Seasonal variations of trihalomethanes (THMs) in water distribution networks of Istanbul City

Seasonal variations of trihalomethane (THM) concentrations were investigated within distribution systems of the Buyukcekmece water treatment plant in Istanbul City (Turkey). The investigation was based on an intensive 30-week sampling program, undertaken during the spring, summer and fall of the year 2003. THMs and other water quality and operational parameters were monitored at points along the distribution system between the treatment plant and the system’s extremity. The results showed that THM concentrations vary significantly between finished waters and water at the distribution network. When water temperature exceeds 24°C in summer, the THM levels are 1.2–1.8 times higher than finished water, while when water temperature is below 15°C in the spring and fall, the measured THM concentrations at the system’s extremity were very rarely higher than 100 µg/L. Finally, THM levels were measured at sampling points representing progressively greater travel times from the plant to the extremity of the distribution system. Multiple regression analysis was also conducted in order to estimate THMs from total organic carbon, temperature, and chlorine dose parameters. The regression model resulted in a R2 value of 0.827.

The results of this investigation demonstrate that seasonal variations of THMs are considerable in BWTP. Such variations are mainly related to the significant changes in water quality and temperature that occur throughout the year in BL water and to the related changes in operational parameters during water treatment. The study results showed that THM concentrations vary significantly between finished waters and water at the distribution network. When water temperature exceeds 24°C in summer, the THM levels are particularly 1.2 to 1.8 times higher than finished water, while when water temperature is below 15°C in the spring and fall, the measured THM concentrations at the distribution system were very rarely higher than 100 µg/L, the regulated limit of the EU. On the other hand, during the study, the highest average concentrations of THMs (143 µg/L) were found in the Esenyurt sample where residual chlorine levels were found to be 0.2 mg/L. The highest THM values are related to low residual-free chlorine. In addition, important differences in THM occurrence were also noted among three seasons. The model for seasonal variation of THMs was also developed from data generated during the sampling program using regression analysis. Modeling results show that, with the use of this model, it is possible to estimate THM levels in sampling points of the distribution system. However, it must be noted that the complexity of THM formation reactions makes it difficult to develop universally applicable models. Future research in this area must be directed to the study of spatial and temporal variations of haloacetic acids (HAAs) which are also not currently regulated in Turkey, and also residence time based modeling studies contribute more benefits to THM modeling efforts in Istanbul.