What is normal water hardness

Everything about water hardness

Hard or soft water - anyone wondering about white residues in the kettle, annoyed by the calcified iron or dealing with the dosage information from detergent manufacturers will sooner or later be confronted with the term "water hardness". However, most consumers cannot usually answer the question about the hardness of the drinking water straight away.

Development of water hardness

Water hardness describes the concentration of calcium and magnesium ions dissolved in the water. Further hardness builders are strontium and barium ions, of which, however, usually only small traces are contained. The hardness builders dissolved in water tend to form insoluble compounds, which include lime and lime soaps in particular. The hardness of the water differs depending on the region and depends on the nature of the geological subsoil. During the passage through soils and / or rock bodies with cavities (aquifers), dissolved hardness builders get into the water. The water hardness mainly arises as carbonate hardness through the absorption of dissolved lime or dolomite (dissolution through carbonic acid), the absorption of calcium and magnesium ions is particularly easy through the dissolution of gypsum. Acid rain and especially the influence of modern agriculture can also contribute to an increase in the degree of hardness. Rainwater, on the other hand, only absorbs a negligible amount of hardness components and therefore has a very low hardness (close to 0).

Complicated equilibrium system

The term water hardness describes a complex system of chemical equilibria that are coupled with one another. The total hardness is the concentration of magnesium and calcium ions in the water. The higher the concentration of calcium and magnesium ions in the water, the higher the degree of hardness. There are different methods of determining the total hardness; complexometric titration is used particularly often. This is a measurement analytical method in which the readily soluble disodium salt of ethylenediaminetetraacetic acid (EDTA) is preferably used. In this process, the total hardness is determined based on the volume of the EDTA solution used. The concentration of hydrogen carbonate is of great importance for the water hardness; this proportion is called carbonate hardness. If water contains an exact amount of carbon dioxide and is in the calcium carbonate-carbonic acid-carbon dioxide equilibrium, it does not separate any lime and, moreover, cannot dissolve any lime. However, this complex equilibrium is temperature-dependent, so deposits (saucepan, kettle, coffee maker, etc.) can form when the water is heated. Another part of the total hardness is called non-carbonate hardness. This is a so-called "permanent hardness" that is bound by anions (sulfates, nitrates, chlorides, etc.) and cannot be removed.

Division into degrees of hardness

In the past, the hardness of drinking water in Germany was given in "degrees of German hardness" (° dH). From 1988, according to the requirements of the legislature, four different hardness ranges were given for the dosage information for cleaning agents and detergents. The legal requirement was the specification "Millimole total hardness per liter":

- 1 "soft" = up to 7.3 ° dH (up to 1.3 millimoles total hardness per liter)
- 2 "medium" = 7.3 to 14 ° dH (1.3 to 2.5 millimoles total hardness per liter)
- 3 "hard" = 14 to 21.3 ° dH (2.5 to 3.8 millimoles total hardness per liter)
- 4 "very hard" = more than 21.3 ° dH (more than 3.8 millimoles total hardness per liter)

A new version of the Detergent and Cleaning Agent Act has been in effect since May 5, 2007. The information now corresponds to European standards and is referred to as “millimoles of calcium carbonate per liter”. Currently only three hardness ranges are mentioned, as grades 3 and 4 have been combined to form a common degree of hardness. The numerical classifications have been omitted and only the subdivisions into "soft", "medium" and "hard" are made:

  • soft = below 1.5 mmol / l (below 8.4 ° dH)
  • medium = 1.5 to 2.5 mmol / l (8.4 to 14 ° dH)
  • hard = over 2.5 mmol / l (over 14 ° dH)

According to the stipulations in Section 8, Paragraph 1, Clause 1 of the WRMG, manufacturers of detergents must be given explicit recommendations on the dosage in grams or milliliters with normal filling of the washing machine with regard to the different degrees of hardness.

Hard or soft - what's the difference?

The optimal water hardness always depends on how the water is used. In general, soft water is more suitable for conventional household use. As already shown, hard water contains a significantly higher proportion of magnesium and calcium, as it comes from regions in which limestone and sandstone predominate. Heating removes carbon dioxide from hard water and lime is precipitated. Because of this, there are some drawbacks to hard water:

  • Influence on the taste and appearance of drinks and food (tea, coffee, etc.)
  • Calcification of household appliances
  • higher consumption of detergents and cleaning agents
  • this higher consumption also results in a higher environmental impact
  • Deposits (lime stains, etc.) in the sanitary area

Soft water comes from areas where rocks such as gneiss, basalt and granite are predominant. It is more suitable in households for all applications in which it is heated (for cooking, washing, etc.). For example, the lower detergent dosage and the significantly lower risk of calcification are advantageous. However, there are disadvantages even with soft water:

  • stronger foaming in detergents
  • poorer removal of soap residue

A water hardness in the upper range of the “soft” degree of hardness (approx. 1.5mmol / l) is considered optimal. With this degree of hardness, soaps have sufficient solubility and, at the same time, consumers benefit from a low incidence of limescale stains.

Determine the water hardness yourself

Knowledge of the hardness of the water in your own place of residence is required so that the detergent manufacturer's information on dosage can be optimally implemented. Information about the water quality is usually provided by the local water supply companies; this information is usually listed on the relevant website. If this information is missing, a quick call to the water utility can provide clarity. A corresponding note can also be found for homeowners on the annually issued water bill. Another way to determine the hardness range is provided by water hardness test strips, which are offered in pharmacies, drug stores and mail order companies. The strips are coated with a special substance and are immersed in cold water for testing. After a short time, discoloration will appear, which will provide information about the hardness of the water based on the information provided by the respective manufacturer. Special tests with titration solutions are somewhat more precise. Anyone who would like to find out more information about water quality in addition to water hardness can have a comprehensive drinking water analysis carried out by a renowned institute. Possible loads with heavy metals (copper, lead, etc.) are also determined in the laboratory, so these analyzes offer interested consumers a comprehensive overview of the quality of the drinking water.

Water hardness in German cities

In the whole of Germany, the average water hardness is 16 ° dH (approx. 2.85 mmol / l) and there are serious differences depending on the region. The following cities, for example, have particularly soft water:

  • Bonn (5.8 ° dH)
  • Bochum (6.7 ° dH)
  • Hamburg (7 ° dH)
  • Essen (7.1 ° dH)
  • Dortmund (7.9 ° dH)

On the other hand, there are also German cities where the residents have to reckon with significantly higher water hardness. Some examples are:

  • Müelte (Geiseltal) in Saxony-Anhalt (45.15 ° dH)
  • Wolfsburg-Vorsfelde (44.80 ° dH)
  • Saarbrücken (27.7 ° dH)
  • Frankfurt am Main (24.72 ° dH)
  • Berlin (19.5 ° dH)

The federal state with the highest average water hardness is Saxony-Anhalt, here the value is around 23.00 ° dH. In contrast, the small federal state of Bremen has the lowest water hardness on average at 8.00 ° dH.