EMERGENCY TREATMENT METHODS

Emergency treatment methods using water sterilizing bags, canteens, and other water containers do not provide for removal of impurities by coagulation and filtration. The entire reliance for rendering the water safe for consumption is placed on the disinfection process. Sufficient chlorine is added to the water so the residual, after 30 minutes of contact, will be at least 5 ppm of total chlorine. Under certain conditions, such as the presence of highly resistant disease-producing microorganisms or adverse environmental conditions, the medical officer will designate such higher residuals as may be necessary.

Boiling is a quick means of disinfecting small quantities of water in the field by individual soldiers. It is likely that all bacteria that produce diseases in man are killed by pasteurization temperatures. But there are some resistant organisms, principally viruses (such as infectious hepatitis), for which water must be boiled to achieve inactivation. A practical minimum standard for altitudes from sea level to 25,000 feet is to bring the water to a rolling boil for 15 seconds. Longer boiling times may be prescribed by the medical officer on the basis of evidence that the minimum is not inactivating all pathogenic microorganisms. Upon cooling, the boiled water should be kept in a covered uncontaminated container. Boiling is obviously a difficult way to disinfect large quantities of water.

Breakpoint chlorination is the application of chlorine to produce a residual of free available chlorine with no combined chlorine present. As chlorine is added, the total residual increases gradually after the initial demand of the water has been satisfied. At some residual concentration, depending on the water treated, free available chlorine reacts with the remaining oxidizable substances (including combined chlorine), and the residual drops sharply. When all combined chlorine has been oxidized by reaction with free available chlorine, the residual, now consisting only of free available chlorine, rises again and continues to increase in direct proportion to increased dosage. The point at which the residual again begins to increase is the breakpoint.

Figure 9-18 shows four typical breakpoint chlorination curves. Note that the curve rises at almost a 45-degree angle after the breakpoint is reached. Reactions are most rapid at pH from 6.5 to 8.5 and with increasing temperatures.

Curve 1 shows a typical breakpoint for water containing a considerable amount of ammonia. During the initial upward rise, chloramines are first formed. The curve rises until sufficient free available chlorine is developed to react with chloramine; then it falls until a point where all ammonia compounds have been oxidized.

With less organic matter in the water, as in curves 2 and 3, free available chlorine is formed sooner, destroying chloramines formed at the early stage. This results in lower combined chlorine residuals and flatter curves before breakpoint.

With practically no organic matter, curve 4 shows the chloramines are neutralized at an early stage by the upswing of the curve.

For some waters containing complex organic compounds, several intermediate breakpoints occur.

Advantages of breakpoint chlorination are high bactericidal efficiency, long-lasting residuals, and low odor and taste characteristics. It can be used only if detention periods are long enough to develop free available chlorine residual. This varies with the organic content of water. In some cases the treated water must be open to the air to permit escape of chloroorganic gases formed.

Tests for ammonia nitrogen will assist in determining the breakpoint. In practice, 10 to 25 times as much chlorine as ammonia nitrogen content may be needed to reach the breakpoint. Breakpoint chlorination, before instead of after filtration, has been found desirable. In surface water supplies with widely varying ammonia nitrogen content, the breakpoint chlorination should not be used unless trained assistance is available to make frequent tests for the breakpoint. With such water quality, the breakpoint curve can change radically in a short time.

Superchlorination is the application of more chlorine than needed for the chlorine residual essential to marginal chlorination. The surplus, which is used to control odors and tastes, is later removed by dechlorination. This method is particularly valuable in surface waters with variable ammonia and organic content. Sulfur dioxide reacts with chlorine to form acids that are neutralized by the natural alkalinity of the water. Sulfur dioxide is fed by equipment similar to that used for chlorine feeding. Activated carbon absorbs the excess chlorine, while aeration removes it by dissipating it to the atmosphere.

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