. It answers "how mineralised is this water," which is one useful question among several. It does not answer "is this water safe to drink," which is the question people think they are asking.

What to do instead

The discipline is the same one a plant applies to any feed water: measure it properly, then treat for what you actually find.

For water you are unsure about — a new borewell, a tanker supply, a source that changed — a one-time lab panel against BIS IS 10500 is worth far more than a meter reading. It resolves the two questions TDS cannot: the specific chemical contaminants (nitrate, fluoride, arsenic, hardness broken out from the bulk), and the microbial count. From there the treatment picks itself. High hardness with safe chemistry needs softening, not necessarily RO. Genuinely high TDS or salinity needs a membrane, sized for the real feed. A microbial problem needs disinfection, which no amount of RO staged for minerals reliably guarantees on its own.

Use TDS for what it is good at — a quick read on how loaded the water is, a way to spot a source that has drifted, a sanity check on whether your RO is still working. Just do not let a single bulk number stand in for knowing what is in your water. The meter that flashes green has not tested for the things that actually put people in a clinic.

If you are trying to make sense of a water report — drinking, process, or industrial feed — and want a second pair of eyes on what the numbers mean and the least treatment that fixes them, that is a short conversation to have with us, or reach us at [email protected] or +91-98100 00233.