Bacteriological Safety

Above all other considerations, water to be used for cooking and drinking must be bacteriologically safe. It must contain none of the bacteria which cause diseases, such as typhoid fever and dysentery, which may be transmitted through water.

Large municipal water systems have the complex facilities and trained personnel o treat water for the destruction of “pathogens,” or disease-causing bacteria, but few private water systems have these resources. Further, individual wells are often subject to unsuspected contamination. Thus, periodic bacteriological tests and preventative water treatment are recommended.

A number of devices and systems can be used to disinfect small water supplies, and each has its own advantages and limitations. Some of these methods are outlined below.

Ultraviolet systems expose the water to light from a special lamp at a specific wavelength which is capable of killing common bacteria. The system adds nothing to the water, produces no tastes or odors, and usually requires only a few seconds of exposure to be effective. Ultraviolet light, however, has no action beyond the point of application, the light penetration of water is shallow (usually only 2-3 inches), suspended solid particles and organic matter can shield organisms against the light, and the ultraviolet lamp must be cleaned frequently to insure proper exposure of the water to the light. Further, there is no simple test available to determine whether or not the system is effective.

Ozone generators are used in some systems to produce small quantities of the gas which is a very strong oxidizing agent, and is effective in killing bacteria with short exposure times. Ozone is also effective in oxidizing organic matter, iron and manganese, and produces no tastes and odors. However, the gas is so active that it must be generated at the point of use, and the equipment does not lend itself to on-off operation or variation in flow rates. Again, there is no simple test to determine whether or not the system is effective.

Several types of devices are available which feed very small amounts of silver into the water supply. Very low levels of silver are effective, and this action is powerful and long-lasting. However, relatively long contact time is necessary, a number of common substances in water interfere with the action of the silver, the silver costs are high, and overdosing can produce undesirable results, such as the discoloration of the skin. There is no simple test to determine whether or not the system is effective.

A relatively new approach is the addition of iodine to the water. This material is very effective even with relatively short contact time. However, relatively high concentrations are necessary, organic matter inhibits the action of iodine, the tastes are objectionable to some persons, iodine is not readily available, and the costs are relatively high. Its long term effects, particularly on children, are not known.

Chlorine Disinfection

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.

Chlorine is known to be effective against bacteria, it requires short to moderate contact time, it is readily available in several forms, and there is a simple test for chlorine residual which is a measure of its effectiveness. It too has its limitations. Its solutions are only moderately stable, organic matter as well as iron and manganese consume chlorine, high chlorine concentrations have objectionable tastes and odors, and even low chlorine concentrations react with some organic compounds to produce very strong, unpleasant tastes and odors. yet in spite of these factors, chlorination is widely used on small private water systems.

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 40°F, 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.

Coliform Bacteria

“Coliform” bacteria are found in the intestines and wastes of humans and some animals and, therefor, their presence in water indicates that the water has been contaminated with such wastes. The coliform bacteria are not pathogens, but serve as “indicators” to show that disease bacteria could be present and that treatment or corrective measures should be taken. The tests for coliform bacteria have become the basis for the evaluation of Bacteriological Safety throughout North America.

Two different procedures may be used for testing water for coliform bacteria. In the older procedure, a small test tube is dropped upside down into a larger tube, a special liquid media is added, and the entire set is sterilized. In the usual approach, measured volumes of water are added to five such tubes, and the tubes are placed in an incubator for 24 to 48 hours. If coliform bacteria are present in the water, they will cause fermentation of the media and the release of some gas. Some of that gas will be trapped in the small, upside-down inner test tube and be visible as a small bubble.

From the number of tubes which show the gas bubbles, a table can be used to estimate the number of coliform bacteria present. This is not a direct count, but is the “Most Probable Number” (or “MPN”) of coliform bacteria per 100 mL (about 4 ounces) of water, based on statistical analysis. The following is an example of the table used when 10 mi volumes of water are added to 5 tubes of the media (see right):

Some laboratories give the MPN in their reports, while others simply indicate the number of negative (no gas) and positive (show gas) tubes with a series of plus and minus signs, +++—, for example, would indicate 2 positive and 3 negative tubes, and from the table, an MPN of 5.1 per 100 mL.

A new procedure for coliform organism tests involves the filtration of a measured volume of water through a sterilized membrane which has pores so small that any bacteria present are retained on the membrane. The membrane is then placed in a small covered dish which contains an absorbent pad saturated with a liquid media. After incubation for 20 hours, each coliform organism originally present will have developed into a “colony,” a clearly visible spot on the membrane which has a characteristic shape and color. By simply counting the colonies, a direct reading of the number of coliform organisms in the volume of water filtered is obtained. The advantage of this method is speed, but high concentrations of iron or suspended matter in the water may interfere with the test. As with the multiple tube method, the results are usually reported as the number of coliform organisms per 100 mL.

Consult your local health department for information about coliform test and sampling procedures.

Coliform & E.coli Bacteria

  • The presence of coliform and/or E.coli bacteria indicates that human and/or animal wastes have entered the water supply
  • Chlorination, along with proper retention is the most common disinfectant method in the world to kill coliform and E.coli bacteria
  • Walkerton, Ontario – 7 people died and over 2,000 became ill after consuming water contaminated with E.coli bacteria

Emergency Disinfection Procedures

If contamination of a water supply is suspected, either of the following emergency measures may be used to disinfect the water sufficiently to make it safe for human consumption. It should be emphasized that these are emergency measures only, and that the cause of the contamination and corrective action should be instituted as soon as possible.


Heat the contaminated water to boiling and let it boil for fifteen minutes prior to use. This should kill any harmful bacteria present in the water.


Add 1 ounce of ordinary household bleach (such as Clorox, Linco, etc.) to 11/2 gallons of the contaminated water and let it stand for several minutes. Although the water will have a strong, objectionable chlorine taste, it will be safe to drink. After allowing sufficient time for disinfection by this method, the chlorine content of the water can be reduced by heating or aerating the water, or allowing it to stand for a longer period of time. For disinfecting higher or lower quantities of contaminated water, use one part of chlorine bleach to 5,000 parts of water by weight.

NOTE: The determination of which treatment method is best should be made only after careful consideration of many factors such as economics, water quality characteristic, the end use to which the water is to be put, temperature variances of the water to be treated, the inherent limitations of the available treatment technology, and others. This determination can best be made by your local water treatment representatives and they should be consulted prior to the purchase and installation of any water treatment equipment.

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