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500
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The Total Dissolved Solids (TDS) Calculator determines the concentration of all dissolved inorganic and organic substances in water using the gravimetric evaporation method. TDS is a primary indicator of water quality and includes minerals, salts, metals, cations, and anions dissolved in water. The evaporation method — the gold standard for TDS measurement — involves filtering a known volume of water, evaporating it to dryness at 180°C, and weighing the residue. This calculator converts the residue mass and sample volume into a concentration expressed in mg/L (equivalent to ppm for dilute aqueous solutions). TDS measurement is essential in drinking water quality assessment, wastewater treatment monitoring, irrigation water evaluation, and industrial process water control. The US EPA secondary drinking water standard recommends TDS below 500 mg/L.
TDS by the gravimetric method is calculated from the mass of residue remaining after evaporation:
$$TDS \,(mg/L) = \frac{m_{residue} \,(mg)}{V_{sample} \,(mL)} \times 1000$$
Where:
The multiplication by 1000 converts the concentration from mg/mL to mg/L. For dilute aqueous solutions (density ≈ 1 g/mL), mg/L is numerically equivalent to ppm. The sample must be filtered through 2.0 μm (or sometimes 0.45 μm) before evaporation to exclude suspended solids. Drying at 180°C is standard (APHA Method 2540C) to remove occluded water while minimizing thermal decomposition of bicarbonates.
TDS classification follows widely accepted guidelines: Excellent (<300 mg/L), Good (300-600 mg/L), Fair (600-900 mg/L), Poor (900-1200 mg/L), and Unacceptable (>1200 mg/L). The WHO guideline for drinking water is 1000 mg/L, while the US EPA recommends ≤500 mg/L. Water below 100 mg/L may taste flat, while water above 500 mg/L may have noticeable mineral taste. Very high TDS (>2000 mg/L) indicates brackish water. TDS does not identify specific dissolved substances — additional testing is needed to determine the ionic composition.
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A 50 mL sample yielding 17.5 mg residue gives TDS of 350 mg/L — Good quality, well within EPA secondary standards.
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TDS of 2500 mg/L indicates heavily mineralized water requiring treatment before discharge or reuse.
TDS includes all dissolved substances that pass through a 2.0 μm (or 0.45 μm) filter and remain after evaporation. This encompasses inorganic salts (calcium, magnesium, sodium, potassium, chloride, sulfate, bicarbonate), dissolved metals, silica, and dissolved organic matter. It does not include suspended particles, colloids, or dissolved gases.
Drying at 180°C (APHA 2540C) converts bicarbonates to carbonates, drives off mechanically occluded water, and removes more organic matter than drying at 103-105°C (which is used for total solids including suspended). The higher temperature gives more consistent results for dissolved solids but may cause some decomposition of volatile organic compounds.
Total solids (TS) includes both dissolved solids (TDS) and suspended solids (TSS). The relationship is TS = TDS + TSS. TDS is measured on filtered samples, while TS is measured on unfiltered samples. TSS is calculated by difference or measured directly by filtering and weighing the residue on the filter.
TDS meters measure electrical conductivity and estimate TDS using a conversion factor (typically 0.5-0.7). They provide quick, convenient readings but are less accurate than the gravimetric method. TDS meters cannot detect non-ionic dissolved substances and the conversion factor varies with water composition. For regulatory compliance, the gravimetric method is preferred.
High TDS results from dissolution of minerals from geological formations, agricultural runoff carrying fertilizers and salts, industrial discharge, urban stormwater, road salt, and natural mineral springs. Seawater has TDS of approximately 35,000 mg/L. Groundwater in arid regions often has elevated TDS due to high evaporation rates.
Not necessarily. Very low TDS water (<50 mg/L) may taste flat, can be corrosive to pipes (leaching metals), and lacks beneficial minerals. Reverse osmosis water typically has 10-50 mg/L TDS. Many people prefer water in the 100-400 mg/L range for taste. The ideal TDS depends on the intended use.
Moderate TDS (200-500 mg/L) provides beneficial minerals for plants. TDS above 1000-2000 mg/L can reduce water uptake through osmotic stress, causing leaf burn and stunted growth. Sensitive crops may be affected at lower levels. Irrigation water quality guidelines specify maximum TDS and specific ion limits for different crop types.
TDS (mg/L) ≈ EC (μS/cm) × conversion factor (0.5-0.7). The exact factor depends on the ionic composition: 0.5 for most natural waters, 0.55-0.7 for mixed salts, and up to 0.8 for high-sulfate waters. This is why conductivity meters can only estimate TDS, not measure it precisely.
For drinking water supplies, monthly to quarterly testing is typical. Industrial processes may require daily or continuous monitoring. Wastewater discharge permits often require weekly TDS testing. Well water should be tested annually or after changes in taste, color, or odor. Continuous TDS monitors are available for critical applications.
Effective TDS removal methods include reverse osmosis (90-99% removal), distillation (95-99%), deionization (90-99%), and electrodialysis (50-90%). Conventional filtration, activated carbon, and UV treatment do not significantly reduce TDS. The choice depends on the TDS level, water volume, cost constraints, and required effluent quality.
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