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Drinking Water Distribution Systems: Assessing and Reducing Risks 6 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. For example, decreases in disinfectant concentrations with travel time through the distribution system could be the result of demand from an external contamination event or it could be due to disinfectant reactions with pipe walls and natural organic matter remaining after treatment. This chapter deals with the various internal processes or events occurring within a distribution system that lead to degradation of water quality, the consequences of those processes, methods for detecting the loss of water quality, operational procedures for preventing these events, and finally, how to restore water quality integrity if it is lost. In many cases, the detection methods and recovery remedies are similar to those discussed in previous chapters. FACTORS CAUSING LOSS OF WATER QUALITY INTEGRITY AND THEIR CONSEQUENCES For water quality integrity to be compromised, specific reactions must occur that introduce undesirable compounds or microbes into the bulk fluid of the distribution system. These reactions can occur either at the solid–liquid interface of the pipe wall or in solution. Obvious microbial examples include the growth of biofilms and detachment of these bacteria within distribution system pipes and the proliferation of nitrifying organisms. Important chemical reactions include the leaching of toxic compounds from pipe materials, internal corrosion, scale formation and dissolution, and the decay of disinfectant residual that occurs over time as water moves through the distribution system. All these interactions are governed by a suite of chemical and physical parameters including temperature, pH, flow regime, concentration and type of disinfectant, the nature and abundance of natural organic matter, pipe materials, etc. Many of these variables may be linked in distribution systems; for example, seasonal increases
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Drinking Water Distribution Systems: Assessing and Reducing Risks in temperature may be accompanied by changes in organic matter, flow regimes, and disinfectant concentrations. As a consequence, attempting to correlate the occurrence of a given event (such as corrosion, microbial growth, disinfectant decay, or DBP formation) within distribution systems to a single variable (such as temperature) is difficult. Biofilm Growth One way in which water quality can be degraded in the distribution system is due to the growth of bacteria on surfaces as biofilms. Virtually every water distribution system is prone to the formation of biofilms regardless of the purity of the water, type of pipe material, or disinfectant used. The extent of biofilm formation and growth, the microbial ecology that develops, and the subsequent water quality changes depend on surface-mediated reactions (e.g., corrosion, disinfectant demand, immobilization of substrates for bacterial growth), mass transfer and mass transport processes, and bulk fluid properties (concentration and type of disinfectants, general water chemistry, organic concentration, etc.). These interactions can be exceedingly complex, which typically means that the mechanisms leading to biofilm growth may not be obvious and are often system specific. Bacteria growing in biofilms can subsequently detach from the pipe walls. Because these organisms must survive in the presence of the disinfectant residual present in the distribution system, the interaction between the suspended organisms and residual is critical. If the residual has decayed due to reactions with compounds in the water or with the pipe wall, intrusion, or other sufficient external contamination, it is possible for attached bacteria to be released into water that contains insufficient disinfectant to cause their inactivation. The potential for this to occur is higher in premise plumbing, which generally has longer water residence times that may lead to very low disinfectant concentrations. Pathogenic Microorganisms 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. In the case of Aeromonas spp., which is included as one of the unregulated “contaminants” to be tested for in the Contaminant Candidate List, it has been shown that drinking
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Drinking Water Distribution Systems: Assessing and Reducing Risks water isolates carry virulence factors directly involved in pathogenesis (Sen and Rogers, 2004). Coliforms and Heterotrophs Another consequence of biofilms is their potential to support the growth and release of organisms of regulatory concern, especially coliforms. Coliforms released from biofilms may result in elevated coliform detection even though physical integrity (i.e., breaches in the distribution system) and disinfectant residual have been maintained (Characklis, 1988; Haudidier et al., 1988; Smith et al., 1990). It should be noted that coliforms arising from biofilms are generally considered to be low risk (see Chapter 2), which is also inferred by EPA’s variance to the Total Coliform Rule for coliforms emanating from biofilms (see page 208). However, coliform regrowth may indirectly present a risk by masking the presence of bacteria introduced in a simultaneous contamination event. If repeated occurrences of coliforms in the distribution system force a utility to notify the public, there can be a loss of consumer confidence and trust in the utility. The regrowth of heterotrophs in biofilms can also be of concern, especially for European communities that are required to monitor their presence. Some U.S. utilities routinely monitor heterotrophs using heterotrophic plate counts (HPC) as a general indicator of microbial quality, and may be required to assess their numbers if chlorine residuals are too low. In general, heterotrophic bacteria are usually not of public health concern, but with the growing immunocompromised population many utilities are interested in minimizing the presence of these organisms in their water. Corrosion and Other Effects In addition to the regrowth issue, biofilms in distribution systems can cause other negative effects on finished water quality. The processes listed here do not require that the organisms detach from the surfaces, since the changes in water quality are due to their metabolic activities as they grow on the surfaces. Bacterial biofilms may contribute to the corrosion of pipe surfaces and their eventual deterioration. Although a considerable amount of corrosion internal to the pipe can be mediated by abiotic factors, it is known that bacteria can both directly and indirectly influence corrosion of metal surfaces. Of particular concern is the pitting of copper that can lead to pinhole leaks in premise plumbing. Geesey et al. (1993) reported that pitting of copper plumbing in four hospitals around the world was likely attributable to bacterial activity. Wagner et al. (1997) have said that biologically produced polymers typical of biofilms create high and low chloride concentration cells, and consequently localized corrosion cells, leading to increased copper corrosion. Laboratory studies have shown that
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Drinking Water Distribution Systems: Assessing and Reducing Risks the presence of bacteria on copper surfaces could accelerate corrosion when compared to an abiotic system (Webster et al., 2000). In other studies, specific organisms were correlated with copper corrosion and could be isolated from pits (Bremer and Geesey, 1991; Bremer et al., 1992). However, other research has shown that organisms alone did not cause copper pitting, and that particulate matter was also required (Walker et al., 1998). Microbes may also influence iron surfaces in distribution systems. Iron bacteria can grow on ferrous metal surfaces (Ridgway et al., 1981), and by virtue of their metabolism may modify the local chemistry at the metal surface which in turn promotes localized corrosion (Victoreen, 1974). As stated by McNeill and Edwards (2001), there are many possible effects of bacterial action and biofilm formation on iron corrosion. These include the production of differential aeration cells (Lee et al., 1980), soluble metal uptake by biofilm polymers (Tuovinen et al., 1980), changes in iron speciation by oxidation or reduction (Shair, 1975; Denisov et al., 1981; Kovalenko et al., 1982; Okereke and Stevens, 1991; Chapelle and Lovely, 1992; Nemati and Webb, 1997), and the production of pH gradients (Tuovinen et al., 1980) or corrosive hydrogen sulfide (Tuovinen et al., 1980; DeAraujo-Jorge et al., 1992). All of these factors can contribute to increased localized corrosion and the deterioration of the pipe material, as well as influencing water quality by causing the release of metal ions or corrosion products and associated problems with water color. Other effects of biofilms are worth noting. As demonstrated in the wastewater industry, it is possible to have nitrifying bacteria present in biofilms, and these organisms could result in nitrification episodes in distribution systems where chloramine is used (Wolfe et al., 1990, and see the section below). Actinomycetes or fungi present in biofilms may result in taste and odor problems (Burman, 1965, 1973; Olson, 1982), which then lead to consumer complaints. Excess biofilm growth can result in the loss of hydraulic capacity by increasing fluid frictional resistance at the pipe wall (see examples in Characklis et al., 1990). Finally, growth of biofilms and the associated organics can create a chlorine demand at the pipe wall. Biologically Stable Water Because this report focuses on distribution system events, it does not delve into failures or breaches at the treatment plant that might allow a breakthrough of contaminated water. Nonetheless, a brief discussion of biologically stable water is warranted, given its potential to reduce the growth of bacteria in the distribution system. Drinking water is generally considered to be biologically stable if it does not support the growth of bacteria in the distribution system. In its broadest sense, biologically stable water restricts growth because it lacks an essential nutrient (nitrogen or phosphorus), is sufficiently low in utilizable organic carbon, or contains adequate disinfectant. Although all of these parameters may influence biofilm growth, the U.S. drinking water industry has typically
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Drinking Water Distribution Systems: Assessing and Reducing Risks viewed biologically stable water as sufficiently low in organic carbon as to limit the proliferation of heterotrophic bacteria. In this context, the general concepts of microbial stable water and maximum regrowth potential are relatively well understood (Rittman and Snoeyink, 1984; Sathasivan et al., 1997). Another mechanism for ensuring biological stability is the maintenance of an adequate disinfectant residual. However, since disinfectants decay in the distribution system, reliance on a residual to ensure biological stability may not be entirely feasible. Within distal portions of the distribution system or within stagnant portions of premise plumbing, disinfectants disappear via reactions with pipe or bulk water or via nitrification. At these locations, any available organics can then be freely utilized by the bacteria present. The reduction of organic carbon to control microbial growth may allow utilities to decrease their reliance on disinfectants. This approach also has the advantage of decreasing the potential for the production of disinfectant byproducts (DBPs). Organic carbon removal is most often accomplished through enhanced coagulation, granular activated carbon filtration, or biological filtration. Although there is controversy surrounding target concentrations of organics that will limit regrowth, some recommendations have been made. van der Kooij et al. (1989) and van der Kooij and Hijnen (1990) showed a correlation between assimilable organic carbon (AOC) and regrowth in a non-disinfected distribution system, and provided evidence for biological stability in the Netherlands when the AOC concentration (Pseudomonas fluorescens P17 + Spirillum NOX) is reduced to 10 µg acetate C eq/L (van der Kooij 1992). LeChevallier et al. (1991) have suggested that coliform regrowth may be controlled by influent AOC levels (P17 + NOX) below 50 µg acetate C eq/L. Based on a field study, LeChevallier et al. (1996) subsequently recommended a level below 100 µg C/L to control regrowth. Servais et al. (1991) have associated biological stability with a biodegradable dissolved organic carbon (BDOC) level of 0.2 mg/L, but Joret et al. (1994) have stated that the value is 0.15 mg/L at 20° C and 0.30 mg/L at 15° C. It should also be noted that organic carbon may not be the limiting nutrient. In Japan and Finland, evidence supports the concept that phosphorus is limiting (Miettinen et al., 1997; Sathasivan et al., 1997; Sathasivan and Ohgaki, 1999; Lehtola and Miettinen, 2001; Keinanen et al., 2002; Lehtola et al., 2002a,b, 2004). In these cases, the addition of phosphate-based corrosion inhibitors may decrease the biological stability of the water and allow for regrowth (Miettinen et al., 1997). This discussion illustrates that the best strategy for creating and maintaining biologically stable water is most likely to be system specific. Each water utility should identify the limiting nutrient and best practices to attain and then maintain biological stability. Changing water quality goals should then keep these factors in mind. For example, the dosing of ammonia during a switch to chloramination would relieve nitrogen limitations to regrowth, whereas dosing of phosphate corrosion inhibitors can relieve phosphate limitations.
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Drinking Water Distribution Systems: Assessing and Reducing Risks Nitrification Biological nitrification is a process in which bacteria oxidize reduced nitrogen compounds (e.g., ammonia) to nitrite and then nitrate. It is associated with nitrifying bacteria in distribution systems and long retention times in water supply systems practicing chloramination. One of the most important problems exacerbated by nitrification is loss of the chloramine disinfectant residual. This occurs because a reduction in ammonia results in an increased ratio of chlorine to ammonia nitrogen. This ratio controls the stability of monochloramine, which is governed by a complex set of reactions (Jafvert and Valentine, 1992; also see following section on loss of disinfectant residual). As the ratio approaches 1.5 on a molar basis, a rapid loss of monochloramine occurs attributable to the eventual oxidation of N(III) to primarily nitrogen gas and the release of more ammonia. The released ammonia can then be further oxidized by the nitrifying organisms, establishing what amounts to a positive feedback loop. Furthermore, the loss of disinfectant residual removes one of the controls on the activity of nitrifiers, and it may also lead to the increased occurrence of microorganisms such as coliforms (Wolfe et al., 1988, 1990) and heterotrophic bacteria. As discussed in NRC (2005), the loss of chloramine residual is the most significant health threat that can result from nitrification. It should be noted, however, that there are other lesser health effects of nitrification that may be important for certain populations. Nitrite and nitrate have been shown to cause methemoglobinemia (blue baby syndrome), an acute response to nitrite that results in a blockage of oxygen transport (Bouchard et al., 1992). Methemoglobinemia affects primarily infants below six months of age, but it may occur in adults of certain ethnic groups (Navajos, Eskimos) and those suffering from a genetic deficiency of certain enzymes (Bitton, 1994). Pregnant women may also be at a higher risk of methemoglobinemia than the general population (Bouchard et al., 1992). A second concern is that nitrate may be reduced to nitrite in the low pH environment of the stomach, reacting with amines and amides to form N-nitroso compounds (Bouchard et al., 1992; De Roos et al., 2003). Nitrosamines and nitrosamides have been linked to different types of cancer, but the intake of nitrate from drinking water and its causal relation to the risk of cancer is still a matter of debate (Bouchard et al., 1992). A study by Gulis et al. (2002) in Slovakia related increased colorectal cancer and non-Hodgkin’s lymphoma to medium (10.1–20 mg/l) and high (20.1–50 mg/l) concentrations of nitrate nitrogen in drinking waters. Similarly, Sandor et al. (2001) showed a correlation between the consumption of waters containing greater than 88 mg/l nitrate nitrogen and gastric cancer. Despite numerous papers (Sandor et al., 2001; Gulis et al., 2002; Kumar et al., 2002; De Roos et al., 2003; Coss et al., 2004; Fewtrell, 2004), the concentration at which nitrate nitrogen in drinking waters presents a health risk is unclear (Fewtrell, 2004). Finally, a lesser but still significant water quality effect of nitrification is a reduction in alkalinity and pH in low
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Drinking Water Distribution Systems: Assessing and Reducing Risks alkalinity waters. This may cause the pH to decrease to the point that corrosion of lead or copper becomes a problem. It is important to recognize that nitrate and nitrite may come from sources other than nitrification. van der Leeden et al. (1990) found that 93 percent of all U.S. water supplies contain less than 5 mg/l nitrate, but noted that these values may be changing as a result of the increased use of nitrate-containing fertilizers. Increased use of chloramination (up to 50 percent of the surface water systems in the United States may use chloramination in the near future as a result of the Stage 1 Disinfectants/Disinfection Byproducts Rule; EPA, 2003) may result in higher levels of nitrate in drinking waters (Bryant et al., 1992), but the increment in nitrate plus nitrite nitrogen from this source would typically be less than 1 mg/L, which is well below the current maximum contaminant level (MCL). Thus, as stated earlier the concern may be predominantly for more susceptible populations (pregnant women, infants, some ethnic groups). Interestingly, although nitrification is a recognized potential problem in water systems practicing chloramination, nitrification control is required or encouraged in only 11 of 34 states that responded to a survey of drinking water programs conducted by the Association of State Drinking Water Administrators in March 2003 (see Table 2-5). This illustrates the need for state agencies to recognize the potential issues associated with chloramination and nitrification, and thereby prepare their utilities to deal with this potentially problematic issue. Leaching All materials in the water distribution system, including pipes, fittings, linings, other materials used in joining or sealing pipes, and internal coatings leach substances into the water. The processes that account for this include corrosion, dissolution, diffusion, and detachment. Taste and odor problems (Burlingame et al., 1994; Khiari et al., 2002) are the most likely outcome of leaching because most substances leaching into water from materials in the distribution system are non-toxic, present only at trace levels, or are in a form unlikely to cause health problems. There are however, a few situations in which leaching may present a substantial health risk. By far the most significant is the leaching of lead from lead pipe, lead-containing solder, and lead service connections. Monitoring of lead in tap water and replacement of these lines are important components of the Lead and Copper Rule. Other materials used in distribution systems that have the potential for leaching include PVC pipes manufactured before about 1977. These are known to leach carcinogenic vinyl chloride into water at levels above the MCL (AWWA and EES, Inc., 2002). Cement materials have, under unusual circumstances, leached aluminum into drinking water at concentrations that caused death in hemodialysis and other susceptible patients (Berend et al., 2001). Because levels of aluminum normally present in drinking water can also threaten this population, the FDA has issued guidance for water purification pre-
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Drinking Water Distribution Systems: Assessing and Reducing Risks treatments in the U.S. for dialysis and other patients (http://www.gewater.com/library/ tp/1111_Water_The.jsp). Asbestos fibers may also be released from asbestos cement; the content of asbestos in water is regulated with an MCL, although utilities are not required to monitor for asbestos in the distribution system. Finally, excessive leaching of organic substances from linings, joints, and sealing materials have occasionally been noted. Some of these substances may support the growth of biofilms (Shoenen, 1986), such that their use should be limited. For new materials, NSF International establishes levels of allowable contaminant leaching through ANSI/NSF Standard 61 (see Chapter 2). However, this standard, which establishes minimum health effect requirements for chemical contaminants and impurities, does not establish performance, taste and odor, or microbial growth support requirements for distribution system components. This is unfortunate because research has shown that distribution system components can significantly impact the microbial quality of drinking water via leaching. Procedures are available to evaluate growth stimulation potential of different materials (Bellen et al., 1993), but these tests are not applied in the United States by ANSI/NSF. Internal Corrosion Internal corrosion manifests as (1) the destruction of metal pipe interiors by both uniform and pitting corrosion (see Chapter 4) and (2) the buildup of scales of corrosion products on the internal pipe wall that hamper the flow of water (see Chapter 5). A large number of water quality parameters such as disinfectant residual, temperature, redox potential, alkalinity, calcium concentration, total dissolved solids concentration, and pH play an important role both in the internal corrosion of pipe materials and the subsequent release of iron. The products of corrosion may appear in water as dissolved and particulate metals, and the particles may cause aesthetic problems because of their color and turbidity if they are present in sufficient concentration. Metals such as lead and copper in tap water are governed by the Lead and Copper Rule; asbestos particles and iron particles with adsorbed chemicals such as arsenic (Lytle et al., 2004) are of concern because of possible health effects. The quality of distributed water must be controlled so that both corrosion and metal release do not cause water quality problems. Scale Formation and Dissolution Scale on pipe surfaces may form in distribution systems for a variety of reasons including precipitation of residual aluminum coagulant after filtration, precipitation of corrosion products, precipitation of corrosion inhibitors, and precipitation of calcium carbonate and silicate minerals. Scale that forms in a thin,
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Drinking Water Distribution Systems: Assessing and Reducing Risks smooth coat that protects the metal pipe by reducing the rate of corrosion is generally desirable, whereas uncontrolled precipitation can reduce the effective diameters of distribution pipes and can create rough surfaces, both of which reduce the hydraulic capacity of the system (as discussed in Chapter 5) and increase the cost of distributing water. In terms of internal contamination events, rough surfaces and scales with reduced metals such as ferrous iron can increase problems with biofilms (Camper et al., 2003). That is, ferrous iron reacts with chlorine and monochloramine, reducing the effective concentration of disinfectant in the vicinity of biofilms. Furthermore, rough surfaces contain niches where microbes can grow without exposure to hydraulic shear. If the scale material is loosely attached to the pipe wall, such as some aluminum precipitates, hydraulic surges can result in substantial increases in the turbidity of tap water. Scales are also important because they can dissolve under some water quality conditions and release metals to the water in the distribution system. For example, Sarin et al. (2003, 2004) showed that iron scales release iron during flow stagnation, which then causes turbid and colored water. Dodge et al. (2002), Valentine and Stearns (1994), and Lytle et al. (2002) showed that uranium, radium-226, and arsenic, respectively, could be adsorbed to iron corrosion scales found in distribution systems. (In order for these metals to accumulate they must be present in the source water.) Lytle et al. (2002) showed that arsenic would accumulate on iron solids in distribution systems even when present in water at concentrations less than 10 µg/L. Aluminum and manganese solids can also adsorb metal contaminants and may subsequently release them because of changes in water quality. Research is needed to fully characterize this potential source of contamination related to internal corrosion and scale dissolution and to find ways to control it. Other Chemical Reactions that Occur as Water Ages Many water distribution systems in the United States experience long retention times or increased water age, in part due to the need to satisfy fire fighting requirements. Although not a specific degradative process, water age is a characteristic that affects water quality because many deleterious effects are time dependent. The most important for consideration here are (1) the loss of disinfectant residuals and (2) the formation of DBPs. The importance of water age is recognized in part by the survey of state drinking water programs where nearly all states that responded to the survey either required or encouraged utilities to minimize dead ends and to have proper flushing devices at remaining dead ends (Table 2-3).
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Drinking Water Distribution Systems: Assessing and Reducing Risks Loss of Disinfectant Residual Maintenance of a disinfectant residual throughout a distribution system is considered an important element in a multiple barrier strategy aimed at maintaining the integrity of a distribution system. It is generally assumed that the presence of a disinfectant is desirable because it may kill pathogenic organisms, and therefore the lack of a disinfectant is an undesirable situation. The absence of a disinfectant residual when one is expected may also indicate that the integrity of the system has been compromised, possibly by intrusion or nitrification. If the disinfectant is chloramine, its decay will produce free ammonia that could promote the onset of nitrification. Understanding the nature of the processes leading to disinfectant losses, especially when those processes lead to excessive decay rates, is important in managing water quality. Loss of disinfectants in distribution systems is typically due to reduction reactions in the bulk water phase and at the pipe–water interface that reduce disinfectant concentration over time, although nitrification (in the case of chloramine) can also play a role. Dissolved constituents that can act as reductants in the aqueous phase include natural organic matter (NOM) and ferrous Fe(II) and manganous Mn(II) ions. These substances may occur in the water either as a result of incomplete removal during treatment, from the corrosion of pipe material (e.g., cast iron), or from the reduction of existing insoluble iron and manganese deposits. Disinfectants may also readily react with reduced forms of iron and manganese oxides typically found on the surface of cast iron pipes as well as with adsorbed NOM (Tuovinen et al., 1980, 1984; Sarin et al., 2001, 2004). Benjamin et al (1996) found that the accumulation of iron corrosion products at the pipe wall and the release of these products into the bulk water led to a deterioration of water quality. There have been several reports that the loss of chlorine residuals in corroded unlined metallic pipes (particularly cast iron) increases with increasing velocity (Powell, 1998; Powell et al., 2000; Grayman et al., 2002; Doshi et al., 2003). Correlative evidence for the role of corrosion in reducing disinfectant residuals was produced by Camper et al. (2003), who studied the interactions between pipe materials, organic carbon levels, and disinfectants using annular reactors with ductile–iron, polyvinyl chloride (PVC), epoxy, and cement-lined coupons at four field sites. They found that iron surfaces supported much higher bacterial populations than other materials. Modeling efforts to understand disinfectant decay have been primarily empirical in nature or semi-mechanistic, and they have mostly addressed non-biological reactions. The primary purpose of these types of models is to serve as a predictive tool in managing water quality. Most modeling research has targeted the relatively fast reactions of free chlorine in the aqueous phase, predicting free chlorine decay versus hydraulic residence time using single system-specific decay coefficients. For example, Vasconcelos et al. (1996) developed several simple empirical mathematical models to describe free chlorine decay. Clark (1998) proposed a chlorine decay and TTHM formation model based on a
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Drinking Water Distribution Systems: Assessing and Reducing Risks competitive reaction between free chlorine and NOM. The model was validated against the Vasconcelos et al. (1996) data sets and found to be as good or better (based on r2 values) than the models examined by Vasconcelos et al. (1996). More sophisticated models have improved predictive management capabilities and are also useful as research tools in the elucidation of fundamental processes. Rossman et al. (1994) developed a chlorine decay model that includes first-order bulk phase and reaction-limited wall demand coefficients; this model is incorporated into EPANET1. The model developed by Clark (1998) was extended to include a rapid and slow reaction component and to study the effect of variables such as temperature and pH (Clark and Sivaganesan, 2001). Further extensions included the formation of brominated byproducts (Clark et al., 2001). McClellan et al. (2000) modeled the aqueous-phase loss of free chlorine due to reactions with NOM by partitioning the NOM into reactive and non-reactive fractions. Other models have incorporated reactions with reactive pipe surfaces that may dominate the loss pathways (Lu et al., 1995; Vasconcelos et al., 1997) as well as bulk phase reactions. Clark and Haught (2005) were able to predict free chlorine loss in corroded, unlined metallic pipes subject to changes in velocity by modeling the phenomena as being governed by mass transfer to the pipe wall where the chlorine was rapidly reduced. Less studied has been the loss of monochloramine in distribution systems. Monochloramine, while generally less reactive than free chlorine, is inherently unstable because it undergoes autodecomposition. While autodecomposition occurs via a complex set of reactions, the net loss of monochloramine occurs according to the stoichiometry: (1) This reaction has been reasonably well studied (Valentine et al., 1998; Vikesland et al., 2000) and can be approximated (in the absence of other reactions) by a simple second-order relationship: (2) where kvcsc is a rate constant describing the second order loss of monochloramine (Valentine et al., 1998). Its derivation involves the simplifying assumption that monochloramine decays by a mechanism involving the rate limiting formation of dichloramine that then rapidly decays. As such, kvcsc is a combination of several fundamental rate constants and the Cl/N ratio. It can be simply calculated and used to predict monochloramine decay in the aqueous phase in the absence of other demand reactions. It should be pointed out that 1 EPANET is a model developed by EPA that performs an extended period simulation of hydraulic and water quality behavior within pressurized pipe networks (see Chapter 7).
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Drinking Water Distribution Systems: Assessing and Reducing Risks known public health significance should be developed, ideally with results available on-line and in real time. The implementation of best practices to maintain water quality (see Chapter 2) is needed until better monitoring approaches can be developed. Standards for materials used in distribution systems need to be updated to address their impact on water quality, and research is needed to develop new materials that will have minimal impacts. Materials standards have historically been designed to address physical/strength properties including the ability to handle pressure and stress. Testing of currently available materials should be expanded to include (1) the potential for permeation of contaminants, and (2) the potential for leaching of compounds of public health concern as well as those that contribute to tastes and odors and support biofilm growth. The results of these tests should be incorporated into the standards in a way that water quality deterioration attributable to distribution system materials is minimized. Also, research is needed to develop new materials that minimize adverse water quality effects such as the high concentrations of undesirable metals and deposits that result from corrosion and the destruction of disinfectant owing to interactions with pipe materials. REFERENCES Ackers, J., M. Brandt, and J. Powell. 2001. Hydraulic characterization of deposits and review of sediment modeling. Report REF. No. 01/DW/03/18. London: UKWIR Ltd. American Public Health Association (APHA). 2005. Standard Methods for the Examination of Water and Wastewater, 21st edition. A. D. Eaton, L. S. Clesceri, E. W. Rice, and A. E. Greenberg (eds.). Washington, DC: APHA. Amy, G. L., P. A. Chadik, and Z. K. Chowdhury. 1987. Developing models for predicting trihalomethane formation potential and kinetics. J. Amer. Water Works Assoc. 79(7):89–97. Antoun, E. N., T. Tyson, and D. Hiltebrand. 1997. Unidirectional flushing: a remedy to water quality problems such as biologically mediated corrosion. In: Proceedings of the AWWA Annual Conference. Denver, CO: AWWA. American Water Works Association (AWWA). 1990. Position statement on chlorine residual. Pp. 196 In: 1995–1996 AWWA Officers and Committee Directory. Denver, CO: AWWA. AWWA. 2006. Fundamentals and Control of Nitrification in Chloraminated Drinking Water Distribution Systems. AWWA Manual M56. Denver, CO: AWWA. AWWA and EES, Inc. 2002. Permeation and leaching. http://www.epa.gov/safewa-ter/tcr/pdf/permleach.pdf. Washington, DC: EPA. Barbeau, B., K. Julienne, V. Gauthier, R. Millette, and M. Provost. 1999. Dead-end flushing of a distribution system: short and long-term impacts on water quality. In: Proceedings of the AWWA Water Quality Technology Conference. Denver, CO: AWWA.
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Representative terms from entire chapter: