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Hardness of water

Information about hardness of water in Torun

Hard water is water that has a high mineral content. This content usually consists of high levels of metal ions, mainly calcium (Ca) and magnesium (Mg) in the form of carbonates, but may include several other metals as well as bicarbonates and sulfates.

A common distinction is made between 'temporary' and 'permanent' hardness.
Temporary hardness is hardness that can be removed by boiling or by the addition of lime (calcium hydroxide). It is caused by a combination of calcium ions and bicarbonate ions in the water.
Permanent hardness is hardness (mineral content) that cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble as the temperature rises.

According to the decree of the Minister of Health of 29 March 2007, concerning required quality of water intend for human consumption, the acceptable hardness of water should not exceed the level 500 CaCO3 mg /dm³ (cubic decimeter).

While hard water is not generally unhealthy, it can cause many potentially costly problems. Hard water causes scaling, which is the precipitation of minerals to form a rock-hard deposit called limescale. Scale can clog pipes and can decrease the life of toilet flushing units and water taps. It can coat the inside of tea and coffee pots, and clog and ruin water heaters. In industry, hard water contributes to scaling in boilers, cooling towers and other industrial equipment. In these industrial settings, water hardness must be constantly monitored to avoid costly breakdowns.

The hardness of water in the water supply system in Torun
range from 229.1 to 276.3 CaCO3/dm³
what corresponds 13.5 – 16.3 dH (German degrees)
and it is an average hard water.

There are several different scales used to describe the hardness of water in different contexts:
• mmol/L
• mg/l as calcium carbonate (CaCO3)
• mg/L calcium carbonate equivalent
• Various obsolete "degrees":
o Clark degrees (°Clark)/English degrees (°E)
- conversion to mg/L calcium: divide by 0.175
o German degrees (°dH)
- conversion to mg/L calcium: divide by 0.14
o French degrees (°f)
- conversion to mg/L calcium: divide by 0.25
o Degrees of general hardness (dGH)
One degree of general hardness corresponds to 10 mg of calcium oxide or magnesium oxide per litre of water

Table 2. Descriptions of hardness


Total hardness


(mg CaCO3/dm³)



(German degrees)

Very soft










Moderately hard










Very hard





Source - Wikipedia

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