AWWA DPOC56499

The occurrence of microbial contamination in drinking water distribution systems can occur as a result of a system breech or as a result of microbial regrowth. Once in the distribution system, cells attach to pipe walls to form a biofilm which can cause biocorrosion of the pipes, noncompliance with drinking water guidelines, and undesirable aesthetic changes in the drinking water. Other repercussions associated with microbial activity in the distribution system include episodes of coliform growth, an increased likelihood of waterborne disease, and an increased nitrate concentratio. Secondary or residual disinfection is the process of applying a disinfectant that provides a residual concentration in the distribution system. The three main disinfectants used for secondary disinfection are chlorine, chloramines, and chlorine dioxide. Each of these disinfectants have advantages and disadvantages, and the type of disinfectant used, as well as the residual required, will depend upon the finished water quality and pipe characteristics. At the same time, suppliers must minimize disinfection byproduct formation, as well as control any unwanted taste and odour episodes that may be associated with secondary disinfection. In North America and some European countries, maintaining a disinfectant residual in the distribution system has been used as a method of controlling biofilm formation in the distribution system, and preventing microbial contamination in the instance of a system breech. This has been done based on the premise that maintaining an adequate disinfectant residual in the distribution system can minimize regrowth of indicator or pathogenic microorganisms, detect microbial intrusion more rapidly than microbial monitoring alone, and reduce the risk of general contamination as a result of a distribution system breech. But what is considered an adequate residual? The objective of this paper to provide an overview of secondary disinfectants, the regulation in Canada and the US, and evaluate the physical and chemical factors that affect the management of a disinfectant residual. Includes 18 references, figure.