Water hardness

What is Water Hardness?

Water hardness is an important property of water that is determined by the concentration of metal ions, such as magnesium and calcium carbonate, as well as bicarbonates and sulphates, in the water. Most drinking water contains calcium, which precipitates as calcium carbonate when heated, also known as limescale or scale, because of the deposits it forms in, for example, kettles. Even without heating the water, the deposits end up on your glassware, shower screen and pipes etc.

Causes and Impact of Water Hardness

The minerals are naturally dissolved as water flows through rock and soil layers, saturating the water with calcium and magnesium ions. In areas with a lot of limestone, much harder tap water is often found.

The hard water then leads to limescale on all wet surfaces. Heating elements work less efficiently due to the layers of limescale. And soaps work less well with hard water. The latter is noticeable in the dosages required for dishwashers and washing machines, but shampoo also works less well with hard water.

Benefits of hard water

Hard water can also have some benefits. Humans need essential minerals to stay healthy, and according to the World Health Organization (WHO), drinking water can help supplement dietary calcium and magnesium intake. This is especially important for people who may not get enough calcium and magnesium in their diet.

Units of Water Hardness

Water hardness is expressed in different units, depending on the country and the application. In Belgium, the French hardness (fH) is usually used, while in the Netherlands the German hardness (dH) is common . However, these units are not standard units in the SI system.

Conversion Table and Typical Values

Standards for Drinking Water

In the Netherlands, quality requirements for drinking water are included in the Drinking Water Decree. From June 2017, the revised water worksheet 4.6 is in force, in which guidelines for water treatment are elaborated.

The Drinking Water Decree only stipulates that water companies may not soften or desalinate drinking water to a level greater than 1 mmol/l (5.6 DH). Water that is too soft, for example, corrodes copper pipes. The hardness is tested on the basis of the 90th percentile of the measurement data. The total hardness is calculated from the number of mmol Ca ²⁺ plus Mg ²⁺ per litre (see conversion table). Until 2011, the Water Supply Decree applied, which also included an upper limit of 2.5 mmol/l (14 DH).

This table shows how the concentrations in mmol/L correspond to those in mg-CaCO3/L (ppm), which helps to understand the water hardness classifications as described in the text

Water hardness in the Netherlands

In the Netherlands, many households have to deal with hard water. However, water hardness varies depending on the region. In general, water in the Netherlands is considered medium hard to hard, with concentrations of calcium and magnesium ions often above 8 dH (German hardness).

Especially in areas where the soil contains a lot of lime, such as parts of Limburg, Brabant and parts of the Randstad, the water can be relatively hard. These areas often have water with higher concentrations of calcium and magnesium ions.

See also: How can you test your water hardness ?