Disinfection water distribution systems

Methods that may be used to disinfect water distribution systems known to be contaminated with Legionella include superheating and hyperchlorination (2). The use of hyperchlorination is Umited by the fact that Legionella is relatively chlorine tolerant and the concentrations required to kill Legionella result in significant corrosion of the plumbing system. Superheating and flush-... 

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. 

Although worries abound over contamination of the water supply, in reality, the task is quite difficult to accomplish. For example, a contaminant can be dumped into a reservoir, but studies show that the chemical does not mix throughout the entire body of water, even after many hours. There are multitudes of chemical and biological agents that can be used to contaminate the water supply, but all contaminants do not behave similarly. Not all contaminants are threats—some become unstable in water, while others require such large quantities to do harm that they could never be dumped without being noticed. Additionally, if a disinfectant residual is maintained in the water distribution system, that residual will react with the contaminant, and the populace will remain relatively safe. It will be extremely difficult for terrorists to successfully contaminate the water supply. 

Nitrite has a similar action to nitrate, but is usually only found at very low concentrations. It is sometimes formed in water distribution systems when monochloramine is used as a residual disinfectant. Nitrite and nitrate need to be considered together, but monitoring for nitrite is difficult because formation will be in the distribution system. Nitrate levels in surface waters can change quite quickly, but levels in groundwater usually change very slowly unless the groundwater is heavily influenced by surface water. 

Momba, M. N. B., et al. (1998). Evaluation of the impact of disinfection processes on the formation of biofilms in potable surface water distribution systems. Water Science Technol. Wastewater Biological Processes, Proc. 199819th Biennial Conf. Int. Assoc, on Water Quality. Part 7, June 21-26,38, 8-9, 283-289. Elsevier Science Ltd., Exeter, England. 

Free chlorine is highly reactive and relatively unstable. Utilities using free chlorine for disinfection have been known to use secondary chlorination stations to maintain residual chlorine concentrations in the potable water distribution system as regulated by the Clean Water Act (CWA). One of the key concerns in using free chlorine for disinfection is that, under certain conditions, free chlorine may react with organic substances in water to form carcinogenic trihalomethanes (THMs) (1,2). 

Ultraviolet treatment does not provide residual bactericidal action, therefore, the need for periodic flushing and disinfection of the water distribution system must be recognized. Some supplies may require routine chemical disinfection, including the maintenance of a residual bactericidal agent throughout the distribution system. 

Chlorine is unique in that it is the only one of the group to possess the residual disinfectant activity in other words, it maintains its protection of the drinking water throughout the distribution pipeline system. All the other disinfectants, with exception of chlorine dioxide, must be followed with a low dose of chlorine in order to preserve the protection in marginal sites of the water distributing systems. 

Maintenance of appropriate levels of disinfectants in water distribution systems is necessary to ensure the safety of potable water supplies. However, reactions of disinfectants with oxidizable species can lead to undesirable reductions in the concentration of the disinfectant. For example, Fe(II) species in aqueous solution can bring losses of monochloramine. The primary reaction leading to loss of monochloramine is a disproportionation reaction. 

Lund, V. and Ormerod, K. 1995. The influence of disinfection processes on biofihn formation in water distribution systems. 29(4) 1013-1021. 

Woolschlager J., Rittmann B., Piriou P., Kiene L., Schwartz B. 2000. A comprehensive disinfection and water quality model for drinking water distribution systems. First World Water Congress of the IWA, Paris, France. ISBN 2-9515416-0-0. FAN 9782951541603. NP-119. 

Relatively few studies have included the effect of chlorine dioxide on biofilms. Characklis (1990) mentions that chlorine dioxide has been successfully used to control biofouling in several industrial environments. Walker and Morales (1997) studied the effect of chlorine dioxide on a mixed population of drinking water bacteria in a continuous culture model which was developed to simulate an industrial water system. The addition of Img/L chlorine dioxide for approximately 18 h was sufficient to reduce the viable counts of the planktonic population by 99.9%, whereas 1.5 mg/L chlorine dioxide was required to achieve a similar reduction in the biofilms, suggesting an enhanced resistance of biofilm bacteria to the biocide. There are indications that continuous disinfection of drinking water using chlorine dioxide provides a certain control of biofilm formation. In a French drinking water distribution system, the presence of chlorine dioxide allowed a limited surface colonization, while in regions where chlorine dioxide was below the detection limit, an increase in biofilm formation occurred (Servais et al., 1995). 

The two chemical species formed by chlorine in water, HOCl and OCl , are known as free available chlorine and are very effective in kilting bacteria and other pathogens. In the presence of ammonia, HOCl reacts with ammonium ion to produce monochloramine (NH2CI), dichloramine (NHCI2), and trichloramine (NCI3), three species collectively called combined available chlorine. Although weaker disinfectants than chlorine and hypochlorite, the chloramines persist in water distribution systems to provide residual disinfection. 

A study was made of the effects of chloramine disinfectants and free chlorine in water on rubber mechanical parts used in water distribution systems. Tests were undertaken to investigate swelling, surface cracking and loss of elasticity and tensile strength of specimens based on NR, SBR, polychloroprene, nitrile mbber, EPDM, butyl mbber, fluoroelastomers and silicone mbbers. 9 refs. 

Wei, X, Ye, B., Wang, W, Yang, L., Tao, X, Hang, Z. 2010. Spatial and temporal evaluations of disinfection by-products in drinking water distribution systems in Beijing, China. Science of the Total Environment 408 4600 606. 

The chloramines are called combined available chlorine. Chlorination practice frequently provides for formation of combined available chlorine which, although a weaker disinfectant than free available chlorine, is more readily retained as a disinfectant throughout the water distribution system. Too much ammonia in water is considered undesirable because it exerts excess demand for chlorine. 

Because of the decomposition of ozone in water, some chlorine must be added to maintain disinfectant throughout the water distribution system. 

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. 

Chlorate and chlorite ions are disinfection by-products (DBPs) from water treatment using chlorine dioxide. Table 6-2 contains data from four water treatment facilities in the United States that use chlorine dioxide as a disinfectant. Source water samples were also analyzed from each facility and no chlorite or chlorate ions were detected. In all water treatment plants, water taken from the distribution system (i.e., water sampled at water treatment plant) had measurable concentrations of both chlorite and chlorate ions. The ranges of concentrations were 15-740 and 21-330 pg/L for chlorite and chlorate, respectively (Bolyard et al. 1993). 

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). 

All water sources may contain natural organic matter, but concentrations (usually measured as dissolved organic carbon, DOC) differ from 0.2 to more than 10 mg L l. NOM is a direct quality problem due to its color and odor, but more important are indirect problems, such as the formation of organic disinfection by-products (DBPs, e. g. M -halomethanes (THMs) due to chlorination), support of bacterial regrowth in the distribution system, disturbances of treatment efficiency in particle separation, elevated requirements for coagulants and oxidants or reductions in the removal of trace organics during adsorption and oxidation, etc. 

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. 

Crayton, C., Camper, A. and Warwood, B. (1997) Evaluation of mixed oxidants for the disinfection and removal of biofilms from distribution systems, Proceedings of the American Water Eorks Association Water Quality Technology Conference, 3A6/1-3A6/17. 

Monochloramine, used as a residual disinfectant for distribution, is usually formed from the reaction of chlorine with ammonia. Careful control of monochloramine formation in water treatment is important to avoid the formation of di- and trichloramines, because these can cause unacceptable tastes and odours. The formation of nitrite as a consequence of microbial activity in biofilms in the distribution system is a possibility when monochloramine is used as a residual disinfectant, particularly if ammonia levels are not sufficiently controlled. 

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