Water used for drinking and cooking should be free of pathogenic (disease causing) microorganisms that cause such illnesses as typhoid fever, dysentery, cholera, and gastroenteritis. Whether a person contracts these diseases from water depends on the type of pathogen, the number of organisms in the water (density), the strength of the organism (virulence), the volume of water ingested, and the susceptibility of the individual. Purification of drinking water containing pathogenic microorganisms requires specific treatment called disinfection.

Although several methods eliminate disease-causing microorganisms in water, chlorination is the most commonly used. Chlorination is effective against many pathogenic bacteria, but at normal dosage rates it does not kill all viruses, cysts, or worms. When combined with filtration, chlorination is an excellent way to disinfect drinking water supplies. This fact sheet discusses the requirements of a disinfection system, how to test the biological quality of drinking water, how to calculate the amount of chlorine needed in a particular situation, chlorination equipment, by-products of disinfection, and alternative disinfection methods.

Testing water for biological quality

The biological quality of drinking water is determined by tests for coliform group bacteria. These organisms are found in the intestinal tract of warm-blooded animals and in soil. Their presence in water indicates pathogenic contamination, but they are not considered to be pathogens. The standard for coliform bacteria in drinking water is "less than 1 coliform colony per 100 milliliters of sample" (< 1/ 100ml).

Public water systems are required to test regularly for coliform bacteria. Private system testing is done at the owner's discretion. Drinking water from a private system should be tested for biological quality at least once each year, usually in the spring. Testing is also recommended following repair or improvements in the well.

Coliform presence in a water sample does not necessarily mean that the water is hazardous to drink. The test is a screening technique, and a positive result (more than 1 colony per 100 ml water sample) means the water should be retested. The retested sample should be analyzed for fecal coliform organisms. A high positive test result, however, indicates substantial contamination requiring prompt action. Such water should not be consumed until the source of contamination is determined and the water purified.

A testing laboratory provides specific sampling instructions and containers. The sampling protocol includes the following:
use sterile sample container and handle only the outside of container and cap;
run cold water for a few minutes (15 minutes) to clear the lines upon collecting sample, immediately cap bottle and place in chilled container if delivery to lab exceeds 1 hour (never exceed 30 hours). Many laboratories do not accept samples on Friday due to time limits.

Chlorine treatment

Chlorine readily combines with chemicals dissolved in water, microorganisms, small animals, plant material, tastes, odors, and colors. These components "use up" chlorine and comprise the chlorine demand of the treatment system. It is important to add sufficient chlorine to the water to meet the chlorine demand and provide residual disinfection.

The chlorine that does not combine with other components in the water is free (residual) chlorine, and the breakpoint is the point at which free chlorine is available for continuous disinfection. An ideal system supplies free chlorine at a concentration of 0.3-0.5 mg/l. Simple test kits, most commonly the DPD calorimetric test kit (so called because diethyl phenylene diamine produces the color reaction), are available for testing breakpoint and chlorine residual in private systems. The kit must test free chlorine, not total chlorine.

Contact time with microorganisms

The contact (retention) time (Table 1) in chlorination is that period between introduction of the disinfectant and when the water is used. A long interaction between chlorine and the microorganisms results in an effective disinfection process. Contact time varies with chlorine concentration, the type of pathogens present, pH, and temperature of the water. The calculation procedure is given below.

Contact time must increase under conditions of low water temperature or high pH (alkalinity). Complete mixing of chlorine and water is necessary, and often a holding tank is needed to achieve appropriate contact time. In a private well system, the minimum-size holding tank is determined by multiplying the capacity of the pump by 10. For example, a 5-gallons-per-minute (gpm) pump requires a 50-gallon holding tank.

Pressure tanks are not recommended for this purpose since they usually have a combined inlet/outlet and all the water does not pass through the tank.
An alternative to the holding tank is a long length of coiled pipe to increase contact between water and chlorine. Scaling and sediment build-up inside the pipe make this method inferior to the holding tank.

Chlorination levels

If a system does not allow adequate contact time with normal dosages of chlorine, superchlorination followed by dechlorination (chlorine removal) may be necessary.

Superchlorination provides a chlorine residual of 3.0-5.0 mg/l, 10 times the recommended minimum breakpoint chlorine concentration. Retention time for superchlorination is approximately 5 minutes. Activated carbon filtration removes the high chlorine residual.

Shock chlorination is recommended whenever a well is new, repaired, or found to be contaminated. This treatment introduces high levels of chlorine to the water. Unlike superchlorination, shock chlorination is a "one time only" occurrence, and chlorine is depleted as water flows through the system; activated carbon treatment is not required. If bacteriological problems persist following shock chlorination, the system should be evaluated. More information regarding shock disinfection can be found.

CHLORINATION GUIDELINES

Chlorine solutions lose strength while standing or when exposed to air or sunlight. Make fresh solutions frequently to maintain necessary residual.

Maintain a free chlorine residual of 0.3-0.5 mg/l after a 10 minute contact time. Measure the residual frequently.

Once the chlorine dosage is increased to meet greater demand, do not decrease it.

Locate and eliminate the source of contamination to avoid continuous chlorination. If a water source is available that does not require disinfection, use it.

Keep records of pertinent information concerning the chlorination system.

Types of chlorine used in disinfection

Public water systems use chlorine in the gaseous form, which is considered too dangerous and expensive for home use. Private systems use liquid chlorine (sodium hypochlorite) or dry chlorine (calcium hypochlorite). To avoid hardness deposits on equipment, manufacturers recommend using soft, distilled, or demineralized water when making up chlorine solutions.

Equipment for continuous chlorination

Continuous chlorination of a private water supply can be done by various methods. The injection device should operate only when water is being pumped, and the water pump should shut off if the chlorinator fails or if the chlorine supply is depleted. A brief description of common chlorination devices follows.

Chlorine Pump (see Fig. 1):

• commonly used, positive displacement or chemical-feed device,
• adds small amount, of chlorine to the water,
• dose either fixed or varies with water flow rates
• recommended for low and fluctuating water pressure,
• chlorine drawn into device then pumped to water delivery line

Figure 1. Pump type (positive displacement) chlorinator
Figure 2. Injector (aspirator) chlorinator

Suction Device:
• line from chlorine supply to suction side of water pump,
• chlorine drawn into water held in well pump,
• dosage uniformity not assured with this system,
• some suction devices inject chlorine directly into well water, increasing contact time between microorganisms and disinfectant; water/chlorine mixture is then drawn into well pump

Aspirator (see Fig. 2):
• simple, inexpensive mechanism,
• requires no electricity,
• vacuum created by water flowing through a tube draws chlorine into a tank where it mixes with untreated water,
• treated solution fed into water system,
• chlorine doses not consistently accurate

Solid feed unit:
• waste treatment and swimming pool disinfection,
• requires no electricity,
• controlled by flow meter,
• device slowly dissolves chlorine tablets to provide continuous supply of chlorine solution

Batch disinfection:
• used for fluctuating chlorine demand,
• three tanks, each holding 2 to 3 days' water supply, alternately filled, treated, and used

Disinfection by-products

Trihalomethanes (THMS) are chemicals that are formed, primarily in surface water, when naturally occurring organic materials (humic and fulvic acids from degradation of plant material) combine with free chlorine. Some of the THMs present in drinking water are chloroform, bromoform, and bromodichloromethane. Since groundwater rarely has high levels of humic and fulvic acids, chlorinated private wells contain much lower levels of these chemicals.

THMs are linked to increases in some cancers, but the potential for human exposure to THMs from drinking water varies with season, contact time, water temperature, pH, water chemistry and disinfection method. Although there is a risk from consuming THMs in chlorinated drinking water, the health hazards of undisinfected water are much greater. The primary standard (maximum contaminant level) for total THMs in drinking water is 0.10 mg/l, and activated carbon filtration removes THMs from water.