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Chlorine Disinfection Contact Time (Basic)

Commercial Treatment System
Commercial Treatment System

Private Well Owner and Small System Operator Guide

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. A new pathogen screening test is Now Available.


Disinfection requirements

Disinfection reduces pathogenic microorganisms in the water to levels designated safe by public health standards. This prevents the transmission of disease.

An effective disinfection system kills or neutralizes all pathogens in the water. It is automatic, simply maintained, safe, and inexpensive. An ideal system treats all the water and provides residual (long-term) disinfection. Chemicals should be easily stored and not make the water unpalatable. State and federal governments require public water supplies to be biologically safe.

The U.S. Environmental Protection Agency (EPA) recently proposed expanded regulations to increase the protection provided by public water systems. Water supply operators will be directed to disinfect and, if necessary, filter the water to prevent contamination from Giardia lamblia, Coliform Bacteria, Viruses, Heterotrophic Bacteria, Turbidity, and Legionella.

Private systems, while not federally regulated, are also vulnerable to biological contamination from sewage, improper well construction, and poor-quality water sources. Since more than 30 million people in the United States rely on private wells for drinking water, maintaining biologically safe water is a major concern.


Testing water for biological quality

The biological quality of drinking water is determined by tests for Total Coliform Bacteria. These organisms are found in the intestinal tract of warm-blooded animals and in the 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 Total 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 to 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 typically includes the following:

  • 1 | We recommend removing any point-of-use devices or aeration devices and then run cold water for a few minutes (15 minutes) at a high velocity to clear the lines.
  • 2 | Use a sterile sample container and handle only the outside of the container and cap. Decontaminate the sampling tap using a chlorine solution or 91% alcohol (depends on the laboratory SOP (standard operating protocol)) or local standard. After decontamination, flush the line at a high velocity and then reduce the flow to a steady stream about the size of a pencil to sample.
  • 3 | Upon collecting the sample, immediately cap the bottle and place it in a chilled container if delivery time to the lab exceeds 1 hour (never exceed 30 hours). Note: Many laboratories do not accept samples on Friday due to time limits.

Note - If you are experiencing an intermittent water quality problem that may be related to the regrowth of bacteria in the lines, we recommend first-flush testing for total coliform, standard plate count, nuisance bacteria, and in some cases, Legionella.

Chlorine treatment

Chlorine readily oxidizes with some chemicals dissolved in water, microorganisms, and plant material, and tends to eliminate tastes, odors, and colors. The oxidation of these components "uses up" chlorine and adds to 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 colorimetric 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. We also recommend monitoring the ORP (Oxidation Reduction Potential) of the water.

For additional info on the Use of ORP Monitoring for Disinfection, check out the papers by the University of California and YSI.

Contact time with microorganisms

The contact (retention) time (Table 1) in chlorination is that period between the introduction of the disinfectant and when the water is used. A long interaction between chlorine and the microorganisms results in an effective disinfection process. The 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 an 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 of 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.

Table 1 | Calculating Contact Time

Minutes required = K / chlorine residual (mg/l)

K values to determine chlorine contact time

Highest pH Lowest Water Temperature for K Factors (°F)
> 50° 45° < 40°
6.5 4 5 6
7 8 10 12
7.5 12 15 18
8 16 20 24
8.5 20 25 30
9 24 30 36

To calculate contact time, use the highest pH and lowest water temperature expected. For example, if the highest pH anticipated is 7.5 and the lowest water temperature is 42 °F, the "K" value (from the table above)  using the formula is 15.

For a chlorine residual of 0.5 mg/L with a pH of 7.5 and lowest temperature of 42 °F, the minimum chlorine contact time is 30 minutes (assuming no other demand).

Minutes Required  = 15/0.5 = 30 minutes

For a chlorine residual of 0.3 mg/l with a pH of 7.5 and lowest temperature of 42 °F, the minimum chlorine contact time is 50 minutes (assuming no other demand).

Minutes Required  = 15/0.3 = 50 minutes

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. For more information regarding shock-disinfection, check out our Shock-Disinfection page.

Chlorination Guidelines

Chlorine solutions lose strength while standing or when exposed to air or sunlight. Make fresh solutions frequently to maintain the 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 and we recommend that you monitor the ORP of the water and the chlorine residual.


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.

Liquid Chlorine Dry Powder Chlorine
Household bleach most common form Dissolved in water
Available chlorine range:
5.25% (domestic laundry bleach)
18% (commercial laundry bleach)
Available chlorine: 4%
Slightly more stable than solutions from dry powder chlorine Stable when stored properly
Fire hazard near flammable materials
Protect from sun, air, and heat Protect from sun and heat
  Solution maintains strength for 1 week


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.

The system normally uses a chlorine pump; these are normally positive-displacement or chemical-feed devices that are either fixed-dosed or flow-dosed. We recommend a flow-based device because this would work best if there is low pressure or a change in the water flow. The other components of the system would include a suction device, and an aspirator with either a chemical-feed system or a batch-system approach.

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 the 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 un-disinfected 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.

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