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Shock Chlorination


Chlorination units are the most common oxidization units sold in North America, they are usually installed as Point of Entry (POE) systems. They are generally placed at the entrance of the water system into the house to service all or most incoming water.

Continuous Chlorination -- Effective continuous disinfection requires the chlorine be consistently and thoroughly mixed with the water and have sufficient contact time to kill all disease-causing and nuisance organisms.

The most common solution the Hypochlorinator, uses a positive displacement chemical feed pump to inject a chlorine solution into the water supply. 

Chlorination is effective at destroying or assisting in the filtration of :

  • bacteria, parasites and other organisms
  • nuisance iron and sulfate reducing bacteria
  • oxidizes soluble iron, manganese and hydrogen sulphide, so they can be filtered out
  • some organics
  • bacteria and viruses

Some pathogens such as Cryptosporidium cysts are not totally eliminated by chlorination. In fact, Cryptosporidium survivor viability is not impaired by passage through chlorination, not even chlorine at concentrations straight out of the bleach bottle.  For this reason chlorination of water susceptible to these pathogens needs to be augmented with a pre-filter of a pore size equal to or less than 3 microns.

The amount of chlorination and the time required for disinfection depends on the concentration of chlorine, temperature, and pH of the water, the amount of organic matter in the water, and the flow rate of the system. Disinfection for most waterborne disease-causing organisms occurs after 20 minutes of contact time when the pH is between 6 and 8, and the free available chlorine residual is in the range of 3.0 to 5.0 parts per million.

Continuous chlorination systems consist of a chemical metering device that feeds chlorine in sufficient amounts to kill bacteria.  Continuous chlorination is expensive and requires frequent and intelligent management. Because chlorination needs to be closely monitored and maintenance is essential, it should not be considered for disease-causing bacteria control until other alternatives have been fully explored.

In layman's terms chlorine feed rates are about 1 cup of 5 percent laundry bleach per 300 gallons of water. This rate depends on water temperature, pH, and pumping rate.  An inexpensive chlorine residual kit (readily and cheaply available from water supply and pool supply distributors) can determine if the feed rate should be adjusted up or down to obtain the proper chlorine residual.   A chlorine residual of about 3 to 5 parts per million should remain to indicate that disinfection is complete.  If chlorine taste is a problem, use a POU carbon filter to remove excess chlorine from drinking water.


Continuous chlorination is described within this page, for a description of shock chlorination follow this link. 

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.

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 residual stays within the 3 to 5 ppm range.

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.

Continuous Chlorination: 

Continuous chlorination is a necessity for surface water supplies such as lakes, springs, ponds or cisterns. When chlorinating to control disease-causing bacteria, you 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.

The piping in home water systems usually provides little contact time. The time between the pump and the nearest faucet is usually less than one minute. 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, enough pipe to fill a bathroom!. Specially baffled tanks are commercially available to increase contact time.


Some people object to the smell and/or taste of very small amounts (0.2 to 0.5)  of chlorine in public systems This smell is much more significant at residual rates of 3 to 5 ppm required within a smaller system.  Activated carbon or charcoal filter can be used to remove the chlorine taste and smell from the water.

For use in the home, chlorine is readily available as sodium hypochlorite commonly known as household bleach. This product contains 5 percent available chlorine. Chlorine is also available as calcium hypochlorite, which is sold in the form of dry pellets with up to 70% chlorine content.

A dry pellet chlorinator are also available Pellets are injected into the well at a calculated rate. This type of system uses the well casing as a retention tank, permitting the chlorine to kill bacteria and oxidize iron and manganese. 

Chlorine will not completely oxidize manganese unless the water is very alkaline (pH is above 9.5). 

Suspected carcinogenic compounds, called Trihalomethanes, result from the partial oxidation of organics by chlorine. To remedy this, activated carbon filtration or reverse osmosis units after the contact tank and just before the point of use faucet should be a part of home chlorination systems.   Because THMs are very seldom associated with groundwater, they are primarily a concern where surface water supplies are used.  THMs can be removed from drinking water through use of an activated carbon filter.

Shock Chlorination -- Shock chlorination is recommended whenever a well is new, repaired, or tested to be contaminated.  Its use is essential after a flood or entrance of surface water into the well.  Before investing in a continuous chlorination system, it is wise to try repeated shock chlorination applications.  Our personal well took three shock applications before testing no longer indicated the presence of biologicals within our water supply.


Chlorination in North America has a 94 year history.  In 1908 Chicago the first, and Jersey city following shortly thereafter were the first North American cities to continuously chlorinate their water in response to typhoid fever outbreaks.  

There are several sites on the web with historical background and information on chlorination

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Last modified: January 12, 2023