Chlorination refers to the application of chlorine for the purposes of oxidation. The forms of chlorine used for chlorination include: chlorine gas, hypochlorous acid (HOCl), hypochlorite ion (OCl), and chloramines or combined chlorine (Mono-, di-, and tri-chloramines). The first three forms of chlorine are known as free chlorine.
Chlorine (Cl) has three valences under normal environmental conditions, -1, 0 and +1. Environmental scientists often refer only to the chlorine forms having 0 and +1 valences as chlorine; they refer to the -1 form as chloride. Chlorine with a valence of 0 (Cl2) and chlorine with a valence of +1 (HOCl) both have the ability to oxidize materials, whereas chlorine at a -1 valence, chloride, is already at its lowest oxidation state and has no oxidizing power.
The functions of chlorination are to disinfect water or wastewater , decolorize waters or fabrics, sanitize and clean surfaces, remove iron and manganese, and reduce odors. The fundamental principle of each application is that due to its oxidizing potential, chlorine is able to effect many types of chemical reactions. Chlorine can cause alterations in DNA, cell-membrane porosity, enzyme configurations, and other biochemicals; the oxidative process can also lead to the death of a cell or virus . Chemical bonds, such as those in certain dyes, can be oxidized, causing a change in the color of a substance. Textile companies sometimes use chlorine to decolorize fabrics or process waters. In some cases, odors can be reduced or eliminated through oxidation. However, the odor of certain compounds, such as some phenolics, is aggravated through a reaction with chlorine. Certain soluble metals can be made insoluble through oxidation by chlorine (soluble Fe2+ is oxidized to insoluble Fe3+), making the metal easier to remove through sedimentation or filtration .
Chlorine is commercially available in three forms; it can also be generated on-site. For treating small quantities of water, calcium hypochlorite (Ca(OCl)2), commonly referred to as high test hypochlorite (HTH) because one mole of HTH provides two OCl- ions, is sometimes used. For large applications, chlorine gas (Cl2) is the most wide used source of chlorine. It reacts readily with water to form various chlorine species and is generally the least expensive source. There are, however, risks associated with the handling and transport of chlorine gas, and these have convinced some to use sodium hypochlorite (NaOCl) instead. Sodium hypochlorite is more expensive than chlorine gas, but less expensive than calcium hypochlorite. Some utilities and industries have generated chlorine on-site for many years, using electrolysis to oxidize chloride ions to chlorine. The process is practical in remote areas where brine, a source of chloride ions, is readily available.
Chlorine has been used in the United States since the early 1900s for disinfection. It is still commonly used to disinfect wastewater and drinking water, but the rules guiding its use are gradually changing. Until recently, chlorine was added to wastewater effluents from treatment plants without great concern over its effects on the environment . The environmental impact was thought to be insignificant since chlorine was being used in such low concentrations. However, evidence has accumulated showing serious environmental consequences from the discharge of even low levels of various forms of chlorine and chlorine compounds, and many plants now dechlorinate their wastewater after allowing the chlorine to react with the wastewater for 30–60 minutes.
The use of chlorine to disinfect drinking water is undergoing a similar review. Since the 1970s, it has been suspected that chlorine and some by-products of chlorination are carcinogenic. Papers published in 1974 indicated that halogenated methanes are formed during chlorination. During the mid-1970s the Environmental Protection Agency (EPA) conducted two surveys of the drinking-water supply in the United States, the National Organics Reconnaissance Survey and the National Organics Monitoring Survey, to determine the extent to which trihalomethanes (THMs) (chloroform, bromodichloromethane, dibromochloromethane, bromoform) and other halogenated organic compounds were present. The studies indicated that drinking water is the primary route by which humans are exposed to THMs and that THMs are the most commonly detected synthetic organic chemicals in United States' drinking water.
Chloroform was the THM found in the highest concentrations during the surveys. The risks associated with drinking water containing high levels of chloroform are not clear. It is known that 0.2 qt (200 ml) of chloroform is usually fatal to humans, but the highest concentrations in the drinking water surveyed fell far below (311 ug/l) this lethal dose. The potential carcinogenic effects of chloroform are more difficult to evaluate. It does not cause Salmonella typhimurium in the Ames test to mutate, but it does cause mutations in yeast and has been found to cause tumors in rats and mice. However, the ability of chloroform to cause cancer in humans is still questionable, and the EPA has classified it and other THMs as probable human carcinogens. Based on these data, the maximum contaminant level for THMs in drinking water is now 100 ug/l. This is an enforceable standard and requires the monitoring and reporting of THM concentrations in drinking water.
There are several ways to test for chlorine, but among the more common methods are iodometric, DPD (N,NDiethyl-p-phenylenediamine) and amperometric. DPD and amperometric methods are generally used in the water and wastewater treatment industry. DPD is a dye which is oxidized by the presence of chlorine, creating a reddish color. The intensity of the color can then be measured and related to chlorine level; the DPD solution can be titrated with a reducing agent (ferrous ammonium sulfate) until the reddish color dissipates. In the amperometric titration method, an oxidant sets up a current in a solution which is measured by the amperometric titrator. A reducing agent (phenylarsine oxide) is then added slowly until no current can be measured by the titrator. The amount of titrant added is commonly related to the amount of chlorine present.
To minimize the problem of chlorinated by-products, many cities in the United States, including Denver, Portland, St. Louis, Boston, Indianapolis, Minneapolis, and Dallas, use chloramination rather than simple chlorination. Chlorine is still required for chloramination, but ammonia is added before or at the same time to form chloramines. Chloramines do not react with organic precursors to form halogenated by-products including THMs. The problem in using chloramines is that they are not as effective as the free chlorine forms at killing pathogens.
Questions still remain about whether the levels of chlorine currently used are dangerous to human health. The level of chlorine in most water supplies is approximately 1 mg/l, and some scientists believe that the chlorinated byproducts formed are not hazardous to humans at these levels. There are some risks, nevertheless, and perhaps the most important question is whether these outweigh the benefits of using chlorine. The final issue concerns the short-term and long-term effects of discharging chlorine into the environment. Dechlorination would be yet another treatment step, requiring the commitment of additional resources. At the present time, the general consensus is that chlorine is more beneficial than harmful. However, it is important to note that a great deal of research is now underway to explore the benefits of using alternative disinfectants such as ozone , chlorine dioxide, and ultraviolet light. Each alternative poses some problems of its own, so despite the current availability of a great deal of research data, the selection of an alternative is difficult.
[Gregory D. Boardman ]
Tchobanoglous, G., and E. D. Schroeder. Water Quality. Reading, MA: Addison-Wesley, 1985.
Chlorination refers to a chemical process that is used primarily to disinfect drinking water and spills of microorganisms . The active agent in chlorination is the element chlorine, or a derivative of chlorine (e.g., chlorine dioxide). Chlorination is a swift and economical means of destroying many, but not all, microorganisms that are a health-threat in fluid such as drinking water.
Chlorine is widely popular for this application because of its ability to kill bacteria and other disease-causing organisms at relatively low concentrations and with little risk to humans. The killing effect occurs in seconds. Much of the killing effect in bacteria is due to the binding of chlorine to reactive groups within the membrane(s) of the bacteria. This binding destabilizes the membrane, leading to the explosive death of the bacterium. As well, chlorine inhibits various biochemical reactions in the bacterium. In contrast to the rapid action of chlorine, other water disinfection methods, such as the use of ozone or ultraviolet light, require minutes of exposure to a microorganism to kill the organism.
In many water treatment facilities, chlorine gas is pumped directly into water until it reaches a concentration that is determined to kill microorganisms, while at the same time not imparting a foul taste or odor to the water. The exact concentration depends on the original purity of the water supply. For example, surface waters contain more organic material that acts to absorb the added chlorine. Thus, more chlorine needs to be added to this water than to water emerging from deep underground. For a particular treatment facility, the amount of chlorine that is effective is determined by monitoring the water for the amount of chlorine remaining in solution and for so-called indictor microorganisms (e.g., Escherichia coli ).
Alternatively, chlorine can be added to water in the form of a solid compound (e.g., calcium or sodium hypochlorite). Both of these compounds react with water, releasing free chlorine. Both methods of chlorination are so inexpensive that nearly every public water purification system in the world has adopted one or the other as its primary means of destroying disease-causing organisms.
Despite this popularity, chlorination is not without drawbacks. Microorganisms such as Cryptosporidium and Giardia form dormant structures called cysts that are resistant to chlorination. The prevalence of these protozoans in worldwide drinking water supplies is increasing. Thus, the effectiveness of chlorination may be compromised in some water systems. As well, adherent bacterial populations of bacteria such as Escherichia coli that form in distribution pipelines are extremely resistant to chlorine, and so can contaminate the disinfected water that flows from the treatment plant to the tap. A third concern with chlorination is the reaction between chlorine and methane gas, which produces one or more chlorinated derivatives. The best known are trichloromethane (chloroform) and tetrachloromethane (carbon tetrachloride). These chlorinated hydrocarbons have been shown to have adverse health effects in humans when ingested in sufficient quantity for a long time.
Furthermore, from an engineering point of view, excess chlorine can be corrosive to pipelines. In older water treatment systems in the United States, for example, the deterioration of the water distribution pipelines is a significant problem to water delivery and water quality .
See also Infection control; Water quality
Disinfection is the most important step in the water treatment process to destroy pathogenic bacteria and other harmful agents. Chlorination is a very common and effective method for the disinfection of drinking water, and it has been the single most important process for assuring the bacteriological safety of potable water supplies. The practice was introduced in Belgium in 1903, and was first used in the United States in 1908 in Chicago. A sharp decline in typhoid deaths was noted following the onset of chlorination, as was also the case with cholera, dysentery, and hepatitis A. Chlorination has also contributed to a major decline in infant mortality rates due to waterborne illness. Waterborne epidemics have virtually disappeared in the industrialized world. When waterborne disease outbreaks have occurred, they have generally been traced back to a failure of the chlorination system.
Chlorine is a very cost-effective disinfection process. Chlorine concentration is generally 1 miligram per liter, which is about 1 part per million. Chlorine can be added directly as chlorine gas, or indirectly as sodium-hypochlorite solution. Chlorine is applied both to drinking water and to wastewater. Chlorine demand is a measure of the chlorine added to the water system that combines with the impurities and is not available for disinfection. The lower the pH of the water, the more effective chlorine is for disinfection. After the chlorine is added to the water, there must be sufficient contact time for the chlorine to effectively destroy the bacteria.
The chlorination system generally includes chlorine storage and feed equipment. In most cases, a metering device (a chlorinator) allows the chlorine to mix via a small side stream of water. In contrast to a short-acting disinfectant (such as ozone), chlorine also has the benefit of being residual in the system. Should a pipe break or another type of accident occur, there is usually enough residual chlorine in the system to provide for protection of the water supply. Chlorine is used extensively when interruptions of water piping or cross-connections occur. Chlorine used at higher amounts will kill all potential organisms present, but at usual treatment levels certain resistant organisms, such as cryptosporidia, may survive. Chlorine is also used to clean reservoirs, basins, wells, and pipes. Algae can also be controlled with the use of chlorine.
Chlorine does have some undesirable characteristics, including imparting undesirable taste and odors to the water, especially when phenol is preset. Also, the reaction of chlorine with the organic material that can be present in the water results in a group of disinfectant by-products, known as the trihalomethanes (THMs). The most common THM is chloroform, which has been shown to cause cancer in laboratory animals. Chlorine itself is highly toxic and must be handled with extreme care at the water treatment facility.
Mark G. Robson
(see also: Disinfection By-Products in Drinking Water; Drinking Water; Water Quality; Water Treatment )
Craun, G. (1993). Safety of Water Disinfection-Balancing Chemical and Microbial Risks. Washington, DC: ILSI Press.
Koren, H., and Bisesi, M. (1995). Handbook of Environmental Health and Safety, 3rd edition, Vol. 2. Boca Raton, FL: Lewis Publishers.
Wallace, M. (1998). Maxcy-Rosenau-Last Public Health and Preventive Medicine. Stamford, CT: Appleton & Lange.
Chlorination is the process in which the element chlorine reacts with some other substance. For micro-organisims, the typical result of chlorination is death. This has made chlorination a popular means of rendering freshwater safe to drink and for recreational uses.
As well, chlorination is a very important chemical reaction both in pure research and in the preparation of commercially important chemical products. For example, the reaction between chlorine and methane gas produces one or more chlorinated derivatives, the best known of which are trichloromethane (chloroform) and tetrachloromethane (carbon tetrachloride). The chlorinated hydrocarbons constitute one of the most commercially useful chemical families, albeit a family surrounded by a myriad of social, political, economic, and ethical issues. One member of that family, as an example, is dichlorodiphenyltrichloroethane (DDT). Although one of the most valuable pesticides ever developed, DDT is now banned in most parts of the world because of its deleterious effects on the environment. As of 2006, however, there is an emerging opinion that DDT’s ability to control mosquitoes argue for its cardful and controlled use in areas of the world where malaria continues to be a problem
Chlorination is best known in connection with its use in the purification of water supplies. Chlorine is widely popular for this application because of its ability to kill bacteria and other disease-causing organisms at relatively low concentrations, with little risk to humans (although breathing chlorine gas in an enclosed space can be fatal), and at relatively low cost. In many facilities, chlorine gas is pumped directly into water until it reaches a concentration of about one ppm (part per million, which is equivalent to one microliter in a liter). The exact concentration depends on the original purity of the water supply. Alternatively, chlorine can be added to water in the form of a solid compound such as calcium hypochlorite or sodium hypochlorite. Both of these compounds react with water releasing free chlorine. Both methods of chlorination are so inexpensive that nearly every public water purification system in the world has adopted one or the other as its primary means of destroying disease-causing organisms.
Chlorination is the process by which the element chlorine reacts with some other substance. Chlorination is a very important chemical reaction both in pure research and in the preparation of commercially important chemical products. For example, the reaction between chlorine and methane gas produces one or more chlorinated derivatives, the best known of which are trichloromethane (chloroform ) and tetrachloromethane (carbon tetrachloride ). The chlorinated hydrocarbons constitute one of the most commercially useful chemical families, albeit a family surrounded by a myriad of social, political, economic, and ethical issues. One member of that family, as an example, is dichlorodiphenyltrichloroethane (DDT). Although one of the most valuable pesticides ever developed, DDT is now banned in most parts of the world because of its deleterious effects on the environment.
The term chlorination is perhaps best known among laypersons in connection with its use in the purification of water supplies. Chlorine is widely popular for this application because of its ability to kill bacteria and other disease-causing organisms at relatively low concentrations and with little risk to humans. In many facilities, chlorine gas is pumped directly into water until it reaches a concentration of about one ppm (part per million). The exact concentration depends on the original purity of the water supply. In other facilities, chlorine is added to water in the form of a solid compound such as calcium or sodium hypochlorite . Both of these compounds react with water releasing free chlorine. Both methods of chlorination are so inexpensive that nearly every public water purification system in the world has adopted one or the other as its primary means of destroying diseasecausing organisms.