Drinking Water Chlorination White Paper

A Review of Disinfection Practices and Issues


TABLE OF CONTENTS

Preface

Executive Summary

Chapter 1 - Introduction

Chapter 2 - Benefits of Chlorine

Chapter 3 - Risks of Waterborne Disease: the Old and the New

Chapter 4 - The Disinfection By-Products Debate

Chapter 5 - Alternative Treatment Processes

Chapter 6 - Control of Disinfection By-Products

Chapter 7 - Industry Commitment to Safety and Public Health Protection

REFERENCES

Preface

The practice of drinking water chlorination is one of the most significant public health advances of the 20th century. Our quality of life depends on the knowledge that when we turn on the tap, the water will be clean and safe. For that reason, over 98% of water treatment facilities in the United States disinfect with chlorine and chlorine-based chemicals.

This paper has been prepared for water utility managers, public health officials and other decision makers to help answer their questions about the role of chlorine in drinking water treatment.

The document received the input and guidance of many technical experts from the chlorine industry and was reviewed by select members of the Chlorine Chemistry Council's Public Health Advisory Board.

Executive Summary

Chlorination has played a critical role in protecting America's drinking water supply from waterborne infectious diseases for 90 years. Drinking water chlorination is one of the most significant advances in public health protection, having virtually eliminated waterborne diseases such as cholera, typhoid, dysentery and hepatitis A in this country.

Over 98% of water supply systems that disinfect drinking water use chlorine because of its germicidal potency, economy and efficiency. In addition, chlorine-based disinfectants are the only major disinfectants with the lasting residual properties that prevent microbial regrowth and provide continual protection throughout distribution from the treatment plant to the tap.

Preventing Waterborne Diseases

Waterborne diseases continue to present challenges to public health officials and water suppliers. Prevention and control of waterborne diseases through source water protection and proper treatment techniques are critically important.

Untreated or inadequately treated drinking water supplies, primarily from surface water sources, contain microorganisms that can cause outbreaks of waterborne diseases. There are new concerns about emerging pathogens, including Cryptosporidium and certain viruses, that might be found even in drinking water treated by conventional methods. In the United States, vulnerable populations - the young, the elderly and those with compromised immune systems - remain at risk for significant illness and even death. The scourge of diarrheal diseases in the developing world, including a five-year cholera epidemic in Latin America, reinforces the conclusions of the American Academy of Microbiology: "The single, most important requirement that must be emphasized is that disinfection of a public water supply should not be compromised."1

The World Health Organization, in its Guidelines for Drinking-Water Quality, further supports the necessity of water disinfection: "Disinfection is unquestionably the most important step in the treatment of water for public supply. ... Efficient disinfection must never be compromised."52

Chlorine Benefits Outweigh Risks

In 1974, scientists discovered that during the water treatment process, chlorine reacts with organic matter in raw water to form disinfection by-products (DBPs). Other disinfectants also form DBPs. Concerns that the presence of these compounds in drinking water may present potential health risks led the U.S. Environmental Protection Agency to propose regulations to control DBPs.

Nevertheless, 25 years of research have failed to establish a direct link between trace amounts of chlorinated DBPs present in tap water and any additional cancer risk in humans. In 1990, the International Agency for Research on Cancer evaluated the body of research concerning the potential health effects of chlorinated drinking water and concluded that it is "not classifiable as to its carcinogenicity to humans."30 Furthermore, the World Health Organization noted that "the risks to health from disinfection by-products are extremely small in comparison with the risks associated with inadequate disinfection, and it is important that disinfection should not be compromised in attempting to control such by-products."52 According to Regli et al., "the risk of death from known pathogens in untreated surface water appears to be at least 100 to 1000 times greater than the risk of cancer from known DBPs in chlorinated drinking water."36

Controlling Disinfection By-Products

Alternatives to chlorination have been suggested, but all alternative methods, with the possible exception of ultraviolet radiation, also form by-products. Little is known about these other by-products compared with our knowledge of chlorine-related DBPs. In addition, because alternative disinfectants cannot provide the residual protection of chlorine-based disinfectants, they must be used in combination with chlorine or chloramines to provide a complete disinfection system.

Disinfection by-products can be reduced by removing DBP precursors and protecting source water where possible from the entry of DBP precursors. Removing organic precursors through enhanced coagulation and changing the point of chlorination to later in the treatment process are examples of measures that can help control by-product formation.

Establishing Priorities

Protecting public health involves establishing priorities. Comparative risk assessment suggests that the prevention of observed waterborne diseases through the control of microbiological contaminants should take precedence over eliminating the hypothetical risks posed by disinfection by-products. To that end, future drinking water regulations should rely on cost-benefit analysis to determine where dollars spent for water treatment will yield the most public health benefits.

The Chlorine Chemistry Council supports research on both the efficacy of chlorine as a disinfectant and a better understanding of the health effects of disinfection by-products.

Chapter 1 - Introduction

Chlorination has played a critical role in protecting America's drinking water supply from waterborne infectious diseases for 90 years. Chlorination began in the early years of this century in Great Britain, where its application sharply reduced typhoid deaths. Shortly after this dramatic success, chlorination and filtration were introduced into the United States, which resulted in the virtual elimination of waterborne diseases such as cholera, typhoid, dysentery and hepatitis A.51 The adoption of drinking water chlorination has been one of the most significant advances in public health protection.52

Table 1. Disinfection Practices in the USA
Disinfectant Percentage*
Chlorine gas 87.0
   No ammonia 67.0
   Ammonia added 20.0
Chlorine & hypochlorite 4.5
Chlorine & chlorine dioxide 3.0
Chlorine & chlorine dioxide & ammonia nitrogen 1.5
Hypochlorite 1.5
Chlorine & hypochlorite & ammonia nitrogen 0.75
Chlorine & chlorine dioxide & hypochlorite 0.37
98.6% use chlorine-based disinfectants
Ozone 0.37
Other 0.75
* Percentage of facilities that disinfect
Source: 1989-1990 AWWA Disinfection Committee Survey of Disinfection Practices

While chlorine's most important attributes are its broad-spectrum germicidal potency and persistence in water distribution systems, its ability to efficiently and economically address many other water treatment concerns has also supported its wide use. Chlorine-based compounds are the only major disinfectants exhibiting lasting residual properties to provide continual protection against microbial regrowth.51

A Water Treatment Primer

Water treatment practices vary in the United States, but there are generally accepted basic techniques. Source water quality predicates the kind of treatment required to provide safe water. Therefore, protecting source water quality is a primary goal.

The treatment choice depends on a number of factors that are site specific and for which adjustments must be made depending on raw water quality. Source water quality and turbidity (particulate matter) levels, water temperature and pH level, and incidence of pathogenic contaminants must be taken into account for treatment decision making.

Conventional, sequential water treatment for surface water proceeds as follows: watershed protection program; pretreatment employing flocculation and sedimentation to remove turbidity, as well as the organic precursors that form by-products; filtration; disinfection at an appropriate concentration (C) for a prescribed time (T) (CT criteria) to destroy harmful organisms; and the addition of chlorine to maintain residual disinfection throughout the distribution system. There may also be a need for prechlorination or rechlorination during storage and/or distribution to ensure that an appropriate residual is maintained throughout the system.

Primary disinfection provides the appropriate CT to inactivate microbial pathogens. Disinfectants proven effective for this purpose include free chlorine, chlorine dioxide and ozone. Secondary disinfection ensures residual protection to control microorganism regrowth or recontamination during water storage and distribution. Either free chlorine or chlorine plus the addition of ammonia to form chloramine accomplishes this task. Because ozone quickly decomposes in water, a chlorine-based disinfectant must be added prior to distribution to provide this second level of protection.

Chapter 2 - Benefits of Chlorine

Chlorine-based chemicals are the disinfectants of choice for treating drinking water. In fact, some 98% of all systems that treat water employ chlorine-based disinfectants. Facilities use chlorine because it does its job extremely well, is safe to use when handled properly and is very cost-effective. After its initial introduction in 1908 in New Jersey, chlorine was adopted as a disinfectant by most water treatment plants in the United States and Canada. More than 200 million Americans and Canadians receive chlorine- disinfected drinking water every day.

Water Treatment Applications

Chlorine's popularity in water disinfection is based on many factors. A 1985 study of the role of chlorine in water treatment conducted by J. Carrell Morris of the Harvard University School of Medicine identified many of chlorine's benefits:31

Potent germicide. The demonstrated use of chlorine reduces the level of disease-causing microorganisms in drinking water to almost immeasurable levels.

Residual qualities. Chlorine produces a sustained residual disinfection action "unique among available large-scale water disinfectants." Chlorine's superiority as a residual disinfectant remains true today. The presence of a sustained residual maintains the hygienicity of the finished drinking water from the treatment plant to the consumer's tap.

Taste and odor control. Chlorination of drinking water reduces tastes and odors. Chlorine oxidizes many naturally occurring substances such as foul-smelling algae secretions and odors from decaying vegetation, resulting in nonodorous, better-tasting drinking water.

Biological growth control. Chlorine's powerful germicidal action eliminates slime bacteria, molds and algae. Chlorine controls these nuisance organisms, which typically can grow in reservoirs, on the walls of transmission water mains and in storage tanks.

Chemical control. Chlorine in water treatment destroys hydrogen sulfide and removes ammonia and other nitrogenous compounds that have unpleasant tastes and hinder disinfection.

Public Health Protection

Chlorinated drinking water's chief benefit is the protection of public health through the control of waterborne diseases. It plays a paramount role in controlling pathogens in water that cause human illness, as evidenced by the virtual absence of waterborne diseases such as typhoid and cholera in developed countries.

Untreated or inadequately treated drinking water supplies remain the greatest threat to public health, especially in developing countries, where nearly half the population drinks contaminated water. In these countries, diseases such as cholera, typhoid and chronic dysentery are endemic and kill young and old alike. In 1990, over three million children under the age of five died of diarrheal diseases. Unfortunately, the availability of safe drinking water supplies in many areas is practically nonexistent, due to poverty, poor understanding of water contamination, and lack of a treatment and delivery infrastructure. International assistance groups, including the World Health Organization and the Pan American Health Organization (PAHO), have long-standing technical assistance and education programs to improve water supply and sanitation practices. It has been estimated that such improvements - including chlorine disinfection - can prevent 25% of all diarrheal outbreaks and reduce childhood mortality by equal levels.17

A recent example of the continuing public health threat from waterborne disease outbreaks occurred in Peru in 1991, where a major causative factor was the absence or inadequacy of drinking water disinfection. This failure to disinfect was partly based on concern about U.S. reports of the detection of disinfection by-products. The result: a five-year epidemic of cholera, its first appearance in the Americas in this century. The epidemic spread to 19 Latin American countries and has been only partially abated through public health interventions supported by PAHOÍs advice and technical assistance. Nearly a million cases and 10,000 deaths have been reported.17

These statistics strongly reinforce the concept that water disinfection must be a primary tool in protecting public health worldwide. As noted by the American Academy of Microbiology, "The single, most important requirement that must be emphasized is that disinfection of a public water supply should not be compromised."1

Balancing Risks

At the 1992 First International Conference on the Safety of Water Disinfection, several researchers described the costs associated with microbiological disease as well as the benefits of illness avoided through water treatment. Real health care savings can be realized from preventing and eliminating microbial con-tamination in drinking water supplies.12

In his conference presentation, Dr. Pierre Payment (University of Quebec) stated that the "social cost of 'mild' gastrointestinal illness in industrialized countries is several orders of magnitude higher than costs associated with acute hospitalized cases." For example, in the United States, annual costs were estimated to be $9.5 billion (1985 dollars) for cases with no consultation with a physician, $2.7 billion for those with consultations, and only $760 million for those requiring hospitalization.35

Dr. Payment presented data estimating that in 1985, about 500,000 hospitalizations and 3,000 deaths were due to gastrointestinal illnesses in the United States, the majority being of unknown origin. His study assumed that these numbers are grossly underestimated due to unreported or unidentified illnesses. Over 13% were due to viral illnesses, 4.9% were bacterial and 1.1% were parasitic. About 80% were presumed noninfectious. One out of ten deaths from gastroenteritis could be due to viruses.

Commenting on Dr. Payment's report, the American Academy of Microbiology noted, "A decrease in morbidity and mortality is not the only benefit which should be considered in a cost-benefit analysis ... The benefits of microbiologically safe water ... go beyond the absence of disease, and affect the productivity of industry, as well as the prices of goods and services."1

At the same conference, a paper by Gunther F. Craun et al. discussed the cost-effectiveness of water treatment for pathogen removal.19 An evaluation of five pathogens and treatment costs shows the favorable economic benefits of preventing infectious waterborne diseases. These benefits were determined based on an annual probability of illness and death, assuming no water treatment, and a cost of $3,000 per illness and $500,000 per death. The effectiveness of water treatment in reducing waterborne diseases depends on the quality of the source water and how the treatment system is operated and maintained.

The table below shows positive benefit-cost ratios associated with the installation of chlorination and conventional water treatment to remove and control pathogens in drinking water. The ratios were arrived at by comparing the probability of foregone disease, using the difference between the disease probabilities with no water treatment and those for various levels of water treatment in communities with populations of 10,000, 100,000 and 500,000.

Table 2. Positive Benefit-Cost Ratios - Water Treatment & Pathogen Removal
Population 10,000 100,000 500,000
Treatment costs only
Good water source
   chlorination alone 50.2 86.2 98.6
   conventional treatment plus chlorination 18.4 39.5 53.1
Poor water source
   chlorination alone 37.6 64.5 73.9
   conventional treatment plus chlorination 17.5 37.5 53.1
Complete water systems
Good water source
   chlorination alone 5.0 8.6 9.9
   conventional treatment plus chlorination 1.8 4.0 5.3
Poor water source
   chlorination alone 3.8 6.5 7.4
   conventional treatment plus chlorination 1.8 3.8 5.3
Worst-case assumptions
Good water source
   chlorination alone 8.0 13.8 15.8
   conventional treatment plus chlorination 2.9 6.3 8.5
Poor water source
   chlorination alone 6.0 10.4 11.0
   conventional treatment plus chlorination 2.8 6.0 8.1

The report concluded that "municipal water systems designed to prevent waterborne infectious disease are one of the most effective investments of public funds that society can make. Even conservative estimates under worst-case conditions show benefit-cost ratios of 3:1 for small systems and 8:1 for large systems. Pathogen-free drinking water is a bargain."19

Regarding comparison of these benefits with potential cancer risks associated with drinking water disinfection, the group noted that the costs of preventing the relatively small carcinogenic risks may not be warranted in light of many other public health risks that should be reduced.

Chapter 3 - Risks of Waterborne Disease: the Old and the New

Waterborne diseases continue to present challenges to public health officials and water suppliers. The presence of disease-causing microorganisms in tap water typically results from poor source water quality, lapses in disinfection and filtration treatment processes, or compromised distribution systems.

In most instances, outbreaks of waterborne diseases occur in water systems with inadequate or no disinfection. However, there are new concerns about emerging pathogens, such as Cryptosporidium, that appear even in high-quality water supplies.18

Waterborne pathogens that cause disease fall into three general classes - bacteria, viruses and parasitic protozoa, each with various identified species: Bacteria and viruses contaminate both surface and groundwater, whereas parasitic protozoa appear predominantly in surface water.41

Table 3. Waterborne Pathogens
Bateria Viruses Protozoa
Campylobacter Norwalk-like Cryptosporidium parvum
Escherichia coli Entero (poliomyelitis,
coxsackie, echo, rotavirus)		 Giarda lamblia
Salmonella (nontyphoid) Hepatitis A Entamoeba histolytica
Shigella Reovirus
Yersinia

Vibrio (noncholera)

Salmonella (typhoid)

Vibrio (cholera)

Legionella

Illnesses Associated with Waterborne Pathogens

All waterborne microbial pathogens are potentially infectious and capable of causing illness depending on the dose and the physical condition of the individuals exposed. It should be stressed that exposure to waterborne pathogens does not always mean infection, nor does infectivity always lead to clinical illness. Although the dose-response mechanism is still not fully understood, scientists estimate that the risk of waterborne microbial illness in the United States is approximately 1 in 1 thousand individuals. Of those infected in the general population, the mortality risk is 1 in 1 thousand (as compared to a mortality risk of 1 in 1 million for uninfected individuals).41

Bacteria and protozoa generally induce gastrointestinal disorders with a wide range of severity. Bacteria also cause life-threatening diseases such as typhoid and cholera. Viruses cause serious diseases such as aseptic meningitis, encephalitis, poliomyelitis, hepatitis, myocarditis and diabetes.35 In addition, gastrointestinal disorders may be attributed to unidentified or unspecified microorganisms. In terms of occurrence, protozoan infections are the most common, followed by bacterial infections and then viral infections.20

For most pathogens, the severity of illness ranges from mild gastrointestinal upset, fever and vomiting, and intermittent diarrhea to chronic diarrhea, dehydration, liver damage, acute respiratory illness, adverse neurological effects, depressed immune systems and death. Most healthy individuals in the general population usually experience only mild gastroenteritis that is easily controlled and of short duration.

On the other hand, certain segments of the population are especially vulnerable to acute illness (morbidity) and can exhibit high death (mortality) rates. These segments include pregnant women, infants, the elderly and those whose immune systems are compromised by cancer, AIDS or the drugs used to treat these and other conditions.16 For example, nursing home studies have shown dramatic increases in diarrheal deaths in individuals over age 55, with mortality rates as high as 1 in 100, or 10 to 100 times greater than in the general population.26

Occurrence

Some occurrence statistics exist for outbreaks of waterborne diseases, but public health and water authorities generally must deal with unreliable estimates. Among the difficulties encountered when trying to determine the extent of waterborne disease occurrence in the United States is that illnesses often go unreported or may only be attributed to unknown causes (etiology).29,35

Waterborne disease outbreaks are estimated to occur three to ten times more often than reported.20 This may be due to patients not seeking medical attention or physicians failing to properly diagnose or test for specific infectious agents. Acute gastroenteritis often is labeled a viral illness, although very limited data exist on the presence of viruses in water supplies. Tracking often falls by the wayside when limited public resources are redirected to other public health needs. In addition, not all states require regular reporting of microbial disease outbreaks.29

Data on waterborne disease outbreaks have been compiled by Gunther F. Craun, a water expert formerly with the U.S. Environmental Protection Agency (EPA). The following tables illustrate occurrence, pathogenic agents identified with cases of illness and causes of outbreaks from 1971 to 1992 in the United States. It should be noted that these data do not include the outbreak of cryptosporidiosis in Milwaukee in 1993.20

Table 4. Etiology of Waterborne Outbreaks in USA, 1971-1992
Etiology Outbreaks Casses of Illness Hospitalized Cases Deaths
Undetermined 341 82,486 253 0
Protozoan 127 43,952 67 0
Bacterial* 98 19,351 720 6
Viral 58 14,167 85 0
Chemical** 60 4,202 45 6
TOTAL 684 164,158 1,170 12
* E. coli, shigellosis
** Not DBPs

Table 5. Etiologic Agents Most Frequently Identified in Waterborne Outbreaks of Infectious Diseases in USA, 1971-1992
Etiologic Agent Outbreaks Cases of Illness
Giardia lamblia 118 26,733
Shigella 57 9,967
Norwalk-like virus 24 10,908
Hepatitis A 29 807
Campylobacter 13 5,257
Salmonella 12 2,370
Cryptosporidium parvum 7 17,194
All others* 23 4,243
TOTAL 283 77,479
*Toxigenic E. coli, Yersinia, rotavirus, S. typhi, V. cholera and others

Table 6. Causes of Waterborne Outbreaks in USA, 1971-92
Causes of Outbreak Percent of Outbreaks
Community Water Systems Non-Community Water Systems
Contamination of distribution system 29% 7%
Inadequate disinfection of
unfiltered surface water		 24% 8%
Inadequate disifection of groundwater 14% 30%
Untreated groundwater 11% 42%
Inadequate filtration of surface water 11% 1%
Miscellaneous; unknown causes 5% 6%
Inadequate chemical feed 3% 1%
Untreated surface water 2% 5%
Inadequate filtration of groundwater 1% 0%
TOTAL 100% 100%

At least 50% of waterborne disease outbreaks are attributed to "unknown etiology." As indicated in Table 5, where the cause has been identified, Giardia lamblia accounts for the highest number of cases. Cryptosporidium parvum ranks second even without post-1992 data. Those numbers will likely grow as the population classified as vulnerable increases.

Craun also matched outbreaks with source water and treatment techniques in community water systems. For systems using surface water, source contamination and treatment deficiencies were identified as the major causative agents. Untreated or inadequately treated groundwater was responsible for 10-14% of all outbreaks during the 1971-92 period. Overall during the period, contaminated, untreated and inadequately treated groundwater was responsible for more outbreaks than contaminated surface water.20

Prevention and Control

Eliminating and preventing contamination of water supplies is critically important. Under EPA's Total Coliform Rule, maximum contaminant levels (MCLs) and routine monitoring have formed the basis for controlling microbiological contamination of public water supplies. EPA uses the presence or absence of "indicator" bacteria, e.g., E.coli, to verify whether drinking water is pathogen-free and safe.

However, epidemiologists have now traced waterborne disease outbreaks to water supplies that did not exceed the MCL for total coliforms or turbidity and met all water quality standards. Cryptosporidium parvum, Giardia lamblia and viruses have been found in disinfected water where indicator bacteria were absent.1

The best-known example of this phenomenon was the 1993 cryptosporidiosis outbreak in Milwaukee that infected over 400,000 residents and killed over 100, most of whom were immunocompromised individuals. That outbreak was traced to a combination of a "spike" of Cryptosporidium oocysts in the raw water, perhaps related to a runoff event from agricultural areas, and process control difficulties. Although turbidity increased in the finished water, the city's water supply did not exceed EPA's coliform MCL, nor were the turbidities in violation at any time.

Another outbreak of cryptosporidiosis occurred in Clark County (Las Vegas), Nevada, in 1994, resulting in 78 cases, of which 61 were AIDS patients. Epidemiologists remain puzzled by this outbreak since no Cryptosporidium oocysts could be detected in either raw or finished water supplies, nor were coliform MCLs exceeded. Furthermore, the area's source water is exceptionally high quality, and the Las Vegas water treatment facility is state-of-the-art.9,28

Drinking water facilities work to achieve the goal of minimal risk in water supplies by removing contaminants and mitigating disease outcomes with a coordinated multi-barrier approach to risk management and public health protection. An EPA report, National Drinking Water Program Redirection Strategy, released in June 1996 highlighted the need for a nationwide commitment to water quality improvement.49

EPA's recommended action steps for the delivery of high-quality drinking water include:

The American Academy of Microbiology adds further recommendations:1

Scientific Research Update: Chloroform Reassessment

The results of promising new research proj-ects released in the last two years are summarized below.

Chlorine and Chloramine

Dr. Gordon R. Finch of the University of Alberta, Canada, released a study of chlorine inactivation of Cryptosporidium in May 1996.24 Dr. Finch evaluated a two-step disinfection approach in which different types of chlorine were applied sequentially - i.e., free chlorine (elemental chlorine, hypochlorous acid or hypochlorite ion) pretreatment followed by monochloramine. He also proposed a disinfection model for use by engineers to design C. parvum control processes for water treatment facilities.

Dr. Finch theorized that chlorine pretreatment sensitized the oocysts to the effects of monochloramines, increasing levels of inactivation. He also found that hypochlorous acid performed significantly better than hypochlorite ion for inactivation.

The study concluded that chlorine followed by chloramines could be used as an alternative methodology with appropriate disinfectant concentrations and contact times. Increasing levels of chlorine pretreatment proportionally reduce the subsequent monochloramine concentration and contact time necessary for a given level of inactivation.

Dr. Finch further suggested that the proposed two-step model for sequential chlorine- monochloramine disinfection could provide a potential disinfection barrier to C. parvum for water systems with adequate contact time.

Chlorine Dioxide

Dr. Finch also described several studies that reported C. parvum inactivation by chlorine dioxide in water at high pH levels and low temperature. More recent evidence suggests that effective inactivation can be achieved with a chlorine dioxide dose followed by free chlorine.25M

Ultraviolet Radiation

A joint venture in the United Kingdom has proposed an ultraviolet (UV) system to inactivate Cryptosporidium and Giardia in drinking water. Rather than treating the water, the system directly treats the oocysts to achieve inactivation. Water enters a treatment chamber and passes through a screen where the oocysts are trapped and irradiated. Using a two-step process, the water flow is reversed, and the oocysts are trapped and irradiated a second time. While additional development is needed, especially with regard to cost and design requirements, initial animal infectivity studies resulted in no infection to laboratory mice after treatment.22

Ozone

Ozone has been used to achieve the inactivation of Cryptosporidium oocysts. After conventional treatment processes (coagulation, flocculation, sedimentation or dissolved air flotation) accomplish 99.0% to 99.9% removal of the oocysts, ozone disinfection can successfully inactivate the rest. Important considerations in designing models for ozone disinfection of Cryptosporidium include water temperature, CT (concentration X time) criteria and residual levels.39

In water systems using ozone, Dr. Finch's research has shown that combining ozone as the primary disinfectant with chlorine or chloramine to furnish a dependable residual disinfectant may provide very effective treatment.23

Drinking water regulatory policy in the United States has, since 1974, focused primarily on mitigating potential health risks associated with chemical contaminants in drinking water supplies.

In 1974, EPA scientists determined that chlorine reacts with certain organic materials during water disinfection to create trihalo-methanes (THMs), including chloroform in particular and lesser amounts of other THMs. 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 EPA to set regulatory limits for these disinfection by-products (DBPs). The agency initiated a review of other chlorination by-products, such as haloacetic acids (HAAs) and haloacetonitriles, with a goal of developing much more comprehensive regulations. Meanwhile, studies also identified by-products associated with other disinfectants - chloramines, chlorine dioxide and ozone.46

Because high levels of disinfection by- products in drinking water are undesirable, cost-effective methods to reduce DBP formation should be employed. However, the microbiological quality of drinking water must always be the top priority.

Chlorine Chemistry Council
12 June 1997