
What Is the TDS Measurement of Water & What Makes Up that Measurement?
Water quality is a topic that touches every industry, home, and life. Among the most referenced indicators of water quality is TDS, or Total Dissolved Solids. Whether you're working in water treatment, managing a pure water system, or simply curious about the water you drink—understanding TDS is essential.
In this article, we’ll break down what TDS is, what it includes, and why it matters. This blog also lays the groundwork for connected topics like pure water systems, water softeners, and how hardness impacts water quality regardless of TDS levels.
What Is TDS in Water?
TDS (Total Dissolved Solids) refers to the combined content of all inorganic and organic substances contained in a liquid—in this case, water. These substances are dissolved and present in molecular, ionized, or micro-granular suspended form.
TDS is typically measured in parts per million (ppm) or milligrams per liter (mg/L). While TDS itself isn’t a direct health hazard, it serves as a quantitative indicator of water quality and purity.
What Makes Up the TDS Measurement?
TDS includes a broad spectrum of dissolved substances, primarily:
1. Inorganic Salts
- Calcium (Ca²⁺)
- Magnesium (Mg²⁺)
- Sodium (Na⁺)
- Potassium (K⁺)
- Chlorides (Cl⁻)
- Sulfates (SO₄²⁻)
- Bicarbonates (HCO₃⁻)
- Nitrates and Phosphates
2. Organic Matter
Includes decaying plant material, industrial waste, and trace organic chemicals. Certain organics can contribute to odor, taste, or even toxicity.
3. Heavy Metals (in trace amounts)
Iron, copper, lead, and arsenic can contribute to TDS and present health or mechanical concerns if levels are elevated.
4. Volatile and Semi-Volatile Compounds
Found in industrial or contaminated settings; may be present in small amounts, though not always captured through standard TDS testing.
How Is TDS Measured?
1. Gravimetric Analysis
A lab-based method where water is evaporated and the residue is weighed.
2. Conductivity Meters
TDS is estimated by measuring the water's electrical conductivity (EC) and applying a conversion factor. This is the most common method used in residential and field applications.
What Does TDS Tell You?
While TDS does not specify what is in the water, it tells you how much of something is there. TDS is a useful proxy for:
- Water palatability (high TDS may taste bitter, salty, or metallic)
- Scaling potential in plumbing and appliances
- Efficiency of filtration or deionization systems
- Baseline safety screening for water quality
💡 Important Note: A low TDS value does not always mean the water is safe or pure. Dangerous contaminants like lead or pesticides can still be present.
What Is Considered an Acceptable TDS Level?
Here’s a quick guideline from the EPA (for drinking water):
- Excellent: 0–150 ppm
- Good: 150–300 ppm
- Fair: 300–500 ppm
- Poor: 500–1200 ppm
- Unacceptable: >1200 ppm
Note: These numbers vary based on the application. Ultrapure water systems may target TDS levels below 1 ppm.
TDS vs. Water Hardness
While often conflated, TDS and water hardness are not the same:
- Hardness refers specifically to calcium and magnesium content.
- TDS includes these minerals but also all other dissolved solids.
This becomes crucial when evaluating pure water systems, as soft water can still have high TDS due to sodium or chlorides.
Related Topics
- What Are Pure Water Systems? — Explore how reverse osmosis, deionization, and distillation reduce TDS.
- What Is a Water Softener? — Learn how softeners work and their impact on hardness vs. TDS.
- How Water Softeners and Hardness Affect Pure Water Systems — Understand why soft water may still hinder pure water system performance.
Final Thoughts
TDS measurement is a foundational concept in understanding water quality. It helps assess how "clean" or "usable" water is for different purposes. But while TDS gives you a number, it's just the beginning of the story.
As you’ll see in our upcoming posts, a true understanding of water quality demands looking at what's in that number and how it interacts with systems designed to treat or purify it.
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