An Evaluation of Methods for Predicting Total Trihalomethane Concentration in Finished Drinking Water
Creators: name: Hull, Daniel T., III
Publisher: name: The University of North Carolina at Chapel Hill University Libraries
Year: 1995

description: In this study, two models were evaluated for predicting the total trihalomethane (TTHM) concentration in finished drinking water for use in assessing THM exposure in health effects studies. Previously, epidemiologists attempting to measure the effects of exposure to THMs have assumed time and location invariant concentrations of these compounds in water systems. In reality, seasonal changes in the precursor material which reacts with the chlorine disinfectant and hydraulic variations within a water system may result in significant variations in TTHM concentration at a customer's tap. The simpler of the two models measures the linear correlation between TTHM concentration and the amount of chlorine consumed between the treatment plant and the customer's tap. The second model utilizes the Environmental Protection Agency's (EPA) Water Treatment Plant (WTP) simulation to predict TTHM concentration based on water quality parameters including pH, total organic carbon concentration, UV absorbance at 254nm, chlorine dose, temperature, bromide concentration, and contact time between the water and chlorine. Because contact time is synonymous with residence time from the point of chlorination through the treatment plant and in the distribution system to the customer's tap, a network water quality model developed by the Environmental Protection Agency was used to determine values of this parameter. Part of the evaluation was an intensive data collection and THM sampling program in the Orange Water and Sewer Authority (OWASA) water system, located in Orange County, North Carolina. Samples were collected from up to seventeen carefully selected locations within this system on six different occasions. Three of the sets of data were collected by another researcher (Kachur 1994) during the warmest months of the year, while the second three were collected by this investigator during the coldest months. Additionally, historical THM compliance data was collected from ten other water systems in North Carolina. Both models were used on the data from the OWASA system. Initial trials of the WTP modelled to the development of an empirical correction factor to account for a consistent underprediction of TTHM concentration by the model. With this correction, the predicted values from the model agreed reasonably well with the observed data. The model did not appear to account for reactions between chlorine and solid-phase organic material, and this remains a need for further research. Also, future applications of this model should include a more precise measurement of all of the input parameters. For the OWASA system, historical and sampled THM data yielded similar results from the chlorine consumption model. Historical data from other water systems across the state which had single treatment facilities and one water source also fitted this linear model quite well when analyzed separately. However, when data from several systems was pooled, one of the assumptions of the linear regression model was violated. Overall, each model has proven to be very system-specific. Although these methods represent an improvement over previous THM exposure assessment procedures, sufficient data must be available from a given system to calibrate either model for use in predicting TTHM concentration.