Drinking water distribution systems

The reduction of aqueous chlorine (HOC1) to chloride by Fe° and other ZVMs [Eq. (5)] has long been known as a major contributor to the decay of residual chlorine disinfectant during distribution in drinking water supply systems that contain metal pipes (e.g., Ref. 82). This reaction can, however, be turned to advantage for the removal of excess residual chlorine, and a variety of proprietary formulations of granular ZVMs are available commercially for this purpose (e.g., KDF Fluid Treatment, Inc. Three Rivers, MI). This application is sometimes called dechlorination, but should not be confused with the dechlorination of organic contaminants, which is discussed below. 

Keywords biofilm drinking water distribution system microbiological water quality... 

Most studies were inspired by questions regarding the fate of pollutants upon detection in spreading basins and upon passage of aquifer systems, before recollection, post-treatment, and distribution as drinking water. 

The goal of filtration in the modem municipal treatment plant is a maximum of 0.1 ntu (nephelometric turbidity unit), which ensures a sparkling, clear water (8). Freedom from disease organisms is associated with freedom from turbidity, and complete freedom from taste and odor requites no less than such clarity. The National Interim Primary Drinking Water Regulations (NIPDWR) requite that the maximum contaminant level for turbidity at the point of entry into the distribution system be 1.0 ntu unless it can be shown that levels up to 5 ntu do not interfere with disinfection, interfere with the maintenance of a chlorine residual in the distribution system, nor interfere with bacteriological analyses. 

Water sample collection techniques differ depending on the source being tested. The minimum number of water samples collected from a distribution system which are examined each month for coliforms is a function of the population. For example, the minimum number required for populations of 1,000 and 100,000 are 2 and 100, respectively. To ascertain compliance with the bacteriological requirements of drinking water standards, a certain number of positive tests must not be exceeded. When 10-ml standard portions are examined, not more than 10 percent in any month should be positive (that is, the upper limit of coliform density is an average of one per 100 ml). 

At present, chlorine dioxide is primarily used as a bleaching chemical in the pulp and paper industry. It is also used in large amounts by the textile industry, as well as for the aching of flour, fats, oils, and waxes. In treating drinking water, chlorine dioxide is used in this country for taste and odor control, decolorization, disinfection, provision of residual disinfectant in water distribution systems, and oxidation of iron, manganese, and organics. The principal use of chlorine dioxide in the United States is for the removal of taste and odor caused by phenolic compounds in raw water supplies. 

In a more recent Lederal Register notice (EPA 1991d), EPA examined the occurrences of lead in source water and distributed water. By resampling at the entry point to the distribution system, few samples were found to contain lead at levels above 5 pg/L. EPA now estimates that approximately 600 groundwater systems may have water leaving the treatment plant with lead levels above 5 pg/L. Based on several data sets, it is estimated that less than 1% of the public water systems in the United States have water entering the distribution system with lead levels above 5 pg/L. These systems are estimated to serve less than 3% of the population that receives drinking water from public systems (EPA 199 Id). 

Lead levels ranging between 10 and 30 pg/L can be found in drinking water from households, schools, and office buildings as a result of plumbing corrosion and subsequent leaching of lead. The combination of corrosive water and lead pipes or lead-soldered joints in either the distribution system or individual houses can create localized zones of high lead concentrations that exceed 500 pg/L (EPA 1989f). 

Allison Trentman and B.J. Kronschnabel of the City of Lincoln, Nebraska, Water Treatment Plant Laboratory take samples of drinking water from a distribution system sampling site. 

In 1966 and 1967, when the use of endrin was not restricted, endrin was detected in 5 of 67 raw water samples from the Mississippi and Missouri Rivers (Schafer et al. 1969). At a later time when endrin use was substantially restricted, an Iowa study of 33 community water supplies using surface water found no detectable concentrations of endrin in the distribution systems (Wnuk et al. 1987). In an extensive water quality monitoring program conducted by the California Department of Health Services, endrin was detected (detection limit not specified) in only 2 of 5,109 public drinking water sources sampled from 1984 to 1992, at mean and maximum concentrations of 0.06 and 0.10 ppb, respectively (Storm 1994). Concentrations did not exceed the Maximum Concentration Level (MCL) of 0.2 ppb. In another recent study, endrin was not detected (detection limit not specified) in 32 samples each of raw water and highly treated reclaimed waste water undergoing evaluation as a possible supplement to raw water sources in San Diego, California (De Peyster et al. 1993). 

NDMA (and other nitrosamines) can dramatically increase in concentration in distribution systems (relative to finished water at the drinking water-treatment plant). For example, an initial level of 67 ng/L in drinking water-treatment plant effluent was shown to increase to 180 ng/L in the distribution system [53]. As a result, measurements taken at water-treatment plants may substantially underestimate the public s exposure to this carcinogen. 

Xi C, Zhang Y, Marrs CF et al (2009) Prevalence of antibiotic resistance in drinking water treatment and distribution systems. Appl Environ Microbiol 75(17) 5714—5718... 

Niquette P, Servais P, Savoir R (2001) Bacterial dynamics in the drinking water distribution system of Brussels. Water Res 35(3) 675-682... 

Koskinen R, Ali-Vehmas T, Kampfer P et al (2000) Characterization of Sphingomonas isolates from Finnish and Swedish drinking water distribution systems. J Appl Microbiol 89(4) 687-696... 

Blanch AR, Galofre B, Lucena F et al (2007) Characterization of bacterial coliform occurrences in different zones of a drinking water distribution system. J Appl Microbiol 102(3) 711-721... 

Further data on the effects of chronic inhalation exposure to 1,4-dichlorobenzene would be useful, especially because chronic exposures to 1,4-dichlorobenzene in the air, in the home, and the workplace are the main sources of human exposure to this chemical. Any further testing of the effects of chronic exposure to 1,4-dichlorobenzene via the oral route should probably be done at lower levels of 1,4-dichlorobenzene than those that have already been used in the NTP (1987) bioassay, and should focus on dose-response relationships involving the hepatic, renal, hematopoietic, central nervous system, and metabolic pathways. Data on the effects of chronic dermal exposure to 1,4-dichlorobenzene may be useful if dermal absorption and systemic distribution of 1,4-dichlorobenzene can be demonstrated from toxicokinetic studies, since chronic dermal exposure to 1,4-dichlorobenzene occurs as a result of bathing and showering in drinking water that contains low levels of this chemical in many U.S. communities. 

The water distribution system in the city of Dayton, OH, uses Southdown concrete water mains to deliver water to its citizens. Routine sampling and testing of Dayton s water supply by the city s Department of Water consistently shows that the levels of metals are well below the Ohio EOA Community Drinking Water Standards, and that these levels have remained constant throughout a nine-year testing period from 1982 to 1990. Because metal leaching has not occurred, there is no reason for concern over the safety of Southdown concrete pipes to transport drinking water. 

As regulated by EPA (as of January 1, 2002), the maximum residual disinfectant level (MRDL) for chlorine dioxide is 0.8 mg/L (EPA 2002g) the maximum contaminant level (MCE) for its oxidation product, chlorite ion, in drinking water is 1.0 mg/L (EPA 2002e). The levels of chlorite ion in distribution system waters have been reported as part of the Information Collection Rule (ICR), a research project used to support the development of national drinking water standards in the United States (EPA 2002d). 

Figure 6-2 illustrates the levels of chlorite ion in drinking water sampled from the distribution system versus the percentage of publically owned treatment works (POTW) facilities in the United States that reported as part of the ICR in 1998. Approximately 16% of this group had levels of chlorite ion over the MCL of 1 mg/L. 

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