Drinking Water Distribution Systems Assessing and Reducing Risks (2006) / Chapter Skim
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6 Water Quality Integrity
6 Water Quality Integrity
Pages 221-268
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From page 221... ...
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.
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From page 222... ...
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.
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From page 223... ...
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)
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From page 224... ...
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)
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From page 225... ...
. Another mechanism for ensuring biological stability is the maintenance of an adequate disinfectant residual.
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From page 226... ...
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)
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From page 227... ...
. 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)
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From page 228... ...
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)
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From page 229... ...
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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From page 230... ...
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 systemspecific decay coefficients. For example, Vasconcelos et al.
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From page 231... ...
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.
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From page 232... ...
. Reaction of trace levels of free chlorine that equilibrate with monochloramine was a key mechanism accounting for slow monochloramine loss due to reaction with NOM.
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From page 233... ...
, cyanogen chloride, and several iodohaloacetic acids, none of which are currently regulated at the federal level. The state of California has, however, established a notification level of 10 ppb in drinking water for NDMA, a potent carcinogen (http://www.dhs.ca.gov/ps/ddwem/chemicals/NDMA/ NDMAindex.htm)
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From page 234... ...
234 DRINKING WATER DISTRIBUTION SYSTEMS: ASSESSING AND REDUCING RISKS HANs, 0.2% Chloropicrin, 0.5% HKs, 0.2% TTHM, 35.6% Unaccounted for TOX , 51.5% THAA, 11.9% Chlorination Chloropicrin, 0.7% HANs, 1.0% HKs, 0.6% TTHM, 3.9% THAA, 10.8% Unaccounted for TOX, 82.9% Chloramination FIGURE 6-1 Comparison of Halogenated DBPs. SOURCE: Richardson et al.
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From page 235... ...
iodohaloacetic acids) are expected to form primarily in distribution systems since chloramine is not usually used during primary disinfection (although ammonia is sometimes added in the treatment plant to stop THM and HAA formation)
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From page 236... ...
Chloroform Bromodichloromethane Dibromochloromethane Bromoform FIGURE 6-2 Changes in total trihalomethanes in a system with free chlorine with water age. SOURCE: Reprinted, with permission, from Baribeau et al.
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From page 237... ...
Routinely monitored parameters such as temperature, pH, disinfectant residual, and even microbial constituents cannot differentiate between external and internal contamination. Other less routinely monitored constituents, including dissolved metals, turbidity, total organic carbon, synthetic organic compounds, or nuisance organisms such as invertebrates may also not be definitive for exter
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From page 238... ...
. Because most drinking waters in the United States contain a total chlorine residual, the taste and odor of tap water might be described as "chlorinous." Whether this is noticeable to the water-consuming public depends on the chlorine species present, the concentration of the residual, and the temperature of the tap water.
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From page 239... ...
Among the compounds most likely to present a taste and odor problem stemming from an external contamination event are gasoline additives or constituents, soluble components of soil, and compounds found in sewage. Changes in taste and odor can occur anywhere in the distribution system that the chlorine residual deteriorates and the water becomes stagnant, such as in storage tanks, at dead-end water mains, and behind closed valves.
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From page 240... ...
In the case of iron, elevated levels are more likely to be associated with secondary standards and aesthetic concerns rather than with a specific public health threat. Disinfectant Residual and Disinfection Byproduct Measurements Measurements of disinfectant residuals and DBP concentrations often accompany one another and are routinely practiced using a number of standard analytical methods.
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From page 241... ...
An increase in heterotrophic plate count may also TABLE 6-3 Usefulness of Water Quality Parameters for Distribution System Nitrification Monitoring Parameter/Usefulness Very Useful Useful Limited Usefulness Total Chlorine Nitrate Dissolved Oxygen Nitrite-N Total ammonia-N TOC Free ammonia-N HPC-R2A Hardness Temperature pH Alkalinity Free Chlorine*
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From page 242... ...
, an absence of nitrifying organisms in the bulk phase does not necessarily indicate the absence of nitrification. Detection of Biological Changes Several biological constituents, some of which are part of compliance monitoring, can be used to detect the loss of water quality integrity due to both internal and external contamination events.
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From page 243... ...
that rely on high-quality water for manufacturing purposes. Coliforms Under ideal circumstances, the presence of coliforms in a drinking water sample should indicate external fecal contamination of the water supply, which is the main premise behind the current Total Coliform Rule.
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From page 244... ...
This indicator is not always specific for fecal contamination, however, because it can be found in soils and sediments as part of the natural flora. Nonetheless, to date it has not been identified as a part of the natural flora of drinking water distribution systems, and as such, may be a reasonable indicator of external contamination regardless of whether it arises from fecal contamination or the soil.
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From page 245... ...
Summary Water quality integrity needs to be evaluated rapidly and, if at all possible, using in-line, real time methods (as discussed in Chapter 7)
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From page 246... ...
In these cases, the equipment is expensive, and for flow cytometry, extensive optimization may be required. Indirect methods could be used to determine water quality integrity; for example adenosine triphosphate (ATP)
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From page 247... ...
for further details on nitrification control. Adequate Disinfection Residual Maintenance of a disinfectant residual in the distribution system is required under the Surface Water Treatment Rule and has been designated as the best available technology for compliance with the Total Coliform Rule.
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From page 248... ...
In addition to the state regulations mentioned above, the literature implicates and in some cases makes suggestions for appropriate disinfectant levels. For example, systems that maintained dead-end free chlorine levels of < 0.2 mg/liter or monochloramine levels of < 0.5 mg/liter had substantially more coliform occurrences than systems that maintained higher disinfectant residuals (LeChevallier et al., 1996)
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From page 249... ...
Using additional points of disinfectant application in the distribution system can reduce the amount of chlorine added at a treatment plant for the purpose of maintaining the distribution system residual. This, in turn, has the potential to limit DBP formation and subsequent exposure of those consumers' drinking water from taps close to the initial source.
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From page 250... ...
(2003) reviewed operating practices at utilities employing booster chloramination with the addition of free chlorine (Martin and Cummings, 1993; Cohen, 1998; Ireland and Knudson, 1998)
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From page 251... ...
The use of phosphate inhibitors to control problems with iron, lead, and copper is widespread, but care must be taken to ensure that the added phosphate does not decrease the biological stability of the water or cause a problem in municipal wastewater treatment plant discharges. Water stagnation is an important cause of many iron release problems, so distribution systems must be designed to maintain flowing water conditions to the extent possible (Sarin et al., 2004)
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From page 252... ...
In addition to the direct effect of leaching, new pipe materials can have a significant indirect effect on internal water quality. For example, they may exert a chlorine demand that can reduce the residual disinfectant in the distribution system and hence degrade the microbial quality of the drinking water (Haas et al., 2002)
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From page 253... ...
In order of frequency cited, the objectives used for flushing were to eliminate colored water, restore disinfectant residual, reduce turbidity, eliminate tastes and odors, reduce the number of bacteria, reduce DBP precursors, remove sediment, comply with regulations, maintain water quality, decrease chlorine demand, respond to customer complaints, reduce corrosion inhibitor build-up, eliminate stale water, respond to animal activity, and eliminate lime deposits. Another survey (summarized in Table 2-5)
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From page 254... ...
In addition to permanent changes in disinfectant, there are instances where short-term switches are practiced. Drinking water utilities using chloramine as a disinfectant residual sometimes temporarily switch to free chlorine both as a
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From page 255... ...
and achieve a free chlorine residual. This strategy is effective because ammonia oxidizing bacteria are sensitive to free chlorine (Baribeau, 2006)
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From page 256... ...
Maintaining water quality integrity in the distribution system is challenging because of the complexity of the system. There are interactions between the type and concentration of disinfectants, corrosion control schemes, operational practices (e.g., flow characteristics, water age, flushing practices)
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From page 257... ...
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 introduced by chloramination, which include nitrification, lower disinfectant efficacy against suspended organisms, and the potential for deleterious corrosion problems.
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From page 258... ...
2006. Fundamentals and Control of Nitrification in Chloraminated Drinking Water Distribution Systems.
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From page 259... ...
1992. Biofilms in drinking water distribution systems.
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From page 260... ...
1999. Characterizing the effect of chorine and chlor amines on the formation of biofilm in a simulated drinking water distribution sys tem.
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From page 261... ...
1997. Characterization of the loose deposits in drinking water distribution systems.
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From page 262... ...
1994. Control of biode gradable organic matter during drinking water treatment.
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From page 263... ...
2004. Microbiology, chemistry and biofilm de velopment in a drinking water distribution system with copper and plastic pipes.
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From page 264... ...
1998. Bench scale investigations of bacte rial regrowth in drinking water distribution systems.
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From page 265... ...
1993. Clostridium perfringens and somatic coliphages as indicators of the efficiency of drinking water treatment for viruses and protozoan cysts.
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From page 266... ...
Chapter 8 In: Fundamentals and Control of Nitrification in Chlorami nated Drinking Water Distribution Systems. AWWA Manual M56.
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From page 267... ...
1995. Taste-and-Odor Prob lems Observed during Drinking Water Treatment.
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From page 268... ...
2002. Decomposition of trihaloacetic acids and forma tion of the corresponding trihalomethanes in drinking water.
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