WaterWorld
Keith Christman
September 1998

What Lies Ahead for Chlorine Disinfection
The risk posed by microbial pathogens is greater than that associated with disinfection byproducts.

The control of disinfection byproducts remains the primary challenge faced by chlorine disinfection of drinking water. Drinking water regulatory policy in the United States is primarily focused on mitigating potential health risks associated with chemical contaminants in drinking water.

In 1974 Environmental Protection Agency scientists determined that during water disinfection chlorine reacts with certain organic materials to create trihalomethanes (THMs), including chloroform in particular. Toxicological studies undertaken on chloroform suggested that it was carcinogenic to laboratory animals, although at levels much higher than those found in drinking water. Fears that THMs could be a potential human carcinogen led the EPA to set regulatory limits for these disinfection byproducts at 100 ppb for systems serving more the 10,000 people.

EPA is expected to release its new Stage I Disinfection Byproduct Rule in November 1998. The rule is designed to reduce levels of disinfection byproducts in drinking water without compromising microbial protection. The rule mandates a process called enhanced coagulation to remove disinfection byproduct precursors. The proposal also sets new Maximum Contaminant Levels for Total Trihalomethanes at 80 ppb, Haloacetic Acids at 60 ppb and bromate at 10 ppb.

The Stage rules were developed with the assistance of a Federal Advisory Committee that included representatives from water utilities, the Chlorine Chemistry Division of the American Chemistry Council, public health officials, environmentalists and other stakeholder groups.

The Committee was cautious about encouraging the use of alternative disinfectants which would produce other unknown byproducts. Committee members were also very cautious about any changes that would encourage utilities to reduce the level of disinfection. There was widespread agreement among members that the risks of microbial pathogens in drinking water must not be allowed to increase.

On March 31, 1998, EPA published a Notice of Data availability on Disinfectants and Disinfection Byproducts. This notice proposed changes to Maximum Contaminant Level Goals (MCLG) for Disinfection Byproducts based on new research.

EPA sets MCLGs at levels at which no known or anticipated adverse effects on health are expected and which allow for an adequate margin of safety. The most important change in this notice impacting chlorine was the increase in the MCLG for chloroform from 0 to 300 ppb. In proposing this change, EPA followed the recommendations of an expert panel convened by the International Life Sciences Institute. The panel concluded that chloroform was "likely to be a carcinogen above a certain dose range, but unlikely to be carcinogenic below a certain dose."

The new standards of 80 ppb for total trihalomethanes including chloroform is consider ably below the MCLG of 300 ppb.

Comparative Risks: Microbial Versus Chemical Contaminants

The task for regulators is to maximize public health protection by managing the relative human health risks of microbiological and chemical contaminants in drinking water. Continuing evidence of waterborne disease occurrence suggests that microbial risks should receive a much higher level of attention than disinfection byproducts. For this reason, The American Academy of Microbiology has recommended that "the health risks posed by microbial pathogens should be placed as the highest priority in water treatment to protect public health."

In a 1993 study submitted to the EPA for the Chlorine Institute during negotiations over the DBP rule, Dr. Robert Tardiff reported results of applying five essential criteria for determining the comparative health risks of microbial and chemical contamination. The five criteria for assessing water-related diseases are: types, incidence, severity, latency, and certainty of occurrence.

Dr. Tardiff's report concluded that the risk of microbial disease is much greater than the risk posed by chemicals suspected of causing cancer in humans. Importantly, there are significant differences in the incidence of disease, the amount of time (latency) between exposure and clinical illness, and the certainty that many people will become ill. Compared to chemical risks, microbial risks are much greater (1,000 to 100.000 times), their latency is very much shorter (days vs. decades), and they will almost certainly cause illness in humans.

A 1994 report published by the International Society of Regulatory Toxicology and Pharmacology stated that "the reduction in mortality due to waterborne infectious diseases, attributed largely to chlorination of potable water supplies. appears to outweigh any theoretical cancer risks (which may be as low as 0) posed by the minute quantities of chlorinated organic chemicals reported in drinking waters disinfected with chlorine."

Although most research attention has focused on the disinfection byproducts of chlorine other chemical disinfectants also produce byproducts when they react with organic matter and other precursors in raw water. Bromate - mainly a byproduct of ozonation of high bromide waters - is being regulated by EPA in the Stage I rule.

Cryptosporidium

Another factor that is likely to impact the choice of primary disinfectants by utilities is the need to address cryptosporidium. Chlorine is not very effective in treating cryptosporidium. Adequate filtration appears to be the best protection from cryptosporidium but in some areas with poor water quality it may be necessary to provide disinfection of cryptosporidium. Chlorine dioxide and ozone have been shown to be effective for inactivating cryptosporidium.

Gordon Finch at the University of Alberta has reported that these disinfectants can be even more effective when used sequentially with chlorine or chloramines. Chlorine dioxide may be the most economical choice for cryptosporidium inactivation given the lower costs of retrofitting a plant for chlorine dioxide relative to ozone. The recent proposal by EPA also made significant changes to restrictions on chlorine dioxide which will allow use of this disinfectant by more utilities.

Groundwater Disinfection

According to the EPA there are over 150,000 groundwater systems in the United States. EPA is currently developing a groundwater rule to address microbial contamination of groundwaters. Groundwater, although filtered by natural processes, is often susceptible to microbial contamination and may need disinfection.

A major groundwater pathogen occurrence study, supported by AWWA Research Foundation and EPA, indicates that about 60 percent of vulnerable wells and about half of wells initially considered not vulnerable have been found to be positive for one or more indicators of fecal contamination in tests for total coliform bacteria, E. coli, coliphage and human viruses. Viruses were found about ten times more often than fecal bacteria, calling into question the adequacy of current coliform monitoring (Macler, Bruce A., Fredrick W. Pontius, Journal AWWA, January 1997)

Groundwater sources also are the source of nearly half of all waterborne disease outbreaks in the United States. In each decade since 1920, 43-56 percent of outbreaks reported in all types of water systems was caused by contaminated, inadequately treated groundwater. In community water systems, inadequate disinfection of groundwater and untreated groundwater were the identified causes of 25 percent of the US waterborne outbreaks reported between 1971 and 1992.

The Environmental Protection Agency is developing a Groundwater Rule to address the public health risks from microbial contamination of groundwater systems. EPA is considering chlorination as one of the likely Best Available Technologies for disinfection along with UV, ozone and ultrafiltration. Another approach that is being considered is requiring measurable chlorine residual in the distribution system.

Chlorine often is the best choice for groundwater disinfection because chlorine is cost-effective, reliable, relatively simple, measurable and provides a residual which helps protect water from microbial contamination all the way to the tap and provides an indicator of contamination in the distribution system. Chlorine-based disinfectants are the only disinfectants that provide this residual protection.

According to the American Water Works Association white paper Chlorine for Drinking Water Disinfection,"Chlorine disinfection technology is far simpler than other disinfection technology. Experience shows that reliable operation using chlorine disinfection can be achieved in treatment plants of all size."

Because groundwaters generally are very low in organic DBP precursors, low levels of disinfection byproducts are generally produced. A survey by the American Water System reported an average groundwater trihalomethane level of 19 ppb compared to 60 ppb for surface waters and a haloacetic acid level of 8 ppb for groundwater and 48 ppb for surface waters. The Environmental Protection Agency also predicts that 88 percent of groundwater systems can meet these MCL's without changes to their treatment processes.

About the Author: Keith Christman is Director of Disinfection and Government Relations at the Chlorine Chemistry Division of the American Chemistry Council. Christman has managed disinfection issues at Chlorine Chemistry Division of the American Chemistry Council for over two years following five years as an Economist for the American Chemistry Council. He has a Masters of Science in Economics from the University of Delaware.