Drinking Water Treatment – Chlorination

Image:Wellhead.jpgChlorination may be effective in removing pathogens, including certain viruses and bacteria, from your well water system.

EFFECTIVE AGAINST: Pathogens, including certain viruses and bacteria.



Chlorine disinfection is a point-of-entry treatment that kills pathogens, including certain viruses and bacteria. In addition, chlorination can offer residual disinfection throughout the household water distribution system.

How Chlorination Works

The effectiveness of chlorination depends on various factors, including water temperature, water pH, water turbidity, general water quality and contact time. The contact time is the time available to complete the reaction between the chlorine and untreated water. A longer contact time results in more effective disinfection. As the chlorine concentration increases, the required contact time decreases. Chlorination is more effective at a high temperature and a low pH. Particles in the water decrease the effectiveness of chlorination, as microorganisms may ‘hide’ behind particles and avoid disinfection.

The quality of the untreated water affects the chlorine demand. Chlorine readily combines with other components dissolved in water, including iron, manganese, hydrogen sulfide, microorganisms, plant material, ammonia, and organic color such as that from decaying peat moss. These things ‘use up’ chlorine so it is important to add sufficient chlorine to meet the demand and still provide residual disinfection. The chlorine that does not combine with other components is available to treat the water.

Chlorine is available in two formulations, as a dry powder or pellet (calcium hypochlorite), or as a liquid (sodium hypochlorite).

Chlorine solutions from powders should be prepared frequently since the strength of the solution decreases gradually after mixing. To avoid hardness deposits on equipment, manufacturers recommend using softened or distilled water when mixing chlorine solutions.


The different types of chlorine disinfection are batch disinfection, simple chlorination, super-chlorination followed by dechlorination, and shock chlorination. They vary in the amount of chlorine used.

Batch disinfection treats water in batches when the chlorine demand fluctuates. It is especially useful for cisterns, holding tanks, or during emergencies or other special situations. Three tanks, each capable of holding a two- to three-day water supply, are alternately filled. The water is treated and used as needed.

Simple chlorination maintains a low level (.3 to 0.5 milligram per liter) of free chlorine residual for the necessary contact time. The residual should be measured at the faucet farthest from the chlorine source.

When the necessary contact time is unattainable, super-chlorination followed by dechlorination (chlorine removal) is an option. Super-chlorination produces a free chlorine residual of 3.0 to 5.0 milligrams per liter -– ten times higher than the residual from simple chlorination. At this concentration, the necessary contact time is reduced to less than five minutes for water at pH 7. Super-chlorinated water has a strong chlorine smell and taste that is removed with an activated carbon filter following chlorine treatment. If the water contains dissolved iron, manganese, or sulfur, it may be necessary to install a cartridge sediment filter before the activated carbon unit to prevent clogging.

Shock chlorination is recommended when a well is new, newly repaired or temporarily contaminated. If bacteriological problems persist after one or two shock chlorination treatments, you may consider a continuous disinfection system or a new water source.


The effectiveness of chlorination depends on the amount of chlorine used. The amount is controlled by adjusting the equipment or by changing the amount of chlorine added for batch disinfection.

Equipment Options

Various kinds of injection devices and pumps are used to chlorinate a private water supply. The injection device should operate only when the water pump is functioning, and the water pump should shut off if the chlorinator fails or if the chlorine supply runs out. When deciding on a location for chlorination equipment, ensure that: electric sources are conveniently located; adequate ventilation is provided to exhaust chemical fumes and cool any motors; the area is relatively free of dust and dirt, which can collect on movable parts and lead to malfunction; the area is protected from excessive sunlight or freezing; the area has easy access for maintenance and refilling; and, if using a chemical tank, the tank is located as close as possible to the feeder.


Maintaining a chlorination system may involve periodically checking for loose, worn, missing, or broken parts; lubricating the entire system semi-annually; cleaning all surfaces showing corrosion; refilling chlorine supplies; and cleaning any clogged orifices. Be sure to unplug any power cords before maintenance.

Special Considerations

Both solid and liquid forms of chlorine can irritate the skin and are poisonous in concentrated form. They must be handled and stored carefully. Chlorine tablets should be stored in a dry location. Both liquid and solid formulations should be stored in their original labeled containers, away from children and animals.

Chlorination for Iron, Manganese, and Hydrogen Sulfide Removal

Iron and manganese do not affect health, but they can make water bitter, stain laundry and fixtures, and discolor water. The U.S. Environmental Protection Agency (EPA) has given a Secondary Drinking Water Standard of 0.3 milligram per liter for iron and 0.05 milligram per liter for manganese. Hydrogen sulfide is a nuisance contaminant that gives water a ‘rotten egg’ odor.


Chlorination devices are point-of-entry devices that oxidize dissolved iron to red particles and dissolve manganese to black particles. Hydrogen sulfide, present in groundwater as a gas with a ‘rotten egg’ odor, oxidizes to yellow sulfur particles upon chlorination. Once oxidized, these contaminants can be filtered by one of several mechanical filtration devices.

Chlorine kills iron, manganese, and sulfur bacteria, which are not hazardous, but can cause a red-brown or black slime that appears in toilet tanks and clogs faucets. The gelatinous mass resulting from chlorination must be removed via filtration. Shock chlorinating the water source before installing iron, manganese, or hydrogen sulfide filtration equipment also kills the bacteria.

How Chlorination Works for Iron, Manganese, and Hydrogen Sulfide Removal

As it moves through rock, groundwater dissolves iron and manganese that occur naturally in the rock. In dissolved form, these minerals are colorless. Chlorine oxidizes iron and manganese into red-brown or black particle and hydrogen sulfide into yellow particles. These particles can then be filtered.

A pH between 6 and 8 is best when chlorinating for iron or hydrogen sulfide removal. Manganese removal is most effective when the pH is greater than 9.5.

A certain amount of contact time between the contaminant and the chlorine is required. Contact usually occurs in the system’s pressure tank, although water might not remain in the tank long enough for complete oxidation.

An alternative to using a tank to achieve an adequate contact time is super-chlorination, which adds much more chlorine than is necessary. This procedure results in more efficient oxidation, but the excess chlorine must be removed with an activated carbon filter.


The effectiveness of chlorination depends on the amount of chlorine added to the water. The amount is controlled by adjusting the equipment used to add the chlorine.


The same type of equipment can be used for disinfection systems and iron, manganese, and hydrogen sulfide removal systems. Filtering devices, necessary to remove particles, require regular backwashing, media replacement or both.

Questions to Ask Before You Buy

Before purchasing a water treatment device, have your water tested at a state certified laboratory to determine the contaminants present. This will help you determine if chlorination is an effective treatment method for your situation. See Questions to Ask Before You Buy A Water Treatment System for more information.

Adapted from: Wagenet,L., K. Mancl, and M. Sailus. (1995). Home Water Treatment. Northeast Regional Agricultural Engineering Service, Cooperative Extension. NRAES-48. Ithaca, NY.