Drinking Water Distribution Systems Assessing and Reducing Risks (2006) / Chapter Skim
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Appendix A: Public Water Supply Distribution Systems: Assessing and Reducing Risks, First Report
Appendix A: Public Water Supply Distribution Systems: Assessing and Reducing Risks, First Report
Pages 345-385
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From page 345... ...
Appendix A PUBLIC WATER SUPPLY DISTRIBUTION SYSTEMS: ASSESSING AND REDUCING RISKS F I R S T R E P O R T Committee on Public Water Supply Distribution Systems: Assessing and Reducing Risks Water Science and Technology Board Division on Earth and Life Studies THE NATIONAL ACADEMIES PRESS Washington, D.C.
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From page 346... ...
Of all these rules, the Total Coliform Rule (TCR) of 1989 explicitly addresses microbial water quality in the distribution system.
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From page 347... ...
Furthermore, in 2003 EPA committed to revising the TCR -- not only to consider updating the provisions about the frequency and location of monitoring, follow-up monitoring after total coliform positive samples, and the basis of the MCL, but also to consider addressing the broader issue of whether the TCR could be revised to encompass "distribution system integrity." That is, EPA is exploring the possibility of revising the TCR to provide a comprehensive approach for addressing water quality in the distribution system environment. To aid in this process, EPA requested the input of the National Academies' Water Science and Technology Board, which was asked to conduct a study of water quality issues associated with public water supply distribution systems and their potential risks to consumers.
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4 -- Identify advances in detection, monitoring and modeling, analytical methods, information needs and technologies, research and develop ment opportunities, and communication strategies that will enable the water supply industry and other stakeholders to further reduce risks as sociated with public water supply distribution systems. This first report relates the committee's progress on Tasks 1 and 2-that is, trends relevant to the deterioration of distribution system water quality and the issues that the committee thinks are the highest priorities for consideration during TCR revision to encompass distribution system integrity.
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From page 349... ...
population, consumer use of bottled water, and installation of home water treatment devices. This is not a comprehensive list of all the factors that may affect water quality and health risks from distributions systems.
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. The trends of aging pipe and increasing numbers of main breaks are of concern because of the potential relationship between waterborne disease outbreaks and main breaks (see the subsequent section on New and Repaired Water Mains)
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examined causes of reported waterborne outbreaks from 1971-1998 and noted that in community water systems, 30 percent of 294 outbreaks were associated with distribution system deficiencies. From 1999 to 2002, there have been 18 reported outbreaks in community FIGURE 2 Waterborne disease outbreaks in community water systems (CWS)
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It is noted that consumer confidence and legal liability may create a disincentive to report outbreaks and even water quality problems. A number of sources show that endemic risks can be greater than epidemics, for example, Frost et al., (1996)
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. USE OF BOTTLED WATER AND HOUSEHOLD WATER TREATMENT DEVICES There has been a dramatic increase in the proportion of the U.S.
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is deteriorating and that health risks associated with distribution system water quality may be increasing. Although the proportion of the U.S.
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based on a series of expert and stakeholder workshops from 2000 to 2003. The nine white papers focused on the following events or conditions that can bring about water quality degradation in public water supply distribution systems: 1.
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However, the committee decided to follow the structure of the EPA white papers in preparing this report, with the recognition that overlaps and difficult-to-separate phenomena exist. Of the issues presented in the nine white papers, cross connections and backflow, new or repaired water mains, and finished water storage facilities were judged by the committee to be of the highest importance based on their associated potential health risks.
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From page 357... ...
Proven technologies and procedures are available to mitigate the impact of cross connections on potable water quality. State plumbing codes define the type of plumbing materials that are approved for use, including cross connection control devices, but whether these codes are adhered to is questionable.
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, states that about 5 percent of reported waterborne disease outbreaks attributed to distribution system deficiencies in the United States over a 27-year period were associated with new or repaired water mains. Over 200,000 water main breaks occur every year (over 555 breaks per day)
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From page 359... ...
(1998) reported that while 90 percent of utilities surveyed said that new mains must meet water quality criteria before they are released back to service, only 29 percent said samples were required to be collected in response to a main break.
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From page 360... ...
or prevent the short circuiting of the water entering and leaving the facility to shorten the water age in the facility. The documented cases of waterborne disease outbreaks and the potential for contamination due to the large number of these facilities make this a high priority distribution system water quality maintenance and protection issue.
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From page 361... ...
and the cost of a plumbing failure to an individual homeowner can exceeded $25,000. The problems of greatest concern within premise plumbing include microbial regrowth, leaching, permeation, infiltration, cross connections, leaks and the resulting indoor mold growth, scaling, and the high costs of failure.
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From page 362... ...
The premise plumbing issue poses unique challenges because there is no obvious single party who could assume responsibility for the problem, which might be best addressed through changes in and enforcement of plumbing codes, third party standards, and public education. Utility involvement in overseeing solutions may be appropriate where distribution system water quality directly
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From page 363... ...
, and metals, and how distribution system operations affect their concentrations. Such training should also include guidance on meeting water quality monitoring and reporting requirements, on how to interpret monitoring results, and on actions that should be taken when "positive" hits are detected (such as increased levels of coliforms or turbidity and decreased or depleted chlorine residuals)
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In 1991, Calderon and Mood studied the impact of point-of-entry devices containing granular activated carbon on microbial water quality and health effects. Because granular activated carbon can enhance the growth of organisms detected by HPC, counts were elevated in the treated water.
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Biofilm management is ideally accomplished by best practices that also reduce the magnitude of other water quality problems such as disinfection byproduct concentration, corrosion, and aesthetic concerns. This is true for both the utility owned distribution system and premise plumbing.
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Like water age, it has a variety of direct and indirect effects on distribution system water quality. In the committee's opinion, the most important problem exacerbated by both nitrification and by long retention times is loss of disinfectant residuals.
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See two sections below for further discussion of other, lower priority effects of water age and nitrification on distribution system water quality. LOW PRESSURE TRANSIENTS AND INTRUSION Ensuring safe distribution of treated water to consumers' taps requires, among other measures, protection from intrusion of contaminants into the distribution system during low pressure transients.
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From page 368... ...
In the event of a large intrusion of pathogens, the chlorine residual normally sustained in drinking water distribution systems may be insufficient to disinfect contaminated water, which can lead to adverse health effects. Low water pressure in distribution systems is a well-known risk factor for outbreaks of waterborne disease (Hunter, 1997)
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APPENDIX A 369 distribution system, and more rigorous applications of existing engineering standards. Although there are currently insufficient data in the literature to indicate whether intrusion from pressure transients is a substantial source of risk to treated distribution system water quality, nevertheless intrusion is inherently a subset of backflow events, has health risks and is, therefore, an important distribution system water quality maintenance and protection issue.
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From page 370... ...
Corrosion in distribution systems, as well as household plumbing, is a complex process still not adequately understood despite much research into the causes. A number of relevant water quality parameters such as disinfectant residual, redox potential, and pH are affected by water age, and these are believed to play an important role in the corrosion of pipe materials and the release of iron, copper, and lead from pipe scales, especially in low alkalinity waters.
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. However, the concentration at which nitrate nitrogen in drinking waters presents a health risk is unclear (Fewtrell, 2004)
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Although there is the potential for water quality degradation as a result of the permeation of plastic pipe, especially in the water's taste and odor aspects, the health impacts associated with such permeation are not expected to be sig
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LEACHING All materials in the water distribution system undergo reactions that introduce substances to the water via a process known as "leaching." Pipes, fittings, linings, and other materials used in joining or sealing pipes leach at least some substances to water through corrosion, dissolution, diffusion, or detachment. Internal coatings in water storage facilities can also leach substances.
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It should be noted that ANSI/NSF Standard 61, which establishes minimum health effect requirements for chemical contaminants and impurities, does not establish performance, taste and odor, or microbial growth support requirements for distribution system components. There is uncertainty regarding what the states require and systems use in terms of the application and testing of ANSI/NSF Standard 61, particularly for cement materials.
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From page 375... ...
Post precipitation can result from introduction of water to distribution systems that is super-saturated with calcium carbonate, from introduction of a phosphate corrosion inhibitor into the filter effluent of an alum coagulation plant creating an aluminum phosphate precipitate, from water that is supersaturated with aluminum hydroxide, from water that is supersaturated with selected silicate minerals, as well as other causes. Post-precipitation causes an increase in pipe roughness and a decrease in effective pipe diameter, resulting in loss of hydraulic capacity accompanied by an increase in energy required to distribute water, in production of biofilms, and in deterioration of the aesthetic quality of tap water.
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From page 376... ...
Cross connections and backflow. Cross connections and backflow events are ranked as the highest priority because of the long history of recognized health risks posed by cross connections, the clear epidemiological and surveillance data implicating these events with outbreaks or sporadic cases of waterborne disease, and the availability of proven technologies to prevent cross connections.
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From page 377... ...
Improperly maintained and operated distribution system storage facilities. Several documented waterborne disease outbreaks and the potential for contamination due to the large number of these facilities makes this a high priority distribution system water quality maintenance and protection issue.
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Intrusion from pressure transients is a subset of the crossconnection and backflow issue. It has associated health risks, and is therefore an important distribution system water quality maintenance and protection issue.
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From page 379... ...
It should be noted that the rate of degradation of distribution system materials will vary from system to system depending on water quality and system operation and maintenance practices, such that the relationship between the age of a given system, its state of deterioration, and risk cannot be easily predicted. Confronting deteriorating infrastructure requires good asset management, including procedures to monitor and assess the condition of the distribution system and water quality changes that occur during distribution.
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2002b. Effects of water age on distribution system water quality.
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2003. The Future Challenge of Controlling Distribution System Water Quality and Protecting Plumbing Infrastructure: Focusing on Consumers.
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1999. Microbial water quality in distribution systems.
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1999. Maintaining and operating finished water storage facilities.
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1990. Water quality.
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2001. National consumer water quality survey fact sheet.
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