Chlorination is the process of adding the element chlorine to water as a method of water purification to make it fit for human consumption as drinking water. Water which has been treated with chlorine is effective in preventing the spread of diseases.
The chlorination of public drinking supplies was originally met with resistance, as people were concerned about the health effectsof the practice. The use of chlorine has greatly reduced the prevalence of waterborne disease as it is effective against almost all bacteria and viruses
Chlorination is also used to sterilise the water in swimming pools and as a disinfection stage in sewage treatment. It can also apply to the addition of chlorine to other elements, such as gold in the formation of gold chloride.

The Benefits of Chlorine

Current scientific data shows that the benefits of chlorinating our drinking water (less disease) are much greater than any health risks from THMs and other by-products. Although other disinfectants are available, chlorine remains the choice of water treatment experts. When used with modern water filtration methods, chlorine is effective against virtually all microorganisms. Chlorine is easy to apply and small amounts of the chemical remain in the water as it travels in the distribution system from the treatment plant to the consumer's tap,This level of effectiveness ensures that microorganisms cannot recontaminate the water after it leaves the treatment plant.

Alternatives to Chlorination

A number of cities use ozone to disinfect their water, because ozonation does not produce THMs. Although ozone is a very effective disinfectant, it breaks down quickly and cannot be used to maintain disinfection in the distribution system. Small amounts of chlorine or other disinfectants still must be added. Renovating water treatment plants so they can use ozone can be expensive.
Examples of other disinfectants include chloramines and chlorine dioxide. Chloramines are weaker disinfectants than chlorine, but are very effective in the distribution system. Chlorine dioxide can be used in the treatment plant, but it is not very effective in the distribution system .
All chemical disinfectants used in drinking water can be expected to form by-products that could affect human health. In general, we know less about the by-products of other disinfectants than about chlorination by-products.

What Impurities Will Chlorination Remove?

Chlorination is a water treatment that destroys disease-causing bacteria, nuisance bacteria, parasites and other organisms. Chlorination also oxidizes iron, manganese and hydrogen sulfide so they can be filtered out.
Disease-causing bacteria can infect humans and animals in several ways. Fecal waste from an infected host frequently carries organisms which cause diseases such as typhoid fever, paratyphoid fever, bacillary dysentery, infectious hepatitis and others. Disease-causing organisms are transmitted from host to host in many ways including through a contaminated water supply. Human and/or livestock populations concentrated together with improperly located or constructed wells may result in contamination of water supplies by sewage or fecal wastes.
There is no substitute for a safe and sanitary water supply. If your water supply becomes contaminated, elimination of the source of contamination is the most permanent solution. Continuous chlorination to kill disease-causing bacteria in a contaminated water source should be a temporary measure used only until you can develop a new, sanitary water supply.

1. Controls Disease-Causing Bacteria: Disease-causing bacteria may enter your well during construction, repair, flooding or as a result of improper construction. Proper chlorination will kill these bacteria. If disease-causing bacteria enter your water supply on a continuous basis, you must eliminate the source or construct a new water supply.
2. Controls Nuisance Organisms: Chlorine treatment will control nuisance organisms such as iron, slime and sulfate-reducing bacteria. Iron bacteria feed on the iron in the water. They may appear as a slimy, dark-red mass in the toilet tank but microscopic examination is needed to confirm their presence. Iron bacteria colonies may break loose from the inside of pipes and flow through faucets to cause stains in laundry, plumbing fixtures, etc. A thorough shock chlorination of the well and water system may destroy all iron bacteria colonies. However, iron bacteria that have penetrated the water-bearing formation will be difficult to eliminate and will likely re-infest the system. In this situation you will need to repeat chlorination treatment periodically.
   
   Other nuisance organisms include slime bacteria and sulfate reducing bacteria which produce a rotten-egg odor. Chlorination will kill or control these bacteria. Nuisance bacteria do not cause disease.
3. Mineral Removal: You can remove large amounts of iron from water by adding chlorine to oxidize the clear soluble iron into the filterable reddish insoluble form. Chlorine helps remove manganese and hydrogen sulfide in the same way. For a complete discussion of iron and manganese removal see Minnesota Extension Service publication "Treatment Systems for Household Water Supplies - Iron and Manganese Removal."

To chlorinate a water supply properly it is necessary to understand chlorine demand, free available chlorine residual and contact time.

1. Chlorine demand is the amount of chlorine required to kill bacteria, oxidize iron or other elements in the water, and oxidize any organic matter that may be present. There is no easy way to determine the amount of chlorine required—chlorine is added until the chlorine odor persists.
2. Free available chlorine residual is the amount of chlorine remaining in the water after the chlorine demand has been met. If the chlorine demand is greater than the amount of chlorine introduced, there will be no free available chlorine residual. Unless a chlorine residual is present, adequate amounts of chlorine have not been added to the water.
   
    Contact time is the amount of time that the chlorine is present in the water. The combination of chlorine residual and contact time determines the effectiveness of the chlorination treatment. The bacterial "kill factor" is defined as the product of free available chlorine residual and contact time. Thus the greater the chlorine residual the shorter the required contact time for bacterial kill.

Neither shock nor continuous chlorination should be used as a substitute for a safe water supply. Wells should be located and constructed in accordance with existing state well codes and accepted sanitary standards. The water system must also be properly constructed to prevent bacterial contamination between the well and the point of use.

Two methods for chlorinating water are:

1. Shock chlorination required for new wells or after well repairs (your well driller or repair person should perform this chlorination), recommended to treat non-recurring bacterial contamination.
2. Continuous chlorination used to treat recurring bacterial contamination problems. This process is similar to that used in municipal water supplies as a preventive measure.

For shock chlorination, an initial chlorine concentration of 50 to 100 parts per million (ppm) with a contact time of at least 6 hours is recommended. To obtain a concentration of 100 ppm you need to know (or estimate) the diameter of your well and the depth of water in the well. Note: Do not use the total depth of the well. The depth of water is the distance from the water surface to the bottom of the well. Your well driller may have this information or you can determine it yourself by lowering a weighted string to the bottom of the well. Table 1 shows the amounts of chlorine bleach to add for an initial concentration of about 100 ppm.

Continuous chlorination is a necessity for surface water supplies such as ponds, springs, lakes or cisterns. You can use chemical feed pumps to inject chlorine into your system. Wire the feed pump to the water pump pressure switch so that the pumps operate simultaneously. Other proportioning devices are available to feed chemicals into the water. If you are chlorinating to control disease-causing bacteria, you will need an alarm device that indicates when the chlorine solution supply needs replenishing.
Because effective organism kill is a function of contact time, the chlorine solution should be injected into the water as close to the source as possible. Adequate contact time for disease-causing bacterial kill depends upon free chlorine residual, water temperature, water pH (acidity), and the specific organism. Continuous chlorination typically uses a chlorine residual of 3 to 5 ppm. This level is considerably higher than that of municipalities (0.2 to 0.5 ppm) where the large distribution system provides a long contact time. This level of residual chlorine may cause a taste and/or odor that is objectionable.
At the 3 to 5 ppm level, adequate contact time for surface water supplies is in the range of 2 to 7 minutes to develop the proper bacterial kill factor. For well water, a contact time of 2 to 3 minutes is normally sufficient. In any case where disease-causing bacteria are involved, tests should be conducted after installation and continue on a regular basis to insure a safe water supply.
Research has shown that there is no dependable contact time calculation that can be used to test the pressure tank of the water system. The piping in home water systems usually provides little contact time. The time between the pump and the nearest faucet is usually one minute or less. A coil of plastic pipe can be used to increase contact time. The length of pipe required depends on the pipe diameter and the flow rate. For example, to achieve a 4-minute contact time with a 10 gallon per minute flow rate, you would need 128 feet of 3 inch nominal diameter pipe or 480 feet of 1.5 inch nominal diameter pipe. Your pipe supplier should be able to assist you in selecting the proper size and length for your required contact time and flow rate.
Continuous chlorination is expensive and requires frequent and intelligent management. It should not be considered for disease-causing bacteria control until other alternatives have been fully explored.