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Disinfection By-Products

Trihalomethanes (THM) are a group of four chemicals that are formed along with other disinfection by products when chlorine or other disinfectants used to control microbial contaminants in drinking water react with naturally occurring organic and inorganic matter in water. The trihalomethanes are chloroform, bromodichloromethane, dibromochloromethane, and bromoform.  EPA has published the Stage 1 Disinfectants and Disinfection Byproducts Rule to regulate total trihalomethanes (TTHM) at a maximum allowable annual average level of 80 parts per billion. 

The four trihalomethanes (THM's) listed below:


CHCl 3


CHClBr 2


CHCl 2 Br

Tribromomethane (bromoform)

CHBr 3

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 100 ug/l.


THM levels tend to increase with pH, temperature, time, and the level of "precursors" present. Precursors are organic material which reacts with chlorine to form THM's.  One way to decrease THM's is to eliminate or reduce chlorination before the filters and to reduce precursors..  There are more precursors present before filtration, so we want to reduce or eliminate the time chlorine is in contact with this water. If some oxidation before filtration is required, an alternative disinfectant like potassium permanganate or peroxide could be considered. Note that this may not be an option if prechlorination is necessary to achieve required CT values. 

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.

Upcoming Regulations

The EPA is considering extensive revisions to the regulations covering disinfection by-products (DBP's). The limit for TTHM's would be lowered to 80 ug/l, and three additional categories of DBP's . This standard will replace the current standard of a maximum allowable annual average level of 100 parts per billion in December 2001 for large surface water public water systems. The standard will become effective for the first time in December 2003 for small surface water and all ground water systems

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.


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