Understanding the factors influencing the formation of disinfection by-products in drinking water: Quantification and Characterisation of aquatic humic substances Huma Sanawar, Josephine Treacy, Paul Vesey Department of Applied Science; School of Science, Engineering and Information Technology School of the Built Environment; Limerick Institute of Technology Email: huma.sanawar@lit.ie; josephine.treacy@lit.ie; paul.vesey@lit.ie Humic acids form a significant fraction of aquatic natural organic matter (NOM). They are known to react with disinfectants during the production of drinking water. Thereby, humic acids act as major precursors to hazardous disinfection by-products (DBPs). Information on the complex chemical structures of humic acids is unresolved and the mechanism of DBP formation is unclear. This research focuses on understanding the extent of DBPs formation under different treatment and source water conditions, including the NOM character of abstraction waters. This poster will explain how humic acids, in the presence of disinfectants, can form toxic by-products such as trihalomethanes (THMs) and haloacetic acids (HAAs). The quantification of humic acids in water using a liquid-liquid extraction procedure followed by UV-Visible spectrophotometric analysis at 520 nm will be described. Humic acids will also be characterised by Infrared Spectroscopy in an effort to understand the molecular nature of NOM in source waters. The importance of characterisation and quantification of humic acids to prevent the formation of DBPs will be demonstrated in this poster. This research will provide support for the regulation of disinfectant by-products with regard to the EU Drinking Water Directive and public health issues.