Trihalomethanes (THMs)

Trihalomethanes  (THMs) are in many water supplies , especially municipal water supplies where it may be formed as a by-product of the reaction of chlorine and organic matter.  The amount of THM formation is based upon several factors including: pH, temperature , chlorine , bromide and humic acid levels,  there are four THMS with the most common bring, chloroform. Chloroform was used an early anesthetic but is no longer used for this purpose because of its toxic side effects.   It is used widely as a solvent in industry.  This group of compounds can affect the nervous system and muscles and may be carcinogenic.

How Are They Regulated?

There are four trihalomethanes which are regulated by the EPA under the Safe Drinking Water Act: chloroform , bromoform , bromodichloromethane and dibromochloromethane. The combination of these four compounds are what is regulated a Total THMs at a level of 0.08 mg/L. The MCL values are the same in the Stage 2 Disinfection By-Product Rule as they were in the Stage 1 DBPR, but compliance with the MCL is based on different calculations . Under Stage 1, compliance is based on a running annual average (RAA). Under Stage 2, compliance is based on a locational running annual average (LRAA), where the annual average at each sampling location in the distribution system is used to determine compliance with the MCLs. The LRAA requirement will become effective April 1, 2012 for systems on schedule 1, October 1, 2012 for systems on schedule 2, and October 1, 2013 for all remaining systems .

The World Health Organization has established different level for each of the trihalomethanes as follows:


  • Chloroform at 0.30 mg/L
  • Bromoform at 0.10 mg/L
  • Bromodichloromethane   0.06 mg/L
  • Dibromochloromethane at 0.10 mg/L
How Do You Measure It?

Trihalomethanes are typically analyzed in a laboratory due to the sophisticated equipment needed to detect these compounds. For many drinking water samples, an approved EPA method must be used . The most common method is EPA 524 .2 which uses Gas Chromatography and Mass Spectrometer. This method can detect levels as low as 0.02 mg/L and higher. Since THMs are formed when the disinfectant chlorine is added to water and reacts with organic matter, the levels can vary greatly over a short period of time.

How Do You Treat It?

One way to reduce trihalomethanes is to reduce the precursors, so you may want to have the water analyzed for total organic carbon or TOC . The higher the level of TOC the greater the chance the chlorine will react creating higher levels of THMs . Many public water supplies are exploring other methods of disinfection to reduce level of THMs including the use of chloramines, ultraviolet light, and chlorine dioxide.

The EPA recommends , “enhanced coagulation” as the best available technology for THM control at treatment plants. This involves the removal of precursors and refers to the process of optimizing the filtration process to maximize removal of precursors . Removal of the precursors is improved by decreasing pH (to levels as low as 4 or 5) and increasing the feed rate of coagulants.  Some public water supplies may have switched to ferric coagulants instead of the traditional alum .

For residential applications the best way to remove THMs is by utilizing some kind of carbon-based filtration . This could be a point of use filter, located right at the tap or a whole house system consisting of a couple of carbon tanks .

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