Total dissolved solids
- Abbreviation
- TDS
- Also known as
- TDS, dissolved solids, salinity
Total dissolved solids (TDS) is the total concentration of minerals, salts and other substances dissolved in water, usually reported in milligrams per liter. It is the standard measure of how salty or mineralized a water is, from drinking water to oilfield brine.
What is total dissolved solids?
Total dissolved solids, or TDS, is the combined amount of everything dissolved in a water sample, mainly inorganic salts such as sodium, calcium, magnesium, chloride, sulfate and bicarbonate, plus smaller amounts of dissolved organic matter. It is reported as a concentration, usually in milligrams per liter (mg/L), which for dilute water is roughly equivalent to parts per million.
TDS is a single, simple number, which makes it useful for classifying water and comparing sources. It does not say which substances are present, and two waters with the same TDS can behave very differently.
TDS ranges
The U.S. Geological Survey classifies water by dissolved solids:
| Classification | TDS (mg/L) |
|---|---|
| Fresh | Less than 1,000 |
| Slightly saline | 1,000 to 3,000 |
| Moderately saline | 3,000 to 10,000 |
| Highly saline | 10,000 to 35,000 |
| Brine | More than 35,000 |
Seawater contains roughly 35,000 mg/L. The Environmental Protection Agency’s secondary, non-enforceable guideline for drinking water is 500 mg/L, based on taste and aesthetic effects. Produced water from oil and gas wells is commonly far saltier than seawater.
How TDS is measured
The reference method evaporates a filtered sample and weighs the residue. In the field, TDS is usually estimated from electrical conductivity, since dissolved ions carry current; the conversion factor depends on the mix of ions, so conductivity-based readings are approximations.
Why TDS matters in oil and gas
- Water handling. High-TDS produced water is costly to treat, which is why most is reused in operations or injected into saltwater disposal wells.
- Scale. Waters rich in calcium, barium, strontium and sulfate can precipitate mineral scale when conditions change or waters mix.
- Chemical performance. Salts can reduce the solubility and effectiveness of treatment chemicals. Many surfactants lose performance or separate from solution in high-TDS brine, especially in the presence of calcium and magnesium, and nonionic surfactants reach their cloud point at lower temperatures in salty water. Chemicals for fracturing fluids and produced water systems are therefore selected and tested for tolerance to the actual brine.
Why TDS matters in industrial water
In cooling towers, boilers and process water, TDS rises as water evaporates and minerals concentrate. High TDS increases the risk of scaling, corrosion and fouling, so systems are managed by bleeding off concentrated water and by chemical treatment. Discharge permits may also limit the TDS of wastewater.
TDS and Locus biosurfactants
Locus describes high-TDS stability as a property of its oilfield biosurfactants, meaning they are intended to keep working in the salty brines where some conventional chemistries lose performance.
Frequently asked questions
It depends on the context. For drinking water, levels above the EPA guideline of 500 mg/L may affect taste. In hydrogeology, water over 1,000 mg/L is considered saline and over 35,000 mg/L brine. Oilfield brines can be many times saltier than seawater.
They are closely related. Salinity refers specifically to dissolved salts, which usually make up most of the TDS, so the terms are often used interchangeably in practice.
Dissolved salts, particularly calcium and magnesium, can reduce the solubility of surfactants, change how they behave at interfaces, and in some cases cause them to separate from solution. Surfactants for high-TDS applications are chosen and tested for salt tolerance.
Sources
- Saline water and salinity; produced water, U.S. Geological Survey
- Secondary drinking water standards, U.S. Environmental Protection Agency