CHEMICAL CHARACTERISTICS OF WATER

The most important chemical characteristics of water are its acidity, alkalinity, hardness, and corrosiveness. Chemical impurities can be either natural, man-made (industrial), or be deployed in raw water sources by enemy forces.

Some chemical impurities cause water to behave as either an acid or a base. Since either condition has an important bearing on the water treatment process, the pH value must be determined. Generally the pH influences the corrosiveness of the water, chemical dosages necessary for proper disinfection, and the ability to detect contaminants.

Hardness

Hardness is caused by the soluble salts of calcium, magnesium, iron, manganese, sodium, sulfates, chlorides, and nitrates. The degree of hardness depends on the type and amount of impurities present in the water. Hardness also depends on the amount of carbon dioxide in solution. Carbon dioxide influences the volubility of the impurities that cause hardness.

The hardness caused by carbonates and bicarbonates is called carbonate hardness. The hardness caused by all others (chlorides, sulfates, nitrates) is called noncarbonated hardness.

Alkalinity is usually equivalent to the carbonate hardness. Sodium, however, also causes alkalinity. In natural waters, sodium is not normally present in appreciable amounts.

Therefore, in natural waters, the alkalinity is equal to the carbonate hardness. After water has been softened, a large amount of sodium remains in the treated water. In softened water, the total alkalinity is the sum of the carbonate alkalinity plus the sodium alkalinity. Hardness is undesirable in that it consumes soap, makes water less satisfactory for cooking, and produces scale in boilers and distillation units.

The following minerals cause hardness in ground and surface waters: l Calcium carbonate. Alkaline and only lightly soluble; causes carbonate hardness and alkalinity in water. l Calcium bicarbonate. Contributes to the alkalinity and carbonate hardness of water. Calcium bicarbonate when heated produces carbon dioxide and calcium carbonate. This calcium carbonate precipitates as scale ib boilers and distillation units.

* Calcium sulfate or gypsum. Causes noncarbonated hardness in water. Being more soluble in cold water than in hot, it separates from the water in boilers and forms scale on the boiler tubes. l Calcium cloride. Cause s noncarbonated hardness in water. In steam boilers and distillation units, the presence of calcium chloride causes chemical reactions that can pit metallic tubing.

l Magnesium carbonate (magnesite) and magnesium bicarbonate. Act the same in water as calcium carbonate and bicarbonate.

. Magnesium sulfate (epsom salts). Adds to the noncarbonated hardness of water and causes boiler scale. In amounts greater than 500 parts per million in drinking water, it acts as a laxative.

. Magnesium chloride. Has the same properties and effects as calcium chloride. However, the magnesium will contribute to the formation of magnesium hydroxide scale on boilers and evaporators.

. Iron. Iron is undesirable because it imparts a rusty color and objectionable taste to water. It also forms crusts in plumbing and piping. When iron is present in water, organisms whose life processes depend on iron compounds may also be present. These organisms may cause tastes and odors and create what is called red water.

l Manganese. While not encountered as often as iron, it is found in both surface and ground water. Its presence in water normally causes a grey or black color. The total concentrations of iron and manganese in potable water should not exceed 0.3 ppm.

Iron and manganese removal is not normally required in the production of field drinking water. Oxidation by aeration, followed by sedimentation and filtration, is the most common method of removing iron and manganese. They are oxidized to insoluble ferric oxide and manganese oxide by this process. The same methods may generally be used to remove both iron and manganese, although when they are present together in water, removal is more difficult. Combinations of iron and manganese with organic matter may require aeration in trickling beds containing coke, followed by sedimentation and filtration. In some cases superchlorination followed by sedimentation and filtration will in itself remove these two substances. The addition of lime, Ca(OH)2, followed by sedimentation and filtration, is another method for removal of these substances.

The concentration of chemical substances present in water for military water supply should not exceed the values shown below. If local conditions or short-term requirements make the use of water containing higher chemical concentrations necessary, authorization must be obtained from the medical officer.

Water softening is the term used to identify the process of treating water supply hardness. Water softening is most likely to be necessary when water is being supplied to laundries and heating units involving boilers and steam equipment.

. Lime-Soda Process. Lime-soda ash softening consists of the application of these materials to the raw water. Lime, Ca(OH)2, reacts with the soluble calcium and magnesium bicarbonates and forms insoluble calcium carbonate and magnesium hydroxide. Soda ash, Na2CO3, reacts with the soluble noncarbonated compounds of calcium and magnesium to precipitate insoluble calcium and magnesium compounds but leaves sodium compounds in solution. The physical operation of adding lime-soda ash and removing the precipitates is similar to that in the conventional coagulation-filtration process for bacteria and turbidity removal.

. Zeolite Process. Zeolites used in water softening are complex compounds of sodium, aluminum, and silica which have the faculty of exchanging bases. They are often called green sand because of the color of natural zeolite. Synthetic zeolites are also available. When water containing calcium and manganese compounds passes over the zeolite, the calcium and manganese are exchanged for the sodium in the zeolite. In this way the water is softened and its sodium content increased. When the sodium of the zeolite is exhausted, it is regenerated by applying a sodium chloride solution. Another exchange is made, and the resulting concentrated solution of calcium and magnesium chloride is discharged to waste. The operating rate varies directly with the thickness of the zeolite bed. The time between regenerations depends on the characteristics of the water and the total amount of water applied. The need for regeneration will be evident when hardness is no longer removed. The zeolite process can only be used on water that has been treated for removal of turbidity.

. Ion Exchange. The ion exchange unit, when run on the sodium cycle, will significantly soften water. The ion exchange unit will also remove such undesirable ions as (hose of manganese and lead.

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