Disinfection is unquestionably the most important step in the treatment of water for drinking-water supplies. The microbial quality of drinking-water should not be compromised because of concern over the potential long-term effects of disinfectants and DBPs. The risk of illness and death resulting from exposure to pathogens in drinking-water is very much greater than the risks from disinfectants and DBPs. Where local circumstances require that a choice be made between microbiological limits or limits for disinfectants and DBPs, the microbiological quality must always take precedence. Efficient disinfection must never be compromised.
The microbiological quality of drinking-water is of paramount importance and must receive priority over any other considerations in relation to drinking-water treatment. However, the use of any chemical disinfectant results in the formation of by-products that themselves may be of health significance. A thorough understanding of how these DBPs form and the factors that control their formation is valuable in achieving a successful balance between satisfactory inactivation of pathogens and the minimization of DBP formation. The microbiological quality of drinking-water should always receive priority over the minimization of DBPs.
Where it is possible, without compromising the microbiological quality of drinking-water, steps should be taken to minimize the concentrations of DBPs produced by the disinfectant(s) in use. Strategies to minimize exposure to DBPs should focus on the elimination of precursors through source water protection. Not only is this often the most efficient method of reducing DBP concentrations, but it will also assist in improving the microbiological quality of the water. Where treatment is required, DBP control strategies should emphasize DBP organic precursor (TOC) removal.
Chlorine and alternative chemical disinfectants (ozone, chlorine dioxide and chloramine) all lead to the formation of DBPs. However, between disinfectants or combinations thereof, there are differences in DBP groups, species and mixtures that may affect human health. Key water quality determinants of DBPs include TOC, bromide and pH. Based on the current knowledge of both occurrence and health effects, the DBPs of most concern include total THMs and THM species, total HAAs and HAA species, bromate and chlorite.
None of the chlorination by-products studied to date is a potent carcinogen at concentrations normally found in drinking-water.
The toxicology of the DBPs suggests that the likelihood of adverse effects is not significantly different between the described disinfectant options.
Toxicological information on mode and mechanism of action of disinfectants and their by-products is the major limitation for understanding the potential health risks at low doses.
Epidemiological studies have not identified an increased risk of cardiovascular disease associated with chlorinated or chloraminated drinking-water.
The hypothesis of a causal relationship between consumption of chlorination by-products and the increased relative risk of any cancer remains an open question. There is insufficient epidemiological evidence to support a causal relationship between bladder cancer and exposures to chlorinated drinking-water, THMs, chloroform or other THM species. The epidemiological evidence is inconclusive and equivocal for an association between colon cancer and long duration of exposure to chlorinated drinking-water, THMs or chloroform. There is insufficient epidemiological information to properly interpret the observed risks for rectal cancer and the risks for other cancers observed in single analytical studies.
The results of currently published studies do not provide convincing evidence that chlorinated water or THMs cause adverse pregnancy outcomes.