Drinking Water Distribution Systems Assessing and Reducing Risks (2006) / Chapter Skim
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1 Introduction
1 Introduction
Pages 15-46
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From page 15... ...
. Prior to the passage of the SDWA, most drinking water utilities concentrated on meeting drinking water standards at the treatment plant, even though it had long been recognized that water quality could deteriorate in the distribution system -- the vast infrastructure downstream of the treatment plant that delivers 15
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From page 16... ...
Environmental Protection Agency (EPA) as meaning that some federal water quality standards should be met at various points within the distribution system rather than at the water treatment plant discharge.
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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 address 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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The characteristics, general maintenance requirements, and desirable features of the basic infrastructure components in a drinking water distribution system are briefly discussed below. Pipes The systems of pipes that transport water from the source (such as a treatment plant)
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. Inclusion of premise plumbing and service lines in the definition of a public water supply distribution system is not common because of their variable ownership, which ultimately affects who takes responsibility for their maintenance.
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Rubber Material Material Prestressed Concrete (RCP) Rubber Material Material Polyvinyl Chloride Rubber Material Material High Density Polyethylene Fused Material Material Molecular Oriented PVC Rubber Material Material Legends: Commercially Available Predominantly in Use FIGURE 1-2 Timeline of pipe technology in the United States.
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From page 21... ...
Much of the water volume in storage tanks is dedicated to fire protection, and unless utilities properly manage their tanks to control water quality, there can be problems attributable to both water aging and inadequate water mixing. Excessive water age can be conducive to depletion of the disinfectant residual, leading to biofilm growth, other biological changes in the water including nitrification, and the emergence of taste and odor problems.
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From page 22... ...
The cost of power for pumping constitutes one of the major operating costs for a water supply. Valves The two types of valves generally utilized in a water distribution system are isolation valves (or stop or shutoff valves)
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Fire hydrants are typically exercised and tested annually by water utility or fire department personnel. Fire flow tests are conducted periodically to satisfy the requirements of the Insurance Services Office or as part of a water distribution system calibration program (ISO, 1980)
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Fire Flow Provision Besides providing drinking water, a major function of most distribution systems is to provide adequate standby fire flow, the standards for which are governed by the National Fire Protection Association (NFPA, 1986)
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Non-conventional water distribution system designs that might address some of these issues are discussed below including decentralized treatment, dual distribution systems, and an approach that utilizes enhanced treatment to solve distribution system water quality problems. These alternative concepts were not part of the committee's statement of task, such that addressing them extensively is beyond the scope of the report.
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in which water supply is optimized by separately treating several components of water and wastewater streams using decentralized treatment units. The approach largely views water supply, treatment, and waste disposal as different aspects of the same integrated system.
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In addition to these costs, multiple or new pipe networks are a necessary part of the design framework for these satellite systems. That is, new piping would be needed from the advanced water treatment system into the household (or industry)
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From page 28... ...
Currently only about 10 percent of the network is having trouble with compliance but it is anticipated as the system expands, more and more of the network will be out of compliance. Enhanced Treatment A third approach to slowing water quality deterioration involves centralized treatment options that can improve the quality of water to such a degree that formation of DBPs and loss of disinfectant residual are minimized.
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, which operates two water treatment plants with a combined design capacity of 900 mgd (3.41 mil m3 per day)
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30 DRINKING WATER DISTRIBUTION SYSTEMS: ASSESSING AND REDUCING RISKS . Plant)
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Petersburg, Florida, a reclaimed water distribution system was placed into operation in 1976, and fire protection is provided from both the potable and reclaimed water lines. San Francisco has a nonpotable system, constructed after the 1906 earthquake, that serves the downtown area to augment fire protection.
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From page 32... ...
This leads to residence times of weeks in traditional systems versus potentially hours in a system comprised of much smaller pipes. In the absence of smaller sized distribution systems, utilities have had to implement flushing programs and use higher dosages of disinfectants to maintain water quality in distribution systems.
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From page 33... ...
For example, if a new nonpotable line is installed alongside an existing potable line, the nonpotable line can draw demand away from the potable line, thereby increasing its detention time and aggravating water quality deterioration in the potable line. Furthermore, if the potable system is still used for fire flow, which generally governs pipe sizing, many of the advantages of the dual system will not be realized.
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Recycled water itself is tertiary treated wastewater. It meets all of the water quality standards for drinking water, but it is high in salt.
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Generally, agencies that set fire flow requirements are not concerned about water quality while drinking water utilities must be concerned about both quality and fire flow capacity. It will be important to better evaluate the effectiveness of alternative fire suppression technologies including automatic sprinkler systems in a wide range of building types, including residences.
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. Owning or controlling its watershed provides an opportunity for a drinking water utility to exercise increased control of its source water quality (Peckenham et al., 2005)
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There is a large range in the type and age of the pipes that make up water distribution systems. The oldest cast iron pipes from the late 19th century are typically described as having an expected average useful lifespan of about 120 years because of the pipe wall thickness (AWWA, 2001; AWWSC, 2002)
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From page 38... ...
As shown in Figure 1-6, an EPA report on water infrastructure needs (EPA, 2002c) predicted that transmission and distribution replacement rates will rise to 2.0 percent per year by 2040 in order to adequately maintain the water infrastructure, which is about four times the current replacement rate according to Grigg (2005b)
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From page 39... ...
water quality integrity, which refers to the maintenance of finished water quality via prevention of internally derived contamination. This division is important because the three types of integrity have different causes of their loss, different consequences once they are lost, different methods for detecting and preventing a loss, and different remedies for regaining integrity.
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From page 40... ...
Additional activities are ongoing, including consideration of a revision of the TCR to provide a more comprehensive approach for addressing the integrity of the distribution system. To assist in this process, EPA requested that the National Academies' Water Science and Technology Board conduct a study of water quality issues associated with public water supply distribution systems and their potential risks to consumers.
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From page 41... ...
evaluate different approaches for characterization of public health risks posed by water quality deteriorating events or conditions that may occur in public water supply distribution systems; and (b) identify and evaluate the effectiveness of relevant existing codes and regulations and identify general actions, strategies, performance measures, and policies that could be considered by water utilities and other stakeholders to reduce the risks posed by water-quality deteriorating events or conditions.
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From page 42... ...
There is not, therefore, an in-depth discussion of drinking water treatment in the report except where it is pertinent to mitigating the risks of degraded water quality in the distribution system. For example, if the lack of disinfectant residual in the distribution system is identified as a risk, the options for mitigating that risk must first consider whether the root cause is inadequate treatment (e.g., insufficient reduction in disinfectant demand)
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From page 43... ...
1986. Introduction to Water Distribution Principles and Practices of Water Supply Operations.
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1998. Modeling water quality in drinking water distribution systems.
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2005. Integrating hydraulic and water quality models with other utility systems: a case study.
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From page 46... ...
2005. Public Water Supply Distribution Systems: Assessing and Reducing Risks, First Report.
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