NRC. 2006. Drinking Water Distribution Systems: Assessing and Reducing Risks Committee on Public Water Supply Distribution Systems: Assessing and Reducing Risks. National Research Council, National Academies Press.
[ According to this Free National Research Council report, most water systems and distribution pipes will be reaching the end of their expected life spans in the next 30 years.
With nearly a million miles of utility water infrastructure, 5 million miles of private home and building infrastructure, 154,000 storage facilities, and more, it will be hard to replace within 30 years, and the EPA estimated the cost would be over $205 billion dollars.
And since this 2006 report, in 2015 the EPA projected even higher costs: $384 billion over 20 years to maintain the nation’s existing drinking water systems, which will require tens of thousands of miles of replacement pipe and thousands of new or renovated plants. The American Water Works Association, an industry-backed group, puts the price even higher — $1 trillion to replace all outdated pipes and meet growth over the next quarter-century.
This is important because one of the main reasons lifespan rose above 50 years last century was clean drinking water. Residents in Flint who drank lead-poisoned water may not only have their lifespan shortened, but their quality of life reduced as well. Being able to harvest your own rainwater and store it is one way to protect yourself. Excerpts from this 404 page document follow. They are not in order. ]
U.S. Water infrastructure is falling apart (my title)
TABLE 4-7 Material Life Expectancies
|Distribution System Component||Typical Life Expectancies,
|Concrete & metal storage tanks||30|
SOURCE: EPA (2004). EPA’s Note: These expected useful lives are drawn from a variety of sources. The estimates assume that assets have been properly maintained.
The extent of water distribution pipes in the United States is estimated to be a total length of 980,000 miles (1.6 x 106 km), which is being replaced at an estimated rate of once every 200 years. Rates of repair and rehabilitation have not been estimated.
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.
In the 1920s the manufacture of iron pipes changed to improve pipe strength, but the changes also produced a thinner wall. These pipes have an expected average life of about 100 years.
Pipe manufacturing continued to evolve in the 1950s and 1960s with the introduction of ductile iron pipe that is stronger than cast iron and more resistant to corrosion. Polyvinyl chloride (PVC) pipes were introduced in the 1970s and high-density polyethylene in the 1990s. Both of these are very resistant to corrosion but they do not have the strength of ductile iron. Post-World War II pipes tend to have an expected average life of 75 years.
In the 20th century, most of the water systems and distribution pipes were relatively new and well within their expected lifespan. However, as is obvious from the above paragraph and recent reports, these different types of pipes, installed during different time periods, will all be reaching the end of their expected life spans in the next 30 years. Indeed, an estimated 26 percent of the distribution pipe in the country is unlined and in poor condition. For example, an analysis of main breaks at one large Midwestern water utility that kept careful records of distribution system management documented a sharp increase in the annual number of main breaks from 1970 (approximately 250 breaks per year) to 1989 (approximately 2,200 breaks per year). Thus, the water industry is entering an era where it must make substantial investments in pipe repair and replacement.
An EPA report on water infrastructure needs predicted that transmission and distribution replacement rates will rise to 2%/year by 2040 in order to adequately maintain the water infrastructure, which is about four times the current replacement rate.
These data on the aging of the nation’s infrastructure suggest that utilities will have to engage in regular and proactive infrastructure assessment and replacement in order to avoid a future characterized by more frequent failures, which might overwhelm the water industry’s capability to react effectively. Although the public health significance of increasingly frequent pipe failures is unknown given the variability in utility response to such events, it is reasonable to assume that the likelihood of external distribution system contamination events will increase in parallel with infrastructure failure rates.
Corrosion and leaching of pipe materials, growth of biofilms and nitrifying microorganisms, and the formation of Disenfectant By-Products (DBPs) are events internal to the distribution system that are potentially detrimental. Furthermore, most are exacerbated by increased water age within the distribution system. External contamination can enter the distribution system through infrastructure breaks, leaks, and cross connections as a result of faulty construction, backflow, and pressure transients.
Repair and replacement activities as well as permeable pipe materials also present routes for exposing the distribution system to external contamination.
All of these events act to compromise the integrity of the distribution system.
The physical integrity of the distribution system is always in a state of change, and the aging of the nation’s distribution systems and eventual need for replacement are growing concerns. Maintaining such a vast physical infrastructure is a challenge because of the complexity of individual distribution systems, each of which is comprised of a network of mains, fire hydrants, valves, auxiliary pumping or booster disinfection substations, storage reservoirs, standpipes, and service lines along with the plumbing systems in residences, large housing projects, high-rise buildings, hospitals, and public buildings. This is further complicated by factors that vary from system to system such as the size of the distribution network for the population served, the predominant pipe material and age of pipelines, water pressure, the number of line breaks each year, water storage capacity, and water supply retention time in the system.
Risks from Drinking Water
- Drinking water can serve as a transmission vehicle for a variety of hazardous agents: enteric microbial pathogens from human or animal fecal contamination (e.g., noroviruses, E. coli O157:H7, Cryptosporidium)
- aquatic microorganisms that can cause harmful infections in humans (e.g., nontuberculous mycobacteria, Legionella)
- toxins from aquatic microorganisms (such as cyanobacteria)
- several classes of chemical contaminants (organic chemicals such as benzene, polychlorinated biphenyls, and various pesticides; inorganic chemicals such as arsenic and nitrates; metals such as lead and copper
- disinfection byproducts or DBPs such as trihalomethanes
- radioactive compounds
Contaminants in drinking water can produce adverse effects in humans due to multiple routes of exposure. In addition to risk from ingestion, exposure can also occur from inhalation and dermal routes. For example, inhalation of droplets containing respiratory pathogens (such as Legionella or Mycobacterium) can result in illness. It is known that DBPs present in drinking water may volatilize resulting in inhalation risk, and these compounds (and likely other organics) may also be transported through the skin (after bathing or showering) into the bloodstream. Reaction of disinfectants in potable water with other materials in the household may also result in indoor air exposure of contaminants; for example Shepard et al. (1996) reported on release of volatile organics in indoor washing machines. Thus, multiple routes of exposure need to be considered when assessing the risk presented by contaminated distribution systems.
It has been recognized for some years that consumers face risk from multiple hazards, and that action to reduce the risk from one hazard may increase the risk from other hazards given the same exposure.
The distribution system is a critical component of every drinking water utility. Its primary function is to provide the required water quantity and quality at a suitable pressure, and failure to do so is a serious system deficiency. Water quality may degrade during distribution because of the way water is treated or not treated before it is distributed, chemical and biological reactions that take place in the water during distribution, reactions between the water and distribution system materials, and contamination from external sources that occurs because of main breaks, leaks coupled with hydraulic transients, and improperly maintained storage facilities, among other things. Furthermore, special problems are posed by the utility’s need to maintain suitable water quality at the consumers tap, and the quality changes that occur in consumers’ plumbing, which is not owned or controlled by the utility. The primary driving force for managing and regulating distribution systems is protecting the health of the consumer, which becomes more difficult as our nation’s distribution systems age and become more vulnerable to main breaks and leaks.
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,
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.
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.
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.
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.
Contamination from cross-connections and back-siphonage 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.
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.
Positive water pressure should be maintained. Low pressures in the distribution system can result not only in insufficient firefighting 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.
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 degraded 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.
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.
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,
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.
The first municipal water utility in the United States was established in Boston in 1652 to provide domestic water and fire protection. The Boston system emulated ancient Roman water supply systems in that it was multipurpose in nature. Many water supplies in the United States were subsequently constructed in cities primarily for the suppression of fires, but most have been adapted to serve commercial and residential properties with water. By 1860, there were 136 water systems in the United States, and most of these systems supplied water from springs low in turbidity and relatively free from pollution. However, by the end of the nineteenth century waterborne disease had become recognized as a serious problem in industrialized river valleys. This led to the more routine treatment of water prior to its distribution to consumers. Water treatment enabled a decline in the typhoid death rate in Pittsburgh, PA from 158 deaths per 100,000 in the 1880s to 5 per 100,000 in 1935
Similarly, both typhoid case and death rates for the City of Cincinnati declined more than tenfold during the period 1898 to 1928 due to the use of sand filtration, disinfection via chlorination, and the application of drinking water standards. It is without a doubt that water treatment in the United States has proven to be a major contributor to ensuring the nation’s public health.
DRINKING WATER DISTRIBUTION SYSTEMS: ASSESSING AND REDUCING RISKS
They span almost 1 million miles in the United States and include an estimated 154,000 finished water storage facilities. As the U.S. population grows and communities expand, 13,200 miles (21,239 km) of new pipes are installed each year.
Because distribution systems represent the vast majority of physical infrastructure for water supplies, they constitute the primary management challenge from both an operational and public health standpoint.
Their repair and replacement represent an enormous financial liability; EPA estimates the 20-year water transmission and distribution needs of the country to be $183.6 billion, with storage facility infrastructure needs estimated at $24.8 billion.
Infrastructure Distribution system infrastructure is generally considered to consist of the pipes, pumps, valves, storage tanks, reservoirs, meters, fittings, and other hydraulic appurtenances that connect treatment plants or well supplies to consumers’ taps. The characteristics, general maintenance requirements, and desirable features of the basic infrastructure components in a drinking water distribution system are briefly discussed below.
The systems of pipes that transport water from the source (such as a treatment plant) to the customer are often categorized from largest to smallest as transmission or trunk mains, distribution mains, service lines, and premise plumbing. Transmission or trunk mains usually convey large amounts of water over long distances such as from a treatment facility to a storage tank within the distribution system. Distribution mains are typically smaller in diameter than the transmission mains and generally follow the city streets. Service lines carry water from the distribution main to the building or property being served. Service lines can be of any size depending on how much water is required to serve a particular customer and are sized so that the utility’s design pressure is maintained at the customer’s property for the desired flows. Premise plumbing refers to the piping within a building or home that distributes water to the point of use. In premise plumbing the pipe diameters are usually comparatively small, leading to a greater surface-to-volume ratio than in other distribution system pipes.
The three requirements for a pipe include its ability to deliver the quantity of water required, to resist all external and internal forces acting upon it, and to be durable and have a long life. The materials commonly used to accomplish these goals today are ductile iron, pre-stressed concrete, polyvinyl chloride (PVC), reinforced plastic, and steel. In the past, unlined cast iron and asbestos cement pipes were frequently installed in distribution systems, and thus are important components of existing systems
If premise plumbing is included, the figure for total distribution system length would increase from almost 1 million miles to greater than 6 million miles.
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. Most drinking water utilities and regulatory bodies only take responsibility for the water delivered to the curb stop, which generally captures only a portion of the service line. The portion of the service line not under control of the utility and all of the premise plumbing are entirely the building owner’s responsibility.
A grid/looped system, which consists of connected pipe loops throughout the area to be served, is the most widely used configuration in large municipal areas. In this type of system there are several pathways that the water can follow from the source to the consumer. Looped systems provide a high degree of reliability should a line break occur because the break can be isolated with little impact on consumers outside the immediate area. Also, by keeping water moving looping reduces some of the problems associated with water stagnation, such as adverse reactions with the pipe walls, and it increases fire-fighting capability. However, loops can be dead-ends, especially in suburban areas like cul-de-sacs, and have associated water quality problems. Most systems are a combination of both looped and branched portions.
Transmission mains are spaced from 1.5 to 2 miles (2,400 to 3,200 m) apart with dual-service mains spaced 3,000 to 4,000 feet (900 to 1,200 m) apart. Service mains should be located in every street.
Storage Tanks and Reservoirs
Storage tanks and reservoirs are used to provide storage capacity to meet fluctuations in demand (or shave off peaks), to provide reserve supply for firefighting use and emergency needs, to stabilize pressures in the distribution system, to increase operating convenience and provide flexibility in pumping, to provide water during source or pump failures, and to blend different water sources. The recommended location of a storage tank is just beyond the center of demand in the service area. Elevated tanks are used most frequently, but other types of tanks and reservoirs include in-ground tanks and open or closed reservoirs. Common tank materials include concrete and steel. An issue that has drawn a great deal of interest is the problem of low water turnover in these facilities resulting in long detention times. 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. Improper mixing can lead to stratification and large stagnant (dead) zones within the bulk water volume that have depleted disinfectant residual. As discussed later in this report, neither historical designs nor operational procedures have adequately maintained high water quality in storage.
Security is an important issue with both storage tanks and pumps because of their potential use as a point of entry for deliberate contamination of distribution systems.
Pumps are used to impart energy to the water in order to boost it to higher elevations or to increase pressure. Pumps are typically made from steel or cast iron. Most pumps used in distribution systems are centrifugal in nature, in that water from an intake pipe enters the pump through the action of a “spinning impeller” where it is discharged outward between vanes and into the discharge piping. The cost of power for pumping constitutes one of the major operating costs for a water supply.
The two types of valves generally utilized in a water distribution system are isolation valves (or stop or shutoff valves) and control valves. Isolation valves (typically either gate valves or butterfly valves) are used to isolate sections for maintenance and repair and are located so that the areas isolated will cause a minimum of inconvenience to other service areas. Maintenance of the valves is one of the major activities carried out by a utility. Many utilities have a regular valve-turning program in which a percentage of the valves are opened and closed on a regular basis. It is desirable to turn each valve in the system at least once per year. The implementation of such a program ensures that water can be shut off or diverted when needed, especially during an emergency, and that valves have not been inadvertently closed. Control valves are used to control the flow or pressure in a distribution system. They are normally sized based on the desired maximum and minimum flow rates, the upstream and downstream pressure differentials, and the flow velocities. Typical types of control valves include pressure-reducing, pressure-sustaining, and pressure-relief valves; flow-control valves; throttling valves; float valves; and check valves. Most valves are either steel or cast iron, although those found in premise plumbing to allow for easy shut-off in the event of repairs are usually brass. They exist throughout the distribution system and are more widely spaced in the transmission mains compared to the smaller-diameter pipes. Other appurtenances in a water system include blow-off and air-release/vacuum valves, which are used to flush water mains and release entrained air. On transmission mains, blow-off valves are typically located at every low point, and an air release/vacuum valve at every high point on the main. Blow-off valves are sometimes located near dead ends where water can stagnate or where rust and other debris can accumulate. Care must be taken at these locations to prevent unprotected connections to sanitary or storm sewers.
Hydrants are primarily part of the firefighting aspect of a water system. Proper design, spacing, and maintenance are needed to insure an adequate flow to satisfy fire-fighting requirements. 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. Fire hydrants are installed in areas that are easily accessible by fire fighters and are not obstacles to pedestrians and vehicles. In addition to being used for firefighting, hydrants are also for routine flushing programs, emergency flushing, preventive flushing, testing and corrective action, and for street cleaning and construction projects. Infrastructure Design and Operation The function of a water distribution system is to deliver water to all customers of the system in sufficient quantity for potable drinking water and fire protection purposes, at the appropriate pressure, with minimal loss, of safe and acceptable quality, and as economically as possible. To convey water, pumps must provide working pressures, pipes must carry sufficient water, storage facilities must hold the water, and valves must open and close properly. Indeed, the carrying capacity of a water distribution system is defined as its ability to supply adequate water quantity and maintain adequate pressure (Male and Walski, 1991). Adequate pressure is defined in terms of the minimum and maximum design pressure supplied to customers under specific demand conditions. The maximum pressure is normally in the range of 80 to 100 psi; for example, the Uniform Plumbing Code requires that water pressure not exceed 80 psi (552 kPa) at service connections, unless the service is provided with a pressure-reducing valve. The minimum pressure during peak hours is typically in the range of 40 to 50 psi (276–345 kPa), while the recommended minimum pressure during fire flow is 20 psi (138 kPa).
Residential Drinking Water Provision
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 for indoor and outdoor purposes. Mayer et al. (1999) evaluated 1,188 homes from 14 cities across six regions of North America and found that 42 percent of annual residential water use was for indoor purposes and 58 percent for outdoor purposes. Outdoor water use varies quite significantly from region to region and includes irrigation. Of the indoor water use, less than 20 percent is for consumption or related activities, as shown below:
- Human Consumption or Related Use – 17.1 %……
- Faucet use – 15.7 %
- Dishwasher – 1.4 %
- Human Contact Only – 18.5 %……………………
- Shower – 16.8 %
- Bath – 1.7 %
- Non-Human Ingestion or Contact Uses – 64.3 %…
- Toilet – 26.7 %
- Clothes Washer – 21.7 %
- Leaks – 13.7 %
- Other – 2.2 %
Most of the water supplied to residences is used primarily for laundering, showering, lawn watering, flushing toilets, or washing cars, and not for consumption. Nonetheless, except in a few rare circumstances, distribution systems are assumed to be designed and operated to provide water of a quality acceptable for human consumption. Normal household use is generally in the range of 200 gallons per day (757 L per day) with a typical flow rate of 2 to 20 gallons per minute (gpm) [7.57–75.7 L per minute (Lpm)]; fire flow can be orders of magnitude greater than these levels, as discussed below.
Fire Flow Provision
Besides providing drinking water, a major function of most distribution systems is to provide adequate standby fire flow,
Fire-flow requirements for a single family house vary from 750 to 1,500 gpm
The duration for which these fire flows must be sustained normally ranges from 3 to 8 hours. In order to satisfy this need for adequate standby capacity and pressure, most distribution systems use standpipes, elevated tanks, and large storage reservoirs. Furthermore, the sizing of water mains is partly based on fire protection requirements set by the Insurance Services Office. (The minimum flow that the water system can sustain for a specific period of time governs its fire protection rating, which then is used to set the fire insurance rates for the communities that are served by the system.) As a consequence, fire-flow governs much of the design of a distribution system, especially for smaller systems. A study conducted by the American Water Works Association Research Foundation confirmed the impact of fire-flow capacity on the operation of, and the water quality in, drinking water networks. It found that although the amount of water used for firefighting is generally a small percentage of the annual water consumed, the required rates of water delivery for firefighting have a significant and quantifiable impact on the size of water mains, tank storage volumes, water age, and operating and maintenance costs. Generally nearly 75 percent of the capacity of a typical drinking water distribution system is devoted to fire fighting.
The effect of designing and operating a system to maintain adequate fire flow and redundant capacity is that there are long transit times between the treatment plant and the consumer, which may be detrimental to meeting drinking water MCLs. Snyder et al. (2002) recommended that water systems evaluate existing storage tanks to determine if modification or elimination of the tanks was feasible. Water efficient fire suppression technologies exist that use less water than conventional standards. In particular, the universal application of automatic sprinkler systems provides the most proven method for reducing loss of life and property due to fire, while at the same time providing faster response to the fire and requiring significantly less water than conventional fire-fighting techniques. Snyder et al. (2002) also recommended that the universal application of automatic fire sprinklers be adopted by local jurisdictions for homes as well as in other buildings. There is a growing recognition that embedded designs in most urban areas have resulted in distribution systems that have long water residence times due to the large amounts of storage required for firefighting capacity. More than ten years ago, Clark and Grayman (1992) expressed concern that long residence times resulting from excess capacity for firefighting and other municipal uses would also provide optimum conditions for the formation of DBPs and the regrowth of microorganisms. They hypothesized that eventually the drinking water industry would be in conflict over protecting public health and protecting public safety.
Because existing water distribution systems are designed primarily for fire protection, the majority of the distribution system uses pipes that are much larger than would be needed if the water was intended only for personal use. 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. This has the unfortunate side effect of increasing DBP formation as well as taste and odor problems, which contribute to the public’s perception that the water quality is poor. Furthermore, large pipes are generally cement-lined or unlined ductile iron pipe typically with more than 300 joints per mile. These joints are frequently not water tight, leading to water losses as well as providing an opportunity for external contamination of finished water.
From an engineering perspective it seems intuitively obvious that it is most efficient to satisfy all needs by installing one pipe and to minimize the number of pipe excavations. This philosophy worked well in the early days of water system development. However, it has resulted in water systems with long residence times (and their negative consequences) under normal water use patterns and a major investment in above-ground (pumps and storage tanks) and belowground (transmission mains, distribution pipes, service connections, etc.) infrastructure. Therefore as suggested in Okun (2005) it may be time to look at alternatives for supplying the various water needs in urban areas such as dual distribution systems.
However, the creation of dual distribution systems necessitates the retrofitting of an existing water supply system and reliance on existing pipes to provide non-potable supply obtained from wastewater or other sources. Large costs would be incurred when installing the new, small diameter pipe for potable water, disconnecting the existing system from homes and other users so that it could be used reliably for only non-potable needs, and other retrofitting measures.
The potential for cross connections or misuse of water supplies of lesser quality is greatly increased in dual distribution systems and decentralized treatment.
Water System Diversity
Water utilities in the United States vary greatly in size, ownership, and type of operation. The SDWA defines public water systems as consisting of community water supply systems; transient, non-community water supply systems; and non-transient, non-community water supply systems. A community water supply system serves year-round residents and ranges in size from those that serve as few as 25 people to those that serve several million. A transient, non-community water supply system serves areas such as campgrounds or gas stations where people do not remain for long periods of time. A non-transient, non-community water supply system serves primarily non-residential customers but must serve at least 25 of the same people for at least six months of the year (such as schools, hospitals, and factories that have their own water supply).
There are 159,796 water systems in the United States that meet the federal definition of a public water system (EPA, 2005b). Thirty-three (33) percent (52,838) of these systems are categorized as community water supply systems, 55 percent are categorized as transient, non-community water supplies, and 12 percent (19,375) are non-transient, non-community water systems. Overall, public water systems serve 297 million residential and commercial customers. Although the vast majority (98 percent) of systems serves less than 10,000 people, almost three quarters of all Americans get their water from community water supplies serving more than 10,000 people. Not all water supplies deliver water directly to consumers, but rather deliver water to other supplies. Community water supply systems are defined as “consecutive systems” if they receive their water from another community water supply through one or more interconnections
Some utilities rely primarily on surface water supplies while others rely primarily on groundwater. Surface water is the primary source of 22 percent of the community water supply systems, while groundwater is used by 78 percent of community water supply systems. Of the non-community water supply systems (both transient and non-transient), 97 percent are served by groundwater. Many systems serve communities using multiple sources of supply such as a combination of groundwater and/or surface water sources. This is important because in a grid/looped system, the mixing of water from different sources can have a detrimental influence on water quality, including taste and odor, in the distribution system.
Water supply systems serving cities and towns are generally administered by departments of municipalities or counties (public systems) or by investor owned companies (private systems). Public systems are predominately owned by local municipal governments, and they serve approximately 78 percent of the total population that uses community water supplies. Approximately 82 percent of urban water systems (those serving more than 50,000 persons) are publicly owned. There are about 33,000 privately owned water systems that serve the remaining 22 percent of people served by community water systems. Private systems are usually investor-owned in the larger population size categories but can include many small systems as part of one large organization. In the small- and medium-sized categories, the privately owned systems tend to be owned by homeowners associations or developers.
Infrastructure Viability over the Long Term
For the purposes of this report, distribution system integrity is defined as having three basic 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. 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. Factors important in maintaining the physical integrity of a distribution system include the maintenance of the distribution system components, such as the protection of pipes and joints against internal and external corrosion and the presence of devices to prevent cross-connections and backflow. Hydraulic integrity depends on, for example, proper system operation to minimize residence time and on preventing the encrustation and tuberculation of corrosion products and biofilms on the pipe walls that increase hydraulic roughness and decrease effective diameter. Maintaining water quality integrity in the face of internal contamination can involve control of nitrifying organisms and biofilms via changes in disinfection practices.
Older industrial cities in the northeast and Midwest United States no longer have industries that use high volumes of water, and they have also experienced major population shifts from the inner city to the suburbs. As a consequence, the utilities have an overcapacity to produce water, mainly in the form of oversized mains, at central locations, while needing to provide water to suburbs at greater distances from the treatment plant. Both factors can contribute to problems associated with high water residence times in the distribution system.
Currently, 51 organic chemicals, 16 inorganic chemicals, seven disinfectants and disinfection byproducts (DBPs), four radionuclides, and coliform bacteria are monitored for compliance with the SDWA. The SDWA does not directly address distribution system contamination for most compounds.
Water Security-related Directives and Laws
Although not a new issue, security has become paramount to the water utility industry since the events of September 11, 2001. The potential for natural, accidental, and purposeful contamination of water supply has been present for decades whether in the form of earthquakes, floods, spills of toxic chemicals, or acts of vandalism.
One of most common means of contaminating distribution systems is through a cross connection. Cross connections occur when a nonpotable water source is connected to a potable water source. Under this condition contaminated water has the potential to flow back into the potable source. Backflow can occur when the pressure in the distribution system is less than the pressure in the nonpotable source, described as backsiphonage. Conditions under which backsiphonage can occur include water main breaks, firefighting demands, and pump failures. Backflow can also occur when there is increased pressure from the nonpotable source that exceeds the pressure in the distribution system, described as backpressure. Backpressure can occur when industrial operations connected to the potable source are exerting higher internal pressure than the pressure in the distribution system or when irrigation systems connected to the potable system are pumping from a separate water source and the pump pressure exceeds the distribution system pressure.
Some states rely solely on plumbing codes to address cross connections and backflow, which is problematic because plumbing codes, in most cases, do not require testing and follow-up inspections of backflow prevention devices.
Houses are built to code but many fall out of compliance due to age and as the code changes. In addition there are no organizations that advise homeowners on how to maintain their plumbing systems such as when flushing is necessary, water temperature recommendations, home treatment devices, etc. (Chaney, 2005).
The barrier must be non-permeable since contaminants can enter through breaks or failures in materials as well as through the materials themselves. Table 4-1 gives examples of the infrastructure components that constitute this physical barrier, what they protect against, and the materials of which they are commonly constructed. A variety of components and materials make up this physical barrier. Four major component types are delineated and referred to repeatedly in this chapter: (1) pipes including mains, services lines, and premise plumbing; (2) fittings and appurtenances such as crosses, tees, ells, hydrants, valves, and meters;
TABLE 4-1 Infrastructure Components, What They Protect Against, and Common Materials
- Pipe. Protects Against Soil, groundwater, sewer exfiltration, surface runoff, human activity, animals, insects, and other life forms. Materials: Asbestos cement, reinforced concrete, steel, lined and unlined cast iron, lined and unlined ductile iron, PVC, polyethylene and HDPE, galvanized iron, copper, polybutylene
- Pipe wrap and coatings. Supporting role in that it preserves the pipe integrity. Material: Polyethylene, bitumastic, cement-mortar
- Pipe linings. Supporting role in that it preserves the pipe integrity. Materials: Epoxy, urethanes, asphalt, coal tar, cement-mortar, plastic inserts
- Service lines. Protects from Soil, groundwater, sewer exfiltration, surface runoff, human activity, animals, insects, and other life forms. Materials: Galvanized steel or iron, lead, copper, chlorinated PVC, crosslinked polyethylene, polyethylene, polybutylene, PVC, brass, cast iron
- Premise (home and building) plumbing. Protects against Air contamination, human activity, sewage and industrial non-potable water. Materials: Copper, lead, galvanized steel or iron, iron, steel, chlorinated PVC, PVC, cross-linked polyethylene, polyethylene, polybutylene
- Fittings and appurtenances (meters, valves, hydrants, ferrules). Protects against Soil, groundwater, sewer exfiltration, surface runoff, human activity, animals, insects, and other life forms. Materials: Brass, rubber, plastic
- Storage facility walls, roof, cover, vent hatch. Protects against Air contamination, rain, algae, surface runoff, human activity, animals, birds, and insects. Materials: Concrete, steel, asphaltic, epoxy, plastics
- Backflow prevention devices. Protects against Nonpotable water. Materials: Brass, plastic
- Gaskets and joints. Protects against Soil, groundwater, sewer exfiltration, surface runoff, human activity, animals, insects, and other life forms. Materials: Rubber, leadite, asphaltic,
Cast iron pipe (lined or unlined) has been largely phased out due to its susceptibility to both internal and external corrosion and associated structural failures. Ductile-iron pipe (with or without a cement lining) has taken its place because it is durable and strong, has high flexural strength, and has good resistance to external corrosion from soils. It is, however, quite heavy, it might need corrosion protection in certain soils, and it requires multiple types of joints. Concrete, asbestos cement, and polyvinyl chloride (PVC) plastic pipe have been used to replace metal pipe because of their relatively good resistance to corrosion. Polyethylene pipe is growing in use, especially for trenchless applications like slip lining, pipe bursting, and directional drilling. High-density polyethylene pipe is the second most commonly used pipe. It is tough, corrosion resistant both internally and externally, and flexible. The manufacturer estimates its service life to be 50 to 100 years
FACTORS CAUSING LOSS OF PHYSICAL INTEGRITY
Losses in physical integrity are caused by an abrupt or gradual alteration in the structure of the material barrier between the external environment and the drinking water, by the absence of a barrier, or by the improper installation or use of a barrier. These mechanisms are summarized in Table 4-2 (which shows that failure is cause by factors such as: Corrosion, permeation, too high internal water pressure or surges, shifting earth, exposure to UV light, stress from overburden, temperature fluctuations, freezing, natural disasters, aging and weathering.
Infrastructure components break down or fail over time due to chemical interactions between the materials and the surrounding environment, eventually leading to holes, leaks, and other breaches in the barrier. These processes can occur over time scales of days to decades, depending on the materials and conditions present. For example, plastic pipes can be very rapidly compromised by nearby hydrophobic compounds (e.g., solvents in the vadose zone that result from surface or subsurface contamination), with the resulting permeation of those compounds into the distribution system through the pipe materials. Both internal and external corrosion can lead to structural failure of pipes and joints, thereby allowing contaminants to infiltrate into the distribution system via leaks or subsequent main breaks. Materials failure can be hastened if the distribution system water pressure is too high, from overburden stresses on pipes, and during natural disasters. Indeed, hurricanes and earthquakes have caused extensive sudden damage to distribution systems, including broken service lines and fire hydrants, pipes disconnected or broken by the uprooting of trees, cracks in cement water storage basins, and seam separations in steel water storage tanks
A second major contributor to the loss of physical integrity is when certain critical components are absent, either by oversight or due to vandalism. For example, the absence of backflow prevention devices and covers for storage facilities can allow external contaminants to enter distribution systems.
Finally, human activity involving distribution system materials can allow contamination to occur such as through unsanitary repair and replacement practices, unprotected access to materials, or the improper handling of materials leading to unintentional damage. One must even consider the installation of flawed materials, which might, for example, be brought about because of a lack of protection of materials during storage and handling. Structural Failure of Distribution System Components Metallic pipe failures are divided generally into two categories: corrosion failures and mechanical failures. Common types of failures for iron mains include: • Bell splits or cracks that require cutting out the joint and replacing it with a mechanical fitting; these are typical for leadite joints • Splits at tees and offsets and other fittings that require replacement • Circumferential cracks or round cracks and holes, more typical in smaller diameter pipe (< 10 in.). These can result from a lack of soil support, causing the pipe to be called upon to act as a beam • Splits or longitudinal cracks or spiral cracks that will blow out. Longitudinal cracks are more common for larger pipe (> 12 in.) and can result from crushing under external loads or from excessive internal pressure • Spiral failures in medium diameter pipe • Shearing failures in large diameter pipe • Pinholes (corrosion hole) caused by internal corrosion • Tap or joint blowout • Crushed pipe
A simpler categorization can be found in Romer et al. (2004), who summarized three types of pipe failures as weeping failures, pipe breaks, and sudden failures. A weeping failure is where a leak allows an unnoticeable exchange of water to and from the surrounding soil. A pipe break includes a hole in the pipe or a disengagement of a bell-and-spigot joint. A sudden failure is the bursting of a pipe wall or shear of the pipe cross section, as would occur for a concrete pipeline, or a blow out, which refers to a complete break in a pipe. Pipe breaks can occur for a myriad of reasons such as normal materials deterioration, joint problems, movement of earth around the pipe, freezing and thawing, internal and external corrosion, stray DC currents, seasonal changes in internal water temperature, heavy traffic overhead including accidents that damage fire hydrants, changes in system pressure, air entrapment, excessive overhead loading, insufficient surge control (such as with water hammer and pressure transients), and errors in construction practices
One overriding factor in determining the potential for pipe failure is the force exerted on the water main. Contributors to this force include changes in temperature, which cause contraction and expansion of the metal and the surrounding soil, the weight of the soil over the buried main, and vibrations on the main caused by nearby activities such as traffic. An important consideration in this regard is the erosion potential of the supporting soil beneath the buried main. In the construction of a main, special sand and soil can be laid beneath it to help it bear external forces. But the movement of water in the ground beneath the main can wash away the finer material and create small or large caverns under the pipe. The force now bearing down on top of the pipe must be taken by the pipe itself, without the help of supporting material underneath. If these forces exceed the strength of the pipe, the main breaks. Most often these breaks occur at the weakest part of the main, i.e., the joint.
The factors that cause pipe failures can compound one another, hastening the process. For example, if a main develops small leaks because of corrosion, water within the distribution system can exfiltrate into the area surrounding the pipe, eroding away the supporting soil. Leakage that undermines the foundation of a water main can also occur from nearby sewer lines, go on essentially unnoticed, and eventually lead to water main collapse
Table 4-3 summarizes common problems that lead to pipe failures for pipes of differing materials. These are some of the principal factors, but they are not the only factors that act individually or in combination to lead to a main break. Other factors could include a street excavation that accidentally disturbs a water main and the misuse of fire hydrants.
Other components of distribution system also experience structural failure, although they have not historically received the attention afforded to pipes.
TABLE 4-3 Most Common Problems that Lead to Pipe Failure for Various Pipe Materials Pipe Material (common sizes) PVC and Polyethylene (4-36 in.) Problems Excessive deflection, joint misalignment and/or leakage, leaking connections, longitudinal breaks from stress, exposure to sunlight, too high internal water pressure or frequent surges in pressure, exposure to solvents, hard to locate when buried, damage can occur during tapping Cast/Ductile Iron (4-64 in,) (lined and unlined) Internal corrosion, joint misalignment and/or leakage, external corrosion, leaking connections, casting/manufacturing flaws Steel (4-120 in.) Internal corrosion, external corrosion, excessive deflection, joint leakage, imperfections in welded joints Asbestos-Cement (4-35 in.) Internal corrosion, cracks, joint misalignment and/or leakage, small pipe can be damaged during handling or tapping, pipe must be in proper soil, pipe is hard to locate when buried Concrete (12-16 to 144-168 in.) (prestressed or reinforced) Corrosion in contact with groundwater high in sulfates and chlorides, pipe is very heavy, alignment can be difficult, settling of the surrounding soil can cause joint leaks, manufacturing flaws
Corrosion as a Major Factor
Corrosion is the degradation of a material by reaction with the local environment. In water distribution systems, the term corrosion refers to dissolution of concrete linings and concrete pipe, as well as to the deterioration of metallic pipe and valves via redox reactions (e.g., iron pipe rusting). Degradation originating from the inside of the pipe via reactions with the potable water is termed internal corrosion. Degradation originating outside the pipe on surfaces contacting moist soil is referred to as external corrosion. Both internal and external corrosion can cause holes in the distribution system and cause loss of pipeline integrity. In some cases holes are formed directly in pipes by corrosion, as is the case with pinholes, but in many other instances corrosion weakens the pipe to the point that it will fail in the presence of forces originating from the soil environment. The type of corrosion and mode of failure causing loss of physical integrity are highly system specific. External corrosion can be exacerbated by a low soil redox potential, low soil pH, stray currents, and dissimilar metals or galvanic corrosion
Internal corrosion is influenced by pH, alkalinity, disinfectant type and dose, type of bacteria present in biofilms, velocity, water use patterns, use of inhibitors, and many other factors.
Some utilities have tried to avoid the issue by using plastic pipe. Even so, unprotected metal materials are regularly used at the present time, illustrating the water industry’s lack of attention to the problem. According to Romer et al. (2004), “approximately 72 percent of the materials reported in use for water mains are iron pipe, approximately two-thirds of the reported corrosion is in corrosive soils, and approximately two-thirds of the corrosion is on the pipe barrel.” In addition, metallic or cementitious pipe are often designed on the basis of their hydraulic capabilities first and foremost, and corrosion resistance is often a secondary consideration. The annual direct costs of corrosion are estimated to be $5 billio for the main distribution system (not counting premise plumbing).
Issues with Service Lines
Recent evidence indicates that service lines (the piping between the water main and the customer’s premises) and their fittings and connections (ferrules, curb stops, corporation stops, valves, and meters) can account for a significant proportion of the leaks in a distribution system
Many galvanized and lead pipe service lines are being replaced with copper or plastic pipe (chlorinated polyvinyl chloride or CPVC). CPVC and copper each have their benefits and weaknesses. Installation of CPVC requires less skill compared to installation of copper, although if workers are not careful installation can result in cracking and damage to CPVC pipe. CPVC is better for corrosive soils and waters, while copper is more resistant to internal biofilm growth. Buried CPVC pipe is difficult to locate compared to metal or copper pipe because it does not conduct electrical current for tracing. CPVC can impart a “plastic” flavor to water while the copper pipe can impart a “metallic” flavor. With CPVC, low levels of vinyl chloride can leach into the water.
Permeation refers to a mechanism of pipe failure in which contaminants external to the pipe materials and non-metallic joints compromise the structural integrity of the materials and actually pass through them into the drinking water. Permeation is generally associated with plastic pipes and with chemical solvents such as benzene, toluene, ethylbenzene, and xylenes (BTEX) and other hydrocarbons associated with oil and gasoline, all of which are easily detected using volatile organic chemical gas chromatography analyses. These chemicals can readily diffuse through the plastic pipe matrix, alter the plastic material, and migrate into the water within the pipe. Such compounds are common in soils surrounding gasoline spills (leaking storage tanks), at abandoned industrial sites, and near bulk chemical storage, electroplaters, and dry cleaners
Human Activities that Lead to Contamination. A second major cause of physical integrity loss is human activity surrounding construction, repair, and replacement that can introduce contamination into the distribution system. Any point where the water distribution system is opened to the atmosphere is a potential source of contamination. This is particularly relevant when laying new pipes, engaging in pipe repairs, and rehabilitating sites.
The average number of main repairs a year for a single utility ranges from 66 to 901 (which corresponds to 7.9–35.6 repairs per 100 miles of pipe per year), it is clear that exposure of the distribution system to contamination during repair is an inescapable reality.
TABLE 4-4 Potential for Contaminant Entry during Water Main Activities Activity Broken service line fills trench during installation Pipe gets dirty during storage before installation Trench dirt gets into pipe during installation Rainwater fills trench during installation Street runoff gets into pipe before installation Pipe is delivered dirty Trash gets into pipe before installation Vandalism occurs at the site Animals get into pipe before installation
The installation process for buried pipe is not the only place where contamination can occur. The storage of pipe, pipe fittings, and valves along roadways or in pipe yards prior to installation can expose them to contamination from soil, storm water runoff, and pets and wildlife. Damage to pipes prior to their installation is also possible, such as during pipe storage and handling or actual manufacturing defects such as surface impurities or nicks.
Similar issues surface for storage facilities that do not have adequate protection to prevent their contamination. There are 154,000 treated water storage facilities in the United States encompassing a variety of types including elevated tanks, standpipes, open and covered reservoirs, underground basins, and hydropneumatic storage tanks. Storage facilities are susceptible to external contamination from birds, insects, other animals, wind, rain, and algae. Indeed, coliform occurrences have been associated with birds roosting in the vent ports of covered water reservoirs. This is most problematic for uncovered storage facilities, although storage facilities with floating covers are also susceptible to bacterial contamination due to rips in the cover from ice, vandalism, or normal operation. Even with covered storage facilities, contaminants can gain access through improperly sealed access openings and hatches or faulty screening of vents and overflows.
The general rule is that there should be a horizontal separation of at least 10 ft (3 m) between water and sewer lines, and that the water line should be at least 1 ft (0.3 m) above the sewer (although variations to this general rule may occur from state to state). This rule, however, is fairly recent in comparison to the average age of the nation’s buried infrastructure.
Birds, and consequently bird excrement, are probably the biggest concern for storage tanks and reservoirs with floating covers. Sea gulls, for example, can be found roosting at storage facilities. Open reservoirs also offer the opportunity for detrimental changes in water quality because of exposure to the atmosphere or sunlight, such as changes in pH, dissolved oxygen, and algal growth. Even when covered, storage facilities can suffer from algal growth on the tops of floating covers that can gain entry into the tank through rips and tears or missing hatches. Algae can also be airborne or carried by birds and gain entry into storage tanks through open hatches and vents. Algae increase the chlorine demand of the stored water, reduce its oxygen content upon their degradation, affect taste and odor, and in some cases release byproducts. Chemical contaminants gain access to storage facilities via air pollution and surface-water runoff into open storage reservoirs. For example, accidental spills of chemicals during truck transport on highways adjacent to reservoirs are a potential threat, and can be very serious if the chemicals are present in a concentrated form and highly toxic. Surface-water runoff into open reservoirs can also introduce pesticides, herbicides, fertilizers, silt, and humic materials from nearby land. The potential for chemical contamination of storage facilities continues to be overlooked in regulations in comparison to microbial contamination.
Even a water utility with a good program of corrosion control and pipe replacement can experience an annual pipe break rate of around 750 to 850 breaks per year
The hydraulic integrity of a water distribution system is defined as its ability to provide a reliable water supply at an acceptable level of service—that is, meeting all demands placed upon the system with provisions for adequate pressure, fire protection, and reliability of uninterrupted supply (Cesario, 1995; AWWA, 2005). Water demand is the driving force for the operation of municipal water systems.
From an infrastructure perspective, a water distribution system is an elaborate conveyance structure in which pumps move water through the system, control valves allow water pressure and flow direction to be regulated, and reservoirs smooth out the effects of fluctuating demands (flow equalization) and provide reserve capacity for fire suppression and other emergencies. All these distribution system components and their operations and complex interactions can produce significant variations in critical hydraulic parameters, such that many opportunities exist for the loss of hydraulic integrity and degradation of service. This, in turn, may lead to serious water quality problems, some of which may threaten public health. One of the most critical components of hydraulic integrity is the maintenance of adequate pressure, defined in terms of the minimum and maximum design pressure supplied to customers under specific demand conditions. Low pressures, caused for example by failure of a pump or valve, may lead to inadequate supply and reduced fire suppression capability or, in the extreme, intrusion of potentially contaminated water. High pressures will intensify wear on valves and fittings and will increase leakage and may cause additional leaks or breaks with subsequent repercussions on water quality. High pressures will also increase external load on water heaters and other fixtures. Pipes and pumps must be sized to overcome the head loss caused by friction at the pipe walls and thus to provide acceptable pressure under specific demands, while sizing of control valves is based on the desired flow conditions, velocity, and pressure differential. A related need is to ensure that pressure fluctuations associated with surge conditions are kept below an acceptable limit. Excessive pressure surges generate high fluid velocity fluctuations and may cause resuspension of settled particles as well as biofilm detachment. A second element of hydraulic integrity is the reliability of supply, which refers to the ability of the system to maintain the desirable flow rate even when components are out of service (e.g., facility outage, pipe break) and is normally accomplished by providing redundancy in the system. Examples include looping of the pipe network and the development of backup sources to ensure multiple delivery points to all areas.
Pipe deterioration resulting in leaks or breaks can lead to a loss of hydraulic integrity because adequate pressures can no longer be maintained.
Aging pipe infrastructure and chronic water main breaks are a common problem for many water utilities. Analysis of water industry data showed that on average, main breaks occur 700 times per day in the United States
Pressure Transients and Changes in Flow Regime
Rapid changes in pressure and flow caused by events such as rapid valve closures or pump stoppages and hydrant flushing can create pressure surges of excessive magnitude. These transient pressures, which are superimposed on the normal static pressures present in the water line at the time the transient occurs, can strain the system leading to increased leakage and decreased system reliability, equipment failure, and even pipe rupture in extreme cases.
High-flow velocities can remove protective scale and tubercles, which will increase the rate of corrosion. Uncontrolled pump shutdown can lead to the undesirable occurrence of water-column separation, which can result in catastrophic pipeline failures due to severe pressure rises following the collapse of the vapor cavities.
Vacuum conditions can create high stresses and strains that are much greater than those occurring during normal operating regimes. They can cause the collapse of thin-walled pipes or reinforced concrete sections, particularly if these sections were not designed to withstand such strains. In less drastic cases, strong pressure surges may cause cracks in internal lining, damage connections between pipe sections, and destroy or cause deformation to equipment such as pipeline valves, air valves, or other surge protection devices. Sometimes the damage is not realized at the time, but may cause the pipeline to collapse in the future, especially if combined with repeated transients. Transient pressure and flow regimes are inevitable. All systems will, at some time, be started up, switched off, or undergo rapid flow changes such as those caused by hydrant flushing, and they will likely experience the effects of human errors, equipment breakdowns, earthquakes, or other risky disturbances
Gullick et al. (2004) studied intrusion occurrences in distribution systems and observed 15 surge events that resulted in a negative pressure. Most were caused by the sudden shutdown of pumps at a pump station because of either unintentional (e.g., power outages) or intentional (e.g., pump stoppage or startup tests) circumstances. Friedman et al. (2004) confirmed that negative pressure transients can occur in the distribution system and that the intruded water can travel downstream from the site of entry. Locations with the highest potential for intrusion were sites experiencing leaks and breaks, areas of high water table, and flooded air-vacuum valve vaults.
Examples of emergency situations include earthquakes, hurricanes, power failures, equipment failures, or transmission main failures. All these activities can result in a reduction in system capacity and supply pressure and changes to the flow paths of water within the distribution system.
Another function of SCADA is the ability to monitor and remotely control local conditions of water system components based on any desired range of operating conditions or set points. For example, a pump can be set to turn on or off automatically when the pressure at a critical location or the water level in a reservoir drops to a specified lower limit or goes above a specified upper limit. Alarms can be set to alert operators when a fault within the system equipment (e.g., equipment operating out of its normal range or overheating of a pump) and any breach in the system hydraulic integrity is detected. For example, extreme fluctuations in pressure and flow readings could result from pressure surges generated from a power failure at a pump station. SCADA could then divert water to the affected region from a different pump station, thus ensuring adequate supply and fire flow protection.
SCADA systems also contain pertinent system operational information required for water distribution network modeling (Cesario, 1995), such as the boundary conditions (e.g., tank water levels, valve and pump statuses and settings) for the network model as well as local flow and pressure conditions.
Water Quality Integrity
As discussed in Chapters 4 and 5, 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 degraded due to transformations that take place within piping, tanks, and premise plumbing. Most measurements of water quality taken within the distribution system cannot differentiate between the deterioration caused by externally vs. internally derived sources.
An obvious risk to public health from distribution system biofilms is the release of pathogenic bacteria. As discussed in Chapter 3, there are instances where opportunistic pathogens have been detected in biofilms, including Legionella, Aeromonas spp., and Mycobacterium spp. Assessing risk from these organisms in biofilms is complicated by the potential for two modes of transmission. Aeromonas spp. causes disease by ingestion, while the other two organisms cause the most severe forms of disease after inhalation.
Coliform Bacteria. Total coliform bacteria (a subset of Gram-negative bacteria) are used primarily as a measure of water treatment effectiveness and can occasionally be found in distribution systems. The origins of total coliform bacteria include untreated surface water and groundwater, vegetation, soils, insects, and animal and human fecal material. Typical coliform bacteria found in drinking water systems include Klebsiella pneumoniae, Enterobacter aerogenes, Enterobacter cloacae, and Citrobacter freundii. Other typical species and genera are shown in Table 3-2. Although most coliforms are not pathogenic, they can indicate the potential presence of fecal pathogens and thus in the absence of more specific data may be used as a surrogate measure of public health risk. Indeed, the presence of coliforms is the distribution system is usually interpreted to indicate an external contamination event, such as injured organism passage through treatment barriers or introduction via water line breaks, cross connections, or uncovered or poorly maintained finished water storage facilities. However, biofilms within distribution systems can support the growth and release of coliforms, even when physical integrity (i.e., breaches in the treatment plant or distribution system) and disinfectant residual have been maintained, such that their presence may not necessarily indicate a recent external contamination event. Coliform regrowth in the distribution system is more likely during the summer months when temperatures are closer to the optimum growth temperatures of these bacteria. Thermotolerant coliforms (capable of growth at 44.5 oC), also termed “fecal coliforms” have a higher association with fecal pollution than total coliforms. And Escherichia coli is considered to be even more directly related to fecal pollution as it is commonly found in the intestinal track of warm-blooded animals.
[Also of interest is TABLE 8-1 Characteristics of U.S. Public and Private Transmission Systems but I don’t have the time to add it to this post ]