Total Dissolved Solids (TDS) in Water: What It Means & Treatment
What Are Total Dissolved Solids?
Total Dissolved Solids (TDS) is a measure of the combined content of all inorganic and organic substances dissolved in water. These dissolved substances include minerals, salts, metals, cations (calcium, magnesium, sodium, potassium), anions (bicarbonate, chloride, sulfate, nitrate), and small amounts of organic matter. TDS is expressed in milligrams per liter (mg/L) or parts per million (ppm).
TDS originates primarily from natural sources through the dissolution of rocks and minerals as water moves through soil and geological formations. Limestone dissolves to contribute calcium and bicarbonate. Gypsum contributes calcium and sulfate. Halite (rock salt) contributes sodium and chloride. The TDS of a water source reflects the geological characteristics of the watershed or aquifer.
Anthropogenic sources of TDS include agricultural runoff carrying dissolved fertilizers, road salt application in cold climates, industrial wastewater discharge, sewage treatment effluent, and urban stormwater runoff. Seawater intrusion in coastal aquifers can dramatically increase TDS, as seawater has a TDS of approximately 35,000 mg/L.
TDS is closely related to electrical conductivity (EC) because dissolved ions conduct electricity. The relationship between TDS and EC depends on the specific ion composition but is commonly approximated as TDS (mg/L) = EC (microsiemens/cm) x 0.55 to 0.70.
Effects of High TDS in Water
TDS affects water quality primarily through aesthetic and operational impacts. Water with TDS above 500 mg/L may have a noticeable salty, bitter, or mineral taste. The specific taste depends on the dominant ions present: sodium chloride produces a salty taste, magnesium sulfate produces a bitter taste, and calcium bicarbonate produces a flat or chalky taste.
High TDS can cause scaling in pipes, water heaters, and industrial equipment when the water is supersaturated with calcium carbonate or calcium sulfate. Elevated TDS also increases corrosivity when certain ions (chloride, sulfate) are dominant, accelerating deterioration of metal plumbing and equipment.
In industrial applications, TDS is a critical parameter. Boiler feed water, cooling water, pharmaceutical manufacturing, electronics fabrication, and food and beverage production all have specific TDS limits. High-purity water for semiconductor manufacturing requires TDS below 0.1 mg/L (measured as resistivity of 18.2 megohm-cm).
The WHO notes that water with TDS above 1,000 mg/L is generally unpalatable, and concentrations above 1,200 mg/L may cause gastrointestinal effects. However, TDS is not a health-based parameter; the health significance depends on the specific dissolved constituents, not the aggregate TDS value.
TDS Guidelines and Classifications
| Regulatory Body / Classification | Standard | TDS (mg/L) |
|---|---|---|
| WHO | Good palatability | < 600 |
| U.S. EPA | Secondary MCL (SMCL) | 500 |
| Water Classification | Fresh water | < 1,000 |
| Water Classification | Brackish water | 1,000 - 10,000 |
| Water Classification | Saline water | 10,000 - 35,000 |
| Water Classification | Seawater | ~35,000 |
The EPA SMCL of 500 mg/L is a non-enforceable aesthetic guideline. There is no health-based EPA MCL for total TDS because the health significance depends on the specific ions present, each of which has its own regulatory standard.
How to Test for TDS
TDS can be measured by two primary methods. The gravimetric method (Standard Method 2540C) involves evaporating a filtered water sample at 180 degrees Celsius and weighing the residue. This is the definitive method and provides the most accurate TDS measurement.
The conductivity method estimates TDS by measuring the electrical conductivity of the water and applying a conversion factor. Handheld TDS meters use this approach and provide quick, inexpensive field measurements. However, the accuracy depends on the appropriateness of the conversion factor for the specific water chemistry.
For comprehensive water quality assessment, a complete ion analysis (calcium, magnesium, sodium, potassium, bicarbonate, chloride, sulfate, nitrate, and silica) is more informative than a single TDS measurement, as it identifies the specific constituents present and their concentrations relative to individual regulatory limits.
Treatment Methods for TDS Reduction
Reverse Osmosis
Reverse osmosis is the most widely used technology for TDS reduction, achieving 95-99% rejection of dissolved salts. RO is effective across the full range of TDS concentrations, from low-TDS freshwater polishing to high-TDS brackish water and seawater desalination. System design, membrane selection, and operating pressure are determined by the feed water TDS and composition.
Distillation
Thermal distillation (multi-effect distillation and multi-stage flash) produces very low TDS water by evaporating and recondensing purified steam. Distillation is energy-intensive but is used in large-scale desalination, particularly in the Middle East, and in applications requiring the highest purity water.
Ion Exchange / Deionization
Mixed bed deionization using cation and anion exchange resins removes virtually all dissolved ions, producing water with resistivity up to 18.2 megohm-cm. Deionization systems are used for laboratory, pharmaceutical, electronics, and boiler feed water applications. Resins require periodic regeneration with acid and caustic.
Electrodialysis
Electrodialysis (ED) and electrodialysis reversal (EDR) use ion-selective membranes and electrical current to separate dissolved ions from water. ED/EDR is particularly efficient for brackish water desalination (TDS 1,000-5,000 mg/L) and offers advantages in water recovery compared to RO for certain applications.
Nanofiltration
Nanofiltration membranes provide partial TDS reduction (50-90%), selectively removing divalent ions while passing most monovalent ions. NF operates at lower pressures than RO and is used for softening and partial desalination applications.
Frequently Asked Questions
What is a good TDS level for drinking water?
The EPA Secondary MCL for TDS is 500 mg/L, which is a non-enforceable aesthetic guideline. The WHO states that water with TDS below 600 mg/L is generally considered good quality. Water with TDS below 300 mg/L is considered excellent. TDS above 1,000 mg/L is generally unpalatable, and above 1,200 mg/L may cause gastrointestinal effects.
Does a TDS meter measure water safety?
No. A TDS meter measures electrical conductivity, which correlates with total dissolved mineral content but does not indicate water safety. Water with low TDS can contain harmful contaminants such as bacteria, pesticides, and volatile organic compounds that do not significantly affect TDS readings. Conversely, water with high TDS may be perfectly safe if the dissolved minerals are within acceptable limits. TDS is a general water quality indicator, not a safety measurement.
What causes high TDS in water?
High TDS in water results from the dissolution of minerals from geological formations (calcium, magnesium, sodium, bicarbonates, sulfates, and chlorides), agricultural runoff carrying fertilizer residues, urban stormwater runoff, industrial discharge, and seawater intrusion in coastal areas. Evaporation in arid climates also concentrates dissolved solids in surface water and groundwater.
Need to Reduce TDS in Your Water?
ForeverPure provides commercial and industrial TDS reduction systems, including brackish water RO, seawater desalination, mixed bed deionization, and complete water purification trains. Our engineering team designs systems based on your feed water TDS, target water quality, flow rate, and application requirements.
Contact ForeverPure for a customized TDS reduction solution.