Investigation of natural organic matter (NOM) character and its removal in a chlorinated and chloraminated system at Rand Water, South Africa

In its natural environment, natural organic matter (NOM) is not problematic. However, during water treatment NOM does affect water quality specifically during the disinfection step, where if NOM is present it reacts with disinfectants resulting in the formation of disinfection by-products. To emphasize the importance of NOM monitoring during potable water treatment this study aimed to characterize NOM and evaluate NOM removal by a conventional water treatment plant considering seasonal trends. NOM was characterized by making use of NOM polarity and specific ultraviolet absorbance. NOM removal was monitored with high-performance size exclusion chromatography, dissolved organic carbon (DOC) and UV₂₅₄ analyses. The polarity rapid assessment method indicated that the hydrophobic and hydrophilic NOM fractions within the surface water increased during a period of heavy rain when floods occurred, but conversely decreased during an average rain season. Although NOM character showed variability during the 5-year study period, seasonal relationship during high and low flow seasons between aromatic NOM and total trihalomethane (TTHM) formation was not evident. Aromatic NOM was not the only precursor to TTHM formation, which stresses the need to implement advanced NOM characterization techniques during NOM monitoring to study reactivity of the individual NOM fraction with the disinfectant used at the water treatment plant.