Most U.S. water utilities use chlorine to comply with CT requirements, and it is general practice to maintain a low-concentration chlorine residual in water distribution systems. This provides essential public health protection by preventing communication of waterborne infectious disease. Chlorine is a strong oxidant that reacts with the natural organic matter (NOM) present in all natural waters. Although chlorine is invaluable in the protection of public health against waterborne disease, some byproducts of the reactions between chlorine and NOM may represent long-term health risks to water consumers. The trihalomethanes (THMs) and haloacetic acids (HAAs) are two important classes of potentially harmful chlorination byproducts that are currently regulated by the USEPA, and therefore of particular interest to water utilities.

Water suppliers that practice chlorine disinfection face conflicting objectives in providing adequate microbial protection while minimizing the formation of harmful chlorinated organic byproducts. Chlorine/NOM reactions may proceed over a period of days under conditions typical in water distribution systems and variations in the source water quality and in environmental conditions such as temperature and pH can affect the rates and extents of byproduct-forming reactions. As a result of chemical and hydraulic dynamics, the chemical composition of water can vary substantially in time and space in distribution systems. Models that can capture these dynamics would be powerful tools for optimizing water quality in distribution systems.

Mechanistic models are expected to be more robust and flexible than purely empirical models. Mechanistic models may also provide insights into the processes being modeled. It is therefore desirable for kinetic models to reflect the underlying chemistry to the extent possible. Mechanistic modeling has been considered impractical due to the complexity of NOM molecules and NOM/chlorine reaction pathways. However, very complex computations have become feasible using personal computers due to recent advances in computer technology, so that more complex models are practical. The goal of this research is to develop a kinetic model that captures the important mechanistic features of chlorine/NOM reactions.

Existing models. Efforts to model chlorine decay in natural waters began before 1950 (1,2) and a number of studies have been conducted where chlorine decay was modeled in the contexts of drinking water, waste water, and power plant cooling water. The most widely used form for modeling chlorine decay has been the first-order (exponential decay) form. Several investigators have presented empirical models for THM formation as a function of other parameters such as UV absorbance, TOC, pH, temperature and bromide concentration (3-7). These are linear models (in log-transformed variables) that have been fit to large experimental data sets by multivariate linear regression procedures. This approach has also been used to model HAA formation (7). Models that include second-order terms (i.e. terms for both NOM site and chlorine concentration) have been proposed by MacNiell (8), Jedas-Hecart et al. (9), and Clark (10, 11).

Vasconcelos et al. (12) compared several models by fitting them to data sets from bottle tests of eleven different treated waters. Goodness of fit (as measured by the adjusted coefficient of determination R²) for the widely used first-order model was generally poor. For other models compared by Vasconcelos et al. (12), reasonably good fits were achieved with wide variations in the parameter values for the various data sets. This lack of robustness of model parameters is a shortcoming of existing models that we hope to improve with a more mechanistic modeling approach. A mechanistic approach where reactant concentrations and important environmental variables are represented explicitly should be well suited for practical applications such "what if" modeling, modeling of "reactive" chlorination byproducts, and modeling distribution systems with multiple sources of different NOM quality or different chlorine doses.

Reaction mechanisms. The aqueous chlorination of organic compounds has been studied by a number of researchers (13-22). The organic substrates investigated included various pure ketones, organic acids, and aromatics as well as organic fractions extracted from natural waters. Some key findings that bear on the development of a mechanistic model are summarized in this section.

Under conditions typical in water treatment, HOCl and OCl^- predominate and Cl₂ is not present in significant concentrations. HOCl is expected to be more reactive as an oxidant and halogenating agent than OCl^- (23). The functional groups within NOM molecules that have been implicated as important chlorine consumers and byproduct precursors include activated aromatics (particularly those with meta hydroxyl substituents) and some oxygen-rich ketones and keto-acids (16-21). For several ketones and keto-acids, the rate of reaction with halogens is first-order in ketone and not sensitive to halogen concentration except at very low levels (13-15). The reason is a rate-limiting initial dissociation of a proton to form enolate, the reactive species. In the classic haloform reaction of halogens with methyl ketones, enolate formation is followed by a faster halogen addition. After the initial halogenation the remaining protons are more acidic so that the second and third ionizations and halogenations proceed rapidly followed by base-catalyzed hydrolysis to form haloform (23).

For orcinol and resorcinol, the O^-substituted species react much faster with chlorine than the corresponding OH substituted species. Nevertheless, the completely protonated species are predominant and are expected to be the most significant participants in chlorine reactions over the pH range of interest in water treatment (22). Acid-base chemistry would play a more important role for aromatics with more acidic substituents, and a first-order rate-limiting step similar to the rate-limiting step of the haloform reaction could be envisioned.

Most of the reaction mechanisms that have been proposed for chloroform formation pass through a trichloromethyl ketone intermediate (16, 19, 21). Chlorine concentration and pH can affect the speciation of products even where reactions of the intermediate are not rate-limiting steps in the overall reaction (21, 24).

The objective is to capture the most important elements of the reaction, i.e. the elements that control the overall reaction rate, and those that affect the proportions of products that are formed. Based on the reaction mechanisms that have been proposed, it seems appropriate to include two conceptual reaction pathways in the model: one with second-order kinetics, and one with a rate-limiting first-order step similar to the haloform reaction. Chlorine consumption and byproduct formation rates will be determined as functions of the reaction rates through these conceptual pathways.

A series of bench-scale experiments was conducted to characterize the effects of temperature, pH, and reactant concentrations on chlorine decay and byproduct formation in a treated water. These experiments were conducted using three samples collected at different times from the same source, filter effluent of the Lake Gaillard Water Treatment Plant, No. Branford, CT. This plant treats an impounded surface water source. The process includes alum coagulation, flocculation, and anthracite/sand filtration. Table 1 gives UV absorbance and TOC values for each of the samples.

Table 1. TOC and UV Absorbance of samples used for bench scale experiments.

A series of bottle tests was conducted with each sample. For these tests, four-liter batches were prepared by adjusting pH and temperature to the desired values (and diluting with deionized water in some cases). Batches were then chlorinated and dispensed into 300 mL bottles. Table 2 shows the conditions used for the bench scale experiments.

Table 2. Bench scale bottle test experiments.