Disinfection By Products in Drinking Water

Cathal Walsh
Environmental Blogger
Tuesday, 2nd April 2019
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Glass of water on desk


Disinfection by products are a family of chemicals formed when these disinfectants react with naturally occurring organic matter and other substances in the source water. DPB’s have heterogeneous structures which are suspected carcinogens as a result of the reaction between NOMs (Natural Organic Matter) and oxidants/disinfectants such as chlorine.  The levels of disinfection by products depend upon the nature of the source water, the type of treatment used to remove particles and organic matter, and the type and concentration of disinfectants.  The levels can also change with the seasons of the year due to the levels of rainfall.

Many disinfection by products are bio accumulative, they are not destroyed by the body and can accumulate in body tissues. High levels of DPB’s in tap water can have possible adverse effects on reproductive health, including low birth weight and miscarriage. Chlorine causes hundreds of DPB’s to form when used as a disinfectant in water. For example. Trihalomethanes (THM) are a group of four chemicals ( Choloform, Bromodichloromethane, Dibromochloromethane and Bromoform).  These are formed along with other disinfection by products when chlorine and other disinfectants are used to control microbial contaminants in drinking water react with naturally occurring organic matter (NOM) and inorganic matter in water. The concentration of the chlorine dose in water can affect the THM formation.  THM formation however can be minimised by avoiding the use of pre-chlorination.

THM’s continue to form in drinking water as long as sufficient disinfectant residuals and reactive precursors are present in water. THM’s have high chemical stability and persist in the water formation. Generally the longer the contact times between the chlorine and the NOM, the greater the amount of THM’s that can be formed.

High THM values usually occur at points in the distribution system with the longest residence time or water age, such as reservoirs, oversized pipes and network dead ends. The rate of THM formation in water increases with increasing temperature.

The EPA has published the stage 1 Disinfectants and Disinfection by products rule to regulate total Trihalomethanes (TTHM) at a maximum allowable annual average level of 80 parts per billion.

Roughly about 500’000 people in Ireland have disinfectant by products in their water.  It is always important to get your water tested regularly to know what chemicals and compounds you are drinking.  

For a mains water test follow the link https://www.h2olabcheck.com/product/fluoridechlorine-test

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