How to Test TDS in Water: Methods, Equipment, and Interpretation Guide

How to Test TDS in Water: Methods, Equipment, and Interpretation Guide

Total Dissolved Solids (TDS) is one of the most commonly measured water quality parameters, essential for evaluating water purity, designing treatment systems, and monitoring membrane performance. Whether you are assessing raw water sources, measuring RO system efficiency, or ensuring drinking water quality, accurate TDS measurement is fundamental. This guide covers TDS testing methods, equipment selection, and data interpretation.

What Is TDS?

TDS represents the total concentration of dissolved inorganic and organic substances in water, including minerals, salts, metals, and organic matter. TDS is typically expressed in milligrams per liter (mg/L) or parts per million (ppm). Common dissolved constituents include calcium, magnesium, sodium, potassium, bicarbonate, chloride, and sulfate.

TDS Measurement Methods

Conductivity Method (Indirect)

The most common and practical method estimates TDS from electrical conductivity (EC) measurements. Dissolved ions conduct electricity, so conductivity correlates with TDS. A conversion factor (typically 0.5-0.7) multiplies the conductivity reading to estimate TDS. This method provides rapid, continuous measurements suitable for process monitoring.

Gravimetric Method (Direct)

The reference standard method (APHA 2540C) involves filtering a known volume of water, evaporating the filtrate at 180°C, and weighing the residue. This provides a direct measurement of TDS but is time-consuming and only suitable for laboratory analysis.

TDS Ranges and Significance

• 0-50 ppm: Ultra-pure water (laboratory grade, semiconductor manufacturing)

• 50-300 ppm: Excellent drinking water quality

• 300-600 ppm: Good drinking water quality (WHO aesthetic guideline: 600 ppm)

• 600-1000 ppm: Acceptable but may taste salty; brackish water threshold

• 1000-10,000 ppm: Brackish water requiring desalination treatment

• 10,000-35,000 ppm: Saline water (seawater averages ~35,000 ppm)

• Above 35,000 ppm: Brine or hypersaline water

TDS and Membrane System Design

TDS directly impacts membrane system design and operation:

• Osmotic pressure: Higher TDS means higher osmotic pressure, requiring more pressure to drive RO permeation. For NaCl, osmotic pressure is approximately 0.7 bar per 1000 ppm

• Energy consumption: Higher feed TDS increases specific energy consumption in RO systems

• Recovery rate: Maximum achievable recovery decreases with increasing feed TDS due to concentration and scaling limits

• Membrane selection: Feed TDS determines whether brackish water RO (BWRO) or seawater RO (SWRO) membranes are appropriate

• Salt rejection monitoring: Comparing feed and permeate TDS tracks membrane integrity and performance over time

Tech Inc. provides TDS/conductivity monitoring as part of our water quality sampling station solutions for membrane treatment plants.

Frequently Asked Questions

Is lower TDS always better for drinking water?

Not necessarily. Very low TDS water (below 50 ppm) may taste flat and lacks essential minerals. WHO notes that drinking water with moderate mineral content (100-300 ppm) may provide nutritional benefits from calcium and magnesium.

How do I calculate RO salt rejection from TDS?

Salt rejection = (1 - TDS_permeate / TDS_feed) × 100%. For example, if feed TDS is 2000 ppm and permeate TDS is 30 ppm, rejection = (1 - 30/2000) × 100% = 98.5%.

What causes TDS to change in source water?

Seasonal variations, rainfall, drought, agricultural runoff, industrial discharge, saltwater intrusion, and geological changes all affect source water TDS. Regular monitoring helps anticipate treatment needs.