Balancing Competing Drinking Water Risks: EPA Preparing Final Microbial and Disinfection Byproduct Rules

Drinking water treatment represents one of the greatest public health achievements in history. Before U.S. cities began routinely chlorinating their drinking water supplies (starting with Chicago and Jersey City in 1908), cholera, typhoid fever, dysentery and hepatitis A killed thousands of residents every year. Where widely adopted, the combination of chlorine, filtration, and other water treatment practices have helped to virtually eliminate these diseases.

Despite this tremendous success, drinking water systems continue to face challenges in assuring that customers receive microbiologically safe water. One challenge is the emergence of pathogens such as Cryptosporidium that are resistant to chlorination and can appear even in high quality source waters. Another challenge is controlling disinfection byproducts (DBPs), chemical compounds formed unintentionally when chlorine and other disinfectants react with natural organic matter in water.

The U.S. Environmental Protection Agency (EPA) has been engaged in a long-term effort both to improve barriers to Cryptosporidium and other emerging pathogens, and to limit exposures to certain DBPs commonly found in chlorinated water. Congress affirmed EPA's Microbial & DBP rulemaking strategy as part of its 1996 Amendments to the Safe Drinking Water Act.

EPA expects to complete key pieces of this strategy by early 2005, when it publishes the Stage 2 Disinfection Byproducts Rule (Stage 2 DBPR) and the Long Term 2 Enhanced Surface Water Treatment Rule (LT2). Both rules are based on an Agreement in Principle, signed in September 2000 by members of a Federal Advisory Committee. This diverse group of experts, including representatives from water utilities, state regulators, public health agencies, environmentalists and other organizations, developed a consensus set of recommendations to reduce DBP levels cost-effectively, while improving protection from microbial contaminants.

LT2: A Toolbox for Microbial Protection

In April 1993, Cryptosporidium caused of the largest reported outbreak of drinking water-related disease in U.S. history. The outbreak affected over 400,000 people in the city of Milwaukee and led to more than 100 deaths. Following the Milwaukee outbreak, EPA has taken a stepwise approach to addressing Cryptosporidium risks

An interim rule published in 1998 covers large drinking waters systems (those serving more than 10,000 persons). It sets a non-enforceable goal (known as a Maximum Contaminant Level Goal, or MCLG) for Cryptosporidium at zero, and requires all filtered systems to achieve a 99% (2 log) reduction of the pathogen. EPA's Long-Term 1 Enhanced Surface Water Treatment Rule (LT1), finalized in 2002, extends these requirements to smaller systems. EPA believes these current treatment requirements will adequately protect customers served by most systems. However, the Agency believes additional treatment is necessary for some potentially vulnerable systems, including unfiltered systems and filtered systems with the highest levels of Cryptosporidium in their source water.

The LT2 applies to all systems that use surface water or ground water under the direct influence of surface water. It requires systems to conduct initial monitoring to determine Cryptosporidium levels in source water. Unfiltered systems must provide at least 99 or 99.9 percent (2 or 3 log) inactivation of Cryptosporidium, depending on the results of their monitoring. Filtered systems are assigned to regulatory "bins," with additional treatment requirements for higher Cryptosporidium levels (see table below).

EPA predicts that the majority of systems will fall into Bin 1 and require no additional treatment. For systems falling into Bins 2-4, the Agency has proposed a range of treatment and management strategies, collectively termed the "microbial toolbox." For each option, EPA specifies a log reduction credit that applies towards the treatment requirements. Systems may select one or more options to achieve the total additional log reduction required for their respective bins. Options include establishing a watershed control program (0.5 log credit), employing bank filtration (0.5 - 1.0 log credit), and using membranes (log credit based on demonstrated removal efficiency). The toolbox also provides credits for systems that adopt alternative disinfection technologies (chlorine dioxide, ozone or UV disinfection) proven to be effective against Cryptosporidium. Log credit is based on demonstration of inactivation with contact time table.

The proposed LT2 also requires disinfection profiling to prevent any backsliding on disinfection efficacy. Disinfection profiling involves assessing the level of disinfection currently provided and then determining the impact that a proposed change in disinfection practice would have on this level. Whatever treatment options are adopted under LT2 or the Stage 2 DBPR, a water system may not reduce the efficacy of its disinfection technologies on any group of pathogens.

Stage 2 DBPR: Reducing Hazards Across Distribution Systems

In the early 1970s, EPA scientists determined that drinking water chlorination could form a group of byproducts known as trihalomethanes (THMs), including chloroform. Concerned that these chemicals may be carcinogenic to humans, EPA set the first regulatory limits for THMs in 1979. While the potential health risks posed by THMs and other DBPs remain uncertain, high levels of DBPs are clearly undesirable, and cost-effective measures are available to reduce them. The first DBP standards, which covered systems serving more the 10,000 people, limited THM levels to 100 parts per billion (ppb).

In December 1998 EPA issued the Stage 1 Disinfectants and Disinfection Byproducts Rule (Stage 1 DBPR) to cover all systems that use chlorine or other disinfection chemicals. The Stage 1 DBPR mandates a process called enhanced coagulation to remove natural organic matter, and thereby reduce the potential for DBPs to form. The rule also sets enforceable Maximum Contaminant Levels (MCLs) for total THMs at 80 ppb and the sum of five Haloacetic Acids (HAAs) at 60 ppb. These MCLs are based on system-wide running annual averages, meaning that concentrations may be higher at certain times and at certain points in the system, as long as the system-wide average for the year is below the MCL.

The primary purpose of the Stage 2 DBPR is to address the uneven level of protection that may result from averaging DBP levels across distribution systems. As recommended by the Federal Advisory Committee, the MCLs for THMs and five HAAs will remain 80 ppb and 60 ppb respectively. However, the rule will base compliance on locational running annual averages. Each system must identify locations within its distribution system that are likely to have the highest DBP concentrations. When the Stage 2 DBPR is fully implemented, no location within a system will be allowed to exceed the limits for regulated byproducts.

A key element of DBP control is to remove natural organic matter prior to disinfection. EPA has published a guidance document for water system operators entitled, Controlling Disinfection Byproducts and Microbial Contaminants in Drinking Water (EPA, 2001). The EPA guidance discusses several processes to remove natural organic matter effectively prior to disinfection, including optimized coagulation processes and membrane technology.

Water system managers may also look at switching from chlorine to alternative disinfectants to reduce formation of THMs and HAAs. However, all chemical disinfectants form some DBPs, and much less is known about the byproducts of these alternatives than is known about chlorination byproducts. A recent EPA study sampled drinking water across the U.S. (disinfected with the different disinfectants and with different water quality, including elevated levels of bromide in the source water). EPA quantified levels of about 50 DBPs considered "high priority" for further study. While the use of alternative disinfectants lowered the levels of the regulated THMs and HAAs (as compared to chlorine), many of the other prioritized DBPs were formed at higher levels with these alternative disinfectants. Through the study, EPA also detected more than 200 previously unidentified DBPs.

Water system officials must also consider additional impacts of modifying their treatment processes. As discussed in an accompanying article [Reducing Lead Levels in Washington, D.C. Drinking Water], a change in disinfection methods implemented to reduce DBP levels in Washington, D.C.'s water system may have increased the corrosivity of the water, resulting in lead leaching from pipes and into the water supplied to some homes. This case highlights the need for regulators and water system officials to take a holistic approach to addressing drinking water quality, rather than a strictly contaminant-by-contaminant approach.

Path Forward

Although the Federal Advisory Committee reached consensus on key provisions of the Stage 2 DBPR and LT2 nearly four years ago, EPA and water systems have significant work left to do. EPA released proposals for both rules in 2003, and solicited comments from interested groups and the general public. The Agency received hundreds of comments, and will take these into account as it prepares its final rules.

Implementation of both rules would be phased in over time. Requirements for large systems would be fully implemented six years after the final rules are published. Smaller systems would have an additional one and a half to two and a half years to meet the standards.

Although EPA must still address many details, the broad framework for the LT2 and Stage 2 DBPR represents a balanced approach to addressing both the acute risks of waterborne disease outbreaks and potential long-term concerns about DBP exposure. Following consensus recommendations from a diverse group of experts, EPA crafted a strategy to achieve three distinct goals 1) reduce exposures from THMs and HAAs, 2) increase protection against emerging microbial threats, and 3) ensure that the tremendous public health gains achieved through current disinfection practices are maintained as water systems address new priorities.