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DISINFECTION BYPRODUCTS: THE NEXT GENERATION
Citation:
Richardson, S D., J E. Simmons, AND G Rice. DISINFECTION BYPRODUCTS: THE NEXT GENERATION. ENVIRONMENTAL SCIENCE & TECHNOLOGY 36(9):198A-205A, (2002).
Impact/Purpose:
(1) Use toxicity-based approach to identify DBPs that show the greatest toxic response. (2) Comprehensively identify DBPs formed by different disinfectant regimes for the 'Four Lab Study'. (3) Determine the mechanisms of formation for potentially hazardous bromonitromethane DBPs.
Description:
Disinfection of drinking water is rightly hailed as a major public health triumph of the 20th Century. Before widespread disinfection of drinking water in the U.S. and Europe, millions of people died from infectious waterborne diseases, such as typhoid and cholera. The microbial pathogens causing these diseases were the targets of chemical disinfection of drinking water beginning in the early 1900s, and those deaths attributable to these waterborne pathogens virtually ceased in developed nations (1). Recent large outbreaks of waterborne illness (cryptosporidiosis in Milwaukee in 1993 and cholera in Peru beginning in 1991) served as dramatic reminders of the need to properly disinfect and control waterborne pathogens in drinking water and suggested the need to continually reevaluate disinfection techniques to improve/ensure drinking water disinfection. This recognition has led to research and use of alternative disinfectant strategies. While pathogenic organisms provide the primary human health risk from drinking water, chemical disinfection by-products (DBPs) also provide an unintended health hazard. Disinfectants, in addition to effectively killing harmful microorganisms, are powerful oxidants and oxidize the organic matter naturally present in most source waters (rivers, lakes, and many groundwaters), forming DBPs. Chlorine, ozone, chlorine dioxide, and chloramine are the most popular disinfectants in use today, and each produces its own suite of chemical DBPs in drinking water (2). Most developed nations have created regulations or guidelines to control DBPs in order to minimize consumers' exposure to hazardous chemical DBPs, while at the same time, maintaining adequate disinfection and control of targeted pathogens. As such, developed nations often have the opportunity to evaluate the nature and the magnitude of human health risks posed by DBPs and implement changes in drinking water treatment when necessary. Despite much research on drinking water DBPs over the last several years, we have only been aware of them since the early 1970s. In 1974, Rook reported the identification of the first DBPs--chloroform and the other trihalomethanes (THMs)--that are formed in chlorinated drinking water (3). In 1976, the U.S. Environmental Protection Agency (EPA) published the results of a national survey which showed that chloroform and the other THMs were ubiquitous in chlorinated drinking water (4). In the same year (1976), the National Cancer Institute published results linking chloroform to cancer in laboratory animals (5). As a result, an important public health issue was born. In 1979, the U.S. EPA issued a regulation to control THMs at 100 micrograms/L (ppb) in drinking water (6); and in 1998, the Stage 1 Disinfectants/Disinfection By-products (D/DBP) Rule was promulgated, which lowered permissible levels of THMs to 80 micrograms/L and regulated five of the haloacetic acids (HAAs), bromate, chlorate, and chlorite for the first time (Table 1) (7) . The Stage 1 D/DBP Rule became effective 3 years following the promulgation of the Rule (2001). With stricter regulations on THMs and new regulations on HAAs, many drinking water utilities are having to change from chlorine to alternative disinfectants (including ozone, chlorine dioxide, and chloramine) to meet the new regulations. However, new issues and problems can result. For example, the use of ozone can significantly reduce (or eliminate) the formation of THMs and HAAs, but can increase the formation of bromate, when elevated levels of bromide are present in source waters. In the almost 30 years since the THMs were identified, DBPs have been actively investigated. Significant research efforts have been directed toward increasing our understanding of DBP formation, occurrence, and health effects. However, although approximately 500 DBPs have been reported in the literature (2), only a small number have been addressed either in quantitative occurrence or health effects studies. The DBPs that have been quantified in drinking water are generally present at low to mid-ppb (micrograms/L) levels. However, more than 50% of the total organic halide (TOX) formed in the chlorination of drinking water is still quantitatively unaccounted for (8), and nothing is known about the potential toxicity of many of the DBPs present in drinking water. Much of the previous health effects research directed toward understanding the effects of chronic exposure to DBPs has focused on cancer and/or mutagenicity. Today, there are new, additional concerns about potential reproductive and developmental effects, and it is now recognized that the currently available single-chemical toxicity studies cannot by themselves explain epidemiologic effects observed in human populations, necessitating the toxicologic study of DBP mixtures. Also, other routes of exposure besides ingestion are now being recognized as significant. For example, recent work by researchers at Health Canada has revealed that a person can receive twice the exposure to THMs through showering (by inhalation) and equivalent exposure through dermal absorption (bathing, washing clothes, etc.) as ingesting 2 liters of water (9). In addition, new human exposure work is being conducted in which blood and urine are being monitored for DBP exposures (10-14). New work is also being conducted to try to uncover the missing fraction of DBPs' (e.g., the >50% TOX not identified in chlorinated drinking water and the >60% of assimilable organic carbon not being identified in ozonated drinking water). Progress is being made in the identification of highly polar, high molecular weight, and other DBPs that have been missed previously. As a result of all these new efforts, DBP research is entering into an entirely new phase ('The Next Generation'). No longer is cancer the only health endpoint detected in epidemiologic studies. No longer are THMs the only DBPs considered for quantitative occurrence studies or toxicity/epidemiologic studies/risk assessments. As researchers continue to tackle this important public health issue, exciting new work is taking place. The article will discuss this next wave of research and the information that can be gained by it.