Disinfection by-products result from chemical reactions between organic and inorganic matter in water with chemical treatment agents during the water disinfection process.
Chlorination disinfection byproducts
Chlorinated disinfection agents such as chlorine and monochloramine are strong oxidizing agents introduced into water in order to destroy pathogenic microbes, to oxidize taste/odor-forming compounds, and to form a disinfectant residual so water can reach the consumer tap safe from microbial contamination. These disinfectants may react with naturally present fulvic and humic acids, amino acids, and other natural organic matter, as well as iodide and bromide ions, to produce a range of DBPs such as the trihalomethanes, haloacetic acids, bromate, and chlorite, and so-called "emerging" DBPs such as halonitromethanes, haloacetonitriles, haloamides, halofuranones, iodo-acids such as iodoacetic acid, iodo-THMs, nitrosamines, and others. Chloramine has become a popular disinfectant in the US, and it has been found to produce N-nitrosodimethylamine, which is a possible human carcinogen, as well as highly genotoxic iodinated DBPs, such as iodoacetic acid, when iodide is present in source waters. Residual chlorine and other disinfectants may also react further within the distribution network — both by further reactions with dissolved natural organic matter and with biofilms present in the pipes. In addition to being highly influenced by the types of organic and inorganic matter in the source water, the different species and concentrations of DBPs vary according to the type of disinfectant used, the dose of disinfectant, the concentration of natural organic matter and bromide/iodide, the time since dosing, temperature, and pH of the water. Swimming pools using chlorine have been found to contain trihalomethanes, although generally they are below current EU standard for drinking water. Concentrations of trihalomethanes of up to 0.43 ppm have been measured. In addition, trichloramine has been detected in the air above swimming pools, and it is suspected in the increased asthma observed in elite swimmers. Trichloramine is formed by the reaction of urea with chlorine and gives the indoor swimming pool its distinctive odor.
Byproducts from non-chlorinated disinfectants
Several powerful oxidizing agents are used in disinfecting and treating drinking water, and many of these also cause the formation of DBPs. Ozone, for example, produces ketones, carboxylic acids, and aldehydes, including formaldehyde. Bromide in source waters can be converted by ozone into bromate, a potent carcinogen that is regulated in the United States, as well as other brominated DBPs. As regulations are tightened on established DBPs such as THMs and HAAs, drinking water treatment plants may switch to alternative disinfection methods. This change will alter the distribution of classes of DBP's.
Occurrence
DBPs are present in most drinking water supplies that have been subject to chlorination, chloramination, ozonation, or treatment with chlorine dioxide. Many hundreds of DBPs exist in treated drinking water and at least 600 have been identified. The low levels of many of these DBPs, coupled with the analytical costs in testing water samples for them, means that in practice only a handful of DBPs are actually monitored. Increasingly it is recognized that the genotoxicities and cytotoxicities of many of the DBPs not subject to regulatory monitoring, are comparatively much higher than those DBPs commonly monitored in the developed world.
Health effects
Epidemiological studies have looked at the associations between exposure to DBPs in drinking water with cancers, adverse birth outcomes and birth defects. Meta-analyses and pooled analyses of these studies have demonstrated consistent associations for bladder cancer and for babies being born small for gestational age, but not for congenital anomalies. Early-term miscarriages have also been reported in some studies. The exact putative agent remains unknown, however, in the epidemiological studies since the number of DBPs in a water sample are high and exposure surrogates such as monitoring data of a specific by-product are used in lieu of more detailed exposure assessment. The World Health Organization has stated that "the risk of death from pathogens is at least 100 to 1000 times greater than the risk of cancer from disinfection by-products " the "risk of illness from pathogens is at least 10 000 to 1 million times greater than the risk of cancer from DBPs".
