Water Quality & RO Guide
What Is TDS in Water & How Does RO Reduce It? A 2025 Practical Guide
TDS tells you how much dissolved material is in water—but not whether that water is safe. This guide explains what the number means, how reverse osmosis lowers it, and how to use a TDS meter without drawing the wrong conclusion.
1. What Exactly Is TDS?
TDS (Total Dissolved Solids) is the sum of inorganic and organic substances dissolved in water. These substances are small enough to pass through a filter with a nominal pore size of about 2 μm. Laboratory TDS is commonly determined by evaporating a filtered sample and weighing the residue; handheld meters estimate TDS from electrical conductivity.
Real-world water analysis example
One AquaClarion field sample in Shandong contained Ca²⁺ 68 ppm, Mg²⁺ 29 ppm, Na⁺ 94 ppm, HCO₃⁻ 210 ppm, SO₄²⁻ 115 ppm and SiO₂ 18 ppm. The calculated TDS was 384 ppm; the field meter read 380 ppm.
2. TDS vs. Hardness vs. Contaminants
These terms describe different properties. TDS is a broad quantity, while hardness mainly reflects calcium and magnesium. A low TDS result does not rule out bacteria, some organic chemicals or other hazards.
| Parameter | Typical unit | What it does not show |
|---|---|---|
| TDS | ppm or mg/L | Identity of each dissolved substance; larger particles and microbes |
| Hardness | ppm as CaCO₃ | Sodium, chloride, nitrate and many other dissolved ions |
| TOC | mg/L as carbon | Non-carbon salts such as sodium chloride |
3. Is Low-TDS Water Always Healthier?
No single TDS target fits every application. Very low-TDS water may taste flat, while higher values can increase mineral taste and scaling. Health and safety depend on the actual substances present—not simply their combined concentration.
Practical bottom line: Treat TDS as a screening and performance metric. Use a certified laboratory analysis when health, process compliance or equipment protection matters.
4. How RO Reduces TDS
Reverse osmosis applies pressure to feed water and drives water molecules through a dense polyamide membrane. Most dissolved ions are rejected and leave in the concentrate stream. Rejection depends on membrane type, temperature, pressure, feed composition, recovery and membrane condition.
Example: (380 − 10) ÷ 380 × 100 = 97.4% rejection.
Why real-world rejection differs from the datasheet
- Datasheet values are measured under controlled pressure, temperature and salt conditions.
- Scaling ions such as CaSO₄ and silica can reduce flux and rejection.
- Higher recovery increases concentration at the membrane surface.
- Aging, oxidation and fouling gradually lower performance.
For a Hydranautics ESPA2-4040 example, nominal NaCl rejection may reach 99.3% under specified test conditions. On a 380 ppm well, field performance can start around 97–98% and decline as the membrane fouls or ages.
5. TDS Reduction: RO vs. Other Technologies
| Technology | Typical TDS reduction | Best use |
|---|---|---|
| RO (polyamide) | 96–99% | Broad dissolved-salt removal |
| Nanofiltration | 50–75% | Partial softening and divalent-ion reduction |
| Ultrafiltration | ≈ 0% | Suspended solids and microorganisms, not dissolved ions |
| Activated carbon | ≈ 0% | Chlorine, taste, odor and selected organics |
| Water softener | ≈ 0% | Exchanges calcium and magnesium for sodium |
6. Why Add Remineralisation After RO?
RO permeate can have very low alkalinity and a neutral or flat taste. For drinking-water systems, a calcite or magnesium-calcium cartridge can raise mineral content and stabilize pH.
In one AquaClarion field adjustment, a dolomite and Corosex blend raised outlet TDS to 28 ppm and pH to 7.8. The right target depends on feed water, contact time, desired taste and local requirements.
7. How to Measure TDS in 30 Seconds
- Calibrate the meter. Use the correct conductivity standard and conversion factor for your instrument.
- Rinse the cup and probe. Rinse three times with the sample to avoid cross-contamination.
- Measure gently. Fill the cup, immerse the probe and stir slowly until the reading stabilizes.
- Record TDS and temperature. Compare feed and RO permeate under similar operating conditions.
8. Case Study: 1,180 ppm Well Water to 38 ppm
Location: Nantong, 15 m well
Challenge: TDS 1,180 ppm, Cl⁻ 450 ppm, SAR 8.2
System: 2.5-inch brackish-water RO, 1.5 m³/h, 38% recovery, antiscalant 3 ppm
- Permeate TDS: 38 ppm
- Calculated rejection: 96.8%
- Specific energy: 0.96 kWh/m³
- Cleaning interval: approximately 90 days at a differential-pressure rise of 0.35 bar
9. Frequently Asked Questions
Does zero-TDS water mean the water is safer?
No. It means the meter detects very little conductive dissolved material. It does not by itself confirm microbiological or chemical safety.
Why does RO water show “TDS creep” after sitting idle?
During shutdown, ions diffuse across the membrane and concentrate in stagnant permeate. Flush the first small volume after restart and measure again once the system stabilizes.
When should an RO membrane be replaced?
Use normalized permeate flow, salt rejection and pressure drop—not TDS alone. Investigate cleaning when performance deteriorates, and replace the membrane when cleaning can no longer restore acceptable output.
What TDS is required for boiler feed water?
The target depends on boiler pressure and the complete water chemistry. High-pressure systems often require RO followed by polishing such as mixed-bed ion exchange or EDI. Confirm the limit with the boiler manufacturer and applicable standard.
Sources & Data Notes
- WHO, Guidelines for Drinking-water Quality, 4th edition with updates.
- František Kožíšek, Health Risks from Drinking Demineralised Water, WHO background paper.
- Membrane performance values should be checked against the current manufacturer datasheet and corrected for actual operating conditions.
Need Help Interpreting Your Water Analysis?
Send AquaClarion your feed-water report, required product-water quality and design flow. We can help select the right RO process and realistic operating targets.
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