How do you measure the quality of drinking water?

A TDS meter records the amount of dissolved solids and displays it as a TDS value in ppm, usually between 0 and 1,000 ppm. TDS stands for “Total Dissolved Solids”, i.e. the sum of all dissolved substances. Technically, the device measures electrical conductivity in microsiemens (µS). Salts, minerals and metals in particular are conductive, because they are present in water as charged particles (ions).

The rule of thumb for the conversion is: 1.2 µS/cm corresponds to around 1 ppm and therefore 1 mg/l (milligrams per litre). A TDS meter thus provides a numerical value quickly and cheaply. Nothing more. This number answers exactly one question: how much dissolved material is in the water.

The ppm value is purely a measure of quantity. It records the dissolved particles but reveals nothing about their nature. And that is exactly what matters for water quality. Calcium and magnesium increase the ppm value, but are valuable for the body. Lead or copper can be problematic even in traces and barely register in the reading.

There is another gap. Four groups of substances that are important for health do not conduct electricity and remain invisible to a TDS meter:

• Pesticides and plant protection products
• Hormones and pharmaceutical residues
• Germs and bacteria
• Microplastics

A low TDS value below 100 ppm is therefore no proof of healthy water. Conversely, a high value initially only means that more substances are dissolved, often minerals. No contamination can be inferred from this. The idea that a low ppm value automatically means good drinking water is a common misconception. If you want to rule out residues of heavy metals, pesticides or hormones, there is no way around a proper analysis.

Tap water typically ranges between 200 and 500 ppm. Mineral water reaches 400 to 700 ppm, whereas spring water often reaches only 30 to 80 ppm. Which range is usual depends heavily on the type of water and the region. The following table shows typical average values:

The official limit values also paint a broad picture. This makes it clear how little the number alone says about quality:

A value within these ranges is considered normal. The taste is also shaped by the mineral content. That is why a higher ppm value is quite desirable in mineral water.

A reverse osmosis filter lowers the TDS value to 0.1 to 25 ppm. Reverse osmosis retains almost all dissolved particles. This often leads to the conclusion that the water is particularly healthy as a result. It is not that simple.

Reverse osmosis removes not only pollutants but also valuable minerals. A very low ppm value therefore mainly indicates low-mineral water. If a little salt gets into the reverse osmosis water, the TDS value rises again immediately, even though the quality remains the same. This is precisely what shows that the measured value alone is no basis for assessing water quality.

A drinking water analysis tests the water in three groups of parameters: chemical, physical and microbiological values. Among other things, it records the concentration of individual pollutants, electrical conductivity, pH value and water hardness. Only then does it become clear which substances are actually present.

For the analysis, a sample is taken directly at the tap and evaluated in the laboratory. Depending on the test kit, more than 20 substances can be detected, including:

• Heavy metals such as lead, copper, nickel, arsenic, chromium and mercury
• Iron, manganese, aluminium, uranium and zinc
• Nitrate, nitrite, sulphate, phosphate, ammonium and fluoride
• Germs such as E. coli, enterococci, coliform bacteria, Pseudomonas and legionella

The German Trinkwasserverordnung (Drinking Water Ordinance) only guarantees quality up to the house connection. Water suppliers accept no guarantee for the last few metres in a building's own pipes. It is precisely there, in old pipes and taps, that much of the contamination arises.

There are two ways to analyse drinking water: a home test from around 20 euros and a professional laboratory analysis from around 150 euros. They differ significantly in accuracy and cost.

A home test is inexpensive, quick and easy to obtain. This is often enough for initial guidance. At the same time, the method is sensitive to errors during sampling. Even careless sampling can distort the result. A laboratory analysis is more reliable, especially if a certified sampler takes the sample themselves. Depending on the scope, it costs around 150 euros, while comprehensive tests with on-site sampling cost considerably more. Providers include specialised water laboratories such as Ivario or the local public health office.

A water test is a targeted examination when there is a specific suspicion of contamination. Six typical situations speak in favour of one:

• The water is cloudy, discoloured or has a noticeable smell.
• The building is an older property with old lead pipes.
• In a new building with copper pipes, the water can be corrosive and absorb copper.
• Infants, pregnant women or immunocompromised people live in the household.
• The water comes from a private well without public monitoring.
• Symptoms occur that could be related to the drinking water.

Infants, older people and immunocompromised people are particularly sensitive to germs. In such cases, an analysis provides clarity. If, on the other hand, you want clean water in the long term, a water filter is usually simpler and cheaper than repeated tests.

A water filter is the direct way to improve quality instead of just measuring it. It retains pollutants at the point of use and leaves the valuable minerals in the water. This way, you influence the quality instead of just reading off a measured value.

The following overview shows which of the three cartridges retains which substances:

This means you can filter appropriately depending on the contamination, from a simple taste filter to protection against germs. For an overview, see the drinking water filters from Alb Filter and the matching filter cartridges. The guide Why filter water explains why water is filtered.

The following neutral specialist sources serve as the basis: