Balancing the risks of waterborne pathogens and disinfection byproducts (DBPs) is an evolving challenge for the modern water treatment professional. Widespread disinfection of drinking water, approximately a century old, is regarded as a major public health victory over typhoid fever, cholera and other waterborne diseases. Only in the past 30 years has science demonstrated a potential "downside" to mass water treatment: the formation of DBPs, families of unwanted compounds resulting from the chemical reaction of disinfectants with the organic matter in natural waters. Increasingly, scientists and regulators are addressing the complex presence of DBPs in treated water. Some of these compounds have been regulated. Now, new research demonstrates that with only an incomplete knowledge of the universe of DBPs and their potential hazards, current regulations may have unintended consequences.
The U.S. Environmental Protection Agency (EPA), through its 1998 Stage 1 Disinfectants and Disinfection Byproducts Rule, attempts to manage DBP risk from chlorinated drinking water by regulating two families of DBPs-trihalomethanes (THMs) and haloacetic acids (HAAs). To reduce the presence of these compounds substantially, many water treatment facilities have begun to substitute chloramines for chlorine as a secondary disinfectant. Some systems are also adopting alternatives such as ozone and chlorine dioxide for primary disinfection, although chlorine remains the most popular choice by far. Employing a different tactic, other systems choose to reduce DBP formation by more effectively removing organic matter in source water prior to disinfection. Unfortunately, there is no "one size fits all" solution to managing DBP levels. Facilities must consider the natural chemistry and quality of their source water and their available resources, and plan accordingly.
But there is complicating news: A 2002 nationwide EPA study demonstrates that certain unregulated DBPs are present in drinking water in concentrations on par with those that are regulated. And while alternative disinfectants reduce the presence of THMs and HAAs relative to chlorination, alternate disinfectants can produce higher levels of unregulated DBPs. For example, chloramination of natural waters containing high levels of bromine results in iodinated (iodine-containing) DBPs, one of which is more toxic to cells of mammals than any DBP ever identified.
Disinfection Byproducts: Many and Varied
Hundreds of DBPs have been reported in the scientific literature since EPA scientists first found low levels of chloroform in chlorinated drinking water in 1974. Thirty years later, EPA estimates that less than half of all chlorinated DBPs have been identified. Most of the successfully characterized DBPs are easily extracted from water using analytical techniques; those more difficult to extract remain a challenge to identify. Although substantial progress has been made in investigating chlorinated HAAs and THMs, the body of knowledge on the large number of DBPs resulting from alternative disinfectants is meager. EPA is attempting to characterize all chemicals formed during water treatment so that it can minimize public exposure to the most potentially hazardous DBPs while still maintaining microbiologically safe, healthful drinking water.
Gathering Data Through the Nationwide Disinfection By-product (DBP) Occurrence Study
The Nationwide Disinfection By-product Occurrence Study was undertaken by EPA to characterize and quantify DBPs formed throughout the United States. Drinking water across the country was sampled. To limit the number of compounds analyzed to a manageable quantity, EPA experts were asked to prioritize, by potential toxicity, over 500 DBPs reported in the literature. The result is a list of approximately 50 priority compounds. EPA's goal is to conduct a complete assessment of DBPs formed by different treatments in various regions of the nation.
An important result of the EPA study is the development of information about DBPs from increasingly popular alternative disinfectants. The table below outlines the significant prioritized DBPs associated with each alternative disinfection technique:
Levels of many of the prioritized DBPs resulting from the treatments listed above are higher than levels of THMs and HAAs resulting from chlorination. These study results have implications for utilities considering replacing chlorination with alternative disinfection methods to meet EPA's DBP regulations. A facility treating source water containing naturally elevated bromine levels, for example, might consider avoiding chloramine treatment.
Understanding the Big Picture
Widespread drinking water treatment remains today no less a public health triumph than it was a century ago. A safe, abundant water supply that virtually eliminates waterborne disease frees a society to pursue greater goals. The immeasurable value of safe water to human life is reflected in the United Nations Millennium Goal to "halve, by 2015, the proportion of people without sustainable access to safe drinking water and basic sanitation."
Today's water treatment professionals are faced with a growing database of complex information on DBPs. Their challenge will be to map out the optimum course for managing these compounds, taking into consideration the unique characteristics of their source water and keeping in mind the first priority--eliminating waterborne pathogens.
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