RO, TDS and Tap Water: What Indian Households Should Know

By the KABO Nutrition Team · fact-checked against cited public-health sources.

TDS is a crude measure of dissolved solids and says almost nothing about whether water is safe — pathogens and many chemical contaminants do not register on it at all. Choose a purifier based on what is actually in your local supply, which usually means getting the water tested rather than guessing.

Key takeaways
  • Low TDS does not mean safe; bacteria and viruses do not show up in a TDS reading.
  • RO removes dissolved salts, heavy metals and much else, along with useful minerals.
  • UV kills microorganisms but removes no dissolved chemicals.
  • Municipal supply is usually chlorinated; borewell and tanker water often are not.
  • Get your supply tested once — it converts the whole decision from guesswork to fact.

What TDS actually measures

Total Dissolved Solids measures the concentration of dissolved substances — mostly calcium, magnesium, sodium, bicarbonates, chlorides and sulphates. It is measured with a cheap conductivity meter, which is why salespeople carry one.

The critical limitation: TDS does not distinguish beneficial minerals from harmful ones, and it does not detect microorganisms at all. Water swimming with bacteria can show a low TDS reading. A demonstration in which a meter reads 40 after purification and 350 before proves nothing about safety.

Indian standards regard TDS up to 500 mg/L as acceptable, with 2000 mg/L as a permissible upper limit where no alternative source exists. Water in the 150–500 range is ordinary and requires no reduction on health grounds.

The three technologies, and what each does

  • RO (reverse osmosis) — removes dissolved salts, heavy metals, fluoride, nitrates and most chemical contaminants. Also removes calcium and magnesium, and wastes a significant volume of water.
  • UV — inactivates bacteria, viruses and protozoa. Removes nothing dissolved; requires clear water and a working lamp.
  • UF (ultrafiltration) — physically filters out bacteria, cysts and suspended matter without electricity. Does not remove dissolved chemicals.
  • Activated carbon — improves taste and odour, removes chlorine and some organic compounds.
  • Sediment filter — removes visible particles; a pre-stage, not a purifier.

Matching the purifier to your supply

This is the decision most households get wrong, usually by installing RO everywhere because it is what gets sold.

  • Municipal treated supply with TDS under about 300 — UV or UF is generally sufficient; RO is unnecessary.
  • Borewell or hard groundwater with high TDS — RO is appropriate.
  • Areas with known fluoride, arsenic or nitrate contamination — RO, and this is not optional.
  • Tanker or irregular supply — assume microbial risk; UV or UF at minimum.
  • Very high TDS plus microbial risk — RO with UV, which is what most combination units provide.

The mineral question, in proportion

RO removes calcium and magnesium along with everything else, which has generated a good deal of concern and a market for “mineral cartridge” add-ons.

In proportion: water contributes a modest share of mineral intake for most people compared with food. If your diet is reasonable, RO water is not going to cause a deficiency. Where drinking water was a meaningful mineral source — in hard-water regions — the change is more noticeable, and a re-mineralisation stage is a reasonable addition rather than a necessity.

The more practical objection to unnecessary RO is water waste: many domestic units reject two to three litres for every litre produced, which in a water-stressed country is not trivial.

Boiling, and what it does and does not do

Rolling-boiling for one minute kills bacteria, viruses and protozoa reliably. It is the cheapest and most robust microbial treatment available, and it is what to fall back on when a purifier fails or during a contamination advisory.

It does not remove dissolved chemicals, heavy metals, fluoride or nitrates. In fact it slightly concentrates them as water evaporates. Boiling is a microbial solution, not a chemical one.

Maintenance is where systems fail

A purifier past its service interval can be worse than none, because a saturated carbon filter or an expired UV lamp gives false confidence.

  • Replace filters on schedule, not when the water tastes different.
  • UV lamps lose output over time and need replacing even while still lit.
  • Clean storage tanks; a purifier feeding a dirty tank achieves nothing.
  • Do not store purified water in unwashed bottles for days.
  • Get the supply tested at a certified laboratory once — it costs little and settles every question above.

Frequently asked questions

What TDS is best for drinking water?

There is no single ideal figure. Indian standards accept up to 500 mg/L, and water in the 150–500 range is entirely ordinary. Very low TDS is not a health goal.

Do I need RO if I get municipal water?

Often not. If the supply is treated and TDS is moderate, UV or UF usually suffices. RO matters where there are dissolved chemical contaminants or high hardness.

Is RO water harmful because it removes minerals?

For most people with a reasonable diet, no. Food supplies the large majority of calcium and magnesium intake. A re-mineralisation stage is optional rather than essential.

References

  1. ICMR-NIN, Dietary Guidelines for Indians (2024). nin.res.in
  2. World Health Organization, Healthy diet fact sheet. who.int

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