Hardness is expressed in degrees, but the actual units of measure vary from country to country. Iron and manganese. Iron and manganese may be found in water depending on the source of the contamination. Iron is present in some waters at the source, but corrosion in pipelines and storage vessels may taint water. If a given source is alkaline, iron in the form of ferrous bicarbonate may be removed by aeration when ferric hydroxide is precipitated and carbon dioxide released.
Aeration also helps to raise the pH by reducing the content of dissolved carbon dioxide.
This section looks at industrial water treatment using lime-soda processing, base exchange and demineralization to remove all dissolved salts. In this method, hydrated lime and sodium carbonate are added to precipitate calcium and magnesium ions as compounds of low solubility. When water containing bicarbonates of calcium and magnesium is treated, it is the calcium carbonate, which is first deposited. It may be noted that, when temporary hardness is eliminated by this method, no salts remain but permanent hardness, an equivalent quantity of sodium salts is left behind. Base or cation exchange.