HARD WATER

Basic Water Characteristic.

The water used by industry for boiler feed or process purposes may be taken from public supplies, or abstracted directly from wells, lakes or rivers. In Great Britain, town mains water will have been treated to render it largely clear and colourless, free from odour, and bacteriologically sterile. It is thus directly suitable for many industrial purposes such as cooling, and in cases where ion exchange treatment is used, town mains water can normally be fed directly into the columns. Many large industrial users abstract water from natural sources for their own use.

Where these are deep wells, the water is normally clear, colourless, and directly treatable by ion exchange: but in cases where water is taken directly from rivers or lakes, clarification by the classical methods of coagulation and filtration is normally necessary. ‘These techniques are fully described elsewhere, and for the purposes of this book, the starting point of all industrial ion exchange processes is a supply of clear water, free from suspended and colloidal matter.

Hard Water

The range of minerals contained in most natural waters is quite limited. The cat ions present are normally calcium, magnesium and sodium, while the anions are mainly chloride, sulphate and bicarbonate, with lower concentrations of nitrate, phosphate and silica. There are also traces of organic matter. For the majority of natural waters, analysis of the ions mentioned above will give the total dissolved solids in the water.

Waters occurring in regions of unusual rock formation may differ considerably from this pattern, but the principles which are to be discussed apply equally to waters of unusual composition. It is customary to refer to waters as ‘hard’ or ‘soft‘. The former are waters containing appreciable concentrations (over 50 p.p.m.) of calcium and magnesium, which in Great Britain have normally been derived from the leaching of’ limestone or dolomitic rocks, by water containing free carbon dioxide. Significant concentrations of bicarbonate are therefore also present. Calcium may be derived from other types of rock, for example gypsum, CaSO4.2H20, which case the principle ions present will be calcium and sulphate.

It is a common convention in the water treatment industry to refer to ions in solution in terms of certain hypothetical combinations.

Thus calcium, magnesium and bicarbonate ions, when present together in solution, are grouped under the term ‘temporary hardness‘, because on heating, all are substantially removed by insoluble carbonates, with loss precipitation of the corresponding of carbon dioxide into the atmosphere.

Calcium and magnesium co-existing with sulphate or chloride are known as ‘permanent hardness‘, since the solutions are stable to heat at normal pressure.