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Drinking Water Distribution Systems: Assessing and Reducing Risks 4 Physical Integrity This chapter focuses on physical integrity—the ability of the distribution system to act as a physical barrier that prevents external contamination from affecting the quality of the internal, drinking water supply. Water distribution system engineers have defined the physical integrity of the distribution system to be its ability to handle external and internal stresses such that the physical material of the system does not fail (Male and Walski, 1991). Here failure is interpreted more broadly to encompass the absence of a critical component, the improper installation of a component, or the installation of an already contaminated component. 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. When considering the replacement of a given component of the distribution system, decision makers must weigh its potential remaining life versus the potential that the component will fail, which could result in costly consequences and compromise the water utility’s service. The physical integrity of the distribution system, from the entry point to the customer’s tap, is a primary barrier against the entry of external contaminants and the loss in quality of the treated drinking water. This barrier includes such materials as the pipe wall and reservoir cover as well as physical connections to nonpotable water sources. 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;
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Drinking Water Distribution Systems: Assessing and Reducing Risks TABLE 4-1 Infrastructure Components, What They Protect Against, and Common Materials Component External Contamination the Barrier Protects Against Materials Used Pipe Soil, groundwater, sewer exfiltration, surface runoff, human activity, animals, insects, and other life forms 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 Polyethylene, bitumastic, cement-mortar Pipe linings Supporting role in that it preserves the pipe integrity Epoxy, urethanes, asphalt, coal tar, cement-mortar, plastic inserts Service lines Soil, groundwater, sewer exfiltration, surface runoff, human activity, animals, insects, and other life forms Galvanized steel or iron, lead, copper, chlorinated PVC, cross-linked polyethylene, polyethylene, polybutylene, PVC, brass, cast iron Premise plumbing Air contamination, human activity, sewage and industrial nonpotable water. Copper, lead, galvanized steel or iron, iron, steel, chlorinated PVC, PVC, cross-linked polyethylene, polyethylene, polybutylene Fittings and appurtenances (meters, valves, hydrants, ferrules) Soil, groundwater, sewer exfiltration, surface runoff, human activity, animals, insects, and other life forms Brass, rubber, plastic Storage facility walls, roof, cover, vent hatch Air contamination, rain, algae, surface runoff, human activity, animals, birds, and insects Concrete, steel, asphaltic, epoxy, plastics Backflow prevention devices Nonpotable water Brass, plastic Liquids Not applicable Oils, greases, lubricants Gaskets and joints Soil, groundwater, sewer exfiltration, surface runoff, human activity, animals, insects, and other life forms Rubber, leadite, asphaltic, plastic
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Drinking Water Distribution Systems: Assessing and Reducing Risks (3) storage facilities including reservoirs (underground, open, and covered), elevated storage tanks, ground level storage tanks, and standpipes; and (4) backflow prevention devices. The materials used by the water industry for these components, particularly pipes, have changed significantly over time (AWWA, 1986; Von Huben, 1999). For example, 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 (Morrison, 2004). 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 (AWWA, 2005a). Chapter 1 discusses the rate of pipe replacement in the United States and notes that much of the current infrastructure is nearing the end of its usable lifetime. 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. 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 (Geldreich, 1996).
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Drinking Water Distribution Systems: Assessing and Reducing Risks TABLE 4-2 What Causes a Loss in Physical Integrity? Component Mechanism of Integrity Loss Alteration in material structure leading to failure Absence of the barrier or material Improper application or installation of the barrier Pipe Corrosion Permeation Too high internal water pressure or surges Shifting earth Exposure to UV light Stress from overburden Temperature fluctuations, freezing Absence of external or internal linings, wraps, coatings to protect the pipe Unsanitary activity duringconstruction, replacement, or repair Unintentional creation of cracks and breaks Use of faulty materials Fitting and appurtenance Corrosion Permeation Appurtenance in a flooded meter or valve pit (absence of appropriate structures) Unsanitary activity during construction, replacement, or repair Unintentional creation of cracks and breaks Use of faulty materials Contact between dissimilar metals Storage facility wall, roof, cover, vent, hatch Corrosion Permeation Natural disasters Failure due to aging and weathering Missing cover, roof, hatch, vent, can lead to unprotected access to the storage facility. Could be unintentional or intentional (vandalism) Unsanitary activity during construction, replacement, or repair Unintentional or intentional creation of cracks and breaks Poor drainage for unoff Use of faulty materials Backflow prevention device Corrosion Missing device will allow a backflow event via a cross connection Use of faulty materials Improper installation Inadequate drainage of meter pit Operational failure
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Drinking Water Distribution Systems: Assessing and Reducing Risks 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. For the purposes of this discussion, pipes are assumed to always be present. 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 (Male and Walski, 1991; Makar, 2002): 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 dam-
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Drinking Water Distribution Systems: Assessing and Reducing Risks age 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 (Male and Walski, 1991). This last factor is especially troubling since it should be entirely preventable. Nonetheless, there is evidence that poor quality workmanship during initial pipe installation can lead to early structural failure of pipes (Clark and Goodrich, 1989). Burlingame et al. (2002) reported on premature (within one year of installation) failures in service lines that resulted from the combination of using hard copper tubing and poor workmanship during cutting and flaring of the ends. AwwaRF (1985) has also reported that failures with copper tubing can be due to poor workmanship. One of the goals of proper installation of water mains is to account for and circumvent these issues; unfortunately, failure to do so translates into a substantial number of unnecessary main breaks. 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 (Morrison, 2004). 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. At most utilities, overall pipe break rates have been relatively low and stable (Damodaran et al., 2005) even though the infrastructure is aging. Other components of distribution system also experience structural failure, although they have not historically received the attention afforded to pipes. For storage facilities, structural failure is less of a problem than external contamination due to the absence or failure of an essential component such as a cover or vent. Fittings and appurtenances can suffer from the effects of corrosion and permeation.
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Drinking Water Distribution Systems: Assessing and Reducing Risks TABLE 4-3 Most Common Problems that Lead to Pipe Failure for Various Pipe Materials Pipe Material (common sizes) Problems PVC and Polyethylene (4–36 in.) 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 (46–4 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 SOURCES: Morrison (2004) and AWWA (1986). 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 (Von Huben, 1999; Szeliga and Simpson, 2002; Romer et al., 2004; Bonds et al., 2005). The life of the pipe is also influenced by the material used, thickness of the pipe wall, use of protective outer wraps or coatings, application of cathodic protection, and backfill materials and techniques. Internal corrosion
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Drinking Water Distribution Systems: Assessing and Reducing Risks 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. Corrosion is not well understood, particularly at the level of the local water utility, such that insufficient attention has been given to its control (see a later section in this chapter). 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 billion (Romer et al., 2004) 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 (AWWA Water Loss Control Committee, 2003). However, much less is known about what causes structural failures in service lines compared to distribution mains and other system components. Possibilities include improper techniques used during installation that damage materials, improper tapping and flaring to make connections, lack of corrosion prevention or use of corrosive backfill material, damage during handling to plastic tubing, and kinks in copper tubing, and excessive velocity. The Uniform Plumbing Code and International Plumbing Code do not clearly address these issues, and local plumbing codes may not either. Many galvanized and lead pipe service lines are being replaced with copper or plastic pipe (chlorinated polyvinyl chloride or CPVC) (Von Huben, 1999). 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. If manufacturers follow American Society for Testing and Materials (ASTM) standards and are ISO 9002 certified, and certification includes NSF
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Drinking Water Distribution Systems: Assessing and Reducing Risks International standards 14 and 61, the adverse conditions above can be minimized. Permeation 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 (Glaza and Park, 1992; Geldreich, 1996). Permeation incidents have occurred at high-risk sites, such as industrial sites and near underground chemical storage tanks, as well as at lower risk residential sites (Holsen et al., 1991). In some cases the integrity of the pipe has been irreversibly compromised, requiring the complete replacement of the contaminated section. Common pipe materials such as PVC, polybutylene, and polyethylene differ in their chemical and physical structure, and thereby differ in their susceptibility to being altered upon exposure to solvents and in permeation rates. In studying BTEX and 1,3-dichlorobenzene, PVC pipe was found to be more permeable than polyethylene pipe unless the polyethylene pipe was altered by the solvents in contact, after which it can become more permeable to the pollutants (Burlingame and Anselme, 1995). 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. For example, a Midwestern water utility experienced a noticeable increase in the heterotrophic bacterial population of water from a newly installed pipe and identified Pseudomonas fluorescens, Ps. Maltophilia, and Ps. putida as the bacteria responsible for the increase (Geldreich, 1996). The same strains of Pseudomonas were recovered from the sand used as an aggregate in making the concrete lining for the new ductile iron pipe, implicating contamination during construction and installation. More recently, workers in Camden, New Jersey,
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Drinking Water Distribution Systems: Assessing and Reducing Risks were cleaning and lining a 30-inch water main when a parallel sewer line from the post-Civil War era broke. Because of the proximity of the sewer line and the possibility of contamination, officials decided to issue a boil-water alert until water quality testing could show that no external contamination had entered the main. Between 1997 and 1999, the Philadelphia water supply measured elevated turbidity (>1 NTU) in about 12 to 14 percent of the samples that were collected from newly installed water mains. This turbidity, or the particulate debris captured on filters, was found to be largely iron oxides and rust (from the existing water mains still in service), vegetable material such as plant roots, and backfill sand. Incidents like these are not uncommon, as revealed in a survey by Pierson et al. (2002), who point out that pipe repair and installation have not been accomplished using the best available sanitary practices. This is captured generally in Table 4-4, which summarizes the survey of distribution system workers at three different utilities (eastern U.S., western U.S., and western Canada) on the potential for external contamination to occur during water main repair and replacement activities. Given that 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) (Clark and Goodrich, 1989), it is clear that exposure of the distribution system to contamination during repair is an inescapable reality. Unsanitary activity during construction, replacement, or repair can also lead to the contamination of fittings and appurtenances. The use of inappropriate or inferior materials, and the contact between dissimilar metals within fittings, can also cause failures where they should not occur. Appurtenances can be improperly installed in a flooded meter or valve pit which can allow contaminants to enter under intrusion or can create corrosive conditions. Backfill sand contaminating a new pipe at a water main construction site. Photo courtesy of Bureau of Laboratory Service, Philadelphia Water Department.
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Drinking Water Distribution Systems: Assessing and Reducing Risks TABLE 4-4 Potential for Contaminant Entry during Water Main Activities Activity Percent of Responses from Workers at 3 Different Utilities (A, B, C) Occurs Often Occurs Sometimes A B C A B C Broken service line fills trench during installation 46 75 56 39 25 33 Pipe gets dirty during storage before installation 53 75 22 43 25 33 Trench dirt gets into pipe during installation 24 100 39 37 0 44 Rainwater fills trench during installation 20 25 5 60 75 83 Street runoff gets into pipe before installation 30 0 11 61 38 67 Pipe is delivered dirty 4 25 17 33 63 22 Trash gets into pipe before installation 24 0 0 56 50 11 Vandalism occurs at the site 15 0 0 35 0 5 Animals get into pipe before installation 0 0 0 11 0 11 SOURCE: Reprinted, with permission, from Pierson et al. (2002). © 2002 by American Water Works Association. New pipe materials are not sterile, whether they have been kept well protected or not. Indeed, according to a survey (Geldreich, 1996) about 18 percent of new pipe, irrespective of pipe material and size, failed upon testing the water to approve it for release. In one case, Geldreich reported the finding of a piece of wood construction material embedded in a new main that contributed to coliform contamination. Thus, new materials need inspection and some form of disinfection before they are exposed to drinking water. The physical cleanliness of new pipe is important to guarantee that post-installation disinfection will be successful (Geldreich, 1996). The installation or rehabilitation of facilities such as storage reservoirs with floating covers must include water quality checks for health and aesthetic considerations and not assume that new materials and their installation will be free of contaminants (Krasner and Means, 1986). 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, stormwater 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. Regardless of where and how materials become contaminated, the hope is that post-installation disinfection will be sufficient to kill any introduced bacteria. This is not always the case, however, as evidenced by a coliform event in Florissant, Missouri in 1984 (Geldreich, 1996). The coliforms detected in a storage tank were thought to be the result of inadequate disinfection following new pipe installation or repair. Unfortunately, contaminated water subsequently passed into the distribution network. No direct public health outcome was reported; however, the “repeated reissuance of boil-water orders caused a loss of confidence” in the water utility by the public (Geldreich, 1996).
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Drinking Water Distribution Systems: Assessing and Reducing Risks larski, 2002). Damodaran et al. (2005) gave an industry average of 0.1 to 0.3 breaks per mile of pipe per year, such that a low break rate would cause 1 to 3 breaks per year per 1,000 people served. Philadelphia tracks the number of breaks experienced for each 1,000 miles of main using a five-year moving average to smooth out the effect of weather variations. Based on historical information dating back to 1930, the average for 2001 was 212 breaks for every 1,000 miles of main—the lowest total in over 45 years and better than the national average of 240 to 270 breaks per 1,000 miles. Nonetheless, even with a water main replacement program that appears to be successful compared to the national average, every year over 600 water main breaks occur. Therefore, procedures need to be in place by which to recover from a failure in a material barrier and minimize the effects on water quality. Table 4-9 summarizes some of the common methods used today to recover from a failure in a material barrier in order to prevent or minimize contamination of the water supply. There are several categories of recovery efforts. First, compromised materials can be cleaned, repaired, rehabilitated, or replaced. For example, leaks and small breaks can be repaired by repair sleeves or by joint sealing compounds. Storage facilities might have to be drained and cleaned following potential contamination. Another form of restoration is to treat the contaminated water. Chlorine and other disinfectants have been used to protect pipes and storage facilities against external microbial contamination, prevent TABLE 4-9 Ways to Recover from a Loss in Physical Integrity Component Mechanism of Integrity Loss Recovery by Pipe Permeation Reline or replace and conduct water quality testing Structural failure (leak) Replace or repair or rehab Structural failure (break) Replace or repair, flush or disinfect, conduct water quality testing Improper installation Replace, reinstall Unsanitary activity Disinfect, flush, and water quality testing Fitting and appurtenance Structural failure Replace, repair, rehab and disinfect Improper installation Reinstall Unsanitary activity Disinfect and flush Storage facility wall, roof, cover, vent, hatch Structural failure (crack, hole) Repair or rehab or replace, disinfect Absence of Install Improper installation Reinstall Unsanitary activity Disinfect, flush, and water quality testing Backflow prevention device Absence of Install Improper installation Reinstall Operational failure Replace or repair
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Drinking Water Distribution Systems: Assessing and Reducing Risks regrowth of nuisance organisms in response to intruded chemicals, prevent further contamination from the installation of a dirty main, and alleviate customer complaints. Both continuous disinfectant residual maintenance throughout the distribution system and dosing a section of the system with disinfectant are common. Third, recovery is often brought about by flushing the contaminated water from the system rather than treating it, generally using hydrant flushing. Although flushing is mentioned sporadically here because it accompanies many of the other recovery techniques, it is treated more comprehensively in Chapters 5 and 6 where hydraulic and water quality integrity are the focus, respectively. In those situations where the absence of a component was the cause for the lack of physical integrity, then simply installing the component is the recovery effort. For example, the installation of backflow prevention devices or changing covers on reservoirs (say from floating to hard covers) should restore integrity. Finally, where operational failure is the problem, devices may also need to be entirely replaced, along with instituting inspections to ensure that failure does not recur. Repairing, Rehabilitating, and Replacing Pipe Common types of repair activities include cutting and plugging the portion of pipe associated with a leak, installing a repair sleeve or clamp, eliminating dead end mains, replacing and repairing valves, adding ferrules, and repairing or replacing hydrants. These activities are discussed extensively in Grigg (2004) and not considered further here. Improvements are being made in locating buried failure sites, excavation, and repair. For example, trenchless methods are being developed and applied, although the technology development is slow. Rehabilitation of pipe involves the recycling and reinforcing of the existing infrastructure in order to prolong its useful life. For example, structural lining can be used to improve the structural integrity of existing pipes and involves placing a watertight structure in immediate contact with the inner surface of a cleaned pipe (Selvakumar et al., 2002; Ellison et al., 2003). The most commonly used structural lining techniques include conventional slip lining (where new PE pipe is structurally able to replace the existing pipe), cured-in-place rehabilitation or inversion lining (which inserts a non-structural material) (Hughes and Conroy, 2002), fold-and-form pipe, and close-fit slip lining (which can use a structural or non-structural replacement material). Selvakumar et al. (2002) provide a detailed description of all these methods along with their costs, benefits, and limitations. Nonstructural rehabilitation of water mains, which does not focus on recovering the physical integrity of distribution systems, includes chemical dosing for corrosion control, cement mortar lining, epoxy resin lining, and thin-walled PE lining (Hughes and Conroy, 2002; Grigg, 2004; Damodaran et al., 2005). Such rehabilitation should be internally inspected to ensure that it is done to standards.
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Drinking Water Distribution Systems: Assessing and Reducing Risks Pipes are candidates for replacement when the pipe is severely deteriorated (e.g., the pipe has suffered a series of breaks), or when additional hydraulic capacity is needed. Box 4-4 discusses the economic considerations that play into the decision to replace a pipe rather than rehabilitate or repair it. Historically, pipeline replacement involved the construction of a new pipeline normally parallel to the one being replaced. Once constructed, the new pipeline was connected to the pipe network and the old pipeline abandoned. This approach normally involved digging a trench, installing the new pipe, backfilling the trench, and final surface restoration. This construction can be very disruptive in built-up areas plus it may be very difficult to find a location to construct a new waterline. As a result, new trenchless technologies have developed which can result in cost savings over the conventional construction methods. Horizontal directional drilling has seen considerable growth as an alternative to open trench construction, especially at crossings of waterways, rail lines, and highways. A drilling bit bores a horizontal hole that is kept open using drilling fluid. Once a predetermined length of hole is completed, a new pipe is pulled back through the horizontal hole. This method is far less disruptive than open trench construction, and in most cases would not interfere with business or residential property access. Another type of trenchless technology that is most useful in areas where it is difficult to install new pipe is pipe-bursting. This technology is similar to horizontal directional drilling, but with pipe-bursting a new pipe is pulled in the same location as the old pipe. A burster is pulled through the old pipe, breaking it apart and making room for the new pipe. The only openings required are at the two ends and at all active service locations. The equipment can install pipe of the same size that is being replaced or a size or two larger. Selvakumar et al. (2002) give a detailed description of pipe bursting, microtunneling, and horizontal directional drilling methods along with their costs, benefits, and limitations. BOX 4-4 Decision-making regarding Replacement vs. Ongoing Repair There now exist fairly good models for making decisions about ongoing repair vs. replacement of infrastructure pipe components (Damodaran et al., 2005), although they do not incorporate public health risk and water quality deterioration. The traditional economic life of a component is the point at which the cost of keeping it in use equals the cost of replacing it. The “cost”, though, has been expanded beyond the utility’s internal costs to include external costs, like the public’s costs associated with the failure of a component (loss of water and business, traffic disruptions, etc.). Expectations for customer service are rising at the same time that repair and replacement costs are rising. Decisions based on internal costs alone often favor ongoing repair over replacement. When external costs (such as the number of households affected by a failure) are counted, replacement begins to be favored over repair. When the break rate for a 20-ft long pipe exceeds once per year then it can become more economical to replace the pipe than repair it (Damodaran et al., 2005). Utilities need guidance on including external costs along with internal costs, and the advantages and disadvantages of replacement methods, so that they can make up-to-date and sound decisions in a timely manner.
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Drinking Water Distribution Systems: Assessing and Reducing Risks Regardless of whether the situation requires repair, rehabilitation, or replacement, there are practices that can minimize the contamination potential, such as maintaining a positive pressure until the repair site is unearthed and cleared. Trench water should be removed before work is done, and street drainage should be provided to keep water and runoff out of the trench. New materials and repaired materials can be sprayed or swabbed with chlorine or appropriate sanitizing agents, as specified in ANSI/AWWA standards C600-99 for the installation of ductile iron mains and C651-99 for the disinfection of mains. During these activities, inspectors or engineers managing the site need to be aware of all issues related to water quality including the type of pipe that can be laid in soils suspected of contamination, the means by which to protect materials during storage, the methods for working in trenches to prevent contamination of materials, and what to do if materials do become contaminated. Prior to the release for use of a new or replaced water main or facility, a water utility will typically conduct water quality testing. Total coliform bacteria have been the most common indicator that the new material is sanitary and did not become contaminated during storage or installation. In addition to total coliform testing, the water utility can also test for turbidity, HPC bacteria, total chlorine residual, pH, and odor, as unsanitary and improper installation practices can affect these parameters. As documented in Table 2-3, 16 of 34 responding states address the storage and handling of pipes, while 29 of 34 address the need for disinfection and water quality testing following installation. Experience has shown, unfortunately, that sanitary practices vary widely. Even well-run utilities can experience a 30 percent failure rate in the approval of new mains based on water quality testing (Burlingame and Neukrug, 1993). Pipe design and construction is usually focused on existing codes (such as depth of installation to prevent freezing) and corrosion protection (such as using plastic pipe or metallic pipe with protective wrap in corrosive soils) but not on sanitary practices and rarely on permeation concerns. Pierson et al. (2001) found that although the ANSI/AWWA standards, particularly C600-99, attempt to address installation or construction practices, there is a general lack of training and the use of requirements for sanitary practices. It is possible for trenches where pipe is being laid or repaired to fill partially with water from broken lines or from precipitation or groundwater. This water can mobilize soil-related contaminants as well as carry contamination itself. Clearly, during emergency repairs or repairs made under less than favorable conditions, it becomes even more difficult to prevent the exposure of materials to environmental contamination. This could be addressed in part by requiring foremen or managers of construction sites to be certified on a regular basis, as it is for the certification of backflow installers and testers. Such training and certification can be provided through third-party organizations (non-water utility agencies) such as the New England Water Works Association and American Society of Sanitary Engineers. Not only would foremen or managers have to know the engineering requirements, but they would also have to record and un-
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Drinking Water Distribution Systems: Assessing and Reducing Risks derstand the issues related to protecting the sanitary condition of the materials and the water supply. Disinfection Haas et al. (1998) reported that interior pipe surfaces are not free of microbial contaminants even under best case conditions. Furthermore, the lack of adequate distribution system maintenance (which includes flushing, disinfecting, and coliform testing of all pipe repairs and pipe replacement activities) has been found to contribute to higher coliform occurrence rates (Clement et al., 2003). Thus, when a new main is installed or a valve is repaired, it is advisable to act as if some level of contamination has occurred to both the water and the materials and to address potential contamination before the affected portion of the water system is returned to use. When the interior of pipe has become contaminated or needs cleaning due to unsanitary activities, disinfection becomes necessary. Pipes can have a significant chlorine demand which reduces the effectiveness of disinfection (Haas et al., 1999). Fortunately, there is a current AWWA standard (C652) governing new pipe disinfection, which sets forth two options. The first is to flush followed by filling the facility/pipe with a strong (> 25 mg/L) chlorine solution and maintaining it for 24 hours providing that a residual of 10 mg/L remains. The second option is contacting the pipe or facility with a 100 mg/L free chlorine solution for at least three hours so that the residual remaining is at least 50 mg/L. The chlorine used for these disinfection operations may be supplied either as solid calcium hypochlorite powder dissolved in water, sodium hypochlorite (liquid bleach) dissolved in water, or gaseous chlorine dissolved in water. These guidelines basically require that a “CT” (product of disinfectant and contact time) of 14,000 (first option) or 9,000 (second option) mg-min/L be achieved. Tests on actual mains indicate that these guidelines are sufficient to yield four logs (99.99 percent) inactivation of heterotrophic plate count (HPC) bacteria (Haas et al., 1998). Where unusually high levels of contamination are suspected, the design “CT” for facility disinfection should be increased. After disinfection, the chlorinated water must be flushed from the system and the adequacy of disinfection checked by microbiological testing. In flushing the heavily chlorinated water, attention must be paid to (1) preventing leakage into the active distribution system if the newly disinfected pipe is connected to the system, (2) the potential impacts on the sewer system if the water is discharged to a sewer, or (3) dechlorinating the water (using sulfur dioxide, sulfite, or bisulfite) if the water is discharged to a surface waterbody so as to minimize adverse impacts to aquatic life.
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Drinking Water Distribution Systems: Assessing and Reducing Risks CONCLUSIONS AND RECOMMENDATIONS The loss of physical integrity of the distribution system—in which the system no longer acts as a physical 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 sources of contamination that increase the risk of negative public health outcomes. The following primary conclusions and recommendations for maintaining and restoring physical integrity to a distribution system are made. 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 facility 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 should go hand-in-hand considering the increasing costs of infrastructure failure and repair and the increasingly stringent water quality standards. Training and certification can be provided through third-party organizations (non-water utility agencies) such as the New England Water Works Association and American Society of Sanitary Engineers. Although it is difficult and costly to perform, condition assessment of buried infrastructure should be a top priority for utilities. Every water utility should maintain a complete, up-to-date inventory of all infrastructure components from storage facilities to pipes to valves to hydrants, including their current condition. Because failure analysis has not generally been embraced by the water community, there is limited information on many of the materials in common use today. Most useful would be a user-friendly guidance manual for utilities regarding the failure mechanisms of different types of infrastructure
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Drinking Water Distribution Systems: Assessing and Reducing Risks materials and how to use the various types of information on the current condition of the pipe to determine its expected lifetime. Finally, as an essential part of condition assessment, every water utility should have in place a leak detection program that includes checking service lines as well as transmission mains. 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, considering its estimated annual direct cost of $5 billion for the main distribution system (not counting premise plumbing). Cross-connection control should be in place for all water utilities. Every utility should have a uniform and consistent cross-connection control program along with adequate support such as regulations or codes, and staffing. The program should at the least provide for service-protection or containment (i.e., making sure that customers cannot backflow contaminants into the public distribution system), and when possible should attempt to eliminate cross connections on customer’s premises. Most if not all technical and administrative information already exists upon which to institute a cross-connection control program. REFERENCES Allbee, S. 2004. A center of excellence—a sensible step on the pathway to excellence in water utility infrastructure management. Underground Infrastructure Management (Nov/Dec.):27–29. American Water Works Association (AWWA). 1986. Introduction to Water Distribution Principles and Practices of Water Supply Operations. Denver, CO: AWWA. AWWA. 1999. Water Audits and Leak Detection, Manual M36, 2nd edition. Denver, CO: AWWA. AWWA. 2003. Water Stats 2002 Distribution Survey CD-ROM. Denver, CO: AWWA. AWWA. 2004. Recommended Practice for Backflow Prevention and Cross-Connection Control, Manual M14, 3rd edition. Denver, CO: AWWA. AWWA. 2005a. Flexible, lightweight PE gaining ground. Opflow (July):24–25. AWWA. 2005b. Ductile-Iron Pipe—Iron and Icon for Durability, Reliability. Opflow (February):14–15.
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Representative terms from entire chapter: