Many water hobbyists and companies that want to promote water filters for tap water use TDS (Total Dissolved Solids) as the main method of testing water quality. The reason is that it is very easy and cheap to measure. The problem is that it is not a good measure and is very limited in terms of water quality parameters.
Below is a simplified explanation of what TDS is and to what extent it should be used as a measure of water quality.
What is TDS in tap water?
TDS consists of inorganic salts and small amounts of organic matter that are dissolved in water. The main constituents are usually the cations calcium, magnesium, sodium and potassium and the anions carbonate, bicarbonate, chloride, sulfate and, particularly in groundwater, nitrate. TDS is expressed in units of mg per unit volume of water (mg/L) or also referred to as parts per million (ppm).
Typically, natural mineral water and tap water can have a TDS value of 100-200 mg/l. In areas with high mineral concentrations, natural tap water could, however, be considerably higher. Water filtration technologies such as reverse osmosis, water distillation and ion exchange can reduce this value to close to zero, while activated carbon filters will not filter out TDS.
So, is a higher TDS level good or bad?
As most bottled mineral water contains higher TDS (e.g. Evian 300 mg/l; San Pellegrino 850 mg/l), this can also be positive for tap water. The WHO and most other water quality regulatory bodies consider values up to 600 mg/l to be completely safe and 2,000 mg/l to be safe for temporary consumption if no other water is available.
Below 600 mg/l there is no scientific evidence that TDS makes any difference, and no health impact has been identified below 2,000 mg/l. To complicate matters somewhat, there is evidence that high concentrations of specific constituents such as calcium can have an impact, but TDS is a very vague indicator of this as it does not specify the different parameters that make up the final number.
In a study by the World Health Organization, a panel of tasters came to the following conclusions about the preferable level of TDS in water (mg/l):
- 50 - 300: Excellent*
- 300 - 600: Good
- 600 - 900: Fair
- 900 – 1,200: Poor
- Above 1,200: Unacceptable
* Tasters in the WHO report noted that water with less than 50 may taste flat, but this is more of an aesthetic concern than a health concern.
For example, tap water in Barcelona ranges from 200-600 mg/l, Madrid 50 mg/l, Palma de Mallorca 400-900 mg/l, Berlin 260-400 mg/l, Paris 200-300 mg/l, Stockholm 73-93 mg/l, London 260 mg/l... and all of them are considered to have safe and healthy drinking water.
It's worth noting that higher TDS waters typically have a heavier flavor and a much more prominent "mouthfeel," a term used by water connoisseurs to describe the overall sensory impression.
Why is TDS a measure of poor water quality?
Why do water filtration companies and hobbyists use high TDS as an argument for water filtration?
The simple reason is that it is cheap and easy to test. A TDS meter can be purchased online for around 50 euros ($55) or less, and then anyone can easily perform a test in seconds at home.
The actual tests that measure chlorine, pesticides, heavy metals and pharmaceuticals, which are important in determining water quality, require sophisticated and expensive laboratories. Your local water company tests water every day for contaminants and is required to provide a water quality report at least annually. This provides a scientific analysis of water quality.
TAPP water filters have been tested for TDS in tap water with values up to 1000. The water tastes good and the water is safe to drink.
What should I do to get an accurate water quality analysis?
In conclusion, water quality testing is important, but think twice before using TDS as an indicator or supplementing it with other technologies. The belief that high TDS in tap water is bad is often based on misconceptions about the negative impact of minerals on the body’s health.
For Spain, you can consult your local water quality report at http://water.tappwater.com.
If you would like to test for potential heavy metal leaks from your building's pipes (such as calcium, magnesium, chlorine, nitrate, sulfate, fluoride, sodium, copper or nickel), you can request a test from a local water laboratory.
What about limestone?
High TDS is often directly correlated with hard water, which means you have a higher risk of limescale. Read more about limescale and how to filter limescale from tap water.
Summary
Unless your tap water TDS value is way outside the recommended range, you don’t need to worry. High TDS in tap water basically means that there is mineral water coming out of your tap. You should be happy.
Sources:
WHO, EU Water Framework, EPA