What Is Total Dissolved Solids?
Total Dissolved Solids represents the combined concentration of all inorganic and organic substances dissolved in water that pass through a 2-micron filter. In practical terms, TDS includes:
- Cations (positively charged ions): Calcium (Ca2+), magnesium (Mg2+), sodium (Na+), potassium (K+), iron (Fe2+/Fe3+), manganese (Mn2+), copper (Cu2+), zinc (Zn2+), lead (Pb2+), arsenic (As3+/As5+)
- Anions (negatively charged ions): Bicarbonate (HCO3-), carbonate (CO3 2-), chloride (Cl-), sulfate (SO4 2-), nitrate (NO3-), fluoride (F-), phosphate (PO4 3-)
- Other dissolved substances: Silica (SiO2), small organic molecules, dissolved gases
TDS is measured by testing water's electrical conductivity. Pure water is a poor electrical conductor; dissolved ions enable current flow. A TDS meter uses two electrodes to measure conductivity and applies a temperature-compensated conversion factor (typically 0.5-0.7, depending on the dominant ions) to estimate TDS in ppm or mg/L. This measurement is an estimate because different ions conduct electricity differently. For precise analysis, laboratory gravimetric testing (evaporating a measured sample and weighing the residue) provides exact results but costs significantly more ($50-100 vs. $15-30 for a handheld meter).
TDS does not include suspended particles (sediment, silt, algae), undissolved organic matter (leaves, bacteria, viruses), or gases that have not ionized (dissolved oxygen, nitrogen). These require separate testing and different treatment technologies.
EPA Standards for TDS
| Standard | TDS Level | Classification |
|---|
| WHO Ideal | Below 300 ppm | Excellent taste and palatability |
| EPA Secondary MCL | 500 ppm | Aesthetic standard (not health-based) |
| WHO Acceptable | 600 ppm | Fairly palatable |
| WHO Poor | 900-1,200 ppm | Not palatable to most people |
| WHO Unacceptable | Above 1,000 ppm | Brackish taste, potential health concerns |
The EPA classifies TDS as a secondary drinking water standard, meaning it is regulated based on aesthetic qualities (taste, appearance) rather than health effects. Public water systems are not required to treat TDS to meet the 500 ppm standard but must monitor and report levels. The EPA's decision to classify TDS as secondary reflects the fact that many dissolved minerals are beneficial, and the primary concern is consumer acceptance rather than health risk.
However, extremely high TDS (above 2,000 ppm) can cause health issues in sensitive populations. High sodium content in TDS may affect individuals on sodium-restricted diets. High sulfate levels (above 250 ppm) can cause diarrhea, particularly in people not accustomed to it. High nitrate levels (above 10 ppm as N) pose a risk to infants under six months.
Sources of TDS in Drinking Water
Natural Mineral Content
As groundwater flows through rock formations, it dissolves minerals through natural weathering processes. Limestone and dolomite deposits contribute calcium and magnesium (hardness). Sandstone adds silica. Granite and volcanic rock leach sodium and potassium. Coastal aquifers may have elevated sodium and chloride from seawater intrusion. Natural TDS in U.S. groundwater ranges from 50 ppm in granite bedrock regions to 2,000 ppm in deep desert aquifers with limited recharge.
Road Salt and Urban Runoff
In northern states, sodium chloride and calcium chloride road de-icing salts enter groundwater through snowmelt runoff. Wells near highways, parking lots, and salt storage facilities can show seasonal TDS spikes of 200-500 ppm above baseline during spring thaw. This is primarily a chloride and sodium increase and can push borderline wells above the 500 ppm standard temporarily.
Agricultural Runoff
Fertilizer application introduces nitrates, phosphates, and potassium into groundwater. Irrigation in arid regions concentrates salts in the soil profile, which then leach into groundwater. The Central Valley of California, Imperial Valley, and parts of the High Plains aquifer show TDS elevations directly linked to intensive agriculture. Fertilizer-derived TDS increases are dominated by nitrate, which is a health concern at levels above 10 ppm.
Industrial Discharge and Mining
Industrial processes can discharge dissolved solids including heavy metals, acids, and process chemicals. Mining operations expose sulfide minerals to oxygen and water, producing sulfuric acid that dissolves additional minerals from rock (acid mine drainage). AMD can increase TDS by thousands of ppm and introduce toxic metals including iron, aluminum, copper, lead, and zinc.
Water Treatment Chemicals
Water softeners add sodium to water (approximately 7.5 mg/L sodium per grain per gallon of hardness removed). Chlorination adds chloride. Fluoridation adds fluoride. These treatment-related contributions are typically small (10-50 ppm) but add to the baseline TDS.
Not All TDS Is Bad: Minerals vs. Contaminants
A common misconception is that all TDS should be removed. This is incorrect. The composition of TDS matters far more than the total number.
Beneficial Minerals in TDS
- Calcium (Ca2+): Essential for bone health, muscle function, and nerve transmission. The WHO recommends minimum 20-30 mg/L calcium in drinking water. Water with 100-300 ppm calcium hardness provides 10-30% of daily calcium requirements.
- Magnesium (Mg2+): Critical for over 300 enzymatic reactions, heart rhythm regulation, and blood pressure control. Epidemiological studies show inverse correlations between magnesium in drinking water and cardiovascular disease mortality.
- Bicarbonate (HCO3-): Acts as a natural buffer, maintaining water pH in the neutral range. Provides alkalinity that may help with acid-base balance in the body.
- Trace minerals: Fluoride (for dental health at 0.7 ppm), copper, zinc, and selenium at trace levels contribute to nutritional requirements.
Harmful Components of TDS
- Sodium (Na+): At high levels, problematic for hypertensive individuals. Above 270 ppm, the EPA recommends alternative water sources for those on sodium-restricted diets.
- Sulfate (SO4 2-): Above 250 ppm, can cause diarrhea and dehydration. Above 500 ppm, laxative effects are pronounced.
- Nitrate (NO3-): Above 10 ppm as nitrogen (44 ppm as nitrate), causes methemoglobinemia (blue baby syndrome) in infants.
- Heavy metals: Lead, arsenic, cadmium, and mercury are harmful at any level above their respective MCLs.
- Chloride (Cl-): Above 250 ppm, imparts salty taste and can corrode plumbing.
Key Insight: Water with 400 ppm TDS composed primarily of calcium, magnesium, and bicarbonate is healthier than water with 200 ppm TDS composed primarily of sodium and chloride. Always test the specific composition of your TDS, not just the total number.
How to Test TDS Levels
Handheld TDS Meter
A digital TDS meter is the fastest and most affordable testing method. Quality meters from brands like HM Digital, ZeroWater, and Milwaukee cost $15-30 and provide readings in seconds. To use: turn on the meter, insert the probes into a water sample (or hold under running water), wait for the reading to stabilize (3-10 seconds), and record the ppm value.
TDS meters are factory-calibrated using a sodium chloride standard solution. If your water contains predominantly calcium, magnesium, or sulfate, the conductivity-to-TDS conversion factor differs from the sodium chloride standard, introducing potential measurement errors of 5-15%. For home use, this error is acceptable. For precise applications, calibrate the meter with a standard solution matching your water's dominant ions.
Laboratory Gravimetric Analysis
The EPA's Standard Method 2540C uses gravimetric analysis for definitive TDS measurement. A precisely measured water sample (typically 100 mL) is filtered through a 2-micron glass fiber filter, evaporated in a weighed dish at 180C, and the remaining residue is weighed. This method provides exact TDS values but requires laboratory equipment and trained technicians. Cost: $50-100 per sample.
Full Water Analysis
For a complete understanding of your water's TDS composition, order a full mineral analysis from a certified laboratory. This test identifies individual ions and their concentrations, typically including: calcium, magnesium, sodium, potassium, iron, manganese, copper, zinc, chloride, sulfate, nitrate, fluoride, bicarbonate, and silica. Cost: $100-250. This information is essential for selecting appropriate treatment and determining whether your TDS contains harmful components.
Reverse Osmosis: Best Method for TDS Reduction
Reverse osmosis is the most practical and effective method for reducing TDS in residential drinking water. RO membranes use applied pressure to force water through a semi-permeable barrier while rejecting dissolved ions.
How RO Removes Dissolved Solids
The RO membrane is essentially a molecular filter with pores approximately 0.0001 microns (0.1 nanometers) in diameter. Dissolved ions, which are hydrated (surrounded by water molecules), have effective diameters of 0.2-1.0 nanometers, too large to pass through the membrane pores. Water molecules (0.27 nanometers) pass through under pressure.
Rejection rates vary by ion:
| Ion Type | Example Ions | Typical Rejection |
|---|
| Divalent cations | Ca2+, Mg2+, Fe2+ | 96-99% |
| Monovalent cations | Na+, K+ | 92-98% |
| Divalent anions | SO4 2-, CO3 2- | 98-99.5% |
| Monovalent anions | Cl-, NO3-, F- | 85-95% |
| Silica (uncharged) | SiO2 | 85-95% |
Overall TDS rejection for a properly functioning residential RO membrane ranges from 90% to 99%. A membrane achieving less than 85% TDS rejection is failing and should be replaced. Measure rejection by dividing the TDS of the RO product water by the TDS of the feed water, then subtracting from 100.
Example: Feed water TDS = 500 ppm, RO water TDS = 25 ppm. Rejection = (1 - 25/500) x 100 = 95%.
Factors Affecting RO TDS Rejection
- Water temperature: For every 1C below 25C (77F), permeate flow decreases approximately 3%. Rejection typically improves at lower temperatures.
- Inlet pressure: Minimum 40 PSI required; optimal rejection at 60-80 PSI. Below 40 PSI, install a booster pump.
- pH: Optimal range is 6.0-8.5. Below pH 4, the membrane hydrolyzes and degrades. Above pH 11, the polyamide layer delaminates.
- TDS level: Higher TDS creates osmotic pressure that must be overcome, requiring more inlet pressure for the same flow rate.
- Membrane age: Rejection declines gradually over 2-3 years. Replace when rejection drops below 85%.
Distillation: 100% TDS Removal
Water distillation provides the most complete TDS removal of any residential method, achieving 99.9% reduction of all dissolved solids, minerals, metals, and non-volatile organic compounds.
The distillation process works by boiling water to produce steam, which rises and leaves all non-volatile dissolved solids behind in the boiling chamber. The steam is then cooled in a condenser coil and collected as pure distilled water. Volatile organic compounds (VOCs) with boiling points below or near water's boiling point (100C at sea level) may vaporize with the steam, which is why quality distillers include an activated carbon post-filter to capture any VOCs that pass through with the steam.
Distillation has significant practical limitations for home use:
- Production rate: Countertop units produce 0.5-1 gallon per hour. Large automatic units produce 8-12 gallons per day.
- Energy consumption: Approximately 3 kWh per gallon. At $0.14/kWh average U.S. rate, energy cost is $0.42 per gallon.
- Flat taste: Completely mineral-free water tastes flat to most people. Adding a remineralization cartridge or a pinch of mineral salt improves taste.
- Maintenance: The boiling chamber accumulates mineral scale that must be cleaned with vinegar or citric acid every 2-4 weeks, depending on TDS level.
Distillation is ideal for laboratory applications, medical devices (CPAP machines, humidifiers), aquariums, and situations requiring the highest water purity. For general drinking water, RO is more practical.
Deionization (DI): Lab-Grade Purity
Deionization uses ion exchange resin to remove all dissolved ions from water, producing resistivity of 18.2 megohm-cm, the theoretical maximum for pure water. DI resin contains both cation and anion exchange sites that capture positive and negative ions respectively, releasing H and OH- in their place, which combine to form pure water.
Mixed-bed DI cartridges (containing both cation and anion resin in one vessel) are used as polishing stages after RO in laboratory and medical applications. A typical "RO/DI" system for aquarium use consists of an RO membrane followed by a DI cartridge, producing water with near-zero TDS.
DI resin has a finite capacity measured in grains or total dissolved solids removed. A standard 10-inch DI cartridge processing feed water with 200 ppm TSD removes approximately 1,500-2,000 grains of total dissolved solids before exhaustion. At 200 ppm (11.7 grains per gallon), this equals roughly 128-171 gallons of production per cartridge.
DI resin is not practical as a standalone residential drinking water treatment due to high operating costs ($1-2 per gallon of treated water for cartridge replacement alone). It is best used as a final polishing stage after RO when ultra-pure water is needed for specific applications.
What Does NOT Remove TDS
Many popular water treatment technologies do not reduce TDS at all. Understanding these limitations prevents disappointment and ensures you select the right technology.
| Technology | TDS Removal | What It Actually Does |
|---|
| Activated Carbon | 0% | Adsorbs chlorine, VOCs, some organic compounds |
| Sediment Filter | 0% | Removes suspended particles above rated micron size |
| UV Purification | 0% | Inactivates bacteria, viruses, and protozoa |
| Water Softener (Ion Exchange) | 0-5% | Exchanges hardness minerals for sodium; total TDS stays similar |
| Ultrafiltration (0.01 micron) | 0-10% | Removes bacteria, viruses, colloids; passes dissolved ions |
| KDF Media | 0-5% | Removes chlorine, heavy metals via redox; minimal TDS impact |
WARNING: If a product claims to reduce TDS using activated carbon, UV light, or standard filtration, it is making a false claim. Only RO, distillation, and deionization meaningfully reduce TDS. Water softeners change the composition of TDS (calcium/magnesium to sodium) but do not reduce the total.
Complete Technology Comparison for TDS Removal
| Technology | TDS Removal | Production Rate | Initial Cost | Operating Cost/Gal | Best Application |
|---|
| Reverse Osmosis | 90-99% | 50-100 GPD | $200-600 | $0.02-0.05 | Residential drinking water |
| Distillation | 99.9%+ | 0.5-1 gal/hr | $100-500 | $0.40-0.50 | Lab/medical, small volume |
| Deionization (DI) | 99.99%+ | 0.5-2 GPM | $100-300 | $1.00-2.00 | Polishing after RO, aquariums |
| Electrodeionization (EDI) | 99.9%+ | 1-10 GPM | $2,000-10,000 | $0.05-0.10 | Commercial/industrial |
| Nanofiltration | 50-85% | 500-2,000 GPD | $800-2,000 | $0.05-0.10 | Partial softening, organics |
Remineralization: Should You Add Minerals Back?
Because RO and distillation remove beneficial minerals along with harmful ones, the resulting water has two potential issues: flat taste and zero mineral content.
Taste Issues
Water with TDS below 50 ppm tastes flat or "empty" to most people because minerals contribute to the perception of flavor. The ideal drinking water TDS for taste is 100-300 ppm. Many bottled mineral waters fall in this range (Evian: 345 ppm, Fiji: 222 ppm, San Pellegrino: 1,109 ppm).
Health Considerations
The WHO raised concerns in a 2009 report that long-term consumption of demineralized water may reduce intake of essential minerals, particularly calcium and magnesium. While diet is the primary source of these minerals, water contributes 5-20% of daily intake for many people. Populations with mineral-poor diets may be more affected by the switch to demineralized water.
Remineralization Options
- Calcite filter: A post-RO cartridge containing calcium carbonate (limestone) dissolves small amounts of calcium into the water, raising TDS by 20-50 ppm and pH to 7.0-8.0. Cost: $30-60, replaces annually.
- Alkaline/Remineralization cartridge: Contains calcite, corosex (magnesium oxide), and sometimes tourmaline or mineral stones. Adds calcium, magnesium, and raises pH to 8.0-9.5. Cost: $40-80, replaces annually.
- Mineral drops: Liquid mineral supplements added to each gallon of purified water. Precise control over mineral content. Cost: $0.10-0.30 per gallon.
Remineralization is optional but recommended for households that consume primarily RO or distilled water. A simple calcite filter provides adequate mineral addition for most people.
Recommended Products for TDS Removal
iSpring RCC7AK 6-Stage Reverse Osmosis System with Alkaline Remineralization
NSF/ANSI 58 certified 75 GPD RO system with 6th-stage alkaline remineralization filter that adds calcium and magnesium back to purified water. Removes 95-99% of TDS including sodium, chloride, sulfate, and heavy metals. Clear first-stage housing for visual monitoring. Annual filter maintenance: $60-80.
View on Amazon →
APEC Water Systems ROES-50 5-Stage Reverse Osmosis System
Made in USA with NSF-certified components. 50 GPD membrane achieves 95-98% TDS rejection. 5-stage filtration with sediment, carbon block, and post-carbon polishing. Leak-free fittings and quick-connect design. 1-year satisfaction guarantee. WQA Gold Seal certified.
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Megahome Countertop Water Distiller (Stainless Steel)
UL-listed countertop distiller producing 1 gallon per 5.5 hours with 100% TDS removal. All-stainless steel interior with glass collection bottle. Automatic shutoff. Includes activated carbon post-filter for VOC reduction. 1-year warranty. Best for lab-quality drinking water in small volumes.
View on Amazon →
HM Digital TDS-EZ Water Quality TDS Tester
Handheld digital TDS meter with range of 0-9990 ppm and +/- 3% accuracy. Features hold function and automatic temperature compensation. Includes carrying case. Essential for monitoring RO system performance and membrane replacement timing. Factory calibrated with 342 ppm NaCl solution.
View on Amazon →
Installation Guide: RO System for TDS Reduction
Step-by-Step: Installing an Under-Sink RO System
- Measure your TDS: Use a handheld TDS meter to establish baseline feed water TDS. This number determines whether RO is necessary and provides a benchmark for future performance testing.
- Check water pressure: RO systems require minimum 40 PSI. Test with a pressure gauge at an outdoor faucet or laundry tub. If below 40 PSI, install a booster pump ($80-150).
- Choose installation location: Under the kitchen sink is standard. Ensure adequate clearance (minimum 15 inches height) and access to cold water supply, drain, and a dedicated faucet location.
- Install the faucet: Drill a 1/2-inch mounting hole if your sink does not have a pre-drilled soap dispenser or spray hose hole. For stainless steel sinks, use a step drill bit with cutting oil.
- Connect the feed water adapter: Install a 1/4-inch saddle valve or T-adapter on the cold water shutoff valve. Do not connect to the hot water line.
- Install the drain saddle: Position the drain saddle on the vertical drain pipe above the P-trap (not on the horizontal arm). Drill the 1/4-inch hole and secure the saddle with the provided bolts.
- Mount the filter assembly: Use the metal bracket to hang the unit on the cabinet wall at a height that allows easy filter changes. Allow 4-6 inches clearance below for filter removal.
- Connect tubing: Match colors: red to feed, black to drain, blue to faucet, white to tank. Insert tubing fully into quick-connect fittings until it bottoms out, then tug gently to confirm the lock.
- Install the storage tank: Position the tank in an accessible location. Pre-pressurize to 5-7 PSI (empty) using a bicycle pump if needed.
- Leak test and flush: Open the feed valve slowly. Check all connections with a dry paper towel. Open the RO faucet and let water run for 2-4 hours to flush preservatives from the system. Do not drink the first tank of water.
- Verify performance: After 24 hours, measure the TDS of the RO water. Calculate rejection rate: (Feed TDS - RO TDS) / Feed TDS x 100. Should be 90% for a new membrane.
Tools needed: Drill with step bit, adjustable wrenches, screwdriver, Teflon tape, pressure gauge, TDS meter. Time estimate: 2-4 hours for first installation. Difficulty: Intermediate DIY.
Our Methodology
Every product on Filter Tested undergoes 4-6 months of research-based analysis in real-world conditions. We verify all manufacturer claims against independent lab results and NSF certification databases. Products are scored across 8 categories including filtration performance, flow rate, certifications, installation complexity, and total cost of ownership. Learn more about how we test.
Frequently Asked Questions
What TDS level is safe to drink?
TDS itself is not a direct health indicator. Water with TDS of 1,000 ppm composed primarily of calcium and magnesium is safer than water with 200 ppm composed of sodium and nitrate. The EPA secondary standard of 500 ppm is based on taste and aesthetics, not health. As a general guideline: below 300 ppm is excellent, 300-500 ppm is good, 500-1,000 ppm is fair to poor taste, and above 1,000 ppm warrants investigation of the specific composition. Above 2,000 ppm, consult a water treatment professional as some components may pose health risks.
Does low TDS water leach minerals from your body?
No, this is a myth. Water with low TDS does not "pull" minerals from your body. Mineral absorption and excretion in the human body are tightly regulated by the kidneys and hormones. Drinking demineralized water does not cause mineral loss through urine or other pathways. However, the WHO notes that populations with marginal dietary mineral intake may receive reduced mineral contributions from water, which is a different concern than active leaching. If you eat a balanced diet, low TDS water poses no health risk.
Why is my TDS meter reading higher after installing a water softener?
This is expected and normal. A water softener exchanges calcium and magnesium (hardness) for sodium. Because sodium has a higher electrical conductivity per equivalent weight than calcium or magnesium, the TDS meter reading may actually increase by 5-15% after softening, even though the total dissolved solids in milligrams per liter remains approximately the same. The TDS meter is measuring conductivity, not actual dissolved mass. A softener does not remove TDS; it changes the composition of the TDS. Only RO, distillation, or DI actually reduce total dissolved solids.
How often should I replace my RO membrane based on TDS?
Measure your RO water TDS monthly with a handheld meter. When the TDS rejection rate drops below 85%, replace the membrane. For example, if your feed water TDS is 400 ppm and new membrane RO water is 20 ppm (95% rejection), replace the membrane when RO water reaches 60 ppm (85% rejection). Most residential RO membranes last 2-3 years with proper pre-filter maintenance. Neglecting sediment and carbon pre-filter changes allows premature membrane fouling and shortens membrane life to 1 year or less.
Can TDS meters detect lead, arsenic, or other toxic metals?
No. A TDS meter cannot distinguish between beneficial minerals and toxic heavy metals. It measures total ionic conductivity only. Water with 200 ppm TDS consisting entirely of lead would read the same as water with 200 ppm of calcium and magnesium. This is why a TDS meter alone is insufficient for water safety assessment. If you suspect toxic metal contamination (due to lead pipes, nearby mining, or agricultural runoff), you must perform specific laboratory testing for those contaminants. Use a TDS meter for monitoring RO system performance and general water quality assessment, not for safety verification.
Is distilled water the same as RO water?
No, though both produce low-TDS water. Distillation removes 99.9% of all dissolved solids by boiling and condensation, including ions that small RO membrane imperfections might miss. RO removes 90-99% of TDS using pressure-driven membrane filtration. Distilled water typically has TDS of 0-2 ppm, while RO water typically has 5-50 ppm depending on the membrane condition and feed water composition. Distillation also removes more bacteria and viruses (though both are effective). RO is faster, more energy-efficient, and more practical for residential use. Distillation produces higher purity but at much lower volume and higher energy cost.
Should I be concerned about TDS in my well water?
Test your well water TDS as part of a comprehensive water analysis every 1-2 years. If TDS is below 500 ppm and your full mineral analysis shows no harmful contaminants, no treatment is needed. If TDS is 500-1,000 ppm, investigate the composition. If it is primarily calcium and magnesium (hardness), a softener addresses the issue. If it includes high sodium, chloride, or sulfate, consider RO for drinking water. If TDS exceeds 1,000 ppm, conduct a full analysis and consult a water treatment professional. High TDS in wells can indicate saltwater intrusion, agricultural contamination, or mineral leaching that may require specific treatment beyond general TDS reduction.