Various Indicators of Water Quality - Water Hardness

Some metal cations in water combine with anions, and during the heating process, due to evaporation and concentration, they easily form scale. This scale, present on heated surfaces, affects heat conduction. The total concentration of these metal ions in water is called water hardness. The most common metal ions in natural water are calcium (Ca2+) and magnesium (Mg2+). They combine with anions in water such as carbonate (Co32-), bicarbonate (HCO3-), sulfate (SO42-), chloride (Cl-), and nitrate (NO3-) to form calcium and magnesium carbonates, bicarbonates, sulfates, chlorides, and nitrates, resulting in hardness. Iron, manganese, and zinc ions also contribute to hardness, but their concentrations in natural water are negligible. Therefore, the total concentration of Ca2+ and Mg2+ is usually considered the water hardness.

Water hardness has a significant impact on boiler water; therefore, water softening or desalination treatment should be performed according to the water quality requirements of various boiler parameters. The commonly used units for hardness are mmol/L or mg/L. The previously used equivalent concentration N is no longer used. For conversion, 1 N = 0.5 mol/L.

Since hardness is not formed by a single metal ion or salt, it is necessary to convert to another salt for a unified comparison standard. It is usually expressed using the mass concentration of CaO or CaCO3. When the hardness is 0.5 mmol/L, it is equal to 28 mg/L of CaO or 50 mg/L of CaCO3. In addition, some countries use German degrees or French degrees to express hardness. 1 German degree is equal to 10 mg/L of CaO, and 1 French degree is equal to 10 mg/L of CaCO3. 0.5 mmol/L is equivalent to 208 German degrees or 5.0 French degrees. The relationships between commonly used hardness units are shown in the table below:

Hardness Unit mmol/L mN German Degree French Degree CaO (mg/L) CaCO3 (mg/L)

mmol/L 1 2 5.6077 10.0086 56.077 100.086

mN 0.5 1 2.8039 5.0043 28.039 50.043

German Degree 0.17833 0.35665 1 1.7848 10.0000 17.848

French Degree 0.09991 0.19983 0.5603 1 5.6029 10.0000

CaO (mg/L) 0.017833 0.025665 0.1000 0.17848 1 1.7848

CaCO3/(mg/L) 0.009991 0.01998 0.05603 0.1000 0.5603 1