Chlorine Decay

CHAPTER 12
Modeling Chlorine Decay and the Formation of Disinfection By-Products (DBPs) in Drinking Water1
Introduction
A major objective of drinking water treatment is to provide microbiologically safe drinking water. The combination of conventional drinking water treatment and disinfection has proved to be one of the major public health advances in modern times.
In the U.S., chlorine is most often the final disinfectant added to treated water for microbiological protection before it is discharged into a drinking water distribution system. However, disinfectants, especially chlorine, react with natural organic matter (NOM) to form disinfection by-products (DBPs), which are considered to be of concern from a chronic exposure point of view.
Drinking water disinfection, therefore, poses the dilemma of a risk tradeoff. Chemical disinfection reduces risk of infectious disease, but the interaction between chemical disinfectants and precursor materials in source water results in the formation of DBPs. Although disinfection of public drinking water has dramatically reduced outbreaks of diseases attributable to waterborne pathogens, the iden-tification of chloroform, a DBP, in drinking water (Rook 1974; Bellar and Lichtenberg 1974) raised questions about possible health risks posed by these DBPs. Since 1974, additional DBPs have been identified, and concerns have intensified about health risks resulting from exposures to DBPs.
All natural waters and even treated drinking water exerts disinfectant demand due to the reactions with NOM and other constituents in water. Therefore, the applied disinfectant dose must be sufficient to meet the inherent demand in the treated water, to provide sufficient protection against microbial infection, and at the same time minimize exposure to DBPs.
Consequently, much research has been invested in attempting to characterize the nature of DBPs and the conditions that govern their formation in drinking water. One aspect of this research is the devel-opment of mathematical models for predicting the decay of chlorine and other disinfectants and for predicting the formation of DBPs themselves.
This chapter reviews current and historical research efforts related to the development of models for predicting the decay of disinfectants and the formation of DBPs. It focuses on chlorine as a disinfec-tant and emphasizes U.S. Environmental Protection Agency (EPA) research efforts in this area. The conditions that govern the interaction of NOM and chlorine and the resulting formation of DBPs are discussed. Research devoted to models for chlorine decay and the formation of DBPs are reviewed. The factors that affect exposure to DBPs are examined, and EPA field research studies that have driven the current research on chlorine decay and DBP formation are presented. The development of EPANET, a state-of-the-art public sector water quality/hydraulic model, is reviewed, along with the evolution of numerical modeling techniques. The topic of storage tanks and their impact on water quality and the public policy issues associated with this research is also discussed.
1Robert M. Clark, Lewis A. Rossman, Mano Sivagesean, Kathleen Schenck: ORD/NRMRL/ WSWRD, AWBERC Mailstop 689, 26 West Martin Luther King Dr., Cincinnati, OH 45268. Corresponding Author: Robert M. Clark, 513-569-7201, clark.robertm@epa.gov.

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