Operator concept
What is TDS in water?
TDS, total dissolved solids, is the total amount of dissolved minerals, salts, and metals in water, measured in mg/L. It is what would be left behind if you evaporated the water away. TDS affects taste, hardness, and how water behaves in treatment. It is a taste-and-appearance concern more than a direct health limit.
What TDS includes
TDS is not one substance. It is the running total of everything dissolved in the water: calcium and magnesium (which also make water hard), sodium, potassium, chlorides, sulfates, and bicarbonates, among others. Pure water would read near zero. Real source water always carries some dissolved minerals, and how much depends on the geology the water moved through.
How operators measure and use it
The quick way to measure TDS is a meter that reads conductivity, because dissolved salts carry an electric current, and more salts mean more conductivity. The reference method evaporates a measured sample and weighs the dry solids. Operators watch TDS because a jump can mean the source changed or something slipped in treatment, and because high TDS shows up as salty or bitter taste and as scale. The EPA lists TDS as a secondary standard, aimed at taste and appearance rather than a health-based limit.
Common questions
- What is TDS in water?
- Total dissolved solids: the total dissolved minerals and salts in water, measured in mg/L. It is roughly the same number in ppm.
- How is TDS measured?
- With a TDS or conductivity meter, since dissolved salts carry an electric current, or by evaporating a sample and weighing the solids left behind.
- Is high TDS bad?
- High TDS can make water taste salty or bitter and can leave scale, but it is mainly a taste-and-appearance issue, not a direct health limit. Very high TDS may point to a source or treatment problem worth checking.
- What is the difference between TDS and hardness?
- Hardness is just the calcium and magnesium in water. TDS is all the dissolved minerals and salts together, so hardness is one part of TDS.