Total Hardness Drinking Water
Water hardness traditionally refers to the measure of water’s capacity to interact with soap. Water that necessitates more effort to lather is termed HARD WATER. It indicates the presence of dissolved polyvalent metallic ions, predominantly calcium and magnesium cations, although other cations, e.g., barium, iron, manganese, strontium, and zinc, also contribute. Hardness is commonly expressed as milligrams of calcium carbonate equivalent per litre, with water containing less than 60 mg of calcium carbonate per litre generally being considered soft. While hardness is primarily caused by cations, it can also be classified in terms of carbonate (temporary) and noncarbonate (permanent) hardness.
Temporary hardness is caused by the presence of dissolved bicarbonate ions (HCO3-) in the water. These ions can be removed through boiling, as they form solid calcium carbonate (CaCO3) precipitates. The resulting scale can be seen in kettles and on fixtures. Boiling the water causes the precipitation of these minerals, which can then be filtered out.
Permanent hardness is caused by the presence of dissolved calcium and magnesium sulphate or chloride ions. These minerals cannot be removed by boiling, and they contribute to the long-term hardness of the water. To address the issues associated with hard water, water-softening techniques can be used. These techniques often involve the removal or exchange of calcium and magnesium ions with sodium ions, making the water “softer.”
The principal natural sources of hardness in water are dissolved polyvalent metallic ions from sedimentary rocks, seepage, and run-off from soils. Calcium and magnesium, the two principal ions, are present in many sedimentary rocks, the most common being limestone and chalk. They are also present in a wide variety of industrial products and are common constituents of food. As mentioned above, a minor contribution to the total hardness of water is also made by other polyvalent ions, e.g., Aluminium, Barium, Iron, Manganese, Strontium, and Zinc.
The taste threshold for the calcium ion is in the range of 100–300 mg/litre, depending on the associated anion, but higher concentrations are acceptable to consumers. Hardness levels above 500 mg/litre are generally considered to be aesthetically unacceptable, although this level is tolerated in some communities.
Concentrations of up to 100 mg of calcium per litre are fairly common in natural sources of water; sources containing over 200 mg of calcium per litre are rare. Magnesium salts are soluble, natural water sources typically containing concentrations of up to 10 mg/litre. Such sources rarely contain more than 100 mg of magnesium per litre, and it is usually calcium hardness that predominates.
FOOD
Virtually all foods contain calcium and magnesium, and dietary intake is the principal route of exposure. Typical diets provide about 1000 mg of calcium per day and 200–400 mg of magnesium per day. Dairy products are a particularly rich source of calcium, whereas magnesium tends to be associated more with meat and foodstuffs of plant origin.
The typical dietary contribution of calcium and magnesium is over 80% of the total daily intake. Of this, approximately 30% of calcium and 35% of magnesium will be absorbed. For calcium and magnesium, the typical contribution from water is 5–20%.
EFFECTS ON HUMANS
There does not appear to be any convincing evidence that water hardness causes adverse health effects in humans. In contrast, the results of several epidemiological studies have suggested that water hardness may protect against disease.
In most large-scale studies, an inverse relationship between the hardness of drinking water and cardiovascular disease has been reported. The results of several studies have suggested that a variety of other diseases are also inversely correlated with the hardness of water, including anencephaly and various types of cancer. Some data suggest that very soft waters with a hardness of less than 75 mg/litre may hurt mineral balance.
EFFECTS ON PLUMBING
Depending on the interaction of other factors, such as pH and alkalinity, water with a hardness above approximately 200 mg/litre may cause scale deposition in the distribution system, as well as increased soap consumption. In contrast, soft water, with a hardness of less than about 100 mg/litre, has a greater tendency to cause corrosion of pipes, resulting in the presence of certain heavy metals, such as cadmium, copper, lead, and zinc, in drinking water. The degree to which this corrosion and solubilization of metals occurs also depends on the pH, alkalinity, and dissolved oxygen concentration.
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