TDS shows up everywhere water quality is discussed — on the display of a RO purifier, in water testing reports, sometimes now as a live reading in a tank monitoring app. Most people know it’s “a water quality number” without knowing what it actually measures or what a given reading means for whether water is safe.
What TDS actually measures
TDS stands for Total Dissolved Solids — the combined amount of minerals, salts, and other dissolved substances in water, measured in parts per million (ppm) or milligrams per litre (mg/L, effectively the same number). It’s measured using a probe that reads the water’s electrical conductivity — dissolved minerals and salts conduct electricity, and more dissolved solids means higher conductivity, which the meter converts into a ppm reading.
Importantly: TDS is not a single measurement of “contamination.” It’s a general indicator of mineral/salt content, which can come from entirely natural sources (groundwater passing through mineral-rich soil) as easily as from problematic ones (industrial or sewage contamination). A high TDS reading tells you something changed — it doesn’t automatically tell you what.
What the ppm ranges generally mean
These are widely-used general guidelines (WHO and BIS reference ranges), not strict cutoffs — context matters, but as a rough guide:
- Below 300 ppm: Generally considered excellent for drinking.
- 300-600 ppm: Good, acceptable range for most drinking water.
- 600-900 ppm: Fair — still commonly consumed, though at the higher end some people notice a taste difference.
- 900-1200 ppm: Poor — usually the point where RO purification is recommended.
- Above 1200 ppm: Unacceptable for drinking without significant treatment.
Much of India, particularly areas relying on groundwater/borewell sources, naturally runs higher than municipal-supplied regions — TDS readings above 500 ppm from a borewell aren’t unusual or necessarily alarming on their own.
What TDS does NOT tell you
This is the most commonly misunderstood part: TDS does not detect bacteria, viruses, or most biological contamination. Water can have excellent (low) TDS and still be biologically unsafe if it’s contaminated with pathogens — TDS measures dissolved minerals/salts, not microbes. It also doesn’t specifically identify heavy metals, pesticides, or most specific chemical contaminants — a comprehensive water safety test needs more than a TDS reading, particularly if there’s a specific contamination concern (industrial runoff, sewage cross-contamination).
TDS is best understood as a useful, continuous general-health indicator for water — good for spotting trends and sudden changes — not a complete safety certification on its own.
Why a sudden TDS spike matters more than the absolute number
A stable reading of 550 ppm day after day is far less concerning than a reading that suddenly jumps from 300 to 800 overnight — the jump itself is the signal, since it usually indicates something changed at the source: a new contamination event, a source switch (e.g., emergency tanker water mixing with existing supply), or infrastructure damage letting in something new. This is why continuous monitoring is more useful than a one-time test — a single test tells you the state at that moment; ongoing monitoring tells you when something changes.
Hard water and TDS
“Hard water” — high in dissolved calcium and magnesium specifically — is one of the most common practical concerns TDS readings help flag. Consistently high TDS, especially in the 600+ range, often correlates with hard water, which causes scale buildup in geysers, washing machines, and pipes over time, and can affect soap lathering and skin/hair. This is a genuinely useful early-warning use of TDS monitoring even when the water is otherwise safe to drink.
Source fingerprinting — an underused capability
Different water sources tend to have distinct, fairly consistent TDS signatures: municipal supply, borewell water, and tanker-delivered water often read differently from each other in a given area. Some monitoring systems use this to automatically identify which source filled a tank on a given day (useful for buildings that switch between multiple sources), and to flag when a fill doesn’t match the expected source’s usual signature — a possible early sign of contamination or an unexpected source mix.
Frequently asked questions
Is low TDS always better?
Not necessarily — extremely low TDS water (from some RO systems, below roughly 50 ppm) lacks the minerals that are actually beneficial in moderate amounts, and some studies suggest very low-TDS water may not taste as good or provide dietary minerals some people rely on. The commonly recommended range for drinking is a middle band, not “as low as possible.”
Does boiling water reduce TDS?
No — boiling kills biological contaminants but doesn’t remove dissolved minerals/salts. TDS actually rises slightly after boiling as water evaporates and the same dissolved solids remain in a smaller volume.
How often should TDS be checked if I’m not using continuous monitoring?
For a stable municipal supply, monthly spot checks are reasonable. For borewell sources, or during monsoon season when supply sources and quality fluctuate more, more frequent checks (weekly) catch issues sooner — which is exactly the gap continuous, automated monitoring closes without requiring manual testing at all.
