Some of the chlorine-consuming agents in the water are nonpathogenic (nondisease-causing organisms), but this bears no relationship to the fact that they contribute to the total chlorine demand of the water. Navy policy requires that for field water supplies, the chlorine demand must be satisfied and chlorine residual must be present.

Residual chlorine is the amount of unreacted chlorine remaining at a specified time after the chlorine compound is added. Chlorine in aqueous solution is highly unstable. It may change quantitatively and qualitatively under numerous conditions, including the presence of other elements or compounds. The total residual chlorine in the water can be chemically divided into several types. .

Total available residual chlorine. This is the sum of the free available chlorine and the combined available chlorine. .

Free available chlorine. This refers to hypochlorous acid and hypochlorite ion present in water. These are the most effective disinfection forms of chlorine. The free available chlorine is a rapid-acting type, important because it can be relied upon to destroy bacteria relatively quickly, and thus is active during the period immediately following chlorination. The relative amount of each present in the water is dependent upon the pH value of the water. It is important to remember that when the pH is raised, the quantity of free available chlorine required to kill the same number of microorganisms increases. With decreasing temperature, the same situation of increasing dosage to maintain the same kill is encountered. If the contact time is varied, then the dosage applied must also be changed. For example, to shorten the contact time the dosage would have to be increased.

Combined available chlorine. This results from the presence of ammonia or organic nitrogen that will react to form simple chloramines. Thus the term combined available chlorine arises from the fact that the chlorine has combined with another substance. Chloramines are a slower acting and less active form of disinfectant. Therefore, a much higher concentration than that of free available chlorine is needed to produce the same germ-destroying effect. The specific chloramines present are also a function of pH.

Chlorine demand in most water is likely to be largely satisfied 10 minutes after chlorine is added. After the first 10 minutes of chlorination, disinfection continues but at a diminishing rate. A standard period of 30 minutes' contact time is used to assure that highly resistant or high disease-producing organisms have been destroyed, providing a high enough dosage has been applied. Given a sufficiently large chlorine content, and if certain other conditions are met, even such special water purification problems as the presence of amoebic cysts or schistosomes will be solved with the 30-minute contact period.

The efficiency of the chemical disinfection process is dependent upon numerous factors. They include the type and concentration of microorganisms, the pH and temperature of the water, the presence of interfering substances, and whether or not the organisms are protected from the disinfection solution by being embedded in tissue cells, or clumps of tissue cells, or other material. Therefore, various concentrations of disinfectants are required. Minimum concentrations of disinfectants are prescribed below. Higher concentrations may frequently be prescribed by the medical officer on the basis of his knowledge of endemic disease or local environmental conditions.

SEABEE-operated mobile and portable water treatment units use coagulation and filtration as a part of the treatment process. They are capable of a high degree of removal of particulate material. When those units are used, sufficient chlorine will be added to the water, preferably before coagulation, so the residual in the finished water after 30 minutes of contact will be at least as much as that indicated by the following table.

If adequate provisions are not made for accurate and frequent measurement of pH, 5.00 mg/1 must be used. The following guidelines were used in developing the above table:

The water to be treated would be natural surface or ground water of average composition and not grossly or deliberately contaminated.