Total Dissolved Solids (TDS) measures the concentration of inorganic salts and trace organic matter dissolved in water. Measured in milligrams per liter (mg/L) or parts per million (ppm), TDS represents substances other than the pure water molecule (H₂O). A reading of 25 ppm is extremely low and is considered exceptionally safe regarding immediate contamination risk. This low value suggests the water has undergone advanced purification, such as reverse osmosis or distillation, removing almost all dissolved substances.
Understanding Total Dissolved Solids
Total Dissolved Solids are composed of various mobile charged ions, primarily including essential minerals and salts such as calcium, magnesium, sodium, potassium, chlorides, sulfates, and bicarbonates. Trace amounts of organic matter and metals also contribute to the total measurement. Water acts as a universal solvent, absorbing molecules from materials it contacts, which is the source of TDS. Sources include the natural dissolution of minerals from rocks and soil, as well as human activities like agricultural runoff and industrial waste. TDS is measured using a digital meter that determines the water’s electrical conductivity, which is then converted into an estimate expressed in ppm or mg/L.
Regulatory Standards for TDS Safety
The 25 ppm TDS level is far below any established regulatory limit for drinking water safety. The U.S. Environmental Protection Agency (EPA) sets a secondary maximum standard of 500 ppm for TDS. This non-enforceable guideline focuses on aesthetic qualities, such as taste, odor, color, and potential scaling in pipes, rather than direct health risks. The World Health Organization (WHO) considers excellent quality water to be below 300 ppm, noting that palatability is affected by TDS. Since TDS is not considered a primary pollutant at moderate levels, the 25 ppm reading confirms the water is highly purified and meets all aesthetic guidelines.
Implications of Extremely Low TDS
While 25 ppm water is exceptionally clean, this low reading indicates that almost all dissolved minerals have been removed, which presents specific implications. Water with very low TDS, often described as “ultra-purified,” can taste flat or bland because it lacks the minerals, such as calcium and magnesium, that contribute to a palatable flavor.
A technical concern with very low TDS water is its potential corrosivity. Highly pure water is considered “soft” and can become aggressive, meaning it may leach metals like lead and copper from older plumbing over time. The water’s interaction with pipes can introduce contaminants, making the final tap water quality dependent on the household’s plumbing material. Furthermore, the removal of minerals means the water offers negligible contribution to the body’s dietary intake of these substances. Relying solely on ultra-low TDS water over a long period may lead to an inadequate intake of micronutrients if the diet is otherwise deficient.
What TDS Does Not Indicate About Water Quality
A low TDS measurement of 25 ppm is not a comprehensive indicator of complete water safety. TDS only measures the total concentration of dissolved solids, meaning it fails to account for substances that are not charged or dissolved. For example, the test does not detect suspended solids, such as silt or rust, which are physically suspended rather than dissolved.
Crucially, a TDS meter cannot identify specific hazardous contaminants that are uncharged or toxic at very low concentrations. These include microorganisms (bacteria and viruses), volatile organic compounds (VOCs), pesticides, and certain heavy metals like lead and arsenic. Lead is often harmful at parts per billion levels, which is too low for a standard TDS meter to register meaningfully against the total dissolved solids. Therefore, a low TDS reading should not provide a false sense of security; comprehensive laboratory testing for specific contaminants remains the only reliable method for assessing total drinking water safety.