The objectives of this study were to demonstrate whether the findings from recent laboratory studies on disinfection by-product (DBP) formation apply to full scale water treatment processes, to understand the interaction between DBP formation and treatment process under works conditions and to gain knowledge on the impact of bromide and nitrogen in the formation of brominated HAAs and nitrogenous DBPs under works conditions. This was achieved through a set of seasonal sampling rounds across 8 water treatment works (WTW), each with different treatment processes (including coagulation, ion-exchange, granular activated carbon, membranes and advanced oxidation) and raw water quality (carbon, bromide, nitrogen, source type). The findings demonstrated a clear way forward for managing total regulatory and toxicological risk associated with DBPs for each WTW. The key message was that for some water sources attention should be given to the formation of brominated HAAs. Control of these DBPs may require an additional precursor removal stage or alternative management of disinfection processes.