Assessment of trihalomethane (THM) precursors using specific ultraviolet absorbance (SUVA) and molecular size distribution (MSD)

It is well known that natural organic matter (NOM) is the major precursor of disinfection by-products (DBPs) when inadequately removed NOM reacts with the disinfectant during chlorination and chloramination. Special interest in the DBP precursors is justified as it is essential to understand the reactivity and character of the precursors responsible for the formation of DBPs. This is vital in order to develop processes that will enhance precursor removal during drinking water treatment. The aim of this study was to investigate the impact of the characteristics of NOM within an oligotrophic water source on the formation of trihalomethanes (THMs) during drinking water treatment at Rand Water. Specific ultraviolet absorbance (SUVA) and molecular size distribution (MSD) were the main NOM characterisation techniques utilised during this study. The MSD was investigated by making use of high-performance size-exclusion chromatography (HPSEC). The MSD results showed a significant correlation between the high-molecular-weight (HMW) fraction of NOM and TTHM formation, specifically during the summer months. The positive correlation existing between SUVA and full scale NOM removal indicates that SUVA can be used to indicate NOM treatability during water treatment. A link between chloroform formation and the HMW fraction of NOM was also observed; however, the formation of bromodichloromethane (BDCM) was not due to the HWM fraction, as indicated by a weak regression coefficient. The results displayed are an indication that the aromatic fraction of NOM was the main precursor to TTHM formation, more prominently during summer.