The operation of ozone contactors needs to be optimized to provide adequate disinfection while minimizing the formation of disinfection by-products. However, measuring the actual disinfection efficiency is impractical in a full-scale system. The objective of this project was to demonstrate of the use of polystyrene microspheres to assess and optimize the performance of full-scale ozone disinfection systems. Specifically, the researchers developed and optimized microspheres with laboratory-scale experiments, demonstrated the use of microsphere method with a full-scale ozone contactor, and applied a mathematical model for the prediction of disinfection efficiency and bromate formation in the full-scale ozone contactor.