Chlorination
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.
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