Understanding Total Dissolved Solids (TDS) in Water

Water is essential for life, but not all water is created equal. When we turn on our taps or purchase bottled water, we often take for granted the complex mixture of substances dissolved within it. One crucial measure of water quality that often goes unnoticed is TDS, or Total Dissolved Solids.

TDS is a key indicator of water composition, affecting everything from taste and appearance to its impact on our health. In this article, we’ll take a closer look at TDS, exploring what it means, why it matters, and how it influences the water we consume every day.

It is important to note that ‘taste’ can be somewhat of a misnomer. This term is used as a convenient way to describe the sensory experience and overall mouthfeel derived from the presence of mineral salts in the water.

What is TDS?

TDS stands for Total Dissolved Solids, a measure of the combined content of all inorganic and organic substances contained in a liquid, typically water. These dissolved solids are primarily minerals, salts, and metals that have dissolved in the water as they move through various environments.

When water travels through the ground or over surfaces, it can pick up small amounts of minerals like calcium, magnesium, potassium, and sodium. Mineral salts within water are the most bioavailable of any foodstuff and regulations are often created to protect the unique mineral composition at source. Water may also collect traces of organic matter, salts, and other substances. All these dissolved particles contribute to the TDS level of the water.

TDS is typically expressed in parts per million (ppm) or milligrams per litre (mg/L). For context, pure distilled water would have a TDS reading close to 0 ppm, while seawater can have a TDS level of more than 30,000 ppm.

It’s important to note that TDS is a quantitative measure of the total dissolved substances, not a qualitative one. This means it tells us how much is dissolved in the water, but not what specific substances are present or whether they are harmful or beneficial.

Types of Total Dissolved Solids

The primary components of TDS are:

Inorganic salts – these make up the majority of TDS and consist of: 

• Positively charged cations: calcium, magnesium, potassium, and sodium

• Negatively charged anions: carbonates, nitrates, bicarbonates, chlorides, and sulphates

While present in smaller amounts, organic matter also contributes to TDS levels.

• Other materials: Metals, minerals, and various ions can also be part of the total dissolved solids in water.

How is TDS Measured?

Understanding how TDS is measured can help assess water suitability for various uses, including drinking, irrigation, and industrial applications. Here are the primary methods used to measure TDS in water:

Electrical Conductivity (EC) Metres

EC metres are the most common and convenient devices for measuring TDS in water. They operate on the principle that the electrical conductivity of water increases as the concentration of dissolved ionised solids increases.

How EC metres work:

• The metre has two electrodes in the water sample.
• A known voltage is applied across these electrodes.
• The metre measures the current that flows between the electrodes.
• Using Ohm’s Law (V = IR), the metre calculates the resistance of the water.
• The inverse of resistance gives the conductivity.

Converting EC to TDS:

• The metre uses a conversion factor to estimate TDS from the EC reading.
• The most common conversion factor is 0.5, so TDS (in ppm) = EC (in µS/cm) × 0.5.
• This factor can vary from 0.55 to 0.8, depending on the composition of dissolved solids.

Gravimetric Method

The gravimetric method is one of the simplest tests and is often performed by environmental laboratories. TDS is measured using a sensitive analytical balance to weigh residual dissolved solids from a water sample after filtration, evaporation, and heating at 180°C.

How the gravimetric method works:

• Filtration: The water sample is passed through a standard glass fibre filter with a pore size of approximately 1-2 micrometres. This filtration step operationally defines what is considered “dissolved” in the context of TDS measurement.
• Evaporation: The filtered water sample is then evaporated, leaving behind only the dissolved solids.
• Heating: The residue is heated to 180°C to ensure complete dryness and to standardise the measurement conditions.
• Weighing: Finally, the remaining solid residue is weighed using the analytical balance.

TDS Measurements: From Very Low to Rich

TDS in drinking water is typically categorised into four main levels: very low, low, medium, and rich. Understanding these levels can help you better interpret water quality and choose water that suits your preferences or needs.

• Very low (0-50 mg/L): AQUAPAX, Avalis, Eira, Glaceau Smartwater, Lauretana, Lurisia, ONE Ethical Water, Voss.
• Low (51-500 mg/L): Acqua Panna, Aqua Carpatica, Belu, Fiji, Evian, Highland Spring, Hildon, Ty Nant and more.
• Medium (501-1500 mg/L): Badoit, Ferrarelle, 22 Artisan, Gaudianello, San Pellegrino, Vals, Uliveto, Vittel.
• Rich (1500+mg/L): Borjomi, Contrex, Hepar, Pedras Salgadas, Vichy Catalan, Vichy Celestins, Vichy St Yorre. 

Remember, while TDS indicates the overall mineral content, it doesn’t specify which minerals are present.

Why Does TDS Matter in Drinking Water?

Total Dissolved Solids (TDS) measurement is crucial for assessing drinking water quality and safety. As water is a universal solvent, it readily absorbs and dissolves particles from these sources, affecting its overall composition.

While elevated TDS levels are not always directly harmful to health, they can significantly impact water quality in several ways. High TDS concentrations can alter the taste of water, making it bitter, salty, or brackish, which may discourage people from drinking enough water, potentially leading to dehydration. This taste alteration can also affect cooking.TDS levels above 1000 ppm can noticeably change food flavours, potentially making dishes taste saltier or more mineral-like, and affecting the texture and appearance of some cooked foods

Can TDS Be Too High or Too Low?

From a health perspective, while most dissolved solids are not directly harmful, extremely high TDS levels (exceeding 1000 mg/L) can render water unfit for human consumption. Some dissolved solids, if present in high concentrations, may pose health risks.

Conversely, very low TDS levels (between 50 and 250 ppm) can also be problematic. Some dissolved solids, such as calcium, magnesium, and certain organic matter, provide beneficial minerals and nutrients. Therefore, a balanced TDS level, typically between 300 and 500 ppm, is often considered ideal for drinking water.

What is the Difference Between TDS and Dry Residue?

Total Dissolved Solids (TDS) and dry residue at 180°C are essentially the same concept, with the terminology often varying based on regional preferences and regulatory language. Both TDS and dry residue refer to the measurement of dissolved substances in water after filtration and evaporation.

While TDS and dry residue at 180°C are equivalent terms referring to the same measurement, they are related to but not exactly the same as electrical conductivity. Conductivity provides an estimate of TDS based on ionic content, but the filtration and drying method remains the most accurate way to determine the true total dissolved solids in a water sample.

TDS and Its Relevance to Your Water Choices

As we wrap up our exploration of Total Dissolved Solids (TDS) in water, it’s clear that TDS is more than just a number; it’s a key indicator of water composition that affects taste, mouthfeel, and potentially long-term health impacts. However, it’s important to recognise that the question isn’t simply whether TDS is good or bad, but rather how it’s relevant to your personal water preferences and needs.

TDS levels can significantly influence the sensory experience of drinking water. Some people prefer water with higher TDS levels, enjoying the rich mineral taste and substantial mouthfeel it provides. Others may favour water with lower TDS, appreciating its lighter, crisper character.

Choosing water based on its TDS level or specific mineral content is a personal decision. What’s most important is that you enjoy the water you’re drinking. Whether you prefer mineral-rich water or opt for more neutral-tasting options, the key is to find a balance that suits your taste preferences and aligns with your health goals.