DISINFECTION BY-PRODUCTS

        The incidence of waterborne diseases has been greatly reduced since the widespread implementation of drinking water disinfection. While a measurable public health benefit has been achieved, other potential risks may have been introduced.

        The presence of of chloroform and other trihalomethanes (THMs) in finished drinking water was first associated with the chlorination of drinking water in 1974. It was discovered that in addition to killing microorganisms, disinfectants react with organic and inorganic substances naturally present in the water to produce a variety of disinfection by-products (DBPs), which include THMs. The DBPs associated with chlorination are THMs, haloacetic acids, haloacetonitrites and halopicrins. Chlorite and chlorate are by-products of chlorine dioxide disinfection, while ozonation may result in bromate formation.

        Certain DBPs have been shown to be detrimental to health in laboratory animal studies. As a result the USEPA regulated the most prevalent DBPs, the trihalomethanes, in 1979, setting the limit at 100 mg/L. The regulations are currently being revised to reduce the limit to 80 mg/L and to introduce a limit of 60 mg/L for haloacetic acids. In addition, bromate will be regulated at 10 mg/L. While haloacetonitriles, halopicrins, and chlorate have been identified as health hazards, further research needs to be conducted prior to identifying appropriate regulatory levels. It is important that a balance be achieved between reducing exposure to DBPs and maintaining control of waterborne diseases through regulatory efforts.

        The available evidence indicate that the health risks from current levels of DBPs in drinking water is low. Additional research, however, is required to fully ascertain the public health risks. Current efforts to reduce the risk of DBP exposure through drinking water include the use of alternative methods of disinfection to chlorine, such as chloramination, ozonation or combinations of the above. While these treatment alternatives effectively reduce DBP formation, the efficiency with regards to removal of resistant pathogens in the water such as viruses and protozoan cysts requires further research.
        The following fact sheets describe the different types of DBPs and the potential health risks associated with each of them. The DBPs discussed are as follows:

- Trihalomethanes
- Haloacetic acids
- Haloacetonitriles
- Halopicrins
- Bromate
- Chlorate/Chlorite




Source: http://www.ci.sf.ca.us/puc/wqfs/tocsec4.htm