Mathematical models for prediction of trihalomethanes in drinking water

Abstract

Use of chlorine for disinfection of water for potable supply is commonly practiced in Australia and elsewhere. Chlorine (Cl2) is an inexpensive and effective disinfectant but will decay in the distribution system due to its reaction with natural organic matter remaining in water after the water has been treated, normally by coagulation and flocculation. The decay kinetics of chlorine is a function of several physico-chemical factors of water quality, including organic content, temperature and pH. In the reaction of chlorine with organic matter, halogenated disinfection by-products (DBPs) can form including trihalomethanes (THMs), haloacetic acids (HAAs) and haloacetonitriles (HANs). These are considered important compounds from adverse human health perspectives with total and individual compound concentrations being regulated in many countries. Because of the importance of halogenated DBPs in drinking water, they have been the subject of much research over recent decades, including modelling of their formation for prediction purposes. Models developed have included statistically fitted ones, incorporating a range of key variables known to affect their formation. However, the mechanisms involved in formation of DBP appear to be numerous, making the development of a model for both total and individual compound formation that can be applied practically, a significant challenge. In this paper, we report a modelling approach to describe the formation of THMs, for total and for the four compound species, chloroform, bromodichloromethane, chlorodibromomethane and bromoform. Data available from chlorine decay experiments that had been conducted on several Australian waters with temperature and pH controlled and at various bromide (Br) concentrations to ~0.7 mg/L were used with for model development. In contrast to models generally reported in the literature where models relate absolute THM values to water quality parameters, here model development is based on standardization of the formation of THM at a nominal set of water quality conditions, i.e. formation of total THMs at 0.1 mg L-1 Br, pH 7 and temperature at 15oC assigned as 100% formation. The impacts of different pH and temperature levels and bromide concentrations on the % formation of total THM (TTHM) were determined. This was done with aim to enable assessment of the impacts of each influencing parameter on total THM formation. From this information, the impacts of these parameters may be evaluated for other waters with different water quality conditions (pH, temperature, Br concentration) in context of chlorine decay following disinfection by chlorination. For modelling of the formation of individual THM compounds, mathematical functions were determined for relationships between Br/Cl2 decay ratios with the percentage formations of chloroform and bromoform, and with the ratios of formation of chlorodibromomethane to bromodichloromethane. From these functions, the absolute formations of each of the four THM species can be estimated from the predicted total THM formed, based on chlorine decay, bromide concentration, temperature and pH of the water. Bromide was found to have a key role in both TTHM and the individual species formed. In this study few data points of controlled temperature and pH were available and consequently simple linear relationships for these were assumed. Future improved model development would require higher resolution data for these parameters.