Drinking Water: Bacteriological Safety and Treatment

Laboratory tests for specific pathogens or disease-producing bacteria are difficult. They may require several days because of the need to grow organism cultures in incubators. This time lag is serious, since water from the test location may be consumed before test results return. A much quicker test is available.

Coliform bacteria are found in the intestines and wastes of humans and some animals. Their presence in water indicates contamination. Coliform bacteria are not pathogens, but serve as indicators to show that disease bacteria may be present, and that treatment or corrective measures should be taken. Tests for coliform bacteria have become the basis for the evaluation of bacteriological safety throughout the United States.

Two tests exist to discover the presence of coliform bacteria in water. In the older procedure, a small test tube dropped upside down into a larger tube has a special liquid media added to it, and the entire set is sterilized. Usually, measured volumes of water are added to five such tubes and the tubes placed in an incubator for 24 to 48 hours. Coliform bacteria present in the water cause fermentation of the media and gas release. Gas trapped in the small, upside-down inner test tube becomes visible as a small bubble.

An estimate of the number of coliform bacteria present in the test sample is based on the number of tubes that show gas bubbles. This is not a direct count, but is the "Most Probable Number" (MPN) of coliform bacteria per 100 milliliters (about 4 ounces) of water, based on statistical analysis. The following table resulted from 10 ml volumes of water added to 5 tubes of the media:

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.

Devices and systems to disinfect small water supplies and their advantages and limitations are outlined below.

An ultraviolet system exposes water to light from a special lamp at a specific wavelength that kills 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, is only effective in water 2-3 inches from the light source, suspended solid particles and organic matter shield organisms from the light, and the ultraviolet lamp must be cleaned frequently. There is no simple test to determine the system's effectiveness.

Ozone generators produce small quantities of a very strong, gaseous, oxidizing agent that is effective in killing bacteria quickly. Odorless, tasteless ozone is also effective in oxidizing organic matter, iron, and manganese. Ozone disinfection has some drawbacks: the gas is so active that it must be generated at the point of use, you must leave the equipment on constantly and it is affected by variations in flow rates. There is no simple test to determine the system's effectiveness.

Feeding very low levels of silver into a water supply is an effective, long-lasting method of disinfection. Disadvantages are a relatively long contact time, common substances in water that interfere with the action of the silver, high silver cost, and undesirable side effects from overdosing. There is no simple test to determine the system's effectiveness.

Iodine addition is a newer approach that is very effective even in a short contact time. High concentrations, organic matter that inhibits iodine action, objectionable tastes, relative unavailability and high cost are limitations of the method. Its long-term effects, particularly on children, are not known.

The use of chlorine and its compounds is undoubtedly the most common disinfection method in the United States. Its properties and limitations are well known, and chlorination is widely accepted by public health authorities. Chlorine is effective against bacteria, it requires short to moderate contact time, and it is readily available in several forms. There is a simple test for chlorine residual to measure its effectiveness. But chlorine solutions are only moderately stable. Organic matter as well as iron and manganese consume chlorine, and high chlorine concentrations have objectionable tastes and odors. Even low chlorine concentrations react with some organic compounds to produce very strong, unpleasant tastes and odors. Despite these limitations, chlorination is widely used in small, private water systems.

Trihalomethanes (THM) are organic compounds formed when water containing organic compounds is disinfected with chlorine. Chloroform, the most common THM, is a suspected carcinogen. Surface water collected in rivers and lakes often forms THM because overland flow picks up large amounts of organic debris. Groundwater is not likely to contain organic matter, but some groundwater supplies can produce THM when chlorinated. However, the chance of forming THM by chlorinating well water is small. Synthetic chemicals usually do not react to form THM.

Health data on THM and other chlorination by-products is sketchy or incomplete; some evidence suggests that they may contribute slightly to the risk of cancer. Public health officials agree, however, that the prevention of major diseases by chlorination outweighs its potential danger as a carcinogen.

In Washington State, few private, unregulated rural water systems use surface water. THM, taste, and odor problems are therefore limited. Where there are such problems, activated carbon filters can be used to adsorb THM, and most tastes and odors.

Chlorine added to water rapidly oxidizes inorganic materials such as dissolved iron and manganese and converts them to insoluble forms. Chlorine also reacts with organic matter, usually breaking it down slowly.

The "chlorine demand" of a water supply is the amount of chlorine consumed in these reactions. The "chlorine residual" is the amount of chlorine that remains in water after the chlorine demand is satisfied. Chlorine residual in water after adequate contact time assures that disinfection is complete.

Injecting chlorine between
pump and tank.

In very large water systems, such as those in towns and cities, the many hours available for contact time allow the use of chlorine residuals as low as 0.2 to 0.4 parts per million* to indicate complete disinfection. At these low concentrations, few people object to chlorine's taste or odor.

In small, private water systems, long contact time is very difficult. Higher chlorine concentrations disinfect much more rapidly. In water clear of iron, turbidity, and organic matter, 5 to 10 ppm of chlorine kills bacteria in only a few seconds.** Water flow through only a few feet of pipe achieves this contact time. A 10 gallon per minute flow in a 3/4-inch pipe represents a travel distance of 7.5 feet in 1 second.

On the other hand, if water contains slow-reacting organic matter, chlorine requires much longer contact time to disinfect. In such cases, water temperature and pH are also important. Table 1 shows the results of one study of contact time and chlorine concentration.***

Table 1 correlates water pH, minimum water temperature and the residual chlorine factor, which is the product of disinfection time and chlorine concentration to ensure bacteria destruction. For example, if a private well has a pH of 7.8 and a minimum water temperature of 40°F, the residual chlorine factor is 20. Any product of contact time multiplied by a residual chlorine concentration not less than 20 insures bacteria destruction. This could be 10 ppm of chlorine after 2 minutes, 5 ppm of chlorine after 4 minutes, or many other combinations.

*Parts per million (ppm) means one part of a substance, by weight, per million parts of water, by weight. This term is gradually being replaced by the practically equivalent term, "milligrams per liter" (mg/L).

**Christian, Barada, and Renn. "Rapid Disinfection of Water with High Concentrations of Hypochlorite." Jour. AWWA, March 1961, p. 307.

***Baumann and Ludwig. "Free Available Chlorine Residuals for Small Nonpublic Water Supplies." Jour. AWWA, Nov. 1962, p. 1379.

How is chlorine applied? If the water from the well is clear and free of organic matter, the following chemicals and equipment are all that are required. A chlorine solution may be prepared from household hypochlorite bleach, strong hypochlorite solutions used by commercial laundries, or from dry powder or tablet forms of calcium hypochlorite. These materials are available in most areas.

A small, positive displacement chemical feed pump suitable for pumping chlorine solutions into water lines wired to operate with a well pump of the same voltage or through a transformer may be used to inject a chlorine solution into the water line between the well pump and the pressure tank. This accurately proportions chlorine solution to water flow. The pressure tank also serves as an excellent mixing vessel. An activated carbon filter in the water line placed after the pressure tank removes any precipitated matter and the bad tastes and odors of high chlorine concentrations. A small sampling valve in a tee ahead of the filter makes checking chlorine concentrations convenient.

Organic matter, additional tanks, coils of hose or tubing, or other equivalent devices installed between the pressure tank and the filter necessitate longer contact time for disinfection. Almost every installation depends upon available space, local costs, and the installer's ingenuity.

Make fresh, diluted chlorine solution every week. Backwash the activated carbon filter periodically to keep it clean. Replace carbon consumed by the chlorine.

All these disinfection systems use mechanical devices subject to breakdown and must have an adequate supply of chlorine solution. These systems should be used only as temporary measures to treat contaminated water, provided no other water source exists. Disinfection equipment is only a temporary safety precaution. If your well is contaminated, find the contamination's source and eliminate it.

The best treatment method for bacterial presence results from careful consideration of such factors as economics, water quality characteristics, water end-use, water temperature variances, and the inherent limitations of treatment technology.

For more information, consult your local health department for advice on water testing and health aspects.

Consult several water treatment company representatives before purchase and installation of any water treatment equipment.

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. These are emergency measures only. Identify the contamination cause and correct it as soon as possible.

BOILING. Heat the contaminated water to boiling. Let it boil for fifteen minutes before use. This should kill any harmful bacteria.

SHOCK CHLORINATION. Add one ounce of ordinary household bleach, such as Clorox or Linco, to 1 1/2 gallons of the contaminated water, or use one part of chlorine bleach to 5,000 parts of water by weight. Let it stand for several minutes. Although the water may have a strong, objectionable chlorine taste, it is safe to drink. After allowing sufficient time for disinfection, 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.

Ronald E. Hermanson, Ph.D., P.E., Washington State University Extension Agricultural Engineer, Pullman

Issued by Washington State University Cooperative Extension and the U.S. Department of Agriculture in furtherance of the Acts of May 8 and June 30, 1914. Cooperative Extension programs and policies are consistent with federal and state laws and regulations on nondiscrimination regarding race, color, gender, national origin, religion, age, disability, and sexual orientation. Trade names have been used to simplify information; no endorsement is intended. Revised May 1991.Subject code 376. A EB0995