TDS in Water: What the Number Means and What It Cannot Tell You
Many people check their water with a TDS meter and draw quick conclusions: if the number drops, the filter works well; if it is high, the water is bad. The reality is more nuanced.
What TDS means
TDS stands for total dissolved solids. It refers to dissolved minerals, salts, and other inorganic substances in water, generally reported in parts per million (ppm). Source: EPA
The EPA includes TDS in its secondary standards, which address aesthetic concerns such as taste and appearance rather than directly measuring drinking-water safety.
Why a low number does not mean “safer”
A handheld TDS meter measures the water’s electrical conductivity and converts it into an estimate of dissolved solids. It does not identify the specific substances present.
Water with low TDS could contain contaminants the meter does not detect, while water with moderate TDS may contain minerals such as calcium and magnesium. Source: WHO
A TDS number alone cannot tell you whether water is safe to drink.
Where TDS is useful
TDS can provide a general reference for how a reverse osmosis system is working: a substantial decrease after treatment is an indication of membrane performance in relation to dissolved solids.
It may also be one point of reference when considering remineralization if you prefer a different mineral level from the water your system produces.
How to use TDS thoughtfully
- Use it to compare water before and after your system, not as an absolute measure of safety.
- Do not assume “zero TDS” means “perfect water.”
- Combine it with information about specific contaminants of interest, such as lead, nitrate, or chlorine.
- If you are concerned about a particular contaminant, request a specific test for that substance, not just TDS.
A tool, not a verdict
A TDS meter is useful as an operating reference, but it does not replace laboratory analysis or certify water safety. Combining it with specific information gives you a more complete picture.
At PFC, we explain what TDS actually measures and how to interpret your system’s results.
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