Modeling chlorine decay and chlorination by-product formation in water treatment and distribution
Water suppliers that practice chlorine disinfection face conflicting objectives in providing adequate microbial protection while minimizing the formation of harmful chlorinated organic by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs). 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. The goal of this research was to develop a mechanism-based model for chlorine/natural organic matter (NOM) reaction kinetics and incorporate it in a computer program for predicting distribution system chlorine, THM and HAA concentrations.
Laboratory experiments were performed to characterize the kinetics of chlorine decay and chlorination by-product formation in treated drinking waters. In these experiments, treated waters were chlorinated under various conditions and chlorine, THM, and HAA levels were monitored as the reactions proceeded. The data from these experiments were used to develop and calibrate a kinetic model. The kinetic model is based on a simplified conceptual reaction mechanism. The form of the model is a system of differential equations that is solved numerically. Good fits (coefficients of determination > 0.90) to data sets that included as many as five different waters chlorinated under a variety of conditions were achieved.
The kinetic model was incorporated into a computer program for predicting distribution system chlorine, THM, and HAA levels. The computer program tracks chlorine, by-product, and reactive NOM site concentrations. Local reaction rates may be computed as functions of one or more substance concentrations. The model was field-tested in the New Haven, Connecticut distribution system. In order to test the model, samples were collected in the distribution system and chlorine, THM, and HAA levels were measured. Computer simulations of the sampling days were created and model predictions were compared to the field measurements. Good agreement (most predictions within ±20% of observations) between field measurements and model predictions was observed, particularly for THMs. There was no apparent bias in the model predictions for chlorine, THMs, or for the di-halogenated HAA species.