Water distribution systems carry drinking water from a centralized treatment plant or well supplies to consumers’ taps. These systems consist of pipes, pumps, valves, storage tanks, reservoirs, meters, fittings, and other hydraulic appurtenances. Spanning almost 1 million miles in the United States, distribution systems represent the vast majority of physical infrastructure for water supplies, and thus constitute the primary management challenge from both an operational and public health standpoint. Public water supplies and their distribution systems range in size from those that can serve as few as 25 people to those that serve several million.
The issues and concerns surrounding the nation’s public water supply distribution systems are many. Of the 34 billion gallons of water produced daily by public water systems in the United States, approximately 63 percent is used by residential customers. More than 80 percent of the water supplied to residences is used for activities other than human consumption such as sanitary service and landscape irrigation. Nonetheless, distribution systems are designed and operated to provide water of a quality acceptable for human consumption. Another important factor is that in addition to providing drinking water, a major function of most distribution systems is to provide adequate standby fire-flow. In order to satisfy this need, most distribution systems use standpipes, elevated tanks, storage reservoirs, and larger sized pipes. The effect of designing and operating a distribution system to maintain adequate fire flow and redundant capacity is that there are longer transit times between the treatment plant and the consumer than would otherwise be needed.
The type and age of the pipes that make up water distribution systems range from cast iron pipes installed during the late 19th century to ductile iron pipe and finally to plastic pipes introduced in the 1970s and beyond. Most water systems and distribution pipes will be reaching the end of their expected life spans in the next 30 years (although actual life spans may be longer depending on utility practices and local conditions). Thus, the water industry is entering an era where it will have to make substantial investments in pipe assessment, repair, and replacement.
Most regulatory mandates regarding drinking water focus on enforcing water quality standards at the treatment plant and not within the distribution system. Ideally, there should be no change in the quality of treated water from the time it leaves the treatment plant until the time it is consumed. However, in reality substantial changes can occur to finished water as a result of complex physical, chemical, and biological reactions. Indeed, data on waterborne disease outbreaks, both microbial and chemical, suggest that distribution systems remain
a source of contamination that has yet to be fully addressed. As a consequence, the U.S. Environmental Protection Agency (EPA) has renewed its interest in water quality degradation occurring during distribution, with the goal of defining the extent of the problem and considering how it can be addressed during rule revisions or via non-regulatory channels. 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. The following statement of task guided the expert committee formed to conduct the study:
Identify trends relevant to the deterioration of drinking water in water supply distribution systems, as background and based on available information.
Identify and prioritize issues of greatest concern for distribution systems based on a review of published material.
Focusing on the highest priority issues as revealed by task #2, (a) 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. Case studies, either at the state or utility level, where distribution system control programs (e.g., Hazard Analysis and Critical Control Point System, cross-connection control, etc.) have been successfully designed and implemented will be identified and recommendations will be presented in their context.
Identify advances in detection, monitoring and modeling, analytical methods, information needs and technologies, research and development opportunities, and communication strategies that will enable the water supply industry and other stakeholders to further reduce risks associated with public water supply distribution systems.
The committee addressed tasks one and two in its first report, which is included as Appendix A to this report. The distribution system issues given highest priority were those that have a recognized health risk based on clear epidemiological and surveillance data, including cross connections and backflow; contamination during installation, rehabilitation, and repair activities; improperly maintained and operated storage facilities; and control of water quality in premise plumbing. This report focuses on the committee’s third and fourth tasks and makes recommendations to EPA regarding new directions and priorities to consider.
This report considers service lines and premise plumbing to be part of the distribution system. Premise plumbing and service lines have longer residence times, more stagnation, lower flow conditions, and elevated temperatures compared to the main distribution system, and consequently can have a profound
effect on the quality of water reaching the consumer. Also, the report focuses on traditional distribution system design, in which water originates from a centralized treatment plant or well and is then distributed through one pipe network to consumers. Non-conventional distribution system designs including decentralized treatment and dual distribution systems are only briefly considered. Such designs, which would be potentially much more complicated than traditional systems, require considerably more study regarding their economic feasibility, their maintenance and monitoring requirements, and how to transition from an existing conventional system to a non-conventional system. Nonetheless, many of the report recommendations are relevant even if an alternative distribution system design is used.
The federal regulatory framework that targets degradation of distribution system water quality is comprised of several rules under the Safe Drinking Water Act, including the Lead and Copper Rule (LCR), the Surface Water Treatment Rule (SWTR), the Total Coliform Rule (TCR), and the Disinfectants/Disinfection By-Products Rule (D/DBPR). The LCR establishes monitoring requirements for lead and copper within tap water samples, given concern over their leaching from premise plumbing and fixtures. The SWTR establishes the minimum required detectable disinfectant residual and the maximum allowed heterotrophic bacterial plate count, both measured within the distribution system. The TCR calls for distribution system monitoring of total coliforms, fecal coliforms, and/or E. coli. Finally, the D/DBPR addresses the maximum disinfectant residual and concentration of disinfection byproducts like total trihalomethanes and haloacetic acids allowed in distribution systems. A plethora of state regulations and plumbing codes also affect distribution system water quality, from requirements for design, construction, operation, and maintenance of distribution systems to cross-connection control programs.
Despite the existence of these rules, programs, and codes, current regulatory programs have not removed the potential for outbreaks attributable to distribution system-related factors. Part of this can be attributed to the fact that existing federal regulations are intended to address only certain aspects of distribution system water quality and not the integrity of the distribution system in its totality. Most contaminants that have the potential to degrade distribution system water quality are not monitored for compliance purposes, or the sampling requirements are too sparse and infrequent to detect contamination events. For example, TCR monitoring encompasses only microbiological indicators and not in real time. With the exception of monitoring for disinfectant residuals and DBPs within the distribution system and lead and copper at the customer’s tap, existing federal regulations do not address other chemical contaminants.
Although it is hoped that state regulations and local ordinances would contribute to public safety from drinking water contamination in areas where federal
regulations are weak, the considerable variation in relevant state programs makes this impossible to conclude on a general basis. For cross-connection control programs, for the design, construction, operation, and maintenance of distribution systems, and for plumbing code components, state programs range from an absolute requirement to simply encouraging a practice to no provision whatsoever. Voluntary programs do exist to fill gaps in the federal and state regulatory requirements for distribution system operation and maintenance, most notably the G200 standard of the American Water Works Association. These programs, if adopted, can help a utility organize its many activities by unifying all of the piecemeal requirements of the federal, state, and local regulations. The following select conclusions and recommendations regarding the effectiveness of existing regulations and codes and the potential for their improvement are made, with additional detail found in Chapter 2.
EPA should work closely with representatives from states, water systems, and local jurisdictions to establish the elements that constitute an acceptable cross-connection control program. State requirements for cross-connection control programs are highly inconsistent, and state oversight of such programs varies and is subject to availability of resources. If states expect to maintain primacy over their drinking water programs, they should adopt a cross-connection control program that includes a process for hazard assessment, the selection of appropriate backflow devices, certification and training of backflow device installers, and certification and training of backflow device inspectors.
Existing plumbing codes should be consolidated into one uniform national code. The two principal plumbing codes that are used nationally have different contents and permit different materials and devices. In addition to integrating the codes, efforts should be made to ensure more uniform implementation of the plumbing codes, which can vary significantly between jurisdictions and have major impacts on the degree of public health protection afforded.
For utilities that desire to operate beyond regulatory requirements, adoption of G200 or an equivalent program is recommended to help utilities develop distribution system management plans. G200 has advantages over other voluntary programs, such as HACCP, in that it is more easily adapted to the dynamic nature of drinking water distribution systems.
PUBLIC HEALTH RISK OF DISTRIBUTION SYSTEM CONTAMINATION
Three primary approaches are available to better understand the human health risks that derive from contamination of the distribution system: risk assessment methods that utilize pathogen occurrence data, waterborne disease outbreak surveillance, and epidemiology studies. Chapter 3 extensively reviews the
available information in each of these categories and its implications for determining public health risk. In the case of pathogen occurrence measurements, our understanding of the microbial ecology of distribution systems is at an early stage. Microbial monitoring methods are expensive, time consuming, require optimization for specific conditions, and currently are appropriate only for the research laboratory. Methods do not exist for routine detection and quantification of most of the microbes on the EPA’s Contaminant Candidate List. Until better methods, dose-response relationships, and risk assessment data are available, pathogen occurrence measurements are best used in conjunction with other supporting data on health outcomes, such as data on enhanced or syndromic surveillance in communities, or from microbial or chemical indicators of potential contamination.
Outbreak surveillance data currently provide more information on the public health impact of contaminated distribution systems. In fact, investigations conducted in the last five years suggest that a substantial proportion of waterborne disease outbreaks, both microbial and chemical, is attributable to problems within distribution systems. The reason for these observations is not clear; outbreaks associated with distribution system deficiencies have been reported since the surveillance system was started. However, there may be more attention focused on the distribution system now that there are fewer reported outbreaks associated with inadequate treatment of surface water. Also, better outbreak investigations and reporting systems in some states may result in increased recognition and reporting of all the risk factors contributing to the outbreak, including problems with the distribution system that may have been overlooked in the past. Contamination from cross-connections and backsiphonage were found to cause the majority of the outbreaks associated with distribution systems, followed by contamination of water mains following breaks and contamination of storage facilities. The situation may be of even greater concern because incidents involving domestic plumbing are less recognized and unlikely to be reported. In general the identified number of waterborne disease outbreaks is considered an underestimate because not all outbreaks are recognized, investigated, or reported to health authorities.
A third approach for estimating public health risk is to conduct an epidemiology study that isolates the distribution system component. The body of evidence from four epidemiological studies does not eliminate the consumption of tap water that has been in the distribution system from causing increased risk of gastrointestinal illness. However, differences between the study designs, the study population sizes and compositions and follow-up periods, and the extent of complementary pathogen occurrence measurements make comparisons difficult. Although all four cohort studies used similar approaches for recording symptoms of gastrointestinal illness, different illness rates were observed, with some more than twice as high as others. One of the major challenges for designing an epidemiology study of health risks associated with water quality in the distribution system is separating the effect of source water quality and treatment from the effect of distribution system water quality.
Although there is a lack of definitive estimates, the available information seems to be implicating contamination of the distribution system in public health risk. This is particularly true for Legionella pneumophila in water systems, for which occurrence data, outbreak data, and epidemiological data are available. In fact, since Legionella was incorporated into the waterborne disease outbreak surveillance system in 2001, several outbreaks have been attributed to the microorganism, all of which occurred in large buildings or institutional settings. As discussed in Appendix A, the committee relied on the limited available outbreak and epidemiological data as well as its best professional judgment to prioritize distribution system contamination events into high, medium, and low priority. Better public health data could help refine distribution system risks and provide additional justification for the prioritization. The following select conclusions and recommendations regarding the public health risks of distribution systems are made, with additional detail found in Chapter 3.
The distribution system is the remaining component of public water supplies yet to be adequately addressed in national efforts to eradicate waterborne disease. This is evident from data indicating that although the number of waterborne disease outbreaks including those attributable to distribution systems is decreasing, the proportion of outbreaks attributable to distribution systems is increasing. Most of the reported outbreaks associated with distribution systems have involved contamination from cross-connections and backsiphonage. Furthermore, Legionella appears to be a continuing risk and is the single most common etiologic agent associated with outbreaks involving drinking water. Initial studies suggest that the use of chloramine as a residual disinfectant may reduce the occurrence of Legionella, but additional research is necessary to determine the relationship between disinfectant usage and the risks of Legionella and other pathogenic microorganisms.
Distribution system ecology is poorly understood, making risk assessment via pathogen occurrence measurements difficult. There is very little information available about the types, activities, and distribution of microorganisms in distribution systems, particularly premise plumbing. Limited heterotrophic plate count data are available for some systems, but these data are not routinely collected, they underestimate the numbers of organisms present, and they include many organisms that do not necessarily present a health risk.
Epidemiology studies that specifically target the distribution system component of waterborne disease are needed. Recently completed epidemiological studies have either not focused on the specific contribution of distribution system contamination to gastrointestinal illness, or they have been unable to detect any link between illness and drinking water. Epidemiological studies of the risk of endemic disease associated with drinking water distribution systems need to be performed and must be designed with sufficient power and resources to adequately address the deficiencies of previous studies.
PHYSICAL, HYDRAULIC, AND WATER QUALITY INTEGRITY
One of the options being considered during revision of the TCR is that it more adequately address distribution system integrity—defined in this report as having three components: (1) physical integrity, which refers to the maintenance of a physical barrier between the distribution system interior and the external environment, (2) hydraulic integrity, which refers to the maintenance of a desirable water flow, water pressure, and water age, taking both potable drinking water and fire flow provision into account, and (3) water quality integrity, which refers to the maintenance of finished water quality via prevention of internally derived contamination. 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. Protection of public health requires that water professionals take all three integrity types into account in order to maintain the highest level of water quality.
The loss of physical integrity of the distribution system—in which the system no longer acts as a barrier that prevents external contamination from deteriorating the internal, drinking water supply—is brought about by physical and chemical deterioration of materials, the absence or improper installation of critical components, and the installation of already contaminated components. When physical integrity is compromised, the drinking water supply becomes exposed to contamination that increases the risk of negative public health outcomes. Most documented cases of waterborne disease outbreaks attributed to distribution systems have been caused by breaches in physical integrity, such as a backflow event through a cross connection or contamination occurring during repair or replacement of distribution system infrastructure. Selected conclusions and recommendations for maintaining and restoring physical integrity to a distribution system are given below. Additional detail is found in Chapter 4.
Storage facilities should be inspected on a regular basis. A disciplined storage facility management program is needed that includes developing an inventory and background profile on all facilities, developing an evaluation and rehabilitation schedule, developing a detailed facility inspection process, performing inspections, and rehabilitating and replacing storage facilities when needed. Depending on the nature of the water supply chemistry, every three to five years storage facilities need to be drained, sediments need to be removed, appropriate rust-proofing needs to be done to the metal surfaces, and repairs need to be made to structures. These inspections are in addition to daily or weekly inspections for vandalism, security, and water quality purposes (such as identifying missing vents, open hatches, and leaks).
Better sanitary practices are needed during installation, repair, replacement, and rehabilitation of distribution system infrastructure. All trades people who work with materials that are being installed or repaired and that come in contact with potable water should be trained and certified for the level of sanitary and materials quality that their work demands. Quality workmanship for infrastructure materials protection as well as sanitary protection of water and materials are critical considering the increasing costs of infrastructure failure and repair and increasingly stringent water quality standards.
External and internal corrosion should be better researched and controlled in standardized ways. There is a need for new materials and corrosion science to better understand how to more effectively control both external and internal corrosion, and to match distribution system materials with the soil environment and the quality of water with which they are in contact. At present the best defense against corrosion relies on site-specific testing of materials, soils, and water quality followed by the application of best practices, such as cathodic protection. Indeed, a manual of practice for external and internal corrosion control should be developed to aid the water industry in applying what is known. Corrosion is poorly understood and thus unpredictable in occurrence. Insufficient attention has been given to its control, especially considering its estimated annual direct cost of $5 billion in U.S. for the main distribution system, not counting premise plumbing.
Maintaining the hydraulic integrity of distribution systems is vital to ensuring that water of acceptable quality is delivered in acceptable amounts. The most critical element of hydraulic integrity is adequate water pressure inside the pipes. The loss of water pressure resulting from pipe breaks, significant leakage, excessive head loss at the pipe walls, pump or valve failures, or pressure surges can impair water delivery and will increase the risk of contamination of the water supply via intrusion. Another critical hydraulic factor is the length of time water is in the distribution system. Low flows in pipes create long travel times, with a resulting loss of disinfectant residual as well as sections where sediments can collect and accumulate and microbes can grow and be protected from disinfectants. Furthermore, sediment deposition will result in rougher pipes with reduced hydraulic capacity and increased pumping costs. Long detention times can also greatly reduce corrosion control effectiveness by impacting phosphate inhibitors and pH management. A final component of hydraulic integrity is maintaining sufficient mixing and turnover rates in storage facilities, which if insufficient can lead to short circuiting and generate pockets of stagnant water with depleted disinfectant residual. Fortunately, water utilities can achieve a high degree of hydraulic integrity through a combination of proper system design, operation, and maintenance, along with monitoring and model-
ing. The following select conclusions and recommendations are made, with additional detail found in Chapter 5.
Water residence times in pipes, storage facilities, and premise plumbing should be minimized. Excessive residence times can lead to low disinfectant residuals and leave certain service areas with a less protected drinking water supply. In addition, long residence times can promote microbial regrowth and the formation of disinfection byproducts. From an operational viewpoint it may be challenging to reduce residence time where the existing physical infrastructure and energy considerations constrain a utility’s options. Furthermore, limited understanding of the stochastic nature of water demand and water age makes it difficult to assess the water quality benefits of reduced residence time. Research is needed to investigate such questions, as well as how to achieve minimization of water residence time while maintaining other facets of hydraulic integrity (such as adequate pressure and reliability of supply).
Positive water pressure should be maintained. Low pressures in the distribution system can result not only in insufficient fire fighting capacity but can also constitute a major health concern resulting from potential intrusion of contaminants from the surrounding external environment. A minimum residual pressure of 20 psi under all operating conditions and at all locations (including at the system extremities) should be maintained.
Distribution system monitoring and modeling are critical to maintaining hydraulic integrity. Hydraulic parameters to be monitored should include inflows/outflows and water levels for all storage tanks, discharge flows and pressures for all pumps, flows and/or pressure for all regulating valves, and pressures at critical points. An analysis of these patterns can directly determine if the system hydraulic integrity is compromised. Calibrated distribution system models can calculate the spatial and temporal variations of flow, pressure, velocity, reservoir level, water age, and other hydraulic and water quality parameters throughout the distribution system. Such results can, for example, help identify areas of low or negative pressure and high water age, estimate filling and draining cycles of storage facilities, and determine the adequacy of the system to supply fire flows under a variety of conditions.
Water Quality Integrity
Breaches in physical and hydraulic integrity can lead to the influx of contaminants across pipe walls, through breaks, and via cross connections. These external contamination events can act as a source of inoculum, introduce nutrients and sediments, or decrease disinfectant concentrations within the distribution system, resulting in a degradation of water quality. Even in the absence of external contamination, however, there are situations where water quality is de-
graded due to transformations that take place within piping, tanks, and premise plumbing. These include biofilm growth, nitrification, leaching, internal corrosion, scale formation, and other chemical reactions associated with increasing water age.
Maintaining water quality integrity in the distribution system is challenging because of the complexity of most systems. That is, there are interactions between the type and concentration of disinfectants used, corrosion control schemes, operational practices (e.g., flow characteristics, water age, flushing practices), the materials used for pipes and plumbing, the biological stability of the water, and the efficacy of treatment. The following select conclusions and recommendations are made, with additional details found in Chapter 6.
Microbial growth and biofilm development in distribution systems should be minimized. Even though the general heterotrophs found in biofilms are not likely to be of public health concern, their activity can promote the production of tastes and odors, increase disinfectant demand, and may contribute to corrosion. Biofilms may also harbor opportunistic pathogens (those causing disease in the immunocompromised). This issue is of critical importance in premise plumbing where long residence times promote disinfectant decay and subsequent bacterial growth and release.
Residual disinfectant choices should be balanced to meet the overall goal of protecting public health. For free chlorine, the potential residual loss and DBP formation should be weighed against the problems that may be introduced by chloramination, which include nitrification, lower disinfectant efficacy against suspended organisms, and the potential for deleterious corrosion problems. Although some systems have demonstrated increased biofilm control with chloramination, this response has not been universal. This ambiguity also exists for the control of opportunistic pathogens.
Standards for materials used in distribution systems should be updated to address their impact on water quality, and research is needed to develop new materials that will have minimal impacts. Materials standards have historically been designed to address physical/strength properties including the ability to handle pressure and stress. Testing of currently available materials should be expanded to include (1) the potential for permeation of contaminants, and (2) the potential for leaching of compounds of public health concern as well as those that contribute to tastes and odors and support biofilm growth. Also, research is needed to develop new materials that minimize adverse water quality effects such as the high concentrations of undesirable metals and deposits that result from corrosion and the destruction of disinfectant owing to interactions with pipe materials.
INTEGRATING APPROACHES TO REDUCING PUBLIC HEALTH RISK FROM DISTRIBUTION SYSTEMS
Because only a few regulations govern water quality in distribution systems, public health protection from contamination arising from distribution system events will require that utilities independently choose to design and operate their systems beyond regulatory requirements. One voluntary standard in particular—the G200 standard for distribution system operation and management—directly addresses the issues highlighted by EPA and characterized as high priority by this committee (see Appendix A).
As for any voluntary program, it may be necessary to create incentives for utilities to adopt G200, for which several options exist. An extreme would be to create federal regulations that require adherence to a prescribed list of activities deemed necessary for reducing the risk of contaminated distribution systems; this list could partly or fully parallel the G200 standard. Another mechanism to capture elements of G200 within existing federal regulations would be via the sanitary surveys conducted by the state and required for some systems every three to five years. Sanitary surveys encompass a wide variety of activities, and could capture those felt to be of highest priority for reducing risk. Several other options are discussed, including (1) making some of the elements of G200 fall under existing federal regulations through the Government Accounting Standards Board, (2) state regulations that require adherence to G200 including building and plumbing codes and design and construction requirements, (3) linking qualification for a loan from the State Revolving Fund to a utility demonstrating that it is adhering to G200, and (4) implementation of G200 as a way to improve a drinking water utilities’ access to capital via better bond ratings.
For small water systems that are resource limited, adherence to the G200 standard or its equivalent may present financial, administrative, and technological burdens. Thus, its adoption should occur using the following guidelines: (1) implement new activities using a step-wise approach; (2) provide technical assistance, education, and training; and (3) develop regulatory, financial, and social incentives. Training materials, scaled for small-size systems, are essential for operators and maintenance crew. Public education can result in an increased awareness and emphasis on the significance of implementing proactive voluntary efforts, which could help to justify increased actions.
Certain elements of G200 deserve more thoughtful consideration because emerging science and technology are altering whether and how these elements are implemented by a typical water utility. Much of the current scientific thrust is in the development of new monitoring techniques, models, and methods to integrate monitoring data and models to inform decision making. The following select conclusions and recommendations relate specifically to these techniques and methods, with additional detail found in Chapter 7.
Distribution system integrity is best evaluated using on-line, real-time methods to provide warning against any potential breaches in sufficient
time to effectively respond and minimize public exposure. This will require the development of new, remotely operated sensors and data collection systems for continuous public health surveillance monitoring. These types of systems should be capable of accurately (with sufficient precision) determining the nature, type, and location/origin of all potential threats to distribution system integrity. The availability, reliability, and performance of on-line monitors are improving, with tools now available for detecting pressure, turbidity, disinfectant residual, flow, pH, temperature, and certain chemical parameters. Although these devices have reached the point for greater full-scale implementation, additional research is needed to optimize the placement and number of monitors.
Research is needed to better understand how to analyze data from online, real-time monitors in a distribution system. A number of companies are selling (and utilities are deploying) multiparameter analyzers. These companies, as well as EPA, are assessing numerical approaches to convert such data into a specific signal (or alarm) of a contamination event—efforts which warrant further investigation. Some of the data analysis approaches are proprietary, and there has been limited testing reported in “real world” situations. Furthermore, when multiple analyzers are installed in a given distribution system, the pattern of response of these analyzers in space provides additional information on system performance, but such spatially distributed information has not been fully utilized. To the greatest degree possible, this research should be conducted openly (and not in confidential or proprietary environments).
ALTERNATIVES FOR PREMISE PLUMBING
Premise plumbing includes that portion of the distribution system associated with schools, hospitals, public and private housing, and other buildings. It is connected to the main distribution system via the service line. The quality of potable water in premise plumbing is not ensured by EPA regulations, with the exception of the Lead and Copper Rule which assesses the efficacy of corrosion control by requiring that samples be collected at the tap after the water has been allowed to remain stagnant.
Virtually every problem previously identified in the main water transmission system can also occur in premise plumbing. However, unique characteristics of premise plumbing can magnify the potential public health risk relative to the main distribution system and complicate formulation of coherent strategies to deal with problems. These characteristics include:
a high surface area to volume ratio, which along with other factors can lead to more severe leaching and permeation;
variable, often advanced water age, especially in buildings that are irregularly occupied;
more extreme temperatures than those experienced in the main distribution system
low or no disinfectant residual, because buildings are unavoidable “dead ends” in a distribution system;
potentially higher bacterial levels and regrowth due to the lack of persistent disinfectant residuals, high surface area, long stagnation times, and warmer temperatures. Legionella in particular is known to colonize premise plumbing, especially hot water heaters;
exposure routes through vapor and bioaerosols in relatively confined spaces such as home showers;
proximity to service lines, which have been shown to provide the greatest number of potential entry points for pathogen intrusion;
higher prevalence of cross connections, since it is relatively common for untrained and unlicensed individuals to do repair work in premise plumbing;
variable responsible party, resulting in considerable confusion over who should maintain water quality in premise plumbing.
Premise plumbing is a contributor to the degradation of water quality, particularly due to microbial regrowth, backflow events, and contaminant intrusion, although additional research is needed to better understand its magnitude. In particular, more extensive sampling of water quality within premise plumbing by utilities or targeted sampling via research is required. The following detailed conclusions and recommendations are given.
Communities should squarely address the problem of Legionella, both via changes to the plumbing code and new technologies. Changes in the plumbing code such as those considered in Canada and Australia that involve mandated mixing valves would seem logical to prevent both scalding and microbial regrowth in premise plumbing water systems. On-demand water heating systems may have benefits worthy of consideration versus traditional large hot water storage tanks in the United States. The possible effects of chloramination and other treatments on Legionella control should be quantified to a higher degree of certainty.
To better assess cross connections in the premise plumbing of privately owned buildings, inspections for cross connections and other code violations at the time of property sale could be required. Such inspection of privately owned plumbing for obvious defects could be conducted during inspection upon sale of buildings, thereby alerting future occupants to existing hazards and highlighting the need for repair. These rules, if adopted by individual states, might also provide incentives to building owners to follow code and have repairs conducted by qualified personnel, because disclosure of sub-standard repair could affect subsequent transfer of the property.
EPA should create a homeowner’s guide and website that highlights the nature of the health threat associated with premise plumbing and mitigation strategies that can be implemented to reduce the magnitude of the risk. As part of this guide, it should be made clear that water quality is regulated only to the property line, and beyond that point responsibility falls mainly on consumers. Whether problems in service lines are considered to be the homeowner’s responsibility or the water utility’s varies from system to system.
Research is needed that specifically addresses potential problems arising from premise plumbing. This includes the collection of data quantifying water quality degradation in representative premise plumbing systems in geographically diverse regions and climates. In addition, greater attention should be focused on understanding the role of plumbing materials. Furthermore, the role of nutrients in distributed water in controlling regrowth should be assessed for premises. Finally, the potential impacts of representative point-of-use and point-of-entry devices need to be quantified. An epidemiological study to assess the health risks of contaminated premise plumbing should be undertaken in high risk communities.