Treatment of a water supply is a safety factor, not a corrective measure. There are a number of ways of purifying water. In evaluating the methods of treatment available, the following points regarding water disinfectants should be considered:
a. A disinfectant should be able to destroy all types of pathogens and in whatever number present in water.
b. A disinfectant should destroy the pathogens within the time available for disinfection.
c. A disinfectant should function properly regardless of any fluctuations in the composition or condition of the water.
d. A disinfectant should not cause the water to become toxic or unpalatable.
e. A disinfectant should function within the temperature range of the water.
f. A disinfectant should be safe and easy to handle.
g. A disinfectant should be such that it is easy to determine its concentration in the water.
h. A disinfectant should provide residual protection against recontamination.
Techniques such as filtration may remove infectious organisms from water. They are, however, no substitute for disinfection. The following are the general methods used for disinfecting water:
Boiling - This involves bringing the water to its boiling point in a container over heat. The water must be maintained at this temperature 15 to 20 minutes. This will disinfect the water. Boiling water is an effective method of treatment because no important waterborne diseases are caused by heat-resisting organisms. Ultraviolet Light - The use of ultraviolet light is an attempt to imitate nature. As you recall, sunlight destroys some bacteria in the natural purification of water. Exposing water to ultraviolet light destroys pathogens. To assure thorough treatment, the water must be free of turbidity and color. Otherwise, some bacteria will be protected from the germ-killing ultraviolet rays. Since ultraviolet light adds nothing to the water, there is little possibility of its creating taste or odor problems. On the other hand, ultraviolet light treatment has no residual effect. Further, it must be closely checked to assure that sufficient ultraviolet energy is reaching the point of application at all times.
Use of Chemical Disinfectants - The most common method of treating water for contamination is to use one of various chemical agents available. Among these are chlorine, bromine, iodine, potassium permanganate, copper and silver ions, alkalis, acids and ozone. 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 water and is not hazardous to store. It apparently does not cause eye irritation among swimmers nor are its odors troublesome.
One of the most widely used disinfecting agents to ensure safe drinking water is chlorine. Chlorine in cylinders is used extensively by municipalities in purification work. However, in this form chlorine gas (Cl2) is far too dangerous for any home purpose. 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 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 sulphide 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 left over -the chlorine that has not been consumed - remains as "chlorine residual". 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. The cyst-like protozoa causing dysentery are most resistant to chlorine. As yet, little is known about viruses, but some authorities place them at neither extreme in resistance to chlorination.
There are three basic terms used in the chlorination process: chlorine demand, chlorine dosage and chlorine residual. Chlorine demand is the amount of chlorine which will reduced or consumed in the process of oxidizing impurities in the water. Chlorine dosage is the amount of chlorine fed into the water. Chlorine residual is the amount of chlorine still remaining in water after oxidation takes place. For example, if a water has 2.0 ppm chlorine demand and is fed into the water in a chlorine dosage of 5.0 ppm, the chlorine residual would be 3.0 ppm.
For emergency purposes, iodine may be used for 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 exposure to 8.0 ppm of iodine is adequate to render a 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 iodine-treated water on the human system. For this reason, its use must be considered only on an emergency basis.
Silver in various forms has been used to destroy pathogens. It can be added to the water as a liquid or through electrolytic decomposition of metallic silver. It has also been fed into water through an absorption process from silver-coated filters. Various household systems have been designed to yield water with a predetermined silver concentration. However, fluctuations in the flow rate often result in wide variations in the amount of silver in the water. In minute concentrations, silver can be highly destructive in wiping out disease-bearing bacteria. While long contact time is essential, silver possesses residual effect that can last for days. Silver does not produce offensive tastes or odors when used in water treatment. Further, organic matter does not interfere with its power to kill bacteria as in the case with free chlorine. Its high cost and the need for long periods of exposure have hindered its widespread acceptance.
Copper ions are used quite frequently to destroy algae in surface waters but these ions are relatively ineffective in killing bacteria.
Disease-bearing organisms are strongly affected by the pH of a water. They will not survive when water is either highly acidic or highly alkaline. Thus, treatment which 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.
There are numerous other agents which have proved to be successful in destroying pathogens. Many of these must still be subjected to prolonged testing with regard to their physiological effect on man. Among these are certain surfactants and chlorine dioxide. There are several types of surfactants which aid in destroying pathogens. The cationic detergents readily kill pathogens. Anionic detergents are only weakly effective in destroying pathogens. Surfactants have not been seriously considered for treating drinking water because of their objectionable flavor and possible toxic effects. Chlorine dioxide has unusually good germ killing power. Up to the present time, no valid tests for its use have been developed because of the lack of means for determining low residual concentrations of this agent. It's such a strong oxidizing agent, a larger residual of chlorine dioxide would probably be needed than is the case with chlorine. At present, chlorination in one form or another is regarded as the most effective disinfectant available for all general purposes. It has full acceptance of health authorities. Still there are certain factors which affect its ability to disinfect waters. These should always be kept in mind. They are:
a. "Free" chlorine residuals are more effective than "combined" or "chloramine" residuals. Disinfection regardless of the type of chlorine becomes more effective with increased residuals. Chloramine is the compound formed by feeding both chlorine and ammonia to the water. This treatment has been used for controlling bacteria growth in long pipelines and in other appliances where its slower oxidizing action is of particular benefit.
b. A pH of 6.0 to 7.0 makes water a far more effective medium for chlorine as a disinfecting agent than do higher pH values of around 9.0 to 10.0.
c. The effectiveness of chlorine residuals increase with higher temperatures within the normal water temperature range.
d. The effectiveness of disinfection increases with the amount of contact time available.
e. All types of organisms do not react in the same way under various conditions to chlorination.
f. An increase in the chlorine demand of a water increases the amount of chlorine necessary to provide a satisfactory chlorine residual.
In order to ensure the destruction of pathogens, the process of chlorination must achieve certain control of at least one factor and, preferably two, to compensate for fluctuations that occur. For this reason, some authorities on the subject stress the fact that the type and concentration of the chlorine residual must be controlled to ensure adequate disinfection. Only this way, they claim, can chlorination adequately take into account variations in temperature, pH, chlorine demand and types of organisms in the water. While possible to increase minimum contact times, it is difficult to do so. Five to ten minutes is normally all the time available with the type of pressure systems normally used for small water supplies. Many experts feel that satisfactory chlorine residual alone can provide adequate control for disinfection. In their opinion, superchlorination-dechlorination does the best job. Briefly, what is this technique and how does it operate?
The success of superchlorination-dechlorination system depends on putting enough chlorine in the water to provide a residual of 3.0 to 5.0 ppm. This is considerably greater than chlorine residual of 0.1 to 0.5 ppm usually found in municipal water supplies when drawn from the tap. A superchlorination-dechlorination systems consists of two basic units. A chlorinator feeds chlorine into raw water. This chlorine feed is stepped up to provide the needed residual. A dechlorinator unit then removes the excess chlorine from the water before it reaches the household taps. The chlorinator should be installed so that it feeds the chlorine into the water before it reaches the pressure tank. A general purpose chemical feed pump will do the job. The size and the placement of the dechlorinator unit depends on the type of treatment necessary. This will usually be an activated carbon filter. If pathogen kill is all that is required, a small dechlorinator can be installed at the kitchen sink. This unit then serves to remove chlorine from water used for drinking and cooking. The advantage in dechlorinating only a part of the water is obvious. A smaller filter unit does the job and since only a small portion of the total water is filtered under such conditions, the unit lasts longer before either servicing or replacement is necessary. Essentially dechlorination is not needed to ensure a safe drinking water. Once the water is chlorinated, the health hazard is gone. The chlorine residual is removed merely to make the water palatable. If the problem is compounded due to the presence or iron and/or manganese, all the water should be filtered. Under such conditions, a larger central filter is necessary and should be placed on the main line after the pressure tank. The prime advantage of the superchlorination-dechlorination process is that it saturates water with enough chlorine to kill bacteria. Simple chlorination sometimes fails of its objective because homeowners may set the chlorine feed rate too low in order to avoid giving their water a chlorine taste. Sodium Dichloroisocyanurate - Sodium Dichloroisocyanurate can sterilize drinking water, swimming pool, tableware and air, or be used for fighting against infectious diseases as routine disinfection, preventive tableware and environmental sterilization in different places, or act as disinfectant in raising silkworm, livestock, poultry and fish. It can also be used to prevent wool from shrinkage, bleach the textile and clean the industrial circulating water, The product has high efficiency and constant performance with no harm to human beings. It enjoys goods reputation both at home and abroad. Table 8 summarizes some of this chemical's properties.
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