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Measure of the concentrations of dissolved ions of calcium and magnesium in the water. Hardness is expressed as equivalent of calcium carbonate in milligrams per litre (mg/litre). Hardness can be also expressed as milliequivalent per litre, German hardness (°dH) or mg/litre of calcium oxide (CaO) according to the following conversion factor: 50 mg/litre CaCO3 = 1 meq/litre = 2.805 (°dH) = 28 mg/litre CaO. @FAO

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