The occurrence of natural organic matter (NOM) in source waters presents a concern in water treatment plants due to formation of toxic disinfection byproducts (DBPs). In this doctoral thesis, the fate of NOM during ozonation, biofiltration, and chlorination was investigated to identify important aspects in these processes that can be manipulated for better DBP control. Specifically, this thesis studied (1) reaction mechanisms of ozone with dissolved organic nitrogen (DON), an important fraction of NOM that forms nitrogenous DBPs, (2) role of ozone and• OH-mediated attack on NOM and DBP formation during chlorination, and (3) impact of ozonation on biodegradability of DBP precursors.The reaction of ozone with DON (Chapter 4) was observed to produce various transformation products including nitrate (NO 3 -) and ammonium (NH 4 + residual after 24 h = 1 -2 mg/L) as follows: THM4 (37%), HAA8 (44%), CH (107%), HK2 (97%), HAN4 (33%), trichloroacetamide (TCAM, 43%), and AOX (27%), but a decrease in concentrations of THNM2 (43%). Coupling ozonation with biofiltration prior to chlorination effectively lowered the formation potentials of all DBPs including CH, HK2, and THNM2, all of which increased after ozonation. The dynamics of DBP formation potentials during BAC filtration at different EBCTs followed first-order reaction kinetics. Minimum steady-state concentrations were attained at an EBCT of 10 -20 min, depending on the DBP species. The rate of reduction in DBP formation potentials varied among individual species before reaching their minimum concentrations. CH, HK2, and THNM2 had the highest rate constants of between 0.5 and 0.6 min -1 followed by HAN4 (0.4 min -1 ), THM4 (0.3 min -1 ), HAA8 (0.2 min -1 ), and AOX (0.1 min -1 ). Relative to concentrations after ozonation, the reduction in formation potential for most DBPs (e.g., at 15 min EBCT) was less than 50% but was higher than 70% for CH, HK2, and THNM2. The formation of bromine-containing DBPs increased with increasing EBCT likely due to an increase in Br -/DOC ratio.-iv- Dr. Wolfgang Gernjak co-advised me during my PhD candidature and played an integral part in all the chapters of this thesis. He was involved in the initial conceptual thinking, to developing research ideas, identifying appropriate experimental design, and interpreting research data. He also provided critical reviews in all the manuscripts. Declaration by authorDr. Maria José Farré, who was my principal advisor in 2013, is recognized for sharing her expertise in disinfection byproduct research (i.e., from laboratory techniques, instrumentation, and theory). Our regular exchange of viewpoints were essential in the conception of research ideas and approaches related to DBPs. She was also highly involved in the writing and review of manuscripts.Prof. Urs von Gunten, who is a world-leading expert in application of ozone for water treatment, was my host professor during my research visit at École Polytechnique Fédérale de Lausanne (EPFL)where I performed the bulk of the experiments repor...