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The four trihalomethanes (THM's) listed below:
are all by-products of chlorination. They are Cancer Group B carcinogens (shown to cause cancer in laboratory animals). Trichloromethane (chloroform) is by far the most common in most water systems. Dibromochloromethane is the most serious cancer risk, (0.6 ug/l to cause a 10-6 cancer risk increase) followed in order by Bromoform (4 ug/l), and Chloroform (6 ug/l).
Current regulations limit the concentration of these 4 chemicals added together (total trihalomethane or TTHM levels) to 80 ug/l.
|Bromodichloromethane||Zero (0.6 ppb)||0.080 mg/L or 80 ppb
(Sum of the concentrations of all four trihalomethanes) as an annual average
|Bromoform||Zero (5 ppb)|
|Dibromochloromethane||0.06 mg/L or 60 ppb|
|Chloroform||0.07 mg/L or 70 ppb|
The EPA has indicated that the best available technology for THM control at treatment plants is removal of precursors through "enhanced coagulation". Enhanced coagulation refers to the process of optimizing the filtration process to maximize removal of precursors. Removal is improved by decreasing pH (to levels as low as 4 or 5), increasing the feed rate of coagulants, and possibly using ferric coagulants instead of alum.
For point of use systems at homes, activated carbon filters are the most effective treatment. Reverse osmosis units will also eliminate trihalomethanes.
Haloacetic acids (monobromoacetic acid, dibromoacetic acid, monochloroacetic acid, dichloroacetic acid, and trichloroacetic acid) are byproducts of chlorination similar to THM's. An MCL for total haloacetic acids of 60 ug/l is expected. Excessive levels can cause nervous system and liver effects.
Chlorite is to be regulated with an MCL set at 1 mg/l. Excessive levels can cause hemolytic anemia.
Bromate, the other newly regulated DBP , is a concern only for systems using ozone. An MCL of 10 ug/l is expected. Excessive levels causes gastrointestinal, kidney, and hearing effects.
Stage 1 Disinfectants and Disinfection Byproducts Rule
More Comprehensive Information on Trihalomethanes
University of Florida
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Glossary of Water Terminology
Note: ug/l is used as an abbreviation for micrograms/liter or parts per billion.
Note: THM's are not a major concern for treating groundwater wells with low organic matter content. THM's form through the partial oxidation of organic material, therefore it is a more important concern to treating surfacewater and/or springs that have a high organic matter content.
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