Page 10: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Turbidity
Part II. Science and Technical Considerations (continued)
9.0 Distribution System
Turbidity, along with other water quality parameters, is frequently monitored to help guide the operation and maintenance of distribution systems. Several studies have also identified turbidity as a useful indicator for assessing the integrity of distribution systems (Kirmeyer et al, 2000, 2001; U.S. EPA 2006c). Consequently, turbidity should be included in routine monitoring of the distribution system so that deviations from normal conditions can be detected. Turbidity within the distribution system can be monitored in conjunction with other parameters, such as pH, disinfectant residual, E. coli, HPC, total coliforms and pressure to obtain a better understanding of the source of turbidity and thus, the appropriate corrective actions to take when turbidity increases are observed.
Turbidity can serve to signal potential contamination problems or difficulties within a distribution system. Increased distribution system turbidity can be indicative of breaches to the distribution system integrity, such as main breaks, backflow, intrusion, cross connections or detachment of biofilm, which may compromise the microbiological quality of the water (Kirmeyer et al., 2000). As discussed in Section 7.1.1, several studies have documented correlations between increasing levels of plate count microorganisms and increased turbidity (Snead et al., 1980; Goshko et al., 1983; Haas et al., 1983, Power and Naby, 1999). Turbidity may also increase due to hydrant opening, system maintenance and repairs, and valve failures. One study noted that turbidity was repeatedly above 1 NTU at a routine sampling site due to a valve that had been closed creating a dead-end condition in the system (Burlingame and Johnson, 2002).
Increased turbidity has also been associated with the release of corrosion products or disturbances of deposits (U.S. EPA, 2006d). Burlingame and Johnson (2002) reported a statistical correlation between distribution system samples with turbidity greater than 2 NTU and iron concentrations greater than 0.3 mg/L. Distribution systems can also experience heavy loading of sediments with the potential for accumulation of inorganic contaminants in the distribution system when source water turbidity is elevated. In particular, groundwater and other unfiltered source waters where the influent turbidity was greater than 1.0 NTU have been identified as systems where significant levels of sediments may be introduced to the distribution system (U.S. EPA, 2006d). Discoloured drinking water in the distribution system is often attributed to the presence of colloidal and particulate iron which can originate from both distribution system materials and source water (Schock and Lytle, 2011).
Although turbidity measurements cannot automatically be used to interpret the safety of water in the distribution system, a goal to maintain turbidity values below 1.0 NTU has been proposed in several publications (Geldriech, 1996; Burlingame and Johnson, 2002; Friedman et al., 2005). In addition, several case studies of utilities using 1.0 NTU or less as a value to trigger an investigation of distribution system integrity have been documented (Kirmeyer et al., 2001; Burlingame and Johnson, 2002). The concept of optimization of the distribution system has recently been introduced to the water industry (Friedman et al., 2005, 2010; Lauer, 2010). An important aspect of distribution system optimization plans is the development of water quality and operating goals. A turbidity level below 1.0 NTU has been identified as one potential goal for utilities to use for routine monitoring and operations, release to service of water mains, maintenance of water quality at dead-ends and consumer acceptance at the tap (Friedman et al., 2005).
Excessive turbidity has often been associated with unacceptable tastes and odours. Turbidity in excess of 5 NTU also becomes visually apparent and may cause consumers to object to the water. In some cases, if the level of turbidity is not addressed and the organic loading is not reduced in advance of applying certain chemicals, other health-related contaminants (e.g., DBPs) may be formed or released. Every effort should be made to keep the turbidity as low as reasonably achievable in the distribution system by minimizing turbidity entering the distribution system and conducting routine maintenance activities such as flushing and cleaning the pipelines.
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