What is TDS?
TDS stands for Total Dissolved Solids. It measures the total concentration of all dissolved substances in water — including minerals (calcium, magnesium, sodium), salts, metals (iron, lead), and other dissolved organic and inorganic compounds. TDS is expressed in ppm (parts per million) or mg/L (milligrams per litre) — these are equivalent.
A TDS reading of 500 ppm means there are 500 milligrams of dissolved solids in every litre of water.
How is TDS Measured?
TDS is measured using a TDS meter — an inexpensive electronic pen-shaped device available for ₹150–₹500 online. It measures electrical conductivity, which correlates with dissolved ion concentration.
How to measure:
- Allow your RO to produce water normally
- Dip the TDS meter into a glass of the water
- Read the value (takes 2–3 seconds to stabilise)
- Compare to your source water TDS to calculate rejection rate
What TDS Levels Are Acceptable?
| TDS Level | Water Quality Assessment |
|-----------|--------------------------|
| 0–50 ppm | Very low — may lack beneficial minerals |
| 50–150 ppm | Excellent — typical RO output range |
| 150–300 ppm | Good — acceptable for drinking |
| 300–500 ppm | Acceptable — BIS standard upper limit |
| 500–1000 ppm | Poor — not recommended for drinking |
| Above 1000 ppm | Very poor — treatment strongly recommended |
The BIS (Bureau of Indian Standards) drinking water standard sets an acceptable TDS limit of 500 ppm, with a permissible limit of 2000 ppm only where no better source is available.
Does Low TDS Mean Pure Water?
Not necessarily. TDS measures *quantity* of dissolved solids — not *which* solids. Water with very low TDS could theoretically still contain dissolved pesticides or organic compounds that a TDS meter won't detect. Conversely, some dissolved minerals (calcium, magnesium) are beneficial.
A TDS meter is a useful tool but should not be your only water quality assessment method.
How to Calculate RO Rejection Rate
Formula: Rejection % = ((Feed TDS − Permeate TDS) / Feed TDS) × 100
Example: Source water at 600 ppm, RO output at 20 ppm → (600−20)/600 × 100 = 96.7% rejection
A healthy RO membrane should maintain at least 90–95% rejection. When rejection drops below 90%, the membrane likely needs replacement.
Key Takeaway
TDS is a quick, useful indicator of water quality — but it's one data point, not the full picture. Use a TDS meter to monitor your RO performance over time; when output TDS rises significantly (rejection drops), it's a strong signal your membrane or pre-filters need attention.