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The use of chlorine and its compounds is undoubtedly the most common
disinfection method in North America. Because it is used in both public
and private systems, a great deal is known about its properties and limitations,
and chlorination is widely accepted by public health authorities.
When chlorine is added to water several things occur. Almost immediately, it will oxidize inorganic materials, such as dissolved iron and manganese, and convert them to insoluble forms. Chlorine will also react with any organic matter present, usually breaking it down into simpler substances. Reactions with organic matter are much slower, and much longer contact between the organic matter and chlorine is necessary for the reactions to be completed. Finally, chlorine will kill bacteria.
The amount of chlorine consumed in these reactions is known as the "chlorine demand" of a water supply. The amount of chlorine remaining in the water after the chlorine demand is satisfied is known as the "chlorine residual." Only if a chlorine residual is found in the water after adequate contact time is there assurance that disinfection has been completed.
In very large water systems, such as those in large towns and cities, many hours of contact time are available to obtain disinfection. In such cases, chlorine residuals as low as 0.2 to 0.4 parts per million1 may be used as an indication of complete disinfection. At these low concentrations, few persons find the taste or odor objectionable.
In small private water systems, it is very difficult to provide such long contact time. However, disinfection can be achieved in much less time if higher concentrations of chlorine are used. If the water is clear of iron, turbidity, and organic matter, 5 to 10 ppm of chlorine will kill the bacteria in only a few seconds.2 This contact time may be achieved by the flow of water through only a few feet of pipe. (A 10 gallon per minute flow in a 3/4" pipe represents a travel distance of 7.5 feet in 1 second.)
On the other hand, if slow reacting organ matter is present in the water, much longer contact time is required. In such cases, water temperature and pH also become important factors. One study of contact time and chlorine concentration developed the following information.
Disinfection Time (minutes) x Chlorine Concentration (ppm)
This means, for example, that if a private well has a pH of 7.8 and a minimum expected water temperature of 40ºF, any product of contact time multiplied by the residual chlorine concentration which is greater than 20 would insure the destruction of bacteria. This could be 10 ppm of chlorine after 2 minutes, 5 ppm of chlorine after 4 minutes, and many other suitable combinations.
How can the chlorine be applied? A number of small positive displacement chemical feed pumps are available which are suitable for pumping chlorine solutions into water lines. The chlorine solutions may be prepared from household hypochlorite bleach, stronger hypochlorite solutions used by commercial laundries, or from dry powder or tablet forms calcium hypochlorite. These materials are available in most areas of the country.
The chemical feed pumps can be electrically wired to operate with the well pump (be sure the voltage is the same, or use a transformer), and the chlorine solution can be injected into the water line between the well pump and the pressure tank. Thus, good proportioning of the chlorine solution to the flow of water can be obtained. The pressure tank also serves as an excellent mixing vessel. An activated carbon filter in the water line following the pressure tank will remove any precipitated matter and the excess chlorine, thus avoiding the bad tastes and odors of high chlorine concentrations. (A small sampling valve in a tee ahead of the filter is convenient for checking chlorine concentrations.)
If the water from the well is clear and free from organic matter, the above equipment is all that is required. On the other hand, if longer contact time is required due to organic matter, additional tanks, coils of hose or tubing, or other equivalent devices may be installed between the pressure tank and the filter. In such situations, almost every installation is different depending upon the space available, local costs, and the ingenuity of the installer.
In any case, dilute chlorine solutions should be made up fresh every week since they gradually lose strength. The activated carbon filter should be backwashed periodically to keep it clean, and additional carbon added as needed to replace that consumed by the chlorine.
The laboratory tests for specific pathogens (disease-producing bacteria) are difficult, and because of the need to grow cultures of the organisms in incubators, may require several days for completion. This time lag is a serious problem since the results are obtained long after the water actually tested may have been consumed. Fortunately, a much shorter approach is available which will indicate whether or not a water has been contaminated with the animal or human wastes which carry the disease organisms.