WATER DISINFECTION METHODS, PART II

VARIOUS CHEMICAL DISINFECTANTS

Chlorine

The most common method of treating water for contamination is to use one of the various chemical agents available. Among these are chlorine, bromine, iodine, potassium permanganate, copper and silver ions, alkalis, acids, and ozone. Let us review them briefly here.

BROMINE

Bromine is an oxidizing agent that has been used quite successfully in the disinfecting of swimming pool waters. It is rated as a good germicidal agent. Bromine is easy to feed into the water and is not hazardous to store. It apparently does not cause eye irritations among swimmers, nor are its odors troublesome.

CHLORINE

One of the most widely used disinfecting agents to ensure safe drinking water is chlorine. Chlorine in cylinders is used extensively by municipalities in water disinfection. However, in this form, chlorine gas (Cl2) is far too dangerous for any home purposes.

For use in the home, chlorine is readily available as sodium hypochlorite (household bleach) which can be used both for laundering or disinfecting purposes. This product contains a 5.25% solution of sodium hypochlorite which is equivalent to 5% available chlorine.

Chlorine is also available as calcium hypochlorite, which is sold in the form of dry granules. In this form, it is usually 70% available chlorine. When calcium hypochlorite is used, this chlorinated lime should be mixed thoroughly and allowed to settle, pumping only the clear solution. For a variety of reasons, not the least of which is convenience, chlorine in the liquid form (sodium hypochlorite) is more popular for household use. Chlorine is normally fed into the water with the aid of a chemical feed pump.

The first chlorine fed into the water is likely to be consumed in the oxidation of any iron, manganese, or hydrogen sulfide that may be present. Some of the chlorine is also neutralized by organic matter normally present in any supply, including bacteria if present. When the "chlorine demand" due to these materials has been satisfied, what's leftover - the chlorine that has not been consumed - remains as a "chlorine residual."

Chlorine. There are three basic terms used in the chlorination process:

  • chlorine demand,
  • chlorine dosage and
  • chlorine residual.

For example, if water has 2.0 ppm chlorine demand, and a chlorine dosage of 5.0 ppm is fed into the water, the chlorine residual would be 3.0 ppm.

The rate of feed is normally adjusted with a chemical feed pump to provide a chlorine residual of 0.5-1.0 ppm after 20 minutes of contact time. This is enough to kill coliform bacteria, but may or may not kill any viruses or cysts which may be present. Such a chlorine residual not only serves to overcome intermittent trace contamination from coliform bacteria but also provides for minor variations in the chlorine demand of the water. The pathogens causing such diseases as typhoid fever, cholera and dysentery succumb most easily to chlorine treatment. Cyst-forming protozoa which cause amoebic dysentery and giardiasis are most resistant to chlorine.

As yet little is known about viruses, but some authorities place them at neither extreme in resistance to chlorination.

IODINE

For emergency purposes iodine may be used for the treatment of drinking water. Much work at present is being done to test the effect of iodine in destroying viruses, which are now considered among the pathogens most resistant to treatment. Tests show that 20 minutes of exposure to 8.0 ppm of iodine is adequate to render potable water. As usual, the residual required varies inversely with contact time. Lower residuals require longer contact time, while higher residuals require shorter contact time. While such test results are encouraging, not enough is yet known about the physiological effects of iodine­treated water on the human system. For this reason, its use must be considered only on an emergency basis.

SILVER

Silver in various forms has been used to inhibit the growth of microorganisms. It is most frequently found combined with activated carbon in filters. When some bacteria species come into contact with this silver, they are rendered inactive. There is disagreement among the experts as to the effectiveness of this process because silver ions in water kill E.coli very well and probably also salmonella, shigella, and vibro bacteria, but it has found lesser effect on viruses, cysts, and other bacteria species. Silver does not produce offensive tastes or odors when used in water treatment. Further, organic matter does not interfere with its effectiveness as is the case with free chlorine. Its high cost, interferences by chlorides and sulfides, need for long periods of exposure, and incomplete bactericidal action has hindered its widespread acceptance.

COPPER

Copper ions are used quite frequently to destroy algae in surface waters. But these ions are relatively ineffective in killing bacteria. Copper sulfate, for example, is also used to kill algae in reservoirs.

ALKALIES AND ACIDS

Disease-bearing organisms are strongly affected by the pH of water. They will not survive when water is either highly acid or highly alkaline. Thus treatment that sharply reduces or increases pH in relation to the normal range of 6.5 to 7.5 can be an effective means of destroying organisms.

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