The hardness of water

 The hardness of water is a measurement that expresses the concentration of the main minerals present in its composition: calcium and magnesium. It is defined by the sum total of the ions of these substances, which are converted to calcium carbonate.

According to the World Health Organization's (WHO) drinking-water quality guidelines, hardness is categorized as follows:
- Soft water: containing calcium carbonate at concentrations below 60 mg/l.
- Moderately hard water: containing calcium carbonate at concentrations between 60 and 120 mg/L.
- Hard water: containing calcium carbonate at concentrations between 120 and 180 mg/L.
- Very hard water: containing calcium carbonate at concentrations above 180 mg/l.

Most of the water in Japan is soft, while in Europe it is predominantly hard. In Brazil, this categorization can vary widely. According to data from the Brazilian Agricultural Research Corporation (Embrapa), São Paulo is classified in the parameters of soft water, while many Amazonian rivers have great mineral wealth, and their water is considered hard.

Hardness also plays a significant role in the perception of the taste of drinking water. With its light, neutral taste, soft water is considered ideal for Japanese cuisine, as it enhances the fifth flavor, known as umami, present in the edible kelp kombu and dashi, broth made with dried bonito fish (katsuobushi) in the form of glutamic acid. Its purity also enhances the aromatic complexity of such beverages as Japanese tea, black tea and whiskey. Hard water, on the other hand, with its more robust flavor profile, with notes of bitterness and astringency, is considered ideal for Western cuisine, being particularly suitable for such dishes as stews, since it neutralizes the more striking odors of beef and facilitates the removal of impurities during boiling.

In Japan, water hardness is regulated by law with a maximum limit of 300 mg/L (300 mg of calcium carbonate per liter of water), since any higher value impairs the formation of soap foam, for example.

In addition to quality standards, there are water quality management items, which are important parameters in the management of water resources. For example, with flavor quality in mind, a target for hardness was set between 10 and 100 mg/L.

Depending on other water characteristics such as pH, hardness above 200 mg/L can cause formation of scale – whitish deposits related to mineral buildup. Common examples are those that appear inside electric kettles. On the other hand, waters with less than 100 mg/L can intensify corrosion in plumbing pipes.

Conditions for Quality Water

These are the parameters for what would be considered quality water, in other words, tasty water (oishii mizu), established in 1984 by the "Research Group Oishii mizu Kenkyūkai”, founded by Japan's Ministry of Health. In addition to hardness, the criteria consider factors such as evaporation residues, free carbon dioxide, dilution level of potassium permanganate, odor intensity, residual chlorine and water temperature.

The hardness of the water that reaches the taps is strongly influenced by its origin and, groundwater, generally tends to have a higher degree of hardness than surface water.

Graphics
Proportion of soft and hard water between the surface water and groundwater that reach the taps throughout Japanese territory

Source: Japan Water Works Association, “Water Quality Database – Japan Faucet Water Quality Distribution Table,” data from fiscal year 2022 (average values).

Water that flows for long periods in limestone-rich regions such as Europe and the Americans acquires a naturally high level of hardness. On the other hand, in countries such as Japan, where the period the water spends underground and the extension of watercourses are shorter, the hardness tends to be considerably lower.
Source: Samaki Takeo, Oishii Mizu, Anzen na Mizu, Nihon Jitsugyo Publishing House (only available in Japanese), 2000.

According to information from the Tokyo Water Department, the average value of the hardness of tap water in the region is approximately 60 mg/L. A detailed analysis from 2022 revealed that the maximum value was 89.7 mg/L in the districts of Nerima and Kōtō. Meanwhile, the lowest value was 19.6 mg/L in the city of Ōme.

Graphics
Comparison of Distribution of Nationwide Tap Water Hardness with Water Hardness as measured by Tokyo Water Department (Fiscal Year 2022)
Source: Japan Water Works Association, “Water Quality Database – Japan Faucet Water Quality Distribution Table,” data from fiscal year 2022 (average values).

This map illustrates the hardness of tap water throughout Japan. The colors indicate the average water hardness value in each province. Reddish tones indicate harder waters, while bluish tones signal less hard waters. The national average is 48.9 mg/L and is represented on the map by the color yellow.