Water Hardness
Hardness is defined as the concentrations
of calcium and magnesium ions expressed in terms of calcium carbonate, which
can be calculated as shown in the equation :
Hardness mg/l = 2,5 [conc. of Ca2+ (mg/l)] + 4,1 [conc. of Mg2+(mg/l)]
The most frequently used standard classifies water supplies is shown in the
following table .:
Table of | Water supply classification |
Hardness | Concentration |
Soft Water |
0 to 17.1 mg/l (0 to 1 grain/gallon ) |
Slightly Hard Water |
17.1 to 51.3 mg/l (1 to 3.5 grains/gallon) |
Moderately Hard Water |
51.3 to 119.7 mg/l (3.5 to 7 grains/gallon) |
Hard Water | 7 to 10.5 grains /gallon (119.7 to 179.55 mg/l) |
Very Hard Water |
over 179.55 mg/l (over 10.5 grains/gallon) |
These minerals in water can cause some everyday problems. They react with soap
and produce a deposit called "soap curd" or scum that remains on the
skin and clothes and, because it is insoluble and sticky, cannot be removed
by rinsing. Soap curd changes the pH of the skin and may cause infection and
irritation. It also remains on the hair making it dull and difficult to manage.
Soap curd picks up the dirt from laundry water and holds it on cloth, contributing
to a gray appearance of white clothes. It is especially troublesome when wash
water is allowed to drain through the clothes.
The use of synthetic detergents may help a little, but the active ingredient in the detergent is partially inactivated by hardness and more detergent must be used for the same cleaning task. Some detergents will produce soap during the reaction with oil or grease on the surface being cleaned and as a result they will also deposit soap curd. A ring around the bathtub and spotting on glassware, chrome, and sinks are constant problems in the presence of hard water. They require additional rinsing and wiping, increasing the time spent on everyday cleaning.
Cooking with hard water can also be difficult, producing scale on pots. Some vegetables cooked in hard water lose color and flavor. Beans and peas become tough and shriveled.
Hard water may also shorten the life of plumbing and water heaters. When water containing calcium carbonate is heated, a hard scale is formed that can plug pipes and coat heating elements. Scale is also a poor heat conductor. With increased deposits on the unit, heat is not transmitted to the water fast enough and overheating of the metal causes failure. Build-up of deposits will also reduce the efficiency of the heating unit, increasing the cost of fuel.
Most natural water supplies contain at least some hardness due to dissolved
calcium and magnesium salts. Other minerals, such as iron, may contribute to
the hardness of water, but in natural water, they are generally present in insignificant
quantities. The total hardness of water may range from trace amounts to hundreds
of milligrams per liter.