المواد الكيميائية المستخدمة كمطهرات للمياه

Water Disinfection Methods.
VARIOUS CHEMICAL DISINFECTANTS

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 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 insure 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 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 left over - 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.

Chlorine demand is the amount of chlorine which will be reduced or consumed in the process of oxidizing impurities in the water. Chlorine demand is the amount of chlorine impurities in the water.

Chlorine dosage is the amount of chlorine fed into the water.

And 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 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 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 iodineÂtreated 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 have 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 a water. They will not survive when water is either highly acid 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.

Water Disinfection Methods..
OTHER AGENTS/ CHLORINATION

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 several types of surfactants which aid in destroying pathogens. The cationic detergents readily kill pathogens. Anionic detergents are only weakly effective in destroying pathogens. Because of their objectionable flavor and possible toxic effects, however, surfactants have not been seriously considered for treating drinking water.

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. Because it is 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:

1. "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. The compound formed by feeding both chlorine and ammonia to the water. This treatment has been used for controlling bacterial growth in long pipe lines and in the applications where its slower oxidizing action is of particular benefit.

2. A pH of 6.0 to 7.0 makes water a far more effective medium for chlorine as a disinfecting agent than to higher pH values of around 9.0 to 10.0.

3. The effectiveness of disinfection increases with the amount of contact time available.

4. The effectiveness of chlorine residuals increases with higher temperatures within the normal water temperature range.

5. All types of organisms do not react in the same way under various conditions to chlorination.

6. An increase in the chlorine demand of a water increases the amount of chlorine necessary to provide a satisfactory chlorine residual.

In order to insure 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 insure adequate disinfection. Only in 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. For this reason these authorities feel that satisfactory chlorine residual alone can provide adequate control for disinfection. In their opinion, therefore, superchlorination-dechlorination does the job best.

Briefly, what is this technique, and how does it operate?

The success of superchlorination-dechlorination depends on putting enough chlorine in the water to provide a residual of 3.0 to 5.0 ppm. This is considerably greater than a chlorine residual of 0.1 to 0.5 ppm usually found in municipal water supplies when drawn from the tap. A superchlorination-dechlorination system consists of two basic units. A chlorinator feeds chlorine into the 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 (such as described in Lesson 5) 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.

Typical layout of superchlorination-dechlorination equipment on a private water system. With dechlorinator on main waterline, this arrangement can be used where iron and/or manganese are present in the water.

This unit then serves to remove chlorine from water used for drinking and cooking. Since many families also drink water from bathroom taps, it may be necessary to install dechlorinators at these locations as well. 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 insure 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 of iron and/or manganese, all the water must be filtered. Under such conditions, a large 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 its objective because homeowners may set the chlorine feed rate too low in order to avoid giving their water a chlorine taste

QUESTIONS AND ANSWERS ON
DECHLORINATING FILTERS
How long will a dechlorinating filter operate?

With a bad iron problem, it may be necessary to service the filter frequently due to the clogging of the filter with precipitated iron. When neither iron or sulfur is present, a filter continues to operate for long periods of time. How long depends on the amount of turbidity in the water and the design of the unit. With reasonably clear water, a dechlorinating filter should be good for 20,000 gallons of water, more than the average household uses for cooking and drinking in a year.

How can you service a dechlorinator?

Some units can be backwashed. Others are so designed that the homeowner can remove the filter from the container and hose it off. Still others are disposable units.

Typical layout of superchlorination-dechlorination equipment on a private water system. With dechlorinator on main water line, this arrangement can be used where iron and/or manganese are present in the water.

What happens when a dechlorinator ceases to operate?

The homeowner quickly knows his unit is not working because he will taste chlorine in the water, or the pressure drop will become excessive due to plugging by turbidity. He should then backwash or replace the filter.

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